Stable self-assembled monolayers

The promise of self-assembled monolayers (SAMs) is far-reaching. Find an organic molecule that binds easily to a metal surface, and by chemically modifying the other end of the molecule, one can produce a functional surface that can be tailored for applications from sensors to drug delivery. The first, and still favorite, SAMs are based on sulfur-containing compounds called thiols. Unfortunately, the metal–sulfur bond is relatively weak, and thiol-based SAMs degrade in days if not hours under light, air, or many common solvents. That instability limits their usefulness, but better options have been hard to come by. Now a class of molecules called N-heterocyclic carbenes (NHCs) is emerging as an alternative to thiols. Carbenes are highly reactive molecules in which a carbon atom forms just two chemical bonds instead of the usual four. Most carbenes exist only as short-lived reaction intermediates, but some NHCs, in which the carbene carbon is part of a ring containing two or more nitrogen atoms, are stable enough to be synthesized in bulk. In 2011 Ulrich Siemeling at the University of Kassel in Germany and colleagues were the first to report NHC-based SAMs on gold surfaces. In 2013 Jeremiah Johnson and colleagues at MIT took the important step of showing that NHCs in a monolayer can be chemically functionalized. And now Cathleen Crudden and J. Hugh Horton of Queen’s University in Canada and colleagues have thoroughly characterized the stability of several NHC-based SAMs. They found that with some NHC structures, including the one shown in the figure, monolayers can withstand boiling solvent, some electrochemical cycling, and pH extremes. (C. M. Crudden et al., Nat. Chem. 6, 409, 2014 .)