Ultrasensitive Detection of Carcinogenic Chromium (VI) Species Below the WHO Limit Using a LaCeO₃/Carbon Black Screen Printed Electrode in Batch Injection Analysis

Abstract

The widespread industrial use of chromium and its subsequent release into the environment as toxic and carcinogenic hexavalent chromium (Cr(VI)) species pose significant risks to human health and the environment. The World Health Organization (WHO) has established a limit of 50 ppb (960 nM) for Cr (VI) in water samples. Developing simple, selective, and separation-free methods for the direct detection of Cr (VI) species in the environment remains a challenging task. Herein, we present a highly crystalline lanthanum cerate/carbon black chemically modified screen-printed electrode (SPE/CB@LaCeO3) as an effective electrochemical system for the high-performance and selective electrochemical reduction of toxic Cr(VI) species in pH 2 KCl-HCl solution. The CB@LaCeO3 composite is characterized by its high-density electroactive sites and enhanced electrical conductivity, which facilitate the efficient diffusion-controlled reduction of Cr(VI) species at a low reduction potential of 0.55 V vs Ag/AgCl. The modified electrode demonstrated stability and resistance to surface fouling during continuous voltammetry analysis of high Cr(VI) concentrations. A batch-injection analysis using a three-in-one screen-printed electrode, comprising carbon-working, silver-ink reference, and CB@LaCeO3 modified carbon working electrodes, exhibited excellent concentration linearity within the ranges of 5-30 ppb and 10-35 ppm, with a low detection limit of 682 ppt (signal-to-noise ratio, 3). This method was not interfered by dissolved oxygen or other common chemicals present in environmental and water systems. The linear range and detection limit achieved in this study surpass those reported in several previous works involving precious metal and organic molecule-based chemically modified electrodes. The analytical method was validated with t-test analysis. To demonstrate the applicability of this new system, batch injection analysis was performed on a wide range of real samples, including water (tap, ground, well, reverse osmosis), consumable products (coffee, tea and milk powders), and tannery effluent, using the standard addition method. This approach yielded accurate and sensitive detection of Cr(VI) species in the samples, with recovery values of approximately 100%.