Self-contained microelectrochemical immunoassay for small volumes using mouse IgG as a model system

A self-contained, microelectrochemical immunoassay on the smallest volumes reported to date (1 muL for the antigen, 1 muL for the secondary antibody-enzyme conjugate, and 200 nL for the electrochemically detected species) has been developed using mouse IgG as a model system in a sandwich-type enzyme-linked immunosorbant assay, which takes less than 30 min to both complete the assembly of immunoassay components onto the antibody-modified surface and detect enzymatically generated species (excluding time for electrochemical cleaning of electrodes). These studies demonstrate the advantage of the close proximity of electrodes to modified surfaces and their application in the analysis of small volumes. Using a 50 mum diameter x 8 mum deep cavity with individually addressable electrodes on a microfabricated chip, the primary antibody was selectively and covalently attached at a gold, recessed microdisk (RMD) at the bottom of the microcavity to the free end of SAMs of either 11-mercaptoundecanoic acid or 11-mercapto-1-undecanol using 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride. Nonspecific adsorption to the surrounding material, polyimide, of the microcavity device was eliminated. Electrochemical desorption was used to confine the immunoassay activity at the RMD. Enzymatic conversion of the substrate p-aminophenyl phosphate to p-aminophenol is detectable in less than 30 s using cyclic voltammetry at a gold, tubular nanoband electrode, which is on the wall of the microcavity and immediately adjacent to the modified RMD. A third electrode, also within the region of the microcavity, served as the pseudoreference/auxiliary electrode. Calibration curves obtained for 1-muL solutions of 5-100 ng/mL of IgG and for 200 nL-solutions of 5 muM to 4 mM of PAP(R) gave detection limits of 4.4 nM (6.4 ng/mL) or 880 fmol (129 pg) for PAPR and 56 fM (9 pg/mL) or 56 zmol (9 fg) for IgG. It is expected that the device may be suitable for analysis with volumes down to tens of picoliters.