Bright Fluorophores in the Second Near-Infrared Window: HgSe/CdSe Quantum Dots

Fluorophores emitting in the NIR-IIb wavelength range
(1.5–1.7
μm) show great potential for bioimaging due to their large tissue
penetration. However, current fluorophores suffer from poor emission
with quantum yields ∼2% in aqueous solvents. In this work,
we report the synthesis of HgSe/CdSe core/shell quantum dots (QDs)
emitting at 1.7 μm through the interband transition. Growth
of a thick shell led to a drastic increase in the photoluminescence
quantum yield, with a value of 63% in nonpolar solvents. The quantum
yields of our QDs and other reported QDs are explained well by a model
of Forster resonance energy transfer to ligands and solvent molecules.
The model predicts a quantum yield >12% when these HgSe/CdSe QDs
are
solubilized in water. Our work demonstrates the importance of a thick
type-I shell to obtain bright emission in the NIR-IIb region.