Nanoparticles silanation

In an effort to restrict the location of semiconductor nanoparticles in LB films and inhibit aggregation, the formation of CdS in LB films of calixarenes was investigated [195]. Limiting areas of 3.0 nm and 1.8 nm were obtained on 0.5 mM CdCli, compatible with the cross-sectional areas of the calixarenes. Y-type LB fdms were prepared at 25 mN m on glass, quartz, and silicon. The substrates had been made hydrophobic by treatment with a silane vapor. After H2S treatment overnight in sealed jars, UV absorbance spectra and XPS data were obtained. The absorption edge for the CdS particles formed in the calixarene LB films transferred at pH 5.5 was 3.3 eV as compared with 2.7 eV for films formed in cad-... 

Zhong X, Yuan R, Chai Y, Liu Y, Dai J, Tang D (2005) Glucose biosensor based on self-assembled gold nanoparticles and double-layer 2d-network (3-mercaptopropyl)-trimethoxy-silane polymer onto gold substrate. Sensor Actuator B 104 191-198... 

This method of silanation, which uses organic solvent without the addition of water, is suitable for highly reactive silane derivatives, such as chlorosilanes, aminosilanes, and methoxysilanes. This procedure will not work for ethoxysilanes, as these compounds are not reactive enough without prior hydrolysis to create the silanol. This method is convenient to use for silica particle modification and for the functionalization of metallic nanoparticles having the requisite—OH groups present (see Chapter 14, Section 5). 

The use of silica particles in bioapplications began with the publication by Stober et al. in 1968 on the preparation of monodisperse nanoparticles and microparticles from a silica alkoxide monomer (e.g., tetraethyl orthosilicate or TEOS). Subsequently, in the 1970s, silane modification techniques provided silica surface treatments that eliminated the nonspecific binding potential of raw silica for biomolecules (Regnier and Noel, 1976). Derivatization of silica with hydrophilic, hydroxylic silane compounds thoroughly passivated the surface and made possible the use of both porous and nonporous silica particles in all areas of bioapplications (Schiel et al., 2006). 

Figure 14.25 The preparation of highly controlled fluorescent silica nanoparticles can be done by first polymerizing APTS that has been covalently modified with an amine-reactive dye to form fluorescent core particles. The core then is capped by a shell of silica by polymerization of TEOS. The shell layer can be further derivatized with silane coupling agents to provide functional groups for conjugation.

The following protocol for modification of silica nanoparticles is based on the method of Zhao et al. (2004), which describes the addition of amine functionalities using trimethoxysilyl-propyldiethylenetriamine. Other functional silane modifications may be done similarly. 

Add 32 mg of silica nanoparticles (fluorescent or plain) to 20 ml of 1 mM acetic acid containing 1 percent trimethoxysilyl-propyldiethylenetriamine with stirring. Other concentrations of silane derivatives used for particle modification typically range from 1 to 5... 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

Synthesis of Si02- and Ti02-nanoparticles hybrids with MWCNTs has also been reported (Fig. 3.19) the reaction proceeds by means of phosphonic acid-modified and alkoxy silane-modified CNTs which can cap the oxides and template their assembly onto the CNT s surface [99]. 

Fig. 3.19 Linkage of Si02 and Ti02 nanoparticles via silane and phosphonic acid bonds, respectively. Adapted with permission from [99], 2002, American Chemical Society.

On the other hand, the work by Yan et al. and Jin et al. using silicon wafers and Ni as catalysts has suggested that bulk silicon would diffuse through the nanoparticles to produce SiNW. In this case, solid silicon in the wafer reacts with Ni catalysts to directly make SiNW. If this is true, it falls into the category of root growth. However, as we will illustrate below, the use of hydrogen in the presence of metal catalysts may activate a new reaction pathway that converts Si in the substrate into silane. As a result, the suggested solid-liquid-solid model may actually be the VLS model at work. 

We also observed growth of SiNW from Au nanoparticles. In this case, H2 was also necessary. Although Au silicides can form at moderate temperatures and can also be reduced by hydrogen to Au nanoparticles and silane, the growth temperature for SiNW was still above 1000°C. It seems that silane was produced in both Au and Co catalytic growth cases. 

We also attempted to produce SiNW from oxidized Si wafers. However, as shown earlier, only a small number of SiNW were produced. This indicates that a thick Si02 layer may not allow the formation of SiNW, even in the presence of H2 and Co nanoparticles. Since Si is oxidized and becomes amorphous, no SAN would form under this condition. This result perhaps implies that SAN may contribute to SiNW growth (e.g., via the production of silane gas). Also, since H2 can reduce Si02 to Si or SiO, and SiO can be evaporated at 1200°C, oxidized Si in the presence of H2 may, in principle, produce SiNW without the use of catalysts. However, the negative result shown here implies that Co nanoparticles may not react well with Si02 to form CoSi2 and thus H2 cannot affect the growth of SiNW. 

SCHEME 10.1. Formation of silane and SiNW by a series of reactions involving Co nanoparticles, hydrogen, and silicon wafers. 

We further suggest that silane is produced and becomes airborne, which then reacts with metal or metal silicide nanoparticles that have not reacted with silicon or with Co silicide nanostructures to produce SiNW, as shown in Scheme 1. We are looking into how to directly prove the existence of silane. 

FIGURE 10.23. Proposed catalytic processes to make SiNW from Co nanoparticles and hydrogen. Si wafers act as the source of silicon. Silane is produced, which then reacts with Co or Co silicide catalysts to make SiNW. [Chem Comm 2005]—Reproduced by permission of The Royal Society of Chemistry, (ref 54)... 

A schematic of the proposed growth model is shown in Fig. 10.23. In this model, Co nanoparticles play a dual catalytic role. On the one hand, they catalyze silane formation by reacting first with silicon to form Co silicides, and then react with hydrogen to form silane while being reduced to Co metal. The second role of Co nanoparticles is their classic catalytic ability of making nanowires by first dissolving the silane and precipitating out Si nanowires. 

Figure 1.1 Stepwise production of metal-particle multilayer arrays. The attachment ofthe Au or Ag nanoparticles onto ITO-modified glass was achieved by using silanes that have an amine terminus group. This modification step allows for further modification with nanoparticles onto the surface of the ITO. After the nanoparticle attachment a redox-active bridging molecule was...

Fig. 4 Schematic illustration of synthesis of multifunctional nanoparticles starting from a w/o microemulsion, b solubilization of fluorescent dye in the microemulsion core, c formation of silica nanoparticle and encapsulation of fluorescent dye, d condensation of silane ligand and chelation of Gd(lll), e post coating with silica, and f extraction of nanoparticles...

Markowitz et al. developed a different approach, again in an attempt to overcome some of the inherent difficulties that arise when imprinted bulk materials are used as catalysts [82], Here, the authors used a template-directed method to imprint an a-chymotrypsin TSA at the surface of silica nanoparticles, prepared with a number of organically modified silanes as functional monomers. Silica particle formation was performed in a microemulsion, where a mixture of a non-ionic surfactant and... 

Fig. 10 Example of a contact-killing and microbe-releasing surface. The scheme shows the design of a two-level dual-functional antibacterial coating containing both quarternary ammonium salts and silver. The coating process begins with LbL deposition of a reservoir made of bilayers of PAH and PAA. (A) Cap region made of bilayers of PAH and SiC>2 nanoparticles (NP) is added to the top. (B) The SiC>2 nanoparticle cap is modified with a quarternary ammonium silane (QAS) PEM polyelectrolyte multilayer. (C) Ag+ is loaded into the coating using the available unreacted carboxylic acid groups in the LbL multilayers. Scheme was reproduced from [138]...

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