Properties of Quantum Dots

Quantum dots (QDs) are nanocrystalline semiconductors composed of an inorganic core (e.g., cadmium, mercury, cadmium selenite, etc.) with a metal shell (e.g., ZnS) that shields the core and renders QDs bioavailable. QDs have diameters ranging from 2 to 10 nm (for core-shell QDs) and 5-20 nm after surface modification (core-shell-conjugate QDs) (Fig. 5a, b). QDs have unique optical and electrical properties (such as their fluorescence emission) and can be tuned from visible to infrared wavelengths depending on their size and composition; especially, they have large absorption coefficients across a wide spectral range and are very bright and photostable enabling their detection for a wide range of applications in vivo (Niemeyer 2001; Alivisatos 1996).

The light-emitting properties of QDs are attributed to quantum effects due to (1) their nanoscale structures and (2) the so-called quantum confinement phenomenon. Quantum confinement is a quantum effect in which the energy levels of a small nanocrystal (smaller than the Bohr exciton radius, about a few nanometers) are quantized, with values directly related to the nanocrystal size (Alivisatos 1996). When exposed to light sources, the cores of QDs absorb incident photons generating electron-hole pairs (characterized by a long lifetime, greater than 10 nanoseconds (Efros and Rosen 2000)). The pair then recombines and emits a less-energetic

Fig. 5 Schematic quantum dot (QD) structures (a) and a TEM image of InAs QDs in a GaAs matrix (b) (Adapted with permission from (Grundemann et al. 1995))

photon in a narrow, symmetric energy band (full width at half maximum is typically from 30 to 50 nm) (Michalet et al. 2005). The range of emission wavelength is 400-1,350 nm for QDs with sizes varying from 2 to 9.5 nm not including functional layers (Michalet et al. 2005). In comparison with organic fluorophores, these quantum-confined particles have exceptionally superior properties, therefore offering exciting new opportunities for in vivo imaging, especially in cancer diagnostics and management (Michalet et al. 2005; Larson et al. 2003).

First, fluorescence wavelengths emitted by QDs have higher levels of brightness than those of traditional fluorophores. Single QDs appear 10-20 times brighter than organic dyes (Gao et al. 2004a). QDs have large extinction coefficients that are an order of magnitude larger than those of most dyes (meaning that they can absorb more incoming light per unit concentration of dye) (Dubertret et al. 2002; Ballou et al. 2004) and high quantum yields (close to 90% (Reiss et al. 2002)) (meaning a high amount of light emitted over that absorbed) (Bailey and Nie 2003).

Second, QDs have large absorption coefficients across a wide spectral range (Niemeyer 2001; Alivisatos 1996). This property allows one to simultaneously excite multiple QDs of different emissions with a single excitation wavelength. Their emission spectra have very distinct and narrow wavelengths, which allow independent labeling and identification of numerous biological targets (Han et al. 2001; Gao and Nie 2003, 2004b). This is very useful in studying tumor pathophysi-ology, which requires one to be able to distinguish and monitor each component of the tumor microenvironment under dynamic conditions. For example, researchers have used two-photon microscopy to image blood vessels within the microenviro-ment of a tumor using PEG-coated QDs. As seen in Fig. 6, good contrast between cells, matrix, and the leaky vascular was evident.

This suggests the use of QDs' fluorescence contrast imaging for noninvasive diagnostics of human tumors.

Another advantage of QDs is that they are highly resistant to photobleaching than their organic counterparts. This feature is of great importance for three-dimensional (3D) imaging where there is a constant bleaching of fluorophores during acquisition of successive layer-by-layer scanning, which compromises the

Fig. 6 (a) Concurrent imaging of both QDs with a 470 nm emission maximum and green fluorescent protein (GFP) provides a clear separation of the vessel from GFP-expressing perivascular cells and (b) vessels (QDs with a 660 nm emission maximum) micelle preparation were imaged simultaneously with the second harmonic generation signal; the image represents a projection of a stack of 20 images at an interval of 2 mm per slice. Scale bars represent 50 mm (Reprinted with permission from reference (Stroh et al. 2005))

Fig. 6 (a) Concurrent imaging of both QDs with a 470 nm emission maximum and green fluorescent protein (GFP) provides a clear separation of the vessel from GFP-expressing perivascular cells and (b) vessels (QDs with a 660 nm emission maximum) micelle preparation were imaged simultaneously with the second harmonic generation signal; the image represents a projection of a stack of 20 images at an interval of 2 mm per slice. Scale bars represent 50 mm (Reprinted with permission from reference (Stroh et al. 2005))

correct reconstruction of 3D structures. For instance, it was demonstrated that, after 80 min of constant illumination, signal intensity from QDs' fluorescently labeled Xenopus embryos stayed unchanged while dextran-labeled controls were completely photobleached (Dubertret et al. 2002).

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