Electrochemical protein chip with arrayed immunosensors with antibodies immobilized in a plasma-polymerized film

Microsystems technology and biosensing

This review addresses the recent developments in miniaturized microsystems or lab-on-a-chip devices for biosensing of different biomolecules: DNA, proteins, small molecules, and cells, especially at the single-molecule and single-cell level. In order to sense these biomolecules with sensitivity we have fabricated chip devices with respect to the biomolecule to be analyzed. The details of the fabrication are also dealt with in this review. We mainly developed microarray and microfluidic chip devices for DNA, protein, and cell analyses. In addition, we have introduced the porous anodic alumina layer chip with nanometer scale and gold nanoparticles for label-free sensing of DNA and protein interactions. We also describe the use of microarray and microfluidic chip devices for cell-based assays and single-cell analysis in drug discovery research.

Channel and Substrate Zone Two-Dimensional Resolution for Chemiluminescent Multiplex Immunoassay

A two-dimensional resolution system of channels and substrate zones was proposed for multiplex immunoassay performed with a designed multichannel chemiluminescent (CL) detection device coupled with a single photomultiplier. Using carcinoma antigen 125 (CA 125), carcinoma antigen 153 (CA 153), carcinoma antigen 199 (CA 199), and carcinoembryonic antigen (CEA) as two couples of model analytes, two couples of capture antibodies were immobilized in two channels, respectively. With a sandwich format, the CL substrates for alkaline phosphatase and horseradish peroxidase were delivered into the channels sequentially to perform a multiplex immunoassay after the sample and tracer antibodies were introduced into the channels for on-line incubation. CA 125, CA 153, CA 199, and CEA could be assayed in the ranges of 0.50-80, 2.0-100, and 5.0-150 U/mL and 1.0-70 ng/mL with limits of detection of 0.15, 0.80, and 2.0 U/mL and 0.65 ng/mL at 3sigma, respectively. The whole assay process including regeneration of the device could be completed in 37 min. The proposed system showed acceptable detection and fabrication reproducibility, and the results obtained were in acceptable agreement with those from parallel single-analyte test of practical clinical sera. This technique provides a new strategy for a simple, automated, and near-simultaneous multianalyte immunoassay.

A disposable two-throughput electrochemical immunosensor chip for simultaneous multianalyte determination of tumor markers

A disposable two-throughput immunosensor array was proposed for simultaneous electrochemical determination of tumor markers. The low-cost immunosensor array was fabricated simply using cellulose acetate membrane to co-immobilize thionine as a mediator and two kinds of antigens on two carbon electrodes of a screen-printed chip, respectively. With two simultaneous competitive immunoreactions the corresponding horseradish peroxidase (HRP) labeled antibodies were captured on the membranes, respectively, on which the immobilized thionine shuttled electrons between HRP and the electrodes for enzymatic reduction of H2O2 to produce detectable signals. The electrochemical and electronic cross-talks between the electrodes could be avoided, which was beneficial to the miniaturization of the array without considering the distance between immunosensors. Under optimal conditions the immunosensor array could be used for fast simultaneous electrochemical detection of CA 19-9 and CA 125 with the limits of detection of 0.2 and 0.4 U/ml, respectively. The serum samples from clinic were assayed with the proposed method and the results were in acceptable agreement with the reference values. The proposed method for preparation of immunosensor array could be conveniently used for fabrication of disposable electrochemical biochip with high throughput and possessed the potential of mass production and commercialization.

A Disposable Multianalyte Electrochemical Immunosensor Array for Automated Simultaneous Determination of Tumor Markers

Background: Automated and convenient multianalyte detection with high throughput is increasingly needed in clinical diagnosis. We developed a disposable 4-by-2 array for programmed simultaneous amperometric immunoassay of 4 tumor markers.

Methods: We used a screen-printed technique, 1-step immobilization method, and flow injection technique. We immobilized carcinoembryonic antigen, α-fetoprotein, β-human choriogonadotropin, and carcinoma antigen 125 as model analytes in a redox mediator–grafted, biopolymer-modified, screen-printed carbon electrode array to capture corresponding horseradish peroxidase-labeled antibodies in competitive immunoreactions. The simultaneous multianalyte immunoassay was automatically carried out to amperometrically monitor the mediator-catalyzed enzymatic response to hydrogen peroxide, which decreased in proportion to the concentrations of analytes in samples.

Results: The multianalyte immunosensor array had a throughput of 60 samples/h and allowed simultaneous detection of carcinoembryonic antigen, α-fetoprotein, β-human choriogonadotropin, and carcinoma antigen 125 in clinical serum samples with concentrations up to 188 μg/L, 250 μg/L, 266 IU/L, and 334 kIU/L, respectively. The detection limits (limits of the blank, mean of blank plus 3 SD) were 1.1 μg/L, 1.7 μg/L, 1.2 IU/L, and 1.7 kIU/L. The inter- and intraassay imprecision (CVs) of the immunosensor arrays were <7.8% and <9.0%, respectively. The immunosensor arrays were stable for 28 days.

Conclusions: This newly constructed immunosensor array provides a simple, automated, simultaneous multianalyte immunoassay with high throughput, short analytical time, and sufficiently low detection limits for clinical application. This method offers the capability of miniaturizing the multianalyte detection device.

Electrochemical immunoassay on a microfluidic device with sequential injection and flushing functions

An integrated microfluidic device with injecting, flushing, and sensing functions was realized using valves that operate based on direct electrowetting. The device consisted of two substrates: a glass substrate with driving and sensing electrodes and a poly(dimethylsiloxane) (PDMS) substrate. Microfluidic transport was achieved using the spontaneous movement of solutions in hydrophilic flow channels formed with a dry-film photoresist layer. The injection and flushing of solutions were controlled by gold working electrodes, which functioned as valves. The valves were formed either in the channels or in a through-hole in the glass substrate. To demonstrate the system's applicability to an immunoassay, the detection of immobilized antigens was performed as a partial simulation of a sandwich immunoassay. Human alpha-fetoprotein (AFP) or an anti-human AFP antibody was immobilized on a platinum working electrode in the chamber using a plasma-polymerized film (PPF). By applying a potential to the injection valves, necessary solutions were injected one by one through the channels into a reaction chamber at the center of the chip and incubated for reasonable periods of time. The solutions were then flushed through the flushing valve and absorbed in a filter paper placed under the device. After incubation with the corresponding antibodies labeled with glucose oxidase (GOD), electrochemical detection was conducted. In both cases, the obtained current depended on the amount of immobilized antigen. The calibration curves were sigmoidal, and the detection limit was 0.1 ng. The developed microfluidic system could potentially be a fundamental component for a micro immunoassay of the next generation.

Multianalyte immunoassay based on insulating-controllable PoPD film at arrayed electrodes integrated on a silicon chip

A novel, simple and label-free multianalyte immunoassay system is presented here by integrating arrayed electrodes on a silicon chip via MEMS. The chip is consisted of six Au disk electrodes, an Au counter electrode and an Ag/AgCl reference electrode. Semi-insulating poly(o-phenylenediamine) (PoPD) was utilized to co-polymerize and immobilize antibodies at the arrayed Au electrodes, and wider linear detection range was obtained than those prepared with completely insulating PoPD. Electrochemical cyclic voltammogram (CV), AC impedance spectroscopy, AFM and fluorescence microscopy were employed to characterize the system. The arrayed electrodes offered exact control of deposition position via electrochemical operation, allowing selectively immobilization of different antibodies at desired positions on a single chip. Specific recognition of antibody (Ab) to corresponding antigen (An) was quantitatively monitored by cyclic voltammograms in the presence of electrochemical redox probe, ferrocene methanol. The proposed immunoassay chips showed sensitive response to three liver fibrosis markers, hyaluronic acid (HA), collagen type IV (IV-C) and lamin (LN) at ng/mL level simultaneously and specifically in a tiny amount of volume, usually 50 microL. The results obtained via chips were well consistent with those obtained by commercial radio immunoassays (RIA).

Ultrasensitive electrical detection of protein using nanogap electrodes and nanoparticle-based DNA amplification

The present study describes an ultrasensitive protein biochip that employs nanogap electrodes and self-assembled nanoparticles to electrically detect protein. A bio-barcode DNA technique amplifies the concentration of target antigen at least 100-fold. This technique requires the establishment of conjugate magnetic nanoparticles (MNPs) and gold nanoparticles (AuNPs) through binding between monoclonal antibodies (2B2), the target antigen, and polyclonal antibodies (GP). Both GP and capture ssDNA (single-strand DNA) bonds to bio-barcode ssDNA are immobilized on the surface of AuNPs. A denature process releases the bio-barcode ssDNAs into the solution, and a hybridization process establishes multilayer AuNPs over the gap surface between electrodes. Electric current through double-layer self-assembled AuNPs is much greater than that through self-assembled monolayer AuNPs. This significant increase in electric current provides evidence that the solution contains the target antigen. Results show that the protein biochip attains a sensitivity of up to 1 pg/ microL.

Mass spectrometry-based "omics" technologies in cancer diagnostics

Many "omics" techniques have been developed for one goal: biomarker discovery and early diagnosis of human cancers. A comprehensive review of mass spectrometry-based "omics" approaches performed on various biological samples for molecular diagnosis of human cancers is presented in this article. Furthermore, the existing and potential problems/solutions (both de facto experimental and bioinformatic challenges), and future prospects have been extensively discussed. Although the use of present omic methods as diagnostic tools are still in their infant stage and consequently not ready for immediate clinical use, it can be envisaged that the "omics"-based cancer diagnostics will gradually enter into the clinic in next 10 years as an important supplement to current clinical diagnostics.

Biosensor developments: application to prostate-specific antigen detection

Prostate-specific antigen (PSA) is the best serum marker currently available for the detection of prostate cancer and is the forensic marker of choice for determining the presence of azoospermic semen in some sexual assault cases. Most current assays for PSA detection are processed on large analyzers at dedicated testing sites, which require that samples be sent away for testing. This leads to delays in patient management and increased administration costs. The recent emphasis placed on the need for point-of-care patient management has led to the development of novel biosensor detection strategies that are suitable for the miniaturization of assays for various targets including PSA. This review highlights the current and novel analytical technologies used for PSA detection, which will benefit clinicians, patients and forensic workers in the future.

Electrochemical immunosensors for the simultaneous detection of two tumor markers

The microfabrication of electrochemical immunosensors for the simultaneous detection of two protein analytes is described. The sensors consisted of two iridium oxide electrodes (1-mm diameter) patterned on a glass substrate. Capture antibodies were immobilized on the porous iridium oxide electrodes by covalent attachment using (3-aminopropyl)triethoxysilane and glutaraldehyde. The spatial separation of the electrodes (2.5 mm) enabled simultaneous electrochemical immunoassays to be conducted without cross-talk between the electrodes. Proteins were measured using electrochemical ELISA, and detection was achieved by electrochemically oxidizing alkaline phosphatase-generated hydroquinone. Sensors for the simultaneous detection of goat IgG and mouse IgG, and for the tumor markers CEA and AFP, were developed. The sensors had detection limits of 1, 2, 1.2, and 1 ng/mL for goat IgG, mouse IgG, CEA, and AFP, respectively.

Simultaneous detection of five indices of hepatitis B based on an integrated automatic microfluidic device

Immunophenotyping evaluation is of particular importance for the clinical diagnosis, therapy, and prognosis of viral hepatitis. In this study, an integrated micro flow device has been developed to detect the differentiated antigens/antibodies for immunophenotyping of viral hepatitis. The sensors were fabricated with plasma-polymerized ethylenediamine film (PPF) and nanometer-sized gold particles (nanogold) on which the different hepatitis B antigens/antibodies (markers) were subsequently immobilized. Monitoring the changes in the potential signals before and after the antigen-antibody interaction provides the basis for an immunoassay that is simple, rapid, and cost-effective. It permits the detection of hepatitis B in the dynamic concentration range of 2 orders of magnitude (10(-6) g x L(-1) - 10(-4) g x L(-1)). Up to 7 successive assay cycles with retentive sensitivity were achieved for the sensors regenerated by 8 M urea. Moreover, the microfluidic device was applied to evaluate a number of practical specimens with analytical results in acceptable agreement with those clinically classified. The newly proposed multiparameter analysis technique provides a feasible alternative tool for the diagnosis and monitoring of hepatitis B.

Signal-Transducing Proteins for Nanoelectronics

This aim of this article is to provide novel paradigms for 21st century nanoelectronics by taking inspiration from the biology of signal transduction events where Nature has solved many complex problems, particularly those concerned with signal integration and amplification.

A tris(2,2′-bipyridyl)cobalt(III)-bovine serum albumin composite membrane for biosensors

A concept based on a novel redox-biocompatible composite protein membrane fabrication, double enzyme membrane modification technique and antibody immobilization, was exploited to develop a highly sensitive amperometric enzyme immunosensor for detection of carcinoembryonic antigen (CEA). In this concept, a solution of bovine serum albumin (BSA) containing horseradish peroxidase (HRP) is coated on the gold electrode in such a way that the first enzyme membrane is achieved. Then tris(2,2'-bipyridyl) cobalt(III) (Co(bpy)(3)3+), as a mediator, was embedded in BSA-HRP composite membrane vis the electrostatic force and hydrophobe functions. Later a self-assembled conductive nano-Au monolayer was constructed onto the resultant electrode surface by electrostatic interaction between the negatively charged nano-Au and positively charged Co(bpy)3(3+). Protein A is used as a binding material to achieve an adjusted (but not random) orientation of the antibodies surface for efficient combination of antigens. Finally, the HRP, was employed to block the possible remaining active sites and avoid the non-specific adsorption, which acts not only as a blocking reagent instead of the commonly used BSA but also as the conventional enzyme-labeling to amplify the response of the antigen-antibody reaction. The immunosensor constructed with the double layer biocatalytic HRP membranes and the desirable Co(bpy)(3)3+/BSA redox-biocompatible composite membrane performed high sensitivity and a wide linear response to CEA in the range of 0.50-80.00 ng/mL with a limit of detection of 0.14 ng/mL, as well as good stability and long-term life.

Flow-Through Multianalyte Chemiluminescent Immunosensing System with Designed Substrate Zone-Resolved Technique for Sequential Detection of Tumor Markers

A novel flow-through immunosensing system for performing a multianalyte chemiluminescent determination in a single run was designed. A new analytical strategy of substrate zone-resolved technique was proposed. Using carcinoma antigen 125 (CA 125) and carcinoembryonic antigen (CEA) as model analytes, the capture antibodies for CA 125 and CEA were immobilized on an UltraBind aldehyde-activated membrane to act as an immunoreactor, to which the mixture of CA 125, CEA, and their corresponding tracers, horseradish peroxidase (HRP)-labeled anti-CA 125 and alkaline phosphatase (ALP)-labeled anti-CEA, was introduced for on-line incubation. The substrates for HRP and ALP were then delivered into the detection cell sequentially to perform substrate zone-resolved immunoassay by a sandwich format. Under optimal conditions, CA 125 and CEA could be assayed in the ranges of 5.0-100 units/mL and 1.0-120 ng/mL, respectively. The whole assay process including incubation, wash, detection, and regeneration could be completed in 35 min. The serum samples from the clinic were assayed with the proposed method, and the results were in acceptable agreement with the reference values. This method and the strategy of substrate zone-resolved technique could be further developed for high-throughput multianalyte immunoassay.

Multiplex Measurement of Seven Tumor Markers Using an Electrochemical Protein Chip

An electrochemical immunosensor for performing multianalyte measurements of tumor markers is described. The sensor consisted of an array of immunosensing electrodes fabricated on a glass substrate. Each electrode contained a different immobilized antigen and was capable of measuring a specific tumor marker using electrochemical enzyme-based competitive immunoassay. Using this arrangement, multiple analytes could be measured simultaneously by performing the technical operations for a single assay. The biosensor was used to measure the concentrations of seven important tumor markers: AFP, ferritin, CEA, hCG-beta, CA 15-3, CA 125, and CA 19-9. The sensor had excellent precision and accuracy and was comparable in performance to single-analyte ELISAs (1.9-8.1% interassay CV; <2 ng/mL (or units/mL) detection limit for most analytes). Multianalyte assays provide significant advantages over single-analyte tests in terms of cost per test, labor, test throughput, and convenience. We anticipate that chip-based sensors, as described herein, will be suitable for the mass production of economical, miniaturized lab-on-a-chip devices that will have applications in a wide range of clinical, environmental, and biodefense applications.

Carbon nanotube amplification strategies for highly sensitive immunodetection of cancer biomarkers

We describe herein the combination of electrochemical immunosensors using single-wall carbon nanotube (SWNT) forest platforms with multi-label secondary antibody-nanotube bioconjugates for highly sensitive detection of a cancer biomarker in serum and tissue lysates. Greatly amplified sensitivity was attained by using bioconjugates featuring horseradish peroxidase (HRP) labels and secondary antibodies (Ab(2)) linked to carbon nanotubes (CNT) at high HRP/Ab(2) ratio. This approach provided a detection limit of 4 pg mL(-)(1) (100 amol mL(-)(1)), for prostate specific antigen (PSA) in 10 microL of undiluted calf serum, a mass detection limit of 40 fg. Accurate detection of PSA in human serum samples was demonstrated by comparison to standard ELISA assays. PSA was quantitatively measured in prostate tissue samples for which PSA could not be differentiated by the gold standard immunohistochemical staining method. These easily fabricated SWNT immunosensors show excellent promise for clinical screening of cancer biomarkers and point-of-care diagnostics.

Localized surface plasmon resonance biosensors

In this review, the most recent progress in the development of noble metal nano-optical sensors based on localized surface plasmon resonance (LSPR) spectroscopy is summarized. The sensing principle relies on the LSPR spectral shifts caused by the surrounding dielectric environmental change in a binding event. Nanosphere lithography, an inexpensive and simple nanofabrication technique, has been used to fabricate the nanoparticles as the LSPR sensing platforms. As an example of the biosensing applications, the LSPR detection for a biomarker of Alzheimer’s disease, amyloid-derived diffusable ligands, in human brain extract and cerebrospinal fluid samples is highlighted. Furthermore, the LSPR sensing method can be modified easily and used in a variety of applications. More specifically, a LSPR chip capable of multiplex sensing, a combined electrochemical and LSPR protocol and a fabrication method of solution-phase nanotriangles are presented here.

Electrochemical coding for multiplexed immunoassays of proteins

An electrochemical immunoassay protocol for the simultaneous measurements of proteins, based on the use of different inorganic nanocrystal tracers is described. The multiprotein electrical detection capability is coupled to the amplification feature of electrochemical stripping transduction (to yield fmol detection limits) and with an efficient magnetic separation (to minimize nonspecific adsorption effects). The multianalyte electrical sandwich immunoassay involves a dual binding event, based on antibodies linked to the nanocrystal tags and magnetic beads. Carbamate linkage is used for conjugating the hydroxyl-terminated nanocrystals with the secondary antibodies. Each biorecognition event yields a distinct voltammetric peak, whose position and size reflects the identity and level, respectively, of the corresponding antigen. The concept is demonstrated for a simultaneous immunoassay of beta(2)-microglobulin, IgG, bovine serum albumin, and C-reactive protein in connection with ZnS, CdS, PbS, and CuS colloidal crystals, respectively. These nanocrystal labels exhibit similar sensitivity. Such electrochemical coding could be readily multiplexed and scaled up in multiwell microtiter plates to allow simultaneous parallel detection of numerous proteins or samples and is expected to open new opportunities for protein diagnostics and biosecurity.

A new immunosensing method by galactose oxidase-mediated electrocatalysis using a virtual beaker array

We report a novel and convenient method for the determination of glycoproteins, especially antibodies, using galactose oxidase (GAO) on the basis of the contents of galactosyl and N-acetylgalactosaminyl residues in carbohydrate chains of glycoproteins. GAO converts galactose residues to their corresponding aldehyde and H(2)O(2), the latter being electroactive and quantifiable by DC amperometry. The method does not require processes such as antibody labeling or the use of enzyme-tagged secondary antibodies. For an array-type immunosensing, the platform surface for antigen immobilization was specially designed by using differentiated surface wetting property of hydrophobic and hydrophilic patterns. We patterned the hydrophobic surface of the poly(dimethylsiloxane) substrate by microcontact printing with the poly(amidoamine) dendrimer ink, providing hydrophilic patterns on a hydrophobic base substrate. By applying aqueous solution on the patterned surface, an array of free-standing water droplets was made. With the prepared virtual beaker array, electrochemical immunosensing was performed by using anti-dinitrophenyl-IgG as a model target protein. From immunoassay with GAO-mediated electrocatalysis, a good correlation in amperometric signal with the target IgG was registered. The total assay time was about 20 min, including antibody recognition and signal registration.

Sequential Determination of Two Proteins by Temperature-Triggered Homogeneous Chemiluminescent Immunoassay

A novel protocol for performing a sequential dual-protein immunoassay, based on a temperature-triggered separation/mixing process and HRP-catalyzed chemiluminescence (CL) detection, is described. In contrast to current multilabel-based detection techniques, a single HRP label is employed in this proposed method. Herein we introduce poly(N-isopropylacrylamide) (PNIP) and magnetic beads as bimolecular immobilizing carriers to separate different targets by taking advantage of thermal response, as demonstrated by sequential detection of human IgG and IgA. PNIP is known to aggregate and precipitate out of water when the temperature is raised above the lower critical solution temperature (LCST) of 31 degrees C; thus, it can be separated from supernatant by centrifugation. Besides, magnetic beads can be separated from PNIP by magnetic force as the temperature is lower than LCST. A homogeneous noncompetitive ELISA was employed, formed by primary antibodies immobilized onto the surface of magnetic beads and PNIP, antigen as IgG and IgA in the sample, and HRP-labeled second antibodies. Moreover, highly sensitive CL detection of HRP was applied, and the detection limits of IgG and IgA were as low as 2.0 and 1.5 ng/mL, respectively. Within the calibrated amount, the protocol had excellent precision within 11% for each target and was comparable in performance to commercial single-analyte ELISAs. Furthermore, the proposed method has been successfully applied to the determination of dual analyte in real samples without cross-reaction, and a good correlation was achieved after comparison with the conventional assay for IgG and IgA in 40 human serum samples.

Potentiometric immunosensor using artificial antibody based on molecularly imprinted polymers

A potentiometric artificial immunosensor based on a molecularly imprinted polymer was prepared as a detecting element in micro total analysis systems with the intent of providing easy clinical analysis. As the structure and transducing mechanism of this sensor are very simple, construction of a single microsensor should be quite easy. Multimicrosensor arrays applicable to several kinds of analytes will be attainable by both changing the template molecule to be imprinted and reducing the sensor size. The response characteristics of this sensor were evaluated by measuring the response potential to serotonin, which was used as a model material. The obtained sensor was highly responsive to serotonin in water but not to tryptamine, acetaminophen, or procainamide. This phenomenon confirms that the sensor recognizes serotonin and that it functions as a specific artificial immunosensor. Quick measurement is possible because the response time, defined as the time required to achieve 95% of the magnitude of the equilibrated signal, correspond to approximately 12 s. The sensor's determination and detection limits were found to be 1 mumol/L and 100 pmol/L, respectively. These results suggest that our strategy can be applied to construction of a potentiometric artificial immunosensor.

Immunoassays based on microelectrodes arrayed on a silicon chip for high throughput screening of liver fibrosis markers in human serum

A novel immunoassays for screening of disease markers in human serum are presented by miniaturizing interdigitated array (IDA) of microelectrodes via micro electro-mechanical system (MEMS) on a silicon chip for multi-channel electrochemical measurement. Different selected antibodies (Abs) are incorporated site-specifically into the electrochemically deposited polypyrrole (PPy) formed on the IDA of the silicon chip, which was characterized by fluorescence microscope photo and the electrochemical quartz crystal microbalance (EQCM) measurements. The selective recognition of Ab to the corresponding antigen (Ag) is monitored through the measurable conductivity change, which is directly visualized by cyclic voltammograms (CVs) in presence of the redox probe, Fe (CN)6(3-/4-). By using the strategy presented here, three liver fibrosis markers, hyaluronic acid (HA), lamin (LN) and collagen type IV (IV-C), are detected simultaneously and specifically at the surface of the chip with calibration curves, y = 21.75 + 0.84x (R = 0.995), y = 57.54 + 0.47x (R = 0.999) and y = 37.92 + 0.28x (R = 0.999), separately. Either the standard or the serum samples can be detected at ng/mL concentration level in a tiny amount of volume, approximately 50 microL. The chip-based immunoassay shows the advantages of high sensitivity, good specificity, high throughput, low sample consumption, and the stability offered via batch production by MEMS as well, which is expected to benefit the multi-target screening of desired clinical analytes.

Structure and biosensor characteristics of complex between glucose oxidase and plasma-polymerized nanothin film

The structure and biosensor characteristics of complex between glucose oxidase (GOD) and plasma-polymerized nanothin film (PPF), in which the thickness is several nanometers, were investigated by atomic force microscopy (AFM) and electrochemical measurement. The GOD molecules were densely adsorbed onto the PPF surface treated by nitrogen plasma and the individual GOD molecules were observed. Subsequently, the GOD densely packed array on the PPF surface was subsequently treated by plasma polymerization (overcoating). AFM image showed that the thicker film gave the smoother surface, indicating that the GOD adsorbed on the surface was embedded more deeply in PPF. The amperometric biosensor characteristics of the GOD-PPF complex on a platinum electrode showed the current increment due to the enzymatic reaction with glucose addition, indicating that enzyme activity was retained although the enzyme has been exposed to the plasma gas related to diffusion of the substrate. This means that under mild exposure to organic plasma, the enzyme does not become seriously dysfunctional. Amperometric biosensor characteristics were strongly affected by monomer and thickness of PPF overcoating related with the diffusion of the substrate (glucose). Considering that the film deposition was performed using microfabrication-compatible organic plasma, our new method for protein architecture has a great potential of enabling high throughput production of bioelectronic devices.

Electrochemical Multianalyte Immunoassays Using an Array-Based Sensor

A novel amperometric biosensor for performing simultaneous electrochemical multianalyte immunoassays is described. The sensor consisted of eight iridium oxide sensing electrodes (0.78 mm(2) each), an iridium counter electrode, and a Ag/AgCl reference electrode patterned on a glass substrate. Four different capture antibodies were immobilized on the sensing electrodes via adsorption. Quantification of proteins was achieved using an ELISA in which the electrochemical oxidation of enzyme-generated hydroquinone was measured. The spatial separation of the electrodes enabled simultaneous electrochemical immunoassays for multiple proteins to be conducted in a single assay without amperometric cross-talk between the electrodes. The simultaneous detection of goat IgG, mouse IgG, human IgG, and chicken IgY was demonstrated. The detection limit was 3 ng/mL for all analytes. The sensor had excellent precision (1.9-8.2% interassay CV) and was comparable in performance to commercial single-analyte ELISAs. We anticipate that chip-based sensors, as described herein, will be suitable for the mass production of economical, miniaturized, multianalyte assay devices.

Protein microarrays on hybrid polymeric thin films prepared by self-assembly of polyelectrolytes for multiple-protein immunoassays

We report here the development and characterization of protein microarrays fabricated on nanoengineered 3-D polyelectrolyte thin films (PET) deposited on glass slide by consecutive adsorption of polyelectrolytes via self-assembly technique. Antibodies or antigens were immobilized in the PET-coated glass slides by electrostatic adsorption and entrapment of porous structure of the 3-D polymer film and thus establishing a platform for parallel analysis. Both antigen and antibody microarrays were fabricated on the PET-coated slides, and direct and indirect immunoassays on protein microarrays for multiple-analyte detection were demonstrated. Microarrays produced on these PET-coated slides have consistent spot morphology and provide performance features needed for proteomic analysis. The protein microarrays on the PET films provide LOD as low as 6 pg/mL and dynamic ranges up to three orders of magnitude, which are wider than the protein microarrays fabricated on aldehyde and poly-L-lysine functionalized slides. The PET films constructed by self-assembly technique in aqueous solution is green chemistry based, cost-effective method to generate 3-D thin film coatings on glass surface, and the coated slide is well suited for immobilizing many types of biological molecules so that a wide variety of microarray formats can be developed on this type of slide.

Electrochemical microdevice with separable electrode and antibody chips for simultaneous detection of pepsinogens 1 and 2

An electrochemical microdevice with separable electrode and antibody chips has been developed and applied to detect atrophic gastritis-related proteins, pepsinogen 1 (PG1) and pepsinogen 2 (PG2), based on sandwich-type enzyme-linked immunosorbent assays (ELISAs) with horseradish peroxidase (HRP)-labeled antibody. To fabricate the electrochemical device for simultaneous analysis of several proteins, the electrode chip with eight electrode elements was assembled along with an antibody chip with eight cavities containing immobilized anti-PG1 or anti-PG2. The immunoreactions occurring in the cavities of the device were detected simultaneously by amperometry. The labeled HRP in the cavity in the presence of hydrogen peroxide catalyzed the oxidation of ferrocenemethanol (FMA) to FMA+, which was detected electrochemically by the electrode chip. The amperometric responses of respective cavities in the device increased with increasing concentration of PG1 or PG2 of 0-50 ng/ml, ensuring the simultaneous detection of PG1 and PG2. The detection limits for both PG1 and PG2 were 0.6 ng/ml (S/N=2). The electrode chip was recovered easily by disassembling the electrochemical device; thereby, it was used repeatedly, whereas the antibody chip was discarded. No marked decrease in electrochemical responses was detected after repeated use. Reuse of the electrode chip is beneficial to reduce costs of protein analysis.

State of the art and recent advances in immunoanalytical systems

This article is an overview the state of the art and the recent developments in immunosensors. Homogeneous immunosensors, heterogeneous immunosensors, integrated immunosensors and biochip format immunosensors are presented, based on optical, electrochemical, magnetic or mechanical detection/transduction systems.

Protein-detecting microarrays: current accomplishments and requirements

The sequencing of the human genome has been successfully completed and offers the chance of obtaining a large amount of valuable information for understanding complex cellular events simply and rapidly in a single experiment. Interestingly, in addressing these proteomic studies, the importance of protein-detecting microarray technology is increasing. In the coming few years, microarray technology will become a significantly promising and indispensable research/diagnostic tool from just a speculative technology. It is clear that the protein-detecting microarray is supported by three independent but strongly related technologies (surface chemistry, detection methods, and capture agents). Firstly, a variety of surface-modification methodologies are now widely available and offer site-specific immobilization of capture agents onto surfaces in such a way as to keep the native conformation and activity. Secondly, sensitive and parallel detection apparatuses are being developed to provide highly engineered microarray platforms for simultaneous data acquisition. Lastly, in the development of capture agents, antibodies are now probably the most prominent capture agents for analyzing protein abundances. Alternative scaffolds, such as phage-displayed antibody and protein fragments, which provide the advantage of increasing diversity of proteinic capture agents, however, are under development. An approach involving recombinant proteins fused with affinity tag(s) and coupled with a highly engineered surface chemistry will provide simple production protocols and specific orientations of capture agents on the microarray formats. Peptides and other small molecules can be employed in screening highly potent ligands as well as in measuring enzymatic activities. Protein-detecting microarrays supported by the three key technologies should contribute in accelerating diagnostic/biological research and drug discovery.

An impedance array biosensor for detection of multiple antibody–antigen interactions

Electrochemical impedance spectroscopy (EIS) combined with a gold electrode array was developed to detect multiple antibody-antigen interactions. Hepatitis B surface antigen (HBsAg), as a model sample, was employed to evaluate the characteristics of the biosensor. The array was fabricated by immobilizing antibodies on the self-assembled molecules surface of the electrodes. The surface characteristics of the array during the binding process including the antibody-antigen conjugation and the sandwich complex with HRP-labeled antibody, as well as the precipitation layer, were characterized by atomic force microscopy (AFM) and electrochemical impedance spectroscopy, respectively. A linear relationship between electron-transfer resistance and the concentrations of HBsAg ranged from 10 pg ml(-1) to 1 ng ml(-1) and the detection limit of 10 pg ml(-1) was obtained. 100 pg ml(-1) antigen samples, such as rat IgG, HBsAg and HBeAg, as well as the antigen mixture, were incubated with the relative antibody-modified electrodes on the array. No obvious cross-talk reaction was observed. All these results confirm the feasibility of applying electrochemical impedance spectroscopy to the electrode array.

Electrochemical biosensors: towards point-of-care cancer diagnostics

Wide-scale point-of-care diagnostic systems hold great promise for early detection of cancer at a curable stage of the disease. This review discusses the prospects and challenges of electrochemical biosensors for next-generation cancer diagnostics. Electrochemical biosensors have played an important significant role in the transition towards point-of-care diagnostic devices. Such electrical devices are extremely useful for delivering the diagnostic information in a fast, simple, and low cost fashion in connection to compact (hand-held) analyzers. Modern electrochemical bioaffinity sensors, such as DNA- or immunosensors, offer remarkable sensitivity essential for early cancer detection. The coupling of electrochemical devices with nanoscale materials offers a unique multiplexing capability for simultaneous measurements of multiple cancer markers. The attractive properties of electrochemical devices are extremely promising for improving the efficiency of cancer diagnostics and therapy monitoring. With further development and resources, such portable devices are expected to speed up the diagnosis of cancer, making analytical results available at patient bedside or physician office within few minutes.

Rapid three-dimensional biointerfaces for real-time immunoassay using hIL-18BPa as a model antigen

With the goal of designing a rapid and affordable system of real-time immune monitoring for future diagnostic applications in sepsis, we have developed a biointerface composed of polyethyleneimine (PEI) and carboxymethylcellulose (CMC) to provide a means of prompt and facile immunoassay. Biointerface assembly is complete within 30 min, with all preparation performed and monitored within the measurement chamber of a quartz crystal microgravimetry with dissipation (QCM-D) sensor. Optimised biointerface composition, as determined by the mass of antibody immobilised, the level of antigen detection, and the amount of non-specific binding of human serum albumin, was determined to consist of a 4.0 mg/mL CMC hydrogel layer cross-linked to a 0.5 mg/mL PEI sub-layer. Tapping mode atomic force microscopy (AFM) in liquid demonstrates highly uniform and smooth surfaces using these hydrogels. Sensitivity of the biointerface for rhIL-18BPa is 400 ng/mL, with detection of 1 microg/mL achievable following 25 surface regenerations. Performance of the biointerface is verified using surface plasmon resonance (SPR), demonstrating the ability of the biointerface to be applied across platforms.

New antibody immobilization method via functional liposome layer for specific protein assays

A specific protein assay system based on functional liposome-modified gold electrodes has been demonstrated. To fabricate such assay system, a liposome layer was initially grown on top of a gold layer. The liposome layer contained two kinds of functional molecules: biotin molecules for the binding sites of streptavidin and N-(10,12-pentacosadiynoic)-acetylferrocene molecules for the facile redox probe in electrochemical detections. Then, streptavidin was attached on the functional liposme-modified layer using the interaction of streptavidin-sbiotin complex. On the streptavidin-attached surface, antibody molecules, anti-human serum albumin antibodies could be immobilized without any secondary antibodies. AFM imaging of the streptavidin-attached liposome surface revealed a uniform distribution of closely packed streptavidin molecules. In situ quartz-crystal microbalance and electrochemical measurements demonstrated that the wanted antibody-antigen reactions should occur with high specificity and selectivity. Our specific antibody assay system, based on a functional liposome modified electrode, can be developed further to yield sophisticated structures for numerous protein chips and immunoassay sensors.

Novel electrochemical sensor system for protein using the aptamers in sandwich manner

Novel electrochemical detection system for protein in sandwich manner using the aptamers was developed. Two different aptamers, which recognize different positions of thrombin, were chosen to construct sandwich type sensing system for protein, and one was immobilized onto the gold electrode for capturing thrombin onto the electrode and the other was used for detection. To obtain the signal, the aptamer for detection was labeled with pyrroquinoline quinone glucose dehydrogenase ((PQQ)GDH), and the electrical current, generated from glucose addition after the formation of the complex of thrombin, gold immobilized aptamer and the (PQQ)GDH labeled aptamer on the electrode, was measured. The increase of the electric current generated by (PQQ)GDH was observed in dependent manner of the concentration of thrombin added, and more than 10nM thrombin was detected selectively. The batch type protein sensing system was constructed using the two different aptamers sandwiching thrombin and it showed linear response to the increase of the thrombin concentration in the range of 40-100 nM.