Synthesis and characterization of transition metal-based polyoxometalates for dual sensing of biomarkers such as nitrite ions and hydrogen sulphide

Abstract

Accelerated industrial development and expansion have led to increasing concentrations of pollutants, such as nitrite ions (NO₂⁻) and hydrogen sulphide (H₂S) in water. Theses biomarkers can be monitored using polyoxometalates, which are known for their versatile redox properties. In this study, we have synthesized three POMs, (NH₄)₈[Cu₂Mo₁₀O₂₂(O₂)₁₂] (1), [Cu(arg)₂]₃[Mo₆SO₂₄] (2) and [Cu(glu)]₂[CuMoO₄(OH)₄] (3), via a one-pot synthesis for the detection of NO₂⁻ ions. Studies such as FT-IR analysis, UV-visible spectra, NMR spectroscopy, EPR spectroscopy, Powder X-ray diffraction and SEM-EDX analysis were used to understand the structure of the POMs. The influence of pH on the electrochemical behavior of Cu-POMs 1, 2 and 3 was analyzed through cyclic voltammetry (CV) and bulk electrolysis at a potential of −0.8 to 1.4 V. The influence of the scan rate was analyzed using a [Fe(CN)₆]^4−/3− redox couple. The electrocatalytic detection of nitrite ions was determined using a potential of −1 to 1.5 V and 0.5 V–1.5 V to record CV and differential pulse voltammetry (DPV). Results show that Cu-POM 1 exhibited higher detection capability over a concentration range of 5 μM–0.1 M. Cu-POM 2 showed detection for the concentration range of 5–400 μM. The antioxidant activity of the POMs followed the trend 2 > 1 > 3. The colorimetric detection of H₂S could be performed over a concentration range of 1–100 mM.