Development of Highly Fluorescent Copper and Silver Chalcogenides for SWIR Imaging via Surface Control

Ag₂S nanoparticles are promising candidates as short-wave infrared (SWIR) imaging agents due to their low toxicity, high photostability, small size, and tunable fluorescent properties in the second near infrared biological window (NIR-II). They have been widely studied for preclinical imaging with high resolution and signal-to-noise ratio. However, Ag₂S nanoparticles suffer from potential heavy-metal toxicity as well as poor fluorescence efficiency due to surface defects. To overcome these limitations, we are optimizing Ag₂S surface treatments while also investigating alternative heavy-metal-free compositions like Cu-based nanocrystals. While copper sulfide nanocrystals are more commonly known for their localized surface plasmon resonance behavior, recent research shows that stoichiometric Cu₂S nanocrystals fluoresce with a peak between 900 to 1100 nm; more effort is needed to optimize their fluorescence properties and stability. A common factor influencing the photophysical behavior of nanoparticles is their surface properties, including the presence of surface defects and the ligands at the interface between the nanoparticle and the surrounding media. Energy transition processes are highly correlated with these interfacial interactions, impacting both radiative and non-radiative recombination and thus photoluminescence. By modifying the ligand coating and shelling, nanoparticles can be protected from defect-derived nonradiative relaxation and energy loss during interaction with the environment.<br/>In this study, we synthesize fluorescent Ag₂S and Cu₂S nanoparticles and manipulate their optical properties through surface treatment. We explore the impact of different shelling and ligands on surface traps, stability, and optical properties. Enhanced fluorescence intensity of Ag₂S demonstrates the removal of surface defects, while surface treatment of Cu₂S reduces surface oxidation, preventing the shift from fluorescent to plasmonic nanocrystals. By acknowledging the impact of surface conditions on nanoparticle behavior, we can pursue the deliberate design of high-quality contrast agents for SWIR imaging.