Biogenic synthesis of silver nanoparticles mediated by Theobroma cacao extract: enhanced antibacterial and photocatalytic activities

We report the efficient biogenic synthesis of silver nanoparticles (Ag NPs) using silver nitrate and extracts of different parts of Theobroma cacao: the husk (h-Ag NPs), pulp (p-Ag NPs), and seed (s-Ag NPs). In addition, we have tested the antibacterial and photocatalytic activities of the Ag-NPs. The Ag NPs obtained from husk, pulp, and seed extracts show variation in the particle size, dispersion, and morphology. UV-visible absorbance measurements reveal surface plasmon resonance bands at 425, 438, and 462 nm for the s-Ag, h-Ag, and p-Ag NPs, respectively. Transmission electron microscopy studies revealed the formation of monodisperse spherical Ag NPs with diameter ranging from 6 to 18 nm. Fourier transform infrared measurements of the as-synthesized Ag NPs indicate differences in the phytochemicals decorating the NP surfaces, which led to differences in the zeta potential, hydrodynamic radius, and polydispersity index. The p-Ag, h-Ag, and s-Ag NPs exhibited photocatalytic activity on exposure to sunlight from sun, achieving 35%, 29%, and 24% degradation of methylene blue (MB) within 60 min, respectively. Further, the p-Ag NPs showed 98.3% MB photodegradation after 180 min. The photocatalytic rate constants for the degradation of MB were also calculated. Finally, we found that the biogenic nanoparticles affect bacterial growth, possibly by causing protein leakage and cell death. The p-Ag NPs showed better antibacterial activity against Bacillus subtilis and Escherichia coli than h-Ag and s-Ag. The photocatalytic and antibacterial activities of the Ag NPs synthesized with T. cacao mainly depend on the particle size and the biomolecules on the surface of the NPs.