Xiaohua Cai

DNA biosensor for the detection of hydrazines.

A double-stranded (ds) DNA-coated carbon paste electrode is employed as a remarkably sensitive biosensor for the detection of hydrazine compounds. The sensor relies on monitoring changes in the intrinsic anodic response of the surface-confined DNA resulting from its interaction with hydrazine compounds and requires no label or indicator. Short reaction times (1-10 min) are sufficient for monitoring part-per-billion levels of different hydrazines. Applicability to untreated natural water samples is illustrated. The response mechanism is discussed, along with prospects of using DNA biosensors for quantitaing other important molecules and elucidating DNA interactions and damage.

An improved voltammetric method for the determination of trace amounts of uric acid with electrochemically pretreated carbon paste electrodes.

Carbon paste electrodes, preanodized in alkaline medium at 1.4 V vs SCE for a short period of time, exhibit a great shift of the oxidation potential of uric acid in cathodic direction and a marked enhancement of its current response, compared to unpretreated electrodes. These effects are dependent on the preanodization potential and the time imposed on the electrodes as well as on the alkalinity of the supporting electrolyte. The enhanced voltammetric response can be used to determine uric acid in the concentration range of 5.0-4.0 x 10(4) mug/1 (3.0 x 10(-8)-2.4 x 10(-4)M) with a detection limit (3sigma) of 2.0 mug/1 (1.2 x 10(-8)M). Ascorbic acid in less than 30-fold excess does not interfere. For multiple determinations (5 runs), the relative standard deviation is 2.1% at a concentration of 1 mg/1 uric acid. The proposed procedure can be used to determine uric acid in human urine and serum without any preliminary treatment of the samples in an accurate, rapid and simple way.

Voltammetric determination of trace amounts of mercury with a carbon paste electrode modified with an anion-exchanger

SummaryA carbon paste electrode modified with a liquid anion exchanger (Amberlite LA2) was used for the voltammetric determination of mercury(II). Mercury is preconcentrated, as tetrachloromercurate(II), onto the surface of the modified electrode only by the ion-exchange effect of the modifier without application of potential. After exchange of the medium the accumulated amount of mercury(II) is determined by differential pulse anodic stripping voltammetry in a blank electrolyte solution. The response depends on the concentration of mercury in the bulk solution, preconcentration time, and other parameters. The detection limit was 1 μg Hg(II)/l when a suitable time for preconcentration was chosen. Preconcentration for 5 min yields a linear calibration graph for concentrations up to 1000 μg Hg(II)/l. The effect of other ions on the determination of mercury and the applicability of the method to the analysis of phenylmercury compounds in pharmaceutical preparations were investigated.

Determination of traces of copper by anodic stripping voltammetry after its preconcentration via an ion-exchange route at carbon paste electrodes modified with vermiculite

Abstract Vermiculite clay mineral, a novel modifier, was exploited to prepare modified carbon paste electrodes (CPEs), showing attractive ability to efficiently preconcentrate trace copper(II) prior to its voltammetric determination. Under open-circuit conditions, copper(II) was preconcentrated onto the surface of a modified electrode via the ion-exchange capability of the modifier. After medium exchange to a pure electrolyte solution (0.1 M NaNO3 + Britton-Robinson buffer, pH 5), the accumulated copper(II) was reduced at −0.7 V vs. Ag/AgCl during an equilibration period of 20 s and then determined by its re-oxidation employing square-wave anodic stripping voltammetry. The parameters and conditions such as pH of the analyte solution and supporting electrolyte, preconcentration time, CPE composition, its activation and regeneration, stripping mode and others, were studied in detail. The detection limit (3σ) of the proposed procedure was found to be 5 × 10−9 mol l−1 of Cu(II) in the analyte solution, for a preconcentration time of 10 min. Applying suitable preconcentration times, a linear calibration graph from 1 × 10−8 mol l−1 to 8 × 10−5 mol l−1 (0.6 μg l−1−5.1 mg l−1) of Cu(II) was established. Multiple determinations (n = 10), following a preconcentration-measurement-regeneration cycle, gave a relative standard deviation of 4.9% at Cu(II) concentration of 5 × 10 −7 mol l −1 . The accuracy of the proposed method was checked by analysing a standard reference material (SRM 1643b).

Simultaneous determination of uric acid, xanthine and hypoxanthine with an electrochemically pretreated carbon paste electrode

A simple method is presented for the simultaneous differential pulse voltammetric determination of uric acid, xanthine and hypoxanthine. It is based on the improved current responses of the three analytes at carbon paste electrodes polarized in a dilute alkaline medium (0.002 mol/l NaOH, 0.1 mol/l NaClO4) at 1.3 V vs. SCE for a short time. Compared with the methods reported in the literature, this procedure has a much wider linear range (2 to 3 orders of magnitude in concentration), lower detection limits (5 to 10 μg l−1) and less interference by ascorbic acid. The electrochemical responses were found to be dependent on the pre-anodization potential and the time imposed on the electrodes as well as on the alkalinity of the supporting electrolyte. The proposed procedure was used to determine uric acid, xanthine and hypoxanthine in human urine without any preliminary treatment.

Voltammetric determination of gold using a carbon paste electrode modified with thiobenzanilide

Gold(III) can be preconcentrated from acidic solution on a carbon paste electrode chemically modified with thiobenzanilide under open-circuit conditions. Using cathodic differential pulse voltammetry, 0.05–6 µg ml–1 of gold can be determined after preconcentration for 1 or 2 min. Except for mercury(II) and platinum group metals, common ions have little effect on the determination of gold(III). The preparation and regeneration of the modified electrode as well as the various methodical parameters for the preconcentration and measurement of gold(III) were investigated.

Screen-printed electrodes for nitrite based on anion-exchanger-doped carbon inks

Disposable voltammetric sensors for nitrite have been microfabricated by screen-printing an anion-exchanger (Aliquat 336)-modified carbon ink. The modifier ability to accumulate low levels of nitrite is not compromised by the screen-printing and low-temperature drying processes. Experimental variables influencing the preconcentration/voltammetric performance are elucidated and optimized. A short (30 s) accumulation period yields a detection limit of 30 µg l–1 nitrite. Monitoring of micromolar levels of nitrite in drinking- and groundwater samples is illustrated. Prospects of mass production of other preconcentrating modified electrodes are discussed.

Indium–tin oxide film electrode as catalytic amperometric sensor for hydrogen peroxide

A commercial indium–tin oxide (ITO) film, deposited on a glass substrate, was investigated for its potential use as a disposable amperometric sensor for hydrogen peroxide determination in aqueous solutions. After a very simple, yet crucial, electrochemical pre-treatment of a fresh ITO film electrode, a stable and sensitive electrocatalytic response to hydrogen peroxide was exhibited. The electrocatalytic character and factors influencing hydrodynamic amperometric performance of the ITO film electrode were examined and discussed. Under optimized conditions, the amperometric signal was linearly proportional to the hydrogen peroxide concentration over the range 8.4 × 10–5–2.1 × 10–3 mol l–1(2.9–71.4 mg l–1) with a detection limit of 5.6 × 10–1 mol l–5 mol l–1(1.9 mg l–1). The electrode also displayed good reproducibility with an sr value of 1.5% for 20 consecutive chronoamperometric measurements of 2.8 × 10–4 mol l–1 hydrogen peroxide.

Voltammetric behaviour of thallium (III) at a solid heterogeneous carbon electrode using ion-pair formation

Thallium in its trivalent state forms very stable [TIX4]– complexes with halides in corresponding acidic media. Tetrahalogenothallates (III) are able to produce ion pairs with suitable positively charged counter-ions in aqueous solutions. These ion pairs, each possessing distinct lipophilic character, could be accumulated directly onto the surface of carbon paste electrodes without applying a potential (open circuit conditions). The nature of the electrode material and of the counter-ion respectively, influences the adsorptive behaviour of the ion pair significantly. Best results were obtained by using a new type of heterogeneous carbon electrode employing a solid binder (phenanthrene) instead of the commonly used pasting liquid. With respect to the counter-ions, tetraphenylphosphonium and tetrabutylammonium proved to be the best. Parameters influencing the optimum conditions for the analytical procedure (pH, counter-ion concentration, etc.) were investigated. The relationship between current signal and concentration of thallium, and its dependence on the accumulation time were studied. The detection limit was found to be ⩽ 10 µg kg–1, signals were reproducible within an sr of 7.7% for a thallium concentration of 1 mg kg–1. The interference of other ionic species was also evaluated. The method was employed for the determination of thallium in lead- and cadmiumcontaining zinc alloys.

Studies on the voltammetric behaviour of a 2-mercaptoimidazole containing carbon paste electrode: determination of traces of silver

The voltammetric behaviour of a 2-mercaptoimidazole (2-MI) containing carbon paste electrode was studied. When mixed to carbon paste as an electrode modifier, 2-MI can be reduced at negative potentials (−1 V vs. SCE), but it does not give a response in the potential range where Ag(0) is oxidized to Ag(I). Silver could be accumulated from 0.1 mol l−1 acetate buffer onto a 2-MI modified carbon paste electrode without a potential applied; after medium exchange, it was reduced at −1 V vs. SCE in 0.1 mol l−1 acetate buffer solution and determined by differential pulse anodic stripping voltammetry. With suitable preconcentration times, the detection limit was 0.1 μg l−1; a linear relation between current and concentration was found to exist within a range of 0.5 to 1000 μg l−1. In the presence of EDTA, common metal ions have no or only little effect on the voltammetric determination of silver.