A Photoresponsive Smart Covalent Organic Framework.

Ordered π-columnar structures found in covalent organic frameworks (COFs) render them attractive as smart materials. However, external-stimuli-responsive COFs have not been explored. Here we report the design and synthesis of a photoresponsive COF with anthracene units as the photoresponsive π-building blocks. The COF is switchable upon photoirradiation to yield a concavo-convex polygon skeleton through the interlayer [4π+4π] cycloaddition of anthracene units stacked in the π-columns. This cycloaddition reaction is thermally reversible; heating resets the anthracene layers and regenerates the COF. These external-stimuli-induced structural transformations are accompanied by profound changes in properties, including gas adsorption, π-electronic function, and luminescence. The results suggest that COFs are useful for designing smart porous materials with properties that are controllable by external stimuli.

Anti-counterfeit nanoscale fingerprints based on randomly distributed nanowires.

Counterfeiting is conducted in almost every industry, and the losses caused by it are growing as today's world trade continues to increase. In an attempt to provide an efficient method to fight such counterfeiting, we herein demonstrate anti-counterfeit nanoscale fingerprints generated by randomly distributed nanowires. Specifically, we prepare silver nanowires coated with fluorescent dyes and cast them onto the surface of transparent PET film. The resulting non-repeatable patterns characterized by the random location of the nanowires and their fluorescent colors provide unique barcodes suitable for anti-counterfeit purposes. Counterfeiting such a fingerprint pattern is impractical and expensive; the cost of replicating it would be higher than the value of the typical target item being protected. Fingerprint patterns can be visually authenticated in a simple and straightforward manner by using an optical microscope. The concept of generating unique patterns by randomness is not limited to the materials shown in this paper and should be readily applicable to other types of materials.

Conjugated organic framework with three-dimensionally ordered stable structure and delocalized π clouds.

Covalent organic frameworks are a class of crystalline organic porous materials that can utilize π-π-stacking interactions as a driving force for the crystallization of polygonal sheets to form layered frameworks and ordered pores. However, typical examples are chemically unstable and lack intrasheet π-conjugation, thereby significantly limiting their applications. Here we report a chemically stable, electronically conjugated organic framework with topologically designed wire frameworks and open nanochannels, in which the π conjugation-spans the two-dimensional sheets. Our framework permits inborn periodic ordering of conjugated chains in all three dimensions and exhibits a striking combination of properties: chemical stability, extended π-delocalization, ability to host guest molecules and hole mobility. We show that the π-conjugated organic framework is useful for high on-off ratio photoswitches and photovoltaic cells. Therefore, this strategy may constitute a step towards realizing ordered semiconducting porous materials for innovations based on two-dimensionally extended π systems.

Mussel-inspired adhesive binders for high-performance silicon nanoparticle anodes in lithium-ion batteries.

Conjugation of mussel-inspired catechol groups to various polymer backbones results in materials suitable as silicon anode binders. The unique wetness-resistant adhesion provided by the catechol groups allows the silicon nanoparticle electrodes to maintain their structure throughout the repeated volume expansion and shrinkage during lithiation cycling, thus facilitating substantially improved specific capacities and cycle lives of lithium-ion batteries.

Microtubes with rectangular cross-section by self-assembly of a short ß-peptide foldamer.

In nature, complex and well-defined structures are constructed by the self-assembly of biomolecules. It has been shown that ß-peptide foldamers can mimic natural peptides and self-assemble into three-dimensional molecular architectures thanks to their rigid and predictable helical conformation in solution. Using shorter foldamers, which can be prepared more easily than longer ones, to form such architectures is highly desirable, but shorter foldamers have been overlooked due to the seemingly inferior number of intramolecular hydrogen bonds to stabilize a folded state in solution. Here we report that a ß-peptide tetramer, although it lacks full helical propensity in solution, does self-assemble to form well-defined microtubes with rectangular cross-section by evaporation-induced self-assembly.

Ultrafast structural dynamics of the photocleavage of protein hybrid nanoparticles.

Protein-coated gold nanoparticles in suspension are excited by intense laser pulses to mimic the light-induced effect on biomolecules that occur in photothermal laser therapy with nanoparticles as photosensitizer. Ultrafast X-ray scattering employed to access the nanoscale structural modifications of the protein-nanoparticle hybrid reveals that the protein shell is expelled as a whole without denaturation at a laser fluence that coincides with the bubble formation threshold. In this ultrafast heating mediated by the nanoparticles, time-resolved scattering data show that proteins are not denatured in terms of secondary structure even at much higher temperatures than the static thermal denaturation temperature, probably because time is too short for the proteins to unfold and the temperature stimulus has vanished before this motion sets in. Consequently the laser pulse length has a strong influence on whether the end result is the ligand detachment (for example drug delivery) or biomaterial degradation.

Surface PEGylation via native chemical ligation.

Native chemical ligation (NCL) is an emerging chemoselective chemistry that forms an amide bond by trans-thioesterification followed by intramolecular nucleophilic rearrangement between thioester and cysteine. The reaction is simple, occurs in a mild aqueous solution, and gives near-quantitative yields of a desired product. Since the first report in 1994, most studies involving the use of NCL have focused on the total synthesis of proteins to address fundamental questions pertaining to many aspects of protein science, such as folding, mirror images, and site-specific labeling of proteins, but applications of the NCL reaction for other areas remain largely unexplored. Herein, we present a facile strategy for surface immobilization of poly(ethylene glycol) (PEG) utilizing the NCL reaction. Surface immobilization of PEG (i.e., PEGylation) plays a key role in preventing nonspecific protein adsorption on surfaces, which is crucial in a wide variety of medical devices. Using cysteine-PEG and thioester-containing phosphonic acid conjugates, we achieved efficient surface PEGylation on titanium surfaces. Ellipsometry, goniometry, and X-ray photoelectron spectroscopy (XPS) unambiguously confirmed the presence of PEGs, which provided nonfouling effects of surfaces. This study indicates that the NCL reaction will be a useful toolkit for surface bioconjugation and functionalization.

Bionanosphere lithography via hierarchical peptide self-assembly of aromatic triphenylalanine.

A nanolithographic approach based on hierarchical peptide self-assembly is presented. An aromatic peptide of N-(t-Boc)-terminated triphenylalanine is designed from a structural motif for the beta-amyloid associated with Alzheimer's disease. This peptide adopts a turnlike conformation with three phenyl rings oriented outward, which mediate intermolecular pi-pi stacking interactions and eventually facilitate highly crystalline bionanosphere assembly with both thermal and chemical stability. The self-assembled bionanospheres spontaneously pack into a hexagonal monolayer at the evaporating solvent edge, constituting evaporation-induced hierarchical self-assembly. Metal nanoparticle arrays or embossed Si nanoposts could be successfully created from the hexagonal bionanosphere array masks in conjunction with a conventional metal-evaporation or etching process. Our approach represents a bionanofabrication concept that biomolecular self-assembly is hierarchically directed to establish a straightforward nanolithography compatible with conventional device-fabrication processes.

Au nanowire-Au nanoparticles conjugated system which provides micrometer size molecular sensors.

We report a new type of molecular sensor using a Au nanowire (NW)-Au nanoparticles (NPs) conjugated system. The Au NW-NPs structure is fabricated by the self-assembly of biotinylated Au NPs on a biotinylated Au NW through avidin; this creates hot spots between NW and NPs that strongly enhance the Raman signal. The number of the Au NPs attached to the NW is reproducibly proportional to the concentration of the avidin, and is also proportional to the measured surface-enhanced Raman scattering (SERS) signals. Since this well-defined NW-NPs conjugated sensor is only a few micrometer long, we expect that development of multiplex nanobiosensor of a few tens micrometer size would become feasible by combining individually modified multiple Au NWs together on one substrate.

Noncovalently netted, photoconductive sheets with extremely high carrier mobility and conduction anisotropy from triphenylene-fused metal trigon conjugates.

Supramolecular assembly of small molecules via noncovalent interaction is useful for bottom-up construction of well-defined macroscopic structures. This approach is attracting increasing interest due to its high potential in manufacturing novel molecular electronic and optoelectronic devices. This Article describes the synthesis and functions of a sheet-shaped assembly from novel triphenylene-fused metal trigon conjugates. These conjugates were recently designed and synthesized by a divergent method and used for the supramolecular self-assembly of sheet-like objects. In contrast to triphenylene, which absorbs photons in ultraviolet region, the triphenylene-fused metal trigon conjugate shows a strong absorption band in the visible region. The metal trigon conjugate emits green photoluminescence with significantly enhanced quantum yield and allows intramolecular energy migration, as a result of extended pi-conjugation over metal sites. It assembles via physical gelation to form noncovalent sheets that collect a wide wavelength range of photons from ultraviolet to visible regions. The noncovalent sheets allow exciton migration and are semiconducting with an extremely large intrinsic carrier mobility of 3.3 cm(2) V(-1) s(-1). They are highly photoconductive, produce photocurrent with a quick response to light irradiation, and are capable of repetitive on-off switching. Moreover, these sheets facilitate a conduction path perpendicular to the sheet plane, thus exhibiting a spatially distinctive anisotropy in conduction. The noncovalent sheet assemblies with these unique characteristics are important for molecular optoelectronic devices based on solution-processed soft materials.

Single nanowire on a film as an efficient SERS-active platform.

Fabricating well-defined and highly reproducible platforms for surface-enhanced Raman scattering (SERS) is very important in developing practical SERS sensors. We report a novel SERS platform composed of a single metallic nanowire (NW) on a metallic film. Optical excitation of this novel sandwich nanostructure provides a line of SERS hot spots (a SERS hot line) at the gap between the NW and the film. This single nanowire on a film (SNOF) architecture can be easily fabricated, and the position of hot spots can be conveniently located in situ by using an optical microscope during the SERS measurement. We show that high-quality SERS spectra from benzenethiol, brilliant cresyl blue, and single-stranded DNA can be obtained on a SNOF with reliable reproducibility, good time stability, and excellent sensitivity, and thus, SNOFs can potentially be employed as effective SERS sensors for label-free biomolecule detection. We also report detailed studies of polarization- and material-dependent SERS enhancement of the SNOF structure.

Density functional and Ab initio study of Cr(CO)n (n = 1-6) complexes.

Cr(CO)n (n = 1-6) systems were studied for all possible spin states using density functional and high-level ab initio methods to provide a more complete theoretical understanding of the structure of species that may form during ligand dissociation of Cr(CO)6. We carried out geometry optimizations for each system and obtained vibrational frequencies, sequential bond dissociation energies (BDE), and total CO binding energies. We also compared the performance of various DFT functionals. Generally, the ground states of Cr(CO)6, Cr(CO)5, and Cr(CO)4, whose spin multiplicity is a singlet, are in good agreement with both previous theoretical results and currently available experimental data. Calculations on Cr(CO)3, Cr(CO)2, and CrCO provide new findings that the ground state of Cr(CO)3 might be a quintet with C2v symmetry instead of a singlet with C3v symmetry, and the ground state of Cr(CO)2 is not a linear quintet, as suggested by previous DFT calculations, but rather a linear septet. We also found that nonet states of Cr(CO)2 and CrCO display partial C-O bond breakage.