Geometry-Assisted Three-Dimensional Superlocalization Imaging of Single-Molecule Catalysis on Modular Multilayer Nanocatalysts

Native diffusion of fluorogenic turn-on dyes accurately report interfacial chemical reaction locations

Single-molecule fluorescence microscopy with "turn-on" dyes that change fluorescent state after a reaction report on the chemistry of interfaces relevant to analytical and bioanalytical chemistry. Paramount to accurately understanding the phenomena at the ultimate detection limit of a single molecule is ensuring fluorophore properties such as diffusion do not obscure the chemical reaction of interest. Here, we develop Monte Carlo simulations of a dye that undergoes reduction to turn-on at the cathode of a corroded iron surface taking into account the diffusion of the dye molecules in a total internal reflection fluorescence (TIRF) excitation volume, location of the cathode, and chemical reactions. We find, somewhat counterintuitively, that a fast diffusion coefficient of D = 10⁸ nm²/s, corresponding to the dye in aqueous solution, accurately reports the location of single reaction sites. The dyes turn on and are present for the acquisition of a single frame allowing for localization before diffusing out of the thin TIRF excitation volume axially. Previously turned-on (i.e., activated) dyes can also randomly hit the surface surrounding the reaction site leading to a uniform increase in the background. Using concentrations that lead to high turnover rates at the reaction site can achieve signal-to-background ratios of  ~100 in our simulation. Therefore, the interplay between diffusion, turn-on reaction rate, and concentration of the dye must be strategically considered to produce accurate images of reaction locations. This work demonstrates that modeling can assist in the design of single-molecule microscopy experiments to understand interfaces related to analytical chemistry such as electrode, nanoparticle, and sensor surfaces.

Superresolved Motions of Single Molecular Catalysts during Polymerization Show Wide Distributions

The motion of single molecular ruthenium catalysts during and after single turnover events of ring-opening metathesis polymerization is imaged through single-molecule superresolution tracking with a positional accuracy of ±32 nm. This tracking is achieved through the real-time incorporation of spectrally tagged monomer units into active polymer chain ends during living polymerization; thus, by design, only active-catalyst motion is detected and imaged, without convolution by inactive catalysts. The catalysts show diverse individualistic diffusive behaviors with respect to time that persist for up to 20 s. Catalysts occupy three mobility populations: quasi-stationary (23%), intermediate (53%, 65 nm), and large (24%, 145 nm) step sizes. Differences in catalyst mobility populations also exist between individual aggregates (p < 0.001). Such differential motion indicates widely different local catalyst microenvironments during the catalytic turnover. These mobility differences are uniquely observable through single-catalyst microscopy and are not measurable through traditional ensemble analytical techniques for characterizing the behavior of molecular catalysts, such as nuclear magnetic resonance spectroscopy. The measured distributions of active molecular catalyst motions would not be readily predictable through modeling or first-principles, and the range likely impacts individual catalyst turnover rate and selectivity. This range plausibly contributes to property distributions observable in bulk polymers, such as molecular weight polydispersity (e.g., 1.9 in this system), leading to a revised understanding of the mechanistic, microscale origins of macroscale polymer properties.

A microwell array-based approach for studying single nanoparticle catalysis with high turnover frequency

Measuring the catalytical activities of single catalysts in the case of high turnover frequency (TOF, realistic conditions) is highly desirable to accurately evaluate the functional heterogeneities among individuals and to understand the catalytic mechanism. Herein, we report a microwell array-based method to in operando measure the photocatalytic kinetics of single CdS nanoparticles (NPs) with high TOF. This was realized by sealing individual CdS NPs into separated micrometer-sized polydimethylsiloxane wells, thus eliminating the diffusion of products among individuals in the case of high concentration of reactants. This method allowed us to monitor the activities of single catalysts with an average TOF up to 2.1 × 10⁵ s^-1. Interestingly, two types of catalytical behaviors were revealed during single CdS photocatalysis: a rapid decline in activity for most CdS NPs and an initial increase in activity followed by a decrease for a minor population of individuals. The developed method will facilitate the investigation of catalytic activities of single particles under realistic conditions and hold great potential in the fields of photo/electro-catalysts, enzymes, functional bacteria, and so on.

Single-Particle Catalytic Analysis by a Photon Burst Counting Technique Combined with a Microfluidic Chip

Single-particle catalytic analysis plays an important role to understand the catalytic mechanism of nanocatalysts. Currently, some methods are used to study the relationship between single-particle catalytic activity and morphology. However, there is still lack of a simple and rapid analysis method for evaluating the catalytic activity of an individual nanocatalyst that freely moves in solution. Here, we proposed a novel single-particle catalytic analysis method for investigating the catalytic activity of a free nanocatalyst. Its working principle is based on the photon burst counting analysis on fluorescent catalytic products of an individual nanocatalyst combined with a microfluidic chip. In this study, we used the reduction reaction of resazurin (RZ) to resorufin (RF) catalyzed by gold nanoparticles (GNPs) as a model. When nonfluorescent RZ molecules in one microchannel of the microfluidic chip mixed with the GNPs flowing in another channel under the control of flow rates, each individual photon burst from the catalytic product RF by GNPs was measured in real time with a constructed flow single-particle catalytic analysis (SPCA) system. With the method, the obtained intensity of each photon burst reflects the capacity of a particle to catalyze RZ molecules into RF(s). The number of photon burst within sampling time reflects the particle number of GNPs with catalytic activity. The experimental conditions including the mixing mode of the nanocatalyst and the substrate, the flow rate, RZ concentration, and detection time were optimized. Finally, the method was successfully used to study the catalytic activity of GNPs with different sizes and morphologies.

Vertical Diffusion of Ions within Single Particles during Electrochemical Charging

Determining the trajectory of ionic transport and diffusion within single electroactive nanomaterials is critical for understanding the charging kinetics and capacity fading associated with ion batteries, with implications for rational design of excellent-performance electrode materials. While the horizontal pathway of mass transport has been feasibly investigated by optical superlocalization methods and electron microscopes, determination on the vertical trajectory has proven a more challenging task. Herein, we developed dual-angle total internal reflection microscopy by simultaneously introducing different angle-dependent illumination depths to trace the optical centroid shifts of nano-objects in the vertical dimension. We first demonstrated the proof of concept by resolving the vertical moving trails of a nanosphere doing Brownian motion and subsequently explored the picture of mass transport in the interior of single Prussian blue (PB) particles during electrochemical cycling. The results indicated that the vertical centroids of single PB particles remained unchanged when ions were inserted or extracted, suggesting an outside-in ionic transport pathway instead of bottom-up trajectory that one would intuitively expect.

Single Molecule Investigation of Nanoconfinement Hydrophobicity in Heterogeneous Catalysis

Nanoconfinement imposes physical constraints and chemical effects on reactivity in nanoporous catalyst systems. In the present study, we lay the groundwork for quantitative single-molecule measurements of the effects of chemical environment on heterogeneous catalysis in nanoconfinement. Choosing hydrophobicity as an exemplary chemical environmental factor, we compared a range of essential parameters for an oxidation reaction on platinum nanoparticles (NPs) confined in hydrophilic and hydrophobic nanopores. Single-molecule experimental measurements at the single particle level showed higher catalytic activity, stronger adsorption strength, and higher activation energy in hydrophobic nanopores than those in hydrophilic nanopores. Interestingly, different dissociation kinetic behaviors of the product molecules in the two types of nanopores were deduced from the single-molecule imaging data.

Three-Dimensional Single Particle Tracking and Its Applications in Confined Environments

Single particle tracking (SPT) has proven to be a powerful technique in studying molecular dynamics in complicated systems. We review its recent development, including three-dimensional (3D) SPT and its applications in probing nanostructures and molecule-surface interactions that are important to analytical chemical processes. Several frequently used 3D SPT techniques are introduced. Especially of interest are those based on point spread function engineering, which are simple in instrumentation and can be easily adapted and used in analytical labs. Corresponding data analysis methods are briefly discussed. We present several important case studies, with a focus on probing mass transport and molecule-surface interactions in confined environments. The presented studies demonstrate the great potential of 3D SPT for understanding fundamental phenomena in confined space, which will enable us to predict basic principles involved in chemical recognition, separation, and analysis, and to optimize mass transport and responses by structural design and optimization.

Tracking Sub-Nanometer Shift in the Scattering Centroid of Single Gold Nanorods during Electrochemical Charging

While conventional wisdom suggests the scattering centroid of a plasmonic nanoparticle reflects its geometric center, here we uncover the dependence of a scattering centroid of a single gold nanorod (AuNR) on its electron density when the geometric features (position and morphology) do not change at all. When periodically altering the electron density of a single AuNR during nonfaradaic charging and discharging processes, the optical centroid of the scattering dot in a series of dark-field images was found to reversibly shift back and forth by ∼0.4 nm, in pace with the sweeping potential. A Fourier-transform-based demodulation method was proposed to determine the centroid displacement as small as 0.1 nm, allowing for validating the generality of the observed phenomenon. The dependence of an optical centroid on the potential was attributed to the displacement of the electron density center as a result of inhomogeneous accumulation of injected electrons on the surface of a single AuNR. Not only does the present work shed light on studying the photon-electron interactions at sub-nanoparticle level, Fourier transform-based demodulation also provides a superior strategy for other fast and reversible processes such as electrochromic and photothermal conversions.

Imaging the chemical activity of single nanoparticles with optical microscopy

Chemical activity of single nanoparticles can be imaged and determined by monitoring the optical signal of each individual during chemical reactions with advanced optical microscopes. It allows for clarifying the functional heterogeneity among individuals, and for uncovering the microscopic reaction mechanisms and kinetics that could otherwise be averaged out in ensemble measurements.

Bimetallic Effect of Single Nanocatalysts Visualized by Super-Resolution Catalysis Imaging

Compared with their monometallic counterparts, bimetallic nanoparticles often show enhanced catalytic activity associated with the bimetallic interface. Direct quantitation of catalytic activity at the bimetallic interface is important for understanding the enhancement mechanism, but challenging experimentally. Here using single-molecule super-resolution catalysis imaging in correlation with electron microscopy, we report the first quantitative visualization of enhanced bimetallic activity within single bimetallic nanoparticles. We focus on heteronuclear bimetallic PdAu nanoparticles that present a well-defined Pd-Au bimetallic interface in catalyzing a photodriven fluorogenic disproportionation reaction. Our approach also enables a direct comparison between the bimetallic and monometallic regions within the same nanoparticle. Theoretical calculations further provide insights into the electronic nature of N-O bond activation of the reactant (resazurin) adsorbed on bimetallic sites. Subparticle activity correlation between bimetallic enhancement and monometallic activity suggests that the favorable locations to construct bimetallic sites are those monometallic sites with higher activity, leading to a strategy for making effective bimetallic nanocatalysts. The results highlight the power of super-resolution catalysis imaging in gaining insights that could help improve nanocatalysts.

Recent progress on single-molecule nanocatalysis based on single-molecule fluorescence microscopy

Understanding the heterogeneous catalytic properties of nanoparticles is of great significance for the development of high efficient nanocatalysts, but the intrinsic heterogeneities of nanocatalysts were always covered in traditional ensemble studies. This issue can be overcome if one can follow the catalysis of individual nanoparticles in real time. This paper mainly summarizes recent developments in single-molecule nanocatalysis at single particle level in Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. These developments include the revealing of catalytic kinetics of different types (plane & edge) of surface atoms on individual Pd nanocubes, the observing of in situ deactivation of individual carbon-supported Pt nanoparticles during the electrocatalytic hydrogen-oxidation reaction, and the measurement of catalytic activation energies on single nanocatalysts for both product formation process and dissociation process, etc. These studies further indicate the advantages or unique abilities of single-molecule methods in the studies of nanocatalysis or even chemical reactions.

Consecutive Reaction to Construct Hierarchical Nanocrystalline CuS "Branch" with Tunable Catalysis Properties

New CuS nanocrystals with a 3D hierarchical branched structure are successfully synthesized through in situ consecutive reaction method with copper foam as template. The formation mechanism of the 3D hierarchical branched structure obtained from the secondary reaction is investigated by adjusting the reaction time. The morphology of CuS nanosheet arrays with the 3D hierarchical branched structure is changed through Cu(2+) exchange. In this method, the copper foam reacted completely, and the as-synthesized CuS@Cu9S5 nanocrystals are firmly grown on the surface of the 3D framework. This tunable morphology significantly influence the physical and chemical properties, particularly catalytic performance, of the materials. The as-obtained material of Cu@CuS-2 with the 3D hierarchical branched structure as catalyst for methylene blue degradation exhibits good catalytic performance than that of the material of Cu@CuS with 2D nanosheets in dark environment. Furthermore, the cation exchange between Cu and Cu(2+) indicates that Cu(2+) in wastewater could be absorbed by Cu@CuS-2 with the 3D hierarchical branched structure. The exchanged resultant of CuS@Cu9S5 retains its capability to degrade organic dyes. This in situ consecutive reaction method may have a significant impact on controlling the crystal growth direction of inorganic material.

Single-Molecule Investigation of Initiation Dynamics of an Organometallic Catalyst

The action of molecular catalysts comprises multiple microscopic kinetic steps whose nature is of central importance in determining catalyst activity and selectivity. Single-molecule microscopy enables the direct examination of these steps, including elucidation of molecule-to-molecule variability. Such molecular diversity is particularly important for the behavior of molecular catalysts supported at surfaces. We present the first combined investigation of the initiation dynamics of an operational palladium cross-coupling catalyst at the bulk and single-molecule levels, including under turnover conditions. Base-initiated kinetics reveal highly heterogeneous behavior indicative of diverse catalyst population. Unexpectedly, this distribution becomes more heterogeneous at increasing base concentration. We model this behavior with a two-step saturation mechanism and identify specific microscopic steps where chemical variability must exist in order to yield observed behavior. Critically, we reveal how structural diversity at a surface translates into heterogeneity in catalyst behavior, while demonstrating how single-molecule experiments can contribute to understanding of molecular catalysts.

An inorganic capping strategy for the seeded growth of versatile bimetallic nanostructures

Metal nanostructures have attracted great attention in various fields due to their tunable properties through precisely tailored sizes, compositions and structures. Using mesoporous silica (mSiO2) as the inorganic capping agent and encapsulated Pt nanoparticles as the seeds, we developed a robust seeded growth method to prepare uniform bimetallic nanoparticles encapsulated in mesoporous silica shells (PtM@mSiO2, M = Pd, Rh, Ni and Cu). Unexpectedly, we found that the inorganic silica shell is able to accommodate an eight-fold volume increase in the metallic core by reducing its thickness. The bimetallic nanoparticles encapsulated in mesoporous silica shells showed enhanced catalytic properties and thermal stabilities compared with those prepared with organic capping agents. This inorganic capping strategy could find a broad application in the synthesis of versatile bimetallic nanostructures with exceptional structural control and enhanced catalytic properties.

Preparation of yolk-shell structured copper oxide@silica oxide spheres and their application in high performance electrochemical sensing of Formoterol fumarate residues in swine feed and tissues

In this paper, we report a facile route to synthesize yolk-shell structured copper oxide@silica oxide (CuO@SiO2) spheres and their application to construct an electrochemical Formoterol fumarate (FF) sensor. The CuO@SiO2 was characterized by means of Fourier transform infrared spectroscopy, X-ray powder diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. Further, FF was electrocatalytically oxidized at the CuO@SiO2 film modified glassy carbon electrode (GCE), which led to a sensitive determination of FF. The oxidation current of FF was linear with concentration in the range of 0.030-10 μM and the detection limit was found to be 5.0 nM (S/N = 3). The observed analytical parameters such as wide linear range, low detection limit and short response time were superior to previously reported FF sensors. Finally, it was demonstrated that the proposed sensor could be used for the selective determination of FF present in swine feed and tissues.