Synthetic Smart Hydrogels Tailored for Sensing in Gaseous Environments

Stimulus-responsive, i.e. smart, hydrogels are polymers capable of exhibiting a reversible volume change in response to external physical and chemical influences, for example pH value, light, temperature or specific analyte molecules. This makes the material a very interesting candidate for novel sensing elements, especially since the properties can be tailored for specific applications from a wide variety of monomers and mixtures thereof, as well as additives [1,2].<br/>The smart hydrogel’s volume change is usually mediated by the uptake and release of liquid and consequently, most applications are centered on liquid environments. However, the hydrogel’s capabilities for selective and sensitive analyte detection hold great potential beyond that and we have therefore started to explore how to harness the material’s properties for sensing in gaseous media. The specific target application of our research is volatile organic compound (VOC) detection in exhaled breath to enable monitoring of disease biomarkers such as acetone.<br/>For this purpose, synthetic acrylamide-based hydrogels were chosen as the model material. In a first step, different polymers comprising polyacrylic acid (PAA), polyacrylamide (PAM), poly(<i>N</i>-isopropylacrylamide) (PNiPAAm) and their co-polymer combinations were investigated with respect to their ability of maintaining a measurable volume change in air with relative humidity variation from 0% to 100%. These studies were performed without and with added acetone gas as test VOC (concentrations: 20 ppm - 100 ppm) and the hydrogel’s swelling response was characterized by gravimetry and piezoresistive pressure sensing. PNiPAAm showed the best responsiveness in terms of humidity range, dependence of swelling response on the target analyte concentration and potential for selectivity [3].<br/>However, these studies also revealed a significant challenge for using hydrogels in a gaseous atmosphere: A large surface area is crucial for a strong and fast response in VOC detection, hence the hydrogel should ideally feature a very porous structure (surface and bulk). While the porosity can be adjusted by the use of pore-forming additives or by ice-templating during polymerization, the pores are likely to collapse after polymerization and subsequent drying in air.<br/>To address this challenge, the second step of our research focused on the development of a procedure enabling the creation of defined porosity which is maintained under the conditions of a gas atmosphere with varying humidity conditions. We found that templating with PEG during polymerization, followed by freeze drying (either at -196°C or -20°C, depending on the desired pore size) and subsequent conditioning in high relative humidity (90 – 100 %) is the key to achieve stable porosity.<br/>The material properties were characterized and compared in scanning electron microscopy and FT-IR analyses, and the volume change performance again evaluated by gravimetry and piezoresistive pressure sensing [4].<br/>These investigations have been conducted for PNiPAAm which was identified as the most promising material candidate for our target application. However, the developed process for tailoring and stabilizing a porous polymer structure in gaseous environments is of general nature and can easily be applied to other types of hydrogel since there are no components or steps specific for PNiPAAm.<br/>To enhance the responsiveness (i.e. swelling strength, time constants, reversibility) of the smart hydrogel even further, the focus of our future work is on the integration of additives such as graphene- and MXene-based materials.<br/> <br/>[1] M. Koetting et al., <i>Mater Sci Eng R Rep.</i> 93:1-49, 2015; doi:10.1016/j.mser.2015.04.001<br/>[2] S. Yang et al., Energy environ mater. 2023; doi:10.1002/eem2.12646<br/>[3] S. Wang et al., Polymer 278:126009, 2023; doi:10.1016/j.polymer.2023.126009<br/>[4] S. Wang et al., Biomacromolecules 2023; doi:10.1021/acs.biomac.3c00738