Design of Water-Soluble Quantum Dots with Novel Surface Ligands for Biological Applications
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Design of Water-Soluble Quantum Dots with Novel Surface Ligands for Biological Applications H. Tetsuo Uyeda1, Igor L. Medintz2, and Hedi Mattoussi1 U.S. Naval Research Laboratory, Washington, DC 20375, 1Division of Optical Sciences, Code 5611, 2Center for Bio/Molecular Science and Engineering, Code 6910
ABSTRACT We have designed a series of organic oligo- and polyethylene glycol (PEG) based surface capping ligands that allow for QD manipulation in aqueous media. We utilized readily available thioctic acid and various oligo- and polyethylene glycols in simple esterification schemes, followed by reduction of the dithiolane to produce multi-gram quantities of capping substrates. Cap exchange of the native trioctyl-phosphine and -phosphine oxide based ligands with the PEGterminated dithiol-alkyl cap readily resulted in aqueous dispersions of QDs that were homogeneous and stable in various pH ranges over an extended period of time. Mixed surface capping strategies utilizing ratios of dihydrolipoic acid to the pegylated dihydrolipoic acid were also prepared. We anticipate that such systems should allow one to covalently attach amine containing biomolecules to nanoparticle systems bearing carboxylates, employing known coupling agents, such as (dimethylamino) propyl-3-ethyl-carbodiimide (EDC). This design and conjugation strategy may facilitate the development of a new generation of QD-bioconjugates, which can be directly utilized in bio-related applications such as sensing and cellular imaging.
INTRODUCTION The use of fluorescent tags to label biological molecules is a general and practical method in biological research. Organic dyes have been employed in single and multiplex detection schemes [1,2]. However, this approach has several limitations. Organic fluorophores tend to have narrow excitation spectra with red tailing broad emission bands. Due to spectral overlap, simultaneous quantitative assessment of the relative amounts of different fluorophores present in the same sample becomes exceedingly difficult [3]. Moreover, variation of the absorptive and emissive properties of dye labeled conjugates requires the use of chemically distinct labels with non-uniform synthesis and conjugation methods. Colloidal semiconductor nanocrystals, or quantum dots (QDs), with unique properties such as high photobleaching threshold, chemical stability, and readily tunable spectral properties have the potential to overcome many of the limitations in biological research encountered by conventional dye systems [4-7]. Recently, the utility of stable water-soluble luminescent colloidal quantum dots (QDs), either pure or conjugated to biomolecular receptors, has been demonstrated in biosensing and cellular imaging applications [4-14]. However, due to the process in which these systems are prepared, the surface properties of the QDs limit one’s ability to manipulate them in aqueous environments and apply simple covalent conjugation techniques to prepare stable versatile QDbioconjugates. Several examples of surface chemistry include t
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