Probes Based on Silica Sol-Gels for Voltammetry in the Absence of Electrolyte in the Sample Phase

Silica sol-gels that are processed at room temperature contain residual solute. By selection of salts that have the properties of a humidistat in the sol, the resulting gel will contain a controlled amount of water, thereby making the resistance of the sol-gel independent of humidity of a contacting gas phase. Further, when the processing conditions result in a mesoporous sol-gel, diffusion coefficients of electroactive species that are encapsulated therein are in the 10^-6 cm²s^-1 range. The latter is illustrated by the cyclic voltammetry of hexacyanoferrate when a solid-state cell is fabricated to incorporate three electrodes in the silica matrix. This design is extended to the voltammetry of gas-phase species by extending the working electrode beyond the sol-gel so that a three-phase boundary is formed. In this manner the detection of carbon monoxide, hydrazine, and hydrogen peroxide by amperometry is achieved.

            This cell design was evaluated as an amperometric detector for gas chromatography. Initial experiments were in the flow injection mode with He flowed at 0.63 mL min^-1 as the carrier gas and100- µL injections of H₂ comprised the sample. Wirh a Pt working electrode at 0.5 V vs. a Pt quasi-reference electrode, linear response to concentration over range 1.7 x 10^-6 g to 7.6 x 10^-6 g was observed. Extensions to 1,2-ethandithiol, phenol, p-cresol, and thioanisole yielded detection limits of 4, 1, 3, and 70 ppmv (k = 3 criterion), respectively, when operating the working electrode at 800 mV. Application to aqueous samples in the absence of supporting electrolyte also were achieved; however, a problem is that leaching of the electrolyte from the sol-gel cell was a limitation. To address this problem, the sol-gel matrix is being modified to contain a macromolar cation (anion) of a size that is greater than the ca. 0.5 nm pore size of the silica. An example is to immobilize 3,4- ethyelenedioxythiophene (EDOT) in the sol-gel and oxidize it to the polymeric form, PEDOT. Results on the polymerization process will be shown, and the efficacy of the resulting composite as a matrix for a probe to determine various species in water in the absence of supporting electrolyte will be presented.