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Natural compounds cholic acid andβ-cyclodextrin are attachedseparately as pendant groups in a copolymer with N,N′-dimethylacrylamides. The formation ofsupramolecular hydrogels is induced by inclusion complex formationbetween cholic acid and β-cyclodextrin moieties as evidencedby rheological analysis and 1H NMR spectroscopy. Storagemodulus of the hydrogel shows a maximum value when the molar ratioof cholic acid to β-cyclodextrin units is adjusted to 1:1. Theconcentration of the hydrogel can be as low as 5.5 wt %. Both shear-thickeningand shear-thinning have been observed when the shear rate graduallyincreases from 0.01 to 100 s–1. The inclusion complexationrenders the gel–sol process reversible under heating and coolingcycles. The self-healing of such hydrogels is observed and confirmedby step-strain rheological measurements. The dynamically reversiblehost–guest complexation provides reasonably good mechanicalproperties of the cross-linked polymer network. The natural originof the constituents may make the hydrogels suitable candidates forbiomedical applications pending further tests. [ABSTRACT FROM AUTHOR]

Copolymers of N-alkylacrylamidesand methacrylatebearing cholic acid pendant groups were synthesized via radical polymerization.The cholic acid pendant groups of such copolymers can form complexeswith β-cyclodextrin, and the effect of complexation on theirthermoresponsive properties was studied. The phase transition temperatures(transition from hydrophilic to hydrophobic state) of the copolymersgradually increase with the addition of β-cyclodextrin, dueto the complexation of the cholic acid guest with the β-cyclodextrinhost. The increase of the phase transition temperature may be reversedby the addition of a competing guest molecule, potassium 1-adamantylcarboxylate.The host–guest complexation provides a straightforward wayto vary the thermoresponsive properties of such copolymers. [ABSTRACT FROM AUTHOR]

The electronic properties of semiconductor quantum dots (QDs) are critically dependent on the nature of the ligand molecules on their surfaces. Here we show the reversible formation of surface electronic trap states in the model system of solid thin films of PbS QDs capped with thiol molecules. As the temperature was increased from cryogenic to room temperature, we discovered a phase transition in the fluorescence spectra from excitonic emission to trap emission. The critical temperature (Tc) of the phase transition scales with molecular length and in each case is close to the bulk melting temperature of the capping molecules. We conclude that an order-disorder transition in the molecular monolayer above Tc introduces surface mobility and the formation of a disordered atomic lead layer at the QD/capping molecule interface, leading to electronic trap formation. [ABSTRACT FROM AUTHOR]

A series of amphiphilic copolymers were synthesized byreversibleaddition–fragmentation chain transfer cyclocopolymerizationof a styrenic monomer with maleic anhydride followed by grafting methoxypoly(ethylene glycol) onto the anhydride groups of the polymer chain.These amphiphilic graft copolymers exhibit multiple responsivenesstoward temperature, pH, and selected cations in aqueous solutions.The cloud points (CP) of the graft copolymers increase with increasinglength of the side chains and with increasing pH value of the solution.The addition of KCl and LiCl to the solutions had a salting-out effectlowering the CPs of the graft copolymers. The addition of NaCl, however,first raised the CP due to the complexation of the crown ether withNa橷 then lowered the CP. The light scattering resultsconfirmed an increase in phase transition temperature at lower concentrationsof NaCl (5 and 10 mM) and then a decrease at a higher concentrationof the sodium salt (100 mM). [ABSTRACT FROM AUTHOR]

A hallmark of cell-surface processes involving glycans is their multivalent interaction with glycan binding proteins (GBPs). Such a multivalent interaction depends critically on the mobility and density of signaling molecules on the membrane surface. While glycan microarrays have been used in exploring multivalent interactions, the lack of mobility and the difficulty in controlling surface density both limit their quantitative applications. Here we apply a fluidic glycan microarray, with glycan density varying for orders of magnitude, to profile cell surface interaction using a model system, the adhesion of Escherichia coli to mannose. We show the quantitative determination of monovalent and multivalent adhesion channels; the latter can be inhibited by nanopartices presenting a high density of mannosyl groups. These results reveal a new E. coli adhesion mechanism: the switching in the FimH adhesion protein avidity from monovalent to multivalent as the density of mobile mannosyl groups increases; such avidity switching enhances binding affinity and triggers multiple fimbriae anchoring. Affinity enhancement toward FimH has only been observed before for oligo-mannose due to the turn on of secondary interactions outside the mannose binding pocket. We suggest that the new mechanism revealed by the fluidic microarray is of general significance to cell surface interactions: the dynamic clustering of simple sugar groups (homogeneous or heterogeneous) on the fluidic membrane surface may simulate the functions of complex glycan molecules. [ABSTRACT FROM AUTHOR]

The article reports on the photoinduced electron transfer. The photoinduced electron transfer is of interest to research fields such as photocatalysis, surface photochemistry and dye-sensitized solar cells. The process of photoinduced electron transfer is divided into two mechanisms, which is either direct or indirect.

Progress in high-throughput MALDI-TOFMS analysis, especially in proteome applications, requires development of practical and efficient procedures for the preparation of proteins and peptides in a form suitable for high acquisition rates. These methods should improve successful identification of peptides, which depends on the signal intensity and the absence of interfering signals. Contamination of MALDI samples with alkali salts results in reduced MALDI peptide sensitivity and causes matrix cluster formation (widely reported for CHCA matrix) observed as signals dominating in the range below mix 1200 in MALDI spectra. One way to remove these background signals, especially for concentrations of peptides lower than 10 fmol/ μL, is to wash matrix/sample spots after peptide cocrystallization on the MALDI plate with deionized water prior to analysis. This method takes advantage of the low water solubility of the CHCA compared to its alkali salts. We report here that the application of some ammonium salt solutions, such as citrates and phosphates, instead of deionized water greatly improves the efficiency of this washing approach. Mother way to reduce matrix cluster formation is to add ammonium salts as a part of the MALDI matrix. The best results were obtained with monoammonium phosphate, which successfully suppressed matrix clusters and improved sensitivity. Combining both of these approaches-the addition of ammonium salts in the CHCA matrix followed by one postcrystallization washing step with ammonium buffer-provided a substantial (∼3-5-fold) improvement in the sensitivity of MALDI-MS detection compared to unwashed sample spots. This sample preparation method resulted in improved spectral quality and was essential for successful database searching for subnanomolar concentrations of protein digests. [ABSTRACT FROM AUTHOR]

Presents information on a study which measures solution diffusion coefficients of robust fluorophores using the method of fluorescence correlation spectroscopy with patterned photoexcitation. Role of molecular diffusion in the design of small-volume detection experiments; Usefulness of measurements of diffusion of florescently labeled biomolecules; Methods for measuring solution-phase diffusion coefficients.

Water drops impacting windshields of high-speed trains and aircraft as well as blades in steam turbine power generators obliquely and at high speeds are difficult to repel. Impacting drops penetrate the void regions of nanotextured and microtextured superhydrophobic coatings, with this pinning resulting in the loss of drop mobility. In order to repel high-speed water drops, we nanotextured polymer surfaces with nanowire bundles separated from their neighbors by microscale void regions, with the nanowires in a bundle separated from their neighbors by nanoscale void regions. Water drops with speeds below a critical speed rebound completely. Water drops with speeds exceeding a critical speed rebound partially, but residual droplets that begin to be pinned undergo a spontaneous dewetting process and slide off. The natural oscillations of residual droplets drive this dewetting process in the interbundle void regions, resulting in a transition from the sticky Wenzel state to the slippery Cassie state without external stimuli. [ABSTRACT FROM AUTHOR]

The use of natural compounds to construct reversible networks is an attractive strategy in biomaterials design. Our design is based on a host-guest pair of natural compounds β-cyclodextrin and cholic acid through the use of a cholic acid dimer tethered with a poly(ethylene glycol) spacer, which subsequently served as a guest cross-linker to afford a hydrogel with copolymers bearing β-cyclodextrin pendants. The hydrogel after incision self-heals rapidly under ambient atmosphere as observed and confirmed by rheological measurements. To endow the hydrogels with reversibility and responsiveness, the addition of a CO2-switchable guest of benzimidazole followed by alternating treatments with CO2 and N2 leads to a reversible sol-gel transition due to the dynamic complexation between the cholic acid and β-cyclodextrin units. The CO2 responsiveness and the natural origin of the constituents make these self-healing hydrogels attractive as smart biomaterials. [ABSTRACT FROM AUTHOR]

Two-way reversible shape memory polymers (2W-SMPs) are highly desirable for many applications. We report for the first time the use of random copolymers with cocrystallizable monomeric units for the preparation of such polymer networks. Homopolymers and random copolymers of ε-caprolactone and ω-pentadecalactone were designed and made by ring-opening polymerization with Candida antarctica lipase B as catalyst. The melting temperatures of these prepolymers may be adjusted by the use of various molar ratios of the comonomers. Upon thiol-ene cross-linking, the polymer network exhibited two-way reversible shape memory effects under both stress-free and stress conditions. The actuation temperature (TA) of the 2W-SMP under stress-free condition can be tuned in a broad range using a selected mixture of prepolymers followed by photo-cross-linking with a multifunctional cross-linker. Increasing the initial stretching stress amplitude led to an increased absolute strain change under both stress-free and stress condition; it led to a reduced relative strain change under stress-free condition, but almost no change under stress condition. The evolution of the microstructure of 2W-SMPs under stress-free condition cycle was studied by the use of X-ray diffraction (2D-WAXD and SAXS). This is the first report on the tuning of TA of 2W-SMPs under stress-free condition by the use of one or two prepolymers to form a chemically cross-linked network. We have also eliminated the use of toxic metal catalysts in the synthesis of polymeric biomaterials. The materials are shown to be capable of performing reversible bending-unbending and coiling-uncoiling motions. [ABSTRACT FROM AUTHOR]

Betulin is a natural triterpene compound with anticarcinogen and antiviral activities. It is conjugated with methacrylate and then copolymerized with a galactose-bearing comonomer by RAFT polymerization to yield both random and block copolymers. These glycopolymers are designed to possess similar molecular weights and monomer compositions for easy comparison. They self-assembled into micelles, as shown by dynamic light scattering (DLS) and transmission electron microscopy. The smaller micelles formed by the random copolymers facilitated the encapsulation of Nile Red and released more of this hydrophobic model compound (46% in 4 days versus 32% released from the block copolymers). These glycopolymers interacted with lectins, such as RCA120, as studied by turbidity assay and DLS. The block copolymers formed larger aggregates and clustered faster than the random copolymers. The betulin-based glycopolymers may serve as biocompatible multifunctional biomaterials and carriers for use in targeted release of drugs. [ABSTRACT FROM AUTHOR]

The article discusses the quantitative application of DNA microarrays in proteomics in the U.S. The difficulty in the application results in a large part from the lack of understanding and control of the chemical and physical processes involved in probe immobilization. A survey of protein microarray literature suggests that one of the major reasons for poor reproducibility is nonuniform spot profile.

Explores how the two-dimensional arrangement of metal ions binding to surface ligand groups may lead to a surface chelate effect. Significance of the surface chelate effect to adsorption on molecular surfaces; Dependence of the surface chelate effect on chemical functionality and monolayer structure; Incorporation of the effect into the design of monolayer-based technologies.

Microarray-based binding assays facilitate the discovery of protein ligands from large collections of small molecules. Hundreds of ligands can be identified, yet only a small portion of them have interfering effects (competitive or noncompetitive) on a specific protein–receptor binding reaction. Further efficient screening of ligands for those with specific modifying effect is needed in order to take the full advantage of throughputs of microarray-based assays for drug discovery. We report a label-free "microarray-in-microplate" assay platform for simultaneous acquisition of at least 32 dose–response curves in a single experiment, each curve having 12 concentration points. When combined with ligand discovery, this makes the microarray-based platform a true high-throughout means of finding inhibitors to specific protein–receptor reactions starting from a large collection of small-molecule libraries. [ABSTRACT FROM AUTHOR]

The thermo-responsiveness of polymersin aqueous media can be tuned by the choice of comonomers used inthe synthesis of block copolymers made of random sequences of thesame comonomers but of different molar ratios. The same syntheticapproach may be applied to other stimuli and we have made diblockrandom copolymers with both pH- and thermo-responsiveness and studiedthe formation of vesicles whose membrane core and coronas may be invertedin aqueous media. Sequential reversible addition–fragmentationchain transfer (RAFT) polymerization was used to prepare well-definedblock copolymers in the form of AnBm-b-ApCq, where A, B, and C are N-n-propylacrylamide (nPA), 2-(diethylamino)ethyl methacrylate (DEAEMA), and N-ethylacrylamide (EA), respectively. This polymer shows interesting“schizophrenic” behavior in aqueous solutions. Bothblocks are thermo-responsive, and one block is pH-responsive in whichthe tertiary amine group of DEAEMA may be protonated at a lower pH.A molecularly dissolved polymer is obtained at neutral pH and ambienttemperature. At pH 7 and 37 °C, the polymer self-assembles intovesicles with the poly(nPA0.8-co-EA0.2) block as the membrane core (mean hydrodynamicdiameter of the vesicles Dh= 148 nm).In an alkaline medium (pH 10) at 25 °C, the membrane core andthe coronas of the vesicles are inverted with poly(nPA0.8-co-DEAEMA0.2) blockforming the core (Dh= 60 nm). In addition,two-step phase transitions are observed in both alkaline and neutralsolutions corresponding to the cloud points of the individual blocks.Here, the random nature of the blocks allows fine-tuning the thermo-responsivenessbased solely on lower critical solution temperatures and its combinationwith pH-sensitivity provides vesicles with switchable membrane coreand corona in aqueous solution. [ABSTRACT FROM AUTHOR]

Thermoresponsive characteristics of oligo(ethylene glycol)derivativesof lithocholic acid (LCA) depend on the hydrophilic/hydrophobic balanceof the compounds. Below a threshold temperature (∼30 °C),one of the derivatives, LCA(EG4)2, self-assemblesin water into hollow nanotubes that form thixotropic gels at highconcentrations. Two concentration regimes were observed in the plateaumoduli (G0∼ G′). At high concentrations, the mobility of the nanotubesis restricted at the ordered microdomains acting as cross-links. Insemidilute aqueous solutions, the linkers between the aggregated domainsin the fragile gel rupture easily, and this effect depends stronglyon dilution. Organic hydrophobic additives perturb the self-assemblingprocess, leading to changes in the two transition temperatures andpoorer mechanical properties of the gel. [ABSTRACT FROM AUTHOR]

Singlet fission, the creation of two triplet excitons from one singlet exciton, is being explored to increase the efficiency of solar cells and photo detectors based on organic semiconductors, such as pentacene and tetracene. A key question is how to extract multiple electron-hole pairs from multiple excitons. Recent experiments in our laboratory on the pentacene/C60 system (Chan, W.-L.; et al. Science2011, 334, 1543-1547) provided preliminary evidence for the extraction of two electrons from the multiexciton (ME) state resulting from singlet fission. The efficiency of multielectron transfer is expected to depend critically on other dynamic processes available to the singlet (S1) and the ME, but little is known about these competing channels. Here we apply time-resolved photoemission spectroscopy to the tetracene/C60 interface to probe one- and two-electron transfer from S1 and ME states, respectively. Unlike ultrafast (~100 fs) singlet fission in pentacene where two-electron transfer from the multiexciton state resulting from singlet fission dominates, the relatively slow (~7 ps) singlet fission in tetracene allows both one- and two-electron transfer from the S1 and the ME states that are in a quantum mechanical superposition. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photoexcited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C60, respectively. Kinetic analysis shows that ~60% of the S1 ME quantum superposition transfers one electron through the S1 state to C60 while 40% undergoes two-electron transfer through the ME state. We discuss design principles at donor/acceptor interfaces for optimal multiple carrier extraction from singlet fission for solar energy conversion. [ABSTRACT FROM AUTHOR]

Block copolymers consisting of two segments of randomcopolymersof N-alkylacrylamides have been synthesized by asequential reversible addition–fragmentation chain transfer(RAFT) polymerization. The copolymers in the form of AnBm-b-ApBqhavebeen made of two blocks of N-n-propylacrylamide(nPA) and N-ethylacrylamide (EA) of different compositionsto obtain polymers with stepwise thermosensitivity. The control ofthe RAFT polymerization was confirmed by studying the kinetics ofthe copolymerization process. The block random copolymer, poly(nPAx-co-EA1–x)-block-poly(nPAy-co-EA1–y), is well-defined and has a low polydispersity. The cloud pointsof the random copolymers can be tuned by varying the chemical compositionof the copolymers. The diblock copolymer exhibited a two-step phasetransition upon heating to 41.5 and 53.0 °C, corresponding tothe cloud points of the individual blocks. Dynamic light scatteringexperiments also showed the stepwise aggregation properties of thecopolymer in aqueous solutions. [ABSTRACT FROM AUTHOR]

The tunneling resistance and electronic structure of metal-molecule-metal junctions based on oligoacene (benzene, naphthalene, anthracene, and tetracene) thiol and dithiol molecules were measured and correlated using conducting probe atomic force microscopy (CP-AFM) in conjunction with ultraviolet photoelectron spectroscopy (UPS). Nanoscopic tunnel junctions (∼10 nm²) were formed by contacting oligoacene self-assembled monolayers (SAMs) on flat Ag, Au, or Pt substrates with metalized AFM tips (Ag, Au, or Pt). The low bias (<0.2 V) junction resistance (R) increased exponentially with molecular length (s), i.e., R = R0 exp(βs), where R0 is the contact resistance and β is the tunneling attenuation factor. The R0 values for oligoacene dithiols were 2 orders of magnitude less than those of oligoacene thiols. Likewise, the β value was 0.5 per ring (0.2 Å-1) for the dithiol series and 1.0 per ring (0.5 Å-1) for the monothiol series, demonstrating that β is not simply a characteristic of the molecular backbone but is strongly affected by the number of chemical (metal-S) contacts. R0 decreased strongly as the contact work function (Φ) increased for both monothiol and dithiol junctions, whereas β was independent of Φ within error. This divergent behavior was explained in terms of the metal-S bond dipoles and the electronic structure of the junction; namely, β is independent of contact type because of weak Fermi level pinning (UPS revealed EF - EHOMO varied only weakly with Φ), but R0 varies strongly with contact type because of the strong metal-S bond dipoles that are responsible for the Fermi level pinning. A previously published triple barrier model for molecular junctions was invoked to rationalize these results in which R0 is determined by the contact barriers, which are proportional to the size of the interfacial bond dipoles, and β is determined by the bridge barrier, EF - EHOMO. Current-voltage (I-V) characteristics obtained over a larger voltage range 0-1 V revealed a characteristic transition voltage Vtrans at which the current increased more sharply with voltage. Vtrans values were generally >0.5 V and were well correlated with the bridge barrier EF - EHOMO. Overall, the combination of electronic structure determination by UPS with length- and work function-dependent transport measurements provides a remarkably comprehensive picture of tunneling transport in molecular junctions based on oligoacenes. [ABSTRACT FROM AUTHOR]

Poly(vinyl alcohol) (PVA) is a cytocompatible synthetic polymer and has been commonly used to prepare hydrogels. Bile acids and β-cyclodextrin are both natural compounds and they form stable host-guest inclusion complexes. They are attached covalently onto a low molecular weight PVA separately. Self-healing hydrogels can be easily formed by mixing the aqueous solutions of these PVA based polymers. The mechanical properties of the hydrogels can be tuned by varying the molar fractions of bile acid units on PVA. The dynamic inclusion complexation of the host-guest pair of the hydrogel allows the self-healing rapidly under ambient atmosphere and their mechanical properties could recover their original values in 1 min after incision. These PVA based polymers exhibited the good cytocompatibility and high hemocompatibility as shown by their biological evaluations. The use of natural compounds for host-guest interaction make such gels especially convenient to use as biomaterials, an advantage over conventional hydrogels prepared through freeze-thaw method. [ABSTRACT FROM AUTHOR]

Acrylamide derivatives of bile acids are chosen as a hydrophobic comonomer to copolymerize with acrylamide via reversible addition and fragmentation chain transfer (RAFT) polymerization to afford a series of copolymers of P(AAm-co-CAA). These copolymers exhibit a sharp and reversible insoluble-soluble transition in water upon heating to a mixing temperature (T mix) related to the upper critical solution temperature (UCST). T mix of these copolymers can be conveniently tuned to a practical temperature range, around 37 °C for biomedical applications. T mix rises with increasing molar fraction of the bile acid-based acrylamide and increasing concentration of the aqueous solution of the copolymers. The addition of a natural host molecule β-cyclodextrin lowered the T mix. The insoluble-soluble transition of the copolymers was also evidenced by dynamic light scattering and transmission electron microscopy. The biocompatible nature of the bile acids and β-cyclodextrins may make these copolymers potentially useful for biomedical applications. [ABSTRACT FROM AUTHOR]

Metastable structural polymorphs can have superior properties and applications to their thermodynamically stable phases, but the rational synthesis of metastable phases is a challenge. Here, a new strategy for stabilizing metastable phases using surface functionalization is demonstrated using the example of formamidinium lead iodide (FAPbI3) perovskite, which is metastable at room temperature (RT) but holds great promises in solar and light-emitting applications. We show that, through surface ligand functionalization during direct solution growth at RT, pure FAPbI3 in the cubic perovskite phase can be stabilized in nanostructures and thin films at RT without cation or anion alloying. Surface characterizations reveal that long-chain alkyl or aromatic ammonium (LA) cations bind to the surface of perovskite structure. Calculations show that such functionalization reduces the surface energy and plays a dominant role in stabilizing the metastable perovskite phase. Excellent photophysics and optically pumped lasing from the stabilized single-crystal FAPbI3 nanoplates with low thresholds were demonstrated. High-performance solar cells can be fabricated with such directly synthesized stabilized phase-pure FAPbI3 with a lower bandgap. Our results offer new insights on the surface chemistry of perovskite materials and provide a new strategy for stabilizing metastable perovskites and metastable polymorphs of solid materials in general. [ABSTRACT FROM AUTHOR]

Interfacial charge separation and recombination at heterojunctions of monolayer transition metal dichalcogenides (TMDCs) are of interest to two-dimensional optoelectronic technologies. These processes can involve large changes in parallel momentum vector due to the confinement of electrons and holes to the K valleys in each layer. Because these high-momentum valleys are usually not aligned across the interface of two TMDC monolayers, how parallel momentum is conserved in the charge separation or recombination process becomes a key question. Here we probe this question using the model system of a type-II heterojunction formed by MoS2 and WSe2 monolayers and the experimental technique of femtosecond pump-probe spectroscopy. Upon photoexcitation specifically of WSe2 at the heterojunction, we observe ultrafast (<40 fs) electron transfer from WSe2 to MoS2, independent of the angular alignment and thus momentum mismatch between the two TMDCs. The resulting interlayer charge transfer exciton decays via nonradiative recombination with rates varying by up to three-orders of magnitude from sample to sample but with no correlation with interlayer angular alignment. We suggest that the initial interfacial charge separation and the subsequent interfacial charge recombination processes circumvent momentum mismatch via excess electronic energy and via defect-mediated recombination, respectively. [ABSTRACT FROM AUTHOR]

The article discusses the doping, synthesis, and transformation of magic-sized semiconductor alloy nanoclusters. Topics include the challenges posed by the synthetic control of multicomponent semiconductor nanoclusters, and the synthesis of alloy clusters containing three or four types of atoms using Lewis acid base reactions.

This Communication describes a new molecular design for the efficient synthesis of donor-acceptor, cove-edge graphene nanoribbons and their properties in solar cells. These nanoribbons are long (∼5 nm), atomically precise, and soluble. The design is based on the fusion of electron deficient perylene diimide oligomers with an electron rich alkoxy pyrene subunit. This strategy of alternating electron rich and electron poor units facilitates a visible light fusion reaction in >95% yield, whereas the cove-edge nature of these nanoribbons results in a high degree of twisting along the long axis. The rigidity of the backbone yields a sharp longest wavelength absorption edge. These nanoribbons are exceptional electron acceptors, and organic photovoltaics fabricated with the ribbons show efficiencies of ∼8% without optimization. [ABSTRACT FROM AUTHOR]

This Communication describes a new molecular design that yields ultranarrowband organic photodetectors. The design is based on a series of helically twisted molecular ribbons as the optoelectronic material. We fabricate charge collection narrowing photodetectors based on four different helical ribbons that differ in the wavelength of their response. The photodetectors made from these materials have narrow spectral response with full-width at half maxima of <20 nm. The devices reported here are superior by approximately a factor of 5 to those from traditional organic materials due to the narrowness of their response. Moreover, the active layers for the helical ribbon-based photodetectors are solution-cast but have performance that is comparable to the state-of-the-art narrowband photodetectors made from methylammonium lead trihalide perovskite single crystals. The ultranarrow bandwidth for detection results from the helical ribbons' high absorption coefficient, good electron mobility, and sharp absorption edges that are defined by the twisted molecular conformation. [ABSTRACT FROM AUTHOR]

Natural compounds glucosamine and cholic acid have been used to make acrylic monomers which are subsequently used to prepare amphiphilic block copolymers by reversible addition-fragmentation chain transfer (RAFT) polymerization. Despite the striking difference in polarity and solubility, three diblock copolymers consisting of glucosamine and cholic acid pendants with different hydrophilic and hydrophobic chain lengths have been synthesized without the use of protecting groups. They are shown to self-assemble into polymeric micelles with a "bitter" bile acid core and "sweet" sugar shell in aqueous solutions, as evidenced by dynamic light scattering and transmission electron microscopy. The critical micelle concentration varies with the hydrophobic/hydrophilic ratio, ranging from 0.62 to 1.31 mg/L. Longer chains of polymers induced the formation of larger micelles in range of 50-70 nm. These micelles can solubilize hydrophobic compounds such as Nile Red in aqueous solutions. Their loading capacity mainly depends upon the hydrophobic/hydrophilic ratio of the polymers, and may be also related to the length of the hydrophilic block. These polymeric micelles allowed for a 10-fold increase in the aqueous solubility of paclitaxel and showed no cytotoxicity below the concentration of 500 mg/L. Such properties make these polymeric micelles interesting reservoirs for hydrophobic molecules and drugs for biomedical applications. [ABSTRACT FROM AUTHOR]

A mixture of a cholic acid dimer with a secondary amine group and formic acid at a molar ratio of 1/1 is regarded as an organic salt, and it self-assembles in aqueous solutions to form monodisperse nanofibers. The nanofibers are separated at low concentrations of the mixture but entangle with each other at high concentrations to form well-dispersed and randomly arranged 3D fibrous networks. Above the minimum gelation concentration of the dimer, the fibrous network is strong enough to gelate the aqueous solutions to form a hydrogel. Hydrogels obtained from the dimer salt at a lower concentration are isotropic and show extinction between crossed polarizers in the polarizing microscope, whereas they become anisotropic (i.e., nematic hydrogels) upon increasing the dimer salt concentration or under physical stirring. The parallel arrangement of nanofibers from randomly directed fibrous networks may be responsible for the formation of such nematic hydrogels. [ABSTRACT FROM AUTHOR]

Organic photodetectors (OPDs) are attractive for their high optical absorption coefficient, broad wavelength tunability, and compatibility with lightweight and flexible devices. Here we describe a new molecular design that enables high performance organic photodetectors. We use a rigid, conjugated macrocycle as the electron acceptor in devices to obtain high photocurrent and low dark current. We make a direct comparison between the devices made with the macrocyclic acceptor and an acyclic control molecule; we find that the superior performance of the macrocycle originates from its rigid, conjugated, and cyclic structure. The macrocycle's rigid structure reduces the number of charged defects originating from deformed sp2 carbons and covalent defects from photo/thermoactivation. With this molecular design, we are able to suppress dark current density while retaining high responsivity in an ultrasensitive nonfullerene OPD. Importantly, we achieve a detectivity of ~1014 Jones at near zero bias voltage. This is without the need for extra carrier blocking layers commonly employed in fullerene-based devices. Our devices are comparable to the best fullerene-based photodetectors, and the sensitivity at low working voltages (<0.1 V) is a record for nonfullerene OPDs. [ABSTRACT FROM AUTHOR]

2D colloidal crystals (CCs) have important applications; however, the fabrication of large-area, high-quality 2D CCs is still far from being trivial, and the fabrication of 2D microgel CCs is even harder. Here, we have demonstrated that they can be facilely fabricated using charge-reversible substrates. The charge-reversible substrates were prepared by modification with amino groups. The amino groups were then protected by amidation with 2,2-dimethylsuccinic anhydride. At acidic pH, the surface charge of the modified substrate will change from negative to positive as a result of the hydrolysis of the amide bonds and the regeneration of the amino groups. 2D microgel CCs can be simply fabricated by applying a concentrated microgel dispersion on the modified substrate. The negatively charged surface of the substrate allows the negatively charged microgel spheres, especially those close to the substrate, to self-assemble into 3D CCs. With the gradual hydrolysis of the amide bonds and the charge reversal of the substrate, the first 111 plane of the 3D assembly is fixed in situ on the substrate. The resulting 2D CC has a high degree of ordering because of the high quality of the parent 3D microgel CC. Because large-area 3D microgel CCs can be facilely fabricated, this method allows for the fabrication of 2D CCs of any size. Nonplanar substrates can also be used. In addition, the interparticle distance of the 2D array can be tuned by the concentration of the microgel dispersion. Besides rigid substrates (such as glass slides, quartz slides, and silicon wafers), flexible polymer films, including polyethylene terephthalate and poly(vinyl chloride) films, were also successfully used as substrates for the fabrication of 2D microgel CCs. [ABSTRACT FROM AUTHOR]

The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tuned by electrostatic gate potentials. Here we report gate-tunable imaging and spectroscopy of monolayer MoS2 by atomic-resolution scanning tunneling microscopy/spectroscopy (STM/STS). Our measurements are performed on large-area samples grown by metal-organic chemical vapor deposition (MOCVD) techniques on a silicon oxide substrate. Topographic measurements of defect density indicate a sample quality comparable to single-crystal MoS2. From gate voltage dependent spectroscopic measurements, we determine that in-gap states exist in or near the MoS2 film at a density of 1.3 × 1012 eV-1 cm-2. By combining the single-particle band gap measured by STS with optical measurements, we estimate an exciton binding energy of 230 meV on this substrate, in qualitative agreement with numerical simulation. Grain boundaries are observed in these polycrystalline samples, which are seen to not have strong electronic signatures in STM imaging. [ABSTRACT FROM AUTHOR]

The excellent intrinsic optoelectronic properties of methylammonium lead halide perovskites (MAPbX3, X = Br, I), such as high photoluminescence quantum efficiency, long carrier lifetime, and high gain coupled with the facile solution growth of nanowires make them promising new materials for ultralow-threshold nanowire lasers. However, their photo and thermal stabilities need to be improved for practical applications. Herein, we report a low-temperature solution growth of single crystal nanowires of formamidinium lead halide perovskites (FAPbX3) that feature red-shifted emission and better thermal stability compared to MAPbX3. We demonstrate optically pumped room-temperature near-infrared (~820 nm) and green lasing (~560 nm) from FAPbI3 (and MABr-stabilized FAPbI3) and FAPbBr3 nanowires with low lasing thresholds of several microjoules per square centimeter and high quality factors of about 1500-2300. More remarkably, the FAPbI3 and MABr-stabilized FAPbI3 nanowires display durable room-temperature lasing under ~108 shots of sustained illumination of 402 nm pulsed laser excitation (150 fs, 250 kHz), substantially exceeding the stability of MAPbI3 (~107 laser shots). We further demonstrate tunable nanowire lasers in wider wavelength region from FA-based lead halide perovskite alloys (FA,MA)PbI3 and (FA,MA)Pb(I,Br)3 through cation and anion substitutions. The results suggest that formamidinium lead halide perovskite nanostructures could be more promising and stable materials for the development of light-emitting diodes and continuous-wave lasers. [ABSTRACT FROM AUTHOR]

We describe the synthesis of two conjugated macrocycles that are formed from the end-to-end linking of stilbenes. We have named these macrocycles cyclostilbenes. The two cyclostilbene isomers created in this study differ in the configuration of the double bond in their subunits. These macrocycles are formed selectively through a stepwise reductive elimination from a tetraplatinum precursor and subsequent photoisomerization. Single-crystal X-ray diffraction reveals the formation of channel architectures in the solid state that can be filled with guest molecules. The cyclostilbene macrocycles emit blue light with fluorescence quantum yields that are high (>50%) and have photoluminescence lifetimes of ~0.8--1.5 ns. The breadth and large Stokes shift in fluorescence emission, along with broad excited-state absorption, result from strong electronic--vibronic coupling in the strained structures of the cyclostilbenes. [ABSTRACT FROM AUTHOR]

Singlet fission (SF) has the potential to significantly enhance the photocurrent in single-junction solar cells and thus raise the power conversion efficiency from the Shockley--Queisser limit of 33% to 44%. Until now, quantitative SF yield at room temperature has been observed only in crystalline solids or aggregates of oligoacenes. Here, we employ transient absorption spectroscopy, ultrafast photoluminescence spectroscopy, and triplet photosensitization to demonstrate intramolecular singlet fission (iSF) with triplet yields approaching 200% per absorbed photon in a series of bipentacenes. Crucially, in dilute solution of these systems, SF does not depend on intermolecular interactions. Instead, SF is an intrinsic property of the molecules, with both the fission rate and resulting triplet lifetime determined by the degree of electronic coupling between covalently linked pentacene molecules. We found that the triplet pair lifetime can be as short as 0.5 ns but can be extended up to 270 ns. [ABSTRACT FROM AUTHOR]

Recent discoveries of highly efficient solar cells based on lead iodide perovskites have led to a surge in research activity on understanding photo carrier generation in these materials, but little is known about trap states that may be detrimental to solar cell performance. Here we provide direct evidence for hole traps on the surfaces of three-dimensional (3D) CH3NH3PbI3 perovskite thin films and excitonic traps below the optical gaps in these materials. The excitonic traps possess weak optical transition strengths, can be populated from the relaxation of above gap excitations, and become more significant as dimensionality decreases from 3D CH3NH3PbI3 to two-dimensional (2D) (C4H9NH3I)2(CH3NH3I)n-1(PbI2)n (n = 1, 2, 3) perovskites and, within the 2D family, as n decreases from 3 to 1. We also show that the density of excitonic traps in CH3NH3PbI3 perovskite thin films grown in the presence of chloride is at least one-order of magnitude lower than that grown in the absence of chloride, thus explaining a widely known mystery on the much better solar cell performance of the former. The trap states are likely caused by electron-phonon coupling and are enhanced at surfaces/interfaces where the perovskite crystal structure is most susceptible to deformation. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) multicellularspheroids (MCSs) mimic thestructure and function of real tissue much better than the conventional2D cell monolayers, however,their application was severely hindered by difficulties in their generation.An ideal method for MCS fabrication should produce spheroids withnarrow size distribution and allow for control over their size. Themethod should also be simple, cheap, and scalable. Here, we use patternednonadhesive poly(2-hydroxyethyl methacrylate) hydrogel films to guidethe self-assembly of cells. The films were fabricated directly inthe wells of cell culture plates. They were patterned spontaneouslyby swelling in water, without the use of any template or specializedfacilities. When cell suspension is added, the cells settle down bygravity to the bottom. Because of the presence of the wrinkling patterncomposed of uniformed microcaves, the cells accumulate to the centerof the microcaves and gradually self-assemble into MCSs. Using thismethod, monodisperse MCSs were generated. The size of the spheroidscan be facilely controlled by the number of cells seeded. The methodis compatible with the conventional monolayer cell culture method.Thousands of spheroids can be generated in a single well. We expectthis method will pave the way for the application of MCSs in variousbiomedical areas. [ABSTRACT FROM AUTHOR]

Singlet fission, the conversion of a singlet excitation into two triplet excitations, is a viable route to improved solar-cell efficiency. Despite active efforts to understand the singlet fission mechanism, which would aid in the rational design of new materials, a comprehensive understanding of mechanistic principles is still lacking. Here, we present the first study of singlet fission in crystalline hexacene which, together with tetracene and pentacene, enables the elucidation of mechanistic trends. We characterize the static and transient optical absorption and combine our findings with a theoretical analysis of the relevant electronic couplings and rates. We find a singlet fission time scale of 530 fs, which is orders of magnitude faster than tetracene (10— 100 ps) but significantly slower than pentacene (80—110 fs). We interpret this increased time scale as a multiphonon relaxation effect originating from a large exothermicity and present a microscopic theory that quantitatively reproduces the rates in the acene family. [ABSTRACT FROM AUTHOR]

The absorption of one photon by a semiconductor material usually creates one electron–hole pair. However, this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron–hole pairs. This process, where a singlet exciton transforms to two triplet excitons, can have quantum yields as high as 200%. Singlet fission may be useful to solar cell technologies to increase the power conversion efficiency beyond the so-called Shockley-Queisser limit. Through time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene, our lab has recently provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state (also called a correlated triplet pair). More importantly, we found that population of the multiexciton state rises at the same time as the singlet state on the ultrafast time scale upon photoexcitation. This observation does not fit with the traditional view of singlet fission involving the incoherent conversion of a singlet to a triplet pair. However, it provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately after optical excitation. [ABSTRACT FROM AUTHOR]

Bile acids are amphiphilic endogenous steroids that actas anionicsurfactants in the digestive tract and aggregate in aqueous solutions.Nonionic surfactants were synthesized by grafting poly(ethylene glycol)chains of various lengths (pegylation) to three bile acids (lithocholic,deoxycholic, and cholic acid) using anionic polymerization. The aggregationproperties of the derivatives were studied with viscosity measurementsand light scattering as well as with steady-state and time-resolvedfluorescence techniques, and the aggregates were visualized by transmissionelectron microscopy to elucidate the effect of pegylation on the aggregationprocess. The fluorescence results showed a good correlation with thecapacity of the bile acid derivatives to solubilize a hydrophobicdrug molecule. The solubilization of ibuprofen depends on the lengthand the number of grafted PEG chains, and the solubilization efficiencyincreases with fewer PEG chains on the bile acid. The results indicatetheir potential for use in the design of new bile acid-based drug-deliverysystems. [ABSTRACT FROM AUTHOR]

The aggregation of peptides into amyloid fibrils plays a crucial role in various neurodegenerative diseases. While it has been generally recognized that fibril formation in vivo may be greatly assisted or accelerated by molecular surfaces, such as cell membranes, little is known about the mechanism of surface-mediated fibrillation. Here we study the role of adsorbed Alzheimer’s amyloid-β peptide (Aβ42) on surface-mediated fibrillation using polymer coatings of varying hydrophobicity as well a supported lipid bilayer membrane. Using single molecule fluorescent tracking and atomic force microscopy imaging, we show that weakly adsorbed peptides with two-dimensional diffusivity are critical precursors to fibril growth on surfaces. This growth mechanism is inhibited on the highly hydrophilic surface where the surface coverage of adsorbed peptides is negligible or on the highly hydrophobic surface where the diffusion constant of the majority of adsorbed peptides is too low. Physical properties that favor weakly adsorbed peptides with sufficient translational mobility can locally concentrate peptide molecules on the surface and promote inter-peptide interaction via two-dimensional confinement, leading to fibrillation at Aβ peptide concentration many orders of magnitude below the critical concentration for fibrillation in the bulk solution. [ABSTRACT FROM AUTHOR]

Infectious diseases are often initiated by microbial adherence that is mediated by the binding of attachment molecules, termed adhesins, to cell surface receptors on host cells. We present an experimental system, oblique-incidence reflectivity difference (OI-RD) microscopy, which allows the detection of novel, low-affinity microbial attachment mechanisms that may be essential for infectious processes. OI-RD microscopy was used to analyze direct binding of the oncopathogen, Helicobacter pylori (H. pylori) to immobilized glycoconjugates in real time with no need for labeling tags. The results suggest the presence of additional Lewis b blood group antigen (Leb) binding adhesins that have not been detected previously. OI-RD microscopy also confirmed the high-affinity binding of H. pylori outer-membrane protein BabA to Leb. The OI-RD microscopy method is broadly applicable to real-time characterization of intact microbial binding to host receptors and offers new strategies to elucidate the molecular interactions of infectious agents with human host cells. [ABSTRACT FROM AUTHOR]

We explored two macromolecular scaffolds, bovine serum albumin (BSA) and polyvinyl alcohol (PVA), as chemically complementary platforms for immobilizing small molecule compounds on functionalized glass slides. We conjugated biotin molecules to BSA and amine-derivatized PVA and subsequently immobilized the conjugates on epoxy-functionalized glass slides through reaction of free amine residues on BSA and PVA with surface-bound epoxy groups. We studied binding reactions of such immobilized small molecule targets with solution-phase protein probes using an oblique-incidence reflectivity difference scanning optical microscope. The results showed that both BSA and amine-derivatized PVA were effective and efficient as carriers of small molecules with NHS residues and fluoric residues and for immobilization on epoxy-coated solid surfaces. A significant fraction of the conjugated small molecules retain their innate chemical activity. [ABSTRACT FROM AUTHOR]

As drug delivery, therapy, and medical imaging are becoming increasingly cell-specific, there is a critical need for high fidelity and high-throughput screening methods for cell surface interactions. Cell membrane-mimicking surfaces, i.e., supported lipid bilayers (SLBs), are currently not sufficiently robust to meet this need. Here we describe a method of forming fluidic and air-stable SLBs through tethered and dispersed cholesterol groups incorporated into the bottom leaflet. Achieving air stability allows us to easily fabricate SLB microarrays from direct robotic spotting of vesicle solutions. We demonstrate their application as cell membrane-mimicking microarrays by reconstituting peripheral as well as integral membrane components that can be recognized by their respective targets. These demonstrations establish the viability of the fluidic and air-stable SLB platform for generating content microarrays in high throughput studies, e.g., the screening of drugs and nanomedicine targeting cell surface receptors. [ABSTRACT FROM AUTHOR]

We apply attenuated total internal reflection Fourier transform infrared (ATIR-FTIR) spectroscopy to directly probe active layers in organic thin film transistors (OTFTs). The OTFT studied uses the n-type organic semiconductor N-N-dioctyl-3,4,9, 10-perylene tetracarboxylic diimide (PTCDI-C8) and a polymer electrolyte gate dielectric made from poly(ethylene oxide) and LiCIO4. FTIR spectroscopy of the device shows signatures of anionic PTCDI-C8 species and broad polaron bands when the organic semiconductor layer is doped under positive gate bias (VG). There are two distinctive doping regions: a reversible and electrostatic doping region for VG ⩽ 2 V and an irreversible and electrochemical doping regime for VG > 2 V. On the basis of intensity loss of vibrational peaks attributed to neutral PTCDI-C8, we obtain a charge carrier density of 2.9 × 1014/cm2 at VG = 2 V; this charge injection density corresponds to the conversion of slightly more than one monolayer of PTCDI-C8 molecules into anions. At higher gate bias voltage, electrochemical doping involving the intercalation of Li+ into the organic semiconductor film can convert all PTCDI-C8 molecules in a 30-nm film into anionic species. For comparison, when a conventional gate dielectric (polystyrene) is used, the maximum charge carrier density achievable at VG = 200 V is ∼4.5 × 1013/cm2, which corresponds to the conversion of 18% of a monolayer of PTCDI-C8 molecules into anions. [ABSTRACT FROM AUTHOR]

Catalytic antibody 15A10 hydrolyzes the benzoyl ester of cocaine to form the nonpsychoactive metabolites benzoic acid and ecgonine methylester. Here, we report biochemical and structural studies that characterize the catalytic mechanism. The crystal structure of the cocaine-hydrolyzing monoclonal antibody (mAb) 15A10 has been determined at 2.35 Å resolution. The binding pocket is fairly shallow and mainly hydrophobic but with a cluster of three hydrogen-bond donating residues (TrpL96, AsnH33, and TyrH35). Computational docking of the transition state analogue (TSA) indicates that these residues are appropriately positioned to coordinate the phosphonate moiety of the TSA and, hence, form an oxyanion hole. Tyrosine modification of theantibody with tetranitromethane reduced hydrolytic activity to background level. The contribution from these and other residues to catalysis and TSA binding was explored by site-directed mutagenesis of 15A 10 expressed in a single chain fragment variable (scFv) format. The TyrH35Phe mutant had 4-fold reduced activity, and TrpL96Ala, TrpL96His, and AsnH33Ala mutants were all inactive. Comparison with an esterolytic antibody D2.3 revealed a similar arrangement of tryptophan, asparagine, and tyrosine residues in the oxyanion hole that stabilizes the transition state for ester hydrolysis. Furthermore, the crystal structure of the bacterial cocaine esterase (cocE) also showed that the cocE employs a tyrosine hydroxyl in the oxyanion hole. Thus, the biochemical and structural data are consistent with the catalytic antibody providing oxyanion stabilization as its major contribution to catalysis. [ABSTRACT FROM AUTHOR]

The article discusses the correlation between highest-occupied molecular orbital (HOMO) alignment and contact resistance in molecular junctions. It used conducting probe atomic force microscopy to measure transport and ultraviolet photoemission spectroscopy to establish the valence electronic structure. The difference in the HOMO positions for the two series of molecules does not appear to affect the length dependence of the junction resistance.