Biologically inspired materials for electro-responsive coatings and the photo-oxidation of water

Abstract

Evolving out of research on biomineralization, a new field devoted to studying the interactions between inorganic materials and proteins is emerging. In natural systems, proteins are responsible for the assembly of complex hierarchical structures such as the nacre of abalone. Tools such as phage and yeast display libraries have enabled the combinatorial screening of peptides against a multitude of materials to which natural systems typically have no exposure. These techniques have yielded peptides that can bind and assemble technologically relevant materials such as gold and CdS. In this work, combinatorial phage and yeast display libraries are used to identify peptide sequences that bind to electrode materials and metal oxides. As in nature, it is observed that the context of a particular peptide dramatically influences its properties. While a peptide sequence may exhibit good adhesion to a particular surface when displayed on yeast, the same peptide may have little affinity towards that same surface when displayed on bacteriophage. To probe the interactions between peptides and materials in a context-free environment, rationally designed synthetic peptides were screened against a number of inorganic materials. A synthetic peptide, covalently linked to either microspheres, quantum dots, or a polymer, was able to mediate adhesion of those entities to electrode surfaces. In nature, proteins play important roles beyond biomineralization. For example, membrane proteins contain voltage-gated ion channels that open and close in response to a voltage bias. Inspired by the electro-responsive activity of ion channels, the interactions between peptides, surfaces and electric fields was investigated. The peptide sequences that exhibited significant adhesion to metal oxides were dominated by positively charged residues. A high voltage, pulsed electric field was used to overcome the inherent negative charge of the metal oxide electrode surface, thereby controlling peptide adhesion to an electrode surface. Drawing further inspiration from the way nature employs peptides, a synthetic photocatalytic system for water oxidation was developed using photosystem II (PSII) as a model. Proteins form the structural scaffold for PSII, assembling dye molecules as well as the metal-oxo catalytic center; furthermore, peptides play an active role in shuttling charge throughout PSIL. The D1 peptide in PSII is an electro-responsive peptide of sorts, releasing plastiquinone upon the two electron reduction of the molecule. The system developed in this work uses: iridium oxide as a metal-oxo catalyst assembled by a peptide expressed on the M13 bacteriophage; metalloporphyrin photosensitizers that are covalently assembled on the protein framework of the bacteriophage; and a synthetic Ce(IV) dipicolinate electron accepting molecule. The electron accepting molecule, developed to fill the role of plastiquinone in PSII, is believed to be the first non-sacrificial electron acceptor capable of driving the metalloporphyrin-sensitized photocatalytic oxidation of water.