Visual detection of H2O2 and melamine based on PW11MO39n− (M = Cu2+, Co2+, Mn2+, Fe3+) and PW9M3O34n− (M = Cu2+, Co2+, Mn2+, Fe3+)

Abstract

The peroxidase-like activity of transition metal-substituted polyoxometalates (POMs) (PW₁₁MO₃₉ⁿ⁻, PW₉M₃O₃₄ⁿ⁻, abbreviated as PW₁₁M and PW₉M₃, where M = Cu²⁺, Co²⁺, Mn²⁺, Fe³⁺) in the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H₂O₂ was evaluated. Among all POMs, PW₁₁Cu and PW₉Cu₃ were the most active due to their high redox potentials. Based on these findings, H₂O₂ could be detected through observation of TMB/UV-Vis signals in a wide pH range of 2.0–8.0 with linear ranges of 8.96 × 10⁻⁵–2.2 × 10⁻³ mM at a detection limit of 6 nM and 1.21 × 10⁻⁵–1.21 × 10⁻³ mM at a detection limit of 0.98 nM corresponding to PW₁₁Cu and PW₉Cu₃, respectively. Raman and electron paramagnetic resonance measurements revealed that melamine addition could improve the peroxidase-like activity of PW₁₁Co (Cu) because of the interaction between PW₁₁Co (Cu) and melamine to generate several OH· radicals. Thus, a novel method for the colorimetric detection of melamine was developed at a detection limit as low as 0.1 and 1.5 μM based on PW₁₁Co using UV-Vis spectroscopy and naked eye observation in a wide pH range of 2.0–8.0. The TMB/H₂O₂/PW₁₁Co system was essentially used for melamine detection in liquid or powder milk, which showed a promising route to construct POM peroxidase-like mimics in the colorimetric detection of H₂O₂ and melamine with a cost-effective and simple synthesis mechanism instead of enzyme and noble metal utilization. PW₁₁Co/graphene enhanced the peroxidase-like activity of PW₁₁Co in the colorimetric detection of melamine to 0.02 μM detection limit with a linear range of 2 × 10⁻⁴–8 × 10⁻³ mM and it can be recycled due to its heterogeneity.