Micromachining Sensors for Electrochemical Measurement in Subnanoliter Volumes

A miniaturized electrochemical cell was produced using lithographic patterning of metals and photopolymerizable polyimide to give a high aspect ratio circular well with clearly defined, clean gold microelectrodes at its bottom. The device, which consisted of a gold working and counter electrode and a Ag|AgI pseudoreference, located in a microchamber of diameter 200 μm and depth 20 μm (0.6 nL), was fabricated in order to analyze very small volumes of solution. An experimental procedure for routinely conducting assays in subnanoliter volumes, involving micropipetting of small quantities of reactants and solvents, was developed in order that the electrochemical characteristics of the device could be defined using the model redox compound ferrocene monocarboxylic acid. The motivation for developing this technology has been to establish a protocol that can be used to make measurements of the responses of single living cells to hormones or metabolic insult, although it is predicted that these general methods may also be of value to those interested in a variety of other fields, including technologies associated with fast-throughput diagnostics. As a consequence, issues concerning solvent evaporation and biocompatibility have also been addressed.