Layered alkylsilanes

The first category is layered organosilanes. Layered alkylsilanes are synthesized from akyltrialkoxysilane [CH3(CH2) iSi(OR)3, R is often CH3 or C2H5] and tetraethyl orthosilicate (TEOS) [62-66]. Alkoxy groups are hydrolyzed and condensed into... 

Figure 5. Schematic structure of a polysiloxane layer obtained by hydrolysis and poly condensation of monomeric alkylsilanes RSiX3 (X = Cl, OCH3, OC2H5).

Apolar stationary phases suffer from hydrolytic instability at pH extremes. The use of mixed phases of long (Cg, Clg) and short (C, C3) chain alkyls produces stationary phases with increased hydrolytic stability.7,8 Crowding of the long alkyl chains does not allow the alkylsilane molecules to deposit in close packing on a smooth or flat surface. Silane molecules polymerize in vertical direction, loosing contact with the silica surface. The insertion of short chain alkyls allows horizontal polymerization of the silane molecules. Thus, alkyl chains are aligned in a parallel way. The stability of the silane layer is increased consequently (figure 8.1). 

Molecular and structural characteristics of methoxysilane SA-layers deposited on inorganic subtrates can be obtained using DRIFT spectroscopy in combination with TOF-SIMS. The hydrolysis rates are strongly influenced by the acid/base properties of the oxide surface. In accord to this, the contribution of residual Si-OCHs units is much lower for SA-layers on the basic alumina. The outmost layers of the polysiloxanes consist of completely hydrolyzed species. In case of methylsilanes, the SIMS spectra indicate the presence of T and T OH silsesquioxanes whereas for higher alkylsilanes the formation of T OH isomers is likely. This hypothesis has to be addressed in future studies with particular focus on structural aspects. 

In attempt to dilute bromoalkyl residues in the bonded layers, modification with mixtures of alkylchlorosilane-bromoalkylchlorosilane (Table 4) was performed. This process is similar to that employed in the so- called multiphase generation. Alkylsilanes taken in excess were believed to result in formation of bonded layer with isolated bromoalkyl groups. However, experimental evidence shows that even with a huge excess of trimethylchlorosilane bromomethylchlorosilane reacts with silica surface by the island-like mechanism. This result is most likely related to enhanced reactivity of bromoalkylchloro-silanes compared to alkylchlorosilanes. To put it in other words, alkylchlorosilane fails to interfere in formation of bonded layer by bromoalkyl groups (Table 4). 

The problems of supramolecular organization and distribution of bonded molecules are of great importance not only for the materials with a low surface coverage. These are also important for the preparation of the most dense bonded layers with the maximum shielding of the surface. Indeed, the complete modification of silica with alkylsilanes and preparation of superhydrophobic surfaces [44,45] are only possible under the conditions of island-like coverage of the surface with a. modifier. 

FIG. 26 Schematics for creating a microscopic spot of immobilized enzymatic activity by localized derivatization of self-assembled monolayers according to (A) GOx on an patterned gold alkanethiolate layer and (B) diaphorase on alkylsilanized glass. (Part A from Ref. 21. Copyright 1997 American Chemical Society. Part B from Ref. 91. Copyright 1997 American Chemical Society.)... 

To assemble the surface-bound membrane structure, we first form a hydrophobic monolayer by using alkylsilanization to covalently attach long-chained hydrocarbon chains to hydroxyl groups in the oxide layer on the electrode surface. Such a hydrophobic surface can be thought of as one leaflet of a membrane bilayer. Alkylsilane-modified surfaces have been widely used as substrates for lipid monolayers deposited by Langmuir-Blodgett techniques. The lipids in these monolayers have mobilities like those of lipids... 

We simultaneously incorporate both lipid and protein by using dialysis to remove detergent from a solubilized lipid-protein mixture in the presence of the alkylsilanated substrate. Under our conditions, from the evidence in this paper and elsewhere (9), the surface structures appear to be single bilayer membranes. Our hypothesis is that the hydrocarbon chains attached to the surface serve as initiation sites for a lipid bilayer membrane to form as the detergent is slowly removed. The model is of a membrane that is anchored to the surface by hydrophobic interactions with the surface-bound hydrocarbon layer. Integral membrane proteins are retained in these structures by their interaction with the hydrophobic core of the membrane without being directly attached to the electrode surface. 

An equivalent circuit can be derived for the surface-bound membrane formed in this work similar in a manner to the approach taken for porous anodic films and porous electrodes (41-46). An equivalent circuit network, proposed in Figure 8a, corresponds to the model in Figure 7. This network has three RC subnetworks that represent the oxide layer, the surface-bound membrane layer, and the double layer. Cox and Rox are the capacitance and resistance of oxide. and Rdl are the double-layer capacitance and the polarization resistance, known as the charge transfer resistance at the membrane-water interface. For the subnetwork of the surface-bound membrane layer, one branch represents a tightly packed alkylsilane and lipid bilayer in series, and the other branch represents the pores and defects through the bilayer. Calk, Clip and Ralk, Rhp are the capacitances and resistances of... 

Electrochemical impedance spectroscopy provides a sensitive means for characterizing the structure and electrical properties of the surface-bound membranes. The results from impedance analysis are consistent with a single biomembrane-mimetic structure being assembled on metal and semiconductor electrode surfaces. The structures formed by detergent dialysis may consist of a hydrophobic alkyl layer as one leaflet of a bilayer and the lipid deposited by dialysis as the other. Proteins surrounded by a bound lipid layer may simultaneously incorporate into pores in the alkylsilane layer by hydrophobic interactions during deposition of the lipid layer. This model is further supported by the composition of the surface-bound membranes and by Fourier transform infrared analyses (9). 

Duchet J et al (1997) Influence of the deposition process on the structure of grafted alkylsilane layers. Langmuir 13(8) 2271... 

Abel and co-workers [106] at Ciba-Geigy Ltd. have reported an automated optical biosensor system. Their device utilizes 5 -biotinylated-16-mer oligonucleotide probes bound to an optical fiber functionalised with avidin to detect complementary oligonucleotides pre-labeled with fluorescein moieties in a total internal reflection fluorescence (TIRF) evanescent wave motif similar to that of Squirrell. Immobilization of nucleic acid probes onto the optical fiber substrate was achieved by functionalisation of the surface with (3-aminopropyl)triethoxysilane (APTES) or mercaptomethyldimethylethoxysilane (MDS). Onto the short alkylsilane layer was... 

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