A General Approach To Study the Thermodynamics of Ligand Adsorption to Colloidal Surfaces Demonstrated by Means of Catechols Binding to Zinc Oxide Quantum Dots

Abstract

A general strategy to study the thermodynamics of ligand adsorption to colloidal surfaces was established. The versatility of our approach is demonstrated by means of catechols binding to ZnO quantum dots (QDs). First, isothermal titration calorimetry (ITC) was used to extract all relevant thermodynamic parameters, namely association constant, enthalpy, entropy, and free energy of ligand binding. Noteworthy, the determined ΔG of −20.3 ± 0.4 kJ mol −1 indicates a strong, reproducible, and exothermic interaction between the catechol anchor group and the oxide particle surface. To confirm the characterization of ligand binding by measuring the heat of adsorption, the free energy was crossvalidated by mass-based adsorption isotherms. A combination of inductively coupled plasma optical emission spectroscopy (ICP-OES) and UV/vis spectroscopy was developed to quantitatively determine the mass of bound catechols with respect to the available particle surface. The association constant K was determined by a Langmuir fit to be 2618 M −1 which leads to ΔG = −19.50 kJ mol −1 according to ΔG = −RTln K. To close the mass balance, analytical ultracentrifugation (AUC) was applied to detect the amount of the free, unbound catechol in solution. Finally, Raman spectroscopy and nuclear magnetic resonance spectroscopy (NMR) were performed to quantify the amount of remaining acetate from particle synthesis and to distinguish bound (chemisorbed) and unbound (physisorbed) catechol. Our results reveal that approximately 65 wt % of acetate is replaced, and physisorbed catechol will not affect the amount of remaining acetate on the ZnO surface. Moreover, no pronounced chemical shift peak as it would be expected for free catechol is observed by NMR at all. This indicates a highly dynamic adsorption−desorption equilibrium between the free and the physisorbed state of catechol on the particle surface. Our concept of combined analytics is seen to be a generally applicable strategy for particle-ligand interfacial studies. It gives detailed insight into thermodynamics, binding states, and ligand composition and is thus considered as an important step toward tailored colloidal surface properties.

A White Light‐Emitting Quantum Dot Complex for Single Particle Level Interaction with Dopamine Leading to Changes in Color and Blinking Profile

“…This was further supported by the results obtained when the quenching of yellow emission of WLE QDC following the addition of benzoquinone (which is a well‐known electron quencher) was observed (Figure S16, Supporting Information). Additionally, the formation of nonfluorescent Zn‐dopamine complex, which also acts as quencher of the emission of ZnO Qdots, via the chelating ability of the catechol part of dopamine toward surface free Zn 2+ ions of WLE QDC may be the other reason for the observed selective quenching of yellow emission of ZnO Qdots, present in WLE QDC . It may be mentioned here that complexation reaction that was performed to get white light from ZnO Qdots following reaction with MSA might not have resulted in the complete coverage of the surface of the Qdots.…”

Quantifying Ligand Exchange Reactions at CdSe Nanocrystal Surfaces

“…While the observed increase in entropy is not obvious from the 1:1 exchange quantified through NMR studies, the aforementioned acetate−catechol exchange on ZnO nanocrystals is also accompanied by a positive ΔS (+13.1 J/mol K). 60 We hypothesize that the positive ΔS value obtained may arise from the mixed ligand shell of oleate and UDA ligands formed under the exchange equilibrium as opposed to the native ligand shell composed solely of oleate ligands. Overall, the energetic parameters obtained in these studies are consistent with previous observations of carboxylic acid ligand exchange reactions for CdSe; a large excess of exchange ligand is required, and these exchange reactions often require heating or sonication to achieve full ligand displacement.…”

“…For comparison, two recent reports utilizing isothermal titration calorimetry to assess ligand adsorption thermodynamics show that the reversible binding of L-type ligands to vacant coordination sites of CdSe/CdZnS core/shell particles is an exothermic process while the exchange of X-type acetate for catechol on ZnO nanocrystals is accompanied by a positive ΔH. 52,60 For the X-type exchange reaction explored here, a value of ΔS = +28 ± 1 J/mol K was obtained from the Van 't Hoff analysis. As the ΔS value is derived from extrapolation to an intercept, entropy values determined from Van 't Hoff analyses are often considered to be especially prone to error; thus, just the sign of the ΔS value is commonly interpreted.…”

Analysis of Particle Size Distributions of Quantum Dots: From Theory to Application

“…and Snee P.T., 2014), more than two decades of intense research on QDs passed, covering various aspects from quantum mechanics and optical properties (Alivisatos A.P., 1996), particle synthesis (Park J. et al, 2007), functionalization and ligand exchange (Alvarado S.R. et al, 2014;Lin W. et al, 2015).…”