Despite the multifaceted biomedical significance of NO, little progress has been achieved so far in the quantitative understanding of the signal transduction mechanisms where NO is involved. To help progress in this area, we propose a simple electrochemical NO sensor here, consisting of a glass sealed platinum microdisk electrode coated with cellulose acetate to reduce both surface fouling by proteins and response to potential interferences. A differential amperometry protocol is optimized to improve selectivity and provide a stationary oxidation state of the platinum surface, which prevents loss in sensitivity during long-term use. We found the oxidation of NO by O2 second order in [NO] with a rate constant of (8.0 +/- 0.4) x 10(6) M(-2) s(-1), in good agreement with literature data obtained by other than electrochemical methods. The release rates of NO detected in cultures of activated macrophages were on the order of 20 pmol/ (10(6)cells s) and correlated well with the nitrite content determined by the spectrophotometric Griess assay.
Voltammetric experiments in sample droplets of 20 microL volume were carried out with gold ring electrodes microfabricated on Pyrex substrates. The droplet studied was centered and kept in place by a hydrophobic ring deposited on the substrate around the ring electrode, which also ensured that each sample assume a semispherical shape. A small hole in the center of the substrate filled with an agar gel membrane served as a junction. A mild jet of humidified nitrogen gas directed tangentially at the droplet caused it to rotate at a high rate. Cyclic voltammetry and constant-potential electrolysis of potassium ferricyanide were used to characterize this rotating sample system and to calculate diffusion layer thickness. The results clearly demonstrate that rotating of a semispherical microsample placed above a stationary ring electrode with a mild gas jet can be as effective as a rotating electrode system. As a practical example, voltammetric stripping analysis of 20-120 pmol mercuric nitrate samples was performed with a calibration of good linearity (r2 = 0.988) and a time constant for exhaustive electrolysis of approximately 2 min.
As a result of industrialization lead is one of the most widely dispersed toxic heavy metals in the environment. There is a pressing need for a reliable, affordable and portable analytical technique for routine determination of lead at trace levels in biological and environmental samples. Despite their potential for portability and low cost, the currently available electrochemical stripping methods still have limited commercial availability. Among the reasons are the relatively large sample volumes and the large amount of reagents needed (1 ± 3 mL), lower than required precision, and the inconvenience of a rotated electrode system. The Rotating Sample System is a unique approach to electrochemical stripping, devised for 20 mL sample droplets utilizing a large surface area electrode. This design combines the advantages of a microelectrode and a rotated electrode system. The semispherical sample drop itself is rotated by a fine gas jet directed at it tangentially, eliminating the need for a sample container. Neither fine moving mechanical parts nor sophisticated controls are required. The detection limit for lead(II) was found low enough and the reproducibility is sufficient for routine determinations in biomedical samples (5 ppb, 6%). The system can support a CDC recommended screening for blood lead levels and an on-site analysis of environmental samples as well. Under suitable conditions calibration free direct determinations can also be performed.
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