Effect of bridging units on the detection performance of Cd{P ₄Mo ₆} ₂-based electrochemical sensors for trace chromium(VI) and tetracycline

Exploring functionally electrochemical sensors for simultaneously detecting inorganic heavy metal ion and organic antibiotic contaminants in aquatic environments is of great significance. Herein, three reduced phosphomolybdates with distinct bridging units were hydrothermally synthesized for electrochemical sensing applications to detect trace levels of Cr(VI) and tetracycline (TC), with formulas of (H₂bib)₂[Cd(H₂O)]₂[Na(H₂O)][Cd(P₄Mo₆O₃₁H_6.5)₂]·13H₂O (<strong>1</strong>), (H₂bib)₂[Cd(bib)(H₂O)]₂[Cd(P₄Mo₆O₃₁H₇)₂]·17H₂O (<strong>2</strong>), and (H₂bib)₂[Cd(H₂O)][Cd(P₄Mo₆O₃₁H₈)₂]·13H₂O (<strong>3</strong>) (bib = 4,4'-bis(imidazolyl)biphenyl). In hybrids <strong>1-3</strong>, the reduced phosphomolybdate clusters [Cd(P₄Mo₆O₃₁)₂]^22- (<em>abbr</em>. Cd{P₄Mo₆}₂) featured chain-like configurations bridged by diverse cationic linkers ({[Cd(H₂O)]₂[Na(H₂O)]}⁵⁺ unit for <strong>1</strong>, [Cd(bib)(H₂O)]²⁺ fragment for <strong>2</strong>, and [Cd(H₂O)]²⁺ for <strong>3</strong>). Owing to the excellent electrochemical redox properties, hybrids <strong>1-3</strong> exhibit remarkable dual-mode electrochemical responses toward Cr(VI) reduction and TC oxidation. The detection limits for Cr(VI) are 18.41 nM for <strong>1</strong>, 30.86 nM for <strong>2</strong>, and 38.90 nM for <strong>3</strong>, with sensitivities of 122.49, 113.95, and 103.19 μA·mM^-1, respectively. For TC detection, the detection limits (LOD) are 6.19 nM for <strong>1</strong>, 9.90 nM for <strong>2</strong>, and 23.52 nM for <strong>3</strong>, with sensitivities of 120.84, 105.44, and 95.95 μA·mM^-1, respectively. The LODs toward Cr(VI) and TC all reach to the “nM” level. This work offers a potential approach for constructing multi-functional electrochemical sensors for the detection of environmental pollutants.