Rapid and accurate monitoring of food freshness to provide consumers with high-quality meat continues to be of tremendous importance to the food industry. In this report, an efficient Fe-doped polydopamine (Fe-PDA) nanozyme with peroxidase-mimicking activity was synthesized by a high-temperature hydrothermal method, and was applied to a spectrophotometric sensing system, which successfully reports the concentration of hypoxanthine (Hx) related to meat freshness. The Fe-PDA nanozyme showed excellent peroxidase simulation activity, which was primarily verified by steady-state kinetics experiments. In the presence of xanthine oxidase (XOD), Hx can react quantitatively with dissolved O2 to generate H2O2, which can be further catalyzed and produce hydroxyl radicals (•OH) under acidic conditions via the Fe-PDA nanozyme and oxidize colorless TMB to blue oxTMB with absorbance at 653 nm. The absorbance at 653 nm expressed a clear linear relationship with hypoxanthine concentration in the range of 5.13-200 μM, and the detection limit was 1.54 μM. This method was further assessed by measuring the recovery of Hx added to meat samples, which showed promising accuracy. Overall, the developed Fe-PDA nanozyme with excellent peroxidase-mimicking activity is cost-effective, high-performance and easy to produce, offering an efficient and low-cost sensing system based on spectrophotometry for meat freshness determination as an alternative to conventional methods.

Fe-Doped polydopamine nanoparticles with peroxidase-mimicking activity for the detection of hypoxanthine related to meat freshness.

Rapid detection of histamine in fish based on the fluorescence characteristics of carbon nitride

Hypoxanthine (Hx) has been considered an essential indicator for aquatic products' freshness, especially in the early stage of spoilage. It could generate hydrogen peroxide (H2O2) and uric acid (UA) with the catalyst of xanthine oxidase (XOD). Existed Hx biosensors are only based on H2O2-triggered signal changes or are unsuitable for on-site freshness assessment. Here, we design an on-site Hx sensing method via the detection of both H2O2 and UA by taking advantage of the specific properties of manganese dioxide (MnO2) nanosheets. MnO2 nanosheets could oxidize colorless 3,3′,5,5′-tetramethylbenzidine (TMB) into yellow TMB2+, which can etch gold nanorods (Au NRs) with vivid color changes. However, in the presence of Hx, MnO2 nanosheets were reduced to Mn2+ rapidly by H2O2 and UA, inhibiting the etching step and mediating color changes. The H2O2 and UA meditated strategy exhibits excellent sensitivity toward Hx with a limit of detection (LOD) of 0.378 µM. A WeChat mini program was also constructed to achieve the on-site evaluation and satisfy the special population' requirement. The proposed method has been successfully used to monitor actual sample spoilage under different storage conditions. We expect this method to provide another insight into the on-site freshness assessment of aquatic products. 

Smartphone-assisted multicolor hypoxanthine sensing for on-site freshness assessment of aquatic products

Enzyme mimics now play a significant role in biochemistry. Especially, peroxidase mimics have been widely used for developing colorimetric sensors of blood glucose. The peroxidase mimics previously reported could not be recycled for reusing and may generate scattering to cause unwanted optical interference when it was used for fabricating colorimetric sensors. We herein prepared a broad-applicable and reusable magnetic enzyme-loading nanoplatform with enhanced peroxidase-like activity by simultaneously loading Fe3O4 nanoparticles (Fe3O4NPs) and palladium nanoparticles (PdNPs) on graphitic carbon nitride (g-C3N4) nanosheets (Fe3O4NPs/PdNPs/g-C3N4). The prepared Fe3O4NPs/PdNPs/g-C3N4 possesses stable and enhanced peroxidase-like activity and good enzyme-loading capacity and can be used to load various natural enzymes to form highly-efficient and stable double-active nanozyme for fabricating colorimetric sensors for the visual detection of small molecules. Especially, the magnetic feature facilitates the magnetic separation of Fe3O4NPs/PdNPs/g-C3N4 from sample solution, which is in favor of recycling and eliminating the optical interference caused by nanozyme in colorimetric sensors. The prepared Fe3O4NPs/PdNPs/g-C3N4 has been successfully used to load glucose oxidase (GOx) and cholesterol oxidase (Chox) to form magnetic peroxidase-GOx and peroxidase-Chox double-active nanozymes, which can be used to fabricate colorimetric methods for the detection of glucose and cholesterol, respectively, with a visual detection limit of 15 μM and a spectrometry detection limit of 1.0 μM. With the developed glucose and cholesterol detection methods, we have successfully detected glucose and cholesterol in serum with a recovery of 98-104% and a RSD (n = 5) < 5%. With high peroxidase-like activity, good stability, reusable features, and broad applicability of loading enzyme, the developed magnetic Fe3O4NPs/PdNPs/g-C3N4 provided a promising approach for fabricating cost-effective, sensitive, and simple colorimetric sensors for the visual detection of various small molecules.

Dual-Loading of Fe3O4 and Pd Nanoparticles on g-C3N4 Nanosheets Toward a Magnetic Nanoplatform with Enhanced Peroxidase-like Activity for Loading Various Enzymes for Visual Detection of Small Molecules.

Cu nanoclusters decorated Ti3C2 nanosheets composite with tetraenzyme mimic activities and the application for smartphone-assisted detection of hypoxanthine.

Rapid detection of hypoxanthine (Hx), an important freshness indicator of aquatic products, is still urgent and challenging. In the present study, a colorimetric method for the rapid detection of Hx in aquatic products was established based on the peroxidase-like activity of cobalt-doped graphite phase carbon nitride (Co-doped-g-C3N4). After calcination of the mixture of melamine, cobalt chloride, potassium chloride and sodium chloride, Co-doped-g-C3N4 was obtained with strong peroxidase-like activity and good dispersity. Through the decomposition of Hx by xanthine oxidase, the generated hydrogen peroxide (H2O2) was detected based on the oxidation reaction of H2O2 and 3,3′,5,5′-tetramethylbenzidine catalyzed by Co-doped-g-C3N4. The content of Hx could be directly measured by monitoring the spectral absorbance at 652 nm with a linear range of 2.50–153.1 mg kg−1 and a detection limit of 1.84 mg kg−1 calculated based on 3σ/K (n = 9). The established method was applied for the detection of Hx content in various aquatic products at different storage times after execution, and the data were validated by the HPLC method. The results showed that the Hx measurement by the established method was sensitive, accurate and reliable, and could be used for the rapid determination of the freshness of aquatic products at an early stage.

Xin Wang

Papers

Colorimetric detection of hypoxanthine in aquatic products based on the enzyme mimic of cobalt-doped carbon nitride