Self-Assembled Peptide Nanotube Films with High Proton Conductivity

Gentle Patterning Approaches toward Compatibility with Bio-Organic Materials and Their Environmental Aspects

Advances in material science, bioelectronic, and implantable medicine combined with recent requests for eco-friendly materials and technologies inevitably formulate new challenges for nano- and micropatterning techniques. Overall, the importance of creating micro- and nanostructures is motivated by a large manifold of fundamental and applied properties accessible only at the nanoscale. Lithography is a crucial family of fabrication methods to create prototypes and produce devices on an industrial scale. The pure trend in the miniaturization of critical electronic semiconducting components has been recently enhanced by implementing bio-organic systems in electronics. So far, significant efforts have been made to find novel lithographic approaches and develop old ones to reach compatibility with delicate bio-organic systems and minimize the impact on the environment. Herein, such delicate materials and sophisticated patterning techniques are briefly reviewed.

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.

Natural biopolymers as proton conductors in bioelectronics

Bioelectronic devices sense or deliver information at the interface between living systems and electronics by converting biological signals into electronic signals and vice-versa. Biological signals are typically carried by ions and small molecules. As such, ion conducting materials are ideal candidates in bioelectronics for an optimal interface. Among these materials, ion conducting polymers that are able to uptake water are particularly interesting because, in addition to ionic conductivity, their mechanical properties can closely match the ones of living tissue. In this review, we focus on a specific subset of ion-conducting polymers: proton (H⁺ ) conductors that are naturally derived. We first provide a brief introduction of the proton conduction mechanism, and then outline the chemical structure and properties of representative proton-conducting natural biopolymers: polysaccharides (chitosan and glycosaminoglycans), peptides and proteins, and melanin. We then highlight examples of using these biopolymers in bioelectronic devices. We conclude with current challenges and future prospects for broader use of natural biopolymers as proton conductors in bioelectronics and potential translational applications.

Long-Range Proton Transport in Films from a Reflectin-Derived Polypeptide

Protein- and peptide-based proton conductors have been extensively studied because of their important roles in biological processes and established potential for bioelectronic device applications. However, despite much progress, the demonstration of long-range proton transport for such materials has remained relatively rare. Herein, we fabricate, electrically interrogate, and physically characterize films from a reflectin-derived polypeptide. The electrical measurements indicate that device-integrated films exhibit proton conductivities with values of ∼0.4 mS/cm and sustain proton transport over distances of ∼1 mm. The accompanying physical characterization indicates that the polypeptide possesses characteristics analogous to those of the parent protein class and furnishes insight into the relationship between the polypeptide's electrical functionality and structure in the solid state. When considered together, our findings hold significance for the continued development and engineering of not only reflectin-based materials but also other bioinspired proton conductors.

ZnCl2 regulated flax-based porous carbon fibers for supercapacitors with good cycling stability

We systemically control the specific surface area and pore structure of flax-based carbon fibers by impregnating with zinc chloride (ZnCl2) solution. The results show that ZnCl2 affects the microstructure and the specific capacitance.

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.

A combined experimental and computational approach reveals how aromatic peptide amphiphiles self-assemble to form ion-conducting nanohelices

Reported here is a combined experimental-computational strategy to determine structure-property-function relationships in persistent nanohelices formed by a set of aromatic peptide amphiphile (APA) tetramers with the general structure K S XEK S , where KS= S-aroylthiooxime modified lysine, X = glutamic acid or citrulline, and E = glutamic acid. In low phosphate buffer concentrations, the APAs self-assembled into flat nanoribbons, but in high phosphate buffer concentrations they formed nanohelices with regular twisting pitches ranging from 9-31 nm. Coarse-grained molecular dynamics simulations mimicking low and high salt concentrations matched experimental observations, and analysis of simulations revealed that increasing strength of hydrophobic interactions under high salt conditions compared with low salt conditions drove intramolecular collapse of the APAs, leading to nanohelix formation. Analysis of the radial distribution functions in the final self-assembled structures led to several insights. For example, comparing distances between water beads and beads representing hydrolysable KS units in the APAs indicated that the KS units in the nanohelices should undergo hydrolysis faster than those in the nanoribbons; experimental results verified this hypothesis. Simulation results also suggested that these nanohelices might display high ionic conductivity due to closer packing of carboxylate beads in the nanohelices than in the nanoribbons. Experimental results showed no conductivity increase over baseline buffer values for unassembled APAs, a slight increase (0.4 × 102 μS/cm) for self-assembled APAs under low salt conditions in their nanoribbon form, and a dramatic increase (8.6 × 102 μS/cm) under high salt conditions in their nanohelix form. Remarkably, under the same salt conditions, these self-assembled nanohelices conducted ions 5-10-fold more efficiently than several charged polymers, including alginate and DNA. These results highlight how experiments and simulations can be combined to provide insight into how molecular design affects self-assembly pathways; additionally, this work highlights how this approach can lead to discovery of unexpected properties of self-assembled nanostructures.