Principles of affinity-based biosensors

Despite the amount of resources that have been invested by national and international academic, government, and commercial sectors to develop affinity-based biosensor products, little obvious success has been realized through commercialization of these devices for specific applications (such as the enzyme biosensors for blood glucose analysis). Nevertheless, the fastest growing area in the biosensors research literature continues to involve advances in affinity-based biosensors and biosensor-related methods. Numerous biosensor techniques have been reported that allow researchers to better study the kinetics, structure, and (solid/liquid) interface phenomena associated with protein-ligand binding interactions. In addition, potential application areas for which affinity-based biosensor techniques show promise include clinical/diagnostics, food processing, military/antiterrorism, and environmental monitoring. The design and structural features of these devices--composed of a biological affinity element interfaced to a signal transducer--primarily determine their operational characteristics. This paper, although not intended as a comprehensive review, will outline the principles of affinity biosensors with respect to potential application areas.

Peer reviewed: update on environmental biosensors

Scientific understanding and technological development are advancing, but commercialization, with a few exceptions, has been slow.

Detection of low dose radiation induced DNA damage using temperature differential fluorescence assay

A rapid and sensitive fluorescence assay for radiation-induced DNA damage is reported. Changes in temperature-induced strand separation in both calf thymus DNA and plasmid DNA (puc 19 plasmid from Escherichia coli) were measured after exposure to low doses of radiation. Exposures of between 0.004 and 1 Gy were measured with doses as low as 0.008 Gy yielding significant responses. The double-strand, sensitive dye PicoGreen was used as an indicator of DNA denaturation. Calibration plots indicate that fluorescence changes corresponding to amounts as low as 1 ng of double stranded DNA (10(6) copies for plasmid puc 19) are detected by this method.

Thick-film electrochemical immunosensor based on stripping potentiometric detection of a metal ion label

A disposable electrochemical immunosensor based on potentiometric stripping analysis (PSA) of a metal tracer and using an entirely on-chip assay format is demonstrated. Challenges associated with the adaptation of earlier stripping voltammetric immunoassays to an on-chip operation, and with meeting the demands of decentralized testing, have been addressed. These include the surface immobilization of the antibody, the replacement of mercury drop electrodes, elimination of the separation and oxygen-removal steps, and the use of quiescent 30-microL sample droplets. Human serum albumin (HSA) and anti-HSA antibody were used as a model system, while bismuth ion served as the metal label. The anti-HSA was immobilized onto the surface of a thick-film electrode, followed by a competition between the Bi-labeled analyte-tracer and the analyte (HSA) for the antibody binding sites. Upon removal of the unbound tracer, Bi3+ was released and detected by PSA. The dynamic concentration range for HSA (0.3-30 micrograms/mL) and the detection limit (0.2 microgram/mL, i.e., 90 fmol in the 30-microL sample) indicate that the greatly simplified protocol does not compromise the performance characteristics of stripping immunoassays. Consequently, this on-chip operation offers great promise for decentralized (clinical and environmental) applications.

Sol-gel-derived thick-film amperometric immunosensors

Sol-gel processing is used for the first time for the preparation of electrochemical immunosensors. One-step sensor fabrication, based on the coupling of sol-gel and screen-printing technologies, is employed. A low-temperature cured ink is prepared by dispersion of rabbit immunoglobulin G (RIgG), graphite powder, and a binder in the sol-gel solution. The enzyme-labeled antibody can readily diffuse toward the encapsulated antigen, which retains its binding properties, and the association reaction is easily detected at the dispersed graphite surface. Use of anti-RIgG labeled with alkaline phosphatase, naphthyl phosphate as the substrate, and amperometric detection at +400 mV (vs Ag/AgCl) results in a low detection limit of 5 ng/mL (32 pM) for the solution antigen. Tailoring the porosity of the ceramic-carbon matrix can be used for tuning the assay performance. The high sensitivity, low cost, durability, and simplicity of the new single-use immunosensors make them well suited for various on-site applications.

The secretory epithelial cells of the choroid plexus employ a novel kinesin-related protein

The proteins of the kinesin superfamily (KIFs) are microtubule-based molecular motors whose functions include the transport of membrane-bound organelles. We have isolated the cDNA encoding a novel kinesin by reverse transcription and polymerase chain reaction using degenerate primers that flank the highly conserved motor domain. The deduced amino acid sequence of this protein shows considerable similarity to both KIF1A and KIF1B thus defining it as a new member of the monomeric KIF1/unc104 family. The C-terminal domain of KIF1D is the most divergent by comparison with the other members of the family, which supports the view that the tail region is responsible for conferring specificity on the interactions of these kinesins with their cargoes. In the adult rat brain KIF1D mRNA is expressed in neurons in the hippocampus and in the Purkinje cells of the cerebellum. However, the levels of KIF1D are particularly high in the choroid plexus which is a polarised epithelium that lines the lateral, third and fourth ventricles. The major function of the epithelial cells in the choroid plexus is to produce cerebrospinal fluid, which suggests that KIF1D plays an important role in their secretory function.

Detection of 2,4-dichlorophenoxyacetic acid using a fluorescence immunoanalyzer

A flow immunoassay method for the measurement of 2,4-dichlorophenoxyacetic acid (2,4-D) was developed. The competitive fluorescence immunoassay relies on the use of antibody- or antigen-coated poly(methyl methacrylate) particles (98 microns diameter) as a renewable solid phase. The assay exhibits a dynamic range of 0.1-100 micrograms l-1 using a monoclonal antibody or alternatively 10 micrograms l-1 to 10 mg l-1 using commercially available antiserum. The assay is demonstrated in buffered saline solution as well as in aquatic environmental media. The relative errors for the environmental matrices were similar to those for the buffer control. The precision of concentration values calculated at 1 mg l-1 (for the assay using antiserum) were +/- 0.28, +/- 0.27 and +/- 0.43 mg l-1 for the buffer, well water and river water matrices, respectively. The method shows cross-reactivity with compounds of closely related structure but little cross-reactivity with compounds dissimilar in structure to 2,4-D. The proposed automated competitive immunoassay method is rapid (between 7 and 15 min per assay), simple and potentially portable.

Peer reviewed: environmental biosensors: a status report

New instruments and methods being developed show promise for continuous, in situ monitoring of toxic compounds.

Characterization of disulfide crosslink formation of human vimentin at the dimer, tetramer, and intermediate filament levels

We have investigated the structural interactions of individual molecules of human vimentin in the soluble state and in filaments. Oxidative crosslinking experiments were conducted with wild-type vimentin aimed at the single cysteine in the helical domain coil 2b, mutated cysteine-free vimentin, and derivatives engineered to carry cysteines in presumed d positions of the heptad amino acid repeats in coils 1a and 2b. We provide conclusive evidence that crosslinking of the cysteine 328 in wild-type vimentin, when in the filamentous or tetrameric forms, occurs outside of the coiled-coil dimer, i.e., between staggered dimer molecules. This occurs despite the close axial register of the dimers and contradicts previous deductions. The extent of crosslinking increases with temperature as well as with the concentration of the crosslinking reagent. We conclude therefore that the cysteines are not in an ideal position for crosslinking but that molecular motion is needed to enhance the reaction. The occurrence of collision complexes, which has been speculated in the literature, does not occur and cannot explain these results. Furthermore, using tailless vimentin with the corresponding mutations, we provide compelling evidence that in type III intermediate filament proteins exchange of individual chains between dimers occurs only if the proteins are incubated in urea at concentrations above 3 M. In 5 M urea, however, the exchange is completed within seconds. The same reaction occurs between human vimentin and mouse desmin at a comparable speed, indicating that both type III intermediate filament proteins have a high affinity for one another at the coiled-coil level.

Truncation mutagenesis of the non-alpha-helical carboxyterminal tail domain of vimentin reveals contributions to cellular localization but not to filament assembly

We have investigated the effect of stepwise truncating the carboxyterminal domain ("tail") of the intermediate filament (IF) protein vimentin of the clawed toad, Xenopus laevis, on filament assembly in vitro and, using cell transfection, in vivo and also on the cellular topology of the structures formed. All truncations examined, except the minimal one missing the last 11 amino acids which made the protein more sensitive to changes of ionic strength, did not significantly alter IF assembly in vitro, as judged by electron microscopy, viscometry and determination of viscoelastic properties with a laser-operated torsion pendulum. Stable transfections of vimentin-free mammalian cells with cDNAs encoding these mutations resulted at 28 degrees C, i.e. the permissive temperature for assembly of Xenopus vimentin, in the formation of extended IF bundle arrays. At 37 degrees C, however, the mutants lacking more than the last 35 amino acids could leave the cytoplasm and accumulated in the nucleus, indicating a certain topogenic element is located in the tail and directs cytoplasmic restriction in the wild-type protein although this does not form IFs under these conditions. Transfer to the nucleus is, however, abolished if the IF-consensus motif at the end of the rod domain is removed, suggesting that this part of the molecule also contributes to nuclear location. Similar results were obtained with human vimentin: While the rod entered the nucleus, headless vimentin, unable to form IFs, remained restricted to the cytoplasm owing to its tail domain. In contrast, tailless human vimentin and tailless mouse desmin, which are fully assembly-competent in vitro, both formed extensive IF arrays in the cytoplasm but did not accumulate in the nucleus. We conclude that in class III IF proteins stepwise deletions in the tail, while not considerably altering IF assembly in vitro, can change the topogenesis of IF proteins and structures in the living cell.

Detection of nicotinic receptor ligands with a light addressable potentiometric sensor

The nicotinic acetylcholine receptor, purified from Torpedo electric organ, was coupled to a light addressable potentiometric sensor (LAPS) to form a LAPS-receptor biosensor. Receptor-ligand complexes containing biotin and urease were captured on a biotinylated nitrocellulose membrane via a streptavidin bridge and detected with a silicon-based sensor. Competition between biotinylated alpha-bungarotoxin and nonbiotinylated ligands formed the basis of this assay. This biosensor detected both agonists (acetylcholine, carbamylcholine, succinylcholine, suberyldicholine, and nicotine) and competitive antagonists (d-tubocurarine, alpha-bungarotoxin, and alpha-Naja toxin) of the receptor with affinities comparable to those obtained using radioactive ligand binding assays. Consistent with agonist-induced desensitization of the receptor, the LAPS-receptor biosensor reported a time-dependent increase in affinity for the agonist carbamylcholine as expected, but not for the antagonists.

Biosensors for environmental monitoring

In this article we will outline several biosensor applications which may fill existing technology gaps in the area of environmental monitoring. The requirements for these environmental biosensors, as well as difficulties in commercialization, are also addressed.

Acetylcholinesterase fiber-optic biosensor for detection of anticholinesterases

An optical sensor for anticholinesterases (AntiChEs) was constructed by immobilizing fluorescein isothiocyanate (FITC)-tagged eel electric organ acetylcholinesterase (AChE) on quartz fibers and monitoring enzyme activity. The pH-dependent fluorescent signal generated by FITC-AChE, present in the evanescent zone on the fiber surface, was quenched by the protons produced during acetylcholine (ACh) hydrolysis. Analysis of the fluorescence response showed Michaelis-Menten kinetics with a Kapp value of 420 microM for ACh hydrolysis. The reversible inhibitor edrophonium (0.1 mM) inhibited AChE and consequently reduced fluorescence quenching. The biosensor response immediately recovered upon its removal. The carbamate neostigmine (0.1 mM) also inhibited the biosensor response but recovery was much slower. In the presence of ACh, the organophosphate (OP) diisopropylfluorophosphate (DFP) at 0.1 mM did not interfere with the ACh-dependent fluorescent signal quenching, but preexposure of the biosensor to DFP in absence of ACh inhibited totally and irreversibly the biosensor response. However, the DFP-treated AChE biosensor recovered fully after a 10-min perfusion with pralidoxime (2-PAM). Echothiophate, a quaternary ammonium OP, inhibited the ACh-induced fluorescence quenching in the presence of ACh and the phosphorylated biosensor was reactivated with 2-PAM. These effects reflected the mechanism of action of the inhibitors with AChE and the inhibition constants obtained were comparable to those from colorimetric methods. The biosensor detected concentrations of the carbamate insecticides bendiocarb and methomyl and the OPs echothiophate and paraoxon in the nanomolar to micromolar range. Malathion, parathion, and dicrotophos were not detected even at millimolar concentrations; however, longer exposure or prior modification of these compounds (i.e., to malaoxon, paraoxon) may increase the biosensor detection limits. This AChE biosensor is fast, sensitive, reusable, and relatively easy to operate. Since the instrument is portable and can be self-contained, it shows potential adaptability to field use.

Oxidation of thiol in the vimentin cytoskeleton

Sublethal doses of H2O2, which induces oxidative stress, cause substantial alteration to the vimentin cytoskeleton in various cell types. We have used a thiol-blot assay to assess thiol status in individual proteins from cell extracts. Vimentin thiol is oxidized in preference to other cytoskeleton proteins. Immunoblot analysis also demonstrated a loss of reactivity to an anti-vimentin monoclonal antibody under non-reducing conditions, possibly due to thiol-group oxidation. During induced oxidative stress a number of proteins become associated with the cytoskeleton extracts.

Pharmacological specificity of a nicotinic acetylcholine receptor optical sensor

The pharmacological specificity of a nicotinic acetylcholine receptor (nAChR) optical biosensor was investigated using three fluorescein isothiocyanate (FITC)-tagged neurotoxic peptides that vary in the reversibility of their receptor inhibition: alpha-bungarotoxin (alpha-BGT), alpha-Naja toxin (alpha-NT), and alpha-conotoxin (GI) (alpha-CNTX). Kinetic analysis of the time course of binding of FITC-neurotoxins to the nAChR-coated fiber gave association rate constants (k+1) of 8.4 x 10(6) M-1 min-1 for FITC-alpha-BGT, 6.0 x 10(6) M-1 min-1 for FITC-alpha-NT and 1.4 x 10(6) M-1 min-1 for FITC-alpha-CNTX. The dissociation rate constants (k-1) for the three neurotoxins were 7.9 x 10(-3) min-1. 4.8 x 10(-2) min-1 and 8.0 x 10(-1) min-1 for FITC-alpha-BGT. FITC-alpha-NT and FITC-alpha-CNTX, respectively. The equilibrium dissociation constant (Kd) values for the three toxins. calculated from these rare constants, were similar to published values obtained from tissue responses or ligand binding assays. The optical signal generated by FITC-alpha-NT binding to the nAChR-coated fiber was effectively quenched by agonists and antagonists of the nAChR but not by most of the tested agonists and antagonists of muscarinic cholinergic, adrenergic, glutamatergic, serotonergic, dopaminergic or GABAergic receptors. Interestingly, 5-hydroxy-tryptamine, haloperidol and (+)cis-methyldioxolane gave significant inhibition of FITC-alpha-NT binding to the immobilized receptor. Equilibrium constants of inhibition (Ki) for d-tubocurarine (d-TC) and carbamylcholine (carb) were determined from competition studies using FITC-alpha-CNTX. FITC-alpha-NT or FITC-alpha-BGT as probes for receptor occupancy. When the more reversible probe FITC-alpha-CNTX was used, the Ki value for d-TC was an order of magnitude lower than those determined using the less reversible probes. Ki values for carb however, were independent of the FITC-toxin probe used.

Effects of receptor concentration, media pH and storage on nicotinic receptor-transmitted signal in a fiber-optic biosensor

The optical signal generated by a nicotinic acetylcholine receptor (nAChR)-based optical biosensor was dependent on the density of toxin binding sites (i.e. receptors) immobilized on the surface of the fiber. The maximum density of nAChR receptors absorbed on the optic fibers, measured using [125I]-alpha-bungarotoxin (the alpha-neurotoxin of the Bungarus snake venom) binding, was 6.2 pmol of receptor sites per fiber. Incubation time, that was required for maximal noncovalent immobilization of the receptor protein on the fiber, was less than 10 min. Immobilization of the nAChR protein on the quartz fiber was affected by pH of the medium, with pH 3.5 as the optimal. Stored in phosphate buffered saline (PBS) or in a dry nitrogen atmosphere, the nAChR optical sensor showed no loss of activity over the first 3 days, then showed a slow but gradual loss in activity (45% over the next 30 days).