Tethered redox probe

To probe the microscopie order of SAMs (here we will treat only nonelectroactive SAMs those containing tethered redox probes will be discussed later) under the conditions used during electrochemieal electron-transfer measurements, functional characterization is performed by cyclic voltammetric techniques. In this subsection, electrochemical blocking effect EBE) and double-layer capacitance (Cai) experiments are detailed. When these experiments are carried out using a struetural probe of molecular scale [30, 44, 45], they provide the greatest sensitivity to defects because the effects of defects and disorder dominate the electrochemical response. 

Figure 5. Self-assembled monolayers as rigid electrochemical spacers. Close packing of the SAM enforces a precise distance between the redox probes and the electrode. (A) In a tethered redox probe system, the probes are covalently attached to the outer surface of the monolayer. The probes may be undiluted (a probe attached to each chain in the monolayer) or mixed (probe-modified chains are diluted with nonelectroactive chains). (B) In a soluble redox probe system, the monolayer typically contains a single component that provides a well-defined separation between the soluble probe and the electrode surface.

Electrochemical characterization of SAMs containing tethered redox probes... 

The density of tethered redox probe at the surface is another useful parameter in electroactive assembly characterization. The amount of redox probe corresponds to the Faradaic charge of the redox process and is measured from the voltammogram using the area ij E under the voltammetric wave. The area is readily converted to the redox probe density (F, in mol cm -) [47]. For well-ordered -alkanethiol SAMs, the total (electroactive plus diluent) surface density is one molecule of adsorbate per 20 A, or about 8 x 10 mol cm [38]. To minimize disorder and electrochemical communication between tethered redox probes, the amount of redox probe should not exceed 10 20 % of the total adsorbate [23]. 

It is often possible to optimize the response of a poorly assembled monolayer. In freshly adsorbed mixed SAMs, electroactive adsorbate is always present at defect sites [45, 74]. A distinct advantage of tethered redox probes over freely diffusing analyte is that a nearly perfect SAM is not required [23, 50, 73, 81, 90, 91]. 

In another study, which extended the understanding of the behavior of both dissolved and tethered redox probes, the electrochemistry of a structurally related series of outer sphere redox complexes was compared at Au-S(CH2)nOH mono-layers (System 5). As bipyridyls replace cyano ligands in Fe(bpy) (CN)6 2 - "... 

The first experiments characterizing DNA-mediated CT over a precisely defined distance between covalently appended redox probes were reported in 1993 [95]. Remarkably, the luminescence of a photoexcited Ru(II) intercala-tor was quenched by a Rh(III) intercalator fixed to the other end of a 15-mer DNA duplex over 40 A away (Fig. 4). Furthermore, non-intercalating, tethered Ru(II) and Rh(III) complexes did not undergo this quenching reaction. In this way the importance of intercalative stacking for efficient CT was demonstrated. 

In electrochemical experiments, the molecular assembly is deposited on a conductive electrode surface that acts as one of the redox partners . The other partner is a redox molecule either tethered to the molecular film or free in solution. In either case, the film ideally provides a well-defined separation between the electrode and the redox probe. The electrode potential is either swept or stepped to increase the driving force for electron transfer. The chief advantage of the electrochemical elec-... 

Mediation of electron transfer between a dissolved redox probe and the electrode by surface-confined redox groups has also been used to assess the integrity of electroactive monolayer assemblies. As an example, the reduction of Ru (NH3)e, normally occurring at about 0.1 V vs. Ag/AgCl, is fully blocked by an assembly containing tethered A-ethylviologen redox probes. However, at the observed reduction potential of the viologen probes (ca. —0.4 V), these probes mediate the reduction of Ru (NH3)g [87]. Similarly, the delayed oxidation of Fe (CN)6 at... 

Figure 9.4. Schematic representation of a impedimetric genosensor [sandwich hybridization assay]. Unmodified PGR products [b] were captured at the sensor interface [a] via sandwich hybridization with the surface-tethered probe and a biotinylated signaling probe. The biotinylated hybrid [c] was then coupled with a streptavidin-alkaline phosphatase conjugate [d] and finally exposed to the substrate solution [e]. The biocatalyzed precipitation of an insulating product [f] blocked the electrical communication between the gold surface and the [Fe(CN]6] redox probe [published by Elsevier in Ref. 50].

The intercalation of polycyclic aromatic compounds into duplex DNA structures was used to develop nucleic acid-based electrochemical sensors.66 For example, the bis-ferrocene-tethered naphthalene diimide (16) was used as a redox-active intercalator to probe DNA hybridization.67 The thiolated probe was assembled on a Au electrode, and the formation of the duplex DNA with the complementary analyte nucleic acid was probed by the intercalation of (16) into the double-stranded nucleic acid structure and by following the voltammetric response of the ferrocene units (Fig. 12.17a). The method enabled the analysis of the target DNA with a sensitivity that corresponded to ca. 1 x 10-20mol. 

Kavanagh P, Leech D (2006) Redox polymer and probe DNA tethered to gold electrodes for enzyme-amplified amperometric detection of DNA hybridization. Anal Chem 78 2710-2716... 

Aptamer based biosensors, for example for recombinant human erythropoietin (as model analyte), can be made more sensitive by amplification with a boronic acid tethered gold nanoparticle that is then associated with an alkaline phosphatase to produce a redox active probe molecule. A similar re-usable bio-immuno-sensor has been suggested for carcinoembryonic antigen. A phenylboronic acid is assembled on gold to (reversibly) bind the antibody horseradish peroxidase conjugate. Interaction with the antigen slows down the hydrogen peroxide reduction. An HIV-1 immunoassay based on electroluminescence has been proposed by Zhou etaV In this process the... 

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