Exploring functional electrochemical sensors for the simultaneous detection of inorganic heavy metal ions and organic antibiotic contaminants in aquatic environments is highly significant. In this study, three reduced phosphomolybdates with distinct bridging units were hydrothermally synthesized for electrochemical sensing of trace levels of Cr(VI) and tetracycline (TC). The formulas are (H₂bib)₂[Cd(H₂O)]₂[Na(H₂O)_0.5][Cd(P₄Mo₆O₃₁H_6.5)₂]·13H₂O (1), (H₂bib)₂[Cd(bib)(H₂O)]₂[Cd(P₄Mo₆O₃₁H₇)₂]·17H₂O (2), and (H₂bib)₂Cd[Cd(P₄Mo₆O₃₁H₈)₂]·13H₂O (3) (bib = 4,4'-bis(imidazolyl)biphenyl). In hybrids 1–3, the reduced phosphomolybdate clusters [Cd(P₄Mo₆O₃₁)₂]²²⁻ (Cd{P₄Mo₆}₂) feature chain-like configurations bridged by diverse cationic linkers: {[Cd(H₂O)]₂[Na(H₂O)_0.5]}⁵⁺ unit for 1, [Cd(bib)(H₂O)]²⁺ fragment for 2, and Cd²⁺ for 3. Hybrids 1–3 exhibit remarkable dual-mode electrochemical responses for Cr(VI) reduction and TC oxidation because of their excellent electrochemical redox properties. The detection limits for Cr(VI) were 18.41 nM for 1, 30.86 nM for 2, and 38.90 nM for 3, with sensitivities of 122.49, 113.95, and 103.19 μA·mM⁻¹, respectively. For TC detection, the detection limits (LODs) were 6.19 nM for 1, 9.90 nM for 2, and 23.52 nM for 3, with sensitivities of 120.84, 105.44, and 95.95 μA·mM⁻¹, respectively. The LODs for Cr(VI) and TC reach the “nanomolar” level. This study offers a potential method for constructing multifunctional electrochemical sensors for detecting environmental pollutants.

The development of a sensitive and efficient detection technology for trace toxic hexavalent chromium (Cr(VI)) in water is a pressing concern. In this study, an hourglass-type phosphomolybdate-based metal–organic network with the formula [Na_0.5Cu_5.5(H₂O)₂(btmbp)₄][Mn(H₂O)₃]₂{Mn[H₆P₄Mo₆O₃₁]₂}·10H₂O (1, btmbp = 4,4'-bis((1H-1,2,4-triazol-1-yl)methyl)biphenyl) was hydrothermally synthesized. The crystal network consists of a ladder-like two-dimensional layered structure constructed by vertical connections of one-dimensional (1D) [Na_0.5Cu_5.5(H₂O)₂(btmbp)₄]⁶⁺ metal–organic chains and 1D inorganic polyanionic chains. Compound 1 exhibits excellent electrochemical property and a wide light absorption range including visible light to accelerate the electron transfer in redox processes. When serving as a photoassisted electrochemical (PAEC) sensor for trace Cr(VI) detection, compound 1 exhibits a high sensitivity of 330.5 μA·μM⁻¹ and a low detection limit of 0.95 nM (98.79 ppt) along with high anti-interference ability and excellent PAEC detection stability, outperforming most reported polyoxometalate-based sensors and equaling noble-metal sensors, far satisfying World Health Organization standards for Cr(VI) concentration in drinking water. This work provides a new photoelectrochemical sensor material for monitoring environmental pollutants.

The exploration of high-efficiency photocatalysts to drive the conversion of highly toxic heavy metal hexavalent chromium (Cr(VI)) in wastewater to low-toxic trivalent chromium (Cr(III)) is of great significance for purifying water that contains emerging contaminants. Herein, four hourglass-type phosphomolybdate-based hybrid networks—(H₂bpe)₂[M(H₂O)₃]₂{M[P₄Mo₆O₃₁H₇]₂}·8H₂O (M = Mn for 1, Co for 2) and (Hbpe)(H₂bpe)Na[M(H₂O)₃]₂{M[P₄Mo₆O₃₁H₇]₂}·9H₂O (M = Mn for 3, Co for 4; {M[P₄Mo₆O₃₁H₇]₂}⁸⁻ (abbr. M{P₄Mo₆}₂); bpe = 1,2-di(4-pyridyl)ethylene)—were hydrothermally synthesized as heterogeneous photocatalysts for Cr(VI) reduction. A structural analysis showed that the four hybrids 1–4 exhibited two-dimensional inorganic sheet-like structures with a 3,6-connected kgd topology built of hourglass phosphomolybdate clusters having different central metal ions, which further interacted with organic bpe cations via abundant hydrogen-bonding interactions to extend the structure to a three-dimensional (3D) supramolecular network. The four hybrids displayed excellent redox properties and wide visible-light absorption. When used as heterogeneous photocatalysts, hybrids 1–4 exhibited excellent photocatalytic activity for Cr(VI) reduction under 10 W white light irradiation, with reduction rates of 91% for 1, 74% for 2, 90% for 3, and 71% for 4, respectively, within 80 min. The Cr(VI) reduction reaction over hybrids 1–4 followed the pseudo first-order kinetics model with reaction rate constants k of 0.0237 min⁻¹ for 1, 0.0143 min⁻¹ for 2, 0.0221 min⁻¹ for 3 and 0.0134 min⁻¹ for 4, respectively. The Mn{P₄Mo₆}₂-based hybrids 1 and 3 showed better photocatalytic performance than the Co{P₄Mo₆}₂-based hybrids 2 and 4, along with excellent recycle stability. This mechanism study shows that the different central metals M in the M{P₄Mo₆}₂ cluster have a considerable impact on photocatalytic performance due to their regulation effect on the electronic structure. This work provides evidence for the important role of the central metal in hourglass-type phosphomolybdate in the regulation of photocatalytic performance, and it brings inspiration for the design of highly efficient photocatalysts based on polyoxometalates.