Size-Dependent Synthesis of Gold Nanoparticles and Their Peroxidase-Like Activity for the Colorimetric Detection of Glutathione from Human Blood Serum

An eco-friendly and economic efficient approach has been developed for the photoinduced synthesis of more stable AuNPs using an aqueous extract of Croton bonplandianum (AEC) as a reducing and capping agent. The reaction mixture of AEC and HAuCl 4 .xH 2 O, when exposed to sunlight turned purple which primarily confirmed the biosynthesis of AuNPs. The biosynthesis was monitored using UV–vis spectroscopy which exhibited a sharp SPR band at 530 nm after 16 min of sunlight exposure. The parameters affecting the synthesis of AuNPs such as sunlight exposure, AEC inoculum dose, and HAuCl 4 .xH 2 O concentration were also optimized. The HR-TEM study revealed that, as the metal ion concentrations increased, the average size and anisotropic nature of the AuNPs increased. The X-ray diffraction pattern of AuNPs synthesized confirmed the formation of face-centered cubic crystal lattice of metallic gold. The involvement of polyphenolics in the synthesis of AuNPs was confirmed by comparing the FTIR analysis of pure tannic acid, AEC, and pre- and postannealed AuNPs. The XPS analysis corroborated the presence of two individual peaks attributed to the Au 4f 7/2 and Au 4f 5/2 binding energies which corresponded to the presence of metallic gold. The AuNPs thus obtained showed peroxidase-like mimicking activity which catalyzed the oxidation of TMB to oxTMB with the development of blue color and absorption spectra at 652 nm. However, the presence of GSH caused further reduction of oxTMB. This detection experiment showed an excellent linear relationship between 1 and 40 μM with a limit of detection of 0.013 μM. In addition to this, the significant recovery of GSH from human blood serum advocated that the developed system was simple and sensitive for the real sample analysis.