Chemical imaging methods that are available or will become available in the future (in italics), for the investigation of catalysts at the single-particle level. The abbreviations of the included characterization techniques are as follows. Vibrational spectroscopy methods (green): DORS, diagonally offset Raman spectroscopy; IRM, infrared microscopy; CRM, confocal Raman microscopy, CARS, coherent anti-Stokes Raman spectroscopy; SRS, stimulated Raman scattering microscopy; SNIM, scanning near-field infrared microscopy; TERS, tip-enhanced Raman spectroscopy. X-ray spectroscopy methods (blue): μ-XAFS, microbeam X-ray absorption fine structure spectroscopy; XRM, X-ray microscopy; XMT, X-ray microtomography; XAS-SNOM, X-ray absorption spectroscopy/scanning near-field optical microscopy. Electronic spectroscopy methods (red): UV-VIS, ultraviolet–visible microscopy; CFM, confocal fluorescence microscopy; 2PFM, two-photon fluorescence microscopy; CLM, chemiluminescence microscopy; PR-DFM, plasmon resonance dark field microscopy; STED, stimulated emission depletion microscopy; STORM, stochastic optical reconstruction microscopy; FPALM, fluorescence photo-activated localization microscopy; SSIM, saturated structured illumination microscopy. X-ray diffraction methods (black): TEDDI, tomographic energy-dispersive diffraction imaging; XRD-CT, X-ray diffraction-computed tomography; μ-XRD/AFM, microbeam X-ray diffraction/atomic force microscopy. Miscellaneous (orange): MRI, magnetic resonance imaging; IFM, interference microscopy; MSI, mass spectrometry imaging; SHG, second-harmonic generation microscopy; MS-SNOM, mass spectrometry/scanning near-field optical microscopy. Adapted with permission from Ref. [4], copyright 2012 Nature Publishing Group

Single-molecule detection of fluorogenic catalytic reactions on single Au nanoparticles. (a) Experimental design using fluorogenic catalytic reaction, surface immobilized catalysts, and total internal reflection fluorescence microscopy; (b) schematic of a prism-based TIRFM and a microfluidic reactor cell made between a slide and a coverslip; (c) a typical image (~18 μm× 18 μm) of fluorescent products on 6 nm pseudospherical Au nanoparticles during catalysis taken at 100 ms per frame; (d) a segment of the fluorescence trajectory from the fluorescence spot marked by the arrow in (c); (e) schematic diagram of the kinetic mechanism of catalysis. Au_m: Au nanoparticle; S: resazurin; P: resorufin. Au_m–S_n represents an Au nanoparticle having n adsorbed substrate molecules. The fluorescence state (on or off) of the nanoparticle is indicated at each reaction stage. Figures 2(a) to 2(d) were reproduced with permission from Ref. [23], copyright 2008 Nature Publishing Group. Figure 2(e) was reproduced with permission from Ref. [29], copyright 2010 American Chemical Society

Super-resolution imaging of single particle nanocatalysis. (a) Fluorogenic reaction converts a nonfluorescent molecule (Amplex Red) to a fluorescent molecule (resorufin) catalyzed by individual Au@mSiO₂ nanorods; (b) fluorescence intensity versus time trajectory for a single Au@mSiO₂ nanorod under catalysis, and wide-field fluorescence pattern during one burst (red circled); (c) subparticle distribution of reactivity map obtained for a gold nanorod. Segmentation of the nanorods clearly shows substrate concentration-dependent turnover rate in different regions; (d) spatially resolved activity quantitation on single Au@mSiO₂ nanoplates; (e) and (f) spatial distributions of fluorescence spots collected from TiO₂ (e) and 14 nm Au/TiO₂ particles (f); (g) super-resolution reaction map showing a single needle like ZSM-22 particle; (h) reconstructed fluorescence maps showing the location of the active intergrowth region on a ZSM-5 crystal; (i) turnover mapping on Ti-MCM-41 particles. Figures 3(a), 3(b), and 3(c) were reproduced with permission from Ref. [34], copyright 2012 Nature Publishing Group. Figure 3(d) was reproduced with permission from Ref. [35], Figs. 3(e) and 3(f) were reproduced with permission from Ref. [40], copyright 2013 American Chemical Society. Figures 3(g), 3(h), and 3(i) were reproduced with permission from Refs. [41,42], copyright 2009, 2010 Wiley

Surface/tip-enhanced Raman spectroscopy for single particle nanocatalysis. (a) Scanning electron microscopy (SEM) image of a single roughened Ag microsphere with nanostructured surface; (b) photo of the gas flow cell for monitoring the surface plasmon assisted catalysis reaction under controlled gas atmosphere; (c) structure schematic of the designed reaction station; (d) time-dependent SERS spectra of 4ATP under continuous 633 nm laser excitation taken every 1 min, and series shown as a color-coded intensity map; (e) schematic overview of the experimental setup for TERS combining confocal Raman and atomic force microscope; (f) time-dependent SERS spectra acquired with the setup for TERS of the photocatalytic reduction of pNTP (top spectrum) to DMAB, and series shown as a color-coded intensity map. Figures 4(a), 4(b), and 4(c) were reproduced with permission from Ref. [54], copyright 2013 Nature Publishing Group. Figures 4(d) and 4(e) were reproduced with permission from Ref. [55], copyright 2012 Nature Publishing Group

Direct localized surface plasmon resonance monitoring of single plasmonic particle nanocatalysis. (a) Scanning electron microscopy (SEM) image of gold decahedron; (b) scattering spectra of nanoparticle vs. time after electron injection by ascorbate ions; (c) spectral shift as a function of time for the catalysis reaction and for the control experiment; (d) evolution of the scattering spectrum of a gold nanorod (aspect ratio 2.87) after a growth solution is added; (e) spectral shift as a function of time for a growth experiment and for a control experiment; (f) predicted surface plasmon position versus aspect ratio for hemispherically capped rods based on DDA results; (g) scheme for the reduction of 4-nitrophenol, catalyzed by gold nanoparticles with ammonia borane as the reducing agent. AB: ammonia borane; 4-NIP: 4-nitrophenol; 4-AMP: 4-aminophenol; (h) and (i) evolution of the scattering spectrum and surface plasmon band position of a gold nanorod (h) and an elongated tetrahexahedral gold nanoparticle (i) as a function of time after introducing water, 10 mM¹⁾<FootNote>

1 mM= 1 mmol?L^–6

</FootNote> AB, and 0.1 mM 4-NIP, sequentially. Figures 5(a) to 5(f) were reproduced with permission from Ref. [67], copyright 2008 Nature Publishing Group. Figures 5(g), 5(h), and 5(i) were reproduced with permission from Ref. [69], copyright 2013 Royal Society of Chemistry

Indirect localized surface plasmon resonance monitoring of single particle nanocatalysis. (a) Hydrogen adsorption on a Pd nanoparticle induces minimal wavelength shift in its scattering spectrum; (b) hydrogen sensing with a plasmonic Au nanostructure-antenna enhanced single Pd nanoparticle; (c) maximum scattering wavelength from the Au antenna depends on hydrogen partial pressure; (d) surface plasmon resonance electrochemical current density images of a single Pt nanoparticle at different potentials; (e) cyclic voltammogram of the same single nanoparticle obtained by integrating the current density over the images in (d); (f) typical cyclic voltammograms of three different single Pt nanoparticles. Figures 6(a) to 6(c) were reproduced with permission from Ref. [72], Figs. 6(d) to 6(f) were reproduced with permission from Ref. [77], copyright 2011, 2012 Nature Publishing Group

Indirect localized surface plasmon resonance monitoring of single particle nanocatalysis. (a) Hydrogen adsorption on a Pd nanoparticle induces minimal wavelength shift in its scattering spectrum; (b) hydrogen sensing with a plasmonic Au nanostructure-antenna enhanced single Pd nanoparticle; (c) maximum scattering wavelength from the Au antenna depends on hydrogen partial pressure; (d) surface plasmon resonance electrochemical current density images of a single Pt nanoparticle at different potentials; (e) cyclic voltammogram of the same single nanoparticle obtained by integrating the current density over the images in (d); (f) typical cyclic voltammograms of three different single Pt nanoparticles. Figures 6(a) to 6(c) were reproduced with permission from Ref. [72], Figs. 6(d) to 6(f) were reproduced with permission from Ref. [77], copyright 2011, 2012 Nature Publishing Group

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