1. Could you explain the significance of your article to the non-specialist? (50-100 words)

We discuss how to detect and study single metal nanoparticles (diameters 1-100 nm) in an optical microscope. Compared to conventional measurements on ensembles of particles, these new methods radically eliminate ensemble averaging, and directly the time-response of the particles. This feature is particularly appealing for metal nanoparticles, because current synthesis methods always yield particles which differ in size, shape, and distribution of crystal and surface defects, whereas these parameters are unique to every single particle. We believe that these techniques will become more and more important in the fields of nanomaterials and biomolecular sciences.

2. What has motivated you to conduct this work?

The common aim of the several groups working on this problem is to better understand the electrical, optical, and mechanical properties of materials at scales much shorter than a micrometer. These properties often differ spectacularly from those of the same materials at larger scales. Another strong motivation is the development of new non-fluorescent labels for single biomolecules such as proteins. Gold nanoparticles are very stable, bio-compatible, and can be detected in live cells down to very small sizes (1.4 nm diameter is the current record for immobilized particles).

3. Where do you see this work developing in the future?

The detection limit will be pushed down to even smaller sizes, in order to facilitate covalent coupling with biomolecules. A broad variety of optical techniques (time-resolved, frequency-resolved, nonlinear optics, photoluminescence, enhanced Raman scattering, etc.) will be applied to probe an ever expanding collection of properties of matter at nanoscales.

4. Are there any particular challenges facing future research in this area?

The main challenge is to address ever smaller particles, so as to obtain reporter signals for ever smaller scales, and labels as little invasive as possible. Although far-field optical methods have progressed enormously in recent years, near-field optics present specific advantages, and perhaps be crucial to detect very small metal clusters.