Zinc-MOF-carbon hollow sphere hybrid nanozyme for highly selective colorimetric detection of mercury ions

Mercury pollution poses significant environmental and health hazards, necessitating rapid, specific, and sensitive detection methods. Here, a novel zinc-based metal-organic framework (PUC-15) was synthesized, followed by the development of a hybrid nanozyme composite (PUC-15@MCHS) by incorporating the PUC-15 into mesoporous carbon hollow spheres, designed for the effective colorimetric detection of mercury ions (Hg²⁺). The synthesized PUC-15 and PUC-15@MCHS materials were fully characterized using SCXRD, PXRD, FTIR, SEM, TEM, BET, etc. The crystal structure of PUC-15 shows a three-fold interpenetrated network with Zn(ii) displaying a distorted tetrahedral coordination environment built by a nitrogen donor from a bridging dipyridyl ligand and three oxygen atoms from two monodentate L₁ carboxylates (L₁ = 5,5'-(1,3,6,8-tetraoxo-1,3,6,8-tetrahydrobenzo[lmn]-3,8-phenanthro-line-2,7-diyl)diisophthalic acid) and one acetate group, respectively. The PUC-15@MCHS acts as a nanozyme that exhibits vigorous peroxidase-mimicking activity, efficiently catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H₂O₂), resulting in a blue oxidized product detectable at 652 nm. Kinetic studies revealed a low Michaelis-Mentan constant (K _m) for H₂O₂ (0.27 mM), indicating high substrate affinity and catalytic efficiency. Zeta potential analysis showed a negative surface charge (-18.6 mV), which enhances electrostatic attraction toward Hg²⁺ ions. XPS confirmed the specific binding interactions between mercury ions and nitrogen/oxygen-containing active sites on the composite surface. The developed sensor exhibited excellent selectivity toward Hg²⁺ in the presence of competing metal ions, with a LOD of 0.59 µM and a good linear response (R ² = 0.978), highlighting PUC-15@MCHS as an efficient and reliable nanozyme for mercury monitoring in environmental samples.