Understanding synthetic route impact on NHCAuNP stability

Gold nanoparticles are a very promising nanomaterial which currently finds applications in sensing, optoelectronics, photonics, catalysis, and even photothermal cancer therapy. However, gold nanoparticles are typically not stable on their own but rely on a protective layer (typically organic surfactants) to stabilize them in solution. Further, to impart functionality for e.g. sensing or catalysis, surface modifications need to be performed. Current technology relies on a sulfur-gold bond to bind active moieties onto the gold nanoparticle surface. Though these interactions are strong, it is not complete inert and nanoparticles can decompose in a complex matrix. Recent advancements address this issue by replacing the sulfur gold bond with a much more stable carbene-gold interaction. Although these novel gold nanoparticles show great promise with regards to thermal and chemical stability, recent reports suggest that the stability of gold nanoparticles not only depends on the inertness of the gold linker strength, but also that the way the nanoparticles are synthesized play a crucial role. We recently showed that N-heterocyclic carbene stabilized gold nanoparticles contain both elemental and gold ions, which might have implications for the stability of the nanoparticles. In this project we therefore aim to investigate in detail the synthetic route impact on the exact composition of such gold nanoparticles. To do so, a series of molecular N-heterocyclic carbene gold complexes will be synthesized and utilized for gold nanoparticles synthesis (bottom up synthesis). At the same time, surface ligand exchange reactions (top down synthesis) will be conducted and the composition of both gold nanoparticles will be characterized in detail. Further, reactivity studies will give an insight into differences between bottom up and top down synthesis. By comparing bottom up vs. top down synthesis approaches we expect to gain significant insight into the gold nanoparticle composition and reactivity. The second part of this project aims to utlize the gained knowledge of gold nanoparticle composition for the development of nanocatalysts. Although N-heterocyclic carbenes are a versatile ligand for molecular catalysis, there are only a few examples utilizing N- heterocyclic carbenes stabilized gold nanoparticles for catalysis. Through mechanistic investigations and through the proposed reactivity studies, this project will pave they way for the rational design of nanocatalysts. To summarize, this project should lead to a detailed understanding of N-heterocyclic carbene stabilized gold nanoparticle formation and stability. Furthermore, rationally designed nanocatalysts will pave the way for novel catalytic processes.

Gold nanoparticles are tiny particles with enormous potential, which currently find applications in sensing, optoelectronics, photonics, catalysis, and even photothermal cancer therapy. One of the most prominent uses is in the COVID-19 antigen test, where the strong interaction of gold nanoparticles with light is utilized as a colored indicator. However, these particles are naturally unstable and require a protective "shield" of molecules to prevent them from aggregating or degrading. Typically, such "shields" rely on a sulfur-gold bond, which while strong, are not completely stable in complex environments, such as biological fluids, where they can be replaced or disrupted, leading to decomposition. Over the past decade, a new class of surface ligands, N-heterocyclic carbenes (NHCs) has emerged to improve nanoparticle stability by replacing the sulfur-gold bond with a more stable carbene-gold interaction. Such carbene-gold interactions impart the gold nanoparticles with increased thermal and chemical stability. However, other reports, and findings by us suggest that the synthetic route to prepare these NHC-stabilized gold nanoparticles significantly influences their overall stability and reactivity. In this study, we systematically investigated commonly employed synthetic routes, ranging from bottom-up synthesis to ligand-exchange reactions. Furthermore, we also studied how changes in the electronic properties of the NHC ligand might influence the properties of the resulting nanoparticles. Through detailed analytical and chemical characterizations, we gained insight into nanoparticle composition and reactivity, and our findings revealed that the manufacturing route is just as important as the single constituents themselves; different methods lead to different compositions and levels of stability and reactivity. Given that NHCs have only been established as a persistent surface ligand in nanoscience about a decade ago, our study significantly contributes to a broader understanding of how NHCs can influence NP composition and how small changes in synthetic procedures can significantly impact the performance of the final NHC-stabilized gold nanoparticles. It is therefore expected that our findings will significantly contribute to the further development of these novel nanomaterials and that these results will also be transferable to other metals beyond gold.