Abstract: We prepared a new carbon film electrode by the electron-cyclotron-resonance (ECR) plasma sputtering method. The film is amorphous-like and different from the glassy carbon (GC), highly oriented pyrolytic graphite and diamond films that have previously been studied for use as electrode materials. We used cyclic voltammetry (CV) and flow injection analysis (FIA) with amperometric detection to compare the electrochemical properties of an ECR-sputtered carbon (SC) film electrode with those of a GC electrode and carbon film deposited by thermal chemical vapor deposition (T-CVD). The ECR-SC film electrode exhibited better electrochemical properties than the GC and T-CVD carbon electrodes, namely, a lower background current, a wider working potential window, and better reproducibility. CV with the ECR-SC film electrode did not reveal any large reduction in rho-nonylphenol (NP) and other alkylphenol (AP) peaks. In contrast, we observed a clear reduction in the CV and FIA peaks when we measured NP with GC and T-CVD carbon electrodes, indicating that there was much less deactivation from the NP oxidation products on the ECR-SC electrode surfaces. The NP calibration curve at the ECR-SC electrode in a flow system showed a linear concentration range of 0.125-10 muM with a detection limit of 50 nM (S/N = 3). (C) 2002 The Electrochemical Society.

Abstract: We have developed a highly sensitive and selective online biosensor for detecting the reduced form of nicotinamide adenine dinucleotide (NADH) produced by the enzymatic reactions of dehydrogenases with Various substrates such as glutamate. The sensor consists of a glassy carbon electrode modified with an osmium-polyvinylpyridine-based bottom layer containing horseradish peroxidase, and a bovine serum albumin (BSA)-gluteraldehyde (Glut) top layer containing NADH oxidase (NOX) or glutamate dehydrogenase (GluDH) and NOX. We assembled the modified electrode in a thin-layer radial flow cell and sample solution was continuously introduced into the cell with a syringe pump. We optimized the sensitivity of the NADH sensor by adjusting the glutaraldehyde amount in the immobilized layer, the applied potential and the pH of buffer solution. We examined the flow-rate effect on the current response and the conversion efficiency of NADH at the modified electrode. As a result, we achieved a sensitivity of 48.8 nA cm-2 µM-1, a detectable concentration range of 25 nM similar to 10 µM and a detection limit of 20 nM (S/N = 3) for the NADH sensor. The interference from ascorbic acid and other electroactive interferents can be greatly reduced since the sensor can be operated below 0 mV versus Ag/AgCl. The NADH sensor is relatively stable since it retains 70% of its original response after I month if stored at 2-8°C in a dry state after use. Furthermore, we fabricated a glutamate sensor by coimmobilizing GluDH and NOX in the BSA-Glut top layer. The detectable glutamate concentration range is from 0.1 to 10 µM and the detection limit is 0.1 µM (S/N = 3). Our glutamate dehydrogenase-based sensor offers good selectivity as regards other amino acids.

Abstract: We achieved improved detection limits for sugars by developing a novel thin film containing 0.8% highly dispersed Ni nanoparticles in disordered graphite-like carbon (Ni-NDC) as a detection electrode for high-performance liquid chromatography. The Ni-NDC film was prepared in one step by a simple radio frequency (rf) sputtering method at a temperature below 200°C. We characterized the film by XPS, TEM, and AFM analysis and found that the average Ni nanoparticle size was 3 nm and that the film consisted of a mixture of Ni, NiO, Ni2O3, and Ni(OH)2. We studied the electrochemical detection of sugars using the 0.8% Ni-NDC film electrode. The film electrode had excellent electrocatalytic ability and good stability compared with a Ni-bulk electrode with regard to the electrooxidation of sugars. We employed the Ni-NDC film as an HPLC detection electrode. We achieved a good separation of four sugars (glucose, fructose, sucrose, lactose) at a relatively low constant detection potential (0.40 V vs Ag/AgCl) and a linearity of over 3 orders of magnitude. We obtained improved detection limits for the investigated sugars, namely, 20, 25, 50, and 37 nM for glucose, fructose, sucrose, and lactose, respectively. This is at least 1 order of magnitude lower than the detection limits obtained with a Ni-bulk electrode with the same measurement condition. The Ni-NDC film electrode also showed good reproducibility with a relative standard deviation of 1.75% for 40 consecutive injections of glucose in a flow system.

Abstract: A method for the highly sensitive determination of acetylcholine (ACh) and choline (Ch) was developed using platinum (Pt) black microarray electrodes as detectors in a microbore liquid chromatography (LC) with a post column enzyme reactor. The electrodes were prepared by plating a gold (Au) film electrode with sub µm Pt-black particles. Since hydrogen peroxide generated by the enzymatic reaction of ACh and Ch was oxidized only at the Pt-black microarray electrode, each Pt-black particle (typically 0.1-0.2 µm in size) operated as an ultramicroelectrode. A high signal-to-noise ratio was achieved because of the high current density at the Pt-black microarray electrodes and because the Au film has a much lower baseline noise than the Pt. Detection limits of 5.7 (ACh) and 6.0 (Ch) fmol were obtained, with a wide linear range. The ACh and Ch signals with an Au film electrode modified with Pt-black particles retained more than 70% of their initial value after 5 days with continuous potential application. This is better stability than for a bare platinum electrode which retained only 40% of its initial response under comparable conditions.