Compact Cysteine-Coated CdSe(ZnCdS) Quantum Dots for in Vivo Applications

Abstract

Semiconductor nanocrystals (quantum dots, QDs) are usually described as fluorophores having remarkable photostability, large absorption cross sections, and tunable emission peaks. Equally important, QDs also serve as versatile nanoscale objects of precisely tunable size and morphology, having exceptionally narrow size distributions. 1, 2 Size is an especially important parameter in the design of nanomaterials for applications in biology, for both in vitro and in vivo applications. In cell labeling applications, for example, size can affect endocytosis or limit access to receptors of interest, such as those in the neuronal synapse. 3 In live animals, particle size can dramatically affect biodistribution and pharmacokinetics. 4, 5 The optimal size depends on the application. For example, we have previously shown that Type II CdTe(CdSe) core (shell) QDs with a hydrodynamic diameter (HD) of ∼19 nm can be used to selectively map sentinel lymph nodes. 6 More recently, we have also shown that 8.7 nm HD Type I InAs(ZnSe) QDs allowed the mapping of multiple lymph nodes and also showed the potential extravasation of QDs from the vasculature. 7 There has been a strong interest in reducing the HD of QDs for in vivo applications as this could increase bioavailability and lead to an improved understanding of clearance mechanisms. In this communication, we demonstrate bio-compatible fluorescent QDs with an exceptionally small HD of ∼6 nm using a CdSe(ZnCdS) core(shell) structure coated with DL-cysteine and show renal clearance of these QDs in rat models.Cysteine-coated nanocrystals have been demonstrated previously with CdS QDs synthesized directly from aqueous solution in the presence of L-cysteine hydrochloride. 8 However, these QDs suffered from low QY (6−8%) and broad fluorescence (FWHM >100 nm) due to emission from surface trap sites. We report the synthesis of high-quality cysteine-coated CdSe(ZnCdS) core(shell) QDs using well-developed nanocrystal synthetic procedures, in which cores are formed through the rapid injection of metal and chalcogenide precursors into hot solvent and then overcoated with a thin shell of a higher band gap material. Following overcoating, the native ligands are exchanged with DL-cysteine to yield water dispersible QDs (QD-Cys).Ligand exchange with cysteine was achieved using a biphasic exchange method in which QDs dispersed in chloroform were mixed with a solution of DL-cysteine in phosphate buffered saline (PBS). This biphasic mixture was stirred vigorously at room temperature, and phase transfer of QDs from the organic to the aqueous phase occurred over ∼2 h, leaving a colorless chloroform layer ( Figure 1a). The QDs were precipitated twice with ethanol and re-dispersed in PBS at pH 7.4 for analysis. QD-Cys as synthesized formed macroscopic aggregates upon standing at room temperature overnight. Storing the samples in the dark at 4 °C only extended jfrangio@bidmc.harvard.edu, mgb@mit.edu. Supporting Information Available: Experimental procedures, transmission electron microscopy data, and ad...

Functionalized Au22 Clusters: Synthesis, Characterization, and Patterning

“…Tsukuda et al synthesized a series of GSH-protected clusters and determined their chemical composition by ESI-MS (36). These clusters are Au 10 (SG) 10 , Au 15 (SG) 13 , Au 18 (SG) 14 , Au 22 (SG) 16 , Au 22 (SG) 17 , Au 25 (SG) 18 , Au 29 (SG) 20 , Au 33 (SG) 22 , and Au 39 (SG) 24 . All of these nine clusters have good stability.…”

“…Mass spectrometry confirmed that Au 28 (SG) 16 was the most abundant species in the synthesis (5). In the recent past, a series of -SG-protected gold clusters with well-defined compositions have been synthesized (36).…”

“…The molecular compositions of these clusters were characterized by electrospray ionization mass spectrometry (ESI-MS). Tsukuda et al (36) characterized the elemental composition of all of the separated entities by ESI-MS and confirmed that the most abundant species is Au 25 (SG) 18 and not Au 28 (SG) 16 , as suggested originally (5). The ligand-exchange reaction of phosphine-stabilized gold clusters (Au 11 ) with GSH gave only Au 25 SG 18 in large yield, suggesting that it is the most stable one among the thiolated clusters from this class (37).…”

Quantum dots–DNA bioconjugates: synthesis to applications

“…First demonstrated by Chan & Nie close to a couple of decades back, the most popular mercapto alkyl acid based ligand is mercaptopropionic acid (MPA) [2,17]. The others include mercaptohexanoic acid (MHA) [21], mercaptoundecanoic acid (MUA) [22] [4,23] and thiol-containing amino acids such as cysteine [23] and D-penicillamine [24,25] have also been used for ligand exchange. These ligands bind to the QD surface via the thiol functionality and terminate in a hydrophilic amine group, which can be further used for conjugation.…”

“…These ligands bind to the QD surface via the thiol functionality and terminate in a hydrophilic amine group, which can be further used for conjugation. It has also been observed in the literature that thioalkyl amine derivatives often result in less stable dispersions of QDs in hydrophilic media; and thus are used in moderation [23,26]. Monothiol-based ligands are widely used for ligand exchange of QDs.…”