Symposium PM4 : Novel Materials, Fabrication Routes and Devices for Environmental Monitoring

Semiconductor gas sensors are widely used for detection of inflammable and toxic gases. To detect such low concentration gases, we have reported materials design including important three functions i.e. receptor function, transducer function and utility factor. Receptor function concerns the ability of the oxide surface to interact with the target gas. In addition, when the surface is loaded with a foreign receptor like PdO, it acts as a receptor stronger than the adsorbed oxygen. In addition, we found that small size of PdO (less than 3nm) shows high sensor response to inflammable gases even under humid condition. Transducer function concerns that the electron transport through the contact can thus be achieved by migration or tunneling of the surface electrons, indifferent to the bulk electrons inside. The device resistance is then inversely proportional to the surface density of electrons. Therefore the sensor response enhances with increasing oxygen partial pressure. For the utility factor, the target gas molecules diffuse the inside of a sensing body while reacting with the oxide surface. The above design and combination of three factors give gas detection in ppb level. However, for practical use, we must pay attention to water vapor poisoning. Under humid condition, it is well known that water molecules adsorbed on the surface give a large effects on the sensitivity and selectivity. For SnO₂, the sensor response deeply concerns oxygen adsorption species which is O^2- and O^- in dry and wet atmosphere, respectively. To enhance the sensor response, we proposed importance of surface modification and nano-size Pd loading on SnO₂. For surface modification, we found that Fe³⁺ and Sb⁵⁺ modifications gave O^2- adsorption species under humid condition. In addition, Pd nano-size loading showed constant sensor response even by changing humidity. Those material designs are important for practical use, and should be introduced to MEMS gas sensors. In the presentation, the above will be shown.

Titanium oxide is a well known photo-catalyst thereby it is expected to be an excellent gas sensor as both are exclusively surface active phenomena. Herein, titanium oxide thin films were prepared using the Pulsed Laser Deposition (PLD) technique. The target used for the ablation was a defect-free phase-pure stoichiometric TiO2 pellet. We observed that mere change in the ablation energy of laser pulse from 200 mJ to 500 mJ can induce substantial variation in surface defect chemistry of the film thereby enhancement in response to toxic H2S. The techniques such as XPS, AFM, STM and UPS have revealed the cause. We present couple of attempts towards further enhancing the toxic H2S response of titanium oxide based sensors on engineering the defects at sensor film surface.
Firstly, the use of different substrates for deposition can induce significant variations in the surface electronic structures of the film. The variation in the surface defect structure is expected due to mismatch between the lattice parameters of titanium oxide and substrate under deposition. Different substrates such as Si, LAO, STO, sapphire, polycrystalline Al2O3, soda-lime glass etc. were used for the film deposition to induce variation at the film surface. Gold pads with 1mm separation were made for Ohmic contacts on these films using the thermal evaporation technique. Testing of H2S response was done using a laboratory made gas sensing set-up. It was observed that films deposited on Si and LAO were found to be highly sensitive to H2S (with response about SR % ~ 100000) than any other substrate at an operating temperature of 100oC with faster response (few seconds) and quicker recovery (~4 min) times at 50 ppm of H2S.
Secondly, doping titanium oxide wherein the mismatch due to the differences in the ionic radii, electro-negativity and valence states between dopant and the host ions can induce different bulk as well as surface active defects. Thin films of titanium oxide doped with different elements such as Cu, Zn, Al, Na, Fe etc. were deposited on LAO substrate and have been tested towards H2S gas response. It was observed that Cu-doped titanium oxide film exhibited extra-ordinary H2S response than any other doped titanium oxide systems. The presently observed H2S response of SR % ~ 1,36,000 at 50 ppm of H2S is the best in the literature for a Cu-doped titanium oxide film on LAO. These films were found to be sensitive to H2S down to sub-ppm level also. The valence states of titanium in the film were +4 and 0. The film composition concluded to be Ti/TiOx and AFM imaging showed increasing surface roughness with vertically aligned short nano-wires on the film. It is inferred that the enhanced H2S response is due to the excessive over layers of chemi-sorbed oxygen on the film surface.
These data will be discussed and presented.
One of the authors TCJ would like to acknowledge DST, Govt. Of India for INSPIRE Faculty Award [IFA13, PH-73].

Ref: ChemRev 2007 107 2891; RSCAdv 2015 5 93081.

Resistive metal oxide (MO_X) devices fail in the selective detection of a specific molecule because they rely on unspecific redox interactions between surface oxygen and gaseous species. Variations of materials, dimensions, crystal phases, decoration of the surface with foreign inorganic atoms, or the synthesis of heterostructures have shown to direct the sensing interaction toward certain targets, but cannot solve the selectivity problem completely, and are mostly unpredictable in their consequences.
To overcome this limitation, the organic components can be employed to modify the surfaces of MO_X, which can provide a huge variety of functional groups and properties and be immobilized as self-assembled monolayers (SAMs). In this sense, by carefully choosing the composition of this functional layer, it should be possible to tailor the reactivity of the sensors at will, mimicking the biological receptors mechanism. Thus, these organic layers can be considered as the most appropriate candidates for this high selection purpose.
Here, we present a sensor system composed of semiconductor surface (SnO₂) with defined organic SAMs in order to accomplish exclusive chemical and electronic conditions for the selective detection of a single gas species. Amine with different carbon chain lengths and classes (i.e., primary and secondary groups), and urea terminated molecules are presented as potential candidates for the selective detection of NO₂.
Moreover, for this proposed approach, it is feasible to drive the new gas-organic reactions with much lower energies compared to those needed for conventional MO_X sensors, due to the weak nature of these interactions. Specifically, visible light is sufficient to subsequently activate and reset the sensor signals.

In recent years, the number of terrorist’s attacks involving improvised explosive devices or IED’s has grown at an alarming rate. With the most recent terrorist attacks in Paris and Brussels, the need for an electronic trace detection system that can monitor these threats and alert the traveling public to these threats is paramount. Recently, nanotechnology has enabled us to achieve higher sensitivity and better selectivity for potential threats used by terrorists. A number of metal oxide nanowires have been incorporated into our gas sensor for use as a catalyst and/or catalyst-support for the detection of explosives at the ppb level. These oxide nanowires including zinc oxide, copper oxide and iron oxide, were directly grown onto the active sensor elements to detect TATP, ammonium nitrate and 2,6-DNT, commonly used explosives in IED’s. These oxide nanowires were characterized using SEM, XRD and XPS. Based on results of testing for these explosives in the vapor phase using our gas sensor, it will be possible to detect a broader array of potential threats as well as detect at even lower levels for those explosives currently being investigated.

The use of ZnO nanostructures as sensing layer in chemiresistive gas sensors is promising, although the limited selectivity, high response/recovery time, high-power consumption, and lack of long-term stability have limited their use in more demanding applications [1]. Nowadays, many strategies have been developed to improve the gas sensing properties of ZnO-based gas sensors, including the use of catalysts and promoters. Specifically, the loading of ZnO with noble metals (e.g., Au, and Pd), that act as sensitizers or promoters, has been considered as an effective method to catalyze the gas-sensing reactions [34].
In this contribution, the preparation and the characterization of chemiresistive gas sensors based on ZnO nanorods, electrochemically functionalized with Au and Pd nanoparticles (NPs), to improve the sub-ppm detection of gaseous pollutants, compared to pristine ones, are reported.
ZnO nanorods were prepared by hydrothermal sol-gel synthesis with further annealing at 550°C [3]; the metal (Au and Pd) NPs were simultaneously synthesized and deposited directly on hydrothermal ZnO nanostructures, previously desiccated, by sacrificial anode electrolysis (SAE) [4], and, further, the functionalized ZnO nanostructures were subjected to stabilization at 550°C. Both, pristine and metal functionalized ZnO nanorods were proposed as active layer in chemiresistive sensors for environmental monitoring to detect pollutant gases (e.g. NO₂, H₂S, NH₃, C₄H₁₀).
The effect of the presence and of the chemical nature of the deposited metal catalyst on the performance of ZnO-based gas sensor (e.g., sensitivity, selectivity and recovery) were analyzed, comparing the sensing results with those of pristine ZnO nanorods. In particular, the gas sensing properties of pristine and metal-functionalized ZnO nanorods were studied at an operating temperature of 300°C towards various pollutant gases in a wide range of concentrations. Specifically, for the detection of NO₂ gas, Au-decorated ZnO nanordos showed the highest selectivity and sensitivity; instead, Pd-doped ZnO nanorods revealed a higher sensitivity and selectivity towards hydrocarbon gases, such as CH₄, and C₄H₁₀, comparing to those reported for the pristine and Au- ZnO-based gas sensor. Therefore, the obtained results have revealed the possibility to tune the sensitivity and selectivity of ZnO-based chemiresistitve gas sensors towards a gas target simply by the presence and type of metallic element deposited as catalyst on ZnO nanorods.
Finally, the sensing mechanisms of pristine and metal-decorated ZnO nanorods towards the analyzed gaseous pollutants have been proposed, considering the effect of their chemical composition and morphology evaluated by XPS and SEM analyses, respectively.

[1] S.M. Kanan et al., Sensors 2006, 9, 8158–8196.
[2] M. Penza et al., Sens. Act. B 1998, 50, 52–59.
[3] E. Dilonardo et al., Beilst. J. Nanotech. 2016, 7, 22.
[4] M.T. Reetz et al., J. Am. Chem. Soc. 1994, 116, 7401.

Electrical sensing of volatile molecule species in our living environments using mobile electronics is an important issue for future electronics. However, conventional gas sensors including semiconductor oxide sensors have been difficult to be integrated into CMOS electronics and/or wearable electronics due to the relatively high energy consumption (~ J/s) and working temperature (200-300°C) to ensure a rapid and reversible sensing performance. Here we report the thermal management of oxide nanowire sensor in both spatial and time domains by utilizing unique thermal properties of nanowires, which are the reduced thermal conductivity and the short thermal relaxation time down to several micro-seconds. Our method utilizes a pulsed self-Joule-heating for suspended SnO₂ nanowire device, which enables not only the gigantic reduction of energy consumption down to ~ 10² pJ/s, but also enhancing the sensitivity for electrical sensing of NO₂ (100 ppb). Furthermore, we demonstrate the applicability of the present method as sensors on flexible polyethylene naphthalate (PEN) substrate. Thus, this proposed thermal management of nanowire sensor in both spatial and time domains offers a strategy for exploring novel functionalities of nanowire-based devices.

The ongoing trend towards global urbanization results in increasing levels of air pollutants in urban areas and megacities, leading to decreasing air quality and creating significant threats to health worldwide. Despite of this global phenomena, the local situation related to indoor and outdoor air quality are quite divergent: While in a large part of mainly western urban areas, outdoor air is assumed to cleaner than indoors, in numerous cities mainly in Asia a higher level of pollutants is present in outdoor air. Existing monitoring networks of air quality in cities, infrastructures and buildings are completely separate systems using different technological approaches. While outdoor monitoring usually uses a limited number of supporting points equipped with analytical grade instrumentation, the market for indoor air quality monitoring is mainly driven cost-driven, so that the detection technologies have been adapted to fulfil minimum requirements at an acceptable price. However the gap in price and performance of low cost sensors and analytical instruments have reduced by substantial innovations in sensing technologies obtained in the recent years. Partially driven by upcoming MEMS technologies, numerous detection methods spreading from solid state sensors, miniaturized optical systems and concentration and separation approaches strive to reach a level of performance, which may open up the way to the large-spread use of environmental sensor systems and new strategies for monitoring and control of air quality, taking into account localized data for indoor and outdoor air. Typical gases to me measured indoors are CO2, rh, VOC with some differentiation, typical gases that characterize the outdoor air quality are NOx, O3, VOC and there is additionally the need to detect particulate matter. We give an overview of our work on this topic, which is based on semiconducting metal oxide sensors, work function based sensors and tunable laser diodes. An appropriate room air ventilation strategy needs to judge both air qualities to decide in the appropriate time for ventilation or the need for filters.

X-ray absorption near edge structure (XANES) and electrical measurements were used to elucidate the local structure and electronic changes of copper (II) oxide (CuO) nanostructures under working conditions. For this purpose, a sample holder layout was developed enabling the simultaneous analysis of the spectroscopic and electrical properties of the sensor material under identical operating conditions. The influence of the different carrier gases (e.g., air and N₂) on the CuO nanostructures behavior under reducing conditions (H₂ gas) was studied to analyze how a particular gas atmosphere can modify the oxidation state of the sensor material in real time as function of operating temperature.

During the past few decades, the single crystalline gold nanoparticles has attracted great attentions as necessary catalytic decorations on the metal oxide nanostructure sensor for effective detection of toxic and hazardous gases due to the high surface-to-volume ratio, excellent chemisorption ability upon target gas molecules exposures, good mobility of charge carriers and good physical and chemical stability under operating temperature. Remarkably enhanced sensitivity and selectivity towards target gas have been achieved when gold nanoparticles were deposited as catalytic decorations on the surface of Zinc Oxide nanowires.¹ However, the catalytic activity of noble metal nanoparticles are found to be irreversible in high temperature region (>400 degree Celsius) due to the sintering effect.² As a result, the chemical metal oxide sensor with noble metal decorations could only be adopted in a relatively low temperature range. Herein, super paramagnetic iron oxide nanoparticles (SPION) were introduced as the gold nanoparticles support forming the ternary ZnO-Fe₂O₃-Au hybrid structure. The enhancement resulted from the noble metal nanoparticles are found to be further optimized towards NO₂ detection in terms of sensitivity and stability. When considering the NO₂ molecules chemisorption, the gold nanoparticles are electron deficient while the Iron Oxide nanoparticles could offer the free electrons generated by the Fe²⁺→Fe³⁺+e^- transition.³ The electron transfer from the iron oxide support to gold mediated by the O₂^- surface species could greatly promote the target gas chemisorption process, and this may result in the significant enhancement of sensitivity in comparison to the metal and metal oxide binary system. In addition, the sintering effect could be efficiently prevented due to the relatively excellent stability of iron oxide nanoparticles support at high temperature. And it enable the zinc oxide nanowires sensor decorated with hybrid nanoparticles function well for high temperature gas detection.

References:
1. Xiang, Q., et al. "Au nanoparticle modified WO3 nanorods with their enhanced properties for photocatalysis and gas sensing." The Journal of Physical Chemistry C 114.5 (2010): 2049-2055.
2. Liu, S., et al. “Preparation of zinc oxide nanoparticle–reduced graphene oxide–gold nanoparticle hybrids for detection of NO₂” RSC Advances 5 (2015):91760-91765.
3. Horváth, D., L. Toth, and L. Guczi. "Gold nanoparticles: effect of treatment on structure and catalytic activity of Au/Fe2O3 catalyst prepared by co-precipitation.” Catalysis letters 67.2-4 (2000): 117-128.

Nanostructures are used as principal components of sensing systems with the final aim of enhancing the “3-S” values of the gas sensors: sensitivity, selectivity and stability. Using nanowires (NWs) as main active components of a sensing system allows to increase the surface to volume ratio and, combined with their highly crystalline nature, a corresponding enhancement of the sensing properties is expected. Moreover, when working with single nanowires, its considerably lower power consumption as compared to their bulk counterpart, attainable by an adequate device layout, allows to match the limits required in mobile gas sensing applications
In this work metal oxide nanowires with a (In_1-xGa_x)₂O₃ stoichiometry have been fabricated via carbothermal reduction using a chemical vapor deposition (CVD) method. The nanowires, that grew according to a vapor-liquid-solid (VLS) method, have been structurally and optically characterized using X-ray diffraction, scanning and transmission electron microscopy and related techniques as well as photoluminescence and Raman spectroscopy. Correlation between shape, crystallinity and optical properties of the formed nanostructures and their chemical composition will be shown and will be discussed and justified based on the known properties of the pure forming materials.
The gas sensing properties of these nanostructured materials have also been tested. For this, the nanowires were removed from the substrates applying sonication, followed by the deposition on top of suspended microhotplates with prepatterned electrodes. Finally, individual nanowires were contacted either by a combination of Focused Electron- (FEBID) and Focused Ion-Beam Induced Deposition (FIBID) techniques or by evaporated contacts defined by Electron Beam Lithography. Nanodevices have been tested towards gases relevant in air quality monitoring, like ethanol, CO and NO₂, as well as towards O₂ and water vapor. The measurements have been carried out at different gas concentrations and operating temperatures.
The results obtained show that, depending on the chemical composition of the nanowires, the response towards the gases under study differs. The results will be discussed and correlated with the morphological and chemical properties of the sensing material.

The aim for environmentally friendly materials produced with low cost production routes is a reality nowadays. Titanium dioxide (TiO₂) fits into this seek, as it has elevated stability and photoactivity, non-toxicity, and earth-abundance. TiO₂ has been extensively studied for applications ranging from photocatalysis, solar cells to sensors [1,2]. In the present study, TiO₂ nanorod arrays were grown on bacterial nanocellulose (BNC), plastic (mylar polyester film) and tracing paper substrates using a hydrothermal method assisted by microwave irradiation without any seed layer. The selected substrates are inexpensive, reliable, recyclable, flexible, lightweight, portable and from different sources. Structural and morphological characterization was carried out by scanning electron microscopy (SEM) coupled with X-ray energy dispersive spectroscopy (EDS), Raman spectroscopy and by X-ray diffraction (XRD). The simple and low temperature synthesis (80 ^oC) route totally covered the substrates, forming uniform TiO₂nanorod arrays. The photo sensitivity of each material was investigated with the time resolved photo current in response to the UV turn on/off. All the materials demonstrated significant increase of conductance after UV irradiation, however the material using BNC as substrate revealed the highest photo sensitivity. This photo detection behaviour observed was confirmed by Kelvin probe force microscopy (KPFM) experiments, where the surface potential of each material varied under dark or UV irradiation, demonstrating higher surface potential shift for the BNC material. The approach developed in this study is an interesting and competitive alternative for the UV sensors employed nowadays, as it results in highly malleable materials, adaptable to different surfaces, despite the use of low cost and low temperature production routes, such as microwave synthesis.

[1] D. Nunes, A. Pimentel, J.V. Pinto, T.R. Calmeiro, S. Nandy, P. Barquinha, L. Pereira, P.A. Carvalho, E. Fortunato, R. Martins, Photocatalytic behavior of TiO2 films synthesized by microwave irradiation, Catalysis Today.

[2] A.A. Haidry, P. Schlosser, P. Durina, M. Mikula, M. Tomasek, T. Plecenik, T. Roch, A. Pidik, M. Stefecka, J. Noskovic, M. Zahoran, P. Kus, A. Plecenik, Hydrogen gas sensors based on nanocrystalline TiO2 thin films, Central European Journal of Physics, 9 (2011) 1351-1356.

It is well-known that metal oxide (MOX) nano-materials for gas sensing applications were originally introduced by Brattein et al. and Heiland in early 1950’s. Until recently, such gas sensors have been used mainly for industrial safety, research communities and environmental monitoring applications. With the emergence of low-cost sensing solutions enabled by improved manufacturing processes, and supplemented by the awareness of global environmental concerns, demand for gas sensors in consumer market space is rapidly growing. It is generally accepted by the researchers and the sensing industries that one of the most convenient and practical ways to monitor environment on a global-scale is to integrate such sensors within a smart phone and wearable devices. However, improving sensitivity, selectivity and stability (SSS), of MOX-based gas sensors have been a challenge for many years and they still remains as critical obstacles for realising high volume production of miniature gas sensors. To detect gases using MOX (such as SnO₂, WO₃ etc.) the material need to be heated to high temperature (300°C to 600°C), and through redox reaction between the target gas and oxide surface, changes in the carrier concentration occurs, causing the electrical resistance to either increase or decrease. This operation is commonly termed and well-understood as chemo-resistive effect. One of the key components of a MOX gas sensor is the heating element. In this presentation we will explore the manufacturing process of a new generation of CMOS, MEMS micro-hotplate heating elements together with the methods available for deposition of MOX sensing material for developing environmental gas sensing solutions. We will describe how such technology can facilitate reliable manufacturing of future low-power; low-cost gas sensors and the problems of SSS are being addressed with integrated on-chip circuits and sensor arrays, together with algorithmic signal processing solutions. We will also look at future trends and prospects for emerging solutions leading to room temperature sensing applications aimed at ultra-low power mobile phone and internet of things applications.

We report on the concept of applying single walled carbon nanotube field effect transistors (SWNT FETs) as functional building blocks in sensors^[1]. Advances in fabrication processes as well as better understanding of the behavior of SWNT FETs has enabled the vision of using individual-tube devices directly for NO₂ gas sensors. Significant progress has been made in understanding the sources of drift and hysteresis, and techniques have been introduced to counteract them, such as suspended device architectures. Reducing the presence of process residues and dielectrics close to the SWNT to the best possible level leads to the suppression of hysteresis and significant improvement in the noise performance^[2],[3],[4], as well as improvement in the cross-sensitivity to humidity. Suspended devices are also attractive for self-heated, low-power architectures^[5]. We will discuss hysteresis and ultra low power sensing. With improving control over fabrication processes gas sensor functional devices operating at extremely low power can be envisioned.

Acknowledgments
Kiran Chikkadi, Matthias Muoth, Wei Liu, Moritz Mattmann, Laura Jenni, Lalit Kumar and Sebastian Eberle for their contributions to SWNT-FET NO₂ sensors research, Support from ETH Zurich (TH 18/03-1, TH 13/05-3), Swiss National Science Foundation (20021-108059/1 and 200021_153292/1) and KTI/CTI (8885.2 PFDP-NM) is gratefully acknowledged.

[1] Hierold, C.; Jungen, A.; Stampfer, C.; Helbling, T., Sens. Actuators, A 2007, 136, pp 51
[2] Chikkadi, K.; Muoth, M.; Roman, C.; Haluska, M.; Hierold, C., Beilstein Journal of Nanotechnology, 2014, 5, pp 2179
[3] Chikkadi, K.; Muoth, M.; Liu, W.; Maiwald, V.; Hierold, C., Sens. Actuators, B 2014, 196, pp 682
[4] Liu, W; Chikkadi, K; Lee S-W.; Hierold, C.; Haluska, M; Sens. Actuators B 2014, 198, pp 479
[5] Chikkadi, K.; Muoth, M.; Maiwald, V.; Roman, C.; Hierold, C., Appl. Phys. Lett. 2013, 103, 223109

In living systems particularly plants, many movements and transports were triggered by water and moisture which are of great importance as an environmental stimulus. Leaves push upwards because of shrinkage upon evaporation of water inside, pine cones open upon water absorption and awns propel into the soil in response to moisture. The principle of these movements relies on the swelling and shrinking of the micro-fluidic water-conducting cellulose fibrils in the plant upon water absorption and evaporation. In these biological materials, hierarchically assembled structures are critical to realize such functionalities. For instance, upon making contact with water, Towel Gourd tendril’s elongation occurs owing to a hierarchy of chirality caused by the arrangement of molecules, microfibrils, cellulose fibrils, cells, tendril filaments to the macroscopic tendril helix in the tendril. A humidity change causes seed pods to open owing to the existence of hierarchical composite materials made of stiff cellulose fibrils embedded in a softer non-cellulosic matrix.

Inspired by the nature, we have designed a novel carbon nanotube fiber with a helical angle to offer a rapid and large contraction and rotation for sensing water and moisture. As the pristine surface is hydrophobic, the hydrophilic surface has been obtained through an oxygen plasma treatment; hierarchically assembled fibers are formed by simply twisting a certain number of primary CNT fibers into a fiber to enhanced its performance. These novel fiber sensors also exhibit a high reversibility in response to water and moisture. They are promising for various applications such as smart windows and louvers for sensing water and moisture.

Stimuli-responsive polymers are a major class of high-performance smart materials with sensitivity to various environmental factors, such as temperature, pH, humidity and certain electromagnetic radiation. With exposure to external stimuli, these materials exhibit unique response in the way of color change, conductivity variation, and shape reforming. Self-immolative polymers are considered as a special type of stimuli-responsive materials as their responses to environmental stimuli are based-on structural degradation instead of intrinsic change of their physical properties. In the event of external triggers, an end-capping group of the polymer backbone is cleaved, followed by spontaneous head-to-tail depolymerization in a domino-like manner and release of functional terminal groups, which are usually active species used for chemical sensing and further application. With incorporation of branched or dendritic linkers, self-immolative polymers are then intended for molecular amplification process in which multiple terminal groups are released with single stimuli-responsive bond cleaved. Such cascade-fashioned depolymerization facilitates sensitivity enhancement and efficiency improvement of signal amplification and chemical sensing. Here, we report a polycarbomate-based self-immolative polymer with sensitivity to multiple stimuli, such as acid, base and UV radiation. The molecular weight was tuned within the range of 2k-20k by variation of reaction time for condensation polymerization. These polymers were also fabricated into microcapsules' shell-walls and coating materials for chemical sensing and self-healing application.

Aromatic and polycyclic aromatic hydrocarbons (PAHs) are known to exhibit strong carcinogenic properties and to be one of the endocrine disrupting chemicals. The current technologies used to detect these molecules (e.g. liquid chromatography, gas chromatography-mass spectrometry) are accurate and sensitive but, at the same times, these techniques are expensive and time-consuming. Therefore, there is a strong demand for instrumentation to allow rapid and low cost analysis of aromatic hydrocarbon compounds. In this context, in the last few years MWCNTs, on the basis of their unique electrical, optical and mechanical properties, are a promising material for developing the new generation of miniaturized, low-power, ubiquitous sensors. Various investigation have also revealed the potentiality in the use of MWCNTs as sensing layer to detect aromatic compounds based on π-π stacking interactions [1], and charge transfers at the interphase that could be improved by the surface decoration of MWCNTs by different metal nanoparticles that permit to tailor the recognition towards a target aromatic compounds [2].
In this contribution, the metal decoration of MWCNTs-based chemiresistors to improve the detection of aromatic pollutants, compared to pristine ones, is reported. Specifically, an electrophoretic process was proposed to deposit, directly on the outer MWCNTs sidewalls, electrochemically-preformed Au or Pd NPs with controlled size.[3,4] Different deposition times were used to tune the metal content on MWCNTs, as revealed by chemical and morphological analyses.
The sensing properties of pristine and doped MWCNTs were evaluated at an operating temperature of 40°C towards various concentrations of aromatic pollutants (m-Xylene and Pyrene), and related to the metal used in the doping, and to its loading. Metal-doped MWCNTs sensors exhibited a very high gas sensitivity, fast response, reversibility, good repeatability, and detection limit, with the MWCNTs sensing properties controlled by the type and loading of used catalyst.
Finally, based on the gas sensing performance of prepared hybrid gas sensors, sensing mechanisms have been proposed, evaluating the effect of their chemical composition.
[1] S. Liu et al., Analytica Chimica Acta 2014, 826, 21–27.
[2] R. Leghrib et al., Proc. Engin. 2010, 5, 2010, 385-388.
[3] E. Dilonardo et al., Sens. Act. B 2016, 223, 417–428.
[4] E. Dilonardo et al., J. Sens. Sens. Syst. 2014, 3, 1–8.

Sensors from relatively cheap nanocomposite materials are always in demand. Nanocomposite foams are becoming popular research area because of their newly found important applications in nanoelectronics, electromechanical systems, sensors, composites, catalysis, energy storage devices, and optics. We have made sensors based on nanocomposites from sheet material (graphite and/or GO) stabilized emulsion systems which do not require any surfactant because of the presence of these sheets. The monomer is used as oil phase in the emulsion system. Graphene and graphene oxide(GO) mixtures are used as sheet materials in different ratios in these emulsion systems. GO, as the name suggests, is an oxidation product of graphite. The oxidation is often done to aid the exfoliation and dispersion of graphitic sheets in water. The fractionation approach can provide more uniform and better characterized GO that would find applications in various different areas. We use different kinds of GO to mix with graphite in the emulsion systems. The emulsion is then polymerized to produce a flexible yet strong polymer nanocomposite.
The process of synthesis of electrically conducting polymer composite is derived by thermodynamically driven graphite re-assembly. These foams nanocomposites are found to absorb oil, but not water. In addition, the electrical conductivity of the composites is found to change with change in the shape of the foam. Swelling or compressing the foam results in a significant change in the conductivity. Altering the material’s dimensions, either by swelling or by compression, leads to a significant change in electrical resistance. Due to the material’s conductivity and oil absorbing capability, it is a good candidate for sensor materials. Furthermore, polymer nanocomposites using Graphite (90%) and GO fraction (10%) mixture are formed. The interesting property about this GO fraction is the presence of both hydrophilic (GO) and hydrophobic (polymer) units at the interface of the foam. GO was added in small amount to introduce oxygen functionalities in the foam nanocomposite, while Graphite was put as a bulk sheet material for emulsion stabilization to prevent major changes in foam’s mechanical & electrical properties. The two hydrophilic and hydrophobic units make this polymer to be useful in many different areas. These polymer nanocomposites can be useful in Bio-applications, sensors, and energy storage devices, etc. In future, different compositions of Graphite and GO can also provide interesting properties in the sensors fabricated from these composites.

Methane is a potent global warming gas and fugitive natural gas is a significant contributor to climate forcing. Mitigation of this threat requires an inexpensive monitoring solution that can be deployed at natural gas wells, compressor stations, and processing facilities, as well as along pipelines. With support from ARPA-E, PARC and NASA Ames Research Center are developing a low-cost, high-performance natural gas well leak detection system based on printed nanomaterials. The system uses distributed arrays of chemical sensors containing variously-modified carbon nanotubes (CNTs) to achieve high sensitivity and high selectivity for methane and other components of natural gas, as well as potential interfering gases. Target gases includes methane, other low-molecular-weight alkanes, hydrogen sulfide, and ammonia, among others.
Sensitivities down to below 1 part per million (1 ppm) for methane and other gases are possible because of the high surface area of the CNTs. The sensors operate at room temperature, and consume very low power, on the order of microwatts. Selectivity is achieved through advanced pattern-matching algorithms. The sensors can achieve extremely low cost through print manufacturing. Leak localization and leak rate estimation are accessible with physics-based probabilistic modeling supported by computational fluid dynamics as well as wind tunnel simulations of gas plume formation and dispersion. A self-forming wireless communication network allows low-effort commissioning and remote data analysis.
The sensor system can be broadly applied at upstream, midstream, and downstream facilities. It also can be adapted to other gases and gas combinations, and utilized for industrial and residential safety, air quality monitoring, and other applications.
An overview of the sensor system as well as recent technical results will be presented.

Isoprene is a carcinogenic and toxic compound extensively applied in synthetic rubber manufacturing.¹ Additionally at lower level, it is exhaled through human breath being a promising non-invasive marker for monitoring of blood cholesterol-lowering therapy.² Chemo-resistive gas sensors based on metal oxides are ideal for the detection of environmental and breath isoprene, as they offer a compact design, a simple operation and low production cost.³ However, suitable materials for selective isoprene sensing are not available yet. Flame spray pyrolysis (FSP) is a great tool to explore new materials with unique sensing characteristics by accessing novel phases, solid solutions and mixed oxides at the nanoscale. That way analyte-selectivite materials such as Cr:WO₃ (acetone)⁴ and Si:MoO₃ (NH₃)⁵ have been developed before.
Here, we present for the first time an isoprene-selective sensor consisting of chemo-resistive Ti-doped ZnO solid solutions. The sensing nanoparticles (10 - 20 nm crystal size) are made by flame spray pyrolysis (FSP) and directly deposited onto sensor substrates forming highly porous films. Stable Ti-doped ZnO solid solutions are obtained for dopant levels up to 10 mol%. At an optimal Ti content (2.5 mol%), isoprene was accurately detected down to 5 ppb with a signal-to-noise ratio > 10 at high relative humidity (90%). Furthermore, this sensor showed superior isoprene selectivity towards omnipresent interferents (including acetone, NH₃ and ethanol). As a result, a compact and inexpensive isoprene detector is presented that can be incorporated easily into portable devices for environmental monitoring and breath diagnostics.⁶

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(5) Güntner, A. T.; Righettoni, M.; Pratsinis, S. E. Sens. Actuators B 2016, 223, 266-273.
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Among the conductive organic polymers, polypyrrole (PPy) is one of the most widely used due to its high electronic conductivity, biocompatibility and good stability in air and aqueous solutions. For many applications the polymer has to be nanostructured in order to have a very high specific surface area. Up to now, polypyrrole nanostructures were primarily obtained using ‘hard’ or ‘soft’ templates. In this presentation, it will be shown that the direct electrodeposition of either oriented nanowire array or nanofiber networks of polypyrrole can be achieved without using any template. These nanostructures which are 40-100 nm in diameter and can have length in the order of several tens of micrometers are synthesized in one step using an electrochemical route.^1,2 Actually, in order to obtain such nanostructures, the oxidation of the pyrrole monomer should be performed in the presence in the solution of jointly weak-acid anions (monohydrogenophosphate) which confer to the solution a basic pH and non-acid anions (perchlorate). The growth mechanism of the polymer nanostructures will be discussed. Notably, it will be highlighted why the presence of weak-acid anions at the electrode-solution interface is essential. At the very beginning of the process, water oxidation which occurs at a potential that is pH-dependent and leads to hydroxyl radicals production and O₂ evolution should take place.³ It will be shown that the interfacial pH which varies during the monomer oxidation due to proton release is the key point of this electrochemical process. Finally, hydrogen gas sensors have been fabricated, PPy interconnected nanofibers were electrodeposited on conductometric interdigitated transducers. These sensors feature fast and reversible responses to hydrogen due to the high porosity of the PPy nanowires-based sensitive films.⁴

References
1- C. Debiemme–Chouvy, Template-free one-step electrochemical formation of polypyrrole nanowire array, Electrochem. Commun. 11 (2009) 298-301.
2- A.Fakhry, F. Pillier, C. Templateless electrogeneration of polypyrrole nanostructures: impact of the anionic composition and pH of the monomer solution. J. Mater. Chem. A 2 (2014) 9859-9865.
3- A. Fakhry, H. Cachet, C. Debiemme-Chouvy, Mechanism of formation of templateless electrogenerated polypyrrole nanostructures, Electrochim. Acta 179 (2015) 297-303.
4- L. Al-Mashat, C. Debiemme-Chouvy, S. Borensztajn, K. Kalantar-Zadeh, W. Wlodarski, Electropolymerized polypyrrole nanowires for hydrogen gas sensing, J. Phys. Chem. C 116 (2012) 13388−13394.

Recently, flexible electronics have attracted significant interests since the electronic components can be integrated on the flexible substrates. Among these devices, flexible UV sensor is an important part of flexible electronics to monitor different environmental parameters which have a direct effect on human health. So, many studies have been carried on an active material to achieve superior flexibility and sensitivity together in flexible UV sensor. However, in parallel to the active material, researches about electrode are also important component in flexible UV sensor. Generally, silver paste and bulk metal films are widely used as electrodes for UV sensors because of their low sheet resistance and stability. However, these opaque and film-like electrodes block the UV rays to reach the active material during an irradiation process and its mechanical stability is also limited for applications in flexible electronics. In order to overcome these problems, we introduce network-structured electrode of copper nanowires (Cu NWs) which has high conductivity and significantly cheaper than other metals. Moreover, net formation of nanowire electrode makes large contact area between surface of the active material and oxygen molecules. Consequently, the sensing properties of UV sensor with Cu NW electrode were improved in comparison to the UV sensor with common bulk electrode.
In this study, we synthesized Cu NWs by low temperature hydrothermal process and applied to ZnO-based UV sensor. Cu NWs as top electrodes were deposited by vacuum filtration. Surface and cross-section of nanomaterials were characterized by using scanning electron microscope. The junction property of UV sensor and UV sensing performance under UV illumination were investigated by using current-voltage measurement. The performance of the UV sensor with the Cu NW electrode showed stable and high photoresponse. Furthermore, even after dynamic bending tests of 5000 cycles at a radius of curvature of 5 mm, UV photodetectors were operated stably.

Keywords: copper nanowire, Zinc oxide nanorod, UV sensor, hydrothermal process, flexible device.

Gold (Au) nanostructures are of great interest due to their applicability in various areas such as catalysis, sensing and optoelectronics. We report the synthesis of density controlled Au NPs by galvanic displacement, using a limited exposure to hydrofluoric (HF) acid. These nanostructures will be used for sensing applications such as environmental monitoring in the future. The synthesis method consists of immersing a silicon (Si) substrate in HF acid for 2 min., followed by immersing the substrate in gold chloride solution for 5 min. These steps are repeated several times to obtain the desired density of the Au nanostructures. A series of experiments was performed to monitor the density of the synthesized nanostructures and the results were correlated to the number of times the above steps were repeated. Thus a good control over the nanostructure density was obtained. The morphology of the Au nanostructures, however, changed from isolated Au nanoparticles to a continuous network of Au nanostructures. Thus in order to obtain a good control over morphology, surfactants like L-cysteine and cetyltrimethylammonium bromide (CTAB) were used in the synthesis process. The inclusion of surfactants provided a uniform morphology of the Au nanostructures. The above experiments were repeated by using copper sulfate as the metal salt to fabricate copper-based nanostructures. The outcomes of these experiments were compared with the synthesized Au nanostructures. The characterization of all of the above nanostructures was performed by using scanning electron microscope (SEM), X-ray diffractometer (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS).

The aim of this work is to investigate the response of nanostructured chemiresistive gas sensors with an active layers based on nanostructured metal oxides (nanowires, nanoporous structures) to a significant group of chemical warfare agents (CWA) - (G-series: sarin, cyclosarin, soman, choking agent: diphosgene) and their simulants - (diethyl-malonate, ethyl-chloroacetate and difenyl-phosphochloridate). The active layers of sensors (based on SnO₂, ZnO, PdO) with thicknesses of 100 - 400 nm were prepared on sensor substrates by (i) sputtering of source metal and subsequent anodic oxidation, (ii) sputtering of source metal and subsequent thermal oxidation in oxygen atmosphere, (iii) thermal evaporation of source material metal and subsequent oxidation in oxygen atmosphere. Sensor platform consists of a planar sensor aluminium oxide substrate with interdigital platinum electrodes.
We present results of morphology investigation of nanostructured oxide films by AFM and SEM microscopy and structural analysis by XRD. Conventional dc-sensitivity of sensors Sdc was evaluated as a ratio of their resistance in air containing given concentration of analyte and that in "pure" air respectively. Detection was tested for concentrations of CWAs up to 50 ppm in air. SnO₂-based sensors feature a high sensitivity towards CWAs, where Sdc reaches a values of 3 - 10. On the other hand ZnO-based sensors exhibit significant selectivity to CWAs, whereas the responses to a common interferents are negligible. We show the connection between material morphology properties (porosity, crystalline size) and detection ability of CWAs.

In the past decades, nanowires or nanofibers have attracted much attention due to their excellent characteristics, such as compact size, small weight and large surface-to-volume ratio. As important components, optical nanowires show a number of interesting features, including strong evanescent field and tight optical confinement, which are beneficial for highly sensitive, rapid and selective sensing in physical, chemical and biomedical areas. To date, various manufacture methods have been developed for optical nanowires fabrication, such as vapor-liquid-solid technique, electrospinning method and fiber pulling. Compared to above methods, femtosecond laser direct writing fabrication method can realize precise structure control, flexible optical devices integration and functional components doping. Here, we reported a femtosecond laser direct writing fabricated protein-based nanowire sensor for biosensing. Specific biotin-detection was realized with the protein nanowire biosensor, which was fabricated by femtosecond laser covalently photo-crosslinking biosensing probe proteins (e.g. avidin, antibodies) with or without mixture of “inert” proteins (e.g. bovine serum albumin, BSA). The fabricated protein nanowires demonstrated outstanding designability and reproducibility with good morphology (roughness average ≤5 nm), various designed sizes (≥150 nm) and complex geometries. Meantime, they presented good waveguiding characteristics for optical biosensing, the transmission windows were around ≈500 and ≈680 nm for the 700-nm diameter protein nanowires. Specific detection of biotin could be achieved with 0.2 ppb or even higher sensitivity for the avidin/BSA nanowire biosensors in aqueous surrounding. The all-protein-based single-nanowire optical biosensors are good candidates for novel environment monitoring, biochemical detection and biomedical analysis.

In this study, a regenerable heavy-metal detecting optical sensor membrane is prepared via the formation of a dynamic peel-and-stick of phthalocyanine (Pc) layer onto the surface of a poly(tetrafluoroethylene)(PTFE) membrane, using thermo-responsive reversible covalent bonding. The optical sensor membrane is designed based on maleimide end-modified Pc layer and furan-modified PTFE membrane, and they are coupled by reversible Diels–Alder (DA) cycloaddition reaction. The combined results of attenuated total reflection Fourier-transform infrared (ATR FT-IR), Nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), UV-Vis spectroscopy and Fluorescence spectroscopy measurements clearly reveal that the maleimide end-modified Pc is properly attached to the furan-modified PTFE membrane surface by DA reaction. The Pc-coupled PTFE membrane shows heavy-metal detecting performance against chromium (Cr) and ferrous (Fe) ion aqueous solution. In this sensing system, notable changes in fluorescence emission intensity are observed. Then, the heavy-metal detecting property is restored after regeneration of the Pc layer through the peel-and-stick process. The optical sensor membrane exhibited good stability and reusability which made it efficient for detection applications.

Metal oxides (MOXs) represent a significant fraction of research for developing solid state gas sensors. Here, the chemical-to-electrical transduction mechanisms between a reducing gas, carbon monoxide (CO), and an n-type metal oxide (SnO2) were theoretically evaluated and confronted to experimental data obtained with individual nanowires. Our analysis revealed that the sensing process has a complex nature with the concomitant contribution to the response of different experimental parameters such as the concentration of the target gas, the working temperature and the intrinsic properties of the nanowires, with a noteworthy role given to the device geometry, which fully determines the characteristics of the sensors. Those nanowires with thinner radii showed better performance not only in terms of the absolute response to a fixed gas concentration, but also bigger exponents in the power law dependence of the response as function of the gas concentration in air. On the basis of these results, it is clear that any meaningful assessment of the sensing properties of MOX nanowires needs to account for the device geometry influence on the final responses.

Nuclear waste repositories are being installed in deep excavated rock formations in some places in Europe to isolate and store radioactive waste. In France, the Callovo-Oxfordian formation (COx) is a possible candidate for nuclear waste storage. This work investigates the applicability of CeO2-based oxides (CeO2, Ce0.8Sm0.2O2 and Ce0.8Zr0.2O2) for monitoring the pH of the COx pore water (T=25oC). The study is limited to the pH range between 5.5 and 13.2, which includes the pH values that have been encountered or are anticipated in the COx formation during its evolution as radioactive waste repository due mainly to alkalinisation, an increase in salinity, and a decrease in redox potential. Screen-printing was done to assemble electrodes and rapidly generate data sets. The core materials (CeO2, Ce0.8Sm0.2O2 and Ce0.8Zr0.2O2) were characterized using a tool of techniques such as SEM, TEM, in situ XRD, XPS, while the electrochemical behavior of CeO2-based screen-printed electrodes (CeO2-based SPEs) was determined by cyclic voltammetry and electrochemical impedance spectroscopy. The use of the electrodes for pH sensing was then evaluated by potentiometric measurements. The feasibility of measuring pH with CeO₂-based SPEs was first tested in NH4Cl/NH3 buffer solutions, leading to electrode calibration over the widest range of pH, from around neutral to basic pH. Experiments were then conducted in NaHCO3/Na2CO3 buffer samples similar to conditions prevailing in the COx formation. Ce0.8 Zr0.2 O2 SPEs exhibit a near-Nernstian behavior (sensitivity −(51 ± 2) mV/pH) in the pH range of 5.5–13.2 at 25 oC. Electrode response was slightly affected by the direction of the pH change. Electrode reliability was clearly demonstrated for pH monitoring. Probes based on the same components, but more durably designed, could be considered for pH measurements in radioactive waste repositories.

Room temperature ionic liquids (RTILs) are a class of organic material that have a salt like nature. They also have the characteristic to be liquid at room temperature and have high breakdown temperatures nearing 300°C. These RTILs are a tunable organic that has been shown previously to be sensitive to different gases via a chemi- or physi- adsorption process. This gas sensitivity is of novel interest as it provides a reversible chemical change that can be utilized in low temperature CO₂ detection systems. Due to their CO₂ detection capabilities, they are of importance in the environmental, industrial, and health fields because of their reversible low temperature detection abilities unlike current high temperature NIR sensors. Through the use of different scientific techniques, it is possible to distinguish the usability of different RTILs as low temperature gas sensors. Through a combination of Fourier transform infrared spectroscopy (FTIR), electrochemical impedance spectroscopy (EIS), and chronoamperometry (CA); it is possible to observe the effects of diffusion and adsorption of the RTILs. FTIR helps explore the bonding behavior between two different RTILs when exposed to different CO₂ concentrations. The EIS measurements give insight into the behavior of the electric double layer (EDL) layer as it reacts to the CO2 in the form of changes in the resistive and capacitive components of the impedance. The current behavior from the CA measurements provides a direct measurement of the capacitive behavior of the system. From this data the static and dynamic diffusion characteristics of two RTILs is analyzed to develop a COMSOL diffusion model, Helmholtz vs multilayer, for better understanding of CO2 as it reacts with these RTILs. Exploration of RTIL volume is also explored to examine the effects of volume on diffusion characteristics and model determination. Through the use of multiple approaches we will provide a deep understanding of the adsorptive and electrical properties of RTILs and their effects on successful detection of CO₂ at different concentrations.

TiO₂ is a well-known photocatalytic material. Its combination with plasmonic nanoparticles (NPs) has been demonstrated in the literature in various ways. A vast majority of the publications have focused on reactions involving photocatalytic decomposition of organic compounds and water splitting. However, there is little evidence on the efficiency of such composite materials in the intermediate photo-oxidation step required to achieve the removal of heavy metals during water purification (e.g. Mn). In the present study we investigate the enhanced photocatalytic activity of TiO₂ (E_g = 3.2 eV) and TiO_xN_y (N content set to provide samples with E_g either 2.8 eV, or 2.4 eV) with optically active Ag and Au NPs towards oxidation of Mn aqueous species (10 mg/L). The photocatalytic templates were immersed on aqueous solutions of Mn(II) oxy-anions. For the illumination of the samples we used a simple white LED lamp. XPS has been used to identify the retention of the metal ions and their oxidation state during photocatalysis. We demonstrate that the performance of the photocatalysts is a strong function of the Eg of the semiconductor, the properties of the NPs and the structure of devices. Two different structures examined where the metallic NPs were either on the top or beneath of the TiO_xN_y layer; in both cases enhanced photocatalytic was observed compared to pure (no NPs) TiO_xN_y and TiO₂. We show that by tailoring the Eg and the size of the NPs it is possible to maximize the photochemical activity of a semiconductor and create more efficient devices for heavy metal purification of water.

Organic thin film transistors (OTFT) offer great potential for use in flexible electronics. Much of this potential lies in the solution processability of the organic polymers enabling both roll coating and printing on flexible substrates and thus greatly reducing the material and fabrication costs. We have fabricated an OTFT in ambient air, on a PET flexible substrate. The printing technique is very similar to the one used for printing newspapers and has low environmental impact in comparison with traditional electronics. This is because is made at lower temperature than you would normally use for baking a cake. After implementing the transistor we build a small demonstrator circuit to operate a printed electrochromic surface powered by an organic solar cell all realized with the same technique. The devices have top gate architecture and were completed by slot-die coating of the organic semiconductor poly-3-hexylthiophene and the dielectric material polyvinylphenol before the gate was applied by screen printing. We explore the footprint and the practically accessible geometry of such devices with a special view toward being able to drive large currents while handling the thermal aspects. We find especially that an elevated operational temperature is beneficial with respect to both transconductance and on/off ratio. We achieve high currents of up to 45 mA at a temperature of 80 °C with an on/off ratio of 100. We observe a significant temperature dependence of the performance which can be explored further in sensing applications.

[1] Francesco Pastorelli, Thomas M. Schmidt, Markus Hösel, Roar R. Søndergaard, Mikkel Jørgensen and Frederik C. Krebs, " The Organic Power Transistor: Roll-to-Roll Manufacture, Thermal Behavior, and Power Handling When Driving Printed Electronics", Volume 18, Issue 1, pages 51–55, January 2016, doi: 10.1002/adem.201500348

High-performance photodetectors are costly due to either the expensive nature of active materials or sophistication in fabrication process required to meet performance targets. In this work, low-cost metal stannate based nanostructures are demonstrated as active materials for high-performance UV photodetectors[1]. A robust, inexpensive, and scalable drop-casting process was successfully developed to fabricate thin film devices composed of amorphous ZnSnO₃ nanocubes and/or polycrystalline Zn₂SnO₄-SnO₂ nanoparticles with different optical and electronic structures[2,3]. Moreover, the Zn₂SnO₄-SnO₂ heterojunction nanoparticles were directly obtained by thermal treatment of amorphous ZnSnO₃ nanocubes[4,5]. Large-area, uniform, and continuous film photodetectors were achieved with pronounced and fast response upon exposure to UV illuminations (λ < 370 nm), showing both rise and decay time less than 1.0 s. More advantages based on these two stannate nanostructures were observed compared with their counterparts, such as ZnO, SnO₂, Zn₂SnO₄, ZnO-SnO₂ individual nanostructure or film photosensors, including high photo-sensitivities (S > 10² and ~ 10³ were achieved for ZnSnO₃ and Zn₂SnO₄-SnO₂ photodetectors, respectively) and excellent responsivity (as high as 0.5 A/W at 5.0 V bias from the Zn₂SnO₄-SnO₂ photosensors) under low light intensity. This work provides a simple, cost-effective and tunable processing strategy to synthesize and apply earth-abundant metal stannate based nanomaterials for high performance UV photodetectors and other optoelectronic devices.


Reference:
1. C. Liu, A. Piyadasa, M. Piech, S. Dardona, Z. Ren and P.-X. Gao J. Mater. Chem. C , 2016
2. H. J. Fan, Y. Yang and M. Zacharias, J. Mater. Chem., 2009, 19, 885–900.
3. J.-W. Zhao, L.-R. Qin and L.-D. Zhang, Solid State Commun., 2007, 141, 663–666.
4. C. Liu, H. Chen, Z. Ren, S. Dardona, M. Piech, H. Gao and P.-X. Gao, Appl. Surf. Sci., 2014, 296, 53–60.
5. C. Liu, R. Roder, L. Zhang, Z. Ren, H. Chen, Z. Zhang, C. Ronning and P.-X. Gao, J. Mater. Chem. A, 2014, 2, 4157.

The sediment interface between surface and ground waters is typically highly bioreactive, hosting reactions that influence aquatic habitat and water quality. Groundwater/surface-water exchange data are often labor intensive to collect, and various methods of data collection apply to different spatial scales. Traditional physical (e.g., seepage meters) and chemical (e.g., pore water sampling) point measurements made at a relative handful of locations in a streambed are typically interpreted at the reach/watershed scale. Conversely, larger-scale tracer experiments integrate fine-scale exchange processes to the reach length, but it is often difficult to identify physically-based controlling parameters of exchange.
Recent advances in environmental sensing technology offer means to directly measure physical properties, such as temperature and electrical conductivity, which may indicate the distribution of important exchange zones and controlling geologic characteristics. For example, fine-scale (tens of centimeters) bulk streambed electrical measurements can be paired with pore-water time series during salt injections to identify zones of less-mobile porosity that may influence contaminant transport, nutrient cycling, and greenhouse gas production. Heat tracing using discrete thermal loggers, infrared cameras, and modified fiber-optic sensors can be used to quantify vertical water flux rate and direction within interface sediments at scales ranging from bedforms to stream reach (hundreds of meters).
Novel applications of surface geophysical techniques can be used to focus the implementation of more time-intensive physical measurements, and to put data collected at disparate scales into a system-wide context. Case study examples will be presented from field sites across the USA that include streams, rivers, lakes, and salt-water environments.

In recent years, great attention has been focused on the detection and treatment of mercury (II) in the environment due to the serious health risk that mercury (II) poses to humans¹. Exposure to mercury, even at very low levels, can cause damage to the brain, heart, lungs, kidney, and immune system, and can result in symptoms such as memory loss, vision problems, deafness, and lack of coordination. Over these years several metholodgies have been reported for the detection and estimation of mercury such as atomic absorption spectroscopy, fluorescence spectroscopy, inductively coupled plasma-mass spectrometry and electrochemical sensing². However, these techniques require elaborate instrumentations and are time consuming. Colorimetric sensing, based on the induced aggregation of nanoparticles, has attracted great interest since it is low cost, simple, and relatively rapid procedure, making it perfect for on-site analysis³. Taking into account these advantages, we have developed a colorimetric sensor for the detection and estimation of mercury based on chitosan-stabilized gold nanoparticles (AuNPs) modified with Pyridine-2,6-dicarboxylic acid (DPA). In the presence of Hg²⁺, DPA induces the aggregation of AuNPs, causing color change of the solution varying from red to blue, depending on the concentration of Hg²⁺. The formation of aggregated AuNPs in the presence of Hg²⁺ was confirmed using transmission electron microscopy (TEM) and UV-Vis spectroscopy. The method exhibits linearity in the range of 300 nM to 5µM and shows excellent selectivity towards Hg²⁺ among seventeen different metal ions and was successfully applied for the detection of Hg²⁺ in spiked river water sample. The developed technique is indeed superior to the existing techniques in that it is a simple detection using naked eye, thereby eliminating the need for sophisticated instruments.

Plasmonic nanoparticles have become an increasingly common research area as well as becoming a key component in many important applications, such as solar energy harvesting, chemical sensing via surface enhanced Raman scattering, cancer treatment and optical encoding of information to name but a few. The main reason behind their adaptability to these and other prominent applications is their unique optical properties that allow for the manipulation of light below the diffraction limit. This effect, known as Local Surface Plasmon Resonance (LSPR), is a resonance phenomenon occurring when the frequency of the incident photons match the frequency of the surface electrons oscillating against the restoring force of the positive nuclei. The optical identity of the LSPR is extremely sensitive to the particle’s size, shape, distribution and the dielectric functions of the metal and surrounding medium. The extreme sensitivity of plasmonic nanoparticle templates to small changes in their environment has resulted in a wide range of plasmonic based surface sensors. Local surface plasmon resonance (LSPR) spectroscopy sensors have been investigated extensively due to their ability to monitor chemical reactions and surface adsorption. However, traditional LSPR spectroscopy sensor structures require the inclusion of a functionalisation step during the sensor fabrication. Here we present the ability of a laser fabricated nanoparticle template to behave as a LSPR spectroscopy sensor without the need of functionalisation. The nanoparticle template was used to monitor the surface adsorption of lead and lead salt from parts per million aqueous solutions of varying lead salt concentrations, via measuring the change in the optical response of the template. The exact quantity of lead and lead salt adsorbed to the sensor surface was identified via XPS. The resulting calibration demonstrates the feasibility of using a non-functionalised laser fabricated plasmonic nanoparticle template as a LSPR spectroscopy sensor for the surface adsorption of lead and lead salt via an optical measurement.

Access to clean water is essential for the health of an urban population. A key component of water quality monitoring is the measurement of the dissolved oxygen (DO) concentration. In-situ, continuous measurements of DO are needed not only for municipal water facilities and wastewater treatment plants, but also within the body of water or sub-terranean aquifer that serves as the drinking water source. Measuring DO can also serve as a surrogate for monitoring the level of many harmful chemicals in drinking water supplies that are regulated by the Environmental Protection Agency. These applications require a cost-effective, environmentally robust and autonomous DO sensing technology that can operate in both flowing water and porous sediment 24/hours a day, 365 days a year.

To this end we have developed an optical DO sensor based on a novel K₂Mo₆Cl₁₄/silicone polymer composite material that is robust in aqueous fluids of diverse composition and can be deployed for months outdoors. The DO concentration is determined by measuring the oxygen quenching of the phosphorescence from inorganic, metal-halide optical indicators. The photophysical properties of the isolated, nano-scale luminophore are preserved during synthesis of the sensing film by immobilizing singly solvated molybdenum clusters within the composite. This is demonstrated by obtaining a linear fit to the phosphorescent emission (Stern-Volmer equation) from both the sensing film and the molybdenum cluster in solution. We report on the unique materials properties of the K₂Mo₆Cl₁₄/silicone composite that have enabled sensor deployment under harsh environmental conditions, as well as the synthetic route employed to cage the molybdenum cluster in a hydrophobic, optically transparent and oxygen permeable polymer.

Field effect devices are highly favorable for gas sensing applications due to their high sensitivity, as they generally exhibit a logarithmic relationship between gas concentration and response (change in current/voltage) which is responsible for the excellent sensitivity at low concentrations. In the last two decades, silicon carbide (SiC) has emerged as the leading candidate for field effect based sensors, and field effect transistors based on SiC (SiC-FETs) have been widely studied as reliable, high-performance, and cost-efficient chemical sensors for high- and room-temperature applications. The choice of the material to be used as the sensing layer is crucial to enhance the device performance in terms of sensitivity, selectivity, stability, and speed of response.
In this work, we present highly sensitive, selective, and cost-efficient gas sensors based on SiC-FET technology for the detection of three specific hazardous volatile organic compounds (VOCs), which are often found in indoor environments in concentrations of health concern. Formaldehyde (CH₂O), naphthalene (C₁₀H₈), and benzene (C₆H₆) are used as target VOCs in the low parts per billion (ppb) concentration range.
Pure metals (Ir, Pt), pure metal oxides (WO₃, V₂O₅), and a combination of metal/metal oxide (Ir/WO₃, Pt/WO₃) were manufactured, tested, and compared as sensing layers.
All investigated materials showed important advantages, but also drawbacks. Iridium (Ir) and platinum (Pt) are among the most effective catalysts for sensing reducing gases. Pure metal-gated SiC-FETs, especially Ir-gated, show the highest sensitivity to the three studied VOCs down to sub-ppb levels, and high stability during long term operation. Such high sensitivity, though, is exhibited towards a range of gas molecules, and selectivity still remain an issue to be solved.
Pure metal oxide based gas sensors, and specifically WO₃-gated SiC-FETs, suffer from short lifetime and low sensitivity due to the wide band gap, high resistivity, and low reactivity of the metal oxide, but the effect of WO₃ enhances selectivity towards C₁₀H₈.
Here we show that the addition of a noble metal (Ir, Pt) to the semiconducting oxide (WO₃) is an effective mean to ensure a much longer lifetime of the sensor, together with high stability during long-term operation and improved selectivity.
The sensitivity of the sensors was investigated also as a function of the electrical operating point of the transistor, i.e., at the linear, onset of saturation, and saturation regions. C₆H₆ was used as the target gas in the 10 to 100 ppb concentration range. The sensor response to C₆H₆ in saturation region resulted up to 52 % higher than that in linear region, whereas it varied from 6 to 8 % in average when the SiC-FET was operated at the saturation region or at the onset of saturation.
Finally, we demonstrate that the sensor selectivity may be increased by temperature cycled operation (TCO) and data evaluation based on multivariate statistics.

Functional nanomaterials with sensing properties are promising for the purpose of environmental gas detection. Nanocrystal-based films have been developed for detection of humidity and gases by means of surface functionalization with organic molecules.^1,2 We have recently prepared a colloidal silicon nanocrystal with a heavily boron and phosphorus doped shell (codoped silicon nanocrystal).³ Infrared spectroscopy of a codoped silicon-nanocrystal film revealed that water molecules are easily bound onto the surface of nanocrystals.⁴ This water affinity leads us to apply a codoped silicon nanocrystal for sensing application.
Here, we study electrical characteristics of codoped silicon-nanocrystal films in various gas environment. We produced a colloidal silicon nanocrystal following our previous paper.³ Si-rich borophosphosilicate films were formed by co-sputtering Si, SiO₂, B₂O₃, and P₂O₅. The films were annealed in a nitrogen atmosphere at 1050 ^oC in order to grow silicon nanocrystals. The average diameter of silicon nanocrystals was 3 nm. The annealed films were mixed with hydrofluoric acid to extract silicon nanocrystals from the films. Silicon nanocrystals were centrifuged and transferred into methanol. Silicon nanocrystal colloids were spin-coated on gold electrodes on fused silica substrates. The gold electrodes have a 200 μm distance between electrodes and a 5 mm width. The current-voltage characteristics were measured by a source measure unit. The measurements were carried out in a vacuum chamber. The condition in a vacuum chamber changed in air, vacuum, or water vapor atmosphere. The conductivity of a codoped silicon nanocrystal solid was around 10^-6 S/cm in air. The conductivity of films decreased 6 orders of magnitude when the atmosphere in the chamber changed from air to vacuum. In contrast, the conductivity of films increased 8 orders of magnitude when the atmosphere changed from vacuum to water vapor. The conductivity did not change largely by introduction of nitrogen or oxygen gases. This change of conductivity indicates us a possible application of a codoped silicon nanocrystal toward a vapor sensor.
¹ Y. Yan, S.C. Warren, P. Fuller, and B.A. Grzybowski, Nat. Nanotechnol. doi:10.1038/nnano.2016.39 (2016).
² E.S. Cho, J. Kim, B. Tejerina, T.M. Hermans, H. Jiang, H. Nakanishi, M. Yu, A.Z. Patashinski, S.C. Glotzer, F. Stellacci, and B. a. Grzybowski, Nat. Mater. 11, 978 (2012).
³ H. Sugimoto, M. Fujii, K. Imakita, S. Hayashi, and K. Akamatsu, J. Phys. Chem. C 117, 11850 (2013).
⁴ M. Sasaki, S. Kano, H. Sugimoto, K. Imakita, and M. Fujii, J. Phys. Chem. C 120, 195 (2016).

Nanowires are employed as building-blocks, and huge efforts are carried out in order to integrate them easily, in order to reduce the fabrication costs and energy of the functional devices. High surface-to-volume ratio and the well controlled physical and chemical properties of nanowires provide excellent features to use them as a sensing part for toxic gases detection and environmental monitoring [1].
This work presents the localized growth of Ge NWs developed by a VLS method on the top of micromembranes equipped with a buried heater. This is performed on a quartz chamber and the heat necessary for the nanowires synthesis is provided by the incorporated heater on the microhotplate.
Ge NWs have been studied as a gas sensor, responding reversibly towards reducing and oxidizing gases by changing the electrical resistance. The experimental measurements show a similar sensing mechanism to the metal oxide materials: the response is importantly reduced when the gas specie to detect is diluted in nitrogen. The surface of the NWs is covered by a Ge oxide, whose thickness is maintained stable by working at a low temperature (<100C). Ge NWs have shown moderate response against the presence of NO2 and CO in a range of several ppm, diluted in synthetic air keeping a constant flow. The maximum response has been observed towards water vapor. The fabricated sensor provides a functional sensor which consumes several mW for both heating and measuring.
[1] S. Barth et al. “Synthesis and applications of one-dimensional semiconductors”, Prog. Mater. Sci. 2010, 55, 563.

There exists a growing interest for developing low-dimensional functional nanoscale materials. As a contribution to these efforts, in this study, for the first time we report flexible one-dimensional GaN and InN materials which are coated on polymeric nanofibrous templates with high degree of conformality by low-temperature plasma-assisted atomic layer deposition (PA-ALD). These flexible InN and GaN nanofiber templates can be used as alternative stable chemical sensing nanomaterials.

Synthesis of electrospun nylon 6,6 nanofibers are achieved by dissolving 8 wt % nylon 6,6 pellets in formic acid at room temperature. Electrospun nylon 6,6 nanofibrous templates with average fiber diameters below 100 nm were introduced into a partly customized PA-ALD reactor utilizing a hollow-cathode plasma source for the deposition of GaN and InN. 1000 cycle GaN and 700 cycle InN self-limiting growth were performed on nanofiber templates with optimized recipies at 200C using triethylgallium and trimethlyindium as metal, and N₂/H₂ and N₂ plasma as nitrogen precursors, respectively. The structural and chemical chacterizations are performed on the as-grown flexible core-shell nylon/GaN and nylon/InN nanofibers with XPS, SEM, TEM, and XRD.

SEM measurements confirmed the bead-free homogenously distributed fiber morphology of electrospun nylon nanofibers. After PA-ALD deposition of GaN and InN at 200C, nanofibrous templates were observed as uniformly coated with preserved shape which confirmed the compatibility of low-temperature PA-ALD process. TEM analysis of nanofibers using digital micrograph program revealed an average nylon 6,6 fiber diameter around 70 nm. Even though columnar growth of GaN results in a rather non-uniform thickness, the average coating thicknesses for GaN and InN were measured as ~27 nm and ~26nm, respectively. Crystallinity of the nylon/III-nitride core-shell nanofiber samples were investigated via powder XRD system, and results showed that nylon 6,6 features alpha phase while hexagonal wurtzite crystal peaks are observed for InN/nylon and GaN/nylon nanofibers at (010), (002), (011), (012), (110), (013), (112), (021), and (010), (002), (011), and (110) planes, respectively. Regarding the chemical composition and bond analysis performed with XPS, nylon 6.6 fibers exhibit %10.94 oxygen, %11.46 nitrogen, and % 77.59 carbon as expected. GaN/nylon spectra showed %8.33 oxygen, %10.88 carbon, %26.86 gallium and %53.93 nitrogen where nitrogen concentration is overestimated due to the overlap with gallium auger peak. On the other hand, XPS measurements of InN/nylon templates resulted in %36.15 indium, %27.1 nitrogen, %22.57 carbon, and %14.18 oxygen.

The resulting one-dimensional high-surface area III-nitride nanostructures demostrate the capacity for stable gas sensing performance when compared with conventional metal-oxide counterparts.

Molecular recognition of adsorbates with very high sensitivity and selectivity is of great interest in chemical and biological sensing. Physical properties of nanoscale systems are very sensitive to external stimuli due to their high surface to volume ratios. Nanosystems such as nanowires have been envisioned as a sensor platform for the next generation of highly sensitive chemical and biological sensors. However, achieving chemical selectivity in nanosystems is a challenging task because of the lack of highly selective receptor molecules that can be immobilized on sensor surfaces. By combining the nanowire sensors with infrared spectroscopy allows high selectivity detection of physisorbed molecules without sacrificing their sensitivity. Nanowires fabricated from wide band gap semiconductor materials such as bismuth ferrite (BiFeO₃) have very high density of surface states in the band gap. Modulating the surface state population via minute thermal changes induced by resonant excitation of the adsorbed molecules using infrared radiation offers an elegant route to an extremely sensitive and highly selective sensor platform. Low thermal mass of the nanowire, together with the availability of a large number of surface states, promote trapping of thermally generated carriers at the unoccupied surface states leading to variations in the electrical impedance parameters. Because of the large surface-to-volume ratio of the nanowire, occupancy of the electronic surface states can significantly influence its electrical properties. Therefore, by monitoring electrical impedance at the electrical resonance frequency of the nanowire system, it is possible to detect femto gram level adsorbates with very high selectivity. This approach offers an orthogonal and universal method for molecular recognition of extremely small concentrations of adsorbates. Examples of environmental sensing applications of this sensor platform will discussed. The question of sensing in complex environments using this technique will be addressed.

The demand for low-cost portable point-of-need (PON) analytical devices has substantially increased in recent years in many different areas. In the field of environmental protection, fast and reliable methods for the monitoring of the air, soil and the water are essential for the rapid intervention and recovery of natural environment. The control of water pollutants, for example, is of particular relevance in Brazil, where more than 1000 rivers are of fundamental social and economic importance and contamination by untreated effluents is a major problem. However, in Brazil and other developing countries, skilled laboratory personnel and sophisticated infrastructures, usually necessary to perform analytical water analysis, are scarce and typically localized in remote regions where the monitoring is needed. This fact makes the development of low-cost sensors an important subject. The development of such PON sensing technology impacts not only the human activities on the natural environment but also the country’s economy. In this presentation, we discuss the fabrication and characterization of a low-cost paper-based platform to create PON sensing elements for environmental monitoring. The paper-based devices comprise three-dimensional (3D) organic conductive tracks deposited through the porous structure of paper [1]. Here, we combine paper microfluidics and gas-phase pyrrole polymerization to chemically synthesize conducting channels embedded in-between the cellulose fibers. The organic material can be functionalized to create sensors and biosensors to transduce both electrical and electrochemical signals. By using the proposed method, foldable conductive structures can be created across the whole paper structure, allowing the electrical connection between both sides of the substrate, increasing the device available area. As a proof of concept, a set of devices including sensors for real-time monitoring of acids in water as well as for the local humidity determination is demonstrated. This method allows the manufacturing of several devices in parallel with high-volume production at real low cost.
Acknowledgements: CAPES, CNPq and FAPESP
[1] Murilo Santhiago, Jefferson Bettini, Sidnei R. Araújo, Carlos C. B. Bufon, ACS Appl. Mater. Interfaces 8, 10661(2016).

Development of microsensors and microfluidics have proceeded in parallel for a number of decades. The incorporation of chemical sensing within microfluidic structures presents specific challenges, however. Nevertheless, microfluidic structures offer unique opportunities to create microenvironments that set up gradients and environmental conditions that are relevant to biomedical or natural biogeochemical systems where chemical imaging is also of interest. As one of the most important electron acceptors in biology, oxygen is of particular interest as a as a factor many processes, down to the pore scale. Gradients of molecular oxygen are of particular interest due to the fact that many important biological processes either occur at interfaces or are regulated by the flux of oxygen and other resources.
Here we describe approaches to sense oxygen concentrations and gradients within microfluidic structures, using localized films of fluorescent oxygen sensing dyes and frequency domain fluorescent lifetime imaging. Of particular interest is to design systems where the fluorescent images also match the microfluidic spatial structure within the device. This is a concern because the simplest conventional approach, sandwiching a polymer film containing an oxygen sensing dye between a cover plate and a structured base plate produces a fluorescent signal across the entire device, including (mostly) areas that have nothing to do with the microfluidic structure. By contrast, we are pursuing approaches where the fluorescent signal is only seen in microchannel areas. Diverse approaches will be contrasted for fabricating and imaging oxygen sensing microfluidic structures, including pore network structures as models for natural systems at the pore scale.

The deposition of the scale leads to unavoidable damage of the equipment parts. Therefore, suspension of oil operations becomes essential in order to replace the damaged parts. In the petroleum industry, such interruptions are escorted by extremely high costs.
The unique properties of CVD graphene have made it an ideal candidate for a new class of sensor systems, which were not possible before. This is due to its 2-d nature (inert, conductive where the conductivity can be modulated when exposed to various ions, molecules, and gases) , mechanically very strong yet flexible so that it can be applied on various shapes and forms; in addition to its tolerance to high temperatures (stable till 700 C). In addition, the electrical resistance of the CVD graphene changes when chemicals in the surrounding covalently or non-covalently interact with the CVD graphene. The absorbed molecules can act as dopants that cause shifting on the fermi energy of the CVD graphene. Moreover, the bonds formed between absorbed chemicals and the CVD graphene change the band-structure of the tube.
The goal of this work is to fabricate FET (field effect transistor) like sensor with a CVD graphene mat as a channel deposited between metal source and drain. The gate turns the transistor on and off with an electric field through the oxide. This structure is a back gate structure in the meaning that the gate will be at the back of the structure, and the sensing materials (CVD graphene) are at the front. Such a sensor will utilize the advantage of gate biasing to affect sensor recovery and reproducibility. There are plenty of reports regarding fabrication of ChemFETs for sensing applications, with gas detection prevailing other types of analytes. Although synthesis of carbon nanomaterials and integration methods vary from one research to another, the sensor configuration is confined within a FET utilizing Si as a back gate. A role of a gate bias to recover CNT-based sensor was demonstrated by other researchers. In our study, we implement CVD graphene sensor device based on FET structure to detect NaCl salt in water solution. The successful operation of such a sensor opens a path to utilizing CVD graphene-based devices for monitoring other scale, including CaCO₃ and BaSO₄ which are common for scale precipitation in the oil industry. CVD graphene is grown in our lab on a copper and used as a sensing material and exfoliation technique was selected to deposit it on a SiO₂ substrate. The electrical response of the as-fabricated sensor was studied by means of measuring changes in the drain current, i.e. the current in the channel between source and drain. The future work will be reported in the conference which includes the characterization for other salts like carbonates and sulfates along with the response due to various sizes.

Nuclear leak will lead to water contamination and thus threaten people’s health or even life. As a result, the development of a water monitoring method, especially for alpha radiation contamination is important. However, conventional methods of alpha detection are too complicated for daily use. Here, we report a portable microfluidic radiation detection system which is composed of a radiation loading chamber, a bacteria trapper, a detection chamber and electrochemical detection electrodes. The detection scheme is simple and reliable. During detection, the bacteria, Deinococcus Radiodurans, are used to generate β-galactosidase upon exposure of radiation. Then we transfer bacteria suspension into the detection chamber and convert β-galactosidase to p-aminophenol (PAP). Amperometry is performed via the electrochemical detection electrodes to measure the concentration of PAP and characterize the detection limit. In order to enhance the sensitivity for detection, a bacteria trapper is assembled at the bottom of the radiation loading chamber to concentrate Deinococcus Radiodurans. Due to the difference of relative permittivity between media and bacteria, negative dielectrophoresis (DEP) is induced in the presence of electric field and the bacteria are driven to the electrode with the highest potential by DEP force. Simulations are conducted to further investigate the trapping performance and optimize the design. Finally a quantitative relationship between measured current and radiation intensity is concluded.

The contribution deals with thin films of phthalocyanines (CuPc, ZnPc, AgPc) used for an indirect detection of taggants of explosives. At first, we prepared phthalocyanine thin films (50 – 100 nm) onto alumina substrates covered with metal nanoparticles (Au, Pd) which were used to adjust electrical resistance of the sensor. Then we investigated morphology and electrical properties (conductivity, impedance, charge carriers mobility) of created nanostructures.
Prepared sensitive layers were used for detection of nitrogen dioxide, carbon dioxide, four most widely used taggants in explosives (2-nitrotoluene, 4-nitrotoluene, 2,4-dinitrotoluene, 2,3-dimethyl-2,3-dinitrobutane), toluene, benzene, alcohol vapors always in two modes: without or with photoactivation (λ = 266 nm; λ = 532 nm). When inactivated, the dc-responses (Sdc = Rair/Rgas) of MePc sensors were negligible or very weak (< 1.2) towards all analytes (concentrations from 1 to 200 ppm) except nitrogen dioxide which is consistent with data reported in the literature.
On the other hand, when we activated the vapors with laser light (with λ = 266 nm) we observed a strong amplification of responses to 2-nitrotoluene (Sdc = 2.5 for 10 ppm of 2-nitrotoluene) whereas the responses to other vapors and gases remained unaffected. We ascribed this amplification to the laser-induced decomposition of 2-nitrotoluene vapors which is accompanied by release of a significant amount of NO2.
This contribution also presents photoregeneration procedure for phthalocyanines from NO2 exposure. NO2 desorption from phthalocyanine sensitive layer is much slower process when compared with the sorption one which results in the disproportion of optimal operating temperature (↓ temperature ↑ dc-response ↑ regeneration time). In this contribution we evaluate the efficiency of photoregeneration recovery process (λ = 266 – 1000 nm) of sensitive layer in comparison with “standard” thermal pulses based recovery process (temporal increases of sensor operating temperature from 40 °C to 120 °C or 150 °C). Application of light with λ = 405 nm allowed to shorten the recovery time from 30 min to 5 min.

Aerogels made of cross-linking sheets of different two-dimensional materials, such as graphene, hexagonal boron nitride (h-BN), molybdenum disulfide (MoS₂) posses high specific surface areas and large pore volumes, which make them potential candidates for high sensitivity gas sensing with or without catalytic nanoparticles. In this poster, I will present the synthesis of graphene aerogels (GA), h-BN aerogels (BNA) and graphene/MoS₂ aerogels (GMA) with or without platinum (Pt) nanoparticles loading and their sensitivity towards hydrogen, propane and NO₂, respectively. These material systems are deposited onto low-power polysilicon microheater/sensor platforms. Pt loaded GA hydrogen sensors consume at little as 2.2 mW of power, have a t₉₀ response and recovery time of 0.97 s and 0.72 s, respectively, and a lower detection limit of approximately 65 ppm. h-BN aerogels decorated with Pt nanoparticles integrated onto a microheater platform allow for calorimetric propane detection. The sensors show faster response and recover times (<2 s) than reported on alumina support, allow for 10 % duty cycling of the microheater with no loss in sensitivity and consume 1.5 mW of power. For GMA on a miroheater platform, at room temperature the sensor exhibits an ultralow detection limit of 50 ppb NO₂. By heating the material to 200 C, the t₉₀ response and recovery times decrease to < 1 min, while retaining the low detection limit.

Over the past decades, metal-organic frameworks (MOFs) have been of significant interest because the combination of metal ions and organic materials can be applied for various applications based on structure and functionalities such as energy storage, photonics, catalysis, and sensors. Due to their morphological properties and adsorption behavior, which are a result of the high specific surface area of nanostructures, they have suggested to synthesize MOFs for gas sensor purposes. In addition, humidity sensing have been attracted a field of industry, health science, food science, and agriculture, which monitored electrical properties in the moisture- sensitive environment. Although, the reversible sorption property of MOFs was good, there electrical properties as electron channel material were limited. Thus, single-walled carbon nanotubes (SWCNTs) were used for an electron channel and can be compensated the limitation of the MOFs. Moreover, electronic devices on flexible substrate have recently developed in the field of wearable technology and system. It can be processed at room temperature and on a different polymer- based materials. This has allowed for the development of a field of electronic devices on flexible, foldable, and disposable substrates which have reasonable performance by comparison with rigid substrates. Furthermore, it is of significance to fabricate the flexible sensor, considering the above-mentioned aspects. MOFs/CNT on the flexible substrates demonstrated promising properties as sensing materials and used for UV sensing, gas sensing, and humidity sensing.
In this study, we investigated the humidity sensors which was fabricated using MOFs with functionalized SWCNTs on flexible membrane filter. The SWCNTs were functionalized with carboxyl group in the acidic solutions, containing sulfuric and nitric acid. The functionalized SWCNTs was filtered on the membrane substrate, and then MOFs was synthesized by adding mixed solution, containing metal electrolytes and trimesic acid(BTC) as metal ions and organic materials on SWCNTs/membrane substrate. The products were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS). The humidity sensing property of the sensors in quartz chamber were carried out by potentiostat which can detect variation of current at 1V bias. It was found that the MOFs/CNT on membrane had excellent properties such as flexibility, response and recovery times, sensitivity, and stability.

Electronic textiles (E-textiles) have emerged due to the growing interest of wearable electronics. Various electrical components such as interconnects, energy harvesting devices, energy storage devices, transistors and sensors are being developed for numerous applications. Among them, gas sensors are useful for alarming the users for hazardous situations for everyday life and industrial workplaces. Hydrogen gas (H₂) is one of the most cleanest and promising energy source, however, the low spark ignition energy (0.02 mJ) and wide flammable range (4~75%) requires accurate and fast leakage detection. In this work, a palladium (Pd) nanoparticle-based fiber-type H₂ sensor is proposed. The fiber is stretchable and conductive which changes its conductivity under H₂ exposure. The Pd nanoparticle-based conductive fiber is fabricated by a series of solution processes. First, Pd ions are inserted inside the polymer fiber by swelling the fiber using Pd salts dissolved in organic solvents. Then, the Pd ions are chemically reduced into nanoparticles by swelling the fibers with reducing agents. The steps can be repeated to increase Pd nanoparticle concentration and conductivity. The resulting fiber is filled with Pd nanoparticles which forms a conductive network inside the polymer. Pure Pd has a lattice constant of 3.90Å, but when it is exposed to H₂, H atoms are incorporated into the surface of the Pd layer resulting in the formation of Pd hydride (PdH_x) with a lattice constant of 4.04Å. Therefore, when the fiber is exposed to H₂, Pd nanoparticles changes to a larger PdH_x nanoparticle which changes the networked percolation, resulting in lower resistance. The sample have been fabricated using various Pd salts and reducing agent with different concentrations. The effect on the Pd nanoparticle concentration and particle size has been analyzed along with their effect on conductivity-strain relation and H₂ sensitivity.

Highly-uniform bilayer and multilayer porous thin films were successfully fabricated using self-assembled soft template and simple sputtering deposition technique. The sensor based on the In₂O₃/CuO bilayer porous thin film shows obviously improved sensing performance to ethanol at the lower working temperature, compared to single layer counterpart sensors. The response of In₂O₃/CuO bilayer sensors exhibit nearly 3 and 5 times higher than those of the single layer In₂O₃ and CuO porous film sensors over the same ethanol concentration, respectively. The sensing mechanism based on p-n hetero-junction, which contributed to the enhanced sensing performance was also experimentally confirmed by a control experiment which the SiO₂ insulation layer was inserted between the In₂O₃ and CuO layers to break the p-n junction. In addition, the sensing performance can be further enhanced by increasing the number of In₂O₃/CuO junction layers. The facile process can be easily extended to the fabrication of other semiconductor oxide gas sensors for practical sensing applications.
In addition, in this talk I would like to briefly report the progress of the electronic noise based on carbon nanostructures assembled in smart cell phone.

The energy needed to activate the chemical processes occurring at the surface of solid-state sensor materials is the main source of power consumption of this technology. In this presentation, we will review recent advancements in thermal and optical methods to supply such energy in an efficient way. On the one hand, self-heating in nanostructures is a promising but barely exploited approach to reduce power consumption into the µW regime. The technical complexity of fabricating suitable devices for such an approach has hampered its further development for nearly a decade. Recent results show that efficient self-heating also occurs in larger systems, which are simpler to fabricate. Moreover, new findings about the origin of the unexpectedly high efficiency of these large systems are opening doors for truly engineering self-heated devices, which are then easy to implement. On the other hand, optically activated conductometric devices have remained dormant as a scientific curiosity over the years. Most of the past limitations originated from the need of ultraviolet (UV) light sources of relatively high costs and without the possibility of miniaturisation. Rapid progress of highly efficient light-emitting diode (LED) technology in the last years now enable low cost, low energy and highly miniaturized LED systems to be transferred to sensor applications. In parallel, the strategies of performing material modifications and surface functionalization relaxed the need of UV, making possible the use of other, more efficient and convenient wavelengths (e.g., 550-400 nm range). A hybrid LED/sensor technology is expected to merge the world of conductometric sensing with optoelectronics, in which further power savings, miniaturization and flexibility can be obtained (e.g., zero power operation) by using active hybride sensor structures.

Gas species recognition through fully optical devices is currently a raising trend over the well-established conductometric approach, as it opens new possibilities especially for in situ recognition of flammable and/or toxic species such as CO, H₂ NO₂ or volatile organic compounds (VOC).
Plasmonic gas sensors are optical sensors that use localized surface plasmons or extended surface plasmons as transducing platform. Surface plasmons are very sensitive to dielectric variations of the environment or to electron exchange and these effects have been exploited for the realization of sensitive gas sensors.
Au nanoparticles (NPs) dispersed in an oxide matrix represent an effective design for a gas sensor’s active material owing to their catalytic and localized surface plasmon resonance (LSPR) properties. Noble metal NPs can exhibit catalytic properties and hence modify the chemical interactions between the oxide surface and the target analyte, thereby improving the sensing process. Moreover, if the metal NPs show a LSPR peak in the visible range (like Au), the nanocomposites can be used as selective optical gas sensors. The variation in the dielectric constant around the LSPR peaks will differ for different gas species, leading to a diverse variation in the optical properties at different wavelengths.
TiO₂ thin films with embedded Au and/or Pt NPs have been obtained by synthesizing high-quality metal and metal oxide colloids and directly spinning a nanocrystalline ink made of colloidal solutions on glass substrates. These TiO₂-Au samples showed fast and reversible changes in optical absorption when exposed to H₂ and CO species at 200°-350°C, with high sensitivities. More impressively, TiO₂-Au-Pt films showed room-temperature response to H₂ and VOC.
Thin films composed of Au NPs dispersed inside a TiO₂-NiO mixed oxide matrix were obtained spin coating a sol-gel solution on a glass substrate and subsequently thermal annealing. These samples show high response to H₂S down to few ppm and almost no interference in response is observed during simultaneous exposure to CO or H₂. For mechanistic studies, experimental evidence using reaction product analysis and thin film surface characterization suggests a direct catalytic oxidation of H₂S over the Au-TiO₂-NiO nanocomposite film.
More recently we developed VOC sensors based on extended surface palsmons which are very sensitive to different alcohol and operate at room temperature.
In this case the sensors consist of a TiO₂ nanoporous matrix deposited above a metallic plasmonic grating, which can support propagating surface slasmon polaritons. The spectral position of the plasmonic resonance dip in the reflectance spectra was monitored and correlated to the interaction with the differet VOC.

This work emphasizes on development of a dual-signal sensing platform and to understand the fundamental sensing mechanisms involved. Polymeric materials and nanomaterials are designed to be used in proposed sensing platform, yielding optical and electrical signals simultaneously. In the proposed sensing platform, optical signal (colorimetric response) could be observed by naked eye in critical analyte concentration; nanomaterials are used to further quantify the analyte at a wider concentration ranges by electrical response. To validate the above, this work focuses on the optical signal improvement in the first place. The optically active polymer – polydiacetylene (PDA) was known as a sensitive polymeric material to various stimuli. However, only few chemical vapors such as tetrahydrofuran changes its color effectively, this work reports a simple and rational methodology to tune the colorimetric property of PDA, offering higher sensitivity and faster response compared to other reported works based on PDA material. Besides, those modified PDA materials show different colorimetric responses to tested chemical vapors including dichloromethane, acetone, chloroform, tetrahydrofuran, hexane, ethanol, and toluene, forming a fingerprint for chemical vapor discrimination. In dual signals sensing platform, Those PDA materials are incorporated with conductive nanomaterial, carbon nanotube (CNT), to develop membrane-based dual signals sensing platform. The study of the interaction between polymeric material and CNT is included in this work, for having a better understanding of sensing mechanism. The results illustrate the feasibility of developing a membrane -based dual signals sensing platform, implying a potential application in environmental monitoring.

Ratiometric pressure sensitive paints (r-PSPs) are all-optical probes for monitoring oxygen flows in the vicinity of complex or miniaturized surfaces. They typically consist of a porous binder embedding mixtures of a reference and a sensor fluorophore exhibiting oxygen-insensitive and oxygen-responsive luminescence, respectively. Here, we realize the first example of an r-PSP based on a single two-colour emitter that removes limitations of r-PSPs based on fluorophore mixtures such as different temperature dependencies of the two fluorophores, cross-readout between the reference and sensor signals and phase segregation. In our paradigm-changing approach, we utilize a novel ‘double-sensor’ r-PSP that features two spectrally-separated emission bands with opposite responses to the O₂ pressure, which boosts the sensitivity with respect to traditional reference-sensor pairs. Specifically, we use two-colour-emitting CdSe/CdS core/shell nanocrystals, exhibiting red and green emission bands from their core and shell states whose intensities are respectively enhanced and quenched in response to the oxygen partial pressure. This leads to strong and reversible ratiometric response at the single particle level and over 100% enhancement in the pressure sensitivity. Our proof-of-concept r-PSPs further exhibit suppressed cross-readout thanks to zero spectral overlap between the core and shell luminescence and temperature independent ratiometric response between 0°C and 70°C.

Colorimetric methods offer a convenient approach for translating a qualitative chemical information from a reaction accompanied by color change into a quantitative measurand of a molecular species. An integrated, mobile colorimetric sensor can be particularly helpful for occasional chemical sensing measurements, such as informal air quality checks or risk assessment after an incident, where complex analytical methods are not available. In these situations, the main requirement is high availability, easy usage, and high specificity towards one single chemical species, combined with low cost. The main limiting factor for the widespread usage of chemical analysis techniques is the need of trained personnel to operate them (either because they involve some complicated manipulation, e.g. liquid chemical reagents, or because the readout is carried out by a complex instrument, e.g. spectrometer). In this contribution we will show how a well stablished colorimetric method can be adapted for easy operation and readout, making them suitable for the unqualified end user.
As a first example, we will present a modified Saltzman reagent method for the selective detection of NO2 in air that can be applied using solid wet substrates, avoiding the manipulation of chemical solutions and simplifying the procedure. Our results show that the method can be adapted to detect NO2 concentrations ranging from 50 ppb to 300 ppm, with measure-to-result times in the range of minutes. Also, an analytic model for the dependence of the test strip color with the gas concentration and the experimental conditions will be presented and used to prove the robustness of the method. We will demonstrate that the color measurement can be carried out with the optical signals of RGB sensors, without losing quantitative performance.
In order to make RGB analysis as simple as possible, we will present a new tool and a protocol to determine the true color composition of an RGB signal, acquired with a conventional color camera. With this method it is possible to determine the color deviations in an image, caused by the hardware used or by the background illumination, and correct them, in order to determine its true color composition in a fully automated way. As an application example, we will show how this automated color calibration procedure can be used to determine quantitatively the presence of NO2 with the colorimetric method described above.
Overall, combining both approaches, we will demonstrate an easy to use methodology to carry out highly-specific chemical sensing with colorimetry in everyday life, with simple and inexpensive tools, opening a whole new strategy for ubiquitously available environmental test kits.

Nanostructured, porous oxides are prominent sensing materials due to the reversible change in resistivity upon changes in the surrounding atmosphere. Nanowires have gained considerable attention in gas sensing devices due to their high surface to volume ratio and high crystallinity. However, the cost effective integration of nanowires in functional devices is usually challenging and costly.
We present a cost effective and simple growth strategy using CMOS-compatible micromembranes containing a buried heating element, which is used for thermally induced chemical vapour growth of SnO₂, WO₃ and Ge. In addition, the buried heater can be used as the heating source for the effective operation as sensor. The small membrane volume and area requires low power (few mW) for both the growth and the operation of the resulting devices. The actual devices contain a porous network of nanowires bridging interdigitated electrodes on top of the membrane for the electrical readout. Secondary deposition products should be negligible, which can be demonstrated by cross-sectioning of the active part of the device. The devices have been successfully used in monitoring changes in CO [1], ammonia [2] and humidity [3] concentrations and show long-term stability. This contribution will address growth strategies and specific considerations for three different materials in regards to their applicability for sensor applications.
References
[1] S. Barth, R. Jimenez-Diaz, J. Sama, J. D. Prades, I. Gracia, J. Santander, C. Cane, A. Romano-Rodriguez. Chem. Commun. 2012, 48, 4734.
[2] J. Sama, S. Barth, G. Domenech-Gil, J. D. Prades, N. Lopez, O. Casals, I. Gracia, C. Cane, A. Romano-Rodriguez, Sens. Actuators B, 2016, 232, 402.
[3] J. Sama, G. Domenech-Gil, M. Seifner, J. Santander, C. Calaza, I. Gracia, S. Barth, A.Romano-Rodriguez, submitted.

The early detection of volatile organic compounds (VOCs) before they reach dangerous concentration levels is essential for reducing environmental health risk and ensuring public safety. Aromatic hazardous air pollutants such as benzene, toluene, ethylbenzene and xylene (BTEX), are classified as carcinogenic to humans, thus the development of new nanotechnologies for the accurate detection and BTEX-component discrimination at very low concentrations is mandatory.
Functionalised semiconducting nanowires (NWs) are excellent building blocks for gas sensing applications. Their one-dimensional transport properties and high surface-to-volume ratio offer excellent room temperature sensitivity at parts-per-billion (ppb) level of VOCs. In addition, solution processing of NW functional inks enables low-cost printing of sensors on flexible substrates. However, full applicability of printing technologies requires efficient NW assembly and functionalization for the fabrication of reliable sensing platforms.
In this work, we investigate the selective deposition of high-quality silicon (Si) NWs field-effect transistors (FETs), functionalised with supramolecular receptors, following printed electronic approaches, and targeting BTEX gas sensing in 60 ppb to 30 ppm range with a capability to selectively sense benzene. Using dielectrophoresis (DEP) combined with fluidic shear forces we demonstrate that NWs with the highest conductivity and lowest defect density are collected at high DEP signal frequencies (>1MHz). Additionally, using DEP, we simultaneously achieve alignment and precise positioning of NWs in respect to the transistor channel gap. All the above capacities lead to scalable, controllable and reproducible fabrication of a highly sensitive and reliable NW FET gas sensing platform.
We present data and discuss sensing of benzene, toluene, ethylbenzene and xylene with Si NWs FETs that are functionalised with gas receptors with selective binding properties to BTEX VOC pollutants, exhibiting high selectivity and sensitivity to benzene vapour levels as low as 60 ppb.
In summary, we analyse the selectivity of our functionalised Si NW FETs and provide an outlook for multi-elemental flexible and printed sensor platform potentially capable of providing a “map” response to various volatile organic compounds in the atmosphere.

The electronic and ionic transport properties of the nanoscopic conductometric chemical sensors are intimately linked to their surface, interfacial and electrode contact properties. The traditional surface science measurements on ionosorption-conduction interplay have been been performent under UHV condition. To bridge the "pressure gap" between real world and model gas sensinng systems we have developed SEM based electron beam induced current (atmospheric-EBIC) microscopy whcih can be conducted under ambient conditions.We show few examples of correlative imaging, spectroscopy and transport measurements on individual working nanodevices using ambient pressure electron micriscopy. We extand this approach to ambient pressure XPS and AES studies. The design of the electron transparent micro-hot plates for in operando measuremtns will be discussed. The perspectives of the in operando device characterization at real world pressures and temperatures will be outlined.