Close proximity effects in microassays with electrochemical detection

by Fakunle, Eyitayo S. O.; Bullard, Leah; Higginbotham, Penny; Henrichs, Andrea; Aguilar, Zoraida P.; Fritsch, Ingrid

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Examples of different geometries of heterogeneous immunoassays integrated with electrochem. detection on a small scale will be presented. Discussion will involve our early designs of microcavities with immunocomponents immobilized on a recessed microdisk surface in 50 μ diam. microcavities on silicon substrates. Because of the close proximity of a nanoband electrode to the immunoassay site (only 4 μ away), reporter mols. can be detected in a short time frame, before having a chance to dil. into the surrounding soln., resulting in high sensitivity and fast response time. We will also describe recent results from assay sites integrated with electrodes within microchannels, which further limit diln. of reporter mols. One design has been fabricated in low-temp. co-fired ceramic (LTCC) materials, which allows three-dimensional structures to be easily made. Another design involves an array of microband electrodes that allow the signal to be amplified by redox cycling. That design is also modular in that the assay site and electrodes are prepd. sep. and combined in the last step, thereby protecting the electrodes from fouling and allowing reuse of the electrodes for addnl. assays. All of these designs have been tested with the model immunoassay for mouse IgG. We are now extending the benefits of close proximity effects in microassays to detect other species, such as microorganisms like Cryptosporidium parvum.