Ag₂S quantum dot theragnostics

Silver sulfide quantum dots (Ag2S QDs) as a theragnostic agent have received much attention because they provide excellent optical and chemical properties to facilitate diagnosis and therapy simultaneously. Ag2S QDs possess brightness and photostability suitable for intense and stable bioimaging. It has been verified via in vitro and in vivo studies that Ag2S QDs do not cause serious toxicity, unlike the first generation of widely used heavy metal-based (cadmium or lead) QDs. In particular, Ag2S QDs emit in the near infrared-II region (NIR-II window) that enables deep tissue penetration and imaging. Furthermore, various chemotherapeutic agents and targeting moieties can be efficiently conjugated to the surface of Ag2S QDs due to advanced technologies in the relevant surface chemistry using covalent bonding, high affinity, and electrostatic interaction. In addition, Ag2S QDs themselves exhibit an anticancer activity based on the photothermal effect. Consequently, Ag2S QDs can function as both a therapeutic agent and an imaging agent in imaging-based diagnosis concurrently, which led to the creation of Ag2S theragnostic nanomaterials. In this review, the synthetic methods, physicochemical properties, bioconjugations, and bioapplications of Ag2S QD theragnostic nanomaterials are discussed in detail.