Use of Overoxidised Polypyrrole as a Chromium(VI) Sensor

Overoxidation of Intrinsically Conducting Polymers

Intrinsically conducting polymers may undergo significant changes of molecular structure and material properties when exposed to highly oxidizing conditions or very positive electrode potentials, commonly called overoxidation. The type and extent of the changes depend on the experimental conditions and chemical environment. They may proceed already at much lower rates at lower electrode potentials because some of the processes associated with overoxidation are closely related to more or less reversible redox processes employed in electrochemical energy conversion and electrochromism. These changes may be welcome for some applications of these polymers in sensors, extraction, and surface functionalization, but in many cases, the change of properties affects the performance of the material negatively, contributing to material and device degradation. This report presents published examples, experimental observations, and their interpretations in terms of both structural and of material property changes. Options to limit and suppress overoxidation are presented, and useful applications are described extensively.

Evaluation of Cr(VI) adsorption on glutaraldehyde crosslinked chitosan beads using cyclic voltammetry employing gold electrode

Hexavalent chromium is one of the toxic heavy metals found in wastewaters from industries like electroplating, leather tanning, and steel manufacturing. The Cr(VI) removal and quantitative detection are among the major concerns from an environmental toxicity point of view. In the present work, we report an effective electrochemical technique for Cr(VI) monitoring based on the utilization of gold electrode. The technique developed is user-friendly, non-destructive, and provides real-time monitoring of Cr(VI) from wastewaters in contrast to other spectroscopic and optical techniques. The technique developed was applied in monitoring Cr(VI) removal from synthetic Cr(VI) samples using chitosan cross-linked glutaraldehyde (C + G) beads. The parameter optimization for Cr(VI) removal using C + G beads was done. The experimental data of Cr(VI) adsorption on C + G beads obtained using cyclic voltammetry were further used for kinetic and thermodynamic studies. Kinetic and thermodynamic studies found that Cr(VI) adsorption follows pseudo-second-order kinetics and Modified Langmuir isotherm. The maximum adsorption capacity of C + G beads for Cr(VI) was found to be 28.65 mg g−1. These results obtained indicate that the cyclic voltammetric technique using gold electrode can be effectively applied for Cr(VI) analysis from wastewaters.

Highly sensitive detection of hexavalent chromium utilizing a sol-gel/carbon nanotube modified electrode

A pyridine-functionalized thin film has been fabricated to selectively preconcentrate Cr(VI) anions for electrochemical detection in the 5-300 μg L-1 range. Glassy carbon electrodes were modified through physical deposition of single-walled carbon nanotubes (SWNTs) on the electrode surface, followed by electrochemical deposition of a sol-gel containing a 2-pyridine functional group. The use of SWNTs has increased sensitivity for Cr(VI) detection in aqueous solutions, providing a detection limit of 0.8 μg L-1.

Quantitative analysis of trace chromium in blood samples. Combination of the advanced oxidation process with catalytic adsorptive stripping voltammetry

A new method for pretreating blood samples for trace Cr analysis is described. The advanced oxidation process (AOP with H2O2 and 5.5-W UV irradiation for 60 min) is used to remove biological/organic species for subsequent analysis. Prior to the AOP pretreatment, acid (HNO3) is used at pH 3.0 to inhibit the enzyme catalase in the blood samples. Catalytic adsorptive stripping voltammetry at a bismuth film electrode gives a Cr concentration of 6.0 +/- 0.3 ppb in the blood samples. This concentration was confirmed by dry-ashing the blood samples and subsequent analysis by atomic absorption spectroscopy. This current method may be used to monitor chromium, a trace metal in humans, and the efficacy and safety of chromium supplements as adjuvant therapy for diabetes.

Optical determination of Cr(VI) using regenerable, functionalized sol-gel monoliths

Transparent, pyridine-functionalized sol-gel monoliths have been formed and their use in Cr(VI) sensing applications is demonstrated. The monoliths were immersed in acidic Cr(VI)-containing solutions, and the Cr(VI) uptake was monitored using UV-vis and atomic absorption spectroscopies. At concentrations at the ppm level, the monoliths exhibit a yellow color change characteristic of Cr(VI) uptake, and this can be measured by monitoring the absorption change at about 350 nm using UV-vis spectroscopy. Concentrations at the ppb level are below the limit of detection using this wavelength of 350 nm for measurement. However, by adding a diphenylcarbazide solution to monoliths that have been previously immersed in ppb-level Cr(VI) solutions, a distinct color change takes place within the gels that can be measured at about 540 nm using UV-vis spectroscopy. Concentrations as low as 10 ppb Cr(VI) can be measured using this method. The monoliths can then be regenerated for subsequent sensing cycles by thorough washing with 6.0M HCl. The factors affecting monolith uptake of Cr(VI) have been explored. In addition, the gels have been characterized using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) measurements.

Electrochemical deposition of sol-gel films for enhanced chromium(VI) determination in aqueous solutions

A pyridine-functionalized sol-gel film has been formed by electrodeposition at a glassy carbon electrode surface. When this protonated film is exposed to a Cr(VI) solution, the Cr(VI) anions are preconcentrated at the electrode surface. Using square wave voltammetry, the Cr(VI) species are reduced to Cr(III), and a peak current corresponding to this reduction is generated at 0.17V. The peak currents can be correlated with the Cr(VI) concentration. The functionalized sol-gel films demonstrated an enhanced sensitivity for Cr(VI) in aqueous solutions, providing a limit of detection at the low ppb level. Interference studies also displayed the high selectivity of the films for Cr(VI), and the system was able to tolerate a large excess of Cr(III) with no adverse affects. The reported electrodeposition method of film formation uses commercially available reagents and yields films quickly and reproducibly. The growth of these sol-gel films was monitored using an electrochemical quartz crystal microbalance (EQCM), and they were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The reported work shows the promise of such an electrode for use in Cr(VI) sensing applications.