The main disadvantages of QDs are due to their composition of heavy metals (e.g., Cd, Pb, and Se which are known to be toxic to vertebrate systems at parts-per-million concentrations (Hardman 2006)) and the instability of uncoated QDs when exposed to UV radiation which leads to the release of heavy metal ions. For example, the free cadmium in solution can bind to sulfhydryl groups of critical mitochondrial proteins. Thiol group inactivation then leads to oxidative stress and mitochondrial dysfunction (Rikans and Yamano 2000). The instability of QDs under UV exposure is due to the fact that the energy of UV radiation is close to that of covalent chemical bonds, therefore UV exposure can dissolve the semiconductor particles in a process known as photolysis, which releases toxic ions such as cadmium ions (Nie et al. 2007). For example, under oxidized (30 min exposure to air) or long UV radiation (2-8 h) conditions, even QD concentrations of 0.0625 mg/mL were found to be highly toxic (Derfus et al. 2004). The mechanisms of oxidation of Cd-based QDs have been suggested to occur via either TOPO-mediated or UV-catalyzed surface oxidation resulting in the removal of surface atoms and release of Cd2+ ions (Derfus et al. 2004).
Generally, QD toxicity depends on the effects of multiple factors from both individual QD physicochemical properties and environmental conditions. It has been shown that QD size, charge, concentration, outer coating bioactivity (i.e., capping material, functional groups, etc.), and oxidative, photolytic as well as mechanical stability are factors that determine QD toxicity (Hardman 2006). For example, investigators have tested the toxicity of CdSe QDs in liver cultures and found that it was dependent on processing parameters during synthesis, exposure to ultraviolet (UV) light, and surface coatings (Derfus et al. 2004). The CdSe (core only, no shell) QDs were found to be noncytotoxic under standard conditions of synthesis and water-solubilization with mercaptoacetic acid (MAA). However, TOPO-coated QDs, which were initially subjected to air for 30 min and then modified with MAA, showed a dramatic dose-dependent decrease in primary hepatocyte viability (from 98 to 21% at a QD concentration of 62.5 mg/mL). These researchers also capped CdSe QDs with 1-2 monolayers of ZnS to study the effect of capping on cytotoxicity. They found that the CdSe core was intact after 12 h of oxidation in air. However, high levels of free Cd in solution (~40 ppm) after 8 h of UV photooxidation (power density of 15 mW/cm2 preceded by 12 h of oxidation in air) were observed. This finding indicates that ZnS capping was effective in eliminating cytotoxicity due to oxidation by air, but did not fully eliminate cytotoxicity induced by UV photooxidation.
To overcome those cytotoxicity problems, QDs have been coated with PEG or encapsulated in micelles (i.e., vesicles formed by aggregation of surfactant molecules dispersed in a liquid colloid), which can limit the release of toxic metals in response to UV light exposure (Gao et al. 2004, 2005; Dubertret et al. 2002; Stroh et al. 2005). More work is still needed to develop optimal coatings and, thus, encapsulation that prevents the release of heavy ions from QDs.
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