Zerovalent Noble Metal Nanoparticles

Nanoscale Properties of Zerovalent Nobel Metal Nanoparticles

For centuries before the terms nanotechnology and nanoparticles were coined, zerovalent noble metal nanoparticles had been known and studied. Many of the beautiful colors of medieval stained-glass windows are the result of small metal oxide nanoparticles in the glass. Particles of different size scatter different wavelengths of light, imparting the different colors to the glass.The photoreduction of silver salts to small colloidal particles has long been a part of the process of image formation in photography. With the growth of bionanotechnology, noble metal nanoparticles have been found to be indispensable building blocks in a variety of systems, ranging from scaffolds for antigen mimics, electron transfer mediators in enzyme-based biosensors to integral components in DNA detection.
AuNPs and AgNPs demonstrate a variety of interesting nanoscale size- and shape-dependent tailoring of their physical and chemical properties making them highly tunable components in catalytic, sensor and biological imaging applications. There has been considerable interest in resonant Rayleigh scattering from AuNPs and AgNPs. Specifically, light scattering from silver nanoparticles made by lithographic or colloidal techniques has been used extensively for biological and chemical analysis.The utility of these plasmon-resonant particles (PRPs) stems from the sensitivity of the localized surface plasmon resonance (LSPR) to local chemical environment and refractive index. These surface plasmon waves are surface electromagnetic waves that propagate parallel along a metal/dielectric interface. Since the wave is on the boundary of the metal and the external medium (air or water for example), these oscillations are very sensitive to any change of this boundary, such as the adsorption of molecules to the metal surface.

Light scattering from PRPs is 106 times more intense than fluorescence emitted from commercially available fluorophores and PRPs do not suffer from photodegradation.30 Moreover, intense scattering facilitates the detection of particles, typically of 30-100nm diameter, using commercially available optical microscopes, which adds to the popularity of this field.
In discussing shape- and size-dependent optical properties of metal nanoparticles, it is important to note that nanoparticle extinction has both light absorption and scattering components, which are size and shape dependent and may or may not have the same wavelength dependence.Theoretical experiments by Schatz and co-workers show the same wavelength dependence for both extinction and scattering for small spherical gold nanoparticles (30nm diameter). For larger diameter nanoparticles (e.g. 100 nm), scattering is enhanced, relative to absorption in the red region of the spectrum. Size and shape dependence on light scattering for AgNPs has been reported. In general, as silver nanoparticles increase in size, their scattering spectra are red shifted.33 Moreover, changes to the shape of AgNPs has a significant effect on their observed light scattering spectrum.
Silver spheres have a maximum scattering peak at ~400nm and pentagons at ~500 nm, and triangles have a peak maximum further red shifted to 750 nm, all of which are size dependent. In addition, the observed light scattering spectra for triangular-shaped particles are blue shifted as the corners of the triangles go from sharp and well-defined to truncated or rounded.With these tunable properties, AuNPs and AgNPs are highly functional components of optical labeling schemes for a wide array of biological systems. On the nanoscale, noble metal nanoparticles have unique electrical properties that can be used in biosensing applications.The ability of AuNPs to store charge as nanoscale capacitors was originally demonstrated by Murray’s group by observing quantized double-layer (QDL) charging peaks for monodisperse AuNPs referred to as gold monolayer protected clusters (MPCs).QDL charging results from a
single electron transfer into or out of the AuNP metallic core through the thiol shell. QDL peaks can be observed for monodisperse MPCs when this single electron transfer causes a change in the MPC potential. Improvements in techniques for obtaining monodisperse MPCs have enhanced the ability to observe sequential QDL peaks, especially for hexanethiol gold MPCs.It is very likely that this same quantitated
behavior can be explained, in concept, by a similar ‘particle-in-a-box’ model as has been done for semiconductor quantum dots.