Recent breakthroughs in biomedical nanotechnology have demonstrated the promising clinical applicability of nanostructures as targeted diagnostic cancer-imaging agents, [1] as aids to optically guided surgery, [2] as smart drug-delivery systems, [3] and in hyperthermia therapy. [4] Of the existing biomedically applied nanotechnologies to date, CdSe-core, ZnS-capped semiconductor nanocrystals (also known as quantum dots, QDs) have been at the forefront of biomedical nanotechnology research. [5][6][7] Questions regarding the in vivo distribution, clearance, metabolism, and toxicity of the QDs have not been thoroughly investigated. Gao et al., Akerman et al., and Ballou et al. have provided the first qualitative glimpses of the in vivo distribution of QDs. [1,8,9] A thorough quantitative analysis of QD in vivo distribution and clearance is required, since this information could lead to i) an improvement in targeting efficiency of QDs for diagnostics, ii) a better understanding of QDs' non-specificity toward tissues, and iii) an assessment of QD distribution and clearance that serves as the basis in determining their toxicity. Therefore, the focus herein is to quantitatively elucidate the in vivo kinetics of QDs. We found that the modification of QDs' surface altered both the QD clearance from plasma and the sequestration of QDs within organs. Also, clearance of QDs via urine and feces was not observed within the experimental duration (ten days), suggesting that the QDs are sequestered in vivo and not cleared.
QDs are particles of dimensions that are smaller than the exciton Bohr radius. They generally have a core size of 2-7 nm, but can be as large as 100 nm when organic shells and/or conjugated biorecognition molecules are placed onto their surface. Recently, there has been interest in determining the in vivo toxicity of QDs, as in vivo exposure to QDs can lead to potential risks, which stem from three basic factors. The first involves the metallic component (e.g., Cd and Zn) of QDs, which are associated with known toxicity. [10,11] The second is due to the high surface-area-to-volume ratio of the QDs, which provides a large available surface for enzymatic degradation and release of metallic ions. [12] The third relates to the size of the QDs: it has been suggested that the small size of nanostructures, such as fullerenes, permits them to enter vital organs such as the brain and cause damage. [13] Currently, no comprehensive study on the in vivo toxicity of QDs exists. Thus far, only in vitro cell-culture experiments on toxicity have been conducted. It has been shown that the breakdown of QDs can lead to metal-induced toxicity within the cells, and this toxicity is highly dependent on the chemical design of