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In this study, CuS/Fe3O4 composite structures were prepared in a two-step process. Initially, we used a simple solvothermal method to synthesize CuS nanostructures. Hydrothermal method was employed to form CuS/Fe3O4 composites. We characterized the particles using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The particles exhibit an oxidase-like activity using the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) as oxidase substrate. The presence of chromium (VI) increases the oxidase-like activity and is dependent on the concentration. The UV/visible absorption intensity of the oxidized TMB (oxTMB) increases with the concentration of chromium (VI) which provides a route to colorimetric determination of chromium (VI). We noticed a limit of detection as small as 60 nM and a wide linear dynamic range of 0–1500 µM. We found a high selectivity of this method for chromium (VI) against potentially interference cations and anions. Its applicability for determination of chromium (VI) in real environmental samples is demonstrated using tap water. This result demonstrates the capability of the method in determination of chromium (VI) in real samples. [ABSTRACT FROM AUTHOR]

[Display omitted] • Oxidation of 3, 3′, 5, 5′-tetramethylbenzidine (TMB) is initiated by chromium (VI) in the presence of CuO. • The method avoids utilization of the unstable and environmental unfriendly H 2 O 2 as oxidizing agent. • Non-expensive and stable material was used for detection of chromium (VI) in contrast to expensive plasmonic nanostructures. Extensive use of chromium makes it one of the major pollutants of water resources. Chromium (VI) in particular is toxic and has detrimental health effects. Because of its high toxicity a tolerable concentration limit of chromium (VI) in drinking water has been recommended. Here we report a colorimetric method for determination of chromium (VI) in water based on the oxidase-like activity of solvothermal synthesized copper oxide. The particles have been characterized by X- ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). Using 3, 3′, 5, 5′- tetramethylbenzidine (TMB) as a chromogenic substrate, CuO exhibited a pronounced oxidase-like activity in the presence of chromium (VI). This method enables successful colorimetric detection of chromium (VI). It demonstrated excellent selectivity for detection of chromium (VI) ions against potentially interfering ions. The method's feasibility to real sample analysis has been proven by testing tap water. Hence, we anticipate that the method can be successfully applied for analysis of chromium (VI) in environmental samples. [ABSTRACT FROM AUTHOR]