Review

Self-assembling peptide semiconductors

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Science  17 Nov 2017:
Vol. 358, Issue 6365, eaam9756
DOI: 10.1126/science.aam9756

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Peptide-based semiconductors

For semiconductors, one often thinks of inorganic materials, such as doped silicon, or aromatic organic polymers and small molecules. Tao et al. review progress in making semiconductors based on self-assembling short peptides. The structures that form show extensive π and hydrogen bonding leading to a range of semiconductor properties, which can be tuned through doping or functionalization of the peptide sequences. These materials may shed light on biological semiconductors or provide an alternative for constructing biocompatible and therapeutic materials.

Science, this issue p. aam9756

Structured Abstract

BACKGROUND

The increasing demand for environmentally friendly organic semiconductors that can be easily fabricated and tuned has inspired scientists to design self-assembling peptide nanostructures with enhanced semiconducting characteristics. Recently designed bioinspired peptide semiconductors display various supramolecular morphologies with diverse optical and electrical properties, including intrinsic fluorescence, which facilitates real-time detection and quantitative assessment of the self-association process without a need for external conjugation. These assemblies have also been studied for their potential use in ferroelectric-related devices and ultrasensitive electrochemical sensors. In addition to their low-cost fabrication and structural diversity, bioinspired self-assembling peptide semiconductors may serve as candidates for advanced interdisciplinary functional nanostructures. Promotion of the design principles of peptide-based supramolecular materials is thus of great interest for both scientific and engineering development.

ADVANCES

Short peptides, specifically those containing aromatic amino acids, can self-assemble into a wide variety of supramolecular structures that are kinetically or thermodynamically stable; the representative models are diphenylalanine and phenylalanine-tryptophan. Different assembly strategies can be used to generate specific functional organizations and nanostructural arrays, resulting in finely tunable morphologies with controllable semiconducting characteristics. Such strategies include molecular modification, microfluidics, coassembly, physical or chemical vapor deposition, and introduction of an external electromagnetic field. Density functional theory simulations have revealed that extensive, directional aromatic interactions and hydrogen-bonding networks lead to the formation of quantum confined domains within the nanostructures, underlying the molecular origin of their intrinsic semiconductivity. These computational studies provide a conceptual framework for the tunability of the semiconductivity of a peptide assembly, and also demonstrate the feasibility of theoretical probing of the mechanisms leading to band gap formation and the subsequent design of building blocks with desired electronic properties. Recent studies have further elucidated some remarkable physicochemical features of the bioinspired supramolecular semiconductors, including absorption spectra characteristic of one-dimensional quantum dots or two-dimensional quantum wells, photoluminescence emission in the visible spectrum, optical waveguiding, temperature-dependent electrical conductivity, ferroelectric (piezoelectric, pyroelectric) properties, and electrochemical properties useful in ultrasensitive detectors and ultracapacitors.

OUTLOOK

Semiconductive materials are at the foundation of the modern electronics and optics industries. Self-assembling peptide nanomaterials may serve as an alternative source for the semiconductor industry because they are eco-friendly, morphologically and functionally flexible, and easy to prepare, modify, and dope. Moreover, the diverse bottom-up methodologies of peptide self-assembly facilitate easy and cost-effective device fabrication, with the ability to integrate external functional moieties. For example, the coassembly of peptides and electron donors or acceptors can be used to construct n-p junctions, and vapor deposition technology can be applied to manufacture custom-designed electronics and chips on various substrates.

The inherent bioinspired nature of self-assembling peptide nanostructures allows them to bridge the gap between the semiconductor world and biological systems, thus making them useful for applications in fundamental biology and health care research. Short peptide self-assemblies may shed light on the roles of protein semiconductivity in physiology and pathology. For example, research into the relationship between the semiconductive properties of misfolded polypeptides characteristic of various neurodegenerative diseases and the resulting symptoms may offer opportunities to investigate the mechanisms controlling such ailments and to develop therapeutic solutions. Finally, self-assembling short peptide semiconductors could be used to develop autonomous biomachines operating within biological systems. This would allow, for example, direct, label-free, real-time monitoring of a variety of metabolic activities, and even interference with biological systems.

Peptide building blocks self-assemble into quantum confined supramolecular semiconductors.

These bioinspired functional materials can serve as organic semiconductors. Their ability to connect the semiconductor field and the biological world will facilitate the incorporation of semiconductivity into fundamental biomedical and health care applications.

Abstract

Semiconductors are central to the modern electronics and optics industries. Conventional semiconductive materials bear inherent limitations, especially in emerging fields such as interfacing with biological systems and bottom-up fabrication. A promising candidate for bioinspired and durable nanoscale semiconductors is the family of self-assembled nanostructures comprising short peptides. The highly ordered and directional intermolecular π-π interactions and hydrogen-bonding network allow the formation of quantum confined structures within the peptide self-assemblies, thus decreasing the band gaps of the superstructures into semiconductor regions. As a result of the diverse architectures and ease of modification of peptide self-assemblies, their semiconductivity can be readily tuned, doped, and functionalized. Therefore, this family of electroactive supramolecular materials may bridge the gap between the inorganic semiconductor world and biological systems.

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