Observation of Multiple, Identical Binding Sites in the Exchange of

Letter pubs.acs.org/NanoLett

Observation of Multiple, Identical Binding Sites in the Exchange of Carboxylic Acid Ligands with CdS Nanocrystals. Xin Li,† Valerie M. Nichols,† Dapeng Zhou,† Cynthia Lim,† George Shu Heng Pau,‡ Christopher J. Bardeen,† and Ming L. Tang*,† †

Department of Chemistry, University of California, Riverside, Riverside, California 92521, United States Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

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S Supporting Information *

ABSTRACT: We study ligand exchange between the carboxylic acid group and 5.0 nm oleic-acid capped CdS nanocrystals (NCs) using fluorescence resonance energy transfer (FRET). This is the first measurement of the initial binding events between cadmium chalcogenide NCs and carboxylic acid groups. The binding behavior can be described as an interaction between a ligand with single binding group and a substrate with multiple, identical binding sites. Assuming Poissonian binding statistics, our model fits both steadystate and time-resolved photoluminescence (SSPL and TRPL, respectively) data well. A modified Langmuir isotherm reveals that a CdS nanoparticle has an average of 3.0 new carboxylic acid ligands and binding constant, Ka, of 3.4 × 105 M−1. KEYWORDS: Nanocrystals, FRET, ligand exchange, binding constant

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assumption is not necessarily valid.27 Evidence for nonlinear relationships between ligand binding and PLQY includes the observation that a single octanethiol molecule can quench the luminescence of CdSe NCs by at least 50%.28 Most recently, Owen and co-workers found that PLQY of CdSe NCs has a strongly superlinear dependence on the surface coverage of cadmium-carboxylates, while below a threshold of 2 carboxylates/nm2, the PLQY was