Bottom-Up Then Top-Down Synthesis of Gold Nanostructures Using Mesoporous Silica-Coated Gold Nanorods.

Gold nanostructures were synthesized by etching away gold from heat-treated mesoporous silica-coated gold nanorods (AuNR@mSiO2), providing an example of top-down modification of nanostructures made using bottom-up methodology. Twelve different types of nanostructures were made using this bottom-up-then-top-down synthesis (BUTTONS), of which the etching of the same starting nanomaterial of AuNR@mSiO2 was found to be controlled by how AuNR@mSiO2 were heat treated, the etchant concentration, and etching time. When the heat treatment occurred in smooth moving solutions in round-bottomed flasks, red-shifted longitudinal surface plasmon resonance (LSPR) was observed, on the order of 10-30 min, indicating increased aspect ratios of the gold nanostructures inside the mesoporous silica shells. When the heat treatment occurred in turbulent solutions in scintillation vials, a blue shift of the LSPR was obtained within a few minutes or less, resulting from reduced aspect ratios of the rods in the shells. The influence of the shape of the glassware, which may impact the flow patterns of the solution, on the heat treatment was investigated. One possible explanation is that the flow patterns affect the location of opened pores in the mesoporous shells, with the smooth flow of solution mainly removing CTAB surfactants from the pores along the cylindrical body of mSiO2, therefore increasing the aspect ratios after etching, and the turbulent solutions removing more surfactants from the pores of the two ends or tips of the silica shells, hence decreasing the aspect ratios after etching. These new stable gold nanostructures in silica shells, bare and without surfactant protection, may possess unique chemical properties and capabilities. Catalysis using heat-treated nanomaterials was studied as an example of potential applications of these nanostructures.

Sealable Spherical Mesoporous Silica Shell Nanoreactors as Fiducial Nanoscale Probes for X-rays.

Molecular reactions in aqueous solutions are often used as dosimetric probes. A major problem with this approach is that other species such as nanoparticles or radical scavenging chemicals can often interfere with these reactions. The results measured in the presence of nanomaterials and scavengers therefore cannot correctly indicate the true dose based on the calibrated results obtained in solutions free of the interfering species. Storing these molecular probes in nanoreactors can overcome this problem. Here we demonstrate for the first time that it is possible to place common probe molecules inside spherical mesoporous silica shells and seal the pores after impregnation for the purpose of using the so-formed nanoreactors as X-ray dose probes. The reactions are isolated from the external environment, while the sealed shells still allow X-rays to freely penetrate through the walls of the nanoreactors. These nanoreactor probes can therefore fiducially report the dose of X-rays, whether the nanoreactors are in solutions, in dry form, or in the presence of scavengers and catalysts in solution.

Sub-monolayer silver loss from large gold nanospheres detected by surface plasmon resonance in the sigmoidal region.

Nanosilver becomes labile upon entering the human body or the environment. This lability creates silver species with antimicrobial properties that make nanosilver attractive as active components in many consumer products, wound dressings, and agricultural applications. Because lability depends strongly on morphology, it is imperative to use a material with constant lability throughout kinetic studies so that accurate lability data can be acquired with efficient detection. Here 2.5nm thick silver was coated onto 90-nm diameter gold nanosphere cores and this surface silver layer was gradually removed by either chemical or X-ray radiation etching. The most sensitive region of a sigmoidal surface plasmon resonance (SPR) response as a function of silver thickness was found for the first time between 0.9- and 1.6-nm thick silver, revealing a new nanosilver standard for lability studies. The SPR peak position detection sensitivity is 8nm (SPR peak shift)/nm (silver thickness change) within this steepest region of the plasmon response curve whereas outside, sensitivity drops to 1nm/nm. Since the centroid of SPR profiles can be discerned with 0.25nm precision, the 8-nm/nm sensitivity means it is possible to detect a 0.3-angstrom or sub-monolayer change in silver thickness. The SPR response simulated by discrete dipole approximation (DDA) was an identical sigmoidal function between 0 and 2nm of silver coating. These findings were supported by several other analytical measurements, which confirmed no silver recoating during these etching processes.

Encapsulation of multiple large spherical silica nanoparticles in hollow spherical silica shells.

Here we present the results of a stepwise synthesis of multiple large silica nanoparticles encapsulated in hollow, micron sized silica shells for future display applications. In the first step, 200-nm diameter silica nanoparticles were modified with 3-(trimethoxysilyl) propylmethacrylate (MPS) coupling agent. These nanoparticles were then embedded in micron-sized polystyrene particles synthesized through dispersion polymerization. To form silica shells on the polymer composite particles, tetraethylorthosilicate (TEOS) was added with cetyltrimethylammonium bromide (CTAB) surfactant. These three steps resulted in the formation of silica shell-covered solid polystyrene particles, each containing multiple silica nanoparticles. In the last step, polystyrene content was removed via calcination to achieve a multiple-silica-core-in-hollow-silica-shell composite structure. Dynamic light scattering (DLS) analysis and transmission electron microscopy (TEM) confirmed the core/shell morphology of the composite structure.