Generation of metal nanoparticles in the presence of oligoproline derivatives

The generation of metal nanoparticles has raised tremendous interest in recent years. They offer a huge variety of possible applications, due to their unique properties which strongly depend on their morphology. The controlled formation of nanoparticles of desired, monodisperse sizes and shapes still remains a challenge.
Herein we present the design and investigation of novel peptidic additives in the formation of different metal nanoparticles. Oligoprolines in different lengths were functionalized with guanidine, imidazole, primary amine, carboxylic acid, indole and pyrrolidinyl groups on the backbone. All of the peptides adopt the well-defined polyproline type II (PPII) helix which was not disturbed by the functionalization on the backbone. The formation reaction was carried out with and without an external reducing agent.
The additives were first investigated in the formation reaction of silver nanoparticles (AgNPs). The peptides bearing guanidine and imidazole groups were able to stabilize monodisperse and spherical nanoparticles using an external reducing agent in different sizes. Applying the additives without external reducing agents led in the case of guanidine- and indole-functionalized peptides also to the formation of monodisperse and spherical AgNPs.
In the presence of amine-functionalized peptides using ascorbic acid as external reducing agents non-spherical shapes were obtained and with pyrrolidinyl-functionalized peptides rods formed, next to spherical nanoparticles.
It was found that not only the functional group that was attached, but also the defined secondary structure was necessary for the peptides to act as additive in the formation reaction to stabilize nanoparticles in defined sizes and shapes.
In the formation of gold nanoparticles (AuNPs), guanidine- and imidazole-functionalized peptides were able to act as additives resulting in monodisperse nanoparticles when an external reducing agent was used. In contrast to the investigations for the formation of AgNPs also with amine-functionalized peptides monodisperse AuNPs were obtained. Without external reducing agent only polydisperse AuNPs were formed.
In the attempt to test the variability of the designed additives, first tests, evaluating the imidazole-functionalized peptides in the formation of platinum nanoparticles (PtNPs) was carried out. This resulted as well in monodisperse nanoparticles.
Furthermore a novel additive was designed inspired by a tripeptide, indentified in studies earlier carried out in our group. Using this peptide bearing two different functional groups it was possible to use it as additive in the formation of AgNPs using weak and strong reducing agents as well as visible light.
These results demonstrate the potential of these peptidic additives in the formation of metal nanoparticles in defined sizes and shapes.