Silver Nanoparticle Optical Properties

Silver is a special material for nanoparticles.  Due to localized surface plasmon resonance (LSPR), silver nanoparticles have remarkable optical properties that are a function of the size, shape, surface coating, and suspension media.

Silver nanoparticles and the effect of size

The optical properties of spherical silver nanoparticles are highly dependent on the nanoparticle diameter.  The spectra of  10 different sizes of NanoXact Silver nanoparticles all at the same mass concentration (0.02 mg/mL) are shown in the figure on the right.  40 nm diameter silver nanoparticles have the highest light extinction efficiency of all silver spheres.  In fact, 40 nm silver spheres have the largest extinction cross section per unit mass of all nanoparticles.

As the size of the silver particles increases, its unique plasmonic signature shifts towards the red region of the visible spectrum and both the dipole and quadrupole peaks are clearly expressed.  The total optical extinction is comprised of absorption and scattering.  At small particle size  silver nanoparticles are primarily absorbing and have a clear yellow color in solution.  As the silver particles get larger, the scattering portion of the extinction increases.  This increase in the scattering component results in the solution becoming more gray in color.

Plasmonics with a pratical use

These unique optical signature of the plasmonic silver nanoparticles provides an easy and reliable way to monitor the stability of silver nanoparticles.  If the silver nanoparticles become destabilized and agglomerate, the visible/near-IR spectra will change.  The peak extinction will drop in intensity, broaden, and often a secondary peak will form at a longer wavelength.  In the figure on the right, sodium carbonate is added to a solution of 50 nm silver particles (20 mM salt concentration).  The rapid change in the optical properties clearly demonstrates that the nanoparticles are agglomerating. By carefully monitoring the visible-near IR spectrum of silver nanoparticles, subtle changes to the agglomeration state can be monitored and tracked.  This is an important quality control tool that provides information on whether the nanoparticle solution has degraded with time.