Heterogeneous Catalysis at the Single-molecule Level: A Quantitative Understanding of the Catalytic Activity of Individual Gold Nanoparticles and its Associated Dynamics

Open Access
Ravi, Venkataramanan
Graduate Program:
Chemical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
December 17, 2013
Committee Members:
  • Robert Martin Rioux Jr., Dissertation Advisor
  • Robert Martin Rioux Jr., Committee Chair
  • Michael John Janik, Committee Member
  • Enrique Daniel Gomez, Committee Member
  • Peter J Butler, Committee Member
  • Heterogeneous Catalysis
  • Gold Nanoparticles
  • Single Molecule Level Measurements
  • Kinetic Analysis
  • Thermodynamics
We use total internal reflection fluorescence (TIRF) microscopy to understand heterogeneous catalysis at the single-molecule level. This study focuses on examining, in detail, the catalytic activity of individual gold (Au) nanoparticles (5.4 ± 0.7, 9.5 ± 0.6, and 19.4 ± 1.1 nm diameter) through single-molecule detection of a model fluorescent reaction: reduction of resazurin (non-fluorescent) to resorufin (fluorescent) in the presence of a suitable reductant (hydroxylamine). We find that for both the product formation and the product generation reaction, all Au nanoparticles follow a Langmuir-Hinshelwood mechanism. We verify the proposed mechanism for kinetic and thermodynamic consistency through temperature-dependent measurements at both the single-molecule and ensemble levels. We quantitatively study the influence of solvent on the reaction system by replacing H2O with D2O. Single-molecule measurements aid in deconvolution of desorption kinetic parameters, which is impossible at the ensemble-level when the desorption step is not rate-limiting. It also provides a detailed understanding of the dispersive kinetics of individual Au nanoparticles and quantifies the heterogeneity in activity among individual nanoparticles. The next phase of our study focuses on deconvoluting the origins of temporal variations in activity present among individual catalytic turnover events on a single Au nanoparticle. We interpret the contributions of intrinsic restructuring and adsorbate-induced restructuring of gold nanoparticles by Reaction Force-Field (ReaxFF) Molecular Dynamics (MD) simulations and an autocorrelation analysis of experimental data. By adding small molecule adsorbates of varying affinity to the Au surface, we probed the contributions of the reactivity for different types of active sites on the observed variations in activity. Strong adsorbates such as thiols were found to chromatographically titrate the Au active sites from higher reactivity to lower reactivity. By varying the coverage of thiols, we modulate the temporal variations in the activity of individual Au nanoparticles. Furthermore, at the single-molecule level, we studied the reorientation dynamics of single resorufin molecules generated on the surface of a 5.4 nm Au nanoparticle. By combining single molecule observations with theoretical calculations, we relate the presence of a slow rise in on-times (rise times) from our fluorescence turnover trajectory to the rotational dynamics of product (resorufin) on the Au surface. We perform an extensive autocorrelation analysis on the observed rise times and determined the activation barrier associated with product reorientation through temperature-dependent single-molecule analysis. Finally, in a separate study, we probe the thermodynamic adsorption profile at a solvated organic-inorganic interface by following the binding and organization of carboxylic acid-terminated alkanethiols of varying chain lengths (C2, C3, and C6) to the surface of Au nanoparticles using isothermal titration calorimetry (ITC). The thermodynamic parameters support a mechanism of step-wise adsorption of thiols to the surface of Au NPs and secondary ordering of the thiols at the organic-inorganic interface. We observe an apparent compensation effect: the negative ΔH is compensated by a negative ΔS as the thiols self-assemble on the Au NP surface. Understanding the thermodynamics of adsorption at nanoparticle surfaces will provide critical insight into the role of ligands in directing size and shape during nanoparticle synthesis since thiols are a common ligand choice (i.e., Brust method). The ITC technique is applicable to a large number of structure-directing ligands and solvent combinations and, therefore, should become an important tool for understanding reaction mechanisms in nanostructure synthesis.