Optical trapping and light-induced agglomeration of gold nanoparticle aggregates

Abstract

This paper demonstrates the optical trapping of micron-sized gold nanoparticle aggregates (GNAs) with a ${\mathrm{TEM}}_{00}$ mode laser at $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and reports the observation of an unusual light-induced agglomeration phenomenon that occurs besides the trapping of the GNAs. The observed agglomerate has a $60--100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ donut-shaped metal microstructure with the rate of formation dependent on the laser power used. Citrate capped gold nanoparticles also show light-induced agglomeration, yielding different sized microstructures from those produced with GNAs. While the observed trapping can be explained by a model including the optical radiation and radiometric forces, the light-induced agglomeration cannot be explained by these two forces alone as the size of the agglomerate is much greater than the waist of the Gaussian beam used in the optical trapping. We attribute the additional cause of the light-induced agglomeration to ion detachment from the surface of the nanoparticles (aggregates) due to light illumination or heating. This is supported by the observation of reversible electrical conductivity changes of the solution of the nanoparticles (aggregates) upon laser illumination or direct heating. Light-induced agglomeration can be useful in the design and fabrication of microstructures from nanomaterials for various device applications.