Control of dispersion-aggregation of nanoparticles by illumination

采用气相流动-溶液捕集器法制备了金纳米颗粒。分散溶剂为2-丙醇，起始颗粒平均粒径为8nm。在氩离子激光的照射下，色散的颜色已由酒红色变为深蓝色或酒红色。结果表明，分散的粒子聚集成分形团簇，并伴有750 nm波段增加、523nm波段减少的光谱变化。光照射引起的色散变化类似于加盐的效果。分散体的稳定性对分散体中电解质的浓度最为敏感，因为离子会影响颗粒球周围的德拜长度。在加入NaCl溶液的瞬间开始了突然的凝固。即随着盐浓度的增加，520 nm处的初始峰值向红色波长区偏移。这种倾向与光凝类似。根据胶体的标准理论，溶胶的稳定性是由带电粒子的范德华引力和库仑斥力之间的平衡决定的。可见光能的凝聚作用是明显的。在Mie共振频率下，金属粒子中的自由电子系统在共振频率处受到强迫振动，引起较大的偶极子振荡。这种大的振荡引起了粒子间的吸引力，同样在正常的范德华力中。因此，相互作用是粒子大小的平方和场的功率的函数。相互作用能在20 nm左右达到最大值，随着尺寸的增大而逐渐衰减。因此，系统必须是一个介观尺度，以观察到一个大的增强。

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.