Capacitive response of PDMS-coated IDE platforms directly exposed to aqueous solutions containing volatile organic compounds

In this work we report on the capacitive response of interdigitated electrodes (IDEs) covered with a thin (~50 μm) polydimethyl siloxane (PDMS; Sylgard®184) film to (1) the deposition and evaporation of drops of pure volatile organic compounds (VOCs) and to (2) the direct exposure to aqueous solutions of these VOCs (up to 1 mM) under continuous flow conditions. The investigated VOCs were chloroform, methyl tert-butyl ether, 1-hexanol, toluene, m-xylene, and n-hexane. It is shown that the capacitive response changes upon absorption of the VOCs are in both cases in line with their polymer affinity and their relative dielectric constants as compared to the one of thin PDMS layers. In addition, the response changes were fully reversible. Chloroform, methyl tert-butyl ether, and 1-hexanol were measured reproducibly with a detection limit of ~0.1 mM. For toluene, m-xylene, and n-hexane the response reproducibility was strongly impaired due to their low water solubility and the lowest measured concentrations were in the order of ~0.2 mM for the aromatics and ~0.009 mM for n-hexane. Within a closed flow cell system, the presence of a bypass circuit and/or a parasitic parallel impedance to the environment (third electrode effect) was observed. In the case of n-hexane and m-xylene additional features of the response profile were observed before reaching equilibrium, revealing the contribution of additional response mechanisms. This could be rationalized by an overshoot of the VOC concentration in the PDMS or by swelling effects. clude, this work reports on the capability of direct detection of organic pollutants in water by capacitive PDMS-coated IDE platforms. It points out the challenges in system design and response interpretation, which facilitates the further development of water pollutant sensoring systems.