Light Mediated Generation of Silver Nanoparticles by Spinach Thylakoids/Chloroplasts

The unique potential of chloroplasts/thylakoids to harness light energy to transport electrons from H2O to various entities was exploited for reduction of Ag+ to generate nanoparticles (NPs). Spinach thylakoids/chloroplasts turned AgNO3 solutions brown in light, but not in dark. Besides showing Ag-NPs specific surface plasmon resonance band, these brown solutions showed presence of 5-30 nm crystalline NPs composed of Ag. Powder X-ray diffraction (PXRD) analysis revealed that Ag-NPs were biphasic composed of face-centered cubic Ag-0 and cubic Ag2O. X-ray photoelectron spectroscopy (XPS) data further corroborated the presence of Ag2O in Ag-NPs. Limited formation of Ag-NPs in dark and increased generation of Ag-0/Ag2O-NPs with increase in light intensity (photon flux density) by thylakoids/chloroplasts, established the role of light-harvesting photosynthetic machinery in generation of Ag-0/Ag2O-NPs. Potential of thylakoids/chloroplasts to generate Ag-NPs from Ag+ on exposure to red and blue wavelength regions of visible light of electromagnetic spectrum, further confirmed the involvement of photosynthetic electron transport in reduction of Ag+ and generation of Ag-NPs. While light energy mediated photosynthetic electron transport donates energized electrons extracted from H2O to Ag+ to form Ag-0-NPs, O-2 released as a by-product during photolysis of H2O oxidizes Ag-0 to form Ag2O-NPs. Our findings furnish a novel, simple, economic and green method that can be exploited for commercial production of Ag-0/Ag2O-NPs.