Self-assembly of N-heterocyclic monolayers on gold and other transition metals
A self-assembled monolayer or SAM is a layer of molecules one molecule thick attached to a surface in an ordered fashion. One very interesting aspect of SAMs is that they can form spontaneously, creating an ordered layer out of disorder. SAMs are a great way of changing a surface’s wetting properties (e.g. to make them repel or adsorb water, depending on the design of the SAM), to protect metals from oxidation (rusting), to separate layers of metals and semiconductors in microelectronic devices, or to act as a scaffold for more reactive molecules, such as in catalysts. SAMs also have a part in biology – the wall of a cell is formed of a lipid bilayer – essentially two SAMs back-to-back.
One of the most commonly used SAMs in chemistry is based on a molecule in which sulfur links to a metal, known as an alkanethiol. These are very easy to use, and form well-ordered layers, particularly on gold. But, while they have been used widely for at least 30 years, they have a very serious drawback: they oxidize rapidly in air, and decompose, sometimes within only a few hours. Thus, they have had limited use in technological applications. We have recently made a very significant breakthrough in SAM technology by replacing the sulfur with a carbon atom in the metal linkage, using a class of molecules known as N-heterocyclic carbenes (NHC). According to first-year chemistry, an NHC just shouldn’t exist: the carbon molecule at the end of the molecule and which eventually links to the metal has a lone pair of electrons attached to it. In carbon chemistry, such an arrangement is usually highly reactive. However, in an NHC, the presence of both electron-withdrawing nitrogen (the N of NHC) and a ring which delocalizes some of the carbons other electrons, leads to some species that are stable enough to leave in a bottle on the shelf. But they are reactive enough to attach to gold, as well as other metals, and turn out to form an extremely stable SAM. The SAMs we have made can survive extremes of pH, boiling in water, and even exposure to oxidizing solutions of hydrogen peroxide.
In our lab, we are making new NHC-based monolayers, testing out their reactions with different metals, studying the kinetics of the deposition of the NHC on the surface, and looking for new applications for these molecules. One application that seems very promising is use in a surface plasmon resonance detector, or SPR. In an SPR machine, the NHC is used to attach a biomolecule to the surface (e.g. an antibody or peptide) and use this to detect various biomarkers (for example, an enzyme associated with the presence of a cancer) in biological samples. SPR machines work well now, but only in highly controlled laboratory environments by highly trained technicians. With our new robust NHC-based monolayers, we hope to show this powerful instrument can be used in the field, by less expert users, expanding the scope of their use in areas such as bioremediation.