Manganese Dioxide Nanosheets Based Fluorescence Sensors And Their Application In The Detection Of Disease Markers

The level of attention people pay to their own health issues is increasing,and many diseases are not only difficult to cure but also expensive to treat.Early detection and further treatment of many diseases can improve the cure rate,alleviate patient suffering,and reduce treatment costs.However,early diagnosis of diseases is challenging mainly due to the lack of obvious symptoms and the lack of sensitive and highly selective diagnostic techniques in the early stages.Therefore,there is an urgent need to develop detection methods with high sensitivity and selectivity,which are of great significance for improving the cure rate and reducing the mortality rate.Fluorescence analysis has been widely applied due to its simplicity,high sensitivity,and good selectivity.This article aims to address the key scientific issues in the analysis and detection of disease biomarkers,such as the interference of matrix components with target detection and the difficulty of accurately quantifying low levels of target substances.Therefore,there is an urgent need to develop fluorescence biosensors with high fluorescence intensity,sensitivity,and selectivity.Based on a review of relevant literature,the specific research content of this paper is as follows:1.A fluorescence turn on assay for alkaline phosphatase activity based on the oxidase-mimicking activity of MnO₂ nanosheetsAlkaline Phosphatase（ALP）is a class of biological enzymes which are widelydistributed in various organs,such as liver,bone,intestine,placenta,and other tissues.Based on the oxidase-mimicking activity of MnO₂ nanosheets（MnO₂ NS）,a fluorescence assay method was developed for the quantitative detection of alkaline phosphatase activity.MnO₂ NS were employed to catalyze the oxidation of TMB into oxTMB.Gold nanoclusters（BSA-AuNCs）with good stability,low light scattering rate and low toxicity were used as the fluorescent probe.The fluorescence emission spectrum of BSA-AuNCs overlapped with the absorption spectrum of oxTMB at about 600 nm.Because of the internal filtration effect（IFE）between oxTMB and BSA-AuNCs,the fluorescence of BSA-AuNCs was quenched.ALP catalyzed the hydrolysis of 2-phospho-L-ascorbic acid（AAP）to form ascorbic acid（AA）.MnO₂ NS could be reduced to Mn2+by AA,which reduced the catalytic activity of MnO₂ NS and prevented the further oxidation of TMB by MnO₂ NS,thus restoring the fluorescence of BSA-AuNCs.Based on the above principles,ALP could be determined with high sensitivity.The minimal concentration of detection was 0.2 mU/mL,and the fluorescence intensity showed a good linear correlation with ALP activity in the range of 0.2-15 mU/mL.This method was not only simple,sensitive and able to detect ALP activity in human serum,but also expanded the application of MnO₂ NS in biological analysis.2.Portable smartphone-assisted ratiometric fluorescence sensor for visual detection of glucoseFluorescence-based visual assays have sparked tremendous attention in on-site detection due to their obvious color gradient changes and high sensitivity.In this study,a novel emission wavelength shift-based visual sensing platform is constructed to detect glucose based on the oxidation of Rhodamine B（RhB）.MnO₂ nanosheets（MnO₂ NS）with strong oxidizing properties were introduced to oxidize RhB,which resulted in a blue shift in the emission wavelength,and a visual color changed of the fluorescence from orange-red to green.The oxidation reaction could be inhibited via reducing and destroying MnO₂ NS by H2O₂,which was produced by the oxidizing procedure of glucose in the presence of glucose oxidase（GOx）.A series of wavelength shifts and fluorescence color variations appeared with the addition of various amounts of glucose.A ratiometric fluorescence glucose sensor with a lowest recorded concentration of 0.25 μmol/L was developed.Meanwhile,test paper-based assays integrated with the smartphone platform were established for the sensing of glucose by means of the significant fluorescence color changes,offering a reliable,sensitive,and portable on-site assay of glucose.3.Ratiometric fluorescent hydrogel sensor for point-of-care monitoring of uric acid based on the MnO₂ nanosheetsTaking advantages of better mechanical stability,excellent biocompatibility and biodegradability,agarose hydrogels have attracted increasing attention in the area of immobilization and encapsulation.In this work,a visual sensing platform based on the blue shift in the emission wavelength of RhB was constructed by using agarose hydrogel as carrier,and the portable detection of uric acid was realized.MnO₂ NS with strong oxidation ability,which could oxidize RhB into its derivatives.As a result,a series of wavelength shifts and fluorescence color variations appeared.In the presence of uric acid（UA）,MnO₂ NS could be reduced to Mn2+,which inhibited the oxidation of RhB.With the addition of different amounts of UA,a series of fluorescence emission wavelength shifts and fluorescence color changes appeared.Based on the above principles,a ratiometric fluorescence sensor for UA was successfully developed,and the minimal concentration of detection was 1.25 μmol/L.In addition,the hydrogel detection system integrated with the smartphonebased readout was constructed for on-site quantitative detection of UA,with minimal concentration of detection was 20 μmol/L.In conclusion,coupling with smartphone and hydrogel,the platform provides a reliable,sensitive and portable tool to on-site identify of UA.