Phosphomolybdates for Dual-Mode Photoelectrochemical Sensing toward Trace Chromium(VI) and Tetracycline

Highly sensitive photoelectrochemical (PEC) sensors for trace carcinogens, such as heavy metal chromium(VI) [Cr(VI)] and antibiotic tetracycline (TC) are crucial. Herein, by integration of photoactive and redox phosphomolybdates with conjugated organic components, types of dual-mode PEC sensors were synthesized for sensing trace Cr(VI) and TC pollutants, with formulas of (H₂bimb)₂[Co₂(bimb)_1.5][Co(H₂O)₄][Co(P₄Mo₆O₃₁H₆)₂]·6H₂O (1), (H₂bib)₂[Co(H₂O)₃][Co₂(H₂O)₅][Co(P₄Mo₆O₃₁H₆)₂]·9H₂O (2), and (H₂bib)₆[Co(Hbib)₂(H₂O)₅][Co(P₄Mo₆O₃₁H₇)₂]₂·15H₂O (3), where bimb represents 1,4-bis(1-imidazolyl)benzene and bib is 4,4'-bis(imidazolyl)bibphenyl. Hybrid 1 consisted of a three-dimensional framework structure constructed by Co{P₄Mo₆}₂ clusters and one-dimensional (1D) {Co-bimb} chains, hybrid 2 exhibited 1D Co ion-bridged Co{P₄Mo₆}₂ chains hydrogen-bonding with [H₂bib]²⁺ cations, and hybrid 3 showed a discrete hybrid structure built upon a Co{P₄Mo₆}₂ cluster modified by the {Co-bib} unit. Hybrids 1-3 displayed wide spectral absorption and excellent electrochemical redox properties, enabling dual-mode PEC responses to Cr(VI) reduction and TC oxidation. For Cr(VI) detection, hybrids 1-3 exhibited high sensitivities of 364.40, 225.72, and 124.29 μA·μM^-1 as well as "nM" level detection limits (LODs) of 4.9, 10.0, and 11.0 nM, respectively. For TC detection, the sensitivities of hybrids 1-3 were 494.72, 308.78, and 174.03 μA·μM^-1 and the LODs were 5.2, 6.1, and 12.9 nM, respectively. This research offers significant insights into designing efficient PEC sensors for the detection of environmental pollutants.