Plasmon Resonant Nanoparticles

• What are plasmon resonant nanoparticles (PRPs)
• How do we study PRPs?
• What are some potential applications for PRPs?

• Publications on Plasmon Resonant Nanoparticles from our group
• A selection of publications on PRPs from other groups
• Recent SPIE conference Power point Presentation with our experimental results

What are Plasmon Resonant Nanoparticles?

PRPs are metallic (silver or gold) nanoparticles, typically 40–100 nm in diameter, which scatter optical light elastically with remarkable efficiency because of a collective resonance of the conduction electrons in the metal (i.e., the surface plasmon resonance). The magnitude, peak wavelength, and spectral bandwidth of the plasmon resonance associated with a nanoparticle are dependent on the particle’s size, shape, and material composition, as well as the local environment. A number of unique plasmon resonant nanoparticles are shown below.

Colloidal Ag Nanoparticles	Colloidal Au Nanoparticles	Metal Nanorods (Ag, Au, Ni)	Composite Metal Nanorods

 

Optical Microscope Spectral Analysis

A high magnification optical microscope is required to study these nanoparticles. Because PRPs scatter optical light with such great efficiency, we are able characterize the plasmon resonance of individual nanoparticles using our optical microscope apparatus. Due to the diffraction limited imaging and sub-wavelength physical size of the nanoparticles, these PRPs appear as bright colorful spots under the optical microscope. Darkfield illumination is necessary to maximize the scattering from the nanoparticles while minimizing the background scattering from the substrate. There are a number of illumination schemes we utilize which meet this requirement, such as standard Darkfield (utilizing the Nikon microscope darkfield objectives), total internal reflection evanescent field illumination and brightfield oil immersion.

Standard Darkfield	TIR Evanescent Field	Brightfield Oil Immersion

The scattered light signal from the nanoparticles is collected by the optical microscope and passed either to an imaging CCD detector, or to a spectrometer. Nanoparticle positioning is achieved using a precision mechanical stage. Optical microscope characterization of individual nanoparticles can be correlated with the transmission electron microscope (TEM) image of the same region using a technique demonstrated below, which we call mapping. We have performed mapping studies correlating the optical signature of the nanoparticles with their physical shape and size. We have characterized a number of nanoparticles shapes and sizes including both Au and Ag spheres, Ag tetrahedrons and pentagons, and homogeneous and composite nanorods of Au, Ag and Ni (publications).

Darkfield Image of PRPs	TEM Image of PRPs

In addition to size, shape and material properties of a nanoparticle, the local dielectric environment also has a strong effect on the plasmon resonance wavelength. As the local index of refraction increases, the nanoparticle plasmon resonance wavelength red-shifts. Using the darkfield microscope apparatus, we have been able to monitor the plasmon resonance peak of individual nanoparticles (Ag spheres and tetrahedrons) as the local dielectric is changed. It is evident from our experiments that the individual nanoparticles can act as sensors of the local dielectric, and that the tetrahedron shaped nanoparticles' plasmon resonance peak position shifts nearly twice as far as the spherical nanoparticle with the same dielectric change.

Applications for Plasmon Resonant Nanoparticles

A number of applications have recently been demonstrated for Plasmon Resonant Particles, and the interest in this field is growing rapidly. It has been proposed that the plasmon resonant nanoparticles could be used as biological-labels (our PNAS publication, other bio-label publications), because their extremely high scattering cross section and unique colors make them easily identifiable optical microscope markers. Additionally, the sensitivity of the plasmon resonance to local delectric means that the nanoparticles can be used as nanoscale biosensors (our Nanoletters publication, other bio-sensor publications). Individual nanoparticles can be monitored with the optical microscope, and the plasmon resonance peak position shifts as bio-molecules are bound to the nanoparticle surface. Various nano-optical devices have also been proposed (nano-optical devices) which exploit the highly localized plasmons possibly enabling extremely small light manipulation.