Self-assembled monolayers thiol-based

F. Frederix, K. Bonroy, W. Laureyn, G. Reekmans, A. Campitelli, W. Dehaen, and G. Maes, Enhanced performance of an affinity biosensor interface based on mixed self-assembled monolayers of thiols on gold. Langmuir 19, 4351-4357 (2003). 

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. 

The fabrication of STM-based molecular junctions have relied - in most reported cases -on the deposition of thiol end-capped molecules as self-assembled monolayers on a flat gold substrate. By using an exceedingly small tip, the measurement of no more than a single or a few molecules becomes possible. A schematic illustration of an STM-based molecular junction is shown in Fig. 10.2. 

Kubiak and coworkers used STM in an ultra-high vacuum environment to study the conductance through self-assembled monolayers of simple Jt-conju-gated, thiol-terminated molecules on gold [21-23, 59] Early work, which focused on the p-xylyldithiol 82 monolayers [22, 23], was later expanded to include also short oligophenylenes (OPs) (81 and 83) that were terminated at either one or both ends by thiol or isocyanide groups [21]. Based on this work, a relatively simple model was presented to account for the observed conductance spectra of the junctions [22],... 

One of the first reported self-assembled monolayers of porphyrins is cobalt(II)-porphyrins 5 and 6 and the analog free-base porphyrins 7 and 8, in which the number and location of the thiol-containing tails are varied systematically [300, 314]. 

Pt(IV) than Pd(n) [49], An interesting system, with asymmetric inorganic membranes, was used for selective metal ion separation. The membrane phase was a self-assembled monolayer of alkyl thiols as a hydrophobic phase for a trialkyl phosphate and phosphine oxide-based metal ion carrier. This organic mixture was attached on alumina porous supports with thin layers of gold. The thin membrane layer gave high fluxes and high selectivity, while metal ions transport was carrier limited [50]. 

On the other hand, it is possible to study electron transfer to an electroactive species held at a fixed distance (10-30 A) from the electrode surface by a suitable spacer, such as an adsorbed monolayer (Section 14.5.2) (75, 76). One approach is based on the use of a blocking monolayer, such as a self-assembled monolayer of an alkane thiol or an insulating oxide film, to define the distance of closest approach of a dissolved reactant to the electrode. This strategy requires knowledge of the precise thickness of the blocking layer and assurance that the layer is free of pinholes and defects, through which solution species might penetrate (Section 14.5). Alternatively, the adsorbed monolayer may itself... 

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