SAMs thiols

Herein the focus is on SAMs of trichlorosilanes and thiols. SAMs of carboxyUc acids are important as a connection between the LB and self-assembly techniques, but studies of their formation and stmcture have been relatively limited. SAMs of carboxyUc acids on AI2O2, AgO, and CuO have also been carried out (113—124). 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

Sulfiir-anchored SAMs and thin films, mostly from organosulfiir precursors, have been discussed at length by a number of authors [10, 181]. SAMs of organosulfiir compounds (thiols, disulfides, sulfides) form on gold substrates by spontaneous adsorption from either the liquid or the vapor phase. A number of experimental factors can affect the formation and structure of SAMs such as choice of solvent, temperature, concentration, immersion time, purity of adsorbate, oxygen concentration in solution, cleanliness, and structure of the adsorbate. Interestingly, the... 

Self-assembled monolayers (SAMs) [8] The layers are formed by heterologous interaction between reactive groups, such as thiols, and noble metals, such as gold or silver. Since the molecules are selectively adsorbed on these metals, film growth stops after the first monolayer is completed. The molecular aggregation is enthalpy driven, and the final structure is in thermodynamic equilibrium. 

For transition and precious metals, thiols have been successfully employed as the stabilizing reagent (capping reagent) of metal nanoparticles [6]. In such cases, various functionalities can be added to the particles and the obtained nanoparticles may be very unique. It is well known that thiols provide good self-assembled monolayers (SAM) on various metal surfaces. When this SAM technique is applied to the nanoparticle preparation, nanoparticles can be covered constantly by functionalized moieties, which are connected to the terminal of thiol compounds. 

Wet preparation of metal nanoparticles and their covalent immobilization onto silicon surface has been surveyed in this manuscript. Thiol-metal interaction can be widely used in order to functionalize the surface of metal nanoparticles by SAM formation. Various thiol molecules have been used for this purpose. The obtained functionalized particles can be purified to avoid the effect of unbounded molecules. On the other hand, hydrogen-terminated silicon surface is a good substrate to be covered by Si-C covalently bonded monolayer and can be functionalized readily by this link formation. Nanomaterials, such as biomolecules or nanoparticles, can be immobilized onto silicon surface by applying this monolayer formation system. 

We investigated the efficiency of NSC expansion on surfaces with EGF-His immobilized in the correct orientation. NSCs were obtained from neurosphere cultures prepared from fetal rat striatum harvested on embryonic day 16. NSCs were cultured for 5 days on EGF-His-immobilized substrates prepared with mixed SAMs of different COOH-thiol contents. Cells adhered and formed network structures at a density that increased with the COOH-thiol content of the surface. As a control, cells were seeded onto surfaces without immobilized EGF-His. This resulted in poor cell adhesion during the entire culture period. In addition, when EGF-His adsorbed to SAMs with 100% COOH-thiol or SAMs with NTA-derivatized COOH that lacked Ni2+ chelation, we observed poor initial cell adhesion, and the cells formed aggregates within 5 days. Interestingly, the substrate used to covalently immobilize EGF-His with the standard carbodiimide chemistry was not a suitable surface for cell adhesion and proliferation. The control experimental results contrasted markedly with results from EGF-His-chelated surfaces. 

We also conducted experiments to compare our culture method with the standard neurosphere culture. In the standard neurosphere culture, cell number increased approximately nine times over 5 days. Immunostaining showed that the neurosphere cultures contained 54 5.3% nestin+ cells and 41 7.4% nestin+ pIIF cells. This demonstrated that the standard neurosphere culmring method was less efficient than EGF-immobilized substrates for selectively expanding NSCs. Thus, the EGF-immobilized substrates prepared from mixed SAMs with 10% COOH-thiol provided the most efficient method for selective NSC expansion. 

Based on those results, we concluded that, when cultured on the EGF-His-immobilized surface prepared from a mixed SAM of 10% COOH-thiol, highly enriched NSC populations could be produced in large quantities. Over a 5-day culture on the substrate, cells were expanded 32 times. These expanded cells consisted of 98% nestin+ cells that retained multipotency for differentiation into neuronal and glial lineages. This suggested that selective expansion could be repeated for large-scale production of highly enriched NSC cells. 

Figure 2.3 SAM surface modification has been done using monothiol and dithiol compounds containing PEG linkers. Useful coatings typically contain mainly PEG-hydroxyl or PEG-monomethyl ether linkers that provide a biocompatible lawn, which prevents nonspecific binding of proteins to the metallic surface. About 10 percent of the surface modifications are done using a longer carboxylate-containing thiol-PEG linker that provides sites for attachment of affinity ligands.

Coupling of affinity molecules to surfaces also can be enhanced by the use of discrete PEG linkers. Nishimura et al. (2005) modified an amino surface with a NHS-PEG -maleimide crosslinker to create a hydrophilic self-assembled monolayer (SAM) surface that was thiol reactive for the conjugation of sulfhydryl-modified RNAs. This array then was used to investigate the binding specificity of synthetic kanamycins with selected RNA sequences to prove the specific interaction of ribosomal RNA with this molecule. The PEG linkers on surfaces provide lower nonspecific binding character than alkyl linkers, when preparing SAM surfaces for affinity interactions. 

B. Ge and F. Lisdat, Superoxide sensor based on cytochrome c immobilized on mixed-thiol SAM with a new calibration method. Anal. Chim. Acta. 454, 53-64 (2002). 

Two examples from literature illustrate this approach nicely. Moore et al.114 assembled thiol-terminated long-chain S204-crown TTF onto Au and Pt surfaces by thiolato-metal bonds (see Figure 12). In the presence of various cations, most successfully Ag+, small differences were observed in the first oxidation potential (typically 60-80 mV). Similar responses were observed in solution state experiments with the same materials. The SAMs were stable when electrochemically cycled over the first oxidation wave but unstable when scanned beyond this point. Liu et al.115,116 prepared SAMs of 45 and 46 on Au substrate. Anchored to the solid surface by four Au S bonds per molecule, these SAMs were stable for hundreds of cycles over the full oxidation range. In response to the presence of Na+ both the TTF oxidation waves were shifted anodically by 55-60 mV. This observation was ascribed to either surface aggregation or cooperativity of neighbouring crown rings. 

Most metals are covered by an oxide (impervious and insulating, or more often, as with Al, defect-ridden). In contrast, gold has no oxide, and has the advantage of making SAMs with thiols. However, Au atoms migrate somewhat after deposition to minimize total energy, and migrate even more under an electric field... 

Fig. 5 Schematic representation of LAJs based on liquid metal electrodes, (a) The two Hg drops junction. The drops are extruded from two microsyringes and covered singularly by similar or different SAMs before being brought in contact, (b) An Hg-drop electrode covered by SAM(l) (usually formed by hexadecane thiol) is brought in electrical contact with a SAM(2) formed on a solid metal surface, (c) A drop of In/Ga eutectic alloy (E-Gain) contacts a SAM formed on a solid electrode surface...

Fig. 6 Schematic representation of an Hg-drop LAJ incorporating SAMs of organic molecules of (a) alkanethiols, (b) oligophenylene thiols and (c) benzylic derivatives of oligophenylene thiols of different length formed on an Ag electrode, (d) Semi-logarithmic plot of measured current at applied bias I 0.5 V vs electrode gap flowing through the a, b, c interfaces...

SAM-covered gold surfaces, these three peaks were assigned to Au-S, C-C, and C-H modes of surface-bound alkanethiolates [30]. The absence of a strong S-H signal at 329 mV suggests that most of the thiol groups have reacted with the gold bottom and top contacts. Peaks are also reproducibly observed at 80, 107, and 186 mV. We note that all alkanethiolate peaks, without exception or omission, occur in the spectra. 

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