Direct Observation Of Au Nanoparticles Interface Self-Assembly With Dark-Field Microscopy

With the development of nanotechnology and the controlled synthesis of nanoparticles, noble metal nanomaterials have accepted much more attention from researchers and been widely applied to the catalysis, sensor, optical imaging and biomedicine et al, due to their remarkable optical properties relating to localized surface plasmon resonance and good photostability and biocompatibility. The so-called self-assembly, refers to the large area and ordered arrays forming technique using the basic structural units (molecules, nanomaterials, micrometer or larger particles). With the study becoming more and more intensive and comprehensive on self-assembly of gold nanoparticles at the interface of two-phase, researchers have come to realize that the in situ nanoparticle dynamic process was very significant to make certain the mechanism of ordered nanoparticle self-assemble arrays. Herein, based on the dark-field microscopy and unique optical properties of gold nanoparticles, we have obtained interested and novel assembled structures of gold nanoparticles by simple solvent evaporation and the dynamic behaviors of nanoparticles in situ. The main points of our work can be summarized as following:(1) In this chapter, owe to the strong optical absorption and scattering properties of large size gold nanorods, they could be directly observed in dark-field vision. We found that the diluted gold nanorods solution can assembled into a multiple-ring structure on a hydrophobic substrate which was obtained by the surface modification as water drying. In addition, we also explored the regulated factors of multiple-ring formation from droplet concentration, particle shape and surface modification, wettability of substrate et al. Experimental results shown that these unique self-assembly structures were controlled by the colloidal concentration, particle shape and substrate wettability.(2) Based on the advantages of dark-field microscopy, not only could we observe the drying results directly, but also we can investigate the whole dynamic drying process of the droplets in situ. Because it is the first time for anisotropic gold nanorods to assembled into multiple-ring pattern with simple solvent evaporation on hydrophobic substrate, the previous reports on the forming mechanism of polycyclic structures cannot fully explain the experimental phenomena we have observed. So in this chapter we illustrated the special multiple-ring forming mechanism from the mechanics, the influence factors of polycyclic structure and particle-particles and particle-substrate interaction respectively according to the droplet whole dynamic process.(3) In virtue of a special micro-closed device and dark-field microscope, we found that the spherical gold nanoparticles would self-assembled into a special lattice structure at the water-air interface as the solvent evaporation slowly. In addition, we also observed the "float" on the water-air interface was very unstable and gradually deforming due to the constant motion of the nanoparticles and unbalanced interface during drying. What's more, with the help of dark-field microscope, we have also monitored the forming and deforming process of the lattice structures in situ and real time.