Ultra-Sensitive Carbon Interdigitated Electrode Arrays for Redox Amplification
Miniaturization of electrodes has played a key role in the advancement of electrochemical measurements. Ultramicroelectrodes (UME) have a critical dimension that is of the same order as the diffusion layer thickness at the electrode, thus reducing the time scale of an electrochemical measurement. In the case of macroelectrodes, the electroactive species diffuses linearly normal to the electrode surface; whereas in UMEs diffusion is radial thereby increasing the current density. This phenomenon makes UMEs extremely advantageous for the following reasons: 1) the steady state is reached much faster, 2) electrochemical kinetic measurements are possible and 3) highly accurate measurements can be done even in nonpolar solvents or in resistive solutions. One such arrangement is that of interdigitated electrode arrays (IDEAs). In IDEAs two working electrodes are held in close proximity such that their diffusion profiles overlap. When specific potential are applied to the two working electrodes the electroactive redox species will cycle (oxidize and reduce) multiple times before diffusing out into the bulk solution of the sample. This redox cycling effect has been exploited in last couple of decades for enhancing current signal and lowering the limit of detection. However, most of the work involves use of noble metal IDEAs, which are expensive, require sophisticated instrumentation, and are difficult to fabricate. In this contribution, we report on simpler fabrication technique that involves patterning of SU-8, a negative photoresist, using conventional UV photolithography followed by pyrolysis in an inert environment to create conductive carbon IDEAs with sub-micron resolution. The sensor performance was electrochemically characterized under different geometries under stationary and flow conditions. The capability of the carbon IDEA sensor was demonstrated by applying them in selective detection of dopamine against ascorbic acid interference, selective amplification of neurotransmitters such as dopamine and epinephrine in their mixture, and finally as an electrochemical immunoassay.