Principal Investigator Moungi Bawendi
For in vivo biological imaging applications, QD materials are chosen based on size, optical properties, and toxicity. The emission wavelength should ideally be in a region of the spectrum where blood and tissue absorb minimally but detectors are still efficient, approximately 700-900 nm in the near-infrared (NIR). In addition, the hydrodynamic size of the QD should be appropriately matched to the biological experiment of interest.
Quantum dots composed of CdTe core enclosed in a shell of CdSe have been developed to extend the fluorescence wavelength into the NIR range. A type-II energy structure is formed by the two semiconductors, the conduction and valence bands of which are positioned such that the holes and electrons are quantum-confined to the core and the shell, respectively; the nanocrystals thus behave very similarly to indirect semiconductors near the band edge. Charge carriers must cross the core-shell interface for radiative recombination, emitting photons with energies dependent on the band offset and hence can be smaller than that the bandgap of either material. Type II CdTe/CdSe quantum dots therefore exhibit widely tunable fluorescence, and wavelengths between 700 to 1000nm have been reported by our group by varying the core size and shell thickness of the nanostructures.
The size and NIR emission of type II CdTe/CdSe quantum dots have been applied in bioimaging studies after being rendered water soluble by coating the nanocrystals using oligomeric phosphine. Having a final hydrodynamic diameter (HD) of 15.8-18.8 nm, these QDs were injected into live animals and were successfully used for selectively mapping the sentinel lymph node (SLN) in rat and pig models. Emission is observed directly through skin, providing a visual surgical guide for SLN resection.
Recently, we synthesized a size series of (InAs)ZnSe (core)shell QDs that emit in the near-infrared and exhibit HD < 10 nm. We have demonstrated their utility in vivo by imaging multiple, sequential lymph nodes and showing extravasation from the vasculature in rat models, neither of which has been achieved before with QDs. Longer emission wavelengths ranging from 750 to 920 nm can be achieved by increasing (1) the core size or (2) the shell thickness or (3) by altering the band offsets between core and shell such as by adding a small amount of Cd to the ZnSe shell. Due to the perceived undesirability of Cd for in vivo imaging, however, we generally obtained longer wavelengths by the first two options. The biological utility of these fluorescent probes resulted from our intentional choice to match the semiconductor material and water-soluble ligand with a desired final HD and emission wavelength.
Most recently, we have been able to reduce the size of QDs further to <6 nm HD using (CdSe)ZnS (core)shell QDs ligand exchanged with DL-cysteine (QD-cys), and observed some interesting new in-vivo behavior. When injected into rats intravenously, QD-cys mainly accumulated in the bladder 4 hr post-injection, demonstrating for the first time renal clearance of semiconductor nanocrystals in rat models.
By radioactively labeling QD-Cys using Tc99m and quantitatively tracking biodistribution as a function of nanocrystal size, a HD of ~5.5 nm was established as the threshold for renal clearance of QDs. Thus far, few nanometer-sized objects are being actively translated to the clinic. The study suggests that to satisfy both patient safety and regulatory review, nanoparticle biodistribution and clearance must be carefully considered. This study provides a foundation for the design and development of biologically targeted nanoparticles for biomedical applications.