| Over the past decade, electrochemical (EC) detection has become a well-established methodology for use in microfabricated lab-on-a-chip (LOC) analysis systems. In the work reported to date, the actual electrodes employed, whether integrated directly onto the microchip during the fabrication process or incorporated off-chip at the end of a fabricated channel, have always had a relatively simple planar structure. However, microfabrication techniques could be used to produce much more sophisticated electrode structures that are theoretically capable of carrying out complex detection and sample processing operations such as pre-concentration, binding, catalysis, etc.; The primary objectives of this study were to: (1) design and fabricate complex three-dimensional (3-D) microelectrodes with high surface areas on LOC systems; and, (2) apply these 3-D microelectrodes to several EC detection scenarios. The 3-D microelectrodes were fabricated on a soda lime glass microchip by a unique sequence of photolithographic and micromechanical steps. This novel technique to make high aspect ratio gold microstructures (as much as 4:1) involved the modification of stud bump geometry with microfabricated silicon molds using an optimized combination of temperature, pressure and time. The microelectrodes that resulted consisted of an array of 20 mum tall and 20 mum wide square pillars placed at the end of a microfabricated electrophoresis channel. The surface area of 3-D microelectrodes could be increased by as much as 50X while collection efficiencies could be increased to ∼100% as compared to ∼30% efficiency for planar microelectrodes (for the neurotransmitters dopamine and catechol).; The 3-D microelectrodes were used both in a stand-alone configuration for direct EC detection of model catecholamine analytes and, more interestingly, in dual electrode configurations for EC sample processing prior to detection downstream at a second planar electrode. In particular, the 3-D electrodes were shown to be capable of carrying out complete (100%) redox conversion of analyte species in either plug or continuous-flow formats. Specific applications of this approach include redox removal of easily oxidized interferants from a sample mixture and pre-concentration/stripping of a trace metal (e.g., Cu 2+). |
