Ferdinand Gonzaga

Silica Shell/Gold Core Nanoparticles: Correlating Shell Thickness with the Plasmonic Red Shift upon Aggregation

Differences in the wavelengths of the surface plasmon band of gold nanoparticles (AuNP)--before and after particle aggregation--are widely used in bioanalytical assays. However, the gold surfaces in such bioassays can suffer from exchange and desorption of noncovalently bound ligands and from nonspecific adsorption of biomolecules. Silica shells on the surfaces of the gold can extend the available surface chemistries for bioconjugation and potentially avoid these issues. Therefore, silica was grown on gold surfaces using either hydrolysis/condensation of tetraethyl orthosilicate 1 under basic conditions or diglyceroxysilane 2 at neutral pH. The former precursor permitted slow, controlled growth of shells from about 1.7 to 4.3 nm thickness. By contrast, 3-4 nm thick silica shells formed within an hour using diglyceroxysilane; thinner or thicker shells were not readily available. Within the range of shell thicknesses synthesized, the presence of a silica shell on the gold nanoparticle did not significantly affect the absorbance maximum (~5 nm) of unaggregated particles. However, the change in absorbance wavelength upon aggregation of the particles was highly dependent on the thickness of the shell. With silica shells coating the AuNP, there was a significant decrease in the absorbance maximum of the aggregated particles, from ~578 to ~536 nm, as the shell thicknesses increased from ~1.7 to ~4.3 nm, because of increased distance between adjacent gold cores. These studies provide guidance for the development of colorimetric assays using silica-coated AuNP.

Au–carbon nanotube composites from self-reduction of Au3+ upon poly(ethylene imine) functionalized SWNT thin films

The combination of single-walled carbon nanotubes (SWNTs) with metal nanoparticles has been an area of recent interest due to the attractive electronic, mechanical, catalytic, and photonic properties of such materials. Although complexes of SWNTs and nanoparticles are known, macroscopic samples in which nanoparticles are uniformly dispersed in high density throughout the nanotube substrate have not been reported. Here, a simple method for the production of carbon nanotube thin films containing Au nanoparticles is described. SWNTs functionalized with highly branched poly(ethylene imine) (Mn = 10 kDa) were found to exhibit impressive aqueous solubility (180 mg L−1), allowing for the formation of homogeneous thin films by vacuum filtration. These films were subsequently functionalized with Au nanoparticle clusters by in-situreduction of HAuCl4 under mild conditions in the absence of additional reducing agents. Using thermogravimetric analysis, it was determined that poly(ethylene imine) functionalized SWNTs contained 17 wt% polymer, which amounts to one polymer chain for every 2600 carbon atoms within the nanotubes. Incubating these films in aqueous HAuCl4 solutions resulted in a high density, uniform distribution of Au nanoparticle clusters along the film surface. In addition, using transmission electron microscopy (TEM) and focused ion beam (FIB) milling within a scanning electron microscope (SEM), nanoparticles were found to be embedded at various depths throughout the film. SEM was employed to determine Au nanoparticle diameter and distribution. The average diameter of the Au clusters could be controlled in the range of 50 to 550 nm.

Biomimetic Synthesis of Gold Nanocrystals Using a Reducing Amphiphile

The first synthesis of a chelating and reactive surfactant derived from citric acid and a short silicone as hydrophobic tail is described. Aqueous solutions of this reactive amphiphile spontaneously induce gold ion reduction, particle nucleation, and further direct crystal growth. The process, both pH and light dependent, occurs through lipid-directed assembly of metal ions, their reduction and subsequent lipid-directed growth to yield ultrathin (approximately 7 nm thick) quasi two-dimensional gold nanocrystals.

Morphology-controlled synthesis of poly(oxyethylene)silicone or alkylsilicone surfactants with explicit, atomically defined, branched, hydrophobic tails.

Silicone surfactants are widely used in commerce because of the unusual surface activity when compared with fluorocarbon or hydrocarbon surfactants. However, most silicone surfactants are comprised of ill-defined mixtures, which preclude the development of an understanding of structure-surface activity relationships. Herein, we report a synthetic strategy that permits exquisite control over silicone structure by using the B(C(6)F(5))(3)-catalyzed condensation of hydro- and alkoxysilanes. Six different, precise hydrophobes were then mated to hydrophilic poly(oxyethylene)s of three different molecular weights by a metal-free click cyclization to generate a library of explicit silicone surfactants. These compounds were calculated to have a relatively linear value range of the hydrophilic-lipophilic balance, ranging from about 8 to about 15. The solubility of some of the compounds was too low to measure a critical micelle concentration (CMC). The others exhibited a broad range of surface tension values at the CMC that depend both on the length of the hydrophilic tail and, more importantly, the nature of the hydrophobic head group. Subtle distinctions in surfactant-related properties, which can be attributed to the three-dimensional structures, can be seen for compounds with comparable numbers of hydrocarbons and silicon groups.

Amphiphilic silicone architectures via anaerobic thiol-ene chemistry.

Despite broad application, few silicone-based surfactants of known structure or, therefore, surfactancy have been prepared because of an absence of selective routes and instability of silicones to acid and base. Herein the synthesis of a library of explicit silicone-poly(ethylene glycol) (PEG) materials is reported. Pure silicone fragments were generated by the B(C(6)F(5))(3)-catalyzed condensation of alkoxysilanes and vinyl-functionalized hydrosilanes. The resulting pure products were coupled to thiol-terminated PEG materials using photogenerated radicals under anaerobic conditions.

Sugar complexation to silicone boronic acids

A new class of surface-active compounds based on the combination of silicones and boronic acids is described. The properties of the compounds can be tuned by manipulation of both the hydrophobic (silicone size and 3D structure) and hydrophilic components (by binding different saccharides to the boronic acid). Stabilization of the four-coordinate boron structure is provided by Tris buffer that also maintains neutral pH to suppress silicone hydrolysis.

Oriented crystallization of ultra-thin (2 nm) gold nanoplatelets inside a reactive hydrophobic polymeric matrix

The synthesis of an amphiphilic polymer based on a polysiloxane backbone with grafted propyloxy-citrate moieties is described. Aqueous solutions of this polymer are able to spontaneously reduce gold cations and direct gold metal crystal growth, yielding ultrathin (2 nm) crystalline nanoplatelets that have an exceptionally high aspect ratio: they are in excess of hundreds of nm in width. The growth process is accelerated by light and significantly controlled by pH: at pHs above 7, particles instead of platelets are formed.

Nearly monodisperse silica microparticles form in silicone (pre)elastomer mixtures.

The formation of silica from a tetraalkoxysilane in a sol-gel process usually requires a highly polar, typically aqueous, medium that aids in the hydrolysis of the silane and leads to electrostatic stabilization of the growing silica particles. Formation of such silica particles in a hydrophobic medium is much more challenging. We report the formation of silica microspheres within silicone oils (hydroxy-terminated poly(dimethylsiloxane), HO-PDMS) during elastomer cure using atmospheric humidity in a one-pot and one-step synthesis. Using tetraethyl orthosilicate (TEOS) as both cross-linker and silica precursor, and aminopropyl-terminated dimethylsiloxane oligomer (AT-PDMS) as a catalytic surfactant, silica particles of low polydispersity formed near or at the air interface of the elastomer: the presence of a hydrophilic polymer, poly(ethylene glycol) (PEG), had an indirect effect on the particle formation, as it assisted with water transmission into the system, which resulted in particle formation over a wider range of parameters and facilitated silicone elastomer cure further away from the air interface. Depending on the relative humidity during cure, the sizes of particles presenting at the air interface varied from ~6-7 μm under ambient conditions (20-30%RH) to ~7-9 μm at high relative humidity (90% RH). The origin of the controlled particle synthesis is ascribed to the relative solubility of the catalyst and the efficiency of water permeation through the silicone matrix. AT-PDMS preferentially migrates to the air interface, as shown by ninhydrin staining, where it both catalyzes alkoxysilane hydrolysis and condensation, and stabilizes the growing silica particles prior to aggregation. Since reactions in the presence of this catalyst are slow, TEOS can migrate from within the pre-elastomer body to the interface faster than water can penetrate the silicone, such that the main locus of hydrolysis/condensation leading both to silica formation and elastomer cross-linking is at the air interface.

Generic, SN2 reactive silicone surfaces protected by poly(ethylene glycol) linkers: facile routes to new materials

The efficacy of biomaterials is frequently dependent upon the interface between a synthetic material and the surrounding biology. While silicones offer many benefits in biomaterials applications, they can suffer from insufficient hydrophilicity. We present a controlled and generic route to the surface modification of silicones that permits the introduction of a passivating poly(ethylene glycol) (PEG) layer capped with biologically relevant molecules. High-density, tosylate-modified, PEG-tethered silicone surfaces are readily prepared. These surfaces provide a generic platform for further surface modification by nucleophilic substitution (SN2). The efficiency of substitution at the surfaces was established using a broad variety of nucleophiles: although triphenylphosphine did not modify the tosylated surface, the surface reaction of primary amines, azide, and a thiol was demonstrated to be highly efficient in organic solvents at several temperatures. Changes in surface chemistry and properties were demonstrated by water contact angle, ATR-FTIR, XPS and 13C solid-state NMR spectroscopy.