Polydimethylsiloxane-Coated Interdigitated Electrodes for Capacitive Detection of Organic Pollutants in Water

Abstract

There is a high motivation to develop smart sensor systems based on surface-engineered sensor elements for environmental and biomedical monitoring purposes. Key characteristics are that the sensors perform reliable, accurate, sensitive, and ideally selective measurements while being low-cost, portable, robust for online and on-field applications, and allowing wireless communication. Polymer-coated interdigitated electrodes (IDEs), which are able to convert a change in the properties of a dielectric material into an electrical signal, have shown promising detection performances in the gaseous phase. Within this concept, the collection of the target compounds, mostly by absorption, results in dielectric changes that can be measured capacitively. In this thesis, we pursue to study this type of sensors when brought in direct contact with an aqueous phase for the capacitive detection of organic pollutants. For this goal, planar IDE structures centred on 1 cm2 large borosilicate glass chips were fabricated with the following electrode dimensions: 221 golden interdigitated fingers of 2 mm in length, 6 ?m in width, with a 3 ?m gap between them, and diagonally arranged leads of 6 ?m in width to the contact pads. IDE chips with and without in-plane guarding electrodes were designed. The IDE chips were wire-bonded to a printed circuit board, from where further connections to the electronic readout device were made. Measurements were performed using the LabVIEW supported universal transducer interface (UTI) by the Smartec Company in the Netherlands. The polymeric sensing layers were prepared using Sylgard®184, a commercially available polydimethylsiloxane (PDMS) product by the Dow Corning Company. The polymer layers were deposited with controlled thicknesses via a spin-coating technique. For polymer layer thicknesses larger than the spatial electrode wavelength, i.e., 18 ?m, the capacitances for IDE chips with and without in-plane guarding electrodes were in the order of ~19 pF and ~20 pF, respectively. The measurement resolution —and hence detection limit— was measured to be ~500 aF. PDMS has beneficial properties for both sensing and water-contact applications as the visco-elastic and hydrophobic silicone network allows quick and reversible diffusion of small molecules, and simultaneously it serves as a water sealant, protecting the electrode structure. The polymer-coated IDE chip also forms the bottom part of a flow cell chamber, which is completed by mounting a Teflon flow cell with a sealing Viton O-ring centred on top of the IDE chip. The flow cell system allows measurements under continuous flow conditions, using a syringe pump with two individual syringe drives. The polymer layer thickness with respect to the spatial electrode wavelength is an important design parameter. The electric sensing field established between the planar IDEs lies primarily out-of-plane. If the polymer layer is sufficiently thick and the electric field is restrained within the layer, only the absorption of pollutants into the layer will be detected. For thin polymer layers, for which the electric field extends beyond the polymer surface, also adsorption and swelling as induced by pollutant uptake will contribute to changes in the capacitive response. We show that the polymer layer thickness also plays a significant insulation role. The out-of-plane electric field of the planar IDEs is geometrically difficult to guard, especially when the chip is being exposed to an electrically challenging environment, such as water. This is —because in contrast to air— water is relatively conductive, especially when it contains dissolved solids, and it has a relatively high dielectric constant. For this reason, the electrical complexity for aqueous systems is enhanced and the chance for parasitic electrical coupling effects to other system parts and the surrounding increases. If the electric sensing field is parasitically coupled through the water phase, then device sensitivity to electrical changes of the aqueous solution is induced. By means of the two-port technique, the UTI is not affected by parasitic capacitances of parallel capacitors to ground, such as introduced, e.g., by the coax cables. Yet, electrical coupling events that lead to a reduction in the IDE capacitance cannot be distinguished. It was found that for the designed system and for an unguarded IDE chip with ~50 ?m thick PDMS coating an increase in the aqueous salt (NaCl) molarity from 0 up to 20 mM results in a capacitance decrease down to ~175 fF. This is attributed to enhanced parasitic coupling to the system environment for increasing solution salinity. The parasitic coupling effect further increased for lower layer thicknesses, showing its role of insulation. We demonstrated how water-enhanced electrical coupling to in-plane guarding electrodes or ideally a third electrode, which connects the water phase with the ground terminal, is a key mechanism in achieving response immunity to further parasitic coupling. This way the device sensitivity to solution conditions can be significantly decreased for reasonable layer thicknesses and without impairing the detection performance. Eventually, we investigated the capacitive response of interdigitated electrodes covered with ~50 ?m thick PDMS films from the moment of deposition until full evaporation of drops of pure volatile organic compounds (VOCs), including chloroform, methyl tert-butyl ether, 1-hexanol, toluene, m-xylene, and n-hexane. Then the direct exposure to aqueous solutions of these VOCs (up to 1 mM) under continuous flow conditions was studied. It is shown that the capacitive response changes upon absorption of the VOCs into the PDMS are in line with their relative dielectric constants as compared to the one of the thin PDMS layer and that the response changes were fully reversible. The response changes were in the low fF range with a detection limit of ~0.1 mM for the more water-soluble VOCs, which include chloroform, methyl tert-butyl ether, and 1-hexanol. However, the response reproducibility decreased for the more hydrophobic organics toluene, m-xylene, and n-hexane, pointing at distribution processes such as evaporation, wetting, and phase separation within the system. Research challenges are the design of innovative polymeric sensing layers that provide both tuned chemical selectivity within an IDE multi-array and sufficient sealing properties when being directly exposed to water. This may require also the design of new surface modification techniques to achieve good adhesion with the transducer substrate. Further research on the detection and partitioning processes will demand the design of fully conditioned and calibrated measuring systems. Finally, the development of more sensitive transducer interfaces and fully integrated smart sensor systems are further requirements to achieve the goal of target detection at governmental water standards.