Abstract
In the last 70 years, the plastic production has exponentially increased, due to the use of plastic in our daily life for a wide range of applications. However, most of the plastics, after their use, are not recycled but rather incinerated or discarded in the environment. Micro and nanoplastics (MPs/NPs) are small plastic particles which can be ubiquitously found in the environment, such as in aquatic systems and in soil. Detection and quantification of MPs/NPs in the environment is important as they are a source of pollution and cause damage to the fauna and flora. To this end, long and not-standardized methods are nowadays being employed. In this context, sensors and biosensors - a new class of analytical devices - able to selectively detect MPs/NPs are especially interesting due to their simplicity, high sensitivity, short analysis time, label-free detection, low fabrication cost, minimal sample preparation, and field applicability. This Ph.D. thesis focuses on the development and evaluation of an electrochemical sensing platform for the detection of NPs in environmental and agri-food samples. The thesis covers every aspect from fabrication to electrical and morphological characterization, focusing on the sensor’s response to simple and more complex NPs solutions. Furthermore, the work covers also studies on the sensor’s functionalization with a specific biorecognition element (a peptide), in order to achieve the desired sensor selectivity to the analyte of interest. The first part of this research work focuses on the development and optimization of the chosen sensing platform: electrolyte-gated field-effect transistor (EG-FET). The EG-FET sensors were fabricated using microfabrication techniques (photolithography followed by thermal evaporation and lift-off) and spray coating to deposit semiconducting carbon nanotubes (CNTs) as sensing material of the devices. The devices were first studied with simple electrolyte solutions, such as deionized-water and artificial seawater, to assess their stability over time. Results proved their stability (i.e. small and negligible change in current with transfer curves) after about 70 minutes from electrolyte addition. To continue, the ability of CNTs and polystyrene nanoplastics (PS-NPs) to interact through non-covalent interactions was exploited as sensing mechanism. Commercially available PS-NPs solution was used. Indeed, there was an increase in corrected on current (*ION ) with increasing PS-NPs concentration. The representative device yielded a sensitivity of 9.68 µA/(1mg/ml) when DI-water was the electrolyte in which PS-NPs were dispersed. When artificial seawater was the electrolyte used, the EG-FETs sensitivity decreased to 6.19 µA/(1mg/ml). PS-NPs and CNTs interaction was studied both with atomic force microscopy (AFM) and with X-ray photoelectron spectroscopy (XPS). With AFM it was proved that NPs were indeed present on top of the CNTs network. Also, with XPS a π-π interaction was hypothesized. The second part of this research focuses on the preparation, in the laboratory, of more complex NPs solutions (resembling more what is found in the environment), to be studied with the developed EG-FETs. Two environmental contaminants were chosen (Hg2+ ions and Methylene blue) and their possibility to be adsorbed on PS-NPs surface was evaluated. Hg2+ showed a binding capacity of 2.04% (±0.2), while Methylene blue had a binding capacity of 11.02% (±1.7). With PS-Hg complexes further studies were done, and with dynamic light scattering it was shown that no aggregates were formed during the adsorption test. With XPS, a chemical interaction between PS-NPs and Hg2+ was proved. The PS-Hg solution was also used to investigate the response of EG-FETs sensors to this more complex solution, compared to pure PS-NPs. After 10 minutes of test, the increase in IDS with pure PS-NPs solution was +22%(±17), while with PS-Hg complexes solution it was +47%(±17). Not only the increase was higher, but for PS-Hg complexes it kept increasing with time, while for pure PS-NPs it reached a stable value after about 20 minutes of test. Finally, to further improve EG-FET-based sensor performance, selectivity has to be achieved, thus the ability of the sensor to respond only to the specific target analyte. A PS-binding peptide was used as biorecognition element, and functionalization protocol was studied through electrochemical analysis. Gate electrodes could indeed be functionalized and peptide concentrations of 5 and 10 µg/ml were the best performing. Indeed using these two concentrations a high increase in impedance was measured through electrochemical impedance spectroscopy, as well as a change in phase angle. A further increase in impedance when PS-NPs was added to the functionalized electrode was seen. This showed that a binding between the grafted peptide and PS-NPs could be occurring and thus the peptide could be used as biorecognition element. The EG-FETs with functionalized gate electrode showed a higher sensitivity towards PS-NPs, 84.7%/(1mg/ml). These biosensors with improved sensitivity, will undergo further testing with other types of NPs, to study also the selectivity of the biosensor.