When nanoparticles are exposed to high salt solutions such as those present in most biologically compatible media, agglomeration can occur. Depending on the nanoparticle material, size, and surface, the agglomeration can happen instantaneously or over a period of days. Once the particles agglomerate, they behave like much larger particles and can have rapid settling rates, and the optical properties of the aggregated particles are typically dramatically different that those of individually dispersed particles.
The stability of nanoparticles with respect to salt depends heavily on the capping agent used to stability the nanoparticles, with sterically bulky polymeric capping agents generally exhibiting greater salt stability than smaller molecular species. The figure below shows the relative stability over time of 20 nm diameter silver nanoparticles in 100 mM NaCl, over a period of 20 days. Citrate- and tannic acid-capped nanoparticles aggregate immediately, while BPEI- and PEG-coated particles were observed to be stable over this period. PVP- and lipoic acid-coated nanoparticles initially showed relatively good stability, but aggregated slowly over this time.
Under certain environmental conditions, silver nanomaterials can be etched to release silver ions, which is of great interest for a variety of applications ranging from silver nanoparticle-based antimicrobial coatings to color-shifting indicators. Generally speaking, exposure to light, oxygen, chloride or other halogen ions, solution pH, and temperatures all influence the rate at which nanoparticles etch, and the rate at which silver ions are released into the surrounding environment.
The presence of chloride or other halogen ions causes an increase in the dissolution rate of silver nanomaterials, presumably either due to the chloride acting as a silver ion “sink” by reacting to form insoluble AgCl and hence removing silver ions from solution, or via catalytic effects in which adsorbed chloride ions affect the redox potential for silver oxidation.
The morphology of the silver nanoparticle also affects the susceptibility to salt addition; silver nanoplates are particularly sensitive to environmental conditions (as compared to spherical silver) because the high surface energy associated with sharp edges and corners at the atomic scale causes the silver ions to be considerably more labile and prone to etching by species in the surrounding media or atomic rearrangement. This change in nanoplate morphology and size can be detected visually and spectroscopiocally, manifested as a blue-shift in the nanoplate plasmon resonance. Such effects can be modulated by the addition of protecting groups on the surface of the silver nanoplates or the use of buffers to control solution conditions.
The optical properties of plasmonic gold and silver nanoparticles change when particles aggregate and the conduction electrons near each particle surface become delocalized and are shared amongst neighboring particles. When this occurs, the surface plasmon resonance shifts to lower energies, causing the absorption and scattering peaks to red-shift to longer wavelengths. UV-Visible spectroscopy can be used as a simple and reliable method for monitoring the stability of nanoparticle solutions. As the particles destabilize, the original extinction peak will decrease in intensity (due to the depletion of stable nanoparticles), and often the peak will broaden or a secondary peak will form at longer wavelengths (due to the formation of aggregates).