Control of the self-assembly behaviors of charged gold nanoparticles

Critical island size, scaling, and ordering in colloidal nanoparticle self-assembly

In order to obtain a better understanding of short-range (SR) and long-range (LR) nanoparticle (NP) interactions during the self-assembly of dodecanethiol-coated Au NPs in toluene via drop drying, we have investigated the dependence of the island density, scaled island-size distribution (ISD), and scaled capture-zone distribution (CZD) on coverage, deposition flux, and NP size. Our results indicate that, while the critical island size is larger than 1 for all NP sizes studied, due to the increase in the strength of the SR attraction between NPs with increasing NP size, both the exponent describing the dependence of the island density on deposition flux and the critical island-size decrease with increasing NP size. We also find that, despite the existence of significant cluster diffusion and coalescence, the ISD is sharply peaked as in epitaxial growth. In particular, for large NP size, we find good agreement between the scaled ISD and epitaxial growth models as well as good agreement between the scaled CZD and scaled ISD. However, for smaller NPs the scaled ISD is less sharply peaked despite the fact that the critical island size is larger. This latter result suggests that in this case additional effects such as enhanced island coalescence or NP detachment from large islands may play an important role. Results for the ordering of NP islands are also presented which indicate the existence of LR repulsive interactions. One possible mechanism for such an interaction is the existence of a small dipole moment on each NP which arises as a result of an asymmetry, driven by surface tension, in the thiol distribution for NPs adsorbed at the toluene-air interface. Consistent with this mechanism, we find good agreement between experimental results for the nearest-neighbor island-distance distribution and simulations which include dipole repulsion.

Induced transition of CdSe nanoparticle superstructures by controlling the internal flow of colloidal solution

The self-assembly behaviors of flow-enhanced CdSe nanoparticle (NP) colloidal systems were investigated, which were systemically prepared by adding ethylene glycol (EG) or acetic acid (AA) to NP suspensions with deionized water (DI water) base. The additive solvents, which had higher boiling points and lower surface tensions than those of the DI water, modified the internal flow of the NP colloidal system, consequently affecting the morphologies of the generated NP superstructures after the full evaporation of their droplets. In flow-enhanced systems, NPs were formed into highly elongated dendrites that stretched from the center region to the edges along the direction of convective flow inside the droplet, while NPs in random drift system were easily aggregated to form cluster-shaped thick dendritic structures. When the volume fraction of EG was increased, the dominant superstructures were changed from dendrites to clusters, which can be mainly attributed to the changes in the dielectric properties of the NP droplets as evaporation proceeded because of the large discrepancy in the vapor pressures of EG and DI water. The balance between the interparticle potentials of electrostatic repulsion and van der Waals attraction was continuously altered, resulting in the formation of clusters with increasing EG ratio. Contrastively, the transition of superstructures could not be observed in the case of colloidal system prepared by mixing DI water and AA, which can be ascribed to the similar vapor pressures of the two solvents; the dielectric properties of the solution mixture was barely changed throughout the steady evaporation process, which resulted in the formation of uniformly distributed highly elongated dendrites. Polarization-dependent imaging experiments and photoluminescence measurements revealed that the stretched dendrites formed under the flow-enhanced conditions showed higher crystallinity than that of the clusters.

Self-assembly of CdTe nanoparticles into dendrite structure: a microsensor to Hg2+

A novel microsensor to Hg(2+) was fabricated through self-assembly of aqueous CdTe nanoparticles (NPs). The morphologies of self-assembly mainly included classical dendrites, straight dendrites, and small islands. The formation process of these morphologies was systematically investigated by using the field emission scanning electron microscope, confocal laser scanning microscope, and atom force microscope instruments, etc. The proposed mechanism showed that the dendrite structure was formed via manipulating the short-range van der Waals interaction and long-range electrostatic interaction, which was realized through altering the ligand and concentration of the CdTe NPs. Furthermore, polymers with positive charges were used to effectively control the morphology of the self-assembly as well as improve the property of photoluminescence. These CdTe dendrites were used as microsensors to Hg(2+), which presented the advantages of low cost, quick detection time, high selectivity, and easy operation.

Improvement of the stability of colloidal gold superparticles by polypyrrole modification

The colloidal gold superparticle (SP)/polypyrrole (PPy) core/shell composites were successfully prepared by oxidative polymerization of pyrrole monomer on the surface of poly(N-vinylpyrrolidone) (PVP)-grafted colloidal gold SPs. These core/shell composites showed strong catalytic activity and excellent stability. Control experiments indicated that the morphology and the thickness of PPy shell were controllable by adjusting the dosage of pyrrole monomer. Meaningfully, the resulting SP/PPy core/shell composites were quite stable in water, which could be stored for more than half a year without damaging their structures. As an example, we demonstrated the use of these composites as catalyst for the reduction of methylene blue (MB) dye with a reducing agent of sodium borohydride. The composites exhibited highly catalytic activity and long-term stability, implying promising applications of SP/PPy composites in catalysis.