The Use of Impedance Spectroscopy for the Characterization of Protein-Modified ISFET Devices: Application of the Method for the Analysis of Biorecognition Processes

A readout circuit realizing electrochemical impedance spectroscopy for FET-based biosensors

Electrochemical impedance spectroscopy (EIS) is a useful approach for modeling the equivalent circuit of biosensors such as field-effect transistor (FET)-based biosensors. During the process of sensor development, laboratory potentiostats are mainly used to realize the EIS. However, those devices are normally not applicable for real use-cases outside the laboratory, so miniaturized and optimized instrumentations are needed. Various integrated circuits (IC) are available that provide EIS, but these make developed systems highly dependent on semiconductor manufacturers, including component availability. In addition, these generally do not meet the instrumentation requirements for FET-based biosensors, thus external circuitry is necessary as well. In this work, an instrumentation is presented that performs EIS between 10 Hz and 100 kHz for FET-based biosensors. The instrumentation includes the generation of the excitation signal, the configuration of the semiconductor and the readout circuit. The readout circuit consists of a transimpedance amplifier with automatic gain adjustment, filter stages, a magnitude and a phase detection circuit. Since magnitude and phase are converted to a DC signal, digitization of the results is trivial without further signal processing steps, minimizing the computational load on the microcontroller. The transmission behavior of the magnitude and phase measurement circuits shows a high linearity for sinusoidal signals. Furthermore, the overall system was tested with resistors, whereby the magnitude measurement error (1.7%) and the phase shift error (1.6°) were determined within the working range of the instrumentation. The functionality of the instrumentation is demonstrated using pH-sensitive field-effect transistors (ISFET) in various solutions.Clinical relevance- Based on the electrochemical impedance spectroscopy of FET-based biosensors such as ImmunoFETs, new point-of-care testing (POCT) devices can be developed that e.g. quantitatively detect the concentration of biomarkers with very low detection limits in body fluids. The instrumentation presented in this work can be part of new generation of diagnostic tools featuring innovative sensor technologies.

New enzymatic assays based on the combination of signal accumulation type of ion sensitive field effect transistor (SA-ISFET) with horseradish peroxidase

Peroxidase is widely used for the detection of secondary reactions during measurements of various enzymatic reactions, such as that of oxidase activity, or as an enzyme for immunoassay. Conventional methods utilizing the enzyme require expensive equipment such as a spectrophotometer to measure the absorption of light by the reaction product. Here, we describe a simple and cost-effective method for measuring enzymatic reactions using a signal accumulation type of ion sensitive field effect transistor (SA-ISFET) sensor capable of detecting the proton changes due to the enzymatic reaction. Using this detection principle, we constructed a detection system combining ABTS, an electron mediator, and a horseradish peroxidase activity detection system. As a result, we could quantitatively measure hydrogen peroxide with excellent reproducibility and linearity. As an application of this tool, we describe an oxidase-peroxidase reaction system for the measurement of glucose, sarcosine, uric acid and lactic acid. In addition, we describe an immunoassay system using a peroxidase-labeled antibody for detection of Escherichia coli. We also describe a prototype for a flow-type ISFET device for continuous and routine measurements.

Silicon Nanowires Field Effect Transistors: A Comparative Sensing Performance between Electrical Impedance and Potentiometric Measurement Paradigms

Potentiometric sensors based on silicon nanowire field effect transistors (SiNW FETs) typically display exquisite sensitivities, but their bioanalytical implementation is limited due to the need for stringent measurement conditions and high-precision readout units. An alternative operation principle where SiNW FETs are operated in a frequency-domain electrical impedimetric approach is promising. However, to date only limited data is available in regard to the sensing performance and translational relevance of this novel approach in comparison to the standard charge detection paradigm. We demonstrate the feasibility of conducting electrical impedimetric FET measurements with a portable unit for the ultrasensitive detection of cancer biomarkers in biospecimens. Compared to standard potentiometric measurements, electrical impedimetric FET measurements yielded significant improvements in biosensing performances, including the limit of detection, sensing resolution, and dynamic range.

CMOS Electrochemical Instrumentation for Biosensor Microsystems: A Review

Modern biosensors play a critical role in healthcare and have a quickly growing commercial market. Compared to traditional optical-based sensing, electrochemical biosensors are attractive due to superior performance in response time, cost, complexity and potential for miniaturization. To address the shortcomings of traditional benchtop electrochemical instruments, in recent years, many complementary metal oxide semiconductor (CMOS) instrumentation circuits have been reported for electrochemical biosensors. This paper provides a review and analysis of CMOS electrochemical instrumentation circuits. First, important concepts in electrochemical sensing are presented from an instrumentation point of view. Then, electrochemical instrumentation circuits are organized into functional classes, and reported CMOS circuits are reviewed and analyzed to illuminate design options and performance tradeoffs. Finally, recent trends and challenges toward on-CMOS sensor integration that could enable highly miniaturized electrochemical biosensor microsystems are discussed. The information in the paper can guide next generation electrochemical sensor design.

Electrical cell-substrate impedance sensing with field-effect transistors is able to unravel cellular adhesion and detachment processes on a single cell level

We introduce a novel technique of impedimetric sensing of cellular adhesion, which might have the potential to supplement the well-known technique of Electrical Cell-substrate Impedance Sensing (ECIS) in cell culture assays. In contrast to the already commercialized ECIS method, we are using ion-sensitive field-effect transistor (ISFET) devices. The standard gold microelectrode size in ECIS is in the range of 100-250 μm in diameter. Reason for this limitation is that when downscaling the sensing electrodes, their effective impedance governed by the metal-liquid interface impedance is becoming very large and hence the currents to be measured are becoming very small reaching the limit of standard instrumentation. This is the main reason why typical assays with ECIS are focusing on applications like cell-cell junctions in confluent cultures. Single cell resolution is barely reachable with these systems. Here we use impedance spectroscopy with ISFET devices having gate dimensions of only 16 × 2 μm(2), which is enabling a real single cell resolution. We introduce an electrically equivalent circuit model, explain the measured effects upon single cell detachment, and present different cellular detachment scenarios. Our approach might supplement the field of ECIS with an alternative tool opening up a route for novel cell-substrate impedance sensing assays with so far unreachable lateral resolution.

Neurodegeneration through oxidative stress: monitoring hydrogen peroxide induced apoptosis in primary cells from the subventricular zone of BALB/c mice using field-effect transistors

Dementia is one of the big medical challenges of our time with Alzheimer's, Huntington's and Parkinson's disease among its most common forms. In year 2000, 4.5 million people were diagnosed with Alzheimer's disease in the United States. In the case of Alzheimer's disease one of many contributing factors is a metabolic imbalance that leads to elevated oxidative stress levels. Consequences of this imbalance can be symptoms like apraxia, agnosia or sundowning. The use of field-effect transistors is a novel approach to study the effects of external stimuli on cells in vitro to provide researchers with a new tool for high resolution and high throughput studies to better understand cellular interaction and the effects of pharmacological compounds. In our study we use ion-sensitive field-effect transistors (FETs) to analyze the apoptosis inducing effects of hydrogen peroxide treatment on primary cells obtained from the subventricular zone of postnatal BALB/c mice. Upon apoptosis, the cell-substrate adhesion of the neurons is gradually weakened until complete detachment. In former studies we used our FET devices to conduct Electrical Cell-substrate Impedance Sensing (ECIS) experiments on the single cell level using morphologically different cell lines. Here we demonstrate that our novel approach of ECIS using FET devices can be expanded to primary neuronal tissue with high prospects for further studies in the field of pharmacological research.

Human T cells monitored by impedance spectrometry using field-effect transistor arrays: a novel tool for single-cell adhesion and migration studies

Cytotoxic T lymphocytes (CTLs) play an important role in the immune system by recognizing and eliminating pathogen-infected and tumorigenic cells. In order to achieve their function, T cells have to migrate throughout the whole body and identify the respective targets. In conventional immunology studies, interactions between CTLs and targets are usually investigated using tedious and time-consuming immunofluorescence imaging. However, there is currently no straightforward measurement tool available to examine the interaction strengths. In the present study, adhesion strengths and migration of single human CD8(+) T cells on pre-coated field-effect transistor (FET) devices (i.e. fibronectin, anti-CD3 antibody, and anti-LFA-1 antibody) were measured using impedance spectroscopy. Adhesion strengths to different protein and antibody coatings were compared. By fitting the data to an electronically equivalent circuit model, cell-related parameters (cell membrane capacitance referring to cell morphology and seal resistance referring to adhesion strength) were obtained. This electronically-assessed adhesion strength provides a novel, fast, and important index describing the interaction efficiency. Furthermore, the size of our detection transistor gates as well as their sensitivity reaches down to single cell resolution. Real-time motions of individually migrating T cells can be traced using our FET devices. The in-house fabricated FETs used in the present study are providing a novel and very efficient insight to individual cell interactions.

Programmable ion-sensitive transistor interfaces. II. Biomolecular sensing and manipulation

The chemoreceptive neuron metal-oxide-semiconductor transistor described in the preceding paper is further used to monitor the adsorption and interaction of DNA molecules and subsequently manipulate the adsorbed biomolecules with injected static charge. Adsorption of DNA molecules onto poly-L-lysine-coated sensing gates (SGs) modulates the floating gate (FG) potential ψ(O), which is reflected as a threshold voltage shift measured from the control gate (CG) V(th_CG). The asymmetric capacitive coupling between the CG and SG to the FG results in V(th_CG) amplification. The electric field in the SG oxide E(SG_ox) is fundamentally different when we drive the current readout with V(CG) and V(ref) (i.e., the potential applied to the CG and reference electrode, respectively). The V(CG)-driven readout induces a larger E(SG_ox), leading to a larger V(th_CG) shift when DNA is present. Simulation studies indicate that the counterion screening within the DNA membrane is responsible for this effect. The DNA manipulation mechanism is enabled by tunneling electrons (program) or holes (erase) onto FGs to produce repulsive or attractive forces. Programming leads to repulsion and eventual desorption of DNA, while erasing reestablishes adsorption. We further show that injected holes or electrons prior to DNA addition either aids or disrupts the immobilization process, which can be used for addressable sensor interfaces. To further substantiate DNA manipulation, we used impedance spectroscopy with a split ac-dc technique to reveal the net interface impedance before and after charge injection.

Monitoring nanoparticle induced cell death in H441 cells using field-effect transistors

In this work we propose the use of field-effect transistors (FETs) to examine the reaction of individual tumor cells to treatment with cell death inducing nanoparticles for future use in cancer therapy.For our analysis the human cancer cell line H441 (a human lung adenocarcinoma epithelial cell line) was cultivated on fibronectin coated FETs and treated with various concentrations of silicon nanoparticles. The cell line was cultivated under standard conditions. The reactions of the cells to the nanoparticles were analyzed via transfer function measurements, microscopic examination and standard MTT viability assays. Microscopic examination showed a clear change of morphology to round cells, which accompanies detachment from the surface of the substrate. Cell detachment could also be observed as a signal shift in the transfer function.The results of our study indicate the applicability of FETs for cancer research and analyzing pharmacological effects of new compounds. In addition our results implicate the usefulness of silicon nanoparticle based compounds in cancer therapy.

Photosensor based on an FET utilizing a biocomponent of photosystem I for use in imaging devices

We have investigated a photosensor that consists of a field emission transistor (FET) utilizing the biocomponent of the photosystem I (PSI) protein complex for use in an imaging device. The PSI was immobilized on a gold electrode via the self-assembling monolayer (SAM) of 3-mercapto-1-propanesulfonic acid sodium salt to obtain a PSI-modified gold electrode. As for the PSI-modified gold electrode, the basic photoresponses originating from the excitation of PSI, including the photocurrent (106 nA) and the photoresponse of the open-circuit voltage (photo-Voc: 28.6 mV), were characterized. Then, the PSI-modified gold electrode was linked to the gate of the FET using a lead line, and the device was successfully driven by the photoelectric signals from the PSI like a voltage follower circuit. Further, we successfully demonstrated that the PSI-based FET acts as a photosensor in imaging devices.

Time-dependent observation of individual cellular binding events to field-effect transistors

Electrolyte-gate field-effect transistors (EG-FETs) gained continuously more importance in the field of bioelectronics. The reasons for this are the intrinsic properties of these FETs. Binding of analysts or changes in the electrolyte composition are leading to variations of the drain-source current. Furthermore, due to the signal amplification upon voltage-to-current conversion even small extracellular signals can be detected. Here we report about impedance spectroscopy with an FET array to characterize passive components of a cell attached to the transistor gate. We developed a 16-channel readout system, which provides a simultaneous, lock-in based readout. A test signal of known amplitude and phase was applied via the reference electrode. We monitored the electronic transfer function of the FETs with the attached cell. The resulting frequency spectrum was used to investigate the surface adhesion of individual HEK293 cells. We applied different chemical treatments with either the serinpeptidase trypsin or the ionophor amphotericin B (AmpB). Binding studies can be realized by a time-dependent readout of the lock-in amplifier at a constant frequency. We observed cell detachment upon trypsin activity as well as membrane decomposition induced by AmpB. The results were interpreted in terms of an equivalent electrical circuit model of the complete system. The presented method could in future be applied to monitor more relevant biomedical manipulations of individual cells. Due to the utilization of the silicon technology, our method could be easily up-scaled to many output channels for high throughput pharmacological screening.

Label-free and reagentless aptamer-based sensors for small molecules

A label free, reagentless aptasensor for adenosine is developed on an ISFET device. The separation of an aptamer/nucleic acid duplex by adenosine leads to the aptamer/adenosine complex that alters the gate potential of the ISFET. The sensitivity limit of the device is 5 x 10-5 M. Also, the immobilization of the aptamer/nucleic acid duplex on an Au-electrode and the separation of the duplex by adenosine mono-phosphate (AMP) enable the electrochemical detection of adenosine by faradaic impedance spectroscopy. The separation of the aptamer/nucleic acid duplex by adenosine and the formation of the aptamer/adenosine complex results in a decrease in the interfacial electron-transfer resistance in the presence of [Fe(CN)6]3-/4- as redox active substrate.

Label-free detection of single nucleotide polymorphisms utilizing the differential transfer function of field-effect transistors

We present a label-free method for the detection of DNA hybridization, which is monitored by non-metallized silicon field-effect transistors (FET) in a microarray approach. The described method enables a fast and fully electronic readout of ex situ binding assays. The label-free detection utilizing the field-effect is based on the intrinsic charge of the DNA molecules and/or on changes of the solid-liquid interface impedance, when biomolecules bind to the sensor surface. With our sensor system, usually a time-resolved, dc readout is used. In general, this FET signal suffers from sensor drift, temperature drift, changes in electrolyte composition or pH value, influence of the reference electrode, etc. In this article, we present a differential ac readout concept for FET microarrays, which enables a stable operation of the sensor against many of these side-parameters, reliable readout and a possibility for a quick screening of large sensor arrays. We present the detection of point mutations in short DNA samples with this method in an ex situ binding assay.

Signal enhancement in electronic detection of DNA hybridization

Electronic detection of the specific recognition between complementary DNA sequences is investigated. DNA probes are immobilized at different lateral positions on a Poly( L -lysine)-coated surface of an integrated silicon transistor array. Hybridization and field effect detection are done with the solid surface immersed in electrolyte solutions. Differential measurements are performed, where DNA hybridization leads to surface potential shifts between the transistors of the array. We experimentally show that these differential signals of hybridization can be enhanced significantly by changing the salt concentration between hybridization and detection.

Direct electrical detection of antigen–antibody binding on diamond and silicon substrates using electrical impedance spectroscopy

The integration of biological molecules with semiconducting materials such as silicon and diamond has great potential for the development of new types of bioelectronic devices, such as biosensors and bioactuators. We have investigated the electrical properties of the antibody-antigen modified diamond and silicon surfaces using electrical impedance spectroscopy (EIS). Frequency dependent measurements at the open-circuit potential show: (a) significant changes in impedance at frequency >10(4) Hz when the surface immobilized IgG was exposed to anti-IgG, and (b) only little or no change when exposed to anti-IgM. Mott-Schottky measurements at high frequency (200 kHz) show that the impedance is dominated by the space charge layer of the semiconducting substrates. Silicon surfaces modified in a similar manner to the diamond surface are compared; n-type and p-type samples show complementary behavior, as expected for a field effect. We also show it is possible to directly observe antigen-antibody interaction at a fixed frequency in real time, and with no additional labeling.

Covalent photochemical functionalization of amorphous carbon thin films for integrated real-time biosensing

Recent studies have demonstrated that carbon, in the form of diamond, can be functionalized with molecular and/or biomolecular species to yield interfaces exhibiting extremely high stability and selectivity in binding to target biomolecules in solution. However, diamond and most other crystalline forms of carbon involve high-temperature deposition or processing steps that restrict their ability to be integrated with other materials. Here, we demonstrate that photochemical functionalization of amorphous carbon films followed by covalent immobilization of DNA yields highly stable surfaces with excellent biomolecular recognition properties that can be used for real-time biological detection. Carbon films deposited onto substrates at 300 K were functionalized with organic alkenes bearing protected amine groups and characterized using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The functionalized carbon surfaces were covalently linked to DNA oligonucleotides. Measurements show very high selectivity for binding to the complementary sequence, and a high density of hybridizing DNA molecules. Samples repeatedly hybridized and denatured 25 times showed no significant degradation. The ability to use amorphous carbon films as a basis for real-time biosensing is demonstrated by coating quartz crystal microbalance (QCM) crystals with a thin carbon film and using this for covalent modification with DNA. Measurements of the resonance frequency show the ability to detect DNA hybridization in real time with a detection limit of <3% of a monolayer, with a high degree of reversibility. These results demonstrate that functionalized films of amorphous carbon can be used as a chemically stable platform for integrated biosensing using only room-temperature processing steps.

Fluidics of a nanogap

We have determined the filling properties of nanogaps with chemically heterogeneous walls. The quantitative criteria we present allow the prediction of the liquid loading of the nanostructure. They can easily be applied in combination with contact-angle measurements on planar substrates of the nanogap materials. We present an application of the theory to a recently developed nanogap biosensor. Chemical force microscopy (CFM) is employed to characterize the initial silanol properties of the gap. The functionality of the complex surface chemistry of the biosensor is demonstrated by the observation of functionalized nanoparticles in the gap with its resulting characteristic current-voltage relationship.

Alkene/diamond liquid/solid interface characterization using internal photoemission spectroscopy

The photochemical attachment of 10-amino-dec-1-ene molecules protected with a trifluoroacetic acid group (TFAAD) on hydrogen-terminated single-crystalline chemical vapor deposited (CVD) diamond is characterized by total photoyield spectroscopy (TPYS), conductivity, Hall-effect, spectrally resolved photoconductivity (SPC), optical transmission experiments, and, for the first time, by in situ internal photoemission (IPE) spectroscopy applied in the spectral regime from 4 to 6 eV on the alkene/diamond (liquid/solid) heterostructures. These experiments are performed on undoped, (100) oriented, single-crystalline CVD diamond films, which contain no grain boundaries and have negligible bulk and surface defect densities. X-ray photoelectron spectroscopy (XPS) is used to investigate the chemical bonding of alkene molecules to diamond. The spectroscopic set of data shows that the photochemical reaction window of H-terminated diamond is shifted below the optical gap of diamond because of the negative electron affinity. In situ IPE experiments reveal electron emission between 4.5 and 5.2 eV. A model is introduced and discussed in which valence-band electrons are optically excited into empty hydrogen-induced surface states of diamond from where they tunnel into empty pi states of alkene molecules. We theoretically discuss the fastest attachment time to achieve a saturated TFAAD layer of about 2 x 10(14) cm(-)(2) on diamond, which is experimentally detected to be 7 h. In the case of direct optical electron excitations from diamond, the bonding efficiency will be one TFAAD molecule attachment arising from about 1600 emitted electrons.

pH-sensitive diamond field-effect transistors (FETs) with directly aminated channel surface

We have introduced pH sensors fabricated on diamond thin films through modification of the surface-terminated atom. We directly modified the diamond surface from hydrogen to amine or oxygen with ultraviolet (UV) irradiation under ammonia gas. The quantified amine site based on the spectra obtained by X-ray photoelectron spectroscopy (XPS) is 26% (2.6 x 10(14) cm(-2)) with UV irradiation for 8h and its coverage is dependent on the UV irradiation time. This directly aminated diamond surface is stable with long-term exposure in air and electrolyte solution. We fabricated diamond solution-gate field-effect transistors (SGFETs) without insulating layers on the channel surface. These diamond SGFETs with amine modified by direct amination are sensitive to pH (45 mV/pH) over a wide range from pH 2 to 12 and their sensitivity is dependent on the density of binding sites corresponding to UV irradiation time on the channel surface.

Functional monolayers for improved resistance to protein adsorption: oligo(ethylene glycol)-modified silicon and diamond surfaces

The interaction of proteins with semiconductors such as silicon and diamond is of great interest for applications such as electronic biosensing. We have investigated the use of covalently bound oligo(ethylene glycol), EG, monolayers on diamond and silicon to minimize nonspecific protein adsorption. Protein adsorption was monitored by fluorescence scanning as a function of the length of the ethylene glycol chain (EG3 through EG6) and the terminal functional group (methyl- versus hydroxyl-terminated EG3 monolayer). More quantitative measurements were made by eluting adsorbed avidin from the surface and measuring the intensity of fluorescence in the solution. The attachment chemistry of the tri(ethylene glycol) molecules and monolayer orientation was studied by X-ray photoelectron spectroscopy. Improvement in the selectivity of surfaces modified with EG functionality was demonstrated in two model biosensing assays. We find that high-quality EG monolayers are formed on silicon and diamond and that these EG3 monolayers are as effective as EG3 self-assembled monolayers on gold at resisting nonspecific avidin adsorption. These results show promise for use of silicon and diamond materials in many potential applications such as biosensing and medical implants.

A sensitive biosensor for lactate based on layer-by-layer assembling MnO2 nanoparticles and lactate oxidase on ion-sensitive field-effect transistors

A sensitive enzyme-based FET biosensor for lactate has been obtained by introducing MnO2 nanoparticles at the gate surface via a layer-by-layer assembling method.

Direct electrical detection of hybridization at DNA-modified silicon surfaces

Electrochemical impedance spectroscopy was used to investigate the changes in interfacial electrical properties that arise when DNA-modified Si(111) surfaces are exposed to solution-phase DNA oligonucleotides with complementary and non-complementary sequences. The n- and p-type silicon(111) samples were covalently linked to DNA molecules via direct Si?C linkages without any intervening oxide layer. Exposure to solutions containing DNA oligonucleotides with the complementary sequence produced significant changes in both real and imaginary components of the electrical impedance, while exposure to DNA with non-complementary sequences generated negligible responses. These changes in electrical properties were corroborated with fluorescence measurements and were reproduced in multiple hybridization-denaturation cycles. The ability to detect DNA hybridization is strongly frequency-dependent. Modeling of the response and comparison of results on different silicon bulk doping shows that the sensitivity to DNA hybridization arises from DNA-induced changes in the resistance of the silicon substrate and the resistance of the molecular layers.

Spatially resolved electronic detection of biopolymers

An integrated array of field-effect transistor structures is used to detect two oppositely charged biopolymers: poly(L-lysine) and DNA. Local deposition of polymer solutions on part of the array induces sizeable variations in the dc current-voltage characteristics of the transistors exposed to the molecular charge. The whole transistor array is measured in the presence of a common electrolyte. Differential signals are studied as a function of electrolyte salt and polymer concentrations. The measurements provide information on the interface electrostatic potentials of the (semiconductor/biopolymer/electrolyte) system and the experimental data are compared to an analytical model which accounts for screening of the adsorbed charge by mobile ions.

Electronic Transduction of HIV-1 Drug Resistance in AIDS Patients

A drug composition consisting of nucleoside reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors (NNRTIs), and protease inhibitors (PIs) is commonly used in AIDS therapy. A major difficulty encountered with the therapeutic composite involves the emergence of drug-resistant viruses, especially to the PIs, regarded as the most effective drugs in the composition. We present a novel bioelectronic means to detect the appearance of mutated HIV-1 exhibiting drug resistance to the PI saquinavir. The method is based on the translation of viral RNA, the association of cleaved or uncleaved Gag polyproteins at an electrode surface functionalized with the respective antibodies, and the bioelectronic detection of the Gag polyproteins associated with the surface. The bioelectronic process includes the association of anti-MA or anti-CA antibodies, the secondary binding of an antibody-horseradish peroxidase (HRP) conjugate, and the biocatalyzed precipitation of an insoluble product on the electronic transducers. Faradaic impedance measurements and quartz crystal microbalance analyses are employed to follow the autoprocessing of the Gag polyproteins. The method was applied to determine drug resistance in infected cultured cells and also in blood samples of consenting AIDS patients. The method described here is also applicable to the determination of drug effectiveness in AIDS patients and to screening of the efficiency of newly developed drugs.

Impedimetric biosensors

Electrochemical impedance spectroscopy (EIS) is a sensitive indicator of a wide variety of chemical and physical properties. An increasing trend towards the development of impedimetric biosensors is being currently observed. Impedimetric techniques have been performed to characterize the fabrication of the biosensors and to monitor the catalyzed reactions of enzymes or the biomolecular recognition events of specific binding proteins, lectins, receptors, nucleic acids, whole cells, antibodies or antibody-related substances. However, little attention has been paid to reviewing this interesting research area. Herein, impedimetric biosensors are reviewed and many novel designs are discussed.

Ion sensitive field effect transducer-based biosensors

Ion-sensitive field effect transistors (ISFETs) have found growing interest in the rapidly developing field of biosensors. The principles, application and new developing techniques of ISFET-based biosensors are reviewed.

Probing antigen-antibody binding processes by impedance measurements on ion-sensitive field-effect transistor devices and complementary surface plasmon resonance analyses: development of cholera toxin sensors

Impedance measurements on ISFET devices are employed to develop new immunosensors. The analysis of the transconductance curves recorded at variable frequencies, upon the formation of antigen-antibody complexes on the ISFET devices, allows determination of the biomaterial film thicknesses. Complementary surface plasmon resonance measurements of analogous biosensor systems, using Au-coated glass slides as support, reveal similar film thicknesses of the biomaterials and comparable detection limits. A dinitrophenyl antigen layer is immobilized on the ISFET gate as a sensing interface for the anti-dinitrophenyl antibody (anti-DNP-Ab). The anti-DNP-Ab is analyzed with a sensitivity that corresponds to 0.1 microg mL(-1). The assembly of the biotinylated anti-anti-DNP-Ab and avidin layers on the base anti-DNP-Ab layer is characterized by impedance measurements. The development of an ISFET-based sensor for the cholera toxin is described. The anti-cholera toxin antibody is immobilized on the ISFET device. The association of the cholera toxin (CT) to the antibody is monitored by the impedance measurements. The detection limit for analyzing CT is 1.0 x 10(-11) M.

Imprinting of nucleotide and monosaccharide recognition sites in acrylamidephenylboronic acid-acrylamide copolymer membranes associated with electronic transducers

Molecular recognition sites for the nucleotides adenosine 5'-monophosphate (1), guanosine 5'-monophosphate (2), cytosine 5'-monophosphate (3), and uridine 5'-monophosphate (4) are imprinted in an acrylamide-acrylamidephenylboronic acid copolymer (5) membrane. The imprinted membranes are assembled on piezoelectric Au quartz crystals or Au electrodes via electropolymerization or on the gate surface of an ISFET device by radical polymerization. The imprinted membranes reveal selectivity toward the imprinted nucleotide, and the association of the respective nucleotides with the recognition sites is transduced by the following: (i) microgravimetric, quartz crystal microbalance (QCM) measurements; (ii) Faradaic impedance analyses, and (iii) potentiometric responses of the ISFET devices. While the microgravimetric QCM measurements reflect the swelling of the polymers upon the association of the nucleotides with the recognition sites, the ISFET response is due to the charging of the polymer membrane as a result of the formation of the nucleotide-boronate complex. The selective detection of the nucleotides may lead to new DNA/RNA sequencing methods. Also, specific recognition sites for beta-D(+)-glucose (6), D(+)-galactose (7), and beta-D(-)-fructose (8) were imprinted in an acrylamide-acrylamidephenylboronic acid copolymer (5) membrane associated with an ISFET device. Selective sensing of the respective monosaccharides is accomplished in the presence of the imprinted membrane-functionalized ISFET devices.

Imprinting of chiral molecular recognition sites in thin TiO2 films associated with field-effect transistors: novel functionalized devices for chiroselective and chirospecific analyses

(R)- or (S)-2-Methylferrocene carboxylic acids, (R)-1 or (S)-1, (R)- or (S)-2-phenylbutanoic acid, (R)-2 or (S)-2, and (R)- or (S)-2-propanoic acid, (R)-3 or (S)-3, can be imprinted in thin TiO2 films on the gate surface of ion-sensitive field-effect transistor (ISFET) devices. The imprinting is performed by hydrolyzing the respective carboxylate TiIV butoxide complex on the gate surface, followed by washing off the acid from the resulting TiO2 film. The imprinted sites reveal chiroselectivity only towards the sensing of the imprinted enantiomer. The chiral recognition sites reveal not only chiroselectivity but also chirospecificity and, for example, the (R)-2-imprinted film is active in the sensing of (R)-2, but insensitive towards the sensing of (R)2-phenylpropanoic acid, (R)-3, which exhibits a similar chirality. Similarly, the (R)-3-imprinted film is inactive in the analysis of (R)-2. The chiroselectivity and chirospecificity of the resulting imprinted films are attributed to the need to align and fit the respective substrates in precise molecular contours generated in the cross-linked TiO2 films upon the imprinting process.