Enhancement of electronic effects at a biomolecule-inorganic interface by multivalent interactions.

The binding of peptides and proteins through multiple weak interactions is ubiquitous in nature. Biopanning has been used to "hijack" this multivalent binding for the functionalization of surfaces. For practical applications it is important to understand how multivalency influences the binding interactions and the resulting behaviour of the surface. Considering the importance of optimization of the electronic properties of surfaces in diverse electronic and optoelectronic applications, we study here the relation between the multivalency effect and the resulting modulation of the surface work function. We use 12-mer peptides, which were found to strongly bind to oxide surfaces, to functionalize indium tin oxide (ITO) surfaces. We show that the affinity of the peptides for the ITO surface, and concurrently the effect on the ITO work function, are linearly affected by the number of basic residues in the sequence. The multivalent binding interactions lead to a peptide crowding effect, and a stronger modulation of the work function for adodecapeptide than for a single basic amino acid functionalization. The bioderived molecular platform presented herein can pave the way to a novel approach to improve the performance of optoelectronic devices in an eco-friendly manner.

Mechanism of Side Chain-Controlled Proton Conductivity in Bioinspired Peptidic Nanostructures.

Bioinspired peptide assemblies are promising candidates for use as proton-conducting materials in electrochemical devices and other advanced technologies. Progress toward applications requires establishing foundational structure-function relationships for transport in these materials. This experimental-theoretical study sheds light on how the molecular structure and proton conduction are linked in three synthetic cyclic peptide nanotube assemblies that comprise the three canonical basic amino acids (lysine, arginine, and histidine). Experiments find an order of magnitude higher proton conductivity for lysine-containing peptide assemblies compared to histidine and arginine containing assemblies. The simulations indicate that, upon peptide assembly, the basic amino acid side chains are close enough to enable direct proton transfer. The proton transfer kinetics is determined in the simulations to be governed by the structure and flexibility of the side chains. Together, experiments and theory indicate that the proton mobility is the main determinant of proton conductivity, critical for the performance of peptide-based devices.

Dynamic Surface Layer Coiled Coil Proteins Processing Analog-to-Digital Information.

Surface layer proteins perform multiple functions in prokaryotic cells, including cellular defense, cell-shape maintenance, and regulation of import and export of materials. However, mimicking the complex and dynamic behavior of such two-dimensional biochemical systems is challenging, and hence research has so far focused mainly on the design and manipulation of the structure and functionality of protein assemblies in solution. Motivated by the new opportunities that dynamic surface layer proteins may offer for modern technology, we herein demonstrate that immobilization of coiled coil proteins onto an inorganic surface facilitates complex behavior, manifested by reversible chemical reactions that can be rapidly monitored as digital surface readouts. Using multiple chemical triggers as inputs and several surface characteristics as outputs, we can realize reversible switching and logic gate operations that are read in parallel. Moreover, using the same coiled coil protein monolayers for derivatization of nanopores drilled into silicon nitride membranes facilitates control over ion and mass transport through the pores, thereby expanding the applicability of the dynamic coiled coil system for contemporary stochastic biosensing applications.

Proton-Conductive Melanin-Like Fibers through Enzymatic Oxidation of a Self-Assembling Peptide.

Melanin pigments have various properties that are of technological interest including photo- and radiation protection, rich coloration, and electronic functions. Nevertheless, laboratory-based synthesis of melanin and melanin-like materials with morphologies and chemical structures that are specifically optimized for these applications, is currently not possible. Here, melanin-like materials that are produced by enzymatic oxidation of a supramolecular tripeptide structures that are rich in tyrosine and have a 1D morphology are demonstrated, that are retained during the oxidation process while conducting tracks form through oxidative tyrosine crosslinking. Specifically, a minimalistic self-assembling peptide, Lys-Tyr-Tyr (KYY) with strong propensity to form supramolecular fibers, is utilized. Analysis by Raman spectroscopy shows that the tyrosines are pre-organized inside these fibers and, upon enzymatic oxidation, result in connected catechols. These form 1D conducting tracks along the length of the fiber, which gives rise to a level of internal disorder, but retention of the fiber morphology. This results in highly conductive structures demonstrated to be dominated by proton conduction. This work demonstrates the ability to control oxidation but retain a well-defined fibrous morphology that does not have a known equivalent in biology, and demonstrate exceptional conductivity that is enhanced by enzymatic oxidation.

The role of CdS doping in improving SWIR photovoltaic and photoconductive responses in solution grown CdS/PbS heterojunctions.

Low cost short wavelength infrared (SWIR) photovoltaic (PV) detectors and solar cells are of very great interest, yet the main production technology today is based on costly epitaxial growth of InGaAs layers. In this study, layers of p-type, quantum confined (QC) PbS nano-domains (NDs) structure that were engineered to absorb SWIR light at 1550 nm (Eg = 0.8 eV) were fabricated from solution using the chemical bath deposition (CBD) technique. The layers were grown on top of two different n-type CdS intermediate layers (Eg = 2.4 eV) using two different CBD protocols on fluoride tin oxide (FTO) substrates. Two types of CdS/PbS heterojunction were obtained to serve as SWIR PV detectors. The two resulting devices showed similar photoluminescence behavior, but a profoundly different electrical response to SWIR illumination. One type of CdS/PbS heterojunction exhibited a PV response to SWIR light, while the other demonstrated a photo-response to SWIR light only under an applied bias. To clarify this intriguing phenomenon, and since the only difference between the two heterojunctions could be the doping level of the CdS layer, we measured the doping level of this layer by means of the surface photo voltage (SPV). This yielded different polarizations for the two devices, indicating different doping levels of the CdS for the two different fabrication protocols, which was also confirmed by Hall Effect measurements. We performed current voltage measurements under super bandgap illumination, with respect to CdS, and got an electrical response indicating a barrier free for holes transfer from the CdS to the PbS. The results indicate that the different response does, indeed, originate from variations in the band structures at the interface of the CdS/PbS heterojunction due to the different doping levels of the CdS. We found that, unlike solar cells or visible light detectors having similar structure, in SWIR photodetectors, a type I heterojunction is formed having a barrier at the interface that limits the injection of the photo-exited electrons from the QC-PbS to the CdS side. Higher n-doped CdS generates a narrow depletion region on the CdS side, with a spike like barrier that is narrow enough to enable tunneling current, leading to a PV current. Our results show that an external quantum efficiency (EQE) of ∼2% and an internal quantum efficiency (IQE) of ∼20% can be obtained, at zero bias, for CBD grown SWIR sensitive CdS/PbS-NDs heterojunctions.

Self-Assembled Peptide Nanotube Films with High Proton Conductivity.

Design flexibility and modularity have emerged as powerful tools in the development of functional self-assembled peptide nanostructures. In particular, the tendency of peptides to form fibrils and nanotubes has motivated the investigation of electron and, more recently, proton transport in their fibrous films. In this study, we present a detailed characterization by impedance spectroscopy of films of self-assembled cyclic octa-d,l-α-peptide self-assembled nanotubes with amine side chains that promote proton transport. We show that the conductivity of the peptide nanotube film, which is in the range of 0.3 mS cm-1, is within the same order of magnitude as that of ultrathin films of Nafion, a benchmark proton conducting polymer. In addition, we show that while slow diffusion processes at the interface are present for both films, additional interface effects occur in the peptide nanotube films at the same rate as their bulk proton transport effects, further limiting charge transport at the interface. Overall, our studies demonstrate the great potential of using peptides as building blocks for the preparation of bioinspired supramolecular proton conducting polymers with improved conductivity with respect to that of natural systems.

Systematic modification of the indium tin oxide work function via side-chain modulation of an amino-acid functionalization layer.

Controlled modification of the semiconductor surface work function is of fundamental importance for improvements in the efficiency of (opto-)electronic devices. Binding amino acids to a semiconductor surface through their common carboxylic group offers a versatile tool for modulation of surface properties by the choice of their side chain. This approach is demonstrated here by tailoring the surface work function of indium tin oxide, one of the most abundant transparent electrodes in organic optoelectronic devices. We find that the work function can be systematically tuned by the side chain of the amino acid, resulting in either an increase or a decrease of the work function, over a large range of ∼250 meV. This side chain effect is mostly due to alteration of the dipole component perpendicular to the surface, with a generally smaller contribution for changes in surface band bending. These findings also shed light on electronic interactions at the interface between proteins and semiconductors, which are of importance for future bio-electronic devices.

Tailor-Made Functional Peptide Self-Assembling Nanostructures.

Noncovalent interactions are the main driving force in the folding of proteins into a 3D functional structure. Motivated by the wish to reveal the mechanisms of the associated self-assembly processes, scientists are focusing on studying self-assembly processes of short protein segments (peptides). While this research has led to major advances in the understanding of biological and pathological process, only in recent years has the applicative potential of the resulting self-assembled peptide assemblies started to be explored. Here, major advances in the development of biomimetic supramolecular peptide assemblies as coatings, gels, and as electroactive materials, are highlighted. The guiding lines for the design of helical peptides, ß strand peptides, as well as surface binding monolayer-forming peptides that can be utilized for a specific function are highlighted. Examples of their applications in diverse immerging applications in, e.g., ecology, biomedicine, and electronics, are described. Taking into account that, in addition to extraordinary design flexibility, these materials are naturally biocompatible and ecologically friendly, and their production is cost effective, the emergence of devices incorporating these biomimetic materials in the market is envisioned in the near future.

Measuring Proton Currents of Bioinspired Materials with Metallic Contacts.

Charge transfer at the interface between the active layer and the contact is essential in any device. Transfer of electronic charges across the contact/active layer interface with metal contacts is well-understood. To this end, noble metals, such as gold or platinum, are widely used. With these contacts, ionic currents (especially protonic) are often neglected because ions and protons do not transfer across the interface between the contact and the active layer. Palladium hydride contacts have emerged as good contacts to measure proton currents because of a reversible redox reaction at the interface and subsequent absorption/desorption of H into palladium, translating the proton flow reaching the interface into an electron flow at the outer circuit. Here, we demonstrate that gold and palladium contacts also collect proton currents, especially under high relative humidity conditions because of electrochemical reactions at the interface. A marked kinetic isotope effect, which is a signature of proton currents, is observed with gold and palladium contacts, indicating both bulk and contact processes involving proton transfer. These phenomena are attributed to electrochemical processes involving water splitting at the interface. In addition to promoting charge transfer at the interface, these interfacial electrochemical processes inject charge carriers into the active layer and hence can also modulate the bulk resistivity of the materials, as was found for the studied peptide fibril films. We conclude that proton currents may not be neglected a priori when performing electronic measurements on biological and bioinspired materials with gold and palladium contacts under high humidity conditions.

Peptide-functionalized semiconductor surfaces: strong surface electronic effects from minor alterations to backbone composition.

The use of non-canonical amino acids is a powerful way to control protein structure. Here, we show that subtle changes to backbone composition affect the ability of a dipeptide to modify solid surface electronic properties. The extreme sensitivity of the interactions to the peptide structure suggests potential applications in improving the performance of electronic devices.

The Strong Influence of Structure Polymorphism on the Conductivity of Peptide Fibrils.

Peptide fibril nanostructures have been advocated as components of future biotechnology and nanotechnology devices. However, the ability to exploit the fibril functionality for applications, such as catalysis or electron transfer, depends on the formation of well-defined architectures. Fibrils made of peptides substituted with aromatic groups are described presenting efficient electron delocalization. Peptide self-assembly under various conditions produced polymorphic fibril products presenting distinctly different conductivities. This process is driven by a collective set of hydrogen bonding, electrostatic, and π-stacking interactions, and as a result it can be directed towards formation of a distinct polymorph by using the medium to enhance specific interactions rather than the others. This method facilitates the detailed characterization of different polymorphs, and allows specific conditions to be established that lead to the polymorph with the highest conductivity.

Sequence dependent proton conduction in self-assembled peptide nanostructures.

The advancement of diverse electrochemistry technologies depends on the development of novel proton conducting polymers. Inspired by the efficacy of proton transport through proteins, we show in this work that self-assembling peptide nanostructures may be a promising alternative for such organic proton conducting materials. We demonstrate that aromatic amino acids, which participate in charge transport in nature, unprecedentedly promote proton conduction under both high and low relative humidity conditions for d,l α-cyclic peptide nanotubes. For dehydrated networks long-range order of the assemblies, induced by the aromatic side chains, is shown to be a dominating factor for promoting conductivity. However, for hydrated networks this order of effect is less significant and conductivity can be improved by the introduction of proton donating carboxylic acid peptide side chains in addition to the aromatic side chains despite the lower order of the assemblies. Based on these observations, a novel cyclic peptide that incorporates non-natural naphthyl side chains was designed. Self-assembled nanotubes of this peptide show greatly improved dehydrated conductivity, while maintaining high conductivity under hydrated conditions. We envision that the demonstrated modularity and versatility of these bio inspired nanostructures will make them extremely attractive building blocks for the fabrication of devices for energy conversion and storage applications, as well as other applications that involve proton transport, whether dry or wet conductivity is desired.

Filling the Green Gap of a Megadalton Photosystem I Complex by Conjugation of Organic Dyes.

Photosynthesis is Nature's major process for converting solar into chemical energy. One of the key players in this process is the multiprotein complex photosystem I (PSI) that through absorption of incident photons enables electron transfer, which makes this protein attractive for applications in bioinspired photoactive hybrid materials. However, the efficiency of PSI is still limited by its poor absorption in the green part of the solar spectrum. Inspired by the existence of natural phycobilisome light-harvesting antennae, we have widened the absorption spectrum of PSI by covalent attachment of synthetic dyes to the protein backbone. Steady-state and time-resolved photoluminescence reveal that energy transfer occurs from these dyes to PSI. It is shown by oxygen-consumption measurements that subsequent charge generation is substantially enhanced under broad and narrow band excitation. Ultimately, surface photovoltage (SPV) experiments prove the enhanced activity of dye-modified PSI even in the solid state.

Influence of Solvent in Controlling Peptide-Surface Interactions.

Protein binding to surfaces is an important phenomenon in biology and in modern technological applications. Extensive experimental and theoretical research has been focused in recent years on revealing the factors that govern binding affinity to surfaces. Theoretical studies mainly focus on examining the contribution of the individual amino acids or, alternatively, the binding potential energies of the full peptide, which are unable to capture entropic contributions and neglect the dynamic nature of the system. We present here a methodology that involves the combination of nonequilibrium dynamics simulations with strategic mutation of polar residues to reveal the different factors governing the binding free energy of a peptide to a surface. Using a gold-binding peptide as an example, we show that relative binding free energies are a consequence of the balance between strong interactions of the peptide with the surface and the ability for the bulk solvent to stabilize the peptide.

Amplification of single molecule translocation signal using ß-strand peptide functionalized nanopores.

Changes in ionic current flowing through nanopores due to binding or translocation of single biopolymer molecules enable their detection and characterization. It is, however, much more challenging to detect small molecules due to their rapid and small signal signature. Here we demonstrate the use of de novo designed peptides for functionalization of nanopores that enable the detection of a small analytes at the single molecule level. The detection relies on cooperative peptide conformational change that is induced by the binding of the small molecule to a receptor domain on the peptide. This change results in alteration of the nanopore effective diameter and hence induces current perturbation signal. On the basis of this approach, we demonstrate here the detection of diethyl 4-nitrophenyl phosphate (paraoxon), a poisonous organophosphate molecule. Paraoxon binding is induced by the incorporation of the catalytic triad of acetylcholine esterase in the hydrophilic domain of a short amphiphilic peptide and promotes ß-sheet assembly of the peptide both in solution and for peptide molecules immobilized on solid surfaces. Nanopores coated with this peptide allowed the detection of paraoxon at the single molecule level revealing two binding arrangements. This unique approach, hence, provides the ability to study interactions of small molecules with the corresponding engineered receptors at the single molecule level. Furthermore, the suggested versatile platform may be used for the development of highly sensitive small analytes sensors.

Introducing charge transfer functionality into prebiotically relevant ß-sheet peptide fibrils.

Incorporation of naphthalene diimide moieties as side chains of short amphiphilic peptide results in the formation of fibrils that exhibit substantial intermolecular π-stacking interactions. These interactions can be manipulated without affecting the structure. The new system is suggested as a first step towards functional self-synthesizing materials.

Force modulated conductance of artificial coiled-coil protein monolayers.

Studies of charge transport through proteins bridged between two electrodes have been the subject of intense research in recent years. However, the complex structure of proteins makes it difficult to elucidate transport mechanisms, and the use of simple peptide oligomers may be an over simplified model of the proteins. To bridge this structural gap, we present here studies of charge transport through artificial parallel coiled-coil proteins conducted in dry environment. Protein monolayers uniaxially oriented at an angle of ∼ 30° with respect to the surface normal were prepared. Current voltage measurements, obtained using conductive-probe atomic force microscopy, revealed the mechano-electronic behavior of the protein films. It was found that the low voltage conductance of the protein monolayer increases linearly with applied force, mainly due to increase in the tip contact area. Negligible compression of the films for loads below 26 nN allowed estimating a tunneling attenuation factor, ß(0) , of 0.5-0.6 Å(-1) , which is akin to charge transfer by tunneling mechanism, despite the comparably large charge transport distance. These studies show that mechano-electronic behavior of proteins can shed light on their complex charge transport mechanisms, and on how these mechanisms depend on the detailed structure of the proteins. Such studies may provide insightful information on charge transfer in biological systems.

Modulating semiconductor surface electronic properties by inorganic peptide-binders sequence design.

The use of proteins and peptides as part of biosensors and electronic devices has been the focus of intense research in recent years. However, despite the fact that the interface between the bioorganic molecules and the inorganic matter plays a significant role in determining the properties of such devices, information on the electronic properties of such interfaces is sparse. In this work, we demonstrate that the identity and position of single amino acid in short inorganic binding protein-segments can significantly modulate the electronic properties of semiconductor surfaces on which they are bound. Specifically, we show that the introduction of tyrosine or tryptophan, both possessing an aromatic side chain which higher occupied molecular orbitals are positioned in proximity to the edge of GaAs valence band, to the sequence of a peptide that binds to GaAs (100) results in changes of both the electron affinity and surface potential of the semiconductor. These effects were found to be more pronounced than the effects induced by the same amino acids once bound on the surface in a head-tail configuration. Furthermore, the relative magnitude of each effect was found to depend on the position of the modification in the sequence. This sequence dependent behavior is induced both indirectly by changes in the peptide surface coverage, and directly, probably, due to changes in the orientation and proximity of the tyrosine/tryptophan side group with respect to the surface due to the preferred conformation the peptide adopts on the surface. These studies reveal that despite the use of short protein oligomers and aiming at a non-natural-electronic task, the well-known relations between the proteins' structure and function is preserved. Combining the ability to tune the electronic properties at the interface with the ability to direct the growth of inorganic materials makes peptides promising building blocks for the construction of novel hybrid electronic devices and biosensors.

Transient fibril structures facilitating nonenzymatic self-replication.

An emerging new direction of research focuses on developing "self-synthesizing materials", those supramolecular structures that can promote their own formation by accelerating the synthesis of building blocks and/or an entire assembly. It was postulated recently that practical design of such systems can benefit from the ability to control the assembly of amphiphilic molecules into nanostructures. We describe here the self-assembly pathway of short amphiphilic peptides into various forms of soluble ß-sheet structures--ß-plates, fibrils, and hollow nanotubes--and their consequent activity as autocatalysts for the synthesis of monomeric peptides from simpler building blocks. A detailed kinetic analysis of both the self-assembly and self-replication processes allows us to suggest a full model and simulate the replication process, revealing that only specific structures, primarily fibrils that are stable within the solution for a time shorter than a few hours, can be active as catalysts. Interestingly, we have found that such a process also induces fibril reproduction, in a mechanism very similar to the propagation of prion proteins by transmission of misfolded states.

Charge transport in vertically aligned, self-assembled peptide nanotube junctions.

The self-assembly propensity of peptides has been extensively utilized in recent years for the formation of supramolecular nanostructures. In particular, the self-assembly of peptides into fibrils and nanotubes makes them promising building blocks for electronic and electro-optic applications. However, the mechanisms of charge transfer in these wire-like structures, especially in ambient conditions, are not yet fully understood. We describe here a layer-by-layer deposition methodology of short self-assembled cyclic peptide nanotubes, which results in vertically oriented nanotubes on gold substrates. Using this novel deposition methodology, we have fabricated molecular junctions with a conductive atomic force microscopy tip as a second electrode. Studies of the junctions' current-voltage characteristics as a function of the nanotube length revealed an efficient charge transfer in these supramolecular structures, with a low current attenuation constant of 0.1 Å(-1), which indicate that electron transfer is dominated by hopping. Moreover, the threshold voltage to field-emission dominated transport was found to increase with peptide length in a manner that depends on the nature of the contact with the electrodes. The flexibility in the design of the peptide monomers and the ability to control their sequential order over the nanotube by means of the layer-by-layer assembly process, which is demonstrated in this work, can be used to engineer the electronic properties of self-assembled peptide nanotubes toward device applications.