Dual Colorimetric and Ratiometric Fluorescent Responses for the Determination of Glutathione Based on Fluorescence Quenching and Oxidase-Like Activity of MnO2 Nanosheets

A dual colorimetric and ratiometric fluorescent assay is constructed for the detection of glutathione (GSH) based on MnO2 nanosheets (MnO2 NS). Amplex Red (AR) and curcumin (CUR) fluorescence-pairing probes have opposite responses to MnO2 NS with fluorescence quenching and oxidase-like property. MnO2 nanosheets can largely quench the fluorescence of curcumin. However, the fluorescence of oxidized AR (oxAR) was obviously raised via the catalytic oxidation reaction of no fluorescent AR. The color of the solution gradually changes from yellow to orange. However, MnO2 nanosheets lose the fluorescence quenching capacity and oxidase-like property after the addition of GSH. Therefore, the fluorescence intensity of curcumin shows an obvious increase and the intensity of oxAR a decrease, accompanied by the restoration of the solution color from orange to yellow. For MnO2 nanosheets, the linear relationship between the absorbance ratio of the two substrates and the GSH concentration (50–15 000 nM) is obtained with the detection limit of 16 nM. Besides, the ratiometric fluorescent method has a good linear range from 10 to 5000 nM for GSH. The detection limit is as low as 4.2 nM. Further, this method can not only be used for the determination of GSH in an aqueous solution but also shows a high responsive ability to GSH in human serum. This result indicates its potential in bioanalysis and disease diagnosis.