Quantifying Ligand Exchange Reactions at CdSe Nanocrystal Surfaces

Abstract

The exchange reactions of X-type ligands at the surface of CdSe nanocrystals have been quantified via 1 H NMR, absorbance, and emission spectroscopies. 1 H NMR was used to quantify displacement reactions of oleate-capped CdSe nanocrystals with carboxylic-acid-, phosphonic-acid-, and thiolterminated ligands that incorporate terminal alkene functionalities. The alkenyl protons of the native oleate ligand and the vinylic protons of the exchange ligand provide spectroscopic handles to quantify both free and surface-bound forms of these ligands. Undec-10-enoic acid was found to undergo an exchange equilibrium with oleate (K eq = 0.83), whereas phosphonic-acid-and thiol-terminated ligands irreversibly displace native oleate ligands. Absorption and emission experiments indicate that the carboxylic acid exchanges occur solely between surface-bound ligands, whereas exchange reactions with phosphonic acids and thiols alter the surface metal atoms of the nanocrystals, presumably through the displacement of Cd(oleate) 2 . These quantifications can be used to guide the selective functionalization of CdSe nanocrystals. ■ INTRODUCTIONResearch in the field of semiconductor quantum dots (QDs) has exploded since the discovery of their quantum size effects in 1983. 1 Their size tunable optical properties have been exploited for applications ranging from photovoltaic cells to solid-state lighting. 2,3 As synthesized, colloidal QDs are composed of an inorganic semiconductor core and an organic ligand shell. These ligands, generally long chain fatty acids, aid in the growth and stabilization of QDs, solubilize QDs in organic solvents, and passivate undercoordinated surface atoms of the QD. However, these native ligands are not ideal for many QD applications, and can be exchanged for other coordinating ligands which may vary in the surface anchoring group, chain length, and chain identity. Ligand exchanges are commonly performed to incorporate functional groups that alter QD solubility, introduce electron transfer partners, or integrate biological receptors. 4−26 The extent of these ligand exchange reactions must be controlled if a limited number of functionalized ligands per nanocrystal is desired.Although ligand exchange reactions are commonly employed, the mechanisms and principles that govern these processes are not explicitly understood. 27 Many studies have been conducted monitoring the surface chemistry of quantum dots using photoluminescence spectroscopy, 1 H NMR spectroscopy, as well as diffusion-ordered NMR spectroscopy (DOSY), nuclear Overhauser effect spectroscopy (NOESY), and 31 P NMR spectroscopy. 10,12,26,28−43 Among the studies focused on oleatecapped metal chalcogenide nanocrystals, Hens and co-workers have employed these NMR methods to examine the details of various ligand exchange reactions. Through these experiments, they determined that phosphonic acids displace oleate ligands with a 1:1 stoichiometry, 31 and that the self-exchange of oleic acid (OA)/oleate at the surface of CdSe QDs involves a proton exchange. 33...

Quantified Binding Scale of Competing Ligands at the Surface of Gold Nanoparticles: The Role of Entropy and Intermolecular Forces

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Electrochemical Characterization of CdSe Monolayers Modified with Glycosilated Molecules

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