Riassunto analitico
Per- and polyfluoroalkyl substances (PFAS) are linear or branched fluorinated organic compounds, composed by one or more chains of carbon-fluorine atoms and a charged terminal group, like carboxylic acid or sulfonate (CnF2n+1X).
Although PFAS are still considered emerging pollutants, the first report about the global diffusion of the perfluorooctane sulfonate (PFOS) by Giesy and Kannan is dated 20011. These substances could spread in sea water, fresh water, and ground water around the world and can easily accumulate thanks to the stability of the carbon-fluorine bond and the hydrophilicity of the charged head.
Recently, the interest on these compounds is increased and the European Commission proposed a new set of laws to control the PFAS diffusion in both drinking water and non-potable water: in particular, the New Drinking Water Directive (2020) introduced the limit of “Total PFAS” of 0.50 μg/l for non-drinking water and the limit for the “SUM of PFAS” of 0.10 μg/l as regards water intended for human consumption.
At the state of art, the golden standard for PFAS detection is the mass spectrometry but it is an expensive technique that requires expertise, standards, and it is time consuming2.
The aim of this thesis is to propose an alternative technique for PFAS detection, faster and more user friendly than mass spectrometry: the new sensing platform is based on an electrolyte gated organic transistor (EGOT) exploiting fluorous-fluorous interactions as sensing driving force.
The sensing capability is due to a metallic electrode, a gold wire used as gate in the EGOT configuration, functionalized by a binary self-assembled monolayer (SAM) formed by a thiolate four units oligo ethylene-glycol (mPEG4-SH) and a perfluorinated molecule: 1H,1H,2H,2H-Perfluorodecanethiol (PFDT). The organic semiconductive channel is made of a thin film of the polymer Poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)] (DPP-DTT) deposed by spin-coating onto interdigitated gold source and drain electrodes. Distilled water has been used as electrolyte3.
Initially, the procedure for the gate electrode functionalization has been standardized. Both electrodeposition and dip coating processes were investigated by potentiometric methods (cyclovoltammetry and electrochemical impedance spectroscopy). Overnight incubation in an ethylic solution of mPEG4-SH 3 mM and PFDT 1 mM was finally selected as functionalization method because it ensures the best EGOT performances and reproducibility.
In a second time, the EGOT configuration was tested, using a source meter unit, to calibrate the device response to solutions containing increasing concentrations of Perfluorooctanoic acid (PFOA) or Perfluorobutanoic acid (PFBA). The goal was to distinguish the PFAS concentrations below and above the law limits (viz. 0.10 μg/l). All the solution were prepared using distilled water.
The device exhibits a good sensitivity in the ng/l target range, and it is able to discriminate a solution under 100 ng/l to one over the limit. Other experiments are necessary to determine the limit of detection and evaluate the interaction with other PFAS, that feature different terminal group or chain length.
This is the first example of a sensor based on an EGOT architecture for the detection of perfluorinated pollutants and could help researchers in the development of a new class of sensors for the detection of PFAS in water and different substrates exploiting the fluorous-fluorous interactions.
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