Selective C-H Bond Activation in Propane with Molecular Oxygen over Cu(I)-ZSM-5 at Ambient Conditions.

Selective activation of C-H bonds in light alkanes under mild conditions is challenging but holds the promise of efficient upgrading of abundant hydrocarbons. In this work, we report the conversion of propane to propylene with ∼95% selectivity on Cu(I)-ZSM-5 with O2 at room temperature and pressure. The intraporous Cu(I) species was oxidized to Cu(II) during the reaction but could be regenerated with H2 at 220 °C. Diffuse reflectance ultraviolet spectroscopy indicated the presence of both Cu+-O2 and Cu2(µ-O2)2+ species in the zeolite pores during the reaction, and electron paramagnetic resonance results showed that propane activation occurred via a radical-mediated pathway distinct from that with H2O2 as the oxidant. Correlation between spectroscopic and reactivity results on Cu(I)-ZSM-5 with different Cu loadings suggests that the isolated intraporous Cu(I) species is the main active species in propane activation.

Enhancing Hydrogen Oxidation and Evolution Kinetics by Tuning the Interfacial Hydrogen-Bonding Environment on Functionalized Platinum Surfaces.

Developing efficient catalytic systems for the hydrogen oxidation and evolution reactions (HOR/HER) is essential in the world's transition to renewable energy. There is a growing recognition that the HOR/HER activity depends on properties of the electrochemical interface, rather than just the composition and structure of the catalyst. Herein, we demonstrate that specifically adsorbed organic additives (theophylline derivatives) could enhance the intrinsic HOR/HER activity in base on polycrystalline Pt by up to a factor of 3 via introducing weakly hydrogen-bonded water, as confirmed by in situ surface enhanced infrared and Raman spectroscopies. Optimal HOR/HER activity is achieved on a 7-n-butyltheophylline decorated Pt surface, which sufficiently disrupts the hydrogen bonding network in the double layer without depleting the interfacial water. This work demonstrates the promise of electrochemical interfacial engineering as a strategy to boost electrocatalytic performance.

Probing the Local Generation and Diffusion of Active Oxygen Species on a Pd/Au Bimetallic Surface by Tip-Enhanced Raman Spectroscopy.

Active oxygen species (AOS) play key roles in many important catalytic reactions relevant to clean energy and environment. However, it remains challenging to characterize the active sites for producing AOS and to image the surface properties of AOS, especially on multicomponent metallic catalyst surfaces. Herein, we utilize tip-enhanced Raman spectroscopy (TERS) to probe the local generation and diffusion of OH radicals on a Pd/Au(111) bimetallic catalyst surface. The reactive OH radicals can be catalytically generated from hydrogen peroxide (H2O2) at the metal surface, which then oxidizes the surface adsorbed thiolate, a reactant that is used as the TERS probe. By TERS imaging of the spatial distribution of unreacted thiolate molecules, we demonstrate that the Pd surface is active for generation of OH radicals and the Pd step edge shows much higher activity than the Pd terrace, whereas the Au surface is inactive. Furthermore, we find that the locally generated OH radicals at the Pd step edge could diffuse to both the Au and the Pd surface sites to induce oxidative reactions, with a diffusion length estimated to be about 5.4 nm. Our TERS imaging with few-nanometer spatial resolution not only unravels the active sites but also characterizes in real space the diffusion behavior of OH radicals. The results are highly valuable to understand AOS-triggered catalytic reactions. The strategy of using reactants with large Raman cross sections as TERS probes may broaden the application of TERS for studying catalysis with reactive small molecules.

Tip-enhanced Raman spectroscopy for nanoscale probing of dynamic chemical systems.

Dynamics are fundamental to all aspects of chemistry and play a central role in the mechanism and product distribution of a chemical reaction. All dynamic processes are influenced by the local environment, so it is of fundamental and practical value to understand the structure of the environment and the dynamics with nanoscale resolution. Most techniques for measuring dynamic processes have microscopic spatial resolution and can only measure the average behavior of a large ensemble of sites within their sampling volumes. Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for overcoming this limitation due to its combination of high chemical specificity and spatial resolution that is on the nanometer scale. Adapting it for the study of dynamic systems remains a work in progress, but the increasing sophistication of TERS is making such studies more routine, and there are now growing efforts to use TERS to examine more complex processes. This Perspective aims to promote development in this area of research by highlighting recent progress in using TERS to understand reacting and dynamic systems, ranging from simple model reactions to complex processes with practical applications. We discuss the unique challenges and opportunities that TERS presents for future studies.

Tip-enhanced Raman spectroscopy for surfaces and interfaces.

Surface and interfaces play key roles in heterogeneous catalysis, electrochemistry and photo(electro)chemistry. Tip-enhanced Raman spectroscopy (TERS) combines plasmon-enhanced Raman spectroscopy with scanning probe microscopy to simultaneously provide a chemical fingerprint and morphological information for the sample at the nanometer spatial resolution. It is an ideal tool for achieving an in-depth understanding of the surface and interfacial processes, so that the relationship between structure and chemical performance can be established. We begin with the background of surfaces and interfaces and TERS, followed by a detailed discussion on some issues in experimental TERS, including tip preparation and TERS instrument configuration. We then focus on the progress of TERS for studying the surfaces and interfaces under different conditions, from ambient, to UHV, solid-liquid and electrochemical environments, followed by a brief introduction to the current understanding of the unprecedented high spatial resolution and surface selection rules. We conclude by discussing the future challenges for TERS practical applications in surfaces and interfaces.

Tip-Enhanced Raman Spectroscopy with High-Order Fiber Vector Beam Excitation.

We investigated tip-enhanced Raman spectra excited by high-order fiber vector beams. Theoretical analysis shows that the high-order fiber vector beams have stronger longitudinal electric field components than linearly polarized light under tight focusing conditions. By introducing the high-order fiber vector beams and the linearly polarized beam from a fiber vector beam generator based on an electrically-controlled acoustically-induced fiber grating into a top-illumination tip-enhanced Raman spectroscopy (TERS) setup, the tip-enhanced Raman signal produced by the high-order fiber vector beams was 1.6 times as strong as that produced by the linearly polarized light. This result suggests a new type of efficient excitation light beams for TERS.

Real-Space Observation of Atomic Site-Specific Electronic Properties of a Pt Nanoisland/Au(111) Bimetallic Surface by Tip-Enhanced Raman Spectroscopy.

Resolving atomic site-specific electronic properties and correlated substrate-molecule interactions is challenging in real space. Now, mapping of sub-10 nm sized Pt nanoislands on a Au(111) surface was achieved by tip-enhanced Raman spectroscopy, using the distinct Raman fingerprints of adsorbed 4-chlorophenyl isocyanide molecules. A spatial resolution better than 2.5 nm allows the electronic properties of the terrace, step edge, kink, and corner sites with varying coordination environments to be resolved in real space in one Pt nanoisland. Calculations suggest that low-coordinate atomic sites have a higher d-band electronic profile and thus stronger metal-molecule interactions, leading to the observed blue-shift of Raman frequency of the N≡C bond of adsorbed molecules. An experimental and theoretical study on Pt(111) and mono- and bi-atomic layer Pt nanoislands on a Au(111) surface reveals the bimetallic effect that weakens with the increasing number of deposited Pt adlayer.

Shell-Isolated Tip-Enhanced Raman and Fluorescence Spectroscopy.

Tip-enhanced Raman spectroscopy can provide molecular fingerprint information with ultrahigh spatial resolution, but the tip will be easily contaminated, thus leading to artifacts. It also remains a great challenge to establish tip-enhanced fluorescence because of the quenching resulting from the proximity of the metal tip. Herein, we report shell-isolated tip-enhanced Raman and fluorescence spectroscopies by employing ultrathin shell-isolated tips fabricated by atomic layer deposition. Such shell-isolated tips not only show outstanding electromagnetic field enhancement in TERS but also exclude interference by contaminants, thus greatly promoting applications in solution. Tip-enhanced fluorescence has also been achieved using these shell-isolated tips, with enhancement factors of up to 1.7×103 , consistent with theoretical simulations. Furthermore, tip-enhanced Raman and fluorescence signals are acquired simultaneously, and their relative intensities can be manipulated by changing the shell thickness. This work opens a new avenue for ultrahigh resolution surface analysis using plasmon-enhanced spectroscopies.

High Uniformity and Enhancement Au@AgNS 3D Substrates for the Diagnosis of Breast Cancer.

Breast cancer appears to be one of the leading causes of cancer-related morbidity and mortality for women worldwide. The accurate and rapid diagnosis of breast cancer is hence critical for the treatment and prognosis of patients. With the vibrational fingerprint information and high detection sensitivity, surface-enhanced Raman spectroscopy (SERS) has been extensively applied in biomedicine. Here, an optimized bimetallic nanosphere (Au@Ag NS) 3D substrate was fabricated for the aim of the diagnosis of breast cancer based on the SERS analysis of the extracellular metabolites. The unique stacking mode of 3D Au@Ag NSs provided multiple plasmonic hot spots according to the theoretical calculations of the electromagnetic field distribution. The low relative standard deviation (RSD = 2.7%) and high enhancement factor (EF = 1.42 × 105) proved the uniformity and high sensitivity. More importantly, the normal breast cells and breast cancer cells could be readily distinguished from the corresponding SERS spectra based on the extracellular metabolites. Furthermore, the clear clusters of SERS spectra from MCF-7 and MDA-MB-231 extracellular metabolites in the orthogonal partial least-squares discriminant analysis plot indicate the distinct metabolic fingerprint between breast cancer cells, which imply their potential clinical application in the diagnosis of breast cancer.

Operando Electrochemical X-ray Diffraction and Raman Spectroscopic Studies Revealing the Alkali-Metal Ion Intercalation Mechanism in Prussian Blue Analogues.

Prussian blue analogues are promising cathode material candidates for aqueous rechargeable metal-ion batteries. Although great efforts have been made on developing materials, there are still rare reports on optimizing cell performance from mechanistic understanding and studies. Here we unveil the alkali-metal-ion intercalation mechanism in Berlin green with a home-built spectroelectrochemical cell for operando X-ray Diffraction (XRD) and Raman spectroscopies, which allows us to obtain the correlated local structure, crystal structure, redox activity, and potential profiles during the charging and discharging processes. We found that the intercalation of Na+ follows a solid solution mechanism leading to a high capacity, and the intercalation of K+ follows a two-phase transition mechanism showing a high voltage. With this understanding, we propose a new strategy using a Na+/K+ hybrid cation electrolyte to realize both high voltage and energy density. This study offers a unique insight for improving the cell performance from the understanding of the reaction mechanism.

Folding and Fracture of Single-Crystal Graphene Grown on a Cu(111) Foil.

A single-crystal graphene film grown on a Cu(111) foil by chemical vapor deposition (CVD) has ribbon-like fold structures. These graphene folds are highly oriented and essentially parallel to each other. Cu surface steps underneath the graphene are along the <110> and <211> directions, leading to the formation of the arrays of folds. The folds in the single-layer graphene (SLG) are not continuous but break up into alternating patterns. A "joint" (an AB-stacked bilayer graphene) region connects two neighboring alternating regions, and the breaks are always along zigzag or armchair directions. Folds formed in bilayer or few-layer graphene are continuous with no breaks. Molecular dynamics simulations show that SLG suffers a significantly higher compressive stress compared to bilayer graphene when both are under the same compression, thus leading to the rupture of SLG in these fold regions. The fracture strength of a CVD-grown single-crystal SLG film is simulated to be about 70 GPa. This study greatly deepens the understanding of the mechanics of CVD-grown single-crystal graphene and such folds, and sheds light on the fabrication of various graphene origami/kirigami structures by substrate engineering. Such oriented folds can be used in a variety of further studies.

Accelerated interfacial proton transfer for promoting electrocatalytic activity.

Interfacial pH is critical to electrocatalytic reactions involving proton-coupled electron transfer (PCET) processes, and maintaining an optimal interfacial pH at the electrochemical interface is required to achieve high activity. However, the interfacial pH varies inevitably during the electrochemical reaction owing to slow proton transfer at the interfacial layer, even in buffer solutions. It is therefore necessary to find an effective and general way to promote proton transfer for regulating the interfacial pH. In this study, we propose that promoting proton transfer at the interfacial layer can be used to regulate the interfacial pH in order to enhance electrocatalytic activity. By adsorbing a bifunctional 4-mercaptopyridine (4MPy) molecule onto the catalyst surface via its thiol group, the pyridyl group can be tethered on the electrochemical interface. The pyridyl group acts as both a good proton acceptor and donor for promoting proton transfer at the interfacial layer. Furthermore, the pK a of 4MPy can be modulated with the applied potentials to accommodate the large variation of interfacial pH under different current densities. By in situ electrochemical surface-enhanced Raman spectroscopy (in situ EC-SERS), we quantitatively demonstrate that proton transfer at the interfacial layer of the Pt catalyst coated with 4MPy (Pt@4MPy) remains ideally thermoneutral during the H+ releasing electrocatalytic oxidation reaction of formic acid (FAOR) at high current densities. Thus, the interfacial pH is controlled effectively. In this way, the FAOR apparent current measured from Pt@4MPy is twice that measured from a pristine Pt catalyst. This work establishes a general strategy for regulating interfacial pH to enhance the electrocatalytic activities.

Recent advances in plasmon-enhanced Raman spectroscopy for catalytic reactions on bifunctional metallic nanostructures.

Metallic nanostructures exhibit superior catalytic performance for diverse chemical reactions and the in-depth understanding of reaction mechanisms requires versatile characterization methods. Plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and tip-enhanced Raman spectroscopy (TERS), appears as a powerful technique to characterize the Raman fingerprint information of surface species with high chemical sensitivity and spatial resolution. To expand the range of catalytic reactions studied by PERS, catalytically active metals are integrated with plasmonic metals to produce bifunctional metallic nanostructures. In this minireview, we discuss the recent advances in PERS techniques to probe the chemical reactions catalysed by bifunctional metallic nanostructures. First, we introduce different architectures of these dual-functionality nanostructures. We then highlight the recent works using PERS to investigate important catalytic reactions as well as the electronic and catalytic properties of these nanostructures. Finally, we provide some perspectives for future PERS studies in this field.

GAS5 knockdown ameliorates apoptosis and inflammatory response by modulating miR-26b-5p/Smad1 axis in cerebral ischaemia/reperfusion injury.

Ischemic stroke (IS) caused by cerebral arterial embolism remains the leading cause of disability and death worldwide. Cerebral ischemia / reperfusion (CI / R) injury is one of the common complications of ischemic stroke. Growth arrest specific transcript 5 (GAS5) has been found to be abnormally expressed in various tumors. However, the role and potential molecular mechanisms of GAS5 in CI / R-induced injury remain unknown. This study established a CI / R injury model in vivo and in vitro. The results showed that the expression of GAS5 was increased in CI / R rats, while miR-26b-5p expression was decreased. Besides, knockdown of GAS5 by siRNA (si-GAS5) reversed CI / R-induced apoptosis and inflammatory responses. Notably, bioinformatics analysis indicated that GAS5 competitively adsorbed miR-26b-5p, and the relationship was further confirmed by pull-down assay. In addition, miR-26b-5p overexpression reversed CI / R-induced apoptosis and inflammatory responses, whereas low expression of miR-26b-5p had the opposite effect. Moreover, TargetScan assay predicted that drosophila mothers against decapentaplegic protein 1 (Smad1) was a target of miR-26b-5p, and miR-26b-5p overexpression inhibited Smad1 expression. Conversely, Smad1 overexpression reversed the inhibitory effect of miR-26b-5p on CI / R-induced apoptosis and inflammatory responses in rats. Collectively, these results indicate that GAS5 knockdown can improve apoptosis and inflammatory responses by modulating the miR-26b-5p / Smad1 axis in CI / R rats.

Role of Adsorption Orientation in Surface Plasmon-Driven Coupling Reactions Studied by Tip-Enhanced Raman Spectroscopy.

In the field of surface plasmon-mediated photocatalysis, the coupling reactions of p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) to produce p, p'-dimercaptoazobenzene (DMAB) are the most widely investigated systems. However, a clear understanding of the structure-function relationship is still required. Here, we used tip-enhanced Raman spectroscopy (TERS) to study the coupling reactions of PATP and PNTP on well-defined Ag(111) and Au(111) surfaces using 632.8 and 532 nm lasers. On Au(111), the oxidative coupling of PATP can proceed under irradiation by a 632.8 nm laser, and the reductive coupling of PNTP can only occur under irradiation by a 532 nm laser. Neither wavelength of laser light can induce the coupling reactions of these two molecules on Ag(111). Density functional theory (DFT) was used to calculate the stable adsorption configurations of PATP and PNTP on Ag(111) and Au(111). Both the adsorption configurations of the two molecules on the surfaces and laser energies were, experimentally and theoretically, found to determine whether the coupling reactions can occur on different substrates. These results may help the rational design of photocatalysts with enhanced reactivity.

Towards super-clean graphene.

Impurities produced during the synthesis process of a material pose detrimental impacts upon the intrinsic properties and device performances of the as-obtained product. This effect is especially pronounced in graphene, where surface contamination has long been a critical, unresolved issue, given graphene's two-dimensionality. Here we report the origins of surface contamination of graphene, which is primarily rooted in chemical vapour deposition production at elevated temperatures, rather than during transfer and storage. In turn, we demonstrate a design of Cu substrate architecture towards the scalable production of super-clean graphene (>99% clean regions). The readily available, super-clean graphene sheets contribute to an enhancement in the optical transparency and thermal conductivity, an exceptionally lower-level of electrical contact resistance and intrinsically hydrophilic nature. This work not only opens up frontiers for graphene growth but also provides exciting opportunities for the utilization of as-obtained super-clean graphene films for advanced applications.

Probing the electronic and catalytic properties of a bimetallic surface with 3†nm resolution.

An atomic- and molecular-level understanding of heterogeneous catalysis is required to characterize the nature of active sites and improve the rational design of catalysts. Achieving this level of characterization requires techniques that can correlate catalytic performances to specific surface structures, so as to avoid averaging effects. Tip-enhanced Raman spectroscopy combines scanning probe microscopy with plasmon-enhanced Raman scattering and provides simultaneous topographical and chemical information at the nano/atomic scale from ambient to ultrahigh-vacuum and electrochemical environments. Therefore, it has been used to monitor catalytic reactions and is proposed to correlate the local structure and function of heterogeneous catalysts. Bimetallic catalysts, such as Pd-Au, show superior performance in various catalytic reactions, but it has remained challenging to correlate structure and reactivity because of their structural complexity. Here, we show that TERS can chemically and spatially probe the site-specific chemical (electronic and catalytic) and physical (plasmonic) properties of an atomically well-defined Pd(sub-monolayer)/Au(111) bimetallic model catalyst at 3 nm resolution in real space using phenyl isocyanide as a probe molecule (Fig. 1a). We observe a weakened N≡C bond and enhanced reactivity of phenyl isocyanide adsorbed at the Pd step edge compared with that at the Pd terrace. Density functional theory corroborates these observations by revealing a higher d-band electronic profile for the low-coordinated Pd step edge atoms. The 3 nm spatial resolution we demonstrate here is the result of an enhanced electric field and distinct electronic properties at the step edges.

A double-lyophilization method for the preparation of CS/GO-COOH scaffold and its application in blood detoxification.

The accumulation of uremic toxins in blood might induce chronic renal failure (CRF). The incidence of CRF was as high as 10%. The traditional therapy for CRF was hemodialysis, which was more effective to remove small molecules, such as urea and creatinine. However, this detoxification method ignored the tissue functional adaption due to the retention of macromolecule uremic toxins. To solve this problem, this paper developed a new kind of chitosan/carboxyl graphite oxide (CS/GO-COOH) scaffold via a double-lyophilization method. Then, this synthetic scaffold was characterized by Fourier transform infrared spectroscopy, scanning electron microscope, hydrophilic test, mechanical property, and in vitro detoxification test. Covalent bonding and hydrogen bonding were formed, indicating the strong interactions between CS and GO-COOH. There were interconnected networks in the synthesized scaffold. The mechanical test suggested that the GO-2500 scaffold had excellent mechanical strength, which was 7.41 ± 0.82 MPa with 25% shrink. What is more, GO-2500 could totally rebound within 1s, after compressed to 90% shrink. The rates of GO-2500 were 1587 ± 60 and 246 ± 10% according to the water uptake and retention data, respectively. Furthermore, the detoxification of GO-2500 to urea, creatinine, VB12, and ß2-m were 67.59 ± 2.31, 39.67 ± 2.95, 31.51 ± 2.62, and 83.82 ± 7.76 mg/g, respectively. The resulting CS/GO-COOH scaffold held great potential for the detoxification of uremic toxins.

Research on a novel poly (vinyl alcohol)/lysine/vanillin wound dressing: Biocompatibility, bioactivity and antimicrobial activity.

Burn wound dressings have played significant roles in daily clinical practice. An "ideal" burn wound dressing is non-adhesion, absorbency and antimicrobial activity. However, such a dressing is currently not available. A novel composite hydrogel was based on poly (vinyl alcohol) (PVA) containing lysine (Lys) and vanillin (V) using freezing-thawing method. The properties of this hydrogel were characterized by environmental scanning electron microscope (ESEM), attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR), tensile testing, differential scanning calorimetry (DSC) and water vapour transmission rate (WVTR). Then the antibacterial activity of PVA/Lys/V composite hydrogel was examined by bacteriolytic plate. In vivo experiment, a burn rat model was used to evaluate the histological analysis of this hydrogel. In results, the Schiff base formed in the three-phase system. It improved the tensile strength and crystallization of the PVA/Lys/V composite hydrogel. Meanwhile, this hydrogel showed excellent bactericidal activities to both gram-negative bacteria (E. coli) and gram-positive bacteria (S. aureus) due to the Schiff base. And the antibacterial activity toward gram-negative bacteria was better than another. On Day 7, 95-100% of the surface areas of PVA/Lys/V composite treated burns were covered with regenerating epidermis. And the new tissue and capillary vessel formed around the wounds after treatment. Therefore, it is suggested that treatment with PVA/Lys/V composite hydrogel will be effective also in patients with burns and other skin wounds.