Kinetics of self-assembled monolayer formation on individual nanoparticles

A rich catalog of C-C bonded species formed in CO2 reduction on a plasmonic photocatalyst

The understanding and rational design of heterogeneous catalysts for complex reactions, such as CO₂ reduction, requires knowledge of elementary steps and chemical species prevalent on the catalyst surface under operating conditions. Using in situ nanoscale surface-enhanced Raman scattering, we probe the surface of a Ag nanoparticle during plasmon-excitation-driven CO₂ reduction in water. Enabled by the high spatiotemporal resolution and surface sensitivity of our method, we detect a rich array of C₁-C₄ species formed on the photocatalytically active surface. The abundance of multi-carbon compounds, such as butanol, suggests the favorability of kinetically challenging C-C coupling on the photoexcited Ag surface. Another advance of this work is the use of isotope labeling in nanoscale probing, which allows confirmation that detected species are the intermediates and products of the catalytic reaction rather than spurious contaminants. The surface chemical knowledge made accessible by our approach will inform the modeling and engineering of catalysts.

Formation of HCN+ in collisions of N+ and N2+ with a self-assembled propanethiol surface on gold

Collisions of N⁺ and N₂⁺ with C3 hydrocarbons, represented by a self assembled monolayer of propanethiol on a polcrystalline gold surface, were investigated by experiments over the incident energy range between 5 eV and 100 eV. For N⁺, formation of HCN⁺ is observed at incident energies of projectile ions as low as 20 eV. In the case of N₂⁺ projectile ions, the yield of HCN⁺ increased above zero only at incident energies of about 50 eV. This collision energy in the laboratory frame corresponds to an activation energy of about 3 eV to 3.5 eV. In the case of N⁺ projectile ions, the yield of HCN⁺ was large for most of the incident energy range, but decreased to zero at incident energies below 20 eV. This may indicate a very small energy threshold for the surface reaction between N⁺ and C3 hydrocarbons of a few tenths of an eV. Such a threshold for the formation of HCN⁺ may exist also for collisions of N⁺ with an adsorbed mixture of hydrocarbon molecules.

Effect of Concentration, Chain Length, Hydrophobicity, and an External Electric Field on the Growth of Mixed Alkanethiol Self-Assembled Monolayers: A Molecular Dynamics Study

Growing functionalized self-assembled monolayers (SAMs) with fewer defects and lower cost is the focus of ongoing investigations. In the present study, molecular dynamics simulations were performed to investigate the process of SAM formation on a gold substrate from mixed alkanethiolates in ethanol solution. Using the mixed-SAM system of 11-mercaptoundecanoic acid (MUA) with either 1-decanethiol (C9CH3) or 6-mercaptohexanol (C6OH) in a 3:7 ratio as the standard SAM model, we systematically investigated the effects of the concentration, chain length, functional group, and an external electric field on SAM growth. The results showed that the initial growth rate and surface coverage of the SAM are dependent on the ligand concentration. At a certain high concentration (about 1.2-1.5 times the minimum concentration), the final surface coverage is optimal. Reducing the chain length and increasing the proportion of hydrophobic diluting molecules are effective ways to improve the surface coverage, but the compositional ligands have to be changed, which may not be desirable for the functional requirements of SAMs. Furthermore, by investigating the behavior of the alkanethiolates and ethanol solvent under an applied external field, we find that a strong electric field with a proper field direction can facilitate the generation of defect-free monolayers. These findings will contribute to the understanding of mixed-SAM formation and provide insight into experimental design for efficient and effective SAM formation.

Dissection of Interaction Kinetics through Single-Molecule Interaction Simulation

The ability to extract kinetic interaction parameters from single-molecule fluorescence resonance energy transfer trajectories without the need for solving complex single-molecule differential equations has the potential to address some of the critical biophysical questions. Here, we provide a noise-free single-molecule interaction simulation (SMIS) tool to give the expected dwell-time distributions and relative populations of each FRET level based on the assigned kinetic model and to dissect kinetic interaction parameters from single-molecule FRET trajectories. The method provides the expected dwell-time distributions, average transition rates, and relative populations of each FRET level based on the assigned kinetic model. By comparing with ground truth data and experimental data, we demonstrated that SMIS is useful to quantify the interaction kinetic rate constants without using the traditional single-molecule analytical solution approach.

Nanoscale optical imaging in chemistry

Single-molecule-level measurements are bringing about a revolution in our understanding of chemical and biochemical processes. Conventional measurements are performed on large ensembles of molecules. Such ensemble-averaged measurements mask molecular-level dynamics and static and dynamic fluctuations in reactivity, which are vital to a holistic understanding of chemical reactions. Watching reactions on the single-molecule level provides access to this otherwise hidden information. Sub-diffraction-limited spatial resolution fluorescence imaging methods, which have been successful in the field of biophysics, have been applied to study chemical processes on single-nanoparticle and single-molecule levels, bringing us new mechanistic insights into physiochemical processes. However, the scope of chemical processes that can be studied using fluorescence imaging is considerably limited; the chemical reaction has to be designed such that it involves fluorophores or fluorogenic probes. In this article, we review optical imaging modalities alternative to fluorescence imaging, which expand greatly the range of chemical processes that can be probed with nanoscale or even single-molecule resolution. First, we show that the luminosity, wavelength, and intermittency of solid-state photoluminescence (PL) can be used to probe chemical transformations on the single-nanoparticle-level. Next, we highlight case studies where localized surface plasmon resonance (LSPR) scattering is used for tracking solid-state, interfacial, and near-field-driven chemical reactions occurring in individual nanoscale locations. Third, we explore the utility of surface- and tip-enhanced Raman scattering to monitor individual bond-dissociation and bond-formation events occurring locally in chemical reactions on surfaces. Each example has yielded some new understanding about molecular mechanisms or location-to-location heterogeneity in chemical activity. The review finishes with new and complementary tools that are expected to further enhance the scope of knowledge attainable through nanometer-scale resolution chemical imaging.

3DStructGen: an interactive web-based 3D structure generation for non-periodic molecule and crystal

BACKGROUND

The increasing number of organic and inorganic structures promotes the development of the "Big Data" in chemistry and material science, and raises the need for cross-platform and web-based methods to search, view and edit structures. Many web-based three-dimensional (3D) structure tools have been developed for displaying existing models, building new models, and preparing initial input files for external calculations. But few of these tools can deal with crystal structures.

RESULTS

We developed a user-friendly and versatile program based on standard web techniques, such as Hyper Text Markup Language 5 (HTML5), Cascade Style Sheet (CSS) and JavaScript. Both non-periodic organic molecule and crystal structure can be visualized, built and edited interactively. The atom, bond, angle and dihedral in a molecule can be viewed and modified using sample mouse operations. A wide range of cheminformatics algorithms for crystal structure are provided, including cleaving surfaces, establishing vacuum layers, and building supercells. Four displayed styles, namely "Primitive cell", "Original", "In-cell" and "Packing" can be used to visualize a unit cell. Additionally, the initial input files for Vienna Ab-initio Simulation Package (VASP) and Gaussian can be obtained by interacting with dialog boxes in 3DStructGen.

CONCLUSIONS

3DStructGen is a highly platform-independent program. It can provide web service independently or can be integrated into other web platforms. Other than local desktop software, it does not require any additional effort to install the system but a web browser supporting HTML5. 3DStructGen may play a valuable role in online chemistry education and pre-processing of quantum calculations. The program has been released under MIT open-source license and is available on: https://matgen.nscc-gz.cn/Tools.html.

Large patternable metal nanoparticle sheets by photo/e-beam lithography

Techniques for micro/nano-scale patterning of large metal nanoparticle sheets can potentially be used to realize high-performance photoelectronic devices because the sheets provide greatly enhanced electrical fields around the nanoparticles due to localized surface plasmon resonances. However, no single metal nanoparticle sheet currently exists with sufficient durability for conventional lithographical processes. Here, we report large photo and/or e-beam lithographic patternable metal nanoparticle sheets with improved durability by incorporating molecular cross-linked structures between nanoparticles. The cross-linked structures were easily formed by a one-step chemical reaction; immersing a single nanoparticle sheet consisting of core metals, to which capping molecules ionically bond, in a dithiol ethanol solution. The ligand exchange reaction processes were discussed in detail, and we demonstrated 20 μm wide line and space patterns, and a 170 nm wide line of the silver nanoparticle sheets.