Near-infrared-emitting semiconductor quantum dots for tumor imaging and targeting

Visualizing tumors with the help of near-infrared-emitting probes allows not only in vivo imaging, but also online analysis during surgical resection. Near-infrared-emitting semiconductor quantum dots (QDs) are highly fluorescent nanocrystals that can be modified to meet the needs of tumor-selective probes by variations in size, composition and internal structure (core-only, type I and type II core-shell QDs). The passive accumulation of probes in tumors, as a result of leaky vasculature, can be achieved with systemically injected QDs designed to circulate in the bloodstream and avoid clearance via the kidneys or the reticuloendothelial system. With the help of chemical strategies, QDs are decorated with surface ligands, including antibodies or peptides that bind antigens on tumor cells or tumor endothelium, which further improves the specificity of accumulation within tumors. Encapsulation of QDs into macromolecular structures allows the in vivo tracking of gene and drug carriers in real time. Considerable research has been undertaken to avoid acute and chronic toxicities of QDs by reducing the dose or replacing toxic elements with more biocompatible materials. This review discusses the benefits and potential disadvantages of QDs, and highlights recent advances in the application of these probes for tumor imaging and targeting.