Nanoprobes for Biomedical Imaging with Tunable Near-Infrared Optical Properties Obtained via Green Synthesis

Ideally, any material used should be nontoxic and produced with safe, inexpensive, and energy-effective processes. In the case of optically active nanoparticles, this is often not the case, as they are frequently composed of hazardous heavy metals and/or produced with methods far from being environmentally friendly. Herein, the preparation of Ag₂S-based nanoparticles via a simple green synthesis route is explored. Aqueous extracts of roasted coffee are used as sources of coordinating molecules. Optimization of the reaction conditions yields dimeric Ag−Ag₂S nanoparticles, whose near-infrared photoluminescence can be switched on via H₂O₂-mediated oxidation. This oxidation transforms suitable photoacoustic contrast agents into fluorescence imaging probes. Theoretical calculations further clarify the role of metallic silver in determining the optical properties of Ag₂S. Overall, it is demonstrated that nanomaterials with tangible applicative potential can be prepared via cost- and energy-effective synthesis strategies that entail benign, renewable chemicals.