Mixed monolayer, thiolated

Fig. 22 Nanoparticles decorated with pro-mesogenic dendronized or bent-core liquid crystal ligands nematic Fe304 mixed monolayer nanoparticles capped with dendronized cyanobiphenyl ligands and oleic acid (17) [132], and mixed monolayer, non-mesogenic gold nanoparticles decorated with bent-core liquid crystal and hexane thiolates (18) [547]...

Scheme 10 Reversible photoswitchable activation/deactivation of the electrical contact between cytochrome c and the electrode and the secondary activation/deactivation of the COx-biocata-lyzed reduction of oxygen using a thiolated nitrospiropyran and thiolated pyridine mixed monolayer as a command interface.

A further approach to controlling electrical communication between redox proteins and their electrode support through a photo-command interface includes photo stimulated electrostatic control over the electrical contact between the redox enzyme and the electrode in the presence of a diffusional electron mediator (Scheme 12).[58] A mixed monolayer, consisting of the photoisomerizable thiolated nitrospiropyran units 30 and the semi-synthetic FAD cofactor 25, was assembled on an Au electrode. Apo-glucose oxidase was reconstituted onto the surface FAD sites to yield an aligned enzyme-layered electrode. The surface-reconstituted enzyme (2 x 10-12 mole cm-2) by itself lacked electrical communication with the electrode. In the presence of the positively charged, protonated diffusional electron mediator l-[l-(dimethylamino)ethyl]ferrocene 29, however, the bioelectrocatalytic functions of the enzyme-layered electrode could be activated and controlled by the photoisomerizable component co-immobilized in the monolayer assembly (Figure 12). In the... 

Nanoparticles are unstable under strongly acidic conditions. Indeed, NPs protected by a mixed monolayer composed of a 1 1 mixture of dodecanethiolate and triethylene glycol thiolates aggregate under strongly acidic conditions. Interestingly, these authors reported that the acid induces the formation of clusters of these MPCs and that the extent and the rate of cluster formation is the result of a subtle balance between hydrophobic and hydrogen bonding interactions between particles and between the particles and the solvent.157... 

A different situation has been observed using mixtures of thiolates with immiscible chains. AuNPs protected by mixed monolayers of different composition made of HS-C8-TEGME and HS-F8-PEG have been investigated by ESR spectroscopy using a radical probe sensitive to the hydrophobicity of the environment.223 The ESR spectral parameters of the probe in the monolayer are identical to those of the probe in a completely fluorinated medium when the ratio (Rcf) between alkyl and perfluoro-alkyl is lower than 2.5. Only at RCp > 2.5, the probe starts to experience the environment of alkyl chains. The experimental results support the phase segregation of perfluoroalkyl thiolates in patches on nanoparticles with a core size of 2.5 to 4.0 nm.223... 

Recent work has been published on the use of SERS reporters for in vivo analysis. In 2008, Shuming Nie published work on the in vivo targeting of tumors in live mice [65]. The SERRS particles consisted of 60 nm gold nanoparticles functionalized with a Raman reporter dye molecule and then stabilized with thiolated polyethylene glycols (PEGs). Targeted SERS nanoparticles were prepared by having a mixed monolayer of thiolated PEG and a heterofunctional thiolated PEG with a carboxylic... 

Other metallic clusters that have been demonstrated to show the QDL effect are palladium [116, 117], silver [118] and copper [119]. Palladium MFCs capped with mixed monolayers of hexanethiolate/dodecanethiolate and ferrocene thiolate ligands are prepared in a manner similar to that employed for gold MFCs. The DPV studies exhibit a quantized charging effect but the current peaks are not as well defined as those observed for Au-MPCs. Capacitance values of the order of 0.35 aF are obtained, indicating smaller core sizes or thicker monolayer dielectrics [116]. 

Exposure of a gold slide to a mixture of two different thiols results in the formation of a mixed monolayer, in which both thiolate moieties are present148. The composition of such a monolayer (usually determined with XPS) is generally different from the composition of solution149. Coadsorption of different thiols provides an easy way to prepare mixed layers with variable controlled concentration of the components, and this method is routinely used to tailor the properties of monolayers. Mixed monolayers with the gradient of two thiols can be prepared using Sephadex gel to prevent diffusion150,151. 

Figure 4.16. Schematic outline of the label-free impedimetric biosensor for thrombin at an aptamer-functionalized Au electrode based on a three-level cascaded signal amplification (a) Formation of a mixed monolayer of thiolated aptamer and 6-mercaptohexanol on the AuE (b) thrombin addition and binding with aptamer (c) binding with Apt-Au-NPs to carry out the first-level signal amplification ( ] (d) the enlargement of the SDS-stabilized Apt-Au-NPs to achieve the second/third-level signal amplification CH/HO and (e) schematic outline of the electron-transfer resistance of different modified electrodes [82] (reprinted with permission of acs],...

The thiolated j -CD forms monolayers with substantial defect densities due to the lack of favorable lateral interactions between the CD molecules. In order to patch the holes in these monolayer assemblies without clogging the CD cavities, we devised procedures for the preparation of mixed monolayers containing both the thiolated P-CD receptors and pentanethiol molecules. These monolayers were characterized by reductive desorption and electrode capacitance measurements. The monolayer blocking properties were also investigated, i.e., the voltammetric response of solution redox probes such as Fe(CN) and Ru(NH3)6 at the monolayer-covered electrodes were recorded. Our data supported the monolayer structure depicted in the Scheme 3 [26]. 

The adsorption kinetics are found to be completely different for the two molecules, AHS and DHS. The rate of dissolution of molecules into solution from fully formed monolayers at room temperature is negligible, so equilibrium carmot be established by desorption and readsorption of monolayer components in the complete mono-layer. Equilibration could proceed through the physisorbed thiol. Rapid equilibration between the physisorbed molecule and the molecules in solution would be followed by relatively slow conversion of the physisorbed thiols to chemisorbed thiolates. If the rate constant for conversion of thiol to surface thiolate is independent of the structure of the thiol, which is likely, a chemisorbed layer would be kinetically trapped. The adsorption rate constant would then be determined by the equilibration between the physisorbed thiol and the thiols in solution, which might explain the observed large differences between AHS and DHS. A similar argument has been used by Bain et al. to explain the observed composition of mixed monolayers. This would explain the major role of the solvent in the adsorption kinetics and possibly also in the resulting film structure. A further consideration is the expected hy-... 

Figure 3. Advancing (filled points) and receding (open points) contact angles of water on mixed monolayers of HS(CH2)nCH3 and HS(CH2)iiOH on gold (diamonds), silver (squares), and copper (circles). The x-axis is the mole fraction of hydroxyl-containing thiolates in the SAM as determined by X-ray photoelectron spectroscopy. The data are plotted as the cosine of die contact angles since these values are related to the interfacial free energy.

Adsorption of asymmetrical disulphides R —SR2 leads to formation of mixed mono-layers, too. As the S—S bond is broken during deposition, both thiolate moieties in the monolayers behave independently109. However, both groups R1 and R2 are present in the same concentration in these monolayers even if R1 is a long-chain alkane and R2 is a bulky t-butyl group157. 

Polycrystalline silicon thin film transistors have also been employed for the detection of DNA hybridization [16]. A mixed self-assembled monolayer of thiolated DNA probes and mercapto-hexanol was immobilized onto the gold gate of an extended gate poly-Si TFT. A shift of the I-V characteristics on the order of 300 mV was obtained upon hybridization of the immobilized probe with a fully complementary strand. The shift is independent of electrode area, so microarrays can be constructed where a known DNA probe is immobilized on each FET. The inherent miniaturization and compatibility with microfabrication technologies make the technique highly promising for the development of low-cost portable devices. 

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