Monolayers covalent attachment

Fig. 29 Film thickness as a function of the layer numbers for MePVP/PSSa multilayers using 7.6 nm (squares) and 22.9 nm (stars) MePVP covalently attached monolayers as the first layer. The solid lines show a linear fit of the dependence of the film thickness on the number of deposited layers...

Research in our laboratory and by Osa and Fujihira showed that it is possible to covalently attach monolayers of chromo-phores to metal-oxide semiconductor surfaces — with no compromise in quantum efficiency to energy conversion compared with dyes adsorbed from solution (9-11). The quantum efficiency for these systems (ratio of photo-generated current to photons adsorbed in the dye layer, ne/np) is quite low, in the range of 10 5 to 10 4 and argues against device applications of these simple modified electrodes without further improvements, such as linear, multielectrode stacks of dye-modified, semi-transparent electrodes (10). 

Figure 1.3 Categorization of materials based on supported ionic liquid films according to the phase behavior of the supported ionic liquid (a) covalently attached monolayer and (b) multilayers of ionic liquid.

Fadeev, A. Y. Kazakevich, Y. V., Covalently Attached Monolayers of Oligo(dimethylsiloxane)s on Silica A Siloxane Chemistry Approach for Surface Modification. Langmuir 2002,18, 2665-2672. 

Formation of the [TMPEDTA]Rull(H20) and [TMPEDTAlRu Pz Molecular Self-Assemblies. The quartz substrate coated with the TMPEDTA monolayer was immersed in a 10 ml degassed solution of Ru (H20)6 2tos (3.6 x 10" M). The temperature of the purple solution was kept at less than 30°C for 6 hours with 20 minutes periodic sonication. The resulting substrates with the covalently attached monolayer of [TMPEDTA]Ru (H20) were quickly rinsed with deionized water and then dipped into a 3.6 x lO" M pyrazine (Pz) solution. The solution again was sonicated 5 times with 1 hour duration. 

Dendrimers have also been found useful in the construction of thin films containing isolated fullerenes [85, 86]. Such materials could eventually find practical application in sensors and/or optoelectronic devices [87]. Generally, monolayers prepared from C60 are ill-defined due to the aggregation tendency of fullerenes. The covalent attachement of C60 to bulky dendritic frameworks... 

For the determination of these compounds a binding inhibition immunoassay, consisting of the competitive immunoreaction of the unbound antibody present in an analyte-antibody mixture with the hapten derivative immobilized at the sensor surface, has been applied. With the aim of assuring the regeneration and reusability of the surface without denaturation of the immobilized molecule, the formation of an alkanethiol monolayer was carried out to provide covalent attachment of the ligand to the functionalized carbodiimide surface in a highly controlled way. For DDT, the assay sensitivity was evaluated in the 0.004 - 3545 pg/l range of pesticide concentration by the determination of the limit of detection 0.3 pg/1 and the I50 value 4.2 pg/1. 

Examples of surface-immobilized mediators are electropolymerized azines for electro-oxidation of The extreme form of this approach is formation of biocatalytic monolayer, comprising a surface-bound mediator species that is itself bound to a single enzyme molecule. Katz et al. report a complete cell based on novel architecture at both electrodes (Figure 7). On the anode side, the FAD center of glucose oxidase is removed from the enzyme shell and covalently attached to a pyrroloquinoline quinone (PQQ) mediator species previously immobilized on a gold surface. The GOx apoenzyme (enzyme with active center removed) is reintroduced in solution and selectively binds to FAD, resulting in a PQQ-... 

Belosludtsev and coworkers (2001) propose that the unmodified probes on the weakly cationic surface, although prevented from diffusing off the surface because of electrostatic interaction, nevertheless are available for hybrid nucleation. They suggest that such probe behavior could be viewed similarly to behavior observed in a liquid crystal matrix. This would be in sharp contrast to models describing the covalent attachment of short oligonucleotides as "oligo lawns" or monolayers of coiled probes (see Figure 3.6). 

Henderson,). ., Feng, S., Ferrence, G.M., Bein, T. and Kubiak, C.P. (1996) Self-assembled monolayers of dithiols, diisoqfanides, and isocyanothiols on gold chemically sticky surfaces for covalent attachment of metal clusters and smdies of interfacial electron transfer. Inorg. 

As mentioned, the photocatalytic activation of Ti02 requires UV irradiation, and hence the semiconductor performance in the solar spectrum is inefficient. A solution to switch the photocatalytic activity to the visible spectral region was described by covalent attachment of an eosin dye monolayer to the semiconductor oxide particles of a Pd-Ti02 catalyst.168 The improved photocatalytic activities, specifically, the efficient formation of formate, are attributed to the effective injection of electrons from the excited dye into the semiconductor conduction band. 

Various other surface chemistries have been used to attach chemical species to electrode surfaces. For example, Watkins et al. activated the carboxylic acid functionalities on carbon electrodes with thionyl chloride and then reacted this surface with amines [17]. Sagiv and his coworkers have recently invented a clever approach for monolayer-by-monolayer deposition of multilayer films based on organosilane chemistry [18]. Finally, Mallouk et al. have also developed a monolayer-by-monolayer approach for synthesizing well-ordered multilayer films [19]. Because of these interesting new synthetic strategies, covalent attachment of functional groups remains an attractive approach for modifying electrode surfaces. 

In this part we will describe recent achievements in the development of biosensors based on DNA/RNA aptamers. These biosensors are usually prepared by immobilization of aptamer onto a solid support by various methods using chemisorption (aptamer is modified by thiol group) or by avidin-biotin technology (aptamer is modified by biotin) or by covalent attachment of amino group-labeled aptamer to a surface of self-assembly monolayer of 11-mercaptoundecanoic acid (11-MUA). Apart from the method of aptamer immobilization, the biosensors differ in the signal generation. To date, most extensively studied were the biosensors based on optical methods (fluorescence, SPR) and acoustic sensors based mostly on thickness shear mode (TSM) method. However, recently several investigators reported electrochemical sensors based on enzyme-labeled aptamers, electrochemical indicators and impedance spectroscopy methods of detection. 

Fig. 2 Schematic cartoon (a), and chemical structures (b) of the general fabrication procedure of a sensitive fluorescent monolayer on glass i) silanation of the glass slide with N-[3-(trimethoxysilyl)propyl]ethylenediamine to form the amino-terminated monolayer, ii) reaction with an amino-reactive fluorophore, iii) covalent attachment of a binding molecule...

Specifically, two different methods for the fabrication of these sensitive monolayers have been used. These methods differ in the technique used for the covalent attachment of the fluorescent probes (Fig. 2, step ii) while the first and the third fabrication steps (silanation (Fig. 2, step i) and binding molecule attachment (Fig. 2, step iii)) are identical for both methods. The first method is a solution-based procedure and the second method is a microcontact printing (/xCP) based procedure. In the solution-based procedure (Sect. 2.2.1), all... 

Fig. 10 Generation of the patterned, sensitive monolayers using microcontact printing. a An amino-terminated monolayer on a glass surface is brought into contact with a poly(dimethylsiloxane) stamp inked with a fluorophore, resulting in the covalent attachment of the fluorophore to the amino-terminated monolayer (b). c Functionalization of the remaining free amino groups with reactive molecules...

While electrochemical experiments provide useful information regarding electron transport through these molecular monolayers, construction of real devices requires formation of a top contact so that solid-state transport measurements can be made. The fabrication of contacts to molecular layers has been the major obstacle to the development of molecular electronic devices, whether based on thiol-based SAMs on gold or covalently attached molecules on silicon. The most popular approach to making contacts involves evaporation of metals onto the molecular layer, which is likely to result in at least partial penetration of the monolayer, and may possibly damage the molecules in the layer. 

---