Mechanism of Hierarchical Plasmonic Biomaterials Engineered Through Peptide-Directed Self-Assembly

Nanomaterials have garnered significant attention for their exceptional physical and chemical properties. Plasmonic metallic nanoparticles, such as gold (AuNPs) and silver (AgNPs), are particularly fascinating because they exhibit a localized surface plasmon resonance (LSPR) band caused by electron oscillation in response to light. This LSPR band, and thus, their electronic and optical properties, can be tuned by altering the size, shape, and surface chemistry of the nanoparticles. In particular, AgNPs possess great scattering and a high molar absorption coefficient, and thus exhibit multiple colors within the visible range, making them invaluable in sensor applications. Hierarchical assembly of nanoparticles is an extension of this phenomena and offers properties and functionalities beyond those of individual nanoparticles such as increased surface area, improved signal amplification, and synergistic interactions that mimic biomolecules. However, self or directed assembly techniques often require complex redox chemistry, limiting widespread application.<br/><br/>In this study, we explore the hierarchical self-assembly of silver biomaterials using a peptide-directed approach to induce diffusion-limited aggregation. We use specific peptide sequences to control interparticle distance and morphology thereby avoiding complex syntheses to provide an accessible and tunable method for building fractal structures. We first synthesized ~20 nm nanospheres stabilized with bis(p-sulfonatophenyl)phenylphosphine dihydrate dipotassium (BSPP-AgNP) to be incubated with various short peptides (3 – 13 residues). Here, the peptides served as bridging motifs, facilitating the formation of fractal structures through noncovalent interactions with the charged surface. Peptides containing arginine, lysine, histidine, phenylalanine, and tryptophan were chosen, reflecting different interactions such as electrostatic, pi-pi stacking, and hydrophobic forces. We systematically investigated the impact of peptide sequence, concentration, and length on the biomaterials' morphology via TEM, DLS, and UV-Vis spectroscopy.<br/><br/>We determined that arginine-rich peptides were especially effective in promoting self-assembly because of the strong electrostatic interactions facilitated by the guanidine group, which can engage in multiple directional interactions with anionic counterparts. These peptides yielded well-defined fractal structures with high fractal dimensions at low concentrations (1 µM). Peptide concentration played a crucial role for all peptides; higher concentrations led to rapid aggregation and a loss of structural control, while lower concentrations failed to induce significant assembly. Optimal peptide concentrations ranged from 1 to 30 µM, balancing the interaction strength and assembly kinetics. Moreover, fractal dimension analysis indicated that monomeric peptides led to the highest fractal dimensions (~ 1.7), while longer peptide sequences resulted in denser aggregation and reduced fractal complexity. We also investigated C₅₀ values, defined as the peptide concentration required to achieve 50% of the maximum assembly intensity. Arginine-based peptides demonstrated a C₅₀ value two times lower than other sequences, reinforcing their superior ability to drive self-assembly. These structures also exhibited substantial colorimetric changes, confirming their potential for sensor applications.<br/><br/>Control experiments with citrate-AgNPs and BSPP-AuNPs showed no fractal assembly, highlighting BSPP's specificity on silver. S/TEM analysis revealed particles merging as a function of time or ligand concentration, confirming that AgNPs behave as soft particles that undergo spontaneous coalescence after modification of their surface properties with charge neutralization and desorption of ligands.<br/><br/>By leveraging the simplicity of peptide design, we present a scalable and tunable approach to building fractal biomaterials with applications in medical sensing and electronic devices.