Green synthesis and characterization of bioinspired silver, gold and platinum nanoparticles and evaluation of their synergistic antibacterial activity after combining with different classes of antibiotics.

A facile one step green synthesis of silver (AgNP), gold (AuNP) and platinum (PtNP) nanoparticles has been attempted using the rind extract of the fruit of Garcinia mangostana L. The rind of the fruit is enriched with wide varieties of natural resources which could be conveniently exploited as alternative for synthesis of nanoparticles eliminating the need for hazardous chemical methods. The formation of gold nanoparticles occurred at room temperature within a few seconds while the formation of silver and platinum nanoparticles was obtained after heating the solution for 20 min at 80 °C. Parameters such as contact time, temperature and pH were optimized to obtain the precise nanoparticles. The green synthesized nanoparticles were characterized using several state-of-the-art techniques like UV-Vis spectroscopic, FT-IR, HR-SEM and HR-TEM, XRD analyses and zeta potential measurements. Attempt has also been made to evaluate the antibacterial activity of the metal nanoparticles before and after combining with commercially used antibiotics as well as free antibiotics against human pathogenic bacteria. The bare silver nanoparticles (AgNP) showed relatively higher antibacterial activity than AuNP and PtNP and this activity was found to be more pronounced against gram negative bacteria than gram positive ones. Interestingly, all the three metal nanoparticles combined with antibiotics showed enhanced antibacterial activity against the pathogenic bacteria which suggested synergism between the nanoparticles and antibiotics. The high point of the present investigation has been that the Bacillus sp. which is highly resistant to streptomycin becomes highly susceptible to the same antibiotic when combined with gold nanoparticles. This particular observation opens up windows for the treatment of antibiotic resistant bacteria after combining with different nanoparticles under clinical set up.