Shaping Gold Nanocrystals in Dimethyl Sulfoxide: Toward Trapezohedral and Bipyramidal Nanocrystals Enclosed by {311} Facets.

The remarkable synthetically tunable structural, electronic, and optical properties of gold nanocrystals have attracted increasing interest and enabled multidisciplinary applications. Over the past decades, nearly all the possible fundamental shapes of faceted Au nanocrystals have been synthesized, except for only one missing-the trapezohedron enclosed by {hkk} facets. In this report, the unprecedented synthesis of trapezohedral Au nanocrystals with {311} crystal facets was realized. Dimethyl sulfoxide (DMSO) was discovered as a solvent for shaping Au nanocrystals with {311} crystal facets for the first time. Mechanistic studies, together with previous DFT and STM studies, attribute the unique role of DMSO to its ambidentate nature, where both sulfur and oxygen of DMSO can coordinate to gold surface, endowing its unique role in stabilizing high-index {311} facets through a "two center bonding" mode. The DMSO-based synthesis provides a new synthetic tool toward the synthesis of a series of unreported Au nanocrystals with new structures. In particular, a new type of gold bipyramids, the octagonal bipyramids, was first synthesized with additional plasmonic tunability while simultaneously retaining their {311} facets. The application of these new Au nanocrystals in surface-enhanced Raman scattering spectroscopy was investigated, and their shape-dependent performances were demonstrated. These results highlight the tremendous potential of using ambidentate molecules as shape- and surface-directing agents for metal nanocrystals and offer the promise of enabling new synthetic tools toward atomically precise control of surface structures of metal nanocrystals.

Enhanced Electrochemical Properties of Li3 VO4 with Controlled Oxygen Vacancies as Li-Ion Battery Anode.

Li3 VO4 , as a promising intercalation-type anode material for lithium-ion batteries, features a desired discharge potential (ca. 0.5-1.0 V vs. Li/Li+ ) and a good theoretical storage capacity (590 mAh g-1 with three Li+ inserted). However, the poor electrical conductivity of Li3 VO4 hinders its practical application. In the present work, various amounts of oxygen vacancies were introduced in Li3 VO4 through annealing in hydrogen to improve its conductivity. To elucidate the influence of oxygen vacancies on the electrochemical performances of Li3 VO4 , the surface energy of the resulting material was measured with an inverse gas chromatography method. It was found that Li3 VO4 annealed in pure hydrogen at 400 °C for 15 min exhibited a much higher surface energy (60.7 mJ m-2 ) than pristine Li3 VO4 (50.6 mJ m-2 ). The increased surface energy would lower the activation energy of phase transformation during the charge-discharge process, leading to improved electrochemical properties. As a result, the oxygen-deficient Li3 VO4 achieved a significantly improved specific capacity of 495 mAh g-1 at 0.1 Ag-1 (381 mAh g-1 for pristine Li3 VO4 ) and retains 165 mAh g-1 when the current density increases to 8 Ag-1 .

Structure-Dependent cis/trans Isomerization of Tetraphenylethene Derivatives: Consequences for Aggregation-Induced Emission.

The isomerization and optical properties of the cis and trans isomers of tetraphenylethene (TPE) derivatives with aggregation-induced emission (AIEgens) have been sparsely explored. We have now observed the tautomerization-induced isomerization of a hydroxy-substituted derivative, TPETH-OH, under acidic but not under basic conditions. Replacing the proton of the hydroxy group in TPETH-OH with an alkyl group leads to the formation of TPETH-MAL, for which the pure cis and trans isomers were obtained and characterized by HPLC analysis and NMR spectroscopy. Importantly, cis-TPETH-MAL emits yellow fluorescence in DMSO at -20 °C whereas trans-TPETH-MAL shows red fluorescence under the same conditions. Moreover, the geometry of cis- and trans-TPETH-MAL remains unchanged when they undergo thiol-ene reactions to form cis- and trans-TPETH-cRGD, respectively. Collectively, our findings improve our fundamental understanding of the cis/trans isomerization and photophysical properties of TPE derivatives, which will guide further AIEgen design for various applications.

Highly Symmetric Gold Nanostars: Crystallographic Control and Surface-Enhanced Raman Scattering Property.

Gold nanostars have attracted widespread interest due to their remarkable properties and broad applications in plasmonics, spectroscopy, biomedicine, and energy conversion. However, current synthetic methods of Au nanostars have limited control over their symmetry; most existing nanostars are characterized by having uncertain number of arms with different lengths and random spatial arrangement. This morphological arbitrariness not only hampers the fundamental understanding of the properties of Au nanostars, but also lead to poor reproducibility in their applications. Here we demonstrate that, by using a robust solution-phase method, Au nanostars with unpreceded degree of symmetry control can be obtained in high yield and with remarkable monodispersity. Icosahedral seeds are used to dictate the growth of 3D evenly distributed arms in an Ih symmetric manner. Alkylamines serve as shape-control agent to regulate the growth of the hexagonal pyramidal arms enclosed by high-index facets. Benefiting from their high symmetry, the Au nanostars exhibit superior single-particle SERS performance compared to asymmetric Au nanostars, in terms of both intensity and reproducibility.

Pd-Pb Alloy Nanocrystals with Tailored Composition for Semihydrogenation: Taking Advantage of Catalyst Poisoning.

Metallic nanocrystals (NCs) with well-defined sizes and shapes represent a new family of model systems for establishing structure-function relationships in heterogeneous catalysis. Here in this study, we show that catalyst poisoning can be utilized as an efficient strategy for nanocrystals shape and composition control, as well as a way to tune the catalytic activity of catalysts. Lead species, a well-known poison for noble-metal catalysts, was investigated in the growth of Pd NCs. We discovered that Pb atoms can be incorporated into the lattice of Pd NCs and form Pd-Pb alloy NCs with tunable composition and crystal facets. As model catalysts, the alloy NCs with different compositions showed different selectivity in the semihydrogenation of phenylacetylene. Pd-Pb alloy NCs with better selectivity than that of the commercial Lindlar catalyst were discovered. This study exemplified that the poisoning effect in catalysis can be explored as efficient shape-directing reagents in NC growth, and more importantly, as a strategy to tailor the performance of catalysts with high selectivity.

Dodecahedral gold nanocrystals: the missing Platonic shape.

Platonic noble metal nanocrystals (NCs) have attracted considerate attention due to their symmetry, aesthetic beauty, and potential applications in catalysis, plasmonics, sensing, and spectroscopy. Although Platonic noble metal NCs with tetrahedral, cubic, octahedral, and icosahedral geometries have been chemically synthesized, the growth of Platonic dodecahedral noble metal NCs remains elusive. Here we propose a crystal structure of Platonic dodecahedral noble metal NCs and show that via a tailored seed-mediated synthetic approach, Platonic dodecahedral Au NCs can be grown from icosahedral multiply twinned Au seeds. By systematically tuning the ratio between {111} and {110} facets grown on the icosahedral Au seeds, NCs with icosahedral, icosidodecahedral, and dodecahedral shapes can be obtained. These shapes represent a family of Au NCs with icosahedral (Ih) symmetry.

One-pot synthesis of CO2-responsive magnetic nanoparticles with switchable hydrophilicity.

CO2-responsive nanoparticles have been attracted increasing interest due to their benign reactions with CO2 that give them gas-switchable properties, which can be easily reversed by mild heating or purging with inert gases. In this work, we have prepared CO2-responsive magnetic nanoparticles in a simple one-pot polyol synthesis using diaminoalkanes as the surfactant. The as-synthesized nanoparticles show excellent reversible aggregation and dispersion in response to alternating purging of N2 and CO2 at room temperature. We found that, among the diaminoalkanes with different chain lengths, 1,8-diaminooctane is the best candidate for the synthesis of CO2-responsive nanoparticles, since it allows good dispersity of the nanoparticles after charging with CO2 and also provides effective aggregation and separation following N2 purging. Moreover, the self-assembly of 1,8-diaminooctane-functionalized nanoparticles can be controlled to form linear aggregates with the assistance of N2 and an external magnetic field, demonstrating an effective response to dual stimuli. This work paves the way for the direct synthesis of a wide range of CO2-responsive nanoparticles.

DNA-assisted assembly of carbon nanotubes and MnO2 nanospheres as electrodes for high-performance asymmetric supercapacitors.

A DNA-assisted assembly approach is developed to fabricate a capacitor-type electrode material, DNA-functionalized carbon nanotubes (CNTs@DNA), and a battery-type electrode material, DNA@CNTs-bridged MnO2 spheres (CNTs@DNA-MnO2), for asymmetric supercapacitors. An energy density of 11.6 W h kg(-1) is achieved at a power density of 185.5 W kg(-1) with a high MnO2 mass loading of 4.2 mg cm(-2). It is found that DNA assembly plays a critical role in the enhanced supercapacitor performance. This is because while DNA molecules functionalize carbon nanotubes (CNTs) via π-π stacking, their hydrophilic sugar-phosphate backbones also promote the dispersion of CNTs. The resultant CNTs@DNA chains can link multiple MnO2 spheres to form a networked architecture that facilitates charge transfer and effective MnO2 utilization. The improved performance of the asymmetric supercapacitors indicates that DNA-assisted assembly offers a promising approach to the fabrication of high-performance energy storage devices.

A light-sensitive protein-based wearable pH biometer.

Bacteriorhodopsin is a biological material with excellent photosensitivity properties. It can directly convert optical signals into electrical signals and is widely used in various biosensors. Here, we present a bR-based wearable pH biometer that can be used to monitor wound infection. The mechanism of the pH-sensitive effect of the bR electrode is explained, which generates a transient photovoltage under light irradiation and a negative photovoltage when the lamp is turned off. Since the photoelectric signal of bR is affected by different pH values, the photovoltage is changed by adjusting the pH value. The ratio (Vn/Vp) of negative photovoltage (Vn) to positive photovoltage (Vp) has a good linear relationship (R2 = 0.9911) in the pH range of 4.0-10.0. In vitro experiments using rats as a model confirmed that this wearable pH biometer can monitor pH changes that occur in wound infection.

Growth of Au@Ag core-shell pentatwinned nanorods: tuning the end facets.

Au@Ag core-shell nanorods with tunable end facets are obtained by coating Au bipyramids (BPs) with Ag. The resultant nanorods exhibit a pentatwinned crystal structure with tips terminated with either {110} or {111} facets. The control over the end facets is achieved by varying the capping agents and tuning the reduction rate of Ag. Specifically, when Ag is reduced slowly, Au@Ag nanorods with flat {110} end facets are formed with cetyltrimethylammonium bromide (CTAB) as the capping agent. If CTAB is replaced with cetyltrimethylammonium chloride (CTAC), Au@Ag nanorods with tips terminated with {111} facets are obtained. However, at a high Ag reduction rate, dumbbell-shaped Au@Ag nanorods are formed, with either CTAB or CTAC as the capping agent. The morphological evolution of the nanorods in each case is closely followed and a growth mechanism is proposed.

Silver nanocube-enhanced far-red/near-infrared fluorescence of conjugated polyelectrolyte for cellular imaging.

We present the study of silver nanocube (Ag NC)-enhanced fluorescence of a cationic conjugated polyelectrolyte (CPE) for far-red/near-infrared fluorescence cell imaging. Layer-by-layer self-assembly of polyelectrolytes on 78 nm Ag NCs is used to control CPE-metal distance and its effect on CPE fluorescence. The highest fluorescence enhancement factor (FEF) is obtained for Ag NCs with two bilayers, corresponding to a CPE-metal spacer thickness of ~6 nm. At the optimal excitation wavelength, the FEF is 13.8 with respect to the control silica nanoparticles (NPs). The fluorescent NPs are further used for cellular imaging studies. The CPE-loaded Ag NCs with two bilayers exhibit excellent image contrast, superior to the control of CPE-silica NP at a similar uptake efficiency. The viability test indicates low cytotoxicity of the CPE-loaded Ag NCs, rendering them as promising cell imaging agents.

Peptide-based nanomaterials: Self-assembly, properties and applications.

Peptide-based materials that have diverse structures and functionalities are an important type of biomaterials. In former times, peptide-based nanomaterials with excellent stability were constructed through self-assembly. Compared with individual peptides, peptide-based self-assembly nanomaterials that form well-ordered superstructures possess many advantages such as good thermo- and mechanical stability, semiconductivity, piezoelectricity and optical properties. Moreover, due to their excellent biocompatibility and biological activity, peptide-based self-assembly nanomaterials have been vastly used in different fields. In this review, we provide the advances of peptide-based self-assembly nanostructures, focusing on the driving forces that dominate peptide self-assembly and assembly mechanisms of peptides. After that, we outline the synthesis and properties of peptide-based nanomaterials, followed by the applications of functional peptide nanomaterials. Finally, we provide perspectives on the challenges and future of peptide-based nanomaterials.

Chiral transformation: from single nanowire to double helix.

We report a new type of water-soluble ultrathin Au-Ag alloy nanowire (NW), which exhibits unprecedented behavior in a colloidal solution. Upon growth of a thin metal (Pd, Pt, or Au) layer, the NW winds around itself to give a metallic double helix. We propose that the winding originates from the chirality within the as-synthesized Au-Ag NWs, which were induced to untwist upon metal deposition.

Fabrication of TiO2 binary inverse opals without overlayers via the sandwich-vacuum infiltration of precursor.

A sandwich-vacuum method was demonstrated for the fabrication of titania (TiO(2)) binary inverse opals with an open surface. In this method, a moisture-stable TiO(2) precursor was backfilled into the interstitial spaces of polystyrene binary colloidal crystals (PS bCCs), which served as a template. Removal of the template by calcination yielded TiO(2) binary inverse opals with a 3D-ordered macroporous (3DOM) structure. Optical reflectance spectra revealed the existence of a pseudostop band gap in the 3DOM TiO(2) samples. The position of the pseudostop band gap shifted to the low-wavelength region as the number ratio of small over large PS spheres was increased in the template. The sandwich-vacuum method proved to be simple and rapid for the fabrication of TiO(2) binary inverse opals without overlayers in large domains. The 3DOM TiO(2) materials were used as a photocatalyst for the degradation of benzoic acid. Results showed that in comparison to TiO(2) nanoparticles prepared under the same sintering conditions, the 3DOM TiO(2) materials displayed enhanced photocatalytic activity.

Gold nanocages: synthesis, properties, and applications.

Noble-metal nanocages comprise a novel class of nanostructures possessing hollow interiors and porous walls. They are prepared using a remarkably simple galvanic replacement reaction between solutions containing metal precursor salts and Ag nanostructures prepared through polyol reduction. The electrochemical potential difference between the two species drives the reaction, with the reduced metal depositing on the surface of the Ag nanostructure. In our most studied example, involving HAuCl(4) as the metal precursor, the resultant Au is deposited epitaxially on the surface of the Ag nanocubes, adopting their underlying cubic form. Concurrent with this deposition, the interior Ag is oxidized and removed, together with alloying and dealloying, to produce hollow and, eventually, porous structures that we commonly refer to as Au nanocages. This approach is versatile, with a wide range of morphologies (e.g., nanorings, prism-shaped nanoboxes, nanotubes, and multiple-walled nanoshells or nanotubes) available upon changing the shape of the initial Ag template. In addition to Au-based structures, switching the metal salt precursors to Na(2)PtCl(4) and Na(2)PdCl(4) allows for the preparation of Pt- and Pd-containing hollow nanostructures, respectively. We have found that changing the amount of metal precursor added to the suspension of Ag nanocubes is a simple means of tuning both the composition and the localized surface plasmon resonance (LSPR) of the metal nanocages. Using this approach, we are developing structures for biomedical and catalytic applications. Because discrete dipole approximations predicted that the Au nanocages would have large absorption cross-sections and because their LSPR can be tuned into the near-infrared (where the attenuation of light by blood and soft tissue is greatly reduced), they are attractive materials for biomedical applications in which the selective absorption of light at great depths is desirable. For example, we have explored their use as contrast enhancement agents for both optical coherence tomography and photoacoustic tomography, with improved performance observed in each case. Because the Au nanocages have large absorption cross-sections, they are also effective photothermal transducers; thus, they might provide a therapeutic effect through selective hyperthermia-induced killing of targeted cancer cells. Our studies in vitro have illustrated the feasibility of applying this technique as a less-invasive form of cancer treatment.

Ultrathin gold nanowires can be obtained by reducing polymeric strands of oleylamine-AuCl complexes formed via aurophilic interaction.

This Communication describes a facile route to the preparation of ultrathin gold nanowires using linear chains formed from [(oleylamine)AuCl] complex via aurophilic interaction. The linear chains, with AuI...AuI bonds as the backbone and surrounded by oleylamines, can group together to form bundles of polymeric strands. When the AuI was reduced to Au0 by reacting with Ag nanoparticles in hexane, the polymeric strands functioned as both the source of Au and the template to mediate the nucleation and growth of Au nanowires. Using this method, we were able to produce Au nanowires with an average diameter of approximately 1.8 nm and an aspect ratio of >1000 in high yields (approximately 70%).

Multishelled Si@Cu Microparticles Supported on 3D Cu Current Collectors for Stable and Binder-free Anodes of Lithium-Ion Batteries.

Silicon has proved to be a promising anode material of high-specific capacity for the next-generation lithium ion batteries (LIBs). However, during repeated discharge/charge cycles, Si-based electrodes, especially those in microscale size, pulverize and lose electrical contact with the current collectors due to large volume expansion. Here, we introduce a general method to synthesize Cu@M (M = Si, Al, C, SiO2, Si3N4, Ag, Ti, Ta, SnIn2O5, Au, V, Nb, W, Mg, Fe, Ni, Sn, ZnO, TiN, Al2O3, HfO2, and TiO2) core-shell nanowire arrays on Cu substrates. The resulting Cu@Si nanowire arrays were employed as LIB anodes that can be reused via HCl etching and H2-reduction. Multishelled Cu@Si@Cu microparticles supported on 3D Cu current collectors were further prepared as stable and binder-free LIB anodes. This 3D Cu@Si@Cu structure allows the interior conductive Cu network to effectively accommodate the volume expansion of the electrode and facilitates the contact between the Cu@Si@Cu particles and the current collectors during the repeated insertion/extraction of lithium ions. As a result, the 3D Cu@Si@Cu microparticles at a high Si-loading of 1.08 mg/cm2 showed a capacity retention of 81% after 200 cycles. In addition, charging tests of 3D Cu@Si@Cu-LiFePO4 full cells by a triboelectric nanogenerator with a pulsed current demonstrated that LIBs with silicon anodes can effectively store energy delivered by mechanical energy harvesters.

Tip-Selective Growth of Silver on Gold Nanostars for Surface-Enhanced Raman Scattering.

Nanogaps as "hot spots" with highly localized surface plasmon can generate ultrastrong electromagnetic fields. Superior to the exterior nanogaps obtained via aggregation and self-assembly, interior nanogaps within Au and Ag nanostructures give stable and reproducible surface-enhanced Raman scattering (SERS) signals. However, the synthesis of nanostructures with interior hot spots is still challenging because of the lack of high-yield strategies and clear design principles. Herein, gold-silver nanoclusters (Au-Ag NCs) with multiple interior hot spots were fabricated as SERS platforms via selective growth of Ag nanoparticles on the tips of Au nanostars (Au NSs). Furthermore, the interior gap sizes of Au-Ag NCs can be facilely tuned by changing the amount of AgNO3 used. Upon 785 nm excitation, single Au-Ag NC350 exhibits 43-fold larger SERS enhancement factor and the optimal signal reproducibility relative to single Au NS. The SERS enhancement factors and signal reproducibility of Au-Ag NCs increase with the decrease of gap sizes. Collectively, the Au-Ag NCs could serve as a flexible, reproducible, and active platform for SERS investigation.

Stretchable V2O5/PEDOT supercapacitors: a modular fabrication process and charging with triboelectric nanogenerators.

Stretchable energy storage devices are of great importance for the viable applications of wearable/stretchable electronics. Studies on stretchable energy storage devices, especially supercapacitors (SCs), have shown encouraging progress. However, challenges still remain in the pursuit of high specific capacitances and facile fabrication methods. Herein, we report a modular materials fabrication and assembly process for stretchable SCs. With a V2O5/PEDOT composite as the active material, the resulting stretchable SCs exhibited high areal specific capacitances up to 240 mF cm-2 and good capacitance retention at a strain of 50%. To demonstrate the facile assembly process, a stretchable wristband was fabricated by simply assembling SC cells in series to deliver a voltage higher than 2 V. Charging the wristband with a triboelectric nanogenerator (TENG) to light an LED was further demonstrated, indicating the potential to integrate our SCs with environmental energy harvesters for self-powered stretchable devices.

Cross-linker mediated formation of sulfur-functionalized V2O5/graphene aerogels and their enhanced pseudocapacitive performance.

The development of efficient synthesis methods for the preparation of vanadium oxide (V2O5)-graphene holds great promise considering the excellent performance of the composite in electrochemical applications. Herein, we report the cross-linking of a V2O5-graphene hybrid via a vanadium-thiourea redox system, which allowed the assembly of graphene oxide functional groups with V2O5 through the reducing ability of thiourea (TU) under room conditions within an impressively short reaction time (20 min). The resulting 3D composite aerogel forms a highly porous architecture of sulfur-functionalized interconnected networks. Such sulfur-functionalized transition metal oxide-graphene-based aerogels are excellent candidates in energy storage applications. When the vanadium oxide-graphene aerogel was evaluated as an electrode for a supercapacitor, a specific capacitance as high as 484.0 F g-1 at 0.6 A g-1 was obtained in a two-electrode cell configuration. This performance is much higher than that of the vanadium oxide-graphene aerogels prepared in the absence of thiourea. The vanadium oxide-graphene aerogel is able to deliver a remarkable energy density of 43.0 Wh kg-1 at a power density of 0.48 kW kg-1 at 0.6 A g-1 and can hold 24.2 Wh kg-1 at a maximum power density of 9.3 kW kg-1 at 10 A g-1. The symmetric supercapacitor assembled from the aerogel can retain 80% of its initial capacitance after 10 000 cycles.