Ultrasensitive detection of chromium (VI) by a “signal-off” cathodic photoelectrochemical sensor based on AgBr0.8I0.2-Ag-CNTs composite

Hexavalent chromium (Cr(VI)) is highly poisonous and can accumulate in the environment. In this work, we developed a photoelectrochemical (PEC) sensor to detect Cr(VI) content. Firstly, a bean-like AgBr0.8I0.2-Ag-CNTs composite was synthesized through light reduction and hydrothermal method. The physicochemical characterization was investigated and the results revealed that AgBr0.8I0.2-Ag-CNTs possessed a wide visible light absorption range and excellent photoelectrochemical activity. Subsequently, a "signal-off" PEC sensor based on AgBr0.8I0.2-Ag-CNTs as cathodic sensing platform and K3[Fe(CN)6] as photoelectron acceptor was proposed. After photoelectrochemical reduction of Cr(VI) ions, Cr(VI) was reduced to cover AgBr0.8I0.2-Ag-CNTs in the form of Cr(OH)3, which severely blocked the photoelectron transfer from AgBr0.8I0.2-Ag-CNTs to K3[Fe(CN)6] and resulted in a falling photocurrent. Compared with previous PEC sensors based on the direct reduction of Cr(VI) to Cr3+, the proposed sensor showed a wider linear range of 0.1 nM ∼ 5.0 µM and a lower limit of detection (LOD) of 50 pM.