Rosmarinic Acid-Capped Silver Nanoparticles for Colorimetric Detection of CN(–) and Redox-Modulated Surface Reaction-Aided Detection of Cr(VI) in Water

[Image: see text] Rosmarinic acid-capped silver nanoparticles (Ro-AgNPs) were prepared and applied as a probe for selective colorimetric detection of cyanide (CN(–)) and chromium(VI) [Cr(VI)] under different conditions in aqueous media. The carbon atom of CN(–) interacts with the AgNPs, and the carbon atom donates electrons from the HOMO to the vacant orbitals of the coordinatively unsaturated surface atom (Ag(0)). After donating electrons, CN(–) attached onto the surface of the nanoparticles becomes very reactive and interacts with dissolved oxygen and generates reactive oxygen species (ROS) such as superoxide (O(2)(–)), singlet oxygen ((1)O(2)), and so forth. In this process, Ag(0) oxidizes to Ag(+) and combines with CN(–) forming water-insoluble AgCN, and the ROS (O(2)(–)) formed reacts with Ag/Ag(+) to form Ag(2)O. The oxidation of Ag(0) to Ag(+) resulted in dissolution of AgNPs, which causes disappearance of the surface plasmon resonance band and color change from yellow to colorless. For detection of Cr(VI), ascorbic acid and CN(–) were added first; the ascorbic acid replaced the rosmarinic acid and then reduced the added Cr(VI) to Cr(III), and, in this process, ascorbic acid was oxidized to dehydroascorbic acid, which moved away from the nanoparticles’ surface. CN(–) then interacted with the surface Ag(0) atom, got activated, and interacted with dissolved oxygen forming Ag(+) and ROS, which then followed the same process as described for CN(–) to form AgCN and Ag(2)O with a color change. The limits of detection were found to be 0.01 and 0.03 μM for CN(–) and Cr(VI), respectively. The material was also used for sensing CN(–) and Cr(VI) in real samples, and the results obtained were satisfactory. For field application, agarose-based strips were prepared by immobilizing the nanoparticles onto the agarose film and successfully used for the detection of CN(–) and Cr(VI) in water.