Alkyl thiol SAM

Fig. 5.1 Scheme for patterned deposition of PbS. (a) Self-assembled monolayer (SAM) of long-chain mercapto-carboxylic acid. On exposure to UV radiation through a mask, the exposed thiol group is oxidized to a sulphonate group (b) that is weakly bound to the An substrate and can be easily rinsed away (c). Subsequent formation of a long-chain alkyl thiol SAM occurs only on the exposed Au (d). CD of PbS occurs only on hydrophilic car-boxylate endgroups and not on hydrophobic methyl groups (e). (See Ref. 41). 

Nuzzo et al. [509] observed that with increasing temperature from 80 to 400 K, the intensity of the VasCH2 band of alkyl thiol SAMs on Au(lll) decreases while that of the VsCH2 band is almost the same, which was attributed to the intrinsic temperature dependences of these modes. However, this effect can also be due to reducing the film symmetry. As follows from Eqs. (3.50) and (3.52), the AasMg value obtained from the -polarized spectrum of a uniaxial film is the same as for the isotropically distributed film material (if intrinsic spectral perturbations, such as the splitting due to the crystal field effect [504], do not redistribute the intensities). On the other hand, biaxial symmetry should change this ratio. 

After our reports, there were a large number of photo-induced electron transfer studies using alkyl thiol SAMs [62-79]. 

Considerable research effort was focused on systems of colloidal gold of which a broad variety of synthetic procedures were reported [140 b, fj. While native colloidal gold solutions are only stable for a restricted time, Brust et al. [141] were able to overcome this problem by developing a simple method for the in situ preparation of alkyl thiol-stabihzed gold nanoparticles. This synthetic route yields air-stable and easy to handle passivated nanoparticles of moderate polydispersity, and is now commonly employed for the preparation of inorganic-organic core-shell composites. Such composites are used as catalytic systems with principally two different functions of the protective 3D-SAM layer. Either the metal nanoparticle core can be used as the catalytically active center and the thiol layer is only used to stabihze the system [142], or the 3D-SAM is used as a Hnker system to chemically attach further catalytic functions [143]. 

On the other hand, the SAM-formation with hydroxy-terminated alkyl- thiols and -disulfides showed a complete passivation of the substrate surface for any protein adsorption. 

After an additional deposition of smooth and thin metallic gold film and coating it with an alkyl thiol self-assembled monolayer (SAM), we were able to decrease this hysteresis to values of around 10°. By using thin and rough metallic films like wet chemically oxidized titanium oxide tmd a lluoralkylsilane SAM, the hysteresis values could be reduced to only 4°. 

If NPs were only capped with citrate ions and the detector electrode surface was covered with alkyl thiols of various lengths, the electron transfer decreased exponentially with an increase in the alkyl chain length for both OS and IS reactions. Thus, the decrease of the overall particle activity (as measured by the magnitude of current transients) was mainly limited by the exponential decay of electron transport from Pt NPs through SAMs to the gold electrode surface (Figure 8.37). 

SAMs containing an azobenzene moiety have also been used to modify the electrochemical properties of electrodes by several investigators. Thus, Uosaki and collaborators constructed a SAM by depositing an alkyl thiol molecule containing ferrocene and azobenzene moieties on gold electrode surface (Scheme 65). They found that the charge-transfer rate of the ferrocene group in the SAM could be reversibly controlled by electro- and photochemical isomerization between ,Z-forms of the azobenzene moiety in the SAM. 

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). 

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. 

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