(583fe) Correlated Imaging of Single Molecule Turnover Events On Individual Microplates: Number of Turnovers Gradient Versus Activity Gradient

Single-molecule methods have played a central role in biophysics, especially in understanding the kinetics and dynamics of individual enzymes.  The study of enzyme dynamics at the single-molecule level is quite important since there are very few techniques that can be used to extract dynamic information from steady-state systems.  Single-molecule kinetic measurements are extremely useful when minority populations of enzymes or catalytic sites on an inorganic or organometallic catalyst are masked by majority populations.  At the ensemble level, this minority species can never be differentiated from the bulk species.  In heterogeneous catalysis, the minority species can be the responsible entity for the observed behavior (although it would never be detected).  Although the techniques of optical microscopy do not allow visualization of the structure of the active site, the resolution of super-resolved optical microscopy techniques are ~10-20 nm, which is sufficient to resolve individual and simultaneous turnover events (inclusive of reaction and desorption) on large particles.  This enables the spatial extent of individual catalytic turnovers and their potential correlation to be examined since the micron-sized particle can be visualized with standard optical microscopy techniques, and the super-resolution methods can resolve the location of the single molecule turnover event with high resolution.

We have synthesized (111)-terminated Au microplates and mapped spatially and temporally turnover events for the reduction of resazurin to resorufin.  A recent report by Chen and co-workers (Nature Nanotech. 7 (2012) 237) demonstrated over anisotropic Au nanostructures with similar shape that gradients in activity (number of turnovers in a particular spatial extent of the particle) existed.  The areal density of turnovers was greatest at the corner of the Au triangular or hexagonal particles, and an apparent gradient in activity occurs from the center of the particle towards its edges.  Using the larger microplates, we have focused our analysis on understanding the influence of spatial location on the rate of turnover, both in terms of individual waiting times for reaction and diffusion rather than the areal dependence of the number of turnovers.  Additionally, we examine the correlation between catalytic events on spatial and temporal scale using standard statistical analyses, such as autocorrelation functions.  We will demonstrate that rate of turnovers does not necessarily correspond with increased rate of reaction at these locations.  Our results have important implications in fundamental heterogeneous catalysis science and ultimately, catalyst design.  Our results imply that not only the identity (i.e., structure and composition), but also the location of the active site on the nanoparticle surface influence its contribution to the observed macroscopic turnover rate.