Control of dispersion-aggregation of nanoparticles by illumination

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Gold nanometer-sized particles were prepared by the gas flow-solution trap method. The dispersing solvent is 2-propanol and average size of starting particles is 8nm. On irradiation of an Ar-ion laser, the color of dispersion has changed from wine red to dark blue otherwise wine red for more than years. It was shown that dispersed particles agglomerated into fractal cluster in concurrent with the spectral change of increase in 750 nm band and decrease in 523nm plasmon band.The change of dispersion by the illumination of light resembles to the effect of the addition of salt. The stability of dispersion is most sensitive to the concentration of electrolyte in the dispersion because ions affect the Debye length around the particle sphere. Sudden coagulation started at the instant of addition of NaCl solution. That is, the initial peak maximum at 520 nm shifted toward red wavelength region upon increasing concentration of salt. This tendency is similar to the photocoagulation.Following the standard theory of colloids, the stability of sols is governed by balancing between van der Waals attraction and Coulombic repulsion of charged particles. It is obvious that the coagulation proceeds with the visible light energy. At Mie resonance frequency, the free electron-system in a metallic particle feels forced vibration at the resonance frequency causing large dipole oscillation. This large oscillation induced interparticle attractive force among the same particle likewise in the normal van der Waals force. As a result, the interaction is a function of the square of size of particles and the power of a field. The interaction energy has its maximum at about 20 nm and gradually decay for a larger size. Hence the system has to be a mesoscopic scale for a large enhancement to be observed.