Hydrosilylation Reactions

Hydrosilylation reactions are probably the best-developed solution-phase method for attaching organic molecules to silicon. In this reaction, a silicon-hydrogen bond on a silicon surface reacts with a double or triple bond in an alkene or alkyne to form a direct silicon-carbon bond. Hydrosilylation reactions for attachment of alkenes to a silicon surface was first demonstrated by Linford and coworkers [87,88]. Analogous reactions involving organosilanes in solution are well known. The first application of this reaction to produce stable colloids of free silicon nanoparticles was probably that by Lie et al. [89]. Figure 4.4 shows schematically how the hydrosilylation is believed to propagate across the silicon surface. The reaction can be initiated by heating, by visible or UV irradiation, or by a catalyst that generates a free radical site on the surface by removing a hydrogen atom. A double bond reacts with this radical site to form a new silicon-carbon bond and generate a free radical at the neighboring carbon that previously participated in the double bond. That carbon can then abstract a hydrogen atom from a neighboring silicon atom to generate a new radical site, which in turn can react with another alkene and repeat the process. Although some details remain unclear, evidence for this mechanism on both crystalline silicon surfaces and porous silicon has been presented, most recently by de Smet et al. [90]. The review by Buriak [84] provides a good overview of hydrosilylation reactions that had been reported in the literature through 2001. The possibility of initiating hydrosilylation reactions using white light, demonstrated for porous silicon by Stewart and Buriak [91], and for H-terminated silicon wafers by Sun et al. [92] may be of particular importance, because it could allow covalent attachment of relatively delicate biomolecules that would not be compatible with high temperature, UV illumination, or some metal-containing catalysts.

Hydrosilylation reactions on hydrogen-terminated crystalline silicon surfaces and porous silicon have recently been applied to attach a variety of biologically relevant entities to these surfaces. Presumably, many of the strategies that have been employed will also work for free hydrogen-terminated

Mechanism Hydrosilylation Surface
FIGURE 4.4 Schematic mechanism for hydrosilylation of a hydrogen-terminated silicon surface with a terminal alkene.

photoluminescent silicon nanocrystals, though most of them have not been demonstrated. Here, we briefly review biologically relevant examples of hydrosilylation, in roughly chronological order. In 1997, Wagner et al. [93] reported biofunctionalization of self-assembled monolayers (SAMs) of 1-octene formed by hydrosilylation of an H-terminated Si(111) surface. They used 254 nm UV-initiated hydrosilylation to form a dense monolayer of 1-octene on the wafer surface. They then used two different strategies to create reactive groups for attaching biomolecules to this alkane monolayer. The first was to react it with TDBA-OSu, an aryl-diazirine crosslinker, which created N-hydroxysuccinimidyl groups at the end of about 10% of the octene chains. Amine-terminated molecules can then be attached to these groups, as they demonstrated using amine-functionalized DNA. A problem with this approach was that it left the surface hydrophobic. The second approach was photoinduced chlorosulfonation of the terminal methyl groups of the octene chains, carried out by exposing them to a dilute Cl2 in SO2 gas mixture under UV illumination. This created reactive sulfonyl chloride groups that were subsequently reacted with ethylenediamine to form a strong sulfonamide bond and leave amine groups on the surface. This approach involving vapor-phase reagents would be more difficult to apply to free nanocrystals.

In 2000, Strother et al. [94] reported attachment of DNA to silicon wafers by a multistep process. They first attached an ester of undecylenic acid to the H-terminated Si(111) surface by UV-driven hydrosilylation. Then they hydrolyzed the ester using potassium t-butoxide in dimethyl sulfoxide (DMSO) to yield a carboxylic acid-covered surface. Then a layer of polylysine was electrostatically bound to the surface through interactions of its terminal amine groups with the carboxylic acid groups. To the amine group of this, they attached a heterobifunctional crosslinker molecule with an amine-reactive N-hydroxysuccinimide ester group at one end and a thiol-reactive maleimide group at the other end. Finally, they attached thiol-terminated DNA to the resulting maleimide groups. That same year, Strother et al. [95] reported another strategy for DNA attachment to H-terminated silicon. In this approach, they first synthesized t-butoxycarbonyl protected 10-aminodec-1-ene. They attached this to an H-terminated Si(001) surface by UV-initiated hydrosilylation. After removal of the t-butoxycarbonyl protecting group, this provided an amine-terminated surface, to which they could attach thiol-terminated DNA with the same heterobifunctional crosslinker mentioned above. Lin et al. [96] carried out a detailed study of this same approach using unprotected and t-butoxycarbonyl protected 1-amino-3-cyclopentene, and found that use of the protected amine was essential. In that study, they also compared three different bifunctional linking molecules for attaching thiol-terminated DNA to the amine groups on the silicon surface.

In a series of publications [97], Horrocks and coworkers synthesized DNA directly on planar and porous silicon substrates. They first used thermally driven hydrosilylation to attach 4,4'-dimethoxytrityl-protected v-undecanol to the silicon surface. Removal of the protecting group then gave a primary alcohol-terminated surface. They used these functionalized flat and porous silicon substrates in a standard DNA synthesizer for solid-state synthesis of oligonucleotides. They also attempted to break up the porous silicon into nanoparticles after the solid-state DNA synthesis to produce DNA-coated nanocrystals. This met with limited success, however, as porous substrates that were sturdy enough to withstand the DNA synthesis were not easily broken up into nanoparticles, and after the DNA attachment, they could not readily be further etched.

de Smet et al. [98] used both thermal and white-light initiated hydrosilylation to attach mixtures of 1-alkenyl saccharides and 1-decene to silicon surfaces in a mixed monolayer. Hart et al. [99] used Lewis acid-catalyzed hydrosilylation to attach hex-5-ynenitrile to the surface of porous silicon. Subsequent treatment with LiAlH4 in ether reduced the nitrile group to a primary amine. Heterobifunctional linker molecules were then used to attach biomolecules of interest, and the PL of the porous silicon was maintained. Cai's group used 254 nm UV-induced hydrosilylation to attach a-oligo(ethylene glycol)-v-alkenes to H-terminated Si(111) surfaces [100]. The resulting oligo(ethylene glycol) surface was hydro-philic and resistant to protein binding. They were able to pattern this film using conductive atomic force microscopy (AFM) and attach avidin on the patterned spots for subsequent protein binding. Xu et al. [101] attached 4-vinylbenzyl chloride to the Si(111) surface by UV-induced hydrosilylation, then used the Cl-terminated surface to initiate atom transfer radical polymerization of a macromonomer from the surface. They then coupled heparin to the -OH groups of the polyethylene glycol-based polymer layer to produce an antithrombogenic surface.

Although many of the above-mentioned investigations used alkenes with a protected amine or carboxyl group at the opposite end, Voicu et al. [102] were able to attach undecylenic acid (10-undecenoic acid) to H-terminated Si(111) selectively at the alkene end, without any apparent reaction of the carboxylic acid group with the Si-H surface. They were then able to convert the carboxylic acid group to the corresponding succinimidyl ester for subsequent linking to primary amines, including amine-terminated DNA.

There are relatively fewer examples of the application of hydrosilylation to free silicon nanoparticles, but the available studies suggest that the chemistry on free nanoparticles is similar to that on porous silicon and silicon wafers. Lie et al. [89] initiated hydrosilylation of silicon nanoparticles thermally, by refluxing porous silicon in a toluene solution of 1-octene, 1-undecene, or other molecules with a terminal alkene group. This yielded stable colloidal dispersions of individual nanocrystals. The hydro-silylation reaction was confirmed by Fourier transform infrared (FTIR) spectroscopy, and the approximate size of the resulting alkylated silicon nanocrystals was determined by time-of-flight mass spectrometry (TOFMS). Under UV excitation, the particles exhibited PL with a peak emission wavelength near 670 nm. In our group we have applied both thermally driven [14,29] and UV-photoinitiated hydrosilylation [15] to photoluminescent silicon particles produced by laser-induced vapor-phase decomposition of silane followed by HF-HNO3 etching. The hydrosilylation reaction was confirmed by FTIR and NMR spectroscopies. The PL of the particles was dramatically stabilized by the attachment of organic molecules to their surfaces. When we attached undecylenic acid to the particles via thermally driven hydrosilylation by refluxing in an ethanol solution, we observed significant oxidation in addition to the desired hydrosilylation reaction [14]. Particles with undecylenic acid or octadecene attached via thermally driven hydrosilylation remained susceptible to PL quenching by amines [29]. However, in more recent work, we have prepared denser monolayers of a variety of alkenoic compounds on nanoparticles with more complete hydrogen termination, and have seen improved resistance to PL quenching [15]. Li and Ruckenstein [103] used UV-driven hydrosilylation to attach acrylic acid to the surface of silicon nanoparticles prepared by this same method and were able to prepare a stable dispersion of them in water that maintained its PL. Warner et al. [49] used platinum-catalyzed hydrosilylation to attach allylamine to blue-emitting silicon quantum dots prepared by the reduction of SiCl4 with LiAlH4 in reverse micelles. They were also able to obtain a stable dispersion in water that maintained its PL. Wang et al. [104] used photoinitiated hydrosilylation to attach 1-octene or 1-hexene to silicon nanocrystals ultrasonically dispersed from porous silicon. They then used TDBA-OSu, an aryl-diazirine crosslinker, which created N-hydroxysuccinimidyl groups at the end of some or all of the surface-grafted alkyl chains. This allowed them to attach amine-functionalized DNA to the silicon nanoparticles. The oligonucleotide-conjugated silicon nanoparticles maintained their PL and formed stable dispersions in water. Thus, it appears that the wide range of strategies based on hydrosilyla-tion reactions that have been developed for flat silicon wafer surfaces and porous silicon can, at least in many cases, also be applied to free silicon nanocrystals. This approach provides stable, covalent linkage of biologically relevant molecules to the nanoparticle surface, which should make the resulting nanostructures very robust.

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