Toxin detection using capacitance measurements on immunospecies grafted onto a semiconductor substrate

A New Analytical Device Based on Gated Ion Channels: A Peptide-Channel Biosensor

Binding of a protein at a ligand-modified ion channel can lead to closure or distortion of the channel, resulting in a digital on/off response of the ion flux. This principle could be applied as a strategy for sensor design using artificial membrane ligands or receptors. Important details of modeling and a technique of realization of the new molecular electronic sensor device are presented. The response of the sensor induced by analyte binding depends on the analyte's size and ability to close or distort the ion channel. Testing different ion channels in a typical single molecule-binding event modifies the membrane current by about 0.2 to 20 pA. The gated channel-based sensor is set up by using elements from different fields. It consists of a stable transmembrane channel (for example, a chemically engineered 6.3 helix peptide antibiotic) with a ligand covalently bound at the peptide channel entrance, an electrochemical sensor chip with a photostructurized hydrophobic polymer frame, a hydrophilic ion-conducting membrane support, a lipid membrane incorporating the engineered ion channel, and a sensitive membrane current amplifier or a sensitive fluorescence monitor. Detection of channel opening or closure can either be obtained directly by monitoring membrane conductivity or by monitoring a transient change in pH or ion concentration within the membrane compartment. This transient change can be induced by various means, and its decay is directly correlated to the ion permeability of the membrane. Nonspecific binding at a lipid membrane does not interfere with the ion flux through ion channels. This is a basic advantage of this principle compared to that of enzyme-linked immunosorbent assay (ELISA), radioimmunoassay (RIA), or hybridization of labeled DNA/RNA probes. Furthermore, because nonspecific binding events are mainly transient, specific binding events directly at the ion channel can be separated from them by evaluating the current peak time scale. Theoretically, the sensitivity of such a device can approach one molecule. Under practical conditions, the sensitivity is limited by the diffusion process of the analyte to the receptor ligand but not by the molecular electronic device. The new type of biosensor combines sensor design and technology from immunoassays and biorecognition assays with automated analysis systems and knowledge from the field of nanotechnology. Based on the detection principle, miniaturized portable sensor chips with either electronic or optical detection are under construction. These may have capabilities exceeding those of existing analytical instruments (for example, ELISA tests, medical analyzer, and high-pressure liquid chromatography [HPLC].

Microcontact-BSA imprinted capacitive biosensor for real-time, sensitive and selective detection of BSA

An analytical method is presented, combining novel microcontact imprinting technique and capacitive biosensor technology for the detection of BSA. Glass cover slips were used for preparation of protein stamps. The microcontact-BSA imprinted gold electrodes were prepared in the presence of methacrylic acid (MAA) and poly-ethylene glycol dimethacrylate (PEGDMA) as the cross-linker by bringing the protein stamp and the gold electrode into contact under UV-polymerization. Real-time BSA detection studies were performed in the concentration range of 1.0 × 10-20-1.0 × 10-8 M with a limit of detection (LOD) of 1.0 × 10-19 M. Cross-reactivity towards HSA and IgG were 5 and 3%, respectively. The electrodes were used for >70 assays during 2 months and retained their binding properties during all that time. The NIP (non-imprinted) electrode was used as a reference. The microcontact imprinting technology combined with the biosensor applications is a promising technology for future applications.

The use of electrochemical impedance spectroscopy for biosensing

This review introduces the basic concepts and terms associated with impedance and techniques of measuring impedance. The focus of this review is on the application of this transduction method for sensing purposes. Examples of its use in combination with enzymes, antibodies, DNA and with cells will be described. Important fields of application include immune and nucleic acid analysis. Special attention is devoted to the various electrode design and amplification schemes developed for sensitivity enhancement. Electrolyte insulator semiconductor (EIS) structures will be treated separately.

Impedance spectroscopy and biosensing

This chapter introduces the basic terms of impedance and the technique of impedance measurements. Furthermore, an overview of the application of this transduction method for analytical purposes will be given. Examples for combination with enzymes, antibodies, DNA but also for the analysis of living cells will be described. Special attention is devoted to the different electrode design and amplification schemes developed for sensitivity enhancement. Finally, the last two sections will show examples from the label-free determination of DNA and the sensorial detection of autoantibodies involved in celiac disease.

The Optimization of Magnetosandwich Assays for the Sensitive and Specific Detection of Proteins in Serum

Over the past decade, the use of magnetic particles (MPs) as labels in magnetic biosensors has attracted increasing interest because it provides a highly sensitive platform that can meet the diagnostic needs that are currently not met by existing technologies. However, preparing magnetic biosensors for a specific diagnostic application is a challenging task, and the (bio)chemical aspects are often neglected. Hence, one of the major remaining bottlenecks in the development of magnetic biosensors is the lack of an optimized magnetosandwich assay for the highly sensitive and specific detection of proteins in complex sample matrices. Therefore, in this article, we report on the impact of several different aspects of magnetosandwich assay development, that is, surface chemistry, MP size, rinsing procedure, sample matrix, and blocking procedure on the total-assay performance using quartz crystal microbalance and optical microscopy analysis. The optimization focused on the diagnostically relevant protein S100betabeta, a marker for stroke and minor head injury. It was observed that small MPs in combination with a strong rinsing and a BSA/Tween-20 blocking allows for the most specific and sensitive detection of S100betabeta in serum over a wide concentration range.

Capacitive biosensor for detection of endotoxin

A capacitive biosensor for the detection of bacterial endotoxin has been developed. Endotoxin-neutralizing protein derived from American horseshoe crab was immobilized to a self-assembled thiol layer on a biosensor transducer (Au). Upon injection of a sample containing endotoxin, a decrease in the observed capacitive signal was registered. Endotoxin could be determined under optimum conditions with a detection limit of 1.0 x 10(-13) M and linearity ranging from 1.0 x 10(-13) to 1.0 x 10(-10) M. Good agreement was achieved when applying endotoxin preparations purified from an Escherichia coli cultivation to the capacitive biosensor system, utilizing the conventional method for quantitative endotoxin determination, the Limulus amebocyte lysate test as a reference. The capacitive biosensor method was statistically tested with the Wilcoxon signed rank test, which proved the system is acceptable for the quantitative analysis of bacterial endotoxin (P<0.05).

Label-Free Impedance Biosensors: Opportunities and Challenges

Impedance biosensors are a class of electrical biosensors that show promise for point-of-care and other applications due to low cost, ease of miniaturization, and label-free operation. Unlabeled DNA and protein targets can be detected by monitoring changes in surface impedance when a target molecule binds to an immobilized probe. The affinity capture step leads to challenges shared by all label-free affinity biosensors; these challenges are discussed along with others unique to impedance readout. Various possible mechanisms for impedance change upon target binding are discussed. We critically summarize accomplishments of past label-free impedance biosensors and identify areas for future research.

Silica nanoparticle tags for capacitive affinity sensors

Capacitive affinity sensors have shown great promise for disposable biosensors but have poor sensitivity as a direct sensor if the analyte molecule is small. In this paper, we report an indirect capacitive affinity sensor with silica nanoparticle tags to enhance the sensitivity for sandwich type of immunoassays. In these experiments, the antibody&#8212;antigen&#8212;antibody complex (as in the standard indirect sandwich immunoassay) is simulated by a human&#8212;IgG-antihuman-IgG complex. Silica nanoparticles were synthesized by controlled hydrolysis of TEOS in alcohol and anti-human IgG antibodies were immobilized on them. The capacitive immunosensor comprised of a deposited gold electrode on oxidized silicon with human IgG immobilized on it. After the immunocomplexation process, the nanoparticles manifest by a significant decrease of capacitance compared to when only antihuman-IgG is incubated.

Capacitance immunosensors based on an array biotape

A new array immunosensing system with high-throughput has been developed, based on the principle of a biotape that could be used to make a biocassette recorder.

A sensitive immunoassay based on electropolymerized films by capacitance measurements for direct detection of immunospecies

Fabrication of a capacitive immunosensor based on electropolymerized polytyramine (Pty) film for the direct detection of human serum albumin (HSA) without any labeling is described. The capacitance change of the heterostructures, Pty films/covalently bonded antibodies/buffered medium, is utilized for monitoring the specific antibody-antigen interaction. The Pty films are ultrathin and the HSA assay is nearly specific. Experimental parameters affecting antibody immobilization and the sensing of HSA are investigated in detail and optimized. This capacitive sensor prepared with the present method can provide high sensitivity. Under the optimized experimental conditions, a linear calibration curve in the concentration range 1.84-368.6 ng/ml when plotted vs the logarithm of the antigen concentration is obtained and the detection limit (S/N=3) is 1.60 ng/ml. After an acidic washing the present system can be used again. The applicability and reliability of the sensor are also demonstrated.

Covalently coupling the antibody on an amine-self-assembled gold surface to probe hyaluronan-binding protein with capacitance measurement

Hyaluronan-binding proteins (HABPs), the important structural components of extracellular matrices, served important structural and regulatory functions during development and in maintaining adult tissue homestats. A sensitive, specific and rapid-responsing immunosensor to probe hyaluronan-binding cartilage protein was presented in this work. The novel immunosensor supplied a label-free detection method for HABP, which was based on measuring the capacitance change in-between the unlabeled HABP (antigen) and rabbit-anti-HABP (Ra-HABP, antibody). The HABP immunosensor was prepared by covalently coupling Ra-HABP on an amine-self-assembled gold surface with glutaraldehyde. The capacitance change corresponding to the concentration of HABP, the target antigen, was evaluated by an electrochemical approach called potentiostatic-step in microseconds. The immunosensor showed a specific response to HABP in the range 10-1000 ng/ml. The presented work supplied a promising clinical screening method.

Impedimetric immunosensor using avidin-biotin for antibody immobilization

The potentialities of an electrodeposited biotinylated polypyrrole film as an immobilisation matrix for the fabrication of impedimetric immunosensors are described. Biotinylated antibody (anti-human IgG), used as a model system, was attached to free biotin groups on the electrogenerated polypyrrole film using avidin as a coupling reagent. This immobilization method allows to obtain a highly reproducible and stable device. The resulting immunosensor has a linear dynamic range of 10-80 ng ml(-1) of antigen and a detection limit of 10 pg ml(-1). Furthermore, this immunosensor exhibited minor loss in response after two regeneration steps.

Some new aspects in biosensors

This paper reviews recent advances in biosensors contributed mainly by our laboratory. The biosensors, based on the new immobilization materials - sol-gel organic-inorganic hybrid materials, cryohydrogel (or organohydrogel) and bilayer lipid membranes, are presented. The analytical performances of the biosensors are discussed. Applications of the biosensors in extreme environment are emphasized. A new generation of biosensors - surface plasmon resonance biosensors and capacitance biosensors, are also described.

Capacitive detection of glucose using molecularly imprinted polymers

A novel glucose biosensor based on capacitive detection has been developed using molecularly imprinted polymers. The sensitive layer was prepared by electropolymerization of o-phenylenediamine on a gold electrode in the presence of the template (glucose). Cyclic voltammetry and capacitive measurements monitored the process of electropolymerization. Surface uncovered areas were plugged with 1-dodecanethiol to make the layer dense, and the insulating properties of the layer were studied in the presence of redox couples. The template molecules and the nonbound thiol were removed from the modified electrode surface by washing with distilled water. A capacitance decrease could be obtained after injection of glucose. The electrode constructed similarly but with ascorbic acid or fructose only showed a small response compared with glucose. The stability and reproducibility of the biosensor were also investigated.

New use of cyanosilane coupling agent for direct binding of antibodies to silica supports. Physicochemical characterization of molecularly bioengineered layers

A simple protocol to fix biological species to silica-based surfaces (silica microbeads and glass slides), using a bifunctional silane reagent (3-cyanopropyl dimethyl chlorosilane), is presented. This silane reagent was used without further derivatization. This system led to strong, but not covalent, linkage of antibodies through their glycosylated regions (OH groups) to solid supports. The use of a microsized sample revealed that the coupling process depends not only on physicochemical interactions but also on steric phenomena, and in this case, it was shown that a molecule acting as a spacer was required for more efficient cell fixation. Here, monoclonal mouse antibodies against the CD45 molecule expressed on rat lymphocytes (MAR anti CD45 Ab) were linked to lymphocytes, and as spacers, sheep anti-mouse antibodies (SAM Ab) were immobilized on silica surfaces, allowing the cells to stick to the floating hollow silica microbeads by simple incubation. Under such conditions, a single microbead can fix several cells. The potential of this hollow, low-density support is in ultrasound applications, for the destruction by cavitation phenomena of cells selectively fixed onto such a support. Such a study can serve as a basic model for various microbiosystems involving cell manipulation.

An immunological interleukine-6 capacitive biosensor using perturbation with a potentiostatic step

An instrument for potentiostatic capacitance measurements, based on perturbation with a potentiostatic step was used. The capacitive sensor consisted of self-assembled sulfur compounds on gold to which antibodies towards Interleukine 6, Il-6, had been immobilized. The biosensor was part of a potentiostatic three-electrode system with an extra reference electrode. Two different methods using epoxy- or carbodiimide coupling of the polyclonal antibodies were compared. The antigen Il-6 could be detected from 5 x 10(-16) to 5 x 10(-13) M with one immobilization method and to more than 5 x 10(-9) M with the other. No labels were necessary since the binding of the antigen was detected directly. The insulating properties of the different layers of the biosensor were investigated.

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).

Capacitance measurements of antibody-antigen interactions in a flow system

Capacitive immunosensors were made by coupling monoclonal antibodies to thioctic acid, which had self-assembled on a gold electrode. Surface areas that were not covered were plugged with 1-dodecanethiol to make the layer dense and insulating. Cyclic voltammetry showed that the hexacyanoferrate redox reactions were blocked by this procedure. The capacitance of the electrode was evaluated from the current transients obtained when a potentiostatic step was applied. The immunosensor was placed in a flow system, and a capacitance decrease could be observed after injection of an unlabeled antigen. It was linear over almost three decades when plotted vs the logarithm of the antigen concentration. Human chorionic gonadotropin hormone could be determined in the range 1 pg/mL-1 ng/mL, with a detection limit of 0.5 pg/mL (15 10(-15) M). A similar response was obtained with immobilized F(ab)2 fragments. No cross-reactivity was observed with the thyrotropic hormone, which has one chain in common with gonadotropin. Monoclonal antibodies toward interleukin-2 immobilized on the immunosensor gave also a response over 1 pg/mL-1 ng/mL, with a detection limit of 1 pg/mL. An immunosensor with monoclonal antibodies toward human albumin gave a calibration curve with lower slope than the other proteins but still with a detection limit of 1 pg/mL.

On-line monitoring of monoclonal antibody production with regenerable flow-injection immuno systems

In this paper two systems for the observation of the production of mouse-IgG during the cultivation of hybridoma cells in a perfusion reactor are presented. The direct immunosystem is based on the detection of changes in capacitance of a dielectric layer (tantalum oxide) on a metal surface (tantalum) when antibodies bind to immobilized anti-antibodies. The sensor consisted of a 25 nm tantalum oxide layer, electrochemically grown onto a laser patternized 1 micron thick tantalum layer. The indirect system is based on an automated fluorimetric sandwich ELISA system with beta-galactosidase conjugated secondary antibodies. Two cultivations of mouse hybridoma cells in a 2-1 perfusion reactor were performed. The first cultivation was monitored with the capacitance system, the second cultivation was monitored with the fluorimetric system.

Study of immunoglobulin G thin layers obtained by the Langmuir-Blodgett method: application to immunosensors

Nowadays, immunosensors play a leading part in the field of bioanalytical chemistry research. As with any biosensor, they need appropriate transducers and a suitable technique to immobilize the active biocomponents. In this study, two transduction modes were chosen: mass effects (quartz microbalance measurements) and geometric and dielectric effects (capacitance measurements). The Langmuir-Blodgett (LB) method appears to be quite suitable for generating biospecific surfaces. This work has focused on the detection of staphylococcal enterotoxin B, the corresponding antibody being immobilized at the surface of fatty acids by a variant of the LB method. The composition of the film and the nature of antibody-fatty acid interactions were studied by means of the two transducers mentioned above. FTIR (Fourier transform infra-red) spectroscopy and protein diagnostic assay. Influence of several parameters (pH, ionic strength, transfer pressure, antibody concentration in the subphase) was investigated. The immobilization rate reached its maximum when experimental conditions allowed optimal electrostatic interactions. In this case, the quartz crystal microbalance response, in air, reached 55 Hz per monolayer of immobilized immunoglobulin G and the equivalent capacitance variation, measured in liquid media, was around 300 pF cm-2. Activity of the biospecific LB films, when binding enterotoxin, was checked by the classical ELISA (enzyme immuno-linked assay) technique.