Relative Functionality of Buffer and Peptide in Gold Nanoparticle Formation

The templated growth of nanoparticles via biological agents, such as peptides, provides an exciting complement to abiotic routes, opening facile means to combine the specificity of biomacromolecules with nanoparticle platforms. The specific role of the peptide sequence, and its state relative to the buffer, is still unclear with respect to the processes underlying nanoparticle formation. By investigation of Au mineralization in two commonly used buffers (sodium borate and (2-hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES)), the role of the phage-display-identified A3 peptide, and its residues, is examined. In a nonreducing buffer (e.g., borate), mineralization is very slow, suggesting that the tyrosine residue in the A3 motif has at most a minor role in reduction of Au(III). In a buffer with substantial reducing capability (e.g., HEPES), the peptide retards nucleation relative to synthetic additives such as poly(ethylene glycol). Furthermore, it also functions to regulate the concentration of free Au in the growth medium, resulting in a diffusion limited process that yields larger nanoparticles with increased peptide concentration. These roles are consistent with the phage-display process that identified the A3 peptide sequence with respect to its binding strength to metal surfaces and not with regard to a specific reduction capability.