Semiconductor Quantum Dots for Sentinel Lymph Node Mapping through In Vivo Fluorescent Imaging

Abstract

Semiconductor quantum dots (QDs) are nanocrystals that are just several nanometers in size, and therefore exhibit marked quantum size effects. Compared with organic dyes and fluorescent proteins, QDs possess interesting optical properties, including broad absorption, sharp size-tunable emission, high quantum yield and strong resistance to photobleaching. Since QDs were used as fluorescent nanoprobes for cell labeling and imaging in 1998, these fluorescent nanomaterials have been widely studied for in vitro and in vivo imaging applications. In vivo imaging using near-infrared (NIR) QDs shows potential for use in cancer diagnosis. Nanoparticles, including QDs, usually quickly accumulate in sentinel lymph nodes (SLNs) by passive targeting after being injected intramuscularly into a body. Therefore, using QD fluorescence for SLN mapping may be a convenient technique compared with other in vivo imaging applications such as tumor target imaging. Importantly, axillary SLNs containing NIR QDs can be imaged directly through deep tissue without surgical management, because the NIR fluorescence of such QDs can penetrate through tissue. In addition, the NIR fluorescence from the QDs in the SLNs can be observed for several hours or even longer. These advantages are benefit for SLN mapping. In this chapter, the optical properties of QDs and their use in animal SLN mapping are described. Future perspectives including the feasibility of clinical translation are also discussed. Semiconductor Quantum Dots and Rods for In Vivo Imaging and Cancer Phototherapy IntroductionSemiconductor QDs, also called semiconductor metalloid-crystal nanostructures, are small nanoparticles that are usually spherical in shape, possess bulk-like crystalline structures and contain around 200-10,000 atoms. The size of QDs ranges from 1 to 10 nm, which is close to or smaller than the dimensions of the exciton Bohr radius. The most commonly used QDs in biomedical research are cadmium (Cd)-based, especially cadmium selenide (CdSe) and cadmium telluride (CdTe). The band gap, which is the energy difference between the top of the valence band and the bottom of the conduction band, of Cd-based QDs is located between 1.7 and 3.5 eV [1]. As a result, they emit bright visible (blue, green, yellow, or red) and near-infrared (NIR) fluorescence from a low energy band. For example, the fluorescent emission spectrum of CdSe QDs can extend to ~700 nm, and that of CdTe QDs can extend to ~800 nm [2,3]. CdSe and CdTe QDs are easy to synthesize in aqueous solution, and the products obtained are water-soluble. They typically display fluorescence with a narrow full width at half-maximum (FWHM), short lifetime and high quantum yield. These features have facilitated the use of CdSe and CdTe QDs as well as those coated with an inorganic shell of zinc sulfide (ZnS) or silica in both in vitro and in vivo fluorescent imaging for biomedical detection. However, Cd is a probable carcinogen, and has a long biological halflife in the body [4]. Therefore, Cd-free QDs have also attracte...