Selective colorimetric sensing of toxic metal cations by green synthesized silver nanoparticles over a wide pH range

Selective colorimetric sensing of potentially health and environmentally hazardous heavy metal ions in aqueous solution was demonstrated using green synthesized silver nanoparticles (AgNPs) from plant extracts, by taking advantage of metal coordinating organic functional groups present in the extracts. AgNPs synthesized using fresh neem leaf extracts (NF-AgNPs) selectively detected Hg2+ whereas sun-dried neem leaf extract based AgNPs (ND-AgNPs) were observed to selectively sense Hg2+ and Pb2+ at micromolar concentrations. Neem bark extract based AgNPs exhibited selective colorimetric sensing of Hg2+ and Zn2+. Similarly, AgNPs synthesized from mango leaf (fresh, MF-AgNPs; sun-dried, MD-AgNPs) and green tea extracts (GT-AgNPs) showed selective colorimetric sensing of Hg2+ and Pb2+ ions. AgNPs obtained using pepper seed extracts exhibited Hg2+, Pb2+ and Zn2+ selective colorimetric sensor properties. Importantly these green synthesized AgNPs selectively detected the presence of hazardous metal ions in aqueous solutions across a wide range of pH values (2.0-11), which is a highly desirable attribute from the perspective of different sources of water pollution. The selective colorimetric sensing of Hg2+, Pb2+ and Zn2+ ions by green synthesized AgNPs over a wide pH range demonstrates the multi-functional utility of plant extracts in green nanotechnology and environmental sensor applications.