Synthesis, Characterization and Biological Applications of Water-Soluble ZnO Quantum Dots

Quantum dots (QDs) or semiconductor nanocrystals are of great interest to fundamental studies but have also potential applications as biological probes (Medintz et al., 2005), fluorescent biosensor (Costa-Fernandez et al., 2006), light-emitting diodes (LEDs) (Lim et al. 2007), and solar cells (Robel et al., 2006). Owing to the effect of quantum confinement, QDs show exceptional physical and chemical properties such as sharp and symmetrical emission spectra, high quantum yield (QY), good photoand chemical stability, and size-dependent emission-wavelength tunability (Bruchez et al., 1998; Chan et al., 1998). For biological labelling, the most studied QDs are the nanocrystals of CdSe and CdTe (Aldeek et al., 2008) and the corresponding core/shell structured QDs (such as CdSe/ZnS, CdTe/ZnS or CdTe/ZnTe) that are more robust against chemical degradation or photooxidation than the parent cores (Law et al., 2009). Recent findings have highlighted the acute toxicity of II-VI semiconductor QDs without an external layer of a nontoxic material on biological systems (Schneider et al., 2009; Dumas et al. 2010). This toxicity results mainly from the decomposition and release of heavy metal ions and formation of reactive oxygen species. The toxicity of cadmium is a concern that will also limit the use of these visible or near IR emitting nanocrystals, especially for applications directly related to human health. Synthesis of low toxicity QDs and especially Cd-free QDs is the most challenging aspect of working with these materials in biological and medical fields. A promising member of the Cd-free QD is ZnO. However, ZnO nanoparticles are not stable in water. This instability is related to their surface luminescent mechanisms. Water molecules are able to attack the luminescent centers on the ZnO surface and destroy them rapidly. This chapter describes the strategies that have been developed over the last years to transfer ZnO QDs in water.