Fluorescent And Colorimetric Sensors For Determination Of Nitrite In Food Based On MnO₂ Nanoparticles,Carbon Quantum Dots And Gold Nanocluster

Nanozyme plays an important role in colorimetric sensing.Transition metal oxide nanozyme has been extensively researched in various fields because of its low cost,high catalytic activity,non-toxicity and environmental friendliness.CQDs have various features such as controllable structure,easy surface modification,adjustable energy band and strong electron transfer ability.Gold nanoclusters（AuNCs）possess excellent hydrophilicity,simple synthesis route and decent biocompatibility.The combination of these nanomaterials to construct highly sensitive sensors has become a promising material for biosensors and biological imaging.Smartphone-based colorimetric and fluorescent sensing facilitates visual detection for targets in food safety.In this paper,the characteristics,synthesis methods of nanozyme,CQDs and AuNCs and their applications were summarized.On this basis,the following works were completed:Ⅰ.Manganese dioxide nanoparticles（MnO₂NPs）with oxidase-like activity were synthesized through one-pot method.Simultaneously,blue fluorescent CQDs and red fluorescent AuNCs were synthesized,respectively.X-ray diffraction,X-ray photoelectron spectroscopy,transmission electron microscope,Fourier transform infrared spectroscopy,fluorescence spectroscopy and ultraviolet visible absorption spectroscopy were used to study the structure,morphology and optical properties of the three nanomaterials.Ⅱ.Colorimetric and fluorescent sensing for three-mode determination of nitrite in food was developed based on MnO₂NPs and CQDs.MnO₂NPs can oxidize 3,3’,5,5’-tetramethylbenzidine（TMB）into a blue oxidation product（TMBox）,which is further oxidized into a yellow diimine derivative by nitrite.The ratio of absorbance at 652 nm to 452 nm A₄₅₂/A₆₅₂ was monitored as signal response for UV–vis detection.The concentration of NO₂^-and A₄₅₂/A₆₅₂ exhibited a good linear relationship(A₄₅₂/A₆₅₂=0.0812 C+0.4775,R²=0.9967)in the range of 0.15～12μg/m L.The LOD and LOQ were 0.047μg/m L and 0.14μg/m L,respectively.Yellow diimine derivatives oxidized by nitrite can quench the fluorescence of CQDs via the internal filtration effect（IFE）.A fluorescence“on-off”probe was constructed for the determination of nitrite.The concentration of NO₂^-and F₀-F presented decent linear relationship in the range of 0.05～20μg/m L（F₀-F=35.5 C+88.4,R²=0.9971）.The LOD and LOQ were 0.014μg/m L and 0.046μg/m L,respectively.The color of the solution under LED lamp and fluorescence color under 365nm ultraviolet lamp were captured by smartphone to realize smartphone-based colorimetry for nitrite.The analytical performances of different detection modes were compared and evaluated.The sensor was applied to determine NO₂^-in food samples with recoveries of 83.3%～106%.The results were also verified by standard spectrophotometric method.The current method was accurate,sensitive,selective,low cost and suitable for the field analysis of nitrite in food and water samples.Ⅲ.2,3-diaminonaphthalene（DAN）can react with NO₂^-to produce naphtholtriazole（NAT）with strong blue fluorescence under acidic conditions.Under excitation at 370 nm,the fluorescence emission peak of NAT is at 435 nm.The fluorescence intensity increased with the addition of nitrite.The red fluorescence of AuNCs at 650nm was gradually quenched by the redox of NO₂^-.Therefore,a ratio fluorescence probe was developed to determine nitrite.Fluorescence intensity ratio at two wavelengths F₄₃₅/F₆₅₀ and the concentration of NO₂^-displayed a good linear relationship in the range of 0.01～9.0μg/m L(F₄₃₅/F₆₅₀=0.31 C+0.25,R²=0.9983).The LOD and LOQ were 2.3μg/L and 8.7μg/L,respectively.The ratio fluorescence probe was successfully applied to the determination of nitrite in water and food with recoveries of 80.0%～103%.Under ultraviolet light,the color of the reaction solution changed gradually from red to pink to purple to blue.A fluorescent paper-based sensor based on the color variation was constructed to achieve the visual and quantitative detection of nitrite by naked-eye and smartphone.