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

A Dual-Enzyme Amplification Loop for the Sensitive Biosensing of Endopeptidases

A rapid and sensitive approach for the detection of endopeptidases via a new analyte-triggered mutual emancipation of linker-immobilized enzymes (AMELIE) mechanism has been developed and demonstrated using a matrix metallopeptidase, a collagenase, as the model endopeptidase analyte. AMELIE involves an autocatalytic loop created by a pair of selected enzymes immobilized on solid substrates via linkers with specific sites that can be proteolyzed by one another. These bound enzymes are spatially separated so that they cannot act upon their corresponding substrates until the introduction of the target endopeptidase analyte that can also cleave one of the linkers. This triggers the self-sustained loop of enzymatic activities to emancipate all the immobilized enzymes. In this proof of concept, signal transduction was achieved by a colorimetric horseradish peroxidase-tetramethylbenzidine (HRP-TMB-H₂O₂) reaction with HRP that are also being immobilized by one of the linkers. The pair of immobilized enzymes were collagenase and alginate lyase, and they were immobilized by an alginate linker and a short peptide chain containing the amino acid sequence of Leu-Gly-Pro-Ala for collagenase. A detection limit of 2.5 pg collagenase mL^-1 with a wide linear range up to 4 orders of magnitude was achieved. The AMELIE biosensor can detect extracellular collagenase in the supernatant of various bacteria cultures, with a sensitivity as low as 10³ cfu mL^-1 of E. coli. AMELIE can readily be adapted to provide the sensitive detection of other endopeptidases.

The promise of graphene-based transistors for democratizing multiomics studies

As biological research has synthesized genomics, proteomics, metabolomics, and transcriptomics into systems biology, a new multiomics approach to biological research has emerged. Today, multiomics studies are challenging and expensive. An experimental platform that could unify the multiple omics approaches to measurement could increase access to multiomics data by enabling more individual labs to successfully attempt multiomics studies. Field effect biosensing based on graphene transistors have gained significant attention as a potential unifying technology for such multiomics studies. This review article highlights the outstanding performance characteristics that makes graphene field effect transistor an attractive sensing platform for a wide variety of analytes important to system biology. In addition to many studies demonstrating the biosensing capabilities of graphene field effect transistors, they are uniquely suited to address the challenges of multiomics studies by providing an integrative multiplex platform for large scale manufacturing using the well-established processes of semiconductor industry. Furthermore, the resulting digital data is readily analyzable by machine learning to derive actionable biological insight to address the challenge of data compatibility for multiomics studies. A critical stage of systems biology will be democratizing multiomics study, and the graphene field effect transistor is uniquely positioned to serve as an accessible multiomics platform.

Detection of cholera toxin with surface plasmon field-enhanced fluorescent spectroscopy

In this work, a biosensor based on surface plasmon field-enhanced florescence spectroscopy (SPFS) method was successfully constructed to detect the truncated form of cholera toxin, that is, its beta subunit (CTX-B). CTX-B is a relatively small molecule (12 kDa) and it was chosen as model analyte for the detection of protein toxins originated from waterborne pathogens. Recognition layer was prepared on gold-coated LaSFN9 glasses modified with 11-mercaptoundecanoic acid (11-MUA). Biotin-conjugated anti-CTX-B polyclonal antibody (B-Ab) was immobilized on streptavidin (SA) layer constructed on the 11-MUA-modified surface. CTX-B amount was determined with direct assay using B-Ab in surface plasmon resonance (SPR) mode and with sandwich assay in SPFS mode using Cy5-conjugated anti-CTX-B polyclonal antibody. Minimum detected CTX-B concentrations were 10 and 0.01 μg/ml with SPR and SPFS, respectively, showing the sensitivity of the SPFS system over the conventional one. The detection was done in 2-6 h, which was faster than both culture and polymerase chain reaction (PCR)-based methods. Stability tests were performed with SA-coated sensors (excluding B-Ab). In this form, the layer was stable after 30 days of storage in phosphate-buffered saline (PBS; 0.01 M, pH = 7.4) at +4°C. B-Ab layer was formed immediately on them before each measurement.

On-Chip Electrochemical Detection of Cholera Using a Polypyrrole-Functionalized Dendritic Gold Sensor

Rapid diagnosis of an infectious disease outbreak in the field is critical for limiting the escalation of an outbreak into an epidemic. Devices suited to point-of-care (POC) diagnosis of cholera must not only demonstrate clinical laboratory levels of sensitivity and specificity but do so in a portable and low-cost manner, with a simplistic readout. We report work toward an on-chip electrochemical immunosensor for the detection of cholera toxin subunit B (CTX), based on a dendritic gold architecture biofunctionalized via poly(2-cyanoethyl)pyrrole (PCEPy). The dendritic electrode has an ∼18× greater surface area than a planar gold counterpart, per electrochemical measurements, allowing for a higher level of detection sensitivity. A layer of PCEPy polymer generated on the dendritic surface facilitated the performance of an electrochemical enzyme-linked immunosorbant assay (ELISA) for CTX on-chip, which demonstrated a detection limit of 1 ng mL^-1, per a signal-to-noise ratio of 2.6. This was more sensitive than detection using a simple planar gold electrode (100 ng mL^-1) and also matched the diagnostic standard optical ELISA, but on a miniaturized platform with electrical readout. The ability to meet POC demands makes biofunctionalized gold dendrites a promising architecture for on-chip detection of cholera.

Cardiac Troponin I: Ultrasensitive Detection Using Faradaic Electrochemical Impedance

An electrochemical biosensor for the detection of cardiac troponin I, cTnI, an important cardiac biomarker, is described. A combination of a novel monoclonal antibody, mAb20B3, and a novel Ir(III)-based metal complex was used for detection using faradaic electrochemical impedance spectroscopy. A limit of detection of 10 ag/mL was achieved, which is significantly lower than established assays. The ability to detect these ultralow concentrations enables rapid and early stage detection of cardiac events and opens up the possibility of developing a point-of-care device.

Nanotechnology-Based Surface Plasmon Resonance Affinity Biosensors for In Vitro Diagnostics

In the last decades, in vitro diagnostic devices (IVDDs) became a very important tool in medicine for an early and correct diagnosis, a proper screening of targeted population, and also assessing the efficiency of a specific therapy. In this review, the most recent developments regarding different configurations of surface plasmon resonance affinity biosensors modified by using several nanostructured materials for in vitro diagnostics are critically discussed. Both assembly and performances of the IVDDs tested in biological samples are reported and compared.

Personalizing Biomaterials for Precision Nanomedicine Considering the Local Tissue Microenvironment

New advances in (nano)biomaterial design coupled with the detailed study of tissue-biomaterial interactions can open a new chapter in personalized medicine, where biomaterials are chosen and designed to match specific tissue types and disease states. The notion of a "one size fits all" biomaterial no longer exists, as growing evidence points to the value of customizing material design to enhance (pre)clinical performance. The complex microenvironment in vivo at different tissue sites exhibits diverse cell types, tissue chemistry, tissue morphology, and mechanical stresses that are further altered by local pathology. This complex and dynamic environment may alter the implanted material's properties and in turn affect its in vivo performance. It is crucial, therefore, to carefully study tissue context and optimize biomaterials considering the implantation conditions. This practice would enable attaining predictable material performance and enhance clinical outcomes.

Target-triggering multiple-cycle amplification strategy for ultrasensitive detection of adenosine based on surface plasma resonance techniques

An ultrasensitive protocol for surface plasma resonance (SPR) detection of adenosine is designed with the aptamer-based target-triggering cascade multiple cycle amplification, and streptavidin-coated Au-NPs (Au NPs-SA) enhancement to enhance the SPR signals. The cascade amplification process consists of the aptamer-based target-triggering nicking enzyme signaling amplification (T-NESA), the nicking enzyme signaling amplification (NESA) and the hybridization chain reaction (HCR), the entire circle amplification process is triggered by the target recognition of adenosine. Upon recognition of the aptamer to target adenosine, DNA s1 is released from the aptamer and then hybridizes with hairpin DNA (HP1). The DNA s1 can be dissociated from HP1 under the reaction of nicking endonuclease to initiate the next hybridization and cleavage process. Moreover, the products of the upstream cycle (T-NESA) (DNA s2 and s3) could act as the "DNA trigger" of the downstream cycle (NESA and HCR) to generate further signal amplification, resulting in the immobilization of abundant Au NPs-SA on the gold substrate, and thus significant SPR enhancement is achieved due to the electronic coupling interaction between the localized surface plasma of Au NPs and the surface plasma wave. This detection method exhibits excellent specificity and sensitivity toward adenosine with a detection limit of 4 fM. The high sensitivity and specificity make this method a great potential for detecting biomolecules with trace amounts in bioanalysis and clinical biomedicine.

Quantitative detection of Vibrio cholera toxin by real-time and dynamic cytotoxicity monitoring

We report here the quantitative detection of Vibrio cholerae toxin (CT) in isolates and stool specimens by dynamic monitoring of the full course of CT-mediated cytotoxicity in a real-time cell analysis (RTCA) system. Four cell lines, including Y-1 mouse adrenal tumor cells, Chinese hamster ovary (CHO) cells, small intestine epithelial (FHs74Int) cells, and mouse adrenal gland (PC12-Adh) cells, were evaluated for their suitability for CT-induced cytotoxicity testing. Among them, the Y-1 line was demonstrated to be the most sensitive for CT-mediated cytotoxicity, with limits of detection of 7.0 pg/ml for purified CT and 0.11 ng/ml for spiked CT in pooled negative stool specimens. No CT-mediated cytotoxicity was observed for nontoxigenic V. cholerae, non-V. cholerae species, or non-V. cholerae enterotoxins. The CT-RTCA assay was further validated with 100 stool specimens consecutively collected from patients with diarrhea and 200 V. cholerae isolates recovered from patients and the environment, in comparison to a reference using three detection methods. The CT-RTCA assay had sensitivities and specificities of 97.5% and 100.0%, respectively, for V. cholerae isolates and 90.0% and 97.2% for stool specimens. For stool specimens spiked with CT concentrations ranging from 3.5 pg/ml to 1.8 ng/ml, the inoculation-to-detection time was 1.12 ± 0.38 h, and the values were inversely correlated with CT concentrations (ρ = -1; P = 0.01). The results indicate that the CT-RTCA assay with the Y-1 cell line provides a rapid and sensitive tool for the quantitative detection of CT activities in clinical specimens.

Amplified surface plasmon resonance based DNA biosensors, aptasensors, and Hg2+ sensors using hemin/G-quadruplexes and Au nanoparticles

Thiolated nucleic acid hairpin nanostructures that include in their stem region a "caged" G-quadruplex sequence, and in their single-stranded loop region oligonucleotide recognition sequences for DNA, adenosine monophosphate (AMP), or Hg(2+) ions were linked to bare Au surfaces or to Au nanoparticles (NPs) linked to Au surfaces. The opening of the hairpin nanostructures associated with the bare Au surface by the complementary target DNA, AMP substrate, or Hg(2+) ions, in the presence of hemin, led to the self-assembly of hemin/G-quadruplexes on the surface. The resulting dielectric changes on the surface exhibited shifts in the surface plasmon resonance (SPR) spectra, thus providing a readout signal for the recognition events. A similar opening of the hairpin nanostructures, immobilized on the Au NPs associated with the Au surface, by the DNA, AMP, or Hg(2+) led to an ultrasensitive SPR-amplified detection of the respective analytes. The amplification originated from the coupling between the localized surface plasmon associated with the NPs and the surface plasmon wave, an effect that cooperatively amplifies the SPR shifts that result from the formation of the hemin/G-quadruplexes. The different sensing platforms reveal impressive sensitivities and selectivities toward the target analytes.

Binding assay for cholera toxin based on sequestration electrochemistry using lactose labeled with an electroactive compound

A simple electrochemical binding assay for cholera toxin (CT) was developed using lactose labeled with daunomycin as an electroactive compound. The labeled lactose (LL) was determined with high sensitivity by adsorptive stripping voltammetry (AdSV). The electrochemical behaviors of LL at glassy carbon (GC), plastic formed carbon (PFC) and carbon nanotubes paste (CNTP) electrode were investigated. The CNTP electrode showed the greatest accumulation capacity for LL. The assay for CT based on the sequestration electrochemistry was demonstrated. The binding event of the LL to CT was detected by the decrease in the electrochemical response of daunomycin as an electroactive label without a separation process to remove the free LL from the one bound with CT before any measurements can be made. The detection limit of the CT assay using the CNTP electrode was 0.5 nM (42 ng mL(-1)).

Molecularly imprinted Au nanoparticles composites on Au surfaces for the surface plasmon resonance detection of pentaerythritol tetranitrate, nitroglycerin, and ethylene glycol dinitrate

Molecularly imprinted Au nanoparticles (NPs) composites are generated on Au-coated glass surfaces. The imprinting process involves the electropolymerization of thioaniline-functionalized Au NPs (3.5 nm) on a thioaniline monolayer-modified Au surface in the presence of a carboxylic acid, acting as a template analogue for the respective explosive. The exclusion of the imprinting template from the Au NPs matrix yields the respective imprinted composites. The binding of the analyte explosives to the Au NPs matrixes is probed by surface plasmon resonance spectroscopy, SPR, where the electronic coupling between the localized plasmon of the Au NPs and the surface plasmon wave leads to the amplification of the SPR responses originating from the dielectric changes of the matrixes upon binding of the different explosive materials. The resulting imprinted matrixes reveal high affinities and selectivity toward the imprinted explosives. Using citric acid as an imprinting template, Au NPs matrixes for the specific analysis of pentaerythritol tetranitrate (PETN) or of nitroglycerin (NG) were prepared, leading to detection limits of 200 fM and 20 pM, respectively. Similarly, using maleic acid or fumaric acid as imprinting templates, high-affinity sensing composites for ethylene glycol dinitrate (EGDN) were synthesized, leading to a detection limit of 400 fM for both matrixes.

EIS microfluidic chips for flow immunoassay and ultrasensitive cholera toxin detection

A flow-injection impedimetric immunosensor for the sensitive, direct and label-free detection of cholera toxin is reported. A limit of detection smaller than 10 pM was achieved, a value thousands of times lower than the lethal dose. The developed chips fulfil the requirement of low cost and quick reply of the assay and are expected to enable field screening, prompt diagnosis and medical intervention without the need of specialized personnel and expensive equipment, a perspective of special relevance for use in developing countries. Since the chip layout includes two sensing areas each one with a 2 × 2 sensor array, our biochips can allow statistical or (alternatively) multiplex analysis of biorecognition events between antibodies immobilized on each working electrode and different antigens flowing into the chamber.

Surface plasmon resonance analysis of antibiotics using imprinted boronic acid-functionalized Au nanoparticle composites

Au nanoparticles (NPs) are functionalized with thioaniline electropolymerizable groups and (mercaptophenyl)boronic acid. The antibiotic substrates neomycin (NE), kanamycin (KA), and streptomycin (ST) include vicinal diol functionalities and, thus, bind to the boronic acid ligands. The electropolymerization of the functionalized Au NPs in the presence of NE, KA, or ST onto Au surfaces yields bisaniline-cross-linked Au NP composites that, after removal of the ligated antibiotics, provide molecularly imprinted matrixes which reveal high sensitivities toward the sensing of the imprinted antibiotic analytes (detection limits for analyzing NE, KA, and ST correspond to 2.00 +/- 0.21 pM, 1.00 +/- 0.10 pM, and 200 +/- 30 fM, respectively). The antibiotics are sensed by surface plasmon resonance (SPR) spectroscopy, where the coupling between the localized plasmon of the NPs and the surface plasmon wave associated with the Au surface is implemented to amplify the SPR responses. The imprinted Au NP composites are, then, used to analyze the antibiotics in milk samples.

Sub-attomolar detection of cholera toxin using a label-free capacitive immunosensor

A label-free immunosensor for the direct detection of cholera toxin (CT) at sub-attomolar level has been developed based on potential-step capacitance measurements. Anti-CT antibody was adsorbed on gold nanoparticles (AuNPs) incorporated on a polytyramine-modified gold electrode. The concentration of CT was determined by detecting the change of capacitance caused by the formation of antibody-antigen complexes. By using AuNPs adsorbed to the sensing surface, the signal was dramatically increased leading to a significantly more sensitive assay. In fact, under optimum conditions the immunosensor could detect CT concentration with a limit of detection of 9 x 10(-20)M or 0.09 aM, with a dynamic range between 0.1 aM and 10 pM. Good analytical reproducibility could be obtained by injecting CT up to 36 times with an RSD of 2.5%. In addition, good performance of the developed immunosensor was achieved when applied to turbid water samples collected from a local stream that were spiked with CT.

Ion-sensitive field-effect transistor for biological sensing

In recent years there has been great progress in applying FET-type biosensors for highly sensitive biological detection. Among them, the ISFET (ion-sensitive field-effect transistor) is one of the most intriguing approaches in electrical biosensing technology. Here, we review some of the main advances in this field over the past few years, explore its application prospects, and discuss the main issues, approaches, and challenges, with the aim of stimulating a broader interest in developing ISFET-based biosensors and extending their applications for reliable and sensitive analysis of various biomolecules such as DNA, proteins, enzymes, and cells.

An ISFET biosensor for the monitoring of maltose-induced conformational changes in MBP

Here we describe an ion sensitive field effect transistor (ISFET) biosensor, which was designed to monitor directly the surface charge of structurally altered maltose binding protein (MBP) upon stimulation with maltose. This study is the first report of the application of a FET biosensor to the monitoring of conformationally changed proteins. Consequently, a significant drop in current on the basis of the charge-dependent capacitance measurement has been clearly observed in response to maltose, but not for the glucose control, thereby indicating that the substrate-specific conformational properties of the target protein could be successfully monitored using the ISFET. Collectively, our results clearly suggest that ISFET provide a high fidelity system for the detection of maltose-induced structural alterations in MBP.

An ultrasensitive chemiluminescence biosensor for cholera toxin based on ganglioside-functionalized supported lipid membrane and liposome

A novel chemiluminescence biosensor based on a supported lipid layer incorporated with ganglioside GM1 was developed for the detection of cholera toxin. The planar supported lipid membrane was prepared as biosensing interface via spontaneous spread of ganglioside-incorporated phospholipid vesicles on the octadecanethiol-coated gold surface. The specific interaction of multivalent CT by ganglioside GM1 molecules enables the biosensor to be implemented via a sandwiched format using a liposome probe functionalized with GM1 and horseradish peroxidase (HRP). Then, the presence of the target CT could be determined via the HRP-catalyzed enhanced chemiluminescence reaction. The developed strategy offers several unique advantages over conventional biosensors in that it allows for an easy construction and renewal of the sensing interface, a small background signal due to low non-specific adsorption of serum constituents on the lipid membrane, and effective immobilization of multiple biocatalytic amplifiers and recognition components via common phospholipid reagents. The developed biosensor was shown to give chemiluminescence signal in linear correlation to CT concentration within the range from 1pgmL(-1) to 1ngmL(-1) with readily achievable detection limit of 0.8pgmL(-1).

Parallel analysis of two analytes in solutions or on surfaces by using a bifunctional aptamer: applications for biosensing and logic gate operations

A bifunctional aptamer that includes two aptamer units for cocaine and adenosine 5'-monophosphate (AMP) is blocked by a nucleic acid to form a hybrid structure with two duplex regions. The blocked bifunctional aptamer assembly is used as a functional structure for the simultaneous sensing of cocaine or AMP. The blocked bifunctional aptamer is dissociated by either of the two analytes, and the readout of the separation of the sensing structure is accomplished by a colorimetric detection, by a released DNAzyme, or by electronic means that use Faradaic impedance spectroscopy or field-effect transistors. In one configuration, the blocked bifunctional aptamer structure is separated by the substrates cocaine or AMP, and the displaced blocker units act as a horseradish peroxidase-mimicking DNAzyme that permits the colorimetric detection of the analytes. In the second system, the blocked bifunctional aptamer hybrid is associated with a Au electrode. The displacement of the aptamer by any of the substrates alters the interfacial electron transfer resistance at the electrode surface, thus providing an electronic signal for the sensing process. In the third configuration, the blocked aptamer hybrid is linked to the gate of a field-effect transistor device. The separation of the complex by means of any of the analytes, cocaine, or AMP alters the gate potential, and this allows the electronic transduction of the sensing process by following the changes in the gate-to-source potentials. The different systems enable not only the simultaneous detection of the two analytes, but they provide a functional assembly that performs a logic gate "OR" operation.

Aspects of recent development of immunosensors

This chapter focuses on the recent developments in the field of immunosensors. Immunosensors incorporate the specific immunochemical reaction with the modern transducers including electrochemical (potentiometric, conductometric, capacitative, impedance, amperometric), optical (fluorescence, luminescence, refractive index), and microgravimetric transducers. These immunosensor devices with dramatic improvements in the sensitivity and selectivity possess the abilities to investigate the reaction dynamics of antibody–antigen binding and the potential to revolutionize conventional immunoassay techniques. With the rapid development of immunological reagents and detection equipments, immunosensors have allowed an increasing range of analytes to be identified and quantified and in particular, simple-to-use, inexpensive, and reliable immunosensing systems have been developed for areas such as outpatient monitoring, large screening programs, and remote environmental surveillance. Immunosensors with lowered detection limits and increased sensitivities have been developed in various fields, particularly in clinical analysis. A noticeable development trend is also observed in the development of immunosensors combining with other techniques such as flow injection analysis (FIA) or capillary electrophoretic (CE) analysis, which complement and improve the present immunoassay methods. Belov et al. have proposed a novel immunophenotyping method for leukemias which uses a cluster of differentiation antibody microarray, and a microarray of enzyme-linked immunosorbent assay has been developed for autoimmune diagnosis of systematic rheumatic disease. Development of microfluidic immunosensor systems for proteomics and drug discovery have also been reported in recent years where the microfluidic system integrates multiple processes in a single device to improve analytical performance by reducing the reagent consumption and the analysis time, and increasing reliability and sensitivity through automation.

Analysis of Dopamine and Tyrosinase Activity on Ion-Sensitive Field-Effect Transistor (ISFET) Devices

Dopamine (1) and tyrosinase (TR) activities were analyzed by using chemically modified ion-sensitive field-effect transistor (ISFET) devices. In one configuration, a phenylboronic acid functionalized ISFET was used to analyze 1 or TR. The formation of the boronate-1 complex on the surface of the gate altered the electrical potential associated with the gate, and thus enabled 1 to be analyzed with a detection limit of 7x10(-5) M. Similarly, the TR-induced formation of 1, and its association with the boronic acid ligand allowed a quantitative assay of TR to be performed. In another configuration, the surface of the ISFET gate was modified with tyramine or 1 to form functional surfaces for analyzing TR activities. The TR-induced oxidation of the tyramine- or 1-functionalized ISFETs resulted in the formation of the redox-active dopaquinone units. The control of the gate potential by the redox-active dopaquinone units allowed a quantitative assay of TR to be performed. The dopaquinone-functionalized ISFETs could be regenerated to give the 1-modified sensing devices by treatment with ascorbic acid.

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.

Glyconanoparticles for the colorimetric detection of cholera toxin

Cholera continues to represent a major threat to human health, particularly in developing countries. Death can be readily avoided when medical treatment is rapidly administered. In order to provide a means of detecting the bacterially secreted toxin, we have developed a simple, yet rapid, bioassay for the cholera toxin. The colorimetric bioassay is based on a specifically synthesized lactose derivative that is self-assembled onto gold nanoparticles of 16 nm diameter. In solution the lactose-stabilized nanoparticles are red in color due to the intense surface plasmon absorption band centered at 524 nm. Cholera toxin (added as the B-subunit) (CTB) binds to the lactose derivative and induces aggregation of the nanoparticles. Upon aggregation, the surface plasmon absorption band broadens and red shifts such that the nanoparticle solution appears a deep purple color. The selectivity of the bioassay stems from the thiolated lactose derivative that mimics the GM(1) ganglioside--the receptor to which cholera toxin binds in the small intestine. Consequently, added metal ions, anions, and a protein, at relevant concentrations, do not induce nonspecific aggregation of the nanoparticles. The simple color change of the bioassay provides a selective means to detect and quantify the cholera toxin within 10 min. The theoretical limit of detection of the bioassay was determined to be 54 nM (3 microg/mL) for CTB. The stability of the lactose-stabilized nanoparticles was established by freeze-drying and then resuspending the particles in water and subsequently measuring CTB in biologically relevant electrolyte solutions. This colorimetric bioassay provides a new tool for the direct measurement of cholera toxin.

Label-free electrochemical detection for aptamer-based array electrodes

An electrochemical impedance spectroscopy method of detection for aptamer-based array electrodes is reported in which the binding of aptamers immobilized on gold electrodes leads to impedance changes associated with target protein binding events. Human IgE was used as a model target protein and incubated with the aptamer-based array consisting of single-stranded DNA containing a hairpin loop. To increase the binding efficiency for proteins, a hybrid modified layer containing aptamers and cysteamine was fabricated on the photolithographic gold surface through molecular self-assembly. Atomic force microscopy analysis demonstrated that human IgE could be specifically captured by the aptamer and stand well above the self-assembled monolayer (SAM) surface. Compared to immunosensing methods using anti-human IgE antibody as the recognition element, impedance spectroscopy detection could provide higher sensitivity and better selectivity for aptamer-modified electrodes. The results of this method show good correlation for human IgE in the range of 2.5-100 nM. A detection limit of 0.1 nM (5 fmol in a 50-microL sample) was obtained, and an average of the relative standard deviation was <10%. The method herein describes the first label-free detection for arrayed electrodes utilizing electrochemical impedance spectroscopy.

Electrochemical strategies for the label-free detection of amino acids, peptides and proteins

Electrochemical methods for the detection of amino acids, peptides, and proteins in a variety of media are reviewed. Label-free strategies in which the detection is based on the inherent electrochemical properties of the analyte are discussed. Various processes such as direct or mediated (in solution or immobilised) redox processes and interfacial ion transfers have been employed for the electrochemical detection and determination of the target analytes. The various methods covered encompass voltammetry at uncoated and modified electrodes and at immiscible liquid-liquid interfaces, potentiometry at polymer membrane electrodes and electrochemical impedance spectroscopy. The determination of the target analytes in complex biological matrices is discussed. The various approaches highlighted here illustrate the rich capabilities of electrochemical methods as simple, low-cost, sensitive tools for the determination of these important biological analytes at trace and ultra-trace levels.

Polarizability and Kerr constant of proteins by boundary element methods

A precise implementation of the boundary element method has been applied to the computation of the polarizability and the Kerr constant of eight soluble proteins. The method is demonstrated to be accurate and precise by comparison with analytical values for spheroids. Two different integral equations have been solved: (1) an exact equation with explicit dielectric constant dependence, and (2) an exact equation for a metallic body. The dielectric dependence for the metallic body case is built in with a generalization of the ellipsoid formula. Both methods agree quantitatively with each other for low relative dielectric constants. A full tensor expression for the Kerr constant yields perfect agreement with experiment for some proteins and badly under reports for the rest. The protein structure is obtained from a crystallographic database and is assumed rigid throughout the TEB measurement. Electrolyte effects are neglected. The Kerr constant is dominated by the protein dipole moment and is quite sensitive to the orientation of the dipole moment relative to the principal axes of the polarizability tensor. Several possible reasons for the large discrepancy between some computed and measured values are discussed.

Determination of Electron Transfer Kinetic Parameters by Fourier Transform Electrochemical Impedance Spectroscopic Analysis

A new attempt to obtain electron transfer kinetic parameters at an electrified electrode/electrolyte interface using Fourier transform electrochemical impedance spectroscopic (FTEIS) analyses of small potential step chronoamperometric currents is presented. The kinetic parameters thus obtained allowed mass transport free voltammograms to be constructed in an overpotential region, where the diffusion limits the electron transfer reaction, using the Butler-Volmer (B-V) relation. The B-V voltammograms clearly distinguish electrode reactions that are not much different in their electron transfer kinetic parameters, thus showing very similar normal linear sweep voltammetric (SCV) behaviors. Electrochemical reduction of p-benzoquinone, which displays nearly the same SCV responses at a gold electrode regardless whether the electrode is covered by a thiolated beta-cyclodextrin self-assembled monolayer, was taken as an example for the demonstration. The results show that the two voltametrically similar systems display very different electron transfer characteristics.