The self-assembly behavior of peptides on inorganic surfaces such as highly oriented pyrolytic graphite (HOPG) remains a highly studied field with numerous applications. Some of these applications include the integration of engineered devices for biomedical purposes. Overall, it is important to understand the types of interactions occurring at protein-surface interfaces, and which of these interactions govern the primary self-assembly behavior of the molecules. A number of experimental methods may be employed to better understand the behavior at these interfaces. Atomic force microscopy (AFM) is a well-established method, and was used to image self-assembly samples of charged peptides on graphite. Fixed concentrations of negatively charged peptides were placed in varying concentrations of KCl solutions to determine the effect of salt concentration on surface coverage and ordering of peptides on HOPG. Additionally, other factors were investigated, including the effect of step edges and incubation time on ordering. AFM images of the peptides were analyzed using Gwyddion to extract the surface coverage and degree of ordering. With increasing salt concentration, it was found that peptide assembly on graphite generally increased in KCl ranges of 1μM to 1mM. Additionally, the degree of peptide ordering increased with this range of salt concentration, displaying that charged peptides are able to pack closer together in the presence of an electrolyte. The electrostatic screening effect can explain this close-packing behavior, in which positively charged K+ ions are able to surround a negatively charged peptide and decrease the effective coulombic force between any two adjacent peptides. Overall, several factors may influence the assembly and ordering of peptides on HOPG, including electrolyte concentration, density of step-edges in the graphite, and charge of the peptide in question. This research has broader implications in the bio-medicial field with applications to engineered devices, and may prove important for future research.

Since its discovery, graphene has become a prominent material of interest for advanced bioelectronic and biomedical applications such as diagnostics, drug delivery, and imaging. This two-dimensional atomically thin sheet of sp2 hybridized carbon has exceptional electronic properties and is well suited for the development of highly sensitive and selective biosensors when paired with biomolecular adlayers. Additionally, by controlling the biomolecular orientation, conformation, and assembly structure of the adlayer, device functionality and performance can be fine-tuned. In this work, we focus on the conformational properties of three graphene-binding peptides, GrBP5-WT, GrBP5-M2 and Truncated GrBP5-M2, that form strongly adhered self-assembled adlayers at graphene surfaces. While these peptides are chemically very similar, experimental observations revealed they demonstrate opposite assembly phenomena upon thermal stimulus. Herein, enhanced sampling molecular dynamics simulations were employed using GROMACS simulation package with the PLUMED plugin to unravel how specific peptide conformations lead to the unexpected assembly behavior. Self-assembling peptide conformations were identified by comparing their computationally derived binding energies to experimental energetic data obtained from a scanning probe microscopy based molecular energetic analysis. The self-assembly structure of peptide on the graphene surface could form a module on the biosensor and used to detect specific proteins or peptides. With a better understanding of the peptide self-assembly mechanism, it would be significant to the development of biosensor.