Near-infrared CIS/ZnS quantum dots for fluorescence imaging in deep tissues

Abstract

Near-infrared (NIR) fluorescence is of great interest for in vivo imaging and therapeutic applications because of low levels of light absorption, scattering, and auto-fluorescence. Compared to organic dyes, quantum dots (QDs) are very attractive for the above applications due to their relatively large absorption coefficient, high quantum yield, less photo bleaching, and large Stokes shift. However, two technical issues need to be resolved for their practical in vivo applications: efficient NIR emission and minimized toxicity. Here we report a sim-ple and reliable method to synthesize highly luminescent, NIR-emitting CuxInyS2/ZnS (CIS/ZnS) core-shell QDs by controlling an off-stoichiometric composition. Thermolytic synthesis is adopted with 1-dodecanethiol as a sulfur source, a stabilizing ligand, and a reaction medium. The most important finding in our work is that increasing the Cu/In ratio enables the tuning of the emission spectrum of CIS/ZnS QDs to a range of NIR with a high quantum yield up to ~ 60 %. The maximum emission wavelengths of the synthesized QDs are 589 nm (QD589) and 726 nm (QD726) at a Cu/In ratio of 0.25 and 1.8, respectively. The imaging sensitivity of the QDs in a deep tissue condition is investigated by intramuscular injection of polymer microspheres incorporating the QDs into a mouse model. Our results show that > 30 % of the original emission of the QD726 can be detected through the biological tissue of 0.9 cm in thickness, while the emission intensity from the QD589 is negligible. Our study shows that investigation on the off-stoichiometric effects of CIS QDs contribute to the development of highly luminescent, NIR-emitting, non-cadmium QDs for practical applications to tissue-level imaging, sensing, and therapeutics.