biomolecules with up to zeptomole sensitivity in bulk experiments 7,8,13 . Moreover, the binding of proteins to single functionalized particles can be followed by dark-field microscopy, by exploiting the dependence of the plasmon resonance wavelength on the refractive index of the particle's surroundings 14,15 . The plasmon resonance wavelength of a metal nanoparticle is also affected by other nanoparticles that are in its immediate environment. When two nanoparticles are brought into proximity, their plasmons couple, which shifts the resonance wavelength depending on the particle separation. Since this effect is well known theoretically 16, 17 and experimentally observed for fixed distances 18, 19 , we sought to explore its use as molecular ruler.We applied this 'plasmon ruler' to study the dynamics of DNA hybridization on a single We employed 40 nm diameter gold and silver nanoparticles. The particle diameter was chosen to ensure a sufficiently intense light scattering signal while minimizing any effects of the particles on proximal biomolecules. However, we subsequently found that a reduction in particle size to 20 nm for silver and 30 nm for gold is possible with the current technique.Particles were illuminated with unpolarized white light and light scattered by individual particles was collected by a darkfield microscope in transmission mode (Fig. 1a) 25 . Upon introduction of streptavidin-functionalized particles into the BSA-biotin coated glass chamber, we immediately observed numerous scattering sources adhering to the chamber surfaces. Nanoparticles were vividly colored: individual silver nanoparticles were blue (Fig. 1b), gold nanoparticles were green ( Fig. 1c), aggregates were red-shifted compared to individual particles (typically by about 50 nm for gold, 150nm for silver), and dust and scratches were white.Our first application of plasmon coupling was to monitor the directed assembly of functionalized particle pairs. We used the surface immobilized particles (Figs. 1b and c) as anchors for single stranded DNA (ssDNA) functionalized particles. The 33-nucleotide ssDNA molecules had a biotin at their 3' end, allowing them to bind to the streptavidin coated anchor particles (Fig. 1a). Shortly after introducing the ssDNA-functionalized particles into the chamber, some scattering centers suddenly changed color due to dimer formation. Silver particles turned from blue to green (Fig. 1b), gold particles turned from green to orange (Fig. 1c). The spectral shift upon dimer formation is considerably larger for the silver particles (102 nm) than for gold particles (23 nm, Fig. 1d). The fraction of surface immobilized particles that captured a ssDNAfunctionalized particle ranged from 10% to 86 % depending on the time the samples had been stored, with fresher particles performing better. Aggregates of more than two particles were easily identified by their intensity and distinct color with multiple peaks in the spectrum (see for example the purple dot in Fig 1b). To avoid these aggregates of more than two partic...