Vladimir M. Mirsky

Conducting polymers in chemical sensors and arrays.

The review covers main applications of conducting polymers in chemical sensors and biosensors. The first part is focused on intrinsic and induced receptor properties of conducting polymers, such as pH sensitivity, sensitivity to inorganic ions and organic molecules as well as sensitivity to gases. Induced receptor properties can be also formed by molecularly imprinted polymerization or by immobilization of biological receptors. Immobilization strategies are reviewed in the second part. The third part is focused on applications of conducting polymers as transducers and includes usual optical (fluorescence, SPR, etc.) and electrical (conductometric, amperometric, potentiometric, etc.) transducing techniques as well as organic chemosensitive semiconductor devices. An assembly of stable sensing structures requires strong binding of conducting polymers to solid supports. These aspects are discussed in the next part. Finally, an application of combinatorial synthesis and high-throughput analysis to the development and optimization of sensing materials is described.

Capacitive monitoring of protein immobilization and antigen–antibody reactions on monomolecular alkylthiol films on gold electrodes

Self-assembled monolayers of omega-mercaptohexadecanoic acid and omega-mercaptohexadecylamine on gold electrodes are stable at neutral pH and display pure capacitive behavior at frequencies around 20 Hz. Different methods of covalent immobilization of proteins on these monolayers are compared. Various reagents including succinimides, thionylchloride, p-nitrophenol and carbodiimides were used to activate the carboxy groups of the adsorbed monolayer of omega-mercaptohexadecanoic acid. Glutaraldehyde, cyanuric chloride and phenylene diisocyanate were used to activate the amino groups of the monolayer of omega-mercaptohexadecylamine. The immobilization of albumin on the activated surface was studied by capacitive measurements. The N-hydroxysuccinimide and carbodiimide methods were identified as most suitable for protein immobilization in that they did not compromise the insulating properties of the alkylthiol layer and led to maximal increase of its dielectric thickness. These approaches were used for a layer-by-layer preparation of a capacitive immunosensor. Specifically, antibodies to human serum albumin were immobilized on the alkylthiol mono-layer. Binding of the antigen led to a decrease of the electrode capacitance. The detection limit of the immunosensor is as low as 15 nM (1 mg/l).

Impedometric herbicide chemosensors based on molecularly imprinted polymers

The technique of grafting polymerization has been used for preparation of thin films of molecularly imprinting polymers on the surface of polypropylene membranes and on hydrophobized gold electrodes. The herbicide desmetryn was used as a template. The solid supports used were hydrophobic, while the polymer was hydrophilic. The adsorbed layer of benzophenone, irradiated by UV-light, initiated a radical polymerization near the surface. Polymer films were characterized by weighing, contact angle measurements and impedance spectroscopy. The electrodes coated with the molecularly imprinted polymers displayed fairly specific binding of desmetryn, as detected by the decrease in the capacitance of the electrode. Only small capacitive effects were observed on addition of terbumeton or atrazine, while metribuzine displayed capacitance decrease similar to desmetryn.

New electroanalytical applications of self-assembled monolayers

New applications of self-assembled monolayers of thiol compounds on gold electrodes are reviewed. They include: (i) exploitation of electrical control of self-assembly of thiol compounds for electrically-addressable immobilization of receptor molecules onto sensor arrays; (ii) a spreader-bar technique for formation of stable nanostructures; and, (iii) use of self-assembled monolayers as selective filters for chemical sensors.

A spreader-bar approach to molecular architecture: formation of stable artificial chemoreceptors.

The destructive influence of lateral diffusion on nanostructured monolayers can be prevented by using the spreader-bar technique. This approach allows the formation of stable artificial receptors for barbituric acid by lateral structuring of a dodecanethiol monolayer with molecular spreader-bars from thiobarbituric acid without chemical polymerization (see schematic representation). The new technique may have applications in chemosensors, affinity chromatography, stereoselective catalysis, and molecular electronics.

Capacitive Creatinine Sensor Based on a Photografted Molecularly Imprinted Polymer

The first reversible chemosensor to creatinine based on artificial chemoreceptors is described. The receptor layer was prepared by molecularly imprinted photopolymerization of acrylamidomethylpropanesulfonic acid and methylenediacrylamide. The polymer layer was grafted onto the surface of gold electrodes coated with an alkanethiol monolayer. Creatinine binding was detected by a decrease in the electrode capacitance. The sensor response is reversible and highly selective: no response to addition of sodium chloride, creatine, urea or glucose were observed. The detection limit for creatinine is 10 µM which is optimal for medical applications.

Investigation of contact and bulk resistance of conducting polymers by simultaneous two- and four-point technique

A conventional resistance measured by two-point technique includes bulk and contact components. The contact resistance can be excluded by a four-point technique. A simultaneous use of the two- and four-point techniques allows to separate the total resistance between two electrodes for the contact and bulk parts. This approach has been used to analyze electrical coupling between metal electrodes of interdigitated structures and chemically sensitive coating formed by polyaniline (PANI). The polymer layers were deposited by electropolymerization on bare gold and platinum electrodes as well as on gold electrodes precoated by a self-assembled monolayer of 4-aminothiophenol. An exposure of the PANI films formed on interdigitated structures to gaseous HCl displays proportional changes of resistance measured by the two- and four- point techniques for platinum electrodes and for the precoated gold electrode, while a strong deviation from this proportionality was observed for the gold electrodes without precoating. The results demonstrate a high contribution of the contact resistance between PANI and bare gold electrodes into the resistance measured by the two-point technique. This contribution is small and therefore not measurable for bare platinum electrodes or for gold electrodes coated by 4-aminothiophenol.

Soluble neuropilin‐2, a nerve repellent receptor, is increased in rheumatoid arthritis synovium and aggravates sympathetic fiber repulsion and arthritis

OBJECTIVE In inflammatory lesions, sympathetic nerve fibers disappear soon after the start of inflammation. We identified sympathetic nerve repellents as possible causal agents in rheumatoid arthritis (RA). On nerve terminals, repellent factors bind to neuropilin-2 and its coreceptor. The aim of this study was to investigate the role of neuropilin-2 in the synovial tissue of patients with RA and patients with osteoarthritis (OA) and in experimental arthritis. METHODS The density of neuropilin-2-positive fibers and cells positive for semaphorin 3F (a sympathetic repellent) was investigated using immunofluorescence staining. Enzyme-linked immunosorbent assay was used to detect soluble neuropilin-2 in body fluids from patients with RA and patients with OA. An axon outgrowth assay and a neuropilin-2 Fc fusion construct (neuropilin-2Fc) were used to investigate semaphorin 3F-induced sympathetic nerve repulsion. In an animal model of type II collagen-induced arthritis, soluble neuropilin-2Fc was studied in vivo. RESULTS The synovial density of neuropilin-2-positive sympathetic nerve fibers was lower in RA than in OA, but the density of cells positive for semaphorin 3F was similar. In synovial fluid, the level of soluble neuropilin-2 was markedly higher in RA compared with OA. Mouse sympathetic ganglia served as an excellent model with which to study semaphorin 3F-induced nerve fiber repulsion. Neuropilin-2 and its coreceptor were present on sympathetic neurons, and semaphorin 3F bound to neuropilin-2Fc (binding constant 96 nmoles/liter). Semaphorin 3F dose-dependently increased sympathetic nerve fiber repulsion (at a 50% maximum response concentration of 160-210 nmoles/liter). In contrast to our expectations, soluble neuropilin-2Fc did not inhibit repulsion but increased the repellent effect of semaphorin 3F. In experimental arthritis, therapy with neuropilin-2Fc aggravated arthritis. CONCLUSION Soluble neuropilin-2 has no antirepellent activity but aggravates sympathetic nerve fiber repulsion and arthritis. Increased shedding of neuropilin-2 is probably an unfavorable sign in RA.

Optimization of capacitive affinity sensors: drift suppression and signal amplification

The detection limit of capacitive affinity sensors based on the gold–alkanethiol system can be improved by optimization of sensor preparation and by signal amplification. The dissociation of the gold–sulfur binding is often a critical point leading to operative errors of such sensors. The stability of self-assembled monolayers prepared with different thiols on gold electrodes in aqueous and organic solvents was studied by the capacitive technique. The results show that monolayers made of 16-mercaptohexadecanoic acid are stable in aqueous solution and can be hardly extracted from a gold surface by ethanol, methanol, or dioxane, while a considerable damage of self-assembled monolayers was observed due to incubation in chloroform or dimethylformamide. In contrast, self-assembled monolayers made from short-chain disulfides or thiols (such as 3,3′-dithio-bis(propionic acid N-hydroxysuccinimide ester) or 11-mercaptoundecanoic acid) displayed a poor stability in aqueous phase. Capacitive affinity sensors based on these short-chain thiols showed a considerable drift of the signal. The use of long-chain thiols resulted in a stable signal; it was applied to compare capacitive effects due to immobilization of different biological molecules and for preparation of different biosensors. The response of capacitive biosensors can be amplified by formation of a sandwich structure. This principle was illustrated by subsequent adsorption of polyclonal anti-HSA after binding of HSA with a sensor for HSA based on monoclonal antibodies.

Chemiresistors based on conducting polymers: A review on measurement techniques

This review covers the development of measurement configurations for chemiresistors based on conducting polymers. The simplest chemiresistors are based on application of a two-electrode technique. Artifacts caused by contact resistance can be overcome by application of a four-electrode technique. Simultaneous application of the two- and four-electrode measurement configurations provides an internal control of sensor integrity. An incorporation of two additional electrodes controlling the redox state of chemosensitive polymers and connecting to the measurement electrodes through liquid or (quasi)solid electrolyte results in a six-electrode technique; an electrically driven regeneration of such sensors allows one to perform fast and completely reversible measurements.