Angew. Chem. Int. Ed. http://dx.doi.org/10.1002/anie.201102882 (2011)

The surfaces of metal nanoparticles are often coated with a single layer of thiol molecules in which the sulphur atoms at the head of the molecules bind to the surface and the molecular tails extend out into the surrounding medium. Such monolayers can also be formed using mixtures of thiols, and these mixed monolayers can undergo phase separations to form nanoscale domains on the surface. By controlling the structure of the domains, the properties of the nanoparticles, such as their solubility, can be modified. However, characterizing the domains is difficult. John McLean, David Cliffel and colleagues at Vanderbilt University have now shown that mass spectrometry can be used to observe and measure phase separation in gold nanoparticle monolayers.

The nanoparticles are first fragmented using the matrix-assisted laser desorption/ionization process, which frees the gold–thiolate complexes that cover the nanoparticle core. The ionized species are then separated in the gas phase by effective surface area, and subsequently by mass-to-charge ratio. This allows mass spectra that contain only the gold–thiolate ions to be collected. From the spectra, specific gold–thiolate species are extracted and their abundances compared with a theoretical model that represents a random distribution of molecules on the nanoparticle surface. Deviations from the model indicate that nanophase separation is present in the nanoparticle monolayer and can be correlated to the formation of domains.

With the technique, the Vanderbilt team are able to efficiently analyse monolayers prepared with different mixtures and ratios of thiols, as well as by different synthetic procedures.