This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.
Nonradiative decay of localized surface plasmons results in the production of hot charge carriers and the generation of heat, both of which can affect the efficiency of plasmon-mediated photoelectrochemical processes. Unfortunately, decoupling the impact of each effect on measured photocurrents is extremely challenging because the relative contribution of the two plasmon decay pathways cannot be controlled or easily measured. Here, we present a methodology for exploring the roles of hot carriers and heat generation on plasmon-mediated photoelectrochemical processes using scanning electrochemical microscopy (SECM). Light is used to drive a redox reaction at a plasmonic substrate, while an ultramicroelectrode tip is positioned close to the substrate to read out both the reaction products and the mass transfer rate of the redox species. By controlling the potential at the tip and substrate electrodes, the roles of photoinduced reactions at the substrate and enhanced mass transport to the tip due to local heating can be isolated and investigated independently. We observe enhanced photo-oxidation at the substrate that is due to both plasmon-generated hot holes as well as a thermal-induced change in the equilibrium potential of the redox molecules. The concentration of the reaction products changes as a function of excitation intensity, showing a linear dependence on hot carrier effects and an exponential dependence for thermal effects, and allowing us to quantify the relative contributions of the two plasmon decay pathways to enhanced photo-oxidation. This SECM approach is suitable for probing a variety of photoactive structures used in photovoltaic and photocatalytic devices.
The impact of heterogeneous surface oxide formation on the electrochemical performance of single silver nanoparticles is explored using in situ superlocalization optical microscopy. Silver nanoparticles are well-known to form a natural oxide layer on their surface, but the effect of this oxide layer on electrochemical reactions is not well understood. Here we track the temporal and spatial dependence of electrodissolution of single silver nanoparticles in order to study the role of surface oxide layers on electrochemical reactions. Heterogeneity in electrodissolution kinetics is observed by following the time-dependent loss in scattering intensity from individual silver nanoparticles using dark-field scattering. Both fast and slow dissolution kinetics are observed, with the dominant pathway changing as a function of applied potential. To understand this, superlocalization imaging is employed to follow the spatial variance of the electrodissolution process and reveals that the silver nanoparticles undergo electrodissolution in either a spatially symmetric or asymmetric manner. We hypothesize that asymmetric dissolution events are due to heterogeneity in the exposed silver sites on the nanoparticle surface because of incomplete surface oxide formation. Polarization-resolved measurements support this hypothesis by revealing anisotropic dissolution of the nanoparticles over time. By tracking the electrodissolution behavior of silver nanoparticles in both the temporal and spatial domains, we provide an improved understanding of how heterogeneity in electrochemical reactions is impacted by nanoparticle surface properties.
Controlled three-dimensional positioning of nanoparticles is achieved by delivering single fluorescent nanoparticles from a nanopipette and capturing them at well-defined regions of an electrified substrate. To control the position of single nanoparticles, the force of the pressure-driven flow from the pipette is balanced by the attractive electrostatic force at the substrate, providing a strategy by which nanoparticle trajectories can be manipulated in real time. To visualize nanoparticle motion, a resistive-pulse electrochemical setup is coupled with an optical microscope, and nanoparticle trajectories are tracked in three dimensions using super-resolution fluorescence imaging to obtain positional information with precision in the tens of nanometers. As the particles approach the substrate, the diffusion kinetics are analyzed and reveal either subdiffusive (hindered) or superdiffusive (directed) motion depending on the electric field at the substrate and the pressure-driven flow from the pipette. By balancing the effects of the forces exerted on the particle by the pressure and electric fields, controlled, real-time manipulation of single nanoparticle trajectories is achieved. The developed approach has implications for a variety of applications such as surface patterning and drug delivery using colloidal nanoparticles.
Single-molecule fluorescence microscopy is used to follow dynamic ligand reorganization on the surface of single plasmonic gold nanorods. Fluorescently labeled DNA is attached to gold nanorods via a gold−thiol bond using a low-pH loading method. No fluorescence activity is initially observed from the fluorescent labels on the nanorod surface, which we attribute to a collapsed geometry of DNA on the metal. Upon several minutes of laser illumination, a marked increase in fluorescence activity is observed, suggesting that the ligand shell reorganizes from a collapsed, quenched geometry to an upright, ordered geometry. The ligand reorganization is facilitated by plasmon-mediated photothermal heating, as verified by controls using an external heat source and simulated by coupled optical and heat diffusion modeling. Using super-resolution image reconstruction, we observe spatial variations in which ligand reorganization occurs at the singleparticle level. The results suggest the possibility of nonuniform plasmonic heating, which would be hidden with traditional ensemble-averaged measurements.
Studying multiple simultaneous electrochemical reactions using typical electrochemical methods is challenging, because the measured current is a convolution of concurrent electrochemical reactions. Thus, to monitor multiple simultaneous electrochemical reactions, secondary techniques, such as imaging or spectroscopy are increasingly useful. Herein we use dark-field optical microscopy to visualize the electrodeposition of silver oxide (Ag x O y ) particles using the Ag + ions generated by the concurrent electrodissolution of individual Ag nanoparticles at high anodic potential. We propose that the formation of Ag x O y particles is based on an aggregative growth mechanism, where electrodeposited Ag x O y nanoclusters aggregate over time to form a larger Ag x O y particle. The electrodeposited Ag x O y particles catalyze water oxidation and decrease the local pH, which alters the reaction equilibrium by hindering continued growth of the Ag x O y particles at 1.2 V and consuming the Ag x O y particles and producing Ag + ions at open circuit. Overall the understanding obtained by imaging these reactions is not possible to decode using the measured ensemble current.
Fluorescence microscopy imaging speed is fundamentally limited by the measurement signal-to-noise ratio (SNR). To improve image SNR for a given image acquisition rate, computational denoising techniques can be used to suppress noise. However, common techniques to estimate a denoised image from a single frame either are computationally expensive or rely on simple noise statistical models. These models assume Poisson or Gaussian noise statistics, which are not appropriate for many fluorescence microscopy applications that contain quantum shot noise and electronic Johnson–Nyquist noise, therefore a mixture of Poisson and Gaussian noise. In this paper, we show convolutional neural networks (CNNs) trained on mixed Poisson and Gaussian noise images to overcome the limitations of existing image denoising methods. The trained CNN is presented as an open-source ImageJ plugin that performs real-time image denoising (within tens of milliseconds) with superior performance (SNR improvement) compared to conventional fluorescence microscopy denoising methods. The method is validated on external datasets with out-of-distribution noise, contrast, structure, and imaging modalities from the training data and consistently achieves high-performance ( > 8 d B ) denoising in less time than other fluorescence microscopy denoising methods.
We report a strategy for the optical determination of tip-substrate distance in nanoscale scanning electrochemical microscopy (SECM) using three-dimensional super-resolution fluorescence imaging. A phase mask is placed in the emission path of our dual SECM/optical microscope, generating a double helix point spread function at the image plane, which allows us to measure the height of emitting objects relative to the focus of the microscope. By exciting both a fluorogenic reaction at the nanoscale electrode tip as well as fluorescent nanoparticles at the substrate, we are able to calculate the tip-substrate distance as the tip approaches the surface with precision better than 25 nm. Attachment of a fluorescent particle to the insulating sheath of the SECM tip extends this technique to nonfluorogenic electrochemical reactions. Correlated electrochemical and optical determination of tip-substrate distance yielded excellent agreement between the two techniques. Not only does super-resolution imaging offer a secondary feedback mechanism for measuring the tip-sample gap during SECM experiments, it also enables facile tip alignment and a strategy for accounting for electrode tilt relative to the substrate.
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