Construction Of Food Fluorescence/Colorimetric Sensor Based On Nano-MnO₂

Biosensors have been widely used in the field of food analysis and detection due to their advantages such as wide application range,fast analysis speed,high specificity,good reproducibility,and the ability to achieve rapid on-site detection.The detection of food poisonous and harmful substances like additives in foods,pesticide and veterinary drug residues,biotoxins and heavy metals is the focus of food analysis and detection.Protease-based biosensors have limited their development in the field of food detection due to enzymes’variability,high price,and poor stability,which has made the enzyme-free biosensors with high sensitivity,wide detection range and strong specificity become a new hot spot.Nano-manganese dioxide has a large specific surface area and quantum size effect.Because of its excellent oxidation ability and catalytic activity,it has been widely used in the construction of biosensors.Based on manganese dioxide nanosheets combining with graphene quantum dots,metals and other materials,several biosensors were construct in this thesis for the detection of tea polyphenols,heavy metal copper and hydrogen peroxide in food,which effectively broadened the application of nano-manganese dioxide materials in the field of food analysis and detection.The main contents and results are summarized as follows:1.Using graphene quantum dots as fluorescent probes,a novel fluorescence detection method for tea polyphenols was constructed based on the effective fluorescence resonance energy transfer（FRET）between manganese dioxide nanosheets（MnO₂NSs）and graphene quantum dots（GQDs）.MnO₂NSs can quench the fluorescence of GQDs,while tea polyphenols can reduce MnO₂NSs to Mn²⁺and restore the fluorescence of the system.The content of tea polyphenols was linearly correlated with the increase of fluorescence intensity in the range of 20-750μg/m L with a good linear relationship,and the minimum detection limit was 12.2 g/m L.The method was applied to the determination of tea polyphenols in tea.The result was close to the national standard method and the recovery rate was 95.5%-103.6%.This method has lower detection limit and wide detection range,with wide applicability and high accuracy.It is expected to be applied to the real-time detection of other antioxidant substances in food.2.Based on the redox reaction between MnO₂NSs and glutathione（GSH）,a fast,economical and sensitive colorimetric analysis method was developed for the qualitative and quantitative detection of copper ions.GSH can oxidize and decompose MnO₂NSs,inhibiting the oxidation of3,3’,5,5’-tetramethylbenzidine（TMB）to blue product（oxTMB）induced by MnO₂NSs,thus the solution is colorless.In the presence of Cu²⁺,the GSH were oxidized to GSSG（oxidized glutathione）and the solution changed from colorless to blue.Under the optimum conditions,the absorption intensity of the blue product（oxTMB）is proporyional to the concentration of Cu²⁺in the range of 10-300 n M with a detection limit of6.9 n M.This sensing system shows high specificity for Cu²⁺in the presence of 10-fold concentration of other ions.Moreover,the system has been successfully applied to the detection of Cu²⁺in real samples and the recovery of spiked Cu²⁺in tap water ranged from 95.2%to 104.5%.Therefore,the strategy based on MnO₂nanosheets is promising for real-time,rapid and highly sensitive detection of Cu²⁺in practical samples.3.AgNPs-BSA-MnO₂NSs with oxidase-like activity were successfully synthesized by redox method,and a new method for H₂O₂colorimetric detection based on AgNPs-BSA-MnO₂NSs was constructed.In this method,AgNPs-BSA-MnO₂NSs are reduced and decomposed by H₂O₂,which results in the aggregation of AgNPs to be light yellow.In the absence of H₂O₂,AgNPs-BSA-MnO₂NSs catalyzes the chromogenic substrate TMB oxidation to blue product.The qualitative and quantitative detection of H₂O₂can be achieved by the color change of the solution.Through a series of optimization of detection conditions,the method has a good detection effect on H₂O₂in a certain concentration.The linear detection range is divided into two segments,0.005-0.2 m M（blue）and0.2-0.35 m M（yellow）,with a minimum detection limit of 2.8μM.At the same time,the experimental results show that this method has high specificity and is suitable for the detection of H₂O₂in actual samples such as pickled chicken feet,pickled bamboo shoot tips,milk,and other samples.It shows that this method can be applied to the rapid and visual detection of H₂O₂in actual samples.