Fabrication of carbon microelectrodes with a micromolding technique and their use in microchip-based flow analyses

Abstract

In this paper, we report a new technique to pattern carbon microelectrodes for use in microfluidics. This technique, termed micromolding of carbon inks, uses poly(dimethylsiloxane) (PDMS) microchannels to define the size of the microelectrode. First, PDMS microchannels of the approximate dimensions desired for the microelectrode are made by soft lithography. The PDMS is then reversibly sealed to a substrate and the microchannels are filled with carbon ink. After a heating step the PDMS mold is removed, leaving a carbon microelectrode with a size slightly smaller than the original PDMS microchannel. The resulting microelectrode (27 µm wide and 6 µm in height) can be reversibly sealed to a PDMS-based flow channel. Fluorescence microscopy showed that no leakage occurred around the chip/electrode seal, even up to flow rates of 10 µL min⁻¹. The electrode was characterized by microchip-based flow injection analysis. Injections of catechol in Hank's Balanced Salt Solution (pH 7.4), showed a linear response from 2 mM to 10 µM (r² = 0.995), with a sensitivity of 56.5 pA µM⁻¹ and an estimated limit of detection of 2 µM (0.27 picomole, S/N=3). Reproducibility of the electrode response was shown by repeated injections (n = 10) of a 500 µM catechol solution, resulting in a RSD of 4.6%. Finally, selectivity was demonstrated by coating the microelectrode with Nafion, a perfluoronated cation exchange polymer. Dopamine exhibited a response at the modified microelectrode while ascorbic acid was rejected by the Nafion-coating. These electrodes provide inexpensive detectors for microfluidic applications while also being viable alternatives to use of other carbon microelectrode materials, such as carbon fibers. Furthermore, the manner in which the microelectrodes are produced will be of interest to researchers who do not have access to state of the art microfabrication facilities.