Advancements in Self-assembled monolayers of N-heterocyclic carbenes

Abstract

Metals are an important part of our modern society. They make up fundamental components of our modern infrastructure including transportation, smart-phones, point-of care devices, electrical equipments and therapeutics (metallic nanoparticles). One of the main challenges we face is effective modulation of surface properties of metals. Modulation of surface properties by formation of strongly bound self assembled monolayers (SAMs) has received considerable interest in last 50 years. Self assembly of alkanethiolates on gold are one of the most studied systems, however, recently N-heterocyclic carbenes (NHCs) have gained considerable interest. NHCs are organic molecules that have been extensively been employed as ligands in transition metal complexes for last few decades. They form stable metal complexes compared to other ligands. In materials chemistry, especially for applications of SAMs, NHCs have important advantages compared to thiolate systems as the sterics and electronic properties of the carbenic carbon can be chemically tuned to serve variety of purposes. Given that they are a new ligand in field of material sciences, many fundamental properties of NHC-based SAMs are not well understood. The work presented here is a detailed study in which I use combination STM, XPS, and RAIRS to study self assembly behaviour of NHCs on various metal surfaces. I have identified that adsorption of NHCs on surface is not straightforward process. I unravelled the reactivity of NHCs with oxidized Cu surfaces (outside of ultra-high vacuum conditions). On Pt and Ru, I observed that NHCs form strong bonds to the surface, but the overall stability of SAMs was highly temperature dependent. The results from these studies have resulted in guidelines for rational design of NHC-based SAMs for coinage and non-coinage metals. Furthermore, I have used these data as guiding principles to design the first example of an NHC based broad spectrum electrochemical biosensor. NHC based biosensors have the ability to detect whole-cell bacteria from cultures as well as in real-life samples, with a linear response over a wide range of concentration. In addition, we have used NHCs for chemical functionalization of graphenes supported on metals. This type of chemical functionalization was found to be dependent on electronic interactions between graphene and metals.