Materials and methods for a miniaturized E-Tongue

The only existing techniques to study the composition of a liquid or to detect anomalies and specific compounds in aqueous solutions are cumbersome tools, available only for expert users. By use of a cross-sensitive array of polymeric sensors, the dimensions of an electronic tongue system can be terrifically reduced, up to the creation of a portable device. Incorporating hexacyanoferrate(II) anions ([Fe(CN)6]4-) in polypyrrole membranes allows the films to achieve sensitivity towards monovalent anions and cations, with quasi-Nernstian slopes, during potentiometric characterizations. Literature research and many experiments brought to the ability of depositing film in an extremely reproducible way, on top of electrode with a diameter of 250um. The polymer membranes can be selectively deposited on conductive surfaces, by a potential-driven technique known as electro-polymerization. Cation sensitivity is obtained applying a constant potential on a working electrode, while response towards negative ions can be achieved sweeping the potential between a negative and a positive value during the polymer deposition: in the latter case, the incorporated anions are expelled during the negative side of the cycle, while the polymer is grown during the positive one. An electrochemical characterization by cyclic voltammetry shows the different effect of soaking films in reference solutions (0.1M KCl) in the case of presence of large anion in the film, and this difference is reinforced by SEM analysis of the polymer surface. The polymer membranes behavior in aqueous solutions is strongly dependent on the electrode surface on top of which they are deposited and on the volume sample in which they are immersed. The best surface on top of which the best potentiometric responses are obtained has been studied, and resulted in sputtered platinum electrode, while a too rough surface, as in the case of carbon screen printed electrodes, strongly influences the membrane response. All the acquired knowledge about polymeric sensitive films towards ionic salts was used to fabricate an electrochemical transducer that is able to classify and distinguish three different commercial beverages, and four brands of mineral waters. With the use of an array made of four sensors, two sensitive towards chloride anions, and two towards potassium and sodium, an accuracy of 95% was obtained in the first case, while for mineral waters the sensor was able to classify correctly each brand two times out of three. Moreover, a 3D cell to be used in an automated system was successfully designed and fabricated, in order to considerably increase the experimental throughput. The achieved results are an advanced starting point for the creation of a self-standing device, able to target many industrial application fields, and that is competitive with the existing technologies.