Single molecule spectroscopy uncovers gold nanoparticles' catalytic secrets

Before you can design the perfect nanoparticle catalyst, you first need to understand the fundamental science that governs their reactivity, say scientists from the US. And they claim to have done just that, using single molecule spectroscopy to reveal that nanocatalyst surface properties respond to changes to the concentration of reactants. 

The team led by Peng Chen at Cornell University, Ithaca, have devised a technique for studying the catalytic properties of individual gold nanoparticles - which gets around the traditional problem with studying nanocatalysts, says Chen. ’No matter how good a synthetic chemist you are, the nanoparticles you make will have slightly different sizes and shapes, and this is why you need to study one particle at a time, so you don’t have to average all of them.’

Using single molecule fluorescence, Chen could ’watch’ single nanoparticles react with single reactants, and quantify the heterogeneity between different particles. ’We knew nanoparticle behaviours were different among at population, but just how different they are has been challenging to quantify before now,’ says Chen.

However, while studying the catalytic redox conversion of resazurin to resorufin, the team also found that each nanoparticle changes its surface catalytic properties in response to the concentration of reactants. ’This has never been discovered before,’ says Chen. ’We found that nanoparticles surfaces can change from "type A" to "type B", and these two types have different reactivities. Type B that occurs at high concentrations is more reactive and binds to reactants weakly, and type A that occurs at low concentrations is lower in reactivity and binds to the reactants strongly.’ It is not yet understood, however, why the switching of behaviours occurs.

The ultimate nanocatalysis would be designed to combine these two types of behaviour, says Chen. ’Ideally you want a nanoparticle that binds to the reactant strongly so you can scavenge all the reactants from solution, and you also want reactivity to be high so the catalysis goes faster.’ 

Look closer

Robert Rioux, an expert in heterogeneous catalysis at Pennsylvania State University, US, says that Chen’s findings ’will have vast implications in terms of designing catalysts’. ’Studies like these will be critical for the design of the materials and understanding how catalysts behave under reaction conditions,’ he adds. 

Until last year - when two new techniques were announced - the catalytic properties of nanoparticles couldn’t be studied on a single molecule level. One of these is Chen’s, and the other, which uses surface plasmon resonance (SPR) spectroscopy, was designed by Paul Mulvaney at the University of Melbourne, Australia. Chen says that the main advantage of his technique is that it ’can monitor reactions one reaction at a time, and count reactions in real time. And Mulvaney’s technique can not do this.’

Another advantage of Chen’s technique, explains Rioux, is that unlike SPR spectroscopy it can be used for any type of metal nanoparticle - not just SPR-responsive metals such as gold and silver. 

Next Chen plans to make use of this advantage and switch from studying gold to platinum nanoparticles - as he feels a better understanding of these is could help for fuel cell applications.

Nina Notman