Novel SERS-Based Detection of Glutathione

Glutathione (GSH) is a vital thiol oxidation inhibitor found abundantly in the intracellular environment, serving as a shield against oxidative stress caused by free radicals and reactive species. Its levels have been closely linked to various diseases, including neurodegenerative conditions like Parkinson’s and Alzheimer’s diseases, as well as cancer. Therefore, the precise detection of GSH holds immense importance for early disease identification and overall human health. Scientists proposed various analytical methods to sensitively detect GSH levels. These methods encompass fluorescence, high-performance liquid chromatography, electrochemical probes, and colorimetry, each with its own set of advantages and limitations. However, it is surface-enhanced Raman scattering (SERS) technology that stands out as a powerful analytical technique for its high sensitivity, exceptional selectivity, rapid detection capabilities, and minimal damage to analytes. Nonetheless, the direct detection of GSH molecules using SERS has remained challenging due to GSH’s low polarization and low Raman cross section characteristics. This limitation has necessitated the development of indirect detection strategies. One such approach involves using other molecules, often referred to as report molecules, which interact with GSH and produce detectable signals in the SERS spectrum. This ingenious approach has opened new avenues for GSH detection. Furthermore, the performance of SERS-based detection relies heavily on the characteristics of the nanostructures used as substrates. Various nanostructures, including core-shelled, porous, flower-like, star-like, sea urchin-like, and dendritic nanostructures, have been explored to enhance SERS sensitivity. These structures create active “hot spots” on their surfaces, amplifying the SERS signals. However, many reported SERS substrates have limitations such as the need for external reductants, complex preparation procedures, high costs, or irregular morphologies, which have hindered their widespread use.

To this account, a new study published in the Biosensors and Bioelectronics Journal, Dr. Yuxin Li, Dr. Ping Li, Dr. Yiqing Chen, Dr. Yue Wu, and led by Professor Jing Wei from Xi’an Jiaotong University developed a novel nanomaterial platform for the indirect detection of GSH using SERS technology. They have addressed the shortcomings of existing methods by developing silver nanoparticles (Ag NPs)/copper-polyphenol colloidal spheres, denoted as CuTA@Ag, with adjustable Ag NPs coverage. The researchers employed an interfacial polyphenol reduction method to create these unique nanostructures. This method involved the deposition of Ag NPs on the metal-polyphenol colloidal spheres, resulting in Ag NPs coverage that could be precisely controlled by manipulating the concentration of the silver precursor. The beauty of this approach lies in its simplicity, as it obviates the need for additional reductants or ligands during the synthesis process. This method not only ensures uniform distribution of Ag NPs but also offers a large specific surface area, generating abundant “hot spots” for SERS enhancement.

The authors characterized CuTA@Ag nanomaterials to understand their properties. They found that it exhibited uniform composited nanospheres with a uniform distribution of Ag NPs, which were confirmed through electron microscopy and elemental mapping. High-resolution transmission electron microscopy and selected-area electron diffraction patterns established the crystalline nature of the silver nanoparticles. X-ray diffraction further confirmed the face-centered cubic silver phase. The researchers also performed X-ray photoelectron spectroscopy analysis, which confirmed the presence of carbon, oxygen, copper, and silver elements in the composites. Nitrogen sorption isotherms revealed mesoporous properties in CuTA@Ag. UV-vis absorption spectra and thermogravimetric curves further validated the size and content of Ag NPs on the surface of CuTA@Ag. To assess the peroxidase-like activity of CuTA@Ag, the researchers conducted a catalytic oxidation reaction of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide. This reaction yielded strong absorption peaks, indicating the successful catalysis by CuTA@Ag. The peroxidase-like activity increased with the growth of Ag NPs on the surface, peaking with CuTA@Ag-iv.

Professor Jing Wei and colleagues evaluated CuTA@Ag as a SERS substrate using rhodamine 6G (R6G) as a probe, and demonstrated excellent enhancement factors, reproducibility, and stability. The substrate’s ability to detect GSH via an indirect strategy was then explored. GSH reduces oxidized TMB (oxTMB), leading to a decrease in the SERS signal. CuTA@Ag-iv was identified as the best-performing substrate due to its optimal enzyme activity and particle density. Additionally, CuTA@Ag-iv demonstrated remarkable reproducibility, batch-to-batch stability, and long-term storage stability. These characteristics are crucial for practical applications in quantitative assays. CuTA@Ag-iv showed minimal variation in Raman intensity, ensuring reliable results in real-world scenarios. One of the most exciting aspects of the study is the practical application of CuTA@Ag-iv for monitoring GSH levels in cancer cells, specifically MCF-7 and MDA-MB-231 cells. According to the authors, the substrate exhibited consistent performance in the presence of cellular buffer, and the measured GSH levels correlated well with those determined using a commercial GSH content determination kit. This demonstrates the potential of the indirect SERS method for the quantification of GSH in real samples, especially within the complex cellular environment. In conclusion, CuTA@Ag-iv, exhibited remarkable peroxidase-like activity, sensitivity, reproducibility, and stability, making it an ideal SERS substrate for GSH detection. The new study opens new avenues for the development of indirect monitoring SERS platforms, not only for GSH but also for various other biological species.