Room temperature self-assembly and nonenzymatic electrochemical sensing performance of transition metal-embedded wheel-shaped tungstophosphates

Under heat-free conditions, three wheel-shaped [P₈W₄₈O₁₈₄]⁴⁰⁻ ({P₈W₄₈})-based polyoxometalates (POMs) were controllably synthesized via the reaction of a hexavacant Dawson-type tungstophosphate, K₁₂[H₂P₂W₁₂O₄₈]·24H₂O (K-{P₂W₁₂}), with simple transition metal (TM) (TM = Mn, Co, and Ni) salts in room temperature aqueous solutions. The molecular formulas of the POMs were Na₂₄[Mn₈(H₂O)₃₂P₈W₄₈O₁₈₄]·58H₂O (1), K₄Na₁₆H₄[Co₈(H₂O)₃₂P₈W₄₈O₁₈₄]·76H₂O (2), and Na₂₀H₄[Ni₈(H₂O)₃₂P₈W₄₈O₁₈₄]·72H₂O (3). It should be noted that eight TM ions introduced into each POM skeleton were connected in a {P₈W₄₈} wheel. To the best of our knowledge, this is the first time that the self-assembly of 3d-TM-{P₈W₄₈} with the internal coordination of all metals in an aqueous solution was achieved without heating. Compounds 1–3 were characterized by elemental, single-crystal X-ray diffraction, and other physical and spectral analyses, and their nonenzymatic electrochemical sensing performance for hydrogen peroxide (H₂O₂) was investigated by cyclic voltammetry. Compared with K₂₈Li₅[H₇P₈W₄₈O₁₈₄]·92H₂O without 3d-TM and most reported POM electrode materials, the proposed 3d-TM-{P₈W₄₈} POMs exhibited higher stability and better activity during H₂O₂ detection. Composite films formed using 1–3 and chitosan (CS) on a glassy carbon electrode exhibited a high sensitivity (1.21 μA∙mmol∙L⁻¹), low detection limit (0.02 mmol∙L⁻¹), and rapid response time (2 s) for H₂O₂ detection. The study findings show that the combination of CS and 1–3 provides an alternative to soluble TM-{P₈W₄₈} compounds, which are not reusable in H₂O₂ detection systems.