Glutathione (GSH) is the most abundant and important biological thiol antioxidant in living cells. It is not only able to directly scavenge free radicals but also is an important component of the glutathione peroxidase system to resist oxidative damage caused by free radicals and reactive oxygen species. Abnormal cellular GSH levels are considered an important biomarker for human health and are associated with the progression of various diseases, such as liver injury, aging, cancer, cystic fibrosis, and neurodegenerative diseases. Therefore, there is an urgent need to develop a simple method to detect GSH concentration. At present, the main detection methods for GSH include colorimetry, mass spectrometry, chromatography, magnetic resonance imaging, Raman spectroscopy, and electrochemical methods, which have been employed to determine glutathione in biological systems. However, some of the above methods have drawbacks such as low sensitivity, slow speed, poor selectivity, complex process, and expensive experimental equipment. Therefore, a fast, highly sensitive, and specific GSH sensor should be developed.

We adopt a two-step method to synthesize UFO-shaped oxidase-like Au@MnO₂ nanoparticles (NPs). Firstly, sodium citrate reacts with chloroauric acid to generate AuNPs, and then the KMnO₄ solution reacts with polyamine hydrochloride solution to form MnO₂ wrapped on the surface of the AuNPs. Then, TEM, SEM, and linear scanning are adopted to characterize the prepared Au@MnO₂ nanomaterials. Next, we optimize the reaction parameters of the sensor, such as pH value, OPD concentration, and incubation time of GSH. For GSH sensing, Au@MnO₂ catalyzes the reaction of o-phenylenediamine (OPD) with oxygen to produce 2,3-diaminophenazine (DAP) with fluorescence. The etching of GSH to MnO₂ results in weakened catalytic ability of the Au@MnO₂ nanoparticles after the addition of GSH, and therefore the fluorescence intensity of DAP exhibits an obvious decrease and realizes the fluorescence sensitive detection of GSH.

In the optimal experimental conditions, the fluorescence peak intensity changes of the system solution at 560 nm are investigated when different concentrations of GSH are added (Fig. 6). As shown in Figs. 6(a) and 6(b), as the concentration of GSH increases, the fluorescence peak at 560 nm gradually decreases. The solution color changes from orange to yellow and then to colorless under ultraviolet light. The fluorescence intensity at 560 nm is fitted and analyzed, as shown in Fig. 6 (c), and a good linear relationship between the fluorescence intensity change at 560 nm and the logarithm of GSH concentration (0.01-10 μmol/L and 50-1000 μmol/L) is acquired with the low detection limit of 0.003 μmol/L. Therefore, quantitative detection of GSH concentration can be achieved based on fluorescence changes. To investigate the influence of the nanoprobe on GSH detection in the presence of other interfering substances (metal ions and amino acids), we conduct interference experiments. The results are shown in Fig. 7. When 5 mmol/L amino acids (tyrosine, lysine, and glutamic acid) and ions (Na⁺, K⁺, and Mg²⁺) are added to the system, compared to the fluorescence intensity of Au@MnO₂-OPD, the added amino acids have little effect on the fluorescence intensity of the system. However, when 0.5 mmol/L GSH is further added to the solution, the fluorescence intensity significantly decreases. This indicates that the system has good selectivity for GSH. To study the detection performance of this method in actual biological systems, we employ this sensor to detect GSH in serum. As shown in Fig. 8, the splendid linear relationship between the fluorescence intensity change at 560 nm and the logarithm of GSH concentration (0.01-10 μmol/L and 50-1000 μmol/L) is obtained. Additionally, for the test results of the same serum sample, our method is compared with the medical ultraviolet enzyme method as a reference standard. As shown in Table 1, this method has high consistency with ultraviolet enzyme method in determining GSH content in serum. This method can be utilized for GSH detection in serum and has great application prospects in clinical diagnosis.

We propose a simple and rapid method for preparing UFO-shaped oxidase-like Au@MnO₂ nanoparticles. Based on the oxidase catalytic performance of this material, a fast, simple, and highly sensitive method for fluorescence detection of GSH is designed, which can detect GSH at concentrations of 0.01-10 μmol/L and 50-1000 μmol/L with a detection limit of 0.003 μmol/L. This analytical method has high selectivity, high sensitivity, simple operation, and short detection time, with broad application prospects in GSH detection.