Abstract: Demonstrated is that self-assembled monolayers anchored to gold by a thiolate can trap an analyte and electrochemical release of the assembly into a carrier allows determination at a downstream detector. Quantitative potentiostatic release of a coumarin derivative is demonstrated by fluorometry. The capture Of PMo12O403- on a monolayer of 4-aminothiophenol followed by oxidative release results in a 4.8 nA peak (1.6-s width at 1 mL min-1) for the four-electron reduction Of PMo12O403-. The sensitivity is increased by forming multilayers on the pre-concentrator. Release of 10 layers of P2W18O626- yields a 12.4 nA peak even though only a one-electron reduction occurs at the potential employed.

Abstract: A self-contained, microelectrochemical immunoassay on the smallest volumes reported to date (1 µL for the antigen, 1 µL 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 immunosorbent 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 µL 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.

Abstract: A new type of chemically modified electrode (CME) was fabricated by electrodeposition of palladium(II)-substituted Dawson type heptadecatungstodiphosphate, K-8[P2W17O61Pd(H2O)] (abbreviated as P2W17Pd in the following), onto a carbon fiber microelectrode (CFME). A pair of waves was observed on the P2W17Pd CFME, which is ascribed to the redox process of the palladium center in the heteropolytungstate. After continuous potential scanning for 30 min in a pH 4.0 buffer, 92% of the original electrode response remained for the P2W17Pd CFME. The P2W17Pd CFME had high electrocatalytic activity for nitrite reduction and exhibited good reproducibility and stability. The catalytic peak current was found to be linear with the nitrite concentration in the range of 1.0 x 10^-7 similar to 1.2 x 10^-3 mol L-1 (at 25°C) with a correlation coefficient of 0.9886 The detection limit (signal/noise = 3) was found to be 2.0 x 10^-8 mol L-1. The response time of the microsensor for nitrite measurement was less than 15 s. For 10 parallel measurements of 1.0 x 10^-5 mol L-1 nitrite, the relative standard deviation (RSD) was found to be 4.5%. The sensitivity of the microsensor was 0.57 nA (µmol l-1)-1. The P2W17Pd CME was applied successfully as an electrochemical detector (ECD) to determine the nitrite level in rat brain by flow injection analysis (FIA) coupled with microdialysis sampling. The linear range was over three orders of magnitude and the detection limit was 3.0 pmol for nitrite determination. The mechanism of the catalytic reaction was also addressed.

Abstract: Adsorption-desorption phenomena of sodium dodecyl sulfate and sodium dodecylbenzenesulfonate at a mercury electrode were studied by flow injection method with measuring differential capacity-time curves. Adsorption of sodium dodecyl sulfate at -0.2 V is irreversible, while that at the potential more negative than -0.4 V is reversible. Adsorption of sodium dodecylbenzenesulfonate is irreversible at the potential morepositive then -0.8 V. It is reversible at the potential more negative than -1.0 V. From these results, adsorption models for the two surfactants were proposed.