Quantum Dots and Their Ligand Passivation

Abstract

Another major category of nanoparticles are quantum dots (QDs), which are semiconductor nanocrystals (*2-100 nm) with unique optical and electrical properties, and widely used in biomedical imaging and the electronics industries [1][2][3][4][5][6][7][8][9]. These II-VI semiconductor nanostructures (II = Zn, Cd; VI = O, S, Se, Te) display outstanding properties distinct from their bulk counterparts like broad excitation bands, large extinction coefficient, tunable emission features, bright photoluminescence, nonlinear optical properties, and high stability against photobleaching and chemicals due to the quantum nanoconfinement. These characteristic features of QDs make them amenable for solar cell components, optical sensors, and optoelectronic devices [1][2][3][4][5][6][7][8][9]. The unique fluorescence spectrum of QDs also renders them optimal fluorophores for biomedical imaging. The advantages of using QDs is that their various physicochemical factors such as particles sizes, atomic compositions are easily controllable, and this adaptability of QDs makes their applications more versatile [10][11][12][13][14][15][16]. More interestingly, inorganic QDs with comparable sizes to many biomolecules have recently been proposed as a potential platform for nanodrug carriers and/or diagnostic agents [17][18][19][20][21][22][23][24][25][26][27][28]. These advances have also raised growing concerns on their potential cytotoxicity and biocompatibility of QDs in biological environment.Recent studies suggest that QD toxicity depends on multiple factors derived from both the inherent physicochemical properties of QDs and environmental [19] QD size, charge, concentration, outer coating material, as well as oxidative, photolytic, and mechanical stability can individually or collectively determine the QD toxicity. For instance, Lovrić et al. [20] found that CdTe QDs coated with mercaptopropionic acid (MPA) and cysteamine were cytotoxic to rat pheochromocytoma cell (PC12) cultures at concentrations of 10 μg/mL. Uncoated CdTe QDs were cytotoxic at 1 μg/mL. Cytotoxicity was more pronounced with smaller positively charged QDs (2.2 ± 0.1 nm) than with larger neutrally charged QDs (5.2 ± 0.1 nm)