Cd/Zn-regulated POM-based complexes for the electrochemical detection and photocatalytic reduction of Cr(VI)

To address the challenge of Cr(VI) pollution in wastewater, based on the unique "electronic sponge" properties of polyoxometalates (POMs), we designed and synthesized two redox-active POM-based complexes: [Cd^Ⅱ₂(dm4bt)₄][GeW₁₂O₄₀] (1) and [Zn^Ⅱ₂(dm4bt)₄][GeW₁₂O₄₀] (2) (dm4bt = 2,2’-dimethyl-4,4’-bithiazole). Complex 1 enables highly sensitive detection of trace Cr(VI) with a detection Limit of 0.21 μM and simultaneously serves as a photocatalyst to reduce Cr(VI) to harmless Cr(III) with 99.85% removal within 15 min. Under the same conditions, complex 2 exhibits a detection Limit of 0.30 μM and the reduction rate of Cr(VI) decreased to 93.90%. Transition metals modulate material performance by tuning both the crystal structure and band structure of the inorganic–organic hybrid materials. The high efficiency of 1 originates from the chemical bond network formed between Cd²⁺ and the terminal oxygens of [GeW₁₂O₄₀]⁴⁻, which constructs a rapid electron pathway and optimizes the energy band structure. Complex 2 exhibits attenuated performance due to restricted electron transport (dominated by hydrogen bond stacking), which is due to the core regulatory effect of the metal coordination modes in governing charge migration. This work overcomes the limitation of electron transport through a structure-performance regulation strategy, providing new ideas for the development of integrated environmental remediation materials.

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