A colorimetric composite film (Cu-PAN@G-CS/PVA) for the detection of H₂S in shrimp meat by indicator displacement was prepared by tape casting method using copper (Ⅱ)-1-(2-pyridylazo)-2-naphthol complex (Cu-PAN) as a H₂S-sensitive material, chitosan (CS) combined with polyvinyl alcohol (PVA) as a film-forming material and graphene as a sensitizing material. The morphology and functional groups of Cu-PAN@G-CS/PVA were investigated by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), and the sensing performance to H₂S was examined by ultraviolet-visible (UV-vis) absorption spectroscopy. Experimental results showed that the color of the Cu-PAN@G-CS/PVA film changed from purplish red to yellow with increasing H₂S concentration. This film was used to monitor the freshness of Penaeus chinensis stored at 4 or 25 ℃. It was shown that the trend of the color difference value (ΔE) of the Cu-PAN@G-CS/PVA film had a good correlation with the total volatile basic nitrogen (TVB-N) content of the sample. Therefore, the Cu-PAN@G-CS/PVA film could accurately and rapidly indicate the freshness of P. chinensis, which may provide a reference for the development of intelligent packaging of aquatic products.

A rapid and sensitive method for the determination of sulfonamide (SA) residues in fish was developed using electro-enhanced solid-phase microextraction (EE-SPME) based on nitrogen-doped three-dimensional graphene foam (NGF) functionalized monolithic column (monolith@NGF, M@NGF) combined with high performance liquid chromatography (HPLC). To improve the enrichment efficiency of SA residues in fish, porous M@NGF was prepared on the surface of stainless-steel wires by in situ polymerization using ionic liquids as the functional monomer and NGF as the conductivity enhancer. The EE-SPME method was applied for the extraction of sulfathiazole (ST), sulfamethazine (SM₂), and sulfadimethoxypyrimidine (SMM) from fish. Adsorption time, agitation speed, adsorption voltage, ionic strength, the pH of sample solution, desorption time, desorption voltage, and the composition of desorption solution were optimized. The results showed that the M@NGF-based EE-SPME method enriched ST, SM₂ and SMM by 74, 58 and 64 folds, respectively, and shortened the adsorption equilibrium time to 35 min. In the linear range of 5–5000 μg/kg, the limits of detection (LODs) of this method were 1.78, 3.16 and 1.84 μg/kg for ST, SM₂ and SMM, respectively, and the limits of quantification (LOQs) were 5 μg/kg for all analytes. Coefficients of determination (R²) for linear regression analysis of this method were all greater than 0.9990, and the recoveries from spiked samples ranged from 79.2% to 110.1%, with relative standard deviations (RSDs) ranging from 1.4% to 9.8% (n = 5). This method solved the problems of low extraction efficiency and serious matrix interference of SA residues in fish, and allowed the rapid, sensitive detection of SA residues in fish.

Encapsulating natural enzymes in metal–organic frameworks (MOFs) can maintain the original biological functions of enzymes in harsh environments. However, the nature of interfacial interactions between a MOF and enzyme is currently unclear, rendering effective regulation of the biocatalytic activity of the enzyme@MOF composite difficult. Differences in the hydrophilicity of MOF carriers are closely related to the conformational changes and catalytic properties of the enzyme. In this study, the catalytic activity, stability, and conformational changes of alkaline phosphatase (ALP) encapsulated in hydrophilic zeolite imidazolate framework-90 (ZIF-90) and hydrophobic ZIF-8 were systematically investigated using experimental methods and molecular dynamics simulations. The results demonstrated that hydrophilic ZIF-90-encapsulated ALP exhibited superior stability and was 2.22-fold more retained catalytically active than hydrophobic ALP@ZIF-8 after 20 cycles of utilization. Moreover, the hydrophilic interface provided by ZIF-90 effectively regulated the structure of ALP to maintain the optimal catalytic conformation of its active center. The practical application of highly bioactive ALP@ZIF-90 was demonstrated by employing it in a self-calibrated colorimetric/fluorescence dual-mode sensing method for the efficient, reliable, and accurate detection of methyl paraoxon. This study provides new insights for improving enzyme immobilization strategies and promoting the rapid development of enzyme@MOF composites for catalytic and sensing applications.