Enantioselective Detection of Gaseous Odorants with Peptide-Graphene Sensors Operating in Humid Environments

Replicating the sense of smell presents an ongoing challenge in the development of biomimetic devices. Olfactory receptors exhibit remarkable discriminatory abilities, including the enantioselective detection of individual odorant molecules. Graphene has emerged as a promising material for biomimetic electronic devices due to its unique electrical properties and exceptional sensitivity. However, the efficient detection of nonpolar odor molecules using transistor-based graphene sensors in a gas phase in environmental conditions remains challenging due to high sensitivity to water vapor. This limitation has impeded the practical development of gas-phase graphene odor sensors capable of selective detection, particularly in humid environments. In this study, we address this challenge by introducing peptide-functionalized graphene sensors that effectively mitigate undesired responses to changes in humidity. Additionally, we demonstrate the significant role of humidity in facilitating the selective detection of odorant molecules by the peptides. These peptides, designed to mimic a fruit fly olfactory receptor, spontaneously assemble into a monomolecular layer on graphene, enabling precise and specific odorant detection. The developed sensors exhibit notable enantioselectivity, achieving a remarkable 35-fold signal contrast between d- and l-limonene. Furthermore, these sensors display distinct responses to various other biogenic volatile organic compounds, demonstrating their versatility as robust tools for odor detection. By acting as both a bioprobe and an electrical signal amplifier, the peptide layer represents a novel and effective strategy to achieve selective odorant detection under normal atmospheric conditions using graphene sensors. This study offers valuable insights into the development of practical odor-sensing technologies with potential applications in diverse fields.

Correction to "Bifurcated Hydrogen Bonds in a Peptide Crystal Unveiled by X-ray Diffraction and Polarized Raman Spectroscopy"

[This corrects the article DOI: 10.1021/acs.cgd.3c00302.].

Differential toxicity and localization of arginine-rich C9ORF72 dipeptide repeat proteins depend on de-clustering of positive charges

Arginine-rich dipeptide repeat proteins (R-DPRs), poly(PR) and poly(GR), translated from the hexanucleotide repeat expansion in the amyotrophic lateral sclerosis (ALS)-causative C9ORF72 gene, contribute significantly to pathogenesis of ALS. Although both R-DPRs share many similarities, there are critical differences in their subcellular localization, phase separation, and toxicity mechanisms. We analyzed localization, protein-protein interactions, and phase separation of R-DPR variants and found that sufficient segregation of arginine charges is necessary for nucleolar distribution. Proline not only efficiently separated the charges, but also allowed for weak, but highly multivalent binding. In contrast, because of its high flexibility, glycine cannot fully separate the charges, and poly(GR) behaves similarly to the contiguous arginines, being trapped in the cytoplasm. We conclude that the amino acid that spaces the arginine charges determines the strength and multivalency of the binding, leading to differences in localization and toxicity mechanisms.

Rational Design and Self-Assembly of Histidine-Rich Peptides on a Graphite Surface

Histidine-rich peptides (HRPs) have been investigated to create functional biomolecules based on the nature of histidine, such as ion binding and catalytic activity. The organization of these HRPs on a solid surface can lead to surface functionalization with the well-known properties of HRPs. However, immobilization of HRPs on the surface has not been realized. Here, we design a series of octapeptides with histidine repeat units, aiming to establish their self-assembly on a graphite surface to produce a highly robust and active nanoscaffold. The new design has (XH)₄, and we incorporated various types of hydrophobic amino acids at X in the sequence to facilitate their interaction with the surface. The effect of the pair of amino acids on their self-assembly was investigated by atomic force microscopy. Contact angle measurement revealed that these assemblies functionalized graphite surfaces with different wetting chemistry. Moreover, the secondary structure of peptides was characterized by Fourier transform infrared spectroscopy (FTIR), which gives us further insights into the conformation of histidine repeat peptides on the surface. Our results showed a new approach to applying histidine-rich peptides on the surface and tuning the self-assembly behavior by introducing different counter amino acids that could be integrated with a wide range of biosensing and biotechnology applications.

Designable peptides on graphene field-effect transistors for selective detection of odor molecules

Gas sensing based on graphene field-effect transistors (GFETs) has gained broad interest due to their high sensitivity. Further progress in gas sensing with GFETs requires to detection of various odor molecules for applications in the environmental monitoring, healthcare, food, and cosmetic industries. To develop the ubiquitous odor-sensing system, establishing an artificial sense of smell with electronic devices by mimicking olfactory receptors will be key. Although the application of olfactory receptors to GFETs is straightforward for odor sensing, synthetic molecules with a similar function to olfactory receptors would be desirable to realize the robust performance of sensing. In this work, we designed three new peptides consisting of two domains: a bio-probe to the target molecules and a molecular scaffold. These peptides were rationally designed based on a motif sequence in olfactory receptors and self-assembled into a molecular thin film on GFETs. Limonene, methyl salicylate, and menthol were employed as representative odor molecules of plant flavors to demonstrate the biosensing of odor molecules. The conductivity change of GFETs against the binding to odor molecules with various concentrations and the dynamic response revealed a distinct signature of three different peptides against individual species of the target molecules. The kinetic response of each peptide exhibited characteristic time constants in the adsorption and desorption process, also supported by the principal component analysis. Our demonstration of the graphene odor sensors with the designed peptides opens a way to establish future peptide-array sensors with multi-sequence of peptide, realizing an odor sensing system with higher selectivity.

Self-assembled GA-Repeated Peptides as a Biomolecular Scaffold for Biosensing with MoS2 Electrochemical Transistors

Biosensors with two-dimensional materials have gained wide interest due to their high sensitivity. Among them, single-layer MoS2 has become a new class of biosensing platform owing to its semiconducting property. Immobilization of bioprobes directly onto the MoS2 surface with chemical bonding or random physisorption has been widely studied. However, these approaches potentially cause a reduction of conductivity and sensitivity of the biosensor. In this work, we designed peptides that spontaneously align into monomolecular-thick nanostructures on electrochemical MoS2 transistors in a non-covalent fashion and act as a biomolecular scaffold for efficient biosensing. These peptides consist of repeated domains of glycine and alanine in the sequence and form self-assembled structures with sixfold symmetry templated by the lattice of MoS2. We investigated electronic interactions of self-assembled peptides with MoS2 by designing their amino acid sequence with charged amino acids at both ends. Charged amino acids in the sequence showed a correlation with the electrical properties of single-layer MoS2, where negatively charged peptides caused a shift of threshold voltage in MoS2 transistors and neutral and positively charged peptides had no significant effect on the threshold voltage. The transconductance of transistors had no decrease due to the self-assembled peptides, indicating that aligned peptides can act as a biomolecular scaffold without degrading the intrinsic electronic properties for biosensing. We also investigated the impact of peptides on the photoluminescence (PL) of single-layer MoS2 and found that the PL intensity changed sensitively depending on the amino acid sequence of peptides. Finally, we demonstrated a femtomolar-level sensitivity of biosensing using biotinylated peptides to detect streptavidin.

Volatile Organic Compound Detection by Graphene Field-Effect Transistors Functionalized with Fly Olfactory Receptor Mimetic Peptides

An olfactory receptor mimetic peptide-modified graphene field-effect transistor (gFET) is a promising solution to overcome the principal challenge of low specificity graphene-based sensors for volatile organic compound (VOC) sensing. Herein, peptides mimicking a fruit fly olfactory receptor, OR19a, were designed by a high-throughput analysis method that combines a peptide array and gas chromatography for the sensitive and selective gFET detection of the signature citrus VOC, limonene. The peptide probe was bifunctionalized via linkage of a graphene-binding peptide to facilitate one-step self-assembly on the sensor surface. The limonene-specific peptide probe successfully achieved highly sensitive and selective detection of limonene by gFET, with a detection range of 8-1000 pM, while achieving facile sensor functionalization. Taken together, our target-specific peptide selection and functionalization strategy of a gFET sensor demonstrates advancement of a precise VOC detection system.

De novo designed peptides form a highly catalytic ordered nanoarchitecture on a graphite surface

Here we demonstrate that short peptides, de novo designed from first principles, self-assemble on the surface of graphite to produce a highly robust and catalytic nanoarchitecture, which promotes peroxidation reactions with activities that rival those of natural enzymes in both single and multi-substrate reactions. These designable peptides recapitulate the symmetry of the underlying graphite surface and act as molecular scaffolds to immobilize hemin molecules on the electrode in a hierarchical self-assembly manner. The highly ordered and uniform hybrid graphite-peptide-hemin nanoarchitecture shows the highest faradaic efficiency of any hybrid electrode reported. Given the explosive growth of the types of chemical reactions promoted by self-assembled peptide materials, this new approach to creating complex electrocatalytic assemblies will yield highly efficient and practically applicable electrocatalysts.

Effect of Multivalency on Phase-Separated Droplets Consisting of Poly(PR) Dipeptide Repeats and RNA at the Solid/Liquid Interface

Dipeptide repeat proteins (DRPs) are considered a significant cause of amyotrophic lateral sclerosis (ALS), and their liquid-liquid phase separation (LLPS) formation with other biological molecules has been studied both in vitro and in vivo. The immobilization and wetting of the LLPS droplets on glass surfaces are technically crucial for the measurement with optical microscopy. In this work, we characterized the surface diffusion of LLPS droplets of the DRPs with different lengths to investigate the multivalent effect on the interactions of their LLPS droplets with the glass surface. Using fluorescence microscopy and the single-particle tracking method, we observed that the large multivalency drastically changed the surface behavior of the droplets. The coalescence and wetting of the droplets were accelerated by increasing the multivalency of peptides in the LLPS system. Our findings on the effect of multivalency on interactions between droplets and glass surfaces could provide a new insight to enhance the understanding of LLPS formation and biophysical properties related to the solid/liquid interface.

Order controls disordered droplets: structure-function relationships in C9orf72-derived poly(PR)

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) have been thought as two distinct neurodegenerative diseases. However, recent genetic screening and careful investigations found the genetic and pathological overlap among these disorders. Hexanucleotide expansions in intron 1 of C9orf72 are a leading cause of familial ALS and familial FTD. These expansions facilitate the repeat-associated non-ATG initiated translation (RAN translation), producing five dipeptide repeat proteins (DRPs), including Arg-rich poly(PR: Pro-Arg) and poly-(GR: Gly-Arg) peptides. Arg is a positively charged, highly polar amino acid that facilitates interactions with anionic molecules such as nucleic acids and acidic amino acids via electrostatic forces and aromatic amino acids via cation-pi interaction, suggesting that Arg-rich DRPs underlie the pathophysiology of ALS via Arg-mediated molecular interactions. Arg-rich DRPs have also been reported to induce neurodegeneration in cellular and animal models via multiple mechanisms; however, it remains unclear why the Arg-rich DRPs exhibit such diverse toxic properties, because not all Arg-rich peptides are toxic. In this mini-review, we discuss the current understanding of the pathophysiology of Arg-rich C9orf72 DRPs and introduce recent findings on the role of Arg distribution as a determinant of the toxicity and its contribution to the pathogenesis of ALS.

Phase separation and toxicity of C9orf72 poly(PR) depends on alternate distribution of arginine

Arg (R)-rich dipeptide repeat proteins (DPRs; poly(PR): Pro-Arg and poly(GR): Gly-Arg), encoded by a hexanucleotide expansion in the C9ORF72 gene, induce neurodegeneration in amyotrophic lateral sclerosis (ALS). Although R-rich DPRs undergo liquid-liquid phase separation (LLPS), which affects multiple biological processes, mechanisms underlying LLPS of DPRs remain elusive. Here, using in silico, in vitro, and in cellulo methods, we determined that the distribution of charged Arg residues regulates the complex coacervation with anionic peptides and nucleic acids. Proteomic analyses revealed that alternate Arg distribution in poly(PR) facilitates entrapment of proteins with acidic motifs via LLPS. Transcription, translation, and diffusion of nucleolar nucleophosmin (NPM1) were impaired by poly(PR) with an alternate charge distribution but not by poly(PR) variants with a consecutive charge distribution. We propose that the pathogenicity of R-rich DPRs is mediated by disturbance of proteins through entrapment in the phase-separated droplets via sequence-controlled multivalent protein-protein interactions.

An improved macromolecular crowding sensor CRONOS for detection of crowding changes in membrane-less organelles under stressed conditions

Membrane-less organelles (MLOs) formed by liquid-liquid phase separation (LLPS) play pivotal roles in biological processes. During LLPS, proteins and nucleotides are extremely condensed, resulting in changes in their conformation and biological functions. Disturbed LLPS homeostasis in MLOs is thought to associate with fatal diseases such as amyotrophic lateral sclerosis. Therefore, it is important to detect changes in the degree of crowding in MLOs. However, it has not been investigated well due to the lack of an appropriate method. To address this, we developed a genetically encoded macromolecular crowding sensor CRONOS (crowding sensor with mNeonGreen and mScarlet-I) that senses the degree of macromolecular crowding in MLOs using a fluorescence resonance energy transfer (FRET) system. CRONOS is a bright biosensor with a wide dynamic range and successfully detects changes in the macromolecular volume fraction in solution. By fusing to the scaffold protein of each MLO, we delivered CRONOS to MLO of interest and detected previously undescribed differences in the degree of crowding in each MLO. CRONOS also detected changes in the degree of macromolecular crowding in nucleolus induced by environmental stress or inhibition of transcription. These findings suggest that CRONOS can be a useful tool for the determination of molecular crowding and detection of pathological changes in MLOs in live cells.

Cosolvents Restrain Self-Assembly of a Fibroin-Like Peptide on Graphite

Controllable self-assembly of peptides on solid surfaces has been investigated for establishing functional bio/solid interfaces. In this work, we study the influence of organic solvents on the self-assembly of a fibroin-like peptide on a graphite surface. The peptide has been designed by mimicking fibroin proteins to have strong hydrogen bonds among peptides enabling their self-assembly. We have employed cosolvents of water and organic solvents with a wide range of dielectric constants to control peptide self-assembly on the surface. Atomic force microscopy has revealed that the peptides self-assemble into highly ordered monolayer-thick linear structures on graphite after incubation in pure water, where the coverage of peptides on the surface is more than 85%. When methanol is mixed, the peptide coverage becomes zero at a threshold concentration of 30% methanol on graphite and 25% methanol on MoS₂. The threshold concentration in ethanol, isopropanol, dimethyl sulfoxide, and acetone varies depending on the dielectric constant with restraining self-assembly of the peptides, and particularly low dielectric-constant protic solvents prevent the peptide self-assembly significantly. The observed phenomena are explained by competitive surface adsorption of the organic solvents and peptides and the solvation effect of the peptide assembly.

Chiral Recognition of Self-Assembled Peptides on MoS2 via Lattice Matching

Chiral recognition of peptides on solid surfaces has been studied for a better understanding of their assembly mechanism toward its applications in stereochemistry and enantioselective catalysis. However, moving from small peptides such as dipeptides, understanding the chiral recognition of larger biomolecules such as oligopeptides or peptides with a larger sequence is challenging. Furthermore, their intrinsic mechanism for chiral recognition in liquid conditions was poorly investigated experimentally. Here, we used in/ex situ atomic force microscopy (AFM) to investigate the chiral recognition of self-assembled structures of l/d-type peptides on molybdenum disulfide (MoS₂). We chose single-layer MoS₂ with a triangular shape as a substrate for the self-assembly of peptides. The facet edges of MoS₂ were utilized as a landmark to identify the crystallographic orientation of their ordered structures. We found both peptide enantiomers formed nanowires on MoS₂ with a mirror symmetry according to the facet edges of MoS₂. From in situ AFM measurements, we found a dimension of a unit cell in the self-assembled structure and proposed a model of lattice matching between peptides and MoS₂ lattice. The lattice matching for chiral recognition was further investigated by changing peptide sequences and surface lattice from MoS₂ to graphite. This work further deepened the understanding of biomolecular chiral recognition and will lead us to rationally design specific morphologies and conformations of chiral self-assembled structures of peptides with expected functions in the future.

Preparation of Perfluorosulfonated Ionomer Nanofibers by Solution Blow Spinning

In this work, we report the preparation of high-purity perfluorosulfonated ionomer (Nafion) nanofibers (NFs) via solution blow spinning (SBS). Fiber formation in solution jet spinning is strongly dependent on the structure of the spinning solution. Upon adding a small amount of poly(ethyleneoxide) (PEO) as a spinning aid to Nafion dispersion, most of the highly ordered Nafion aggregate disappeared, allowing the stable production of bead-free and smooth high-purity NFs (Nafion/PEO = 99/1) by SBS. The microstructure of the blowspun Nafion NFs differed from that of electrospun NFs. In the blowspun NFs, incomplete microphase separation between hydrophilic (ionic) and hydrophobic domains was observed, but the crystallization of CF2-CF2 chains was enhanced owing to the high extensional strain rate and rapid solidification during SBS. These findings provide fundamental information for the preparation and characterization of blowspun Nafion NFs.

Diffusion of LLPS Droplets Consisting of Poly(PR) Dipeptide Repeats and RNA on Chemically Modified Glass Surface

The liquid-liquid phase separation (LLPS) of proteins and RNA molecules has emerged in recent years as an important physicochemical process to explain the organization of membrane-less organelles in living cells and cellular functions and even some fatal neurodegenerative diseases, such as Amyotrophic Lateral Sclerosis (ALS) due to the spontaneous condensation and growth of LLPS droplets. In general, the characterization of LLPS droplets has been performed by optical microscopy, where we need transparent substrates. By virtue of the liquid and wetting properties of LLPS droplets on a glass surface, there have been some technical protocols recommended to immobilize droplets on the surfaces. However, interactions between LLPS droplets and glass surfaces still remain unclear. Here, we investigated the surface diffusion of LLPS droplets on the glass surface to understand the interactions of droplets in a dynamic manner, and employed chemically modified glass surface with charges to investigate their Coulombic interaction with the surface. Using the single-particle tracking method, we first analyzed the diffusion of droplets on an untreated glass surface. Then, we compared the diffusion modes of LLPS droplets on each substrate and found that there were two major states of droplets on a solid surface: fix and diffusion mode for the LLPS droplet diffusion. While untreated glass showed a diffusion of droplets mainly, chemically modified glass with positive charges exhibited droplets fixed on the surface. It could arise from the Coulombic interaction between droplets and solid surface, where LLPS droplets have a negative ζ-potential. Our findings on the dynamics of LLPS at the solid/liquid interface could provide a novel insight to advance fundamental studies for understanding the LLPS formation.

Fibroin-like Peptides Self-Assembling on Two-Dimensional Materials as a Molecular Scaffold for Potential Biosensing

Self-assembled peptides have revealed uniform ordering on two-dimensional (2D) materials such as mica, graphene, and MoS₂ so far. These peptides are expected to be utilized as a molecular scaffold for biosensing based on 2D materials. However, the stability of the peptide structures on 2D materials under liquid has not been evaluated, and some of the previously reported peptides may have instability under water. In this work, by mimicking an amino-acid sequence of silk protein, we successfully developed peptide sequences that can maintain ordered nanostructures even after rinsing with deionized water. The structural stability was also proven under electrochemical bias, which is crucial as a biomolecular scaffold for practical biosensing with 2D materials. The stability probably arises from its β-sheet-like structures with improved intermolecular interactions and binding to the surface of 2D materials, resulting in the formation of stable domains of ordered peptide structures. Our peptides showed their ability to immobilize probe molecules for biosensing and inhibit nonspecific adsorption through their co-assembly process. Interestingly, we found two structural phases in the self-assembled structures, where only one of the phases reveals a binding affinity to target molecules.

Electrochemical Control of Peptide Self-Organization on Atomically Flat Solid Surfaces: A Case Study with Graphite

The nanoscale self-organization of biomolecules, such as proteins and peptides, on solid surfaces under controlled conditions is an important issue in establishing functional bio/solid soft interfaces for bioassays, biosensors, and biofuel cells. Electrostatic interaction between proteins and surfaces is one of the most essential parameters in the adsorption and self-assembly of proteins on solid surfaces. Although the adsorption of proteins has been studied with respect to the electrochemical surface potential, the self-assembly of proteins or peptides forming well-organized nanostructures templated by lattice structure of the solid surfaces has not been studied in the relation to the surface potential. In this work, we utilize graphite-binding peptides (GrBPs) selected by the phage display method to investigate the relationship between the electrochemical potential of the highly ordered pyrolytic graphite (HOPG) and peptide self-organization forming long-range-ordered structures. Under modulated electrical bias, graphite-binding peptides form various ordered structures, such as well-ordered nanowires, dendritic structures, wavy wires, amorphous (disordered) structures, and islands. A systematic investigation of the correlation between peptide sequence and self-organizational characteristics reveals that the presence of the bias-sensitive amino acid modules in the peptide sequence has a significant effect on not only surface coverage but also on the morphological features of self-assembled structures. Our results show a new method to control peptide self-assembly by means of applied electrochemical bias as well as peptide design-rules for the construction of functional soft bio/solid interfaces that could be integrated in a wide range of practical implementations.

Water stability of self-assembled peptide nanostructures for sequential formation of two-dimensional interstitial patterns on layered materials

Sequential-assembly of LEY and GrBP5 peptides on a graphite surface.

Selective detection of target proteins by peptide-enabled graphene biosensor

Direct molecular detection of biomarkers is a promising approach for diagnosis and monitoring of numerous diseases, as well as a cornerstone of modern molecular medicine and drug discovery. Currently, clinical applications of biomarkers are limited by the sensitivity, complexity and low selectivity of available indirect detection methods. Electronic 1D and 2D nano-materials such as carbon nanotubes and graphene, respectively, offer unique advantages as sensing substrates for simple, fast and ultrasensitive detection of biomolecular binding. Versatile methods, however, have yet to be developed for simultaneous functionalization and passivation of the sensor surface to allow for enhanced detection and selectivity of the device. Herein, we demonstrate selective detection of a model protein against a background of serum protein using a graphene sensor functionalized via self-assembling multifunctional short peptides. The two peptides are engineered to bind to graphene and undergo co-assembly in the form of an ordered monomolecular film on the substrate. While the probe peptide displays the bioactive molecule, the passivating peptide prevents non-specific protein adsorption onto the device surface, ensuring target selectivity. In particular, we demonstrate a graphene field effect transistor (gFET) biosensor which can detect streptavidin against a background of serum bovine albumin at less than 50 ng/ml. Our nano-sensor design, allows us to restore the graphene surface and utilize each sensor in multiple experiments. The peptide-enabled gFET device has great potential to address a variety of bio-sensing problems, such as studying ligand-receptor interactions, or detection of biomarkers in a clinical setting.

Torsion-sensing material from aligned carbon nanotubes wound onto a rod demonstrating wide dynamic range

A rational torsion sensing material was fabricated by wrapping aligned single-walled carbon nanotube (SWCNT) thin films onto the surface of a rod with a predetermined and fixed wrapping angle without destroying the internal network of the SWCNTs within the film. When applied as a torsion sensor, torsion could be measured up to 400 rad/meter, that is, more than 4 times higher than conventional optical fiber torsion sensors, by monitoring increases in resistance due to fracturing of the aligned SWCNT thin films.

Hierarchical three-dimensional layer-by-layer assembly of carbon nanotube wafers for integrated nanoelectronic devices

We report a general approach to overcome the enormous obstacle of the integration of CNTs into devices by bonding single-walled carbon nanotubes (SWNTs) films to arbitrary substrates and transferring them into densified and lithographically processable "CNT wafers". Our approach allows hierarchical layer-by-layer assembly of SWNTs into organized three-dimensional structures, for example, bidirectional islands, crossbar arrays with and without contacts on Si, and flexible substrates. These organized SWNT structures can be integrated with low-power resistive random-access memory.

Controlling the surface chemistry of graphite by engineered self-assembled peptides

The systematic control over surface chemistry is a long-standing challenge in biomedical and nanotechnological applications for graphitic materials. As a novel approach, we utilize graphite-binding dodecapeptides that self-assemble into dense domains to form monolayer-thick long-range-ordered films on graphite. Specifically, the peptides are rationally designed through their amino acid sequences to predictably display hydrophilic and hydrophobic characteristics while maintaining their self-assembly capabilities on the solid substrate. The peptides are observed to maintain a high tolerance for sequence modification, allowing control over surface chemistry via their amino acid sequence. Furthermore, through a single-step coassembly of two differently designed peptides, we predictably and precisely tune the wettability of the resulting functionalized graphite surfaces from 44° to 83°. The modular molecular structures and predictable behavior of short peptides demonstrated here give rise to a novel platform for functionalizing graphitic materials that offers numerous advantages, including noninvasive modification of the substrate, biocompatible processing in an aqueous environment, and simple fusion with other functional biological molecules.

Controlling self-assembly of engineered peptides on graphite by rational mutation

Self-assembly of proteins on surfaces is utilized in many fields to integrate intricate biological structures and diverse functions with engineered materials. Controlling proteins at bio-solid interfaces relies on establishing key correlations between their primary sequences and resulting spatial organizations on substrates. Protein self-assembly, however, remains an engineering challenge. As a novel approach, we demonstrate here that short dodecapeptides selected by phage display are capable of self-assembly on graphite and form long-range-ordered biomolecular nanostructures. Using atomic force microscopy and contact angle studies, we identify three amino acid domains along the primary sequence that steer peptide ordering and lead to nanostructures with uniformly displayed residues. The peptides are further engineered via simple mutations to control fundamental interfacial processes, including initial binding, surface aggregation and growth kinetics, and intermolecular interactions. Tailoring short peptides via their primary sequence offers versatile control over molecular self-assembly, resulting in well-defined surface properties essential in building engineered, chemically rich, bio-solid interfaces.

Electrical detection of biomolecular adsorption on sprayed graphene sheets

The binding affinities of graphite-binding peptides to a graphite surface were electrically characterized using sprayed graphene field effect transistors (SGFETs) fabricated with solution exfoliated graphene. The binding affinities of these peptides were also characterized using atomic force microscopy (AFM) and mechanically exfoliated graphene field effect transistors (GFETs) to confirm the validity of the SGFET platform. Binding constants obtained via GFET and AFM were comparable with those observed using SGFETs. The sprayed graphene film serves as a scalable platform to study biomolecular adsorption to graphitic surfaces.

A stretchable carbon nanotube strain sensor for human-motion detection

Devices made from stretchable electronic materials could be incorporated into clothing or attached directly to the body. Such materials have typically been prepared by engineering conventional rigid materials such as silicon, rather than by developing new materials. Here, we report a class of wearable and stretchable devices fabricated from thin films of aligned single-walled carbon nanotubes. When stretched, the nanotube films fracture into gaps and islands, and bundles bridging the gaps. This mechanism allows the films to act as strain sensors capable of measuring strains up to 280% (50 times more than conventional metal strain gauges), with high durability, fast response and low creep. We assembled the carbon-nanotube sensors on stockings, bandages and gloves to fabricate devices that can detect different types of human motion, including movement, typing, breathing and speech.

Impurity Gettering in Silicon by Thin Polycrystalline Films

Capability of impurity gettering by thin polycrystalline films on the backside of silicon wafer was evaluated by minority-carrier diffusion length. Cu was gettered easily during usual cooling after high temperature annealing. On the other hand, intentional slow cooling or low temperature annealing was necessary for effective Fe gettering. The gettering efficiency for Fe increased with lowering the annealing temperature when Fe was diffused sufficiently. From the quantitative consideration of Fe gettering, we propose the model of impurity gettering based on the chemical equilibrium of impurity reaction in polysilicon films. It was also expected that gettering efficiency increased with the thickness of polysilicon film.

Dual porosity single-walled carbon nanotube material

We present a dual porosity CNT material with a seamless connection between highly porous aligned nanotubes and lowly porous closely packed nanotubes by using capillary action of liquids. Various approaches were developed to fabricate diverse structures using toothpicks, liquid thin films, bubbles, vapors, and superink jet printing. The dual porosity material showed low wear and was useful as a sliding electrical contact.

Mechanical properties of beams from self-assembled closely packed and aligned single-walled carbon nanotubes

To demonstrate the potential for microelectromechanical systems, nanotube beams composed from self-assembled closely packed and aligned single-walled carbon nanotubes were fabricated and their mechanical properties were measured. We found that the nanotube beams behave as a cohesive, rigid, and elastic body with a sound velocity of 10,100 m/s.

Integrated three-dimensional microelectromechanical devices from processable carbon nanotube wafers

In order to be useful as microelectromechanical devices, carbon nanotubes with well-controlled properties and orientations should be made at high density and be placed at predefined locations. We address this challenge by hierarchically assembling carbon nanotubes into closely packed and highly aligned three-dimensional wafer films from which a wide range of complex and three-dimensional nanotube structures were lithographically fabricated. These include carbon nanotube islands on substrates, suspended sheets and beams, and three-dimensional cantilevers, all of which exist as single cohesive units with useful mechanical and electrical properties. Every fabrication step is both parallel and scalable, which makes it easy to further integrate these structures into functional three-dimensional nanodevice systems. Our approach opens up new ways to make economical and scalable devices with unprecedented structural complexity and functionality.

Biexciton gain and the Mott transition in GaAs quantum wires

Optical gain and the Mott transition in GaAs quantum wires were studied via simultaneous measurements of absorption and photoluminescence (PL). We observed well-separated PL peaks assigned to excitons (X) and biexcitons (XX) even at densities where optical gain existed. A sharp optical gain first appeared when the XX peak overtook the X peak, indicating the gain origin of biexciton-exciton population inversion. The XX peak eventually changed to a broad peak of plasma, and a broad gain due to plasma was observed as the Mott transition was completed.

One-dimensional band-edge absorption in a doped quantum wire

Low-temperature photoluminescence-excitation spectra are studied in an n-type modulation-doped T-shaped single quantum wire with a gate to tune electron densities. With a nondegenerate one-dimensional (1D) electron gas, the band-edge absorption exhibits a sharp peak structure induced by the 1D density of states. When the dense 1D electron gas is degenerate at a low temperature, we observe a Fermi-edge absorption onset without many-body modifications.

Dispersion and separation of small-diameter single-walled carbon nanotubes

The dispersion of small-diameter single-walled carbon nanotubes (SWNTs) produced by the CoMoCAT method in tetrahydrofuran (THF) with the use of amine was studied. The absorption, photoluminescence, and Raman spectroscopies showed that the dispersion and centrifugation process leads to an effective separation of metallic SWNTs from semiconducting SWNTs. Since this method is simple and convenient, it is highly applicable to an industrial utilization for widespread applications of SWNTs.

T-shaped GaAs quantum-wire lasers and the exciton Mott transition

T-shaped GaAs quantum-wire (T-wire) lasers fabricated by the cleaved-edge overgrowth method with molecular beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal are measured to study the laser device physics and fundamental many-body physics in clean one-dimensional (1D) systems. A current-injection T-wire laser that has 20 periods of T-wires in the active region and a 0.5 mm long cavity with high-reflection coatings shows a low threshold current of 0.27 mA at 30 K. The origin of the laser gain above the lasing threshold is studied with the high-quality T-wire lasers by means of optical pumping. The lasing energy is about 5 meV below the photoluminescence (PL) peak of free excitons, and is on the electron-hole (e-h) plasma PL band at a high e-h carrier density. The observed energy shift excludes the laser gain due to free excitons, and it suggests a contribution from the e-h plasma instead. A systematic micro-PL study reveals that the PL evolves with the e-h density from a sharp exciton peak, via a biexciton peak, to an e-h-plasma PL band. The data demonstrate an important role of biexcitons in the exciton Mott transition. Comparison with microscopic theories points out some problems in the picture of the exciton Mott transition.

Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as super-capacitor electrodes

We present a rational and general method to fabricate a high-densely packed and aligned single-walled carbon-nanotube (SWNT) material by using the zipping effect of liquids to draw tubes together. This bulk carbon-nanotube material retains the intrinsic properties of individual SWNTs, such as high surface area, flexibility and electrical conductivity. By controlling the fabrication process, it is possible to fabricate a wide range of solids in numerous shapes and structures. This dense SWNT material is advantageous for numerous applications, and here we demonstrate its use as flexible heaters as well as supercapacitor electrodes for compact energy-storage devices.

84% Catalyst Activity of Water-Assisted Growth of Single Walled Carbon Nanotube Forest Characterization by a Statistical and Macroscopic Approach

We propose a statistical and macroscopic analysis to estimate the catalyst activity of water-assisted growth (super-growth) of single-walled nanotubes (SWNT) and to characterize SWNT forests. The catalyst activity was estimated to be 84% (+/-6%), the highest ever reported. The SWNT forest was found to be a very sparse material where SWNTs represent only 3.6% of the total volume. This structural sparseness is believed to play a critical role in achieving highly efficient growth.