Gold-coated silica nanoparticles

Schematic diagrams for the synthetic schemes are illustrated in Fig. 15.5. To briefly summarize, silica nanoparticles were synthesized by the Stober-method and then coated with a Sn layer, which acts as a linker site for gold deposition. Gold layers were coated on the Sn-functionaUzed silica nanoparticles by reduction of HAuCLt. Finally, the gold-coated silica nanoparticles were covered...

Schematic diagram for the synthesis of gold-coated silica nanoparticles. 

Experimental scheme for preparation of PPV nanocomposites doped with gold-coated silica nanoparticles. Reprinted with permission from Y.T. Limefa/., Synth. Mat 128,133 (2002). Copyright 2002 Elsevier. 

Experimentally, Thomas and coworkers found that hydrocarbon-coated gold nanoparticles, with a diameter of 3.5 nm, segregated to the interface between the microdomains of poly(styrene-btock-ethylene propylene) (PS-fe-PEP) copolymer, whereas larger hydrocarbon-coated silica nanoparticles (21.5nm in diameter) were located at the center of the PEP domains [79], In the absence of specific enthalpic interactions between the two types of nanoparticles and the polymer matrix, the result suggests a profound influence of entropic contributions to the self-organization process. For large particles, the decrease in conformational entropy of the respective polymer subchains after particle sequestration is dominant, whereas for smaller particles, the decrease in entropy is outweighed by the particle translational entropy. 

The Stober method can be used to form core-shell silica nanoparticles when a presynthesized core is suspended in a water-alcohol mixture. The core can be a silica nanoparticle or other types of nanomaterials [46, 47]. If the core is a silica nanoparticle, before adding silicon alkoxide precursors, the hydroxysilicates hydrolyzed from precursors condense by the hydroxide groups on the surface of the silica cores to form additional layers. If the core is a colloid, surface modification of the core might be necessary. For example, a gold colloid core was modified by poly (vinylpyrrolidone) prior to a silica layer coating [46]. 

Figure 16.6 DNA-coated gold-capped mesoporous silica nanoparticle loaded with (3-estradiol, employed for the simultaneous delivery of a gene and its promoter into plant cells.

M. Giersig, T. Ung, L. M. Liz-Marzin, and P. Mulvaney, Direct observation of chemical reactions in silica-coated gold and silver nanoparticles, Adv. Mater. 9,570-575 (1997). 

Nanotechnology has led to very efficient versions of liposomes. Tiny hollow spheres only nanometers in diameter hold even tinier capsules of medicine. The spheres are made of silica covered with gold nanoparticles and when they are coated with antibodies they attach to tumor cells. The spheres are sensitive to light of specific wavelengths and when the light is applied, either heat up and destroy the tumor, or burst, releasing the drugs within the capsules directly into the tumor. 

Figure 5.9 Synthesis of gold nanoparticles within Zr02. A gold colloid is first prepared then coated with silica using a modified Stober process [75]. The silica is removed using NaOH. Reproduced with permission from [74],...

Figure 14.24 Fluorescent silica nanobubbles have been created using gold nanoparticle seeds that initially are coated by adsorption with a fluorescent dye. The particles then are capped by a layer of silica by polymerizing TEOS and entrapping the dye molecules within it. Finally, the gold core is dissolved by reaction with cyanide, leaving behind hollow fluorescent silica nanobubbles.

Beside planar surfaces, Patten et al. [310-312] and Hallensleben et al. [313, 314, 322] prepared weU-defined nanocomposites by ATRSIP of from silica and gold nanoparticles. After spin-coating a solution of the polymer decorated particles onto mica, Hallensleben was able to image the single particles with the individual grafted polymer chains (Pig. 9.33). 

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