At the Nano-bio Interface- Dr. Mattoussi
Surface-functionalization strategies. We focus on developing versatile surface-functionalization strategies that can be applied to an array of inorganic nanocrystals (NCs). Our approach is based on cap exchange with modular multifunctional ligands. We have recently designed a new set of multifunctional and multidentate oligomer (OligoPEG) ligands, made of a central short backbone on which were laterally grafted several short poly(ethylene glycol) (PEG) moieties appended with either thioctic acid (TA) or dihydrolipoic acid (DHLA). Here the insertion of PEG promotes water solubility, while TA and DHLA groups provide strong anchoring onto Au nanoparticles (AuNPs) and ZnS-overcoated semiconductor quantum dots (QDs). The new OligoPEG ligands also allow easy integration of tunable functional and reactive groups (e.g., azide or amine) within their structures, which opens up the possibility of bioconjugation of the NCs to specific biomolecules. Using the same rationales, we prepared and tested another set of multicatechol- and PEG-derivatized oligomers, OligoPEG-Dopa. These OligoPEG ligands provide rapid ligand exchange and the resulting nanoparticles exhibit great colloidal stability over a broad pH range and in the presence of excess electrolytes (Fig. 1). See Langmuir 28, 2761−2772 (2012). DOI: 10.1021/la203968t; and ACS Nano 6, 389–399 ( 2012). nullDOI: 10.1021/nn203735b.
QDs as fluorescent platforms coupled to redox-active complexes. We have over the past several years investigated the effects of arraying florescent dyes and/or redox active complexes around the surface of luminescent QDs on their optical and spectroscopic properties. In one recent example, we used QDs cap-exchanged with a mixture of DHLA-PEG-OCH3 and DHLA-PEG-NH2 (bidentate PEGylated ligands) exhibiting pH-independent emission over pH 4-10, and coupled them to dopamine–isothiocyanate (dopamine-ITC). This permitted us to vary the number of dopamines per QD, while maintaining the average separation distance fixed. We investigated the effects of tuning the number of dopamine groups, the pH, and the importance of oxygen (O2) on the optical and spectroscopic properties of the QD-dopamine conjugates. We measured pH-dependent PL loss combined with substantial shortening of the QD exciton lifetime, and found that oxygen present plays a crucial role in the measured changes of the fluorescence properties of these QD-dopamine conjugates. This system allowed us to show the presence of new mechanisms of charge transfer interactions between luminescent QDs and metal complexes. In particular, it showed that photoexcited QDs interact with two forms of dopamine in the medium: the reduced catechol acting as charge donor (to the QDs) and oxidized quinone acting as an acceptor (from the QDs). See J. Am. Chem. Soc.134, 6006−6017 (2012) DOI: 10.1021/ja300724x.
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