Quantum Dot Cellular Uptake and Toxicity in the Developing Brain: Implications for Use as Imaging Probes.

Nanometer-sized luminescent semiconductor quantum dots (QDs) have been utilized as imaging and therapeutic agents in a variety of disease settings, including diseases of the central nervous system. QDs have several advantages over traditional fluorescent probes including their small size (5-10 nm), tunable excitation and emission spectra, tailorable surface functionality, efficient photoluminescence, and robust photostability, which are ideal characteristics for in vivo imaging. Although QDs are promising imaging agents in brain-related applications, no systematic evaluation of QD behavior in brain-relevant conditions has yet been done. Therefore, we sought to investigate QD colloidal stability, cellular uptake, and toxicity in vitro, ex vivo, and in vivo in the brain environment. We found that QD behavior is highly dependent on surface functionality and that treatment of cultured organotypic whole hemisphere (OWH) slices with QDs results in dose-dependent toxicity and metallothionein increase, but no subsequent mRNA expression level changes in inflammatory cytokines or other oxidative stress. QDs coated with poly(ethylene glycol) (PEG) were protected from aggregation in neurophysiologically relevant fluids and in tissue, allowing for greater penetration. Importantly, QD behavior differed in cultured slices as compared to monolayer cell cultures, and behavior in cultured slices aligned more closely with that seen in vivo. Irrespective of surface chemistry and brain-relevant platform, non-aggregated QDs were primarily internalized by microglia in a region-dependent manner both in slices and in vivo upon systemic administration. This knowledge will help guide further engineering of candidate QD-based imaging probes for neurological application.

On-Demand Sonochemical Synthesis of Ultrasmall and Magic-Size CdSe Quantum Dots in Single-Phase and Emulsion Systems.

The sonochemical synthesis of CdSe quantum dots (QDs) in a single-liquid bulk phase and in an emulsion-based system is presented. Reactions utilized cadmium oleate and trioctylphosphine selenide precursors and were monitored as a function of sonication time under controlled temperature conditions to isolate the effects of cavitation from those of bulk temperature changes. QD synthesis was found to be slow in the single-phase liquid system (i.e., 1-octadecene) but greatly accelerated in the dispersed system (i.e., emulsions of 1-octadecene in ethylene glycol). It is hypothesized that the emulsion system increases the cavitation efficiency while also delivering acoustic energy in closer proximity to the precursor molecules. The capacity of CdSe production in the emulsion system was found to be 3.8 g/(L h), which is comparable to the typical hot-injection synthesis of CdSe QDs and can likely be further optimized. While the single-phase solvent system was found to produce ultrasmall QDs that exhibit broadband white-light emission, the emulsion system was found to produce well-defined magic-size clusters (MSCs) with photoluminescence quantum yield as high as 34%. Differences in synthesis rate and product properties from the emulsion and single-phase systems were probed by X-ray diffraction, electron microscopy, UV-visible (vis) and photoluminescence spectroscopy, and small-angle X-ray scattering (SAXS). Finally, precise temporal control of the QD synthesis was demonstrated via on-off cycling of the ultrasound waves.