Controlling Long-Range Ordered Self-assembly of Solid-Binding Peptide Monolayers on Atomically Flat Layered Materials
Starkebaum, David Alan
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Solid-binding short peptides offer great promise as molecular building blocks in nanotechnology and nanomedicine. Some of these peptides can form self-organized nanostructures on solid surfaces due to highly specific coordination of inter-molecular forces enabled by conformational changes in the peptide. This study aims to examine how the organization of self-assembled monolayers formed by a phage display selected “wild-type” graphite binding peptide (GrBP5-WT) change with solution conditions, such as pH and ionic strength. The surface coverage and crystallinity of these peptide monolayers were shown to increase when incubated in 1mM sodium phosphate. In contrast, GrBP5-WT incubated in 1mM sodium hydroxide showed significantly decreased coverage, and no long-range-ordered structures. Zeta potential measurements of aqueous graphite powder dispersions showed a pH-dependent negative surface charge, which increased in magnitude when GrBP5-WT was added. A peptide mutant (GrBP5-M9) was designed by replacing two carboxylate residues with polar, but non-charged, amide residues. The mutant peptide formed crystalline nanostructures on graphite, which were unaffected by changes to the ionic strength or pH, and did not contribute additional negative charge to the graphite dispersion zeta potential. This showed that a simple mutation to a phage-display selected solid-binding peptide can eliminate its sensitivity to buffer and pH changes, facilitating the formation of more predictable bio/nano interfaces towards the development more robust biosensors and bioreactors. Self-assembly of GrBP5-WT and two other mutants (M6 and M8) was also shown on a variety of different atomically-flat 2D solid substrates, including CVD graphene on copper, and exfoliated BN, MoS2, MoSe2, WS2, and WSe2 on SiO2/Si. Although long-range ordered structures were shown on each substrate material, subtle differences in the patterns formed on each substrate indicate an important influence of the underlying crystal structure on the peptide nanostructure. The ready formation of ordered nanostructures opens the door for an investigation of the physical properties of number of hybrid nanomaterials. In particular, solid-binding peptides were shown to induce a molecular doping effect on the photoluminescence of single-layer MoSe2 (a 2D semiconductor with a direct band-gap in the visible light spectrum). Peptide self-assembly was also found to be sensitive to the presence of polymer residues commonly used in lithographic processing (such as PMMA). Indium microsoldering was investigated as a means to prepare electronic devices (such as graphene field-effect transistors) without contaminating the substrate.