Model for Adsorption of Ligands to Colloidal Quantum Dots with Concentration-Dependent Surface Structure

Abstract

A study of the adsorption equilibrium of solution-phase CdS quantum dots (QDs) and acid-derivatized viologen ligands (N-[1-heptyl],N'-[3-carboxypropyl]-4,4'-bipyridinium dihexafluorophosphate, V(2+)) reveals that the structure of the surfaces of the QDs depends on their concentration. This adsorption equilibrium is monitored through quenching of the photoluminescence of the QDs by V(2+) upon photoinduced electron transfer. When modeled with a simple Langmuir isotherm, the equilibrium constant for QD-V(2+) adsorption, K(a), increases from 6.7 × 10(5) to 8.6 × 10(6) M(-1) upon decreasing the absolute concentration of the QDs from 1.4 × 10(-6) to 5.1 × 10(-8) M. The apparent increase in K(a) upon dilution results from an increase in the mean number of available adsorption sites per QD from 1.1 (for [QD] = 1.4 × 10(-6) M) to 37 (for [QD] = 5.1 × 10(-8) M) through desorption of native ligands from the surfaces of the QDs and through disaggregation of soluble QD clusters. A new model based on the Langmuir isotherm that treats both the number of adsorbed ligands per QD and the number of available binding sites per QD as binomially distributed quantities is described. This model yields a concentration-independent value for K(a) of 8.7 × 10(5) M(-1) for the QD-V(2+) system and provides a convenient means for quantitative analysis of QD-ligand adsorption in the presence of competing surface processes.

Diameter Dependent Electron Transfer Kinetics in Semiconductor–Enzyme Complexes

“…This is critical to evaluating ET kinetics from optical spectroscopy measurements, which exhibit a strong dependence on binding stoichiometries. 33,34 Electron Transfer Kinetics. As noted above, the observed decrease in CdTe lifetime with increasing CaI concentration is attributed to the addition of a photo-driven ET pathway from CdTe f CaI.…”

Quenching Dynamics in CdSe Nanoparticles: Surface-Induced Defects upon Dilution.

“…(The case of a binomial distribution of m values is treated in reference 14 for the static quenching). The probability that a site is active for quenching is p. The number of quenching sites per particle varies from 0 to m, leading to m+1 different populations of fluorescent QDs in proportions that follow the binomial distribution:…”

“…proposed a double binomial distribution to describe (i) the number of available sites per QD and (ii) the partial occupation of these sites by acid-derivatized viologen ligands. 14 The use of a Poisson distribution of quenchers for the analysis of time resolved fluorescence have been done by Tachiya 15 . These authors have included the binomial distribution of quenchers to analyze the electron transfer rate between QDs and viologen, assuming an exponential kinetics.…”

Exciton Coupling of Surface Complexes on a Nanocrystal Surface

“…[18][19][20][21][22][23] It provides an opportunity to control the surface density of a certain ligand coordinated to the NC surface by ligand exchange. One method is to add a controlled amount of free ligand to the NC solution to give a final equilibrium with a desired degree of ligand exchange.…”