Nanofluidic Scattering Microscopy for Single Particle Catalysis

Abstract: Heterogeneous catalysis concerns material formulations – catalysts – that can assist a chemical reaction by improving its rate or selectivity, by lowering activation barriers and altering the energy landscape. The core of catalysts are often tiny metal particles that provide a large reaction surface at a small volume, as well as low coordination sites whose type and abundance depends on particle size and shape. Hence, such nanoparticles come in a broad spectrum of sizes and shapes when studied in a so-called ensemble comprised of millions of particles. Ensemble experimental techniques therefore often disregard the structural heterogeneity of nanoparticles as the measurements of such samples provide information on the averaged response of the heterogeneous ensemble. Consequently, “superparticles” with exceptional performance may be overlooked, or erroneous structure-function correlations may be established. To overcome the ensemble averaging problem, various techniques for single particle catalysis have been developed. The approaches include methods like fluorescence microscopy, X-ray diffraction and scattering, electron microscopy and plasmonic sensing. These methods have in common that they detect electrons or photons that report either on the reactant molecules consumed, the product molecules formed, changes to the catalyst particle itself, or temperature changes that the reaction evokes in the particle surrounding. However, none of the experimental methods provide direct single particle activity information without either using plasmonic enhancement effects that may also impact the studied reaction itself and limit the range of catalyst materials that can be studied or using fluorescence that limits reaction conditions to ultralow concentrations and to a narrow range of reactions. The overarching goal of the work presented in this thesis has been to develop an optical microscopy technique that can quantitatively measure catalytic activity, and in the longer term even selectivity, of a single nanoparticle without the limitations of existing single particle methods. At the core of this method that we call Nanofluidic Scattering Microscopy are nanofluidic channels that can accurately control the transport of reagents to and from a single catalytically active particle localized inside the channels. As a second key trait, the unique light scattering properties of nanochannels render them highly sensitive to refractive index changes of the fluid inside them. Hence, when a catalytic reaction alters the molecular composition of the fluid in the channel, its light scattering characteristics change and reveal in this way the catalytic performance of the nanoparticle. In this thesis, I describe my winding journey towards the first successful implementation of Nanofluidic Scattering Microscopy, where I characterize the catalytic activity in terms of turnover frequency of single colloidal Pt nanoparticles trapped inside nanofluidic channels during the H2O2 decomposition reaction. The experiments reveal that ligands covering the particle surface distinctly impact the activity.