Electrolyte-Gated Field Effect Transistors-Based Sensors for Nanoplastics Detection—Use of Different Fabrication Techniques and Semiconductor Materials to Investigate Sensors Sensitivity

Plastics accumulating in the environment are nowadays of big concern for aquatic systems and for the living organisms populating them. In this context, nanoplastics (NPs) are considered the major and most dangerous contaminants because of their small size and their active surface, which brings them to interact with a variety of other molecules. Despite the importance of detecting NPs, currently available methods rely on bulky and expensive techniques, such as spectroscopy.<br/>The aim of this work is to advance the field of electrochemical sensors in the context of environmental monitoring, by demonstrating that they can be employed for the detection of NPs. In particular, we investigated the use of a novel, fast, and easy-to-use electrolyte-gated field-effect-transistor (EG-FET) based sensor for the detection of polystyrene NPs (PS-NPs) – chosen as model material.<br/>First, random networks of carbon nanotubes (CNTs) were selected as the semiconducting material of choice, considering their ability to form non-covalent interactions with PS-NPs. Indeed, in our CNT EG-FETs (EG-CNTFET) devices, interaction between NPs and CNTs caused a change in the electric double layers. Changes of on current (I_ON) (compared to a baseline value) were calculated and the corrected I_ON (*I_ON) was obtained. A linear increase in the *I_ON of the EG-CNTFETs, with a sensitivity of 9.68 μA/(1mg/ml) and a linear range of detection from 0.025 to 0.25 mg/ml were observed. A π-π interaction was hypothesized to take place between the two materials, as indicated by X-ray photoelectron spectroscopy analysis. Using artificial seawater as electrolyte, to mimic a real-case scenario, a linear increase in *I_ON was also observed, with a sensitivity of 6.19 μA/(1mg/ml), proving the possibility to use the developed sensor in more complex solutions, as well as in low concentrations.<br/>Next, the production and characterization of more complex NPs solutions, resembling more realistic environmental conditions, were evaluated. It was proved that NPs absorbed mercury ions (Hg²⁺) on their surface, with a binding capacity of 2.04 %. This more complex NPs solution (NP-Hg complexes) was used to evaluate the EG-CNTFET sensitivity. No clear differences compared to pure NPs solution were observed, showing that the developed sensors could be used even with a more complex analyte.<br/>Finally, a different semiconductor material, poly[2,5-(2-octyl-dodecyl)- 3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b] thiophene)](DPP–DTT), was ink-jet printed, to use electrolyte-gated organic field-effect-transistor (EGOFETs) for this application. A linear increase in the *I_ON of the EGOFETs, with a sensitivity of 1.3 μA/(1mg/ml) and a linear range of detection from 0.01 to 0.25 mg/ml were observed. Further enhancement of the EGOFET sensors, by utilizing a peptide as biorecognition element, were then evaluated. Through cyclic voltammetry, a decrease of current in the reduction peak was observed, with increasing peptide concentrations, proving the effective grafting of the peptide on the gate electrode. With electrochemical impedance spectroscopy, an increase in Z (impedance) was observed with increasing peptide concentrations, which correlates to an increase in the charge transfer resistance. With peptide concentration of 10 µg/ml, an increase of 1500 % compared to bare gold was shown. The developed functionalization protocol is now being transferred to a final EGOFET device, and studies on the sensitivity are being conducted.<br/>This study offers a starting point for future exploitation of electrochemical sensors for NP detection and identification.