Optimizing the Synthesis of Red- to Near-IR-Emitting CdS-Capped CdTe_xSe_1-x Alloyed Quantum Dots for Biomedical Imaging

Advancements in biomedical imaging require the development of optical contrast agents at an emission region of low biological tissue absorbance, fluorescence, and scattering. This region occurs in the red to near-IR (>600 nm) wavelength window. Quantum dots (Qdots) are excellent candidates for such applications. However, there are major challenges with developing high optical quality far-red- to near-IR-emitting Qdots (i.e., poor reproducibility, low quantum yield, and lack of photostability). Our aim is to systematically study how to prepare alloyed CdTe_xSe_1-x with these properties. We discovered that the precursor concentrations of Te-to-Se and growth time had major impacts on the Qdot's optical properties. We also learned that the capping of these alloyed Qdots were difficult with ZnS but feasible with CdS because of the ZnS's lattice mismatch with the CdTe_xSe_1-x. These systematic and basic studies led to the optimization of synthetic parameters for preparing Qdots with high quantum yield (>30%), narrow fluorescence full width at half-maxima (<50%), and stability against photobleaching (>10 min under 100W Hg lamp excitation with a 1.4 numerical aperture 60× objective) for biomedical imaging and detection. We further demonstrate the conjugation of biorecognition molecules onto the surface of these alloyed Qdots and characterize their use as contrast agents in multicolored and ultrasensitive imaging.