Green synthesis of gold nanoparticles using marine algae and evaluation of their catalytic activity

The hazardous effects of current nanoparticle synthesis methods have steered researchers to focus on developing newer eco-friendly methods for synthesizing nanoparticles using non-toxic chemicals. Owing to the diverse applications of nanoparticles in various fields such as catalysis, medicine, diagnostics, and sensors, several novel green approaches have been explored for synthesizing nanoparticles using different natural sources such as plants, algae, bacteria, and fungi. Hence, in the present work, a green method for the synthesis of gold nanoparticles (AuNPs) under ambient conditions using aqueous extracts of marine brown algae is reported and the synthesized AuNPs were evaluated for their catalytic efficiency. The aqueous extracts of algae comprise reducing as well as capping agents required for the formation of AuNPs. The Fourier transform infrared spectra of the extracts revealed the presence of compounds having hydroxyl groups that are largely responsible for the reduction of auric chloride to AuNPs at room temperature. Results from high-resolution transmission electron microscopy and dynamic light scattering studies suggested that most of the biosynthesized AuNPs are nearly spherical in shape with an average size in the range of 27–35 nm. High negative values of zeta potential measurement confirmed the stability of AuNPs. Moreover, the reduction kinetics of AuNPs studied by UV–visible spectrophotometry showed that they have good catalytic efficiency in the degradation of dyes as well as reduction of nitro compounds in the presence of sodium borohydride as reducing agent. This simple process for the biosynthesis of gold nanoparticles is rapid, cost-effective and eco-friendly. The formation of AuNPs was observed with the change of pale yellow gold solution to ruby red color of gold nanoparticles and confirmed by surface plasmon spectra using UV–visible spectroscopy. Nanoparticles synthesized through such environmentally benign routes can be used for synthesizing many other metal nanoparticles as well as for a wide range of biomedical applications, for commercial production on a large scale and also can be used as efficient catalysts for different organic reactions. Graphical abstract