A new class of far-red and near-infrared biological labels based on alloyed semiconductor quantum dots

Ternary cadmium selenium telluride quantum dots (CdSe _1-xTe _x, x = mole fraction of tellurium) have been prepared for potential use as constant-size biolabels with tunable fluorescence emission in the far-red and near-infrared (650-850 nm) spectral range. In contrast to particle size tuning reported for binary dots, we show that molar composition can be used to tune the optical and electronic properties of alloyed semiconductor nanocrystals without changing the particle size. A surprising finding is a nonlinear relationship between the composition and the absorption/emission energies, leading to new properties not obtainable from the parent CdSe and CdTe binary systems. Coating the alloy cores with a higher band-gap material such as CdS improves the fluorescence efficiencies to about 40-60% at room temperature and allows the preparation of water-soluble and biocompatible alloyed dots at similar quantum yields. A cadmium-rich surface is found to improve mercapto ligand binding and the long-term stability of water-soluble dots.