(17b) Unraveling the Process of Ligand Adsorption to Heterogeneous Colloidal Substrates By Means of Catechols Binding to ZnO Nanoparticles (Invited)
Although being of major importance for various applications, the characterization of nanoparticle (NP) surfaces in dispersions, especially with respect to ligand exchange reaction, is still an open and highly challenging question. Therefore a general strategy to study the thermodynamics of ligand adsorption to colloidal surfaces was established by means of catechol (ethyl-3,4-dihydroxybenzoate, esterCAT) binding to ZnO NPs . This combination represents a technically relevant model system as the catechol anchor group is a promising candidate anchor in applications like dye sensitized solar cells (DSSCs) .
First, isothermal titration calorimetry (ITC) was used to extract all relevant thermodynamic parameters, namely association constant, enthalpy, entropy and free energy of the ligand binding. To confirm the characterization of ligand binding by measuring the heat of adsorption, the free energy was cross-validated by mass-based adsorption isotherms. To close the mass balance, analytical ultracentrifugation (AUC) was applied to detect the amount of free, unbound catechol in solution. Then, Raman spectroscopy and nuclear magnetic resonance spectroscopy (1H NMR) were performed to quantify the replaced amount of acetate which is the ligand from synthesis with esterCAT (65 %) and to distinguish bound (chemisorbed) and unbound (physisorbed) esterCAT. Finally, based on a collection of all our results, the in-depth picture of ligand binding to the esterCAT functionalized ZnO colloid was obtained.
In the second part of our work, this toolbox was applied to tailor the ZnO surface by CAT derivatives with different functionalities, namely hydrogen (pyroCAT), tert-butyl group (tertCAT), aromatic ring (naphCAT), ester group (esterCAT), and nitro group (nitroCAT). The results showed that the ZnO surface is highly heterogeneous and that the binding enthalpies of the CAT molecules follow the order of tertCAT < pyroCAT < naphCAT < esterCAT < nitroCAT, which is in agreement with the electronegativity of the tail groups. Moreover, the visible emission induced by defect sites on ZnO NPs surfaces was quenched in order of the binding strength of CAT molecules. Thus, by means of the toolbox, the correlations between electronegativity of molecules, binding enthalpies to the ZnO surface, and the photoluminescence (PL) properties of ZnO NPs were unraveled.
Finally, the kinetics of the ligand-induced defect quench were investigated in detail by two photon fluorescence (2-PF) spectroscopy, again using esterCAT as model functionality. It was found that at different ligand concentrations the fluorescence leveled off rapidly to a plateau within three seconds. Moreover, a full PL quench was observed at esterCAT concentrations as small as 0.25 monolayers (whereby 1 monolayer refers to the amount of molecules needed to cover the totally available ZnO surface area) provided, a concentration where maximum/equilibrium coverage of ZnO NPs with CAT molecules is clearly not yet reached. However, for liquid-borne ZnO NPs, there are various heterogeneities like different kinds of defects, surface occupied by solvent, as well as surface occupied by synthesis residuals or hydroxyl groups. Among all those, by 2-PF only the kinetics of esterCAT molecules binding to the defect sites were resolved whereby for Langmuir fitting it had to be assumed that the fluorescence intensity is proportional linearly with the number of defect sites. However, then the rate constant for defect saturation became accessible.
In conclusion, first, a widely-applicable strategy to study the thermodynamics of ligand adsorption to colloidal surfaces was established. Then, the toolbox was applied to tailor colloidal ZnO NP surfaces by CAT derivatives with different electronegativity. Finally, the electronegativity of ligands was linked with the binding enthalpy and PL quenching of NPs. We believe that such correlations are an important step towards a more general way of selecting and designing ligand molecules for surface functionalization. This allows establishing strategies for tailored colloidal NP surfaces beyond design on a case by case basis.
The authors want to thank the funding of Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence âEngineering of Advanced Materialsâ and project PE 427/29-1.
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