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Plasmonic photocatalysis uses the light-induced resonant oscillation of free electrons in a metal nanoparticle to concentrate optical energy for driving chemical reactions. By altering the joint electronic structure of the catalyst and reactants, plasmonic catalysis enables reaction pathways with improved selectivity, activity, and catalyst stability. However, designing an optimal catalyst still requires a fundamental understanding of the underlying plasmonic mechanisms at the spatial scales of single particles, at the temporal scales of electron transfer, and in conditions analogous to those under which real reactions will operate. Thus, in this review, we provide an overview of several of the available and developing nanoscale and ultrafast experimental approaches, emphasizing those that can be performed in situ. Specifically, we discuss high spatial resolution optical, tip-based, and electron microscopy techniques; high temporal resolution optical and x-ray techniques; and emerging ultrafast optical, x-ray, tip-based, and electron microscopy techniques that simultaneously achieve high spatial and temporal resolution. Ab initio and classical continuum theoretical models play an essential role in guiding and interpreting experimental exploration, and thus, these are also reviewed and several notable theoretical insights are discussed. 

The presence of dark states causes fluorescence intermittency of single molecules due to transitions between “on” and “off” states. Genetically encodable markers such as fluorescent proteins (FPs) exhibit dark states that make several super-resolved single-molecule localization microscopy (SMLM) methods possible. However, studies quantifying the timescales and nature of dark state behavior for commonly used FPs under conditions typical of widefield and total internal reflection fluorescence (TIRF) microscopy remain scarce and pre-date many new SMLM techniques. FusionRed is a relatively bright red FP exhibiting fluorescence intermittency and has thus been identified as a potential candidate for SMLM. We herein characterize the rates for dark-state conversion and the subsequent ground-state recovery of FusionRed and its 2.5-fold brighter descendent FusionRed L175M M42Q (FusionRed-MQ) at low irradiances (1–10 W cm −2 ), which were previously unexplored experimental conditions. We characterized the kinetics of dark state transitions in these two FPs by using single molecule blinking and ensemble photobleaching experiments bridged with a dark state kinetic model. We find that at low irradiances, the recovery process to the ground state is minimally light-driven and FusionRed-MQ has a 1.3-fold longer ground state recovery time indicating a conformationally restricted dark-state chromophore in comparison to FusionRed. Our studies indicate that the brighter FusionRed-MQ variant exhibits higher dark state conversion rates with longer ground state recovery lifetimes, thus it is potentially a better candidate for SMLM applications than its progenitor FusionRed.