A Novel Approach to High-Performance Aliovalent-Substituted Catalysts-2D Bimetallic MOF-Derived CeCuOx Microsheets.

Mixed transition metal oxides (MTMOs) have enormous potential applications in energy and environment. Their use as catalysts for the treatment of environmental pollution requires further enhancement in activity and stability. This work presents a new synthesis approach that is both convenient and effective in preparing binary metal oxide catalysts (CeCuOx ) with excellent activity by achieving molecular-level mixing to promote aliovalent substitution. It also allows a single, pure MTMO to be prepared for enhanced stability under reaction by using a bimetallic metal-organic framework (MOF) as the catalyst precursor. This approach also enables the direct manipulation of the shape and form of the MTMO catalyst by controlling the crystallization and growth of the MOF precursor. A 2D CeCuOx catalyst is investigated for the oxidation reactions of methanol, acetone, toluene, and o-xylene. The catalyst can catalyze the complete reactions of these molecules into CO2 at temperatures below 200 °C, representing a significant improvement in performance. Furthermore, the catalyst can tolerate high moisture content without deactivation.

Scalable and ultrafast epitaxial growth of single-crystal graphene wafers for electrically tunable liquid-crystal microlens arrays.

The scalable growth of wafer-sized single-crystal graphene in an energy-efficient manner and compatible with wafer process is critical for the killer applications of graphene in high-performance electronics and optoelectronics. Here, ultrafast epitaxial growth of single-crystal graphene wafers is realized on single-crystal Cu90Ni10(1 1 1) thin films fabricated by a tailored two-step magnetron sputtering and recrystallization process. The minor nickel (Ni) content greatly enhances the catalytic activity of Cu, rendering the growth of a 4 in. single-crystal monolayer graphene wafer in 10 min on Cu90Ni10(1 1 1), 50 folds faster than graphene growth on Cu(1 1 1). Through the carbon isotope labeling experiments, graphene growth on Cu90Ni10(1 1 1) is proved to be exclusively surface-reaction dominated, which is ascribed to the Cu surface enrichment in the CuNi alloy, as indicated by element in-depth profile. One of the best benefits of our protocol is the compatibility with wafer process and excellent scalability. A pilot-scale chemical vapor deposition (CVD) system is designed and built for the mass production of single-crystal graphene wafers, with productivity of 25 pieces in one process cycle. Furthermore, we demonstrate the application of single-crystal graphene in electrically controlled liquid-crystal microlens arrays (LCMLA), which exhibit highly tunable focal lengths near 2 mm under small driving voltages. By integration of the graphene based LCMLA and a CMOS sensor, a prototype camera is proposed that is available for simultaneous light-field and light intensity imaging. The single-crystal graphene wafers could hold great promising for high-performance electronics and optoelectronics that are compatible with wafer process.

Modular functionalization of crystalline graphene by recombinant proteins: a nanoplatform for probing biomolecules.

Graphene, as well as other two-dimensional materials, is a promising candidate for use in bioimaging, therapeutic drug delivery, and bio-sensing applications. Here, we developed a protocol to functionalize graphene with recombinant proteins using genetically encoded SpyTag-SpyCatcher chemistry. SpyTag forms a covalent isopeptide bond with its genetically encoded partner SpyCatcher through spontaneous amidation under physiological conditions. The functionalization protocol developed is based on the use of short proteins as a linker, where two graphene-binding-peptides (GBPs) are attached to both ends of SpyTag (referred to as GStG), followed by the covalent conjugation with SpyCatcher-fusion proteins. The proposed method enables the decoration of crystalline graphene with various proteins, such as fluorescent proteins and affibody molecules that bind to cancerous cells. This scheme, which takes advantage of the cleanness of single-crystal graphene and the robustness of SpyTag-SpyCatcher chemistry, provides a versatile platform on which to study the biomolecule-surface and cell-substrate interactions and, indeed, may lead to a new way of designing biomedical devices. The interaction between peptides and graphene was clearly shown using molecular dynamics simulation and proven using specially designed experiments.

Reactions of SO2 and NH3 with epoxy groups on the surface of graphite oxide powder.

Graphite oxide powder was obtained using the modified Hummers' method and characterized using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The XPS results indicated that the epoxy groups were the main functional groups on the graphite oxide powder surface. The graphite oxide powder was then reacted with SO2 and NH3 gases, respectively, at 25 °C. The XPS and ToF-SIMS analyses of the surface of the reacted graphite oxide powder showed that the reactions mainly occurred in the epoxy groups. Bisulfate and amine groups were formed on the surface of the graphite oxide powder after the reactions between the graphite oxide powder and SO2 and NH3 gases. This work demonstrates a new method of removing SO2 and NH3 gases using graphite oxide powder.

Transition temperature of poly(methyl methacrylate) determined by time-of-flight secondary ion mass spectrometry and contact angle measurements.

The surface chain conformations of poly(methyl methacrylate) (PMMA) at different temperatures were extensively studied by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Similar to our previous experimental studies on polystyrene (PS) and poly(2, 3, 4, 5, 6-pentafluorostyrene) (5FPS), a transition temperature (TT) could be identified through the principal component analysis (PCA) of the ToF-SIMS spectra obtained from the PMMA samples annealed at different temperatures. Interestingly, our results show that the TT depended on molecular weight and was about 50-60˚C below the bulk glass transition temperature (Tg) and therefore could possibly be related to the surface glass transition temperature (TgS). These results were confirmed by contact angle measurements. ToF-SIMS results showed higher peak intensities of several low-mass oxygen-containing positive ions, hydrocarbon positive ions and OCH3- negative ion at higher temperatures, which can be interpreted by a higher surface concentration of methoxy groups at the surface.

Mussel-Inspired Adhesive and Tough Hydrogel Based on Nanoclay Confined Dopamine Polymerization.

Adhesive hydrogels are attractive biomaterials for various applications, such as electronic skin, wound dressing, and wearable devices. However, fabricating a hydrogel with both adequate adhesiveness and excellent mechanical properties remains a challenge. Inspired by the adhesion mechanism of mussels, we used a two-step process to develop an adhesive and tough polydopamine-clay-polyacrylamide (PDA-clay-PAM) hydrogel. Dopamine was intercalated into clay nanosheets and limitedly oxidized between the layers, resulting in PDA-intercalated clay nanosheets containing free catechol groups. Acrylamide monomers were then added and in situ polymerized to form the hydrogel. Unlike previous single-use adhesive hydrogels, our hydrogel showed repeatable and durable adhesiveness. It adhered directly on human skin without causing an inflammatory response and was easily removed without causing damage. The adhesiveness of this hydrogel was attributed to the presence of enough free catechol groups in the hydrogel, which were created by controlling the oxidation process of the PDA in the confined nanolayers of clay. This mimicked the adhesion mechanism of the mussels, which maintain a high concentration of catechol groups in the confined nanospace of their byssal plaque. The hydrogel also displayed superior toughness, which resulted from nanoreinforcement by clay and PDA-induced cooperative interactions with the hydrogel networks. Moreover, the hydrogel favored cell attachment and proliferation, owning to the high cell affinity of PDA. Rat full-thickness skin defect experiments demonstrated that the hydrogel was an excellent dressing. This free-standing, adhesive, tough, and biocompatible hydrogel may be more convenient for surgical applications than adhesives that involve in situ gelation and extra agents.

A Mussel-Inspired Conductive, Self-Adhesive, and Self-Healable Tough Hydrogel as Cell Stimulators and Implantable Bioelectronics.

A graphene oxide conductive hydrogel is reported that simultaneously possesses high toughness, self-healability, and self-adhesiveness. Inspired by the adhesion behaviors of mussels, our conductive hydrogel shows self-adhesiveness on various surfaces and soft tissues. The hydrogel can be used as self-adhesive bioelectronics, such as electrical stimulators to regulate cell activity and implantable electrodes for recording in vivo signals.

Super stretchable hydrogel achieved by non-aggregated spherulites with diameters <5 nm.

The scope of hydrogel applications can be greatly expanded by the improvement of mechanical properties. However, enhancement of nanocomposite hydrogels (NC gels) has been severely limited because the size of crosslinking nanoparticles is too large, at least in one dimension. Here we report a new strategy to synthesize non-aggregated spherulite nanoparticles, with diameters <5 nm, in aqueous solution, and their enhancement to hydrogel. The stress and stretch ratio at rupture of our NC gel are 430 and 121 KPa with only 40-p.p.m. nanoparticle content. The NC gel containing 200-p.p.m. nanoparticles can revert to 90% of its original size after enduring 100-MPa compressive stress. Our results demonstrate that the suppression of nanoparticle size without aggregation helps to establish a super stretchable and high-toughness hydrogel network at very low inorganic content.

Fast Single-Cell Patterning for Study of Drug-Induced Phenotypic Alterations of HeLa Cells Using Time-of-Flight Secondary Ion Mass Spectrometry.

A facile single-cell patterning (ScP) method was developed and integrated with time-of-flight secondary ion mass spectrometry (TOF-SIMS) for the study of drug-induced cellular phenotypic alterations. Micropatterned poly(dimethylsiloxane) (PDMS) stencil film and centrifugation-assisted cell trapping were combined for the preparation of on-surface single-cell microarrays, which exhibited both high site occupancy (>90%) and single-cell resolution (>97%). TOF-SIMS is a surface-sensitive mass spectrometry and is increasingly utilized in biological studies. Here we demonstrated, for the first time, its successful application in high-throughput single-cell analysis. Drug-induced phenotypic alterations of HeLa cells in the early stage of apoptosis were investigated using TOF-SIMS. The major molecular sources of variations were analyzed by principle component analysis (PCA).

Surface Characterization of Polymer Blends by XPS and ToF-SIMS.

The surface properties of polymer blends are important for many industrial applications. The physical and chemical properties at the surface of polymer blends can be drastically different from those in the bulk due to the surface segregation of the low surface energy component. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary mass spectrometry (ToF-SIMS) have been widely used to characterize surface and bulk properties. This review provides a brief introduction to the principles of XPS and ToF-SIMS and their application to the study of the surface physical and chemical properties of polymer blends.

Mussel-inspired adhesive and transferable free-standing films by self-assembling dexamethasone encapsulated BSA nanoparticles and vancomycin immobilized oxidized alginate.

This study developed an adhesive and transferable free-standing (FS) film with dual function of osteoinductivity and antibacterial activity, which was obtained by sequentially assembling vancomycin immobilized oxidized sodium alginate and dexamethasone encapsulated chitosan coated BSA nanoparticles on a poly-dopamine layer. The FS films enabled the dual release of vancomycin and dexamethasone. The FS films had excellent osteoinductivity and antibacterial activity by cell culture and antibacterial assay. The FS film was detached from substrates and transferred to non-fouling surfaces by a wet transfer method, which demonstrated that the adhesive FS film is potential to modify biopolymers with non-fouling surfaces in mild and biocompatible conditions for biomedical applications.

Crystallization-Induced Redox-Active Nanoribbons of Organometallic Polymers.

Polymer/inorganic functional nanostructures are essential for the fabrication of high-performance nanodevices in the future. The synthesis of hybrid nanostructures is hindered by complicated synthetic protocols or harsh conditions. Herein, we report a facile and scalable method for the synthesis of organometallic polymer nanoribbons through crystallization of polymers capped with a ferrate complex. Nanoribbons consisted of a single crystalline polymer lamella coated with a redox-active ferrate complex on both sides. The nanoribbons had a width of approximately 70 nm and a thickness of 10 nm. With the merit of highly ordered crystalline structures of polymers and functional coating layers, as well as a highly anisotropic nature, the nanoribbons are useful in nanodevices and biosensors.

Insights into the aggregation/deposition and structure of a polydopamine film.

Surface-adherent polydopamine (PDA) films as multifunctional coatings can be easily deposited onto a wide range of materials through dopamine self-polymerization. However, a lack of in-depth understanding of PDA aggregation and deposition processes and definite structure elucidation of PDA make it challenging to tailor the surface characteristic and functionality of the PDA films. Herein, we demonstrate that the surface characteristics of the PDA films can be readily tuned by controlling the competitive interplay between PDA aggregation in solution and deposition on the substrate. Moreover, a structural investigation of the PDA films using analytical tools such as X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) allows us to propose a new structure model for the PDA building block. The (DHI)2/PCA trimer complex, which consists of two 5,6-dihydroxyindole (DHI) units and one pyrrolecarboxylic acid (PCA) moiety, is definitely identified as a primary building block of PDA, and its formation is steered by covalent interactions in the initial stages of polymerization. In latter stages, the (DHI)2/PCA trimer complexes are further linked primarily through noncovalent interactions to build up the supramolecular structure of PDA. This study provides new insights into the mechanisms of PDA buildup.

Detection of surface mobility of poly (2, 3, 4, 5, 6-pentafluorostyrene) films by in situ variable-temperature ToF-SIMS and contact angle measurements.

Poly (2, 3, 4, 5, 6-pentafluorostyrene) (5FPS) was prepared by bulk radical polymerization. The spin-cast films of this polymer were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) at various temperatures ranging from room temperature to 120°C. Principal component analysis (PCA) of the ToF-SIMS data revealed a transition temperature (T(T)) at which the surface structure of 5FPS was rearranged. A comparison between the results of the PCA of ToF-SIMS spectra obtained on 5FPS and polystyrene (PS) indicate that the pendant groups of 5FPS and PS moved in exactly opposite directions as the temperature increased. More pendant groups of 5FPS and PS migrated from the bulk to the surface and verse versa, respectively, as the temperature increased. These results clearly support the view that the abrupt changes in the normalized principal component 1 value was caused by the surface reorientation of the polymers and not by a change in the ion fragmentation mechanism at temperatures above the T(T). Contact angle measurement, which is another extremely surface sensitive technique, was used to monitor the change in the surface tension as a function of temperature. A clear T(T) was determined by the contact angle measurements. The T(T) values determined by contact angle measurements and ToF-SIMS were very similar.

Crystallization-Induced Hybrid Nano-Sheets of Fluorescent Polymers with Aggregation-Induced Emission Characteristics for Sensitive Explosive Detection.

Fluorescent organic hybrid nanosheets were generated by crystallization of polymers capped with luminogenic molecules exhibiting aggregation-induced emission characteristics (AIE). During crystallization of polymers, AIE molecules were expelled out of lamellar crystals of polymers, and finally resided on the surface. The fluorescent nanosheets with dangling AIE molecules showed sensitive and specific response to explosives. Such polymer crystallization-induced fluorescent nanomaterials offers a unique avenue to fabricate functional nanomaterials with AIE molecule-enriched domains for potential applications in nanodevices, biological engineering, and so on.

Evidence of enhanced mobility at the free surface of supported polymer films by in situ variable-temperature time-of-flight-secondary ion mass spectrometry.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra of polystyrene (PS) films supported on silicon wafers were obtained at temperatures ranging from room temperature to 100 °C. Principal component analysis (PCA) of the TOF-SIMS data revealed a transition temperature (TT) at which the surface structure of PS was rearranged. The TT of a 120-nm thick PS (weight-average molecular weight of 3,000 g/mol) thin film was determined to be about 36 °C, which is approximately 30 °C lower than the bulk glass transition temperature (Tg) of that PS. Similar TTs were observed on PSs with different molecular weights. As the TT is strongly related to the Tg and dependent on the molecular weight, it is believed that the TT determined by TOF-SIMS is related to the surface glass transition temperature (Tg(S)) measured by other techniques. This suggests that TOF-SIMS combined with PCA can be used to determine the Tg(S) of polymer films. Furthermore, the detailed PCA analyses indicate that the phenyl groups of PS tended to move away from the surface at temperatures above TT. This conclusion was further confirmed by contact angle and XPS measurements.

Hollow interior structure of spin-coated polymer thin films revealed by ToF-SIMS three-dimensional imaging.

Surface patterns were observed on spin-coated poly(bisphenol A decane ether) (BA-C10) films prepared with chloroform and tetrahydrofuran as the solvents. The interior structure of these surface patterns were analyzed using a time-of-flight secondary ion mass spectrometry (ToF-SIMS) equipped with a bismuth cluster source for ion imaging and a C(60)(+) cluster source for depth profiling. For the first time, the surface patterns have been shown to be hollow rather than solid using ToF-SIMS three-dimensional (3D) analysis and optical techniques. Moreover, the microarea depth profiling analysis indicated that the hollow structure was sandwiched between two polymer layers rather than sitting on the substrate. The height of the hollow structure and the thicknesses of the polymer layers above and below the hollow structure were also estimated from the depth profiling results.

Time-of-flight-secondary ion mass spectrometry and principal component analysis: determination of structures of lamellar surfaces.

In this article, we addressed the applicability of time-of-flight secondary ion mass spectrometry (TOF-SIMS) to examine the effects of molecular weight and of flexible-segment length on the polymer chain arrangement at the folding surfaces of the lamellae. Poly(bisphenol A-etheralkane) (Cn) contains both rigid aromatic and flexible aliphatic CH(2) segments. The number of CH(2) units per flexible segment, n, varies from 8 to 12. Principal component analysis (PCA) of TOF-SIMS data revealed the chemical and structural variations of the folding surfaces of these polymers and identified the ion peaks contributing to these variations. We highlighted the discriminating power of PCA to distinguish the structural conformations of the amorphous and flat-on lamellar surfaces of these polymers. PCA loadings analyses showed that relatively more flexible structures were deposited on the folding surfaces when the flexible-segment length increased from 8 to 10 CH(2) units. The concentration of short loops at folding surfaces and the disorder of folding surfaces increased when the molecular weight increased. All these results led us to conclude that TOF-SIMS has great potential for probing the chemical composition of the folding surfaces of polymers.

Induction of molecular organization of oligomers by low-energy electrons.

We reveal that a beam of low-energy electrons (18 eV) can directly trigger long-range molecular ordering of an amorphous, semi-flexible oligomer in a few minutes without the prerequisite of pre-orientation. A strong endothermic transition was detected with a micro-thermal analyzer on the areas that had been exposed to the electron irradiation while the areas that were shielded from the irradiation by a protective mask remained amorphous as usual. This result suggests that long-range molecular ordering only develops in the area of the oligomer film under electron irradiation. This is the first-time effort to use electron irradiation to control the long-range ordering of an amorphous organic thin film above the glass transition temperature.

Control of the fold surface conformation of the lamellae of an oligomer.

Small-angle X-ray scattering revealed that a semirigid oligomer of bisphenol-A-co-ether-octane with a monodisperse chain length is capable of forming ciliated-folded, once-folded, ciliated-extended and fully extended lamellar structures. Isothermal crystallization studies suggested a sequence of structures with increasing crystallization temperature, from a ciliated-folded to a once-folded form and then to a ciliated-extended form as the degree of supercooling is decreased. The crystal surface thus changed from octane cilia to bisphenol A segments and then back to octane cilia as the lamellar structure changed. The results of time-of-flight secondary ion mass spectrometry analyses strongly supported the fold structural models.