Oxidase-Mimic Activities of Copper-Based Particles for Colorimetric Determination of Chromium (VI)

Abstract

Extensive use of chromium (VI) in industrial processes such as leather tanning, textile dyeing and metal plating among others has led to it being one of the major pollutants that are discharged to water sources. Chromium (VI) is a toxin with detrimental health effects to mammals. Because of its high toxicity, it is recommended that its concentration in water be limited and therefore organizations such as the World Health Organization (WHO) has set a limit of 0.005 ppm as its maximum level in waste water, the French regulations set its maximum level in drinking water as 0.5 mg/ L and the US Environmental Protection Agency also have a limit of 0.05 mg/ L in drinking water. Techniques have since been developed for detection of Cr (VI) and they include mass spectroscopy, chromatography and also colorimetric methods such as Localized Surface Plasmon Resonance (LSPR) among others but they encounter shortcomings such as complexity, high cost, the need for controlled particle- analyte distance and so on. Therefore herein a hydrothermal method was employed to synthesize copper- based nanoparticles and techniques such as energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X- ray diffraction (XRD) were employed for characterization purposes. The nanoparticles which include CuO and CuS/Fe3O4 were found to possess enzyme- mimic activities which can be used for detection of Cr (VI) colorimetrically and hence address the shortcomings of the above mentioned techniques. This technique involves the oxidation of 3, 3’, 5, 5’- tetramethylbenzidine (TMB) when introduced to a system containing the nanoparticles and chromium (VI). Chromium (VI) promotes the oxidase-like activity of CuO and CuS/Fe3O4 using oxygen as an oxidant such that a deep blue colored 3,3′,5,5′-tetramethylbenzidine diamine (oxTMB) solution was formed by oxidation of a colorless 3, 3’, 5,5’- tetramethylbenzidine (TMB) solution rapidly. The intensities of oxTMB increased proportionally with concentration of chromium (VI) that effects sensitive detection of chromium (VI) by colorimetry. When CuS/Fe3O4 nanocomposites are present; with optimum conditions of pH 4.0, 2.5 mg catalyst load and 6 mM concentration of TMB, LoD as low as 0.060 μM and a broad linear range of 0- 1500 μM were obtained. Using CuO as oxidase-like catalyst LoD of 0.079 μM and a linear range of 0.1- 650 μM were acquired using 2.0 mg of the particles, buffer of pH 4.0 and 10 mM of TMB. The methods proved to be selective of chromium (VI) among XIII other interfering cations and anions. The assays were applied for analysis of chromium (VI) using tap water and proved to be feasible for real sample analysis. These methods proved to be able to address the shortcomings of the above mentioned techniques since they are cheap, rapid and simple compared to the other techniques.