A molecular ruler based on plasmon coupling of single gold and silver nanoparticles

Abstract

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...

Tunable plasmonic lattices of silver nanocrystals

“…Over the past decade, materials research has focused on self-assembly 15 , hard-and soft-templating 16,17 , and evaporation-based 18 approaches to overcome this challenge of assembly at the nanoscale. Molecular assembly strategies for metal nanoparticles such as the use of DNA-linkages 19,20 , selectively binding ligands 21 , or molecular scaffolding 22 have been shown to generate complexes of nanoparticles, but with ordering only for small groupings. For application in surface-enhanced Raman sensing or for device integration, large-scale nanoparticle assemblies are needed.…”

Plasmonic Ruler at the Liquid–Liquid Interface

“…12 An additional drawback of a solid state system becomes clear when introducing the exciting concept of a plasmonic ruler. [13][14][15] The coupling between excitations of localized plasmons in NPs can allow for precise spatial information. This concept has been demonstrated between 2D arrays, 10 particle dimers 14 at solid interfaces and tethered NPs in bulk solution.…”

Photothermal Methods for Single Nonluminescent Nano-Objects

“…The upper detection limit is determined by the condition of weak plasmon coupling between particles. When the average distance between the small AuNPs is comparable to their size, the optical response of AuNPs is modified (43). Furthermore, the possibility of detecting much smaller AuNPs (D = 1.4 nm) should increase the dynamic range obtained with 20 nm AuNPs significantly (Figure 4c; 12).…”