Realtime Observation Of Gold Nanoparticle Self-assembly At Water-air Interfaces Using Differential Interference Contrast Microscopy

Realtime observation of the self-assembly process of nanoaparticles at two-phase interfaces is very important for preparing and understanding of controlled nanostructures. Optical microscopy techniques have been widely applied in various aeras, such as cell biology, biomedicine, and single molecular detection. Among them, we are particularly interested in tracking the motion of single/individual metal nanoparticle. In this work, we monitored the dynamics of indivudal gold nanoparticle (GNP) at the water-air(w/a) interface in realtime at single molecule level using Differential Interference Contrast (DIC) microscopy. In DIC microscopy, the subject is probed by a pair of lineally polarized light rays separated by a small shear distance. The response is detectable only when there is an optical path difference between two rays, uauslly resulting from sample thickness and refrective index difference. Therefore, DIC is able to detect nanometer sized objects if the optical properties are distant from the surroundings and the background singnal can be minimized because the homogenous media can not produce measurable optical path difference. The depth of field of DIC is very shallow, so one can easily distinguish particles that are at the interface from those that in the solution. So the self-assmbly process can be easily tracked. This thesis includes the following three sections:(1) Dynamic process of evaporation mediated self-assembly of gold nanoparticles (GNPs) at the water-air interfaceHigh Contrast Differential Interference Contrast (HC-DIC) microscopy was used for real time observation of the w/a interface of a drop of gold colloid solution during its evaporation. GNPs were found to be trapped at the w/a interface and gradually assembled into clusters. The dynamic cluster formation process was driven by evaporation induced electrostatic repulsion decrease and can be divided into two stages. During the whole period, individual particles moved in random patterns that were indistinguishable from those produced by computer simulation, but interactions with surrounding particles and clusters caused a more directional flow motion. The observed dynamics of GNP cluster formation is more like the reaction-limited cluster formation model where the sticking probability is less than unity and anisotropic repulsion is responsible for the low dimensionality of GNP clusters formed. The assembly process was found to be affected by many different factors, such as the hydrophilicity of the substrate, the envaporation rate of the solvent, the particle density in the solution, room tempaerature, etc.(2) Resolving power of DIC on single gold nanoparticlesDuring the investigation of the first part, we observed that many uniform and sphereical particles appear during the early stage of evaporation under the high contrast DIC microscope. They looked like single gold nanoaprticles but their diffusion coefficient was much smaller. Whether they are single nanoparticles or not is crucial to the explanation of GNP self-assmbly behavior. In order to study that, we modified the GNP with SH labeled DNA and then perfomed the same experiment. Interestingly, we can observe only a few nanopartiles during the early envaporation stage but can track lots of particles when the drop of the sample is close to dry, and those particles are sphereical and uniform also. We also compared the DIC images of a 18 nm GNP solution and a 105 nm GNP solution. Although the latter has a much lower concentration than the former, there were more particles appear at the w/a interface of the 105 nm GNP solution than that of the 18 nm solution. These results proved that single GNP moving at the w/a interface can not be observed using DIC microscopy. Those single-NP-like particles are actually small GNP clusters with similar size. This conclusion was further confirmed using dark-field microscopy.(3) GNP self-assembly process under different conditionsBased on previous experiments, we found that there were mainly three factors that significantly affect the self-assembly process. They are 1) the flux rate of GNPs toward the water-air interface,2) the GNP density at the interface, and 3) the local charge and charge distribution at the GNP surface. These three factors interacted with each other. For example, by change the envaporagtion rate, both the flux rate of GNPs to the interface and the the ionic strength of the solution were affected. And the ionic strength is directly related to the suface charge of nanoparticles. The density of nanoparticle can affect the chance of collision beween nanoparticles. In this part, we perfomed three experiments:firstly, we tracked the self-assemly process of a concentrated GNP sample; secondly, we studied the evaporation of a drop of GNP solution with 2% iso-butanol; finally, we studied the assembly process of a diluted sample. Our results demonstrated that the self-assmbly process varies under different conditions.