Evaluation of the molecular recognition of monoclonal and polyclonal antibodies for sensitive detection of 2,4,6-trinitrotoluene (TNT) by indirect competitive surface plasmon resonance immunoassay

Emergent Biosensing Technologies Based on Fluorescence Spectroscopy and Surface Plasmon Resonance

The purpose of this work is to provide an exhaustive overview of the emerging biosensor technologies for the detection of analytes of interest for food, environment, security, and health. Over the years, biosensors have acquired increasing importance in a wide range of applications due to synergistic studies of various scientific disciplines, determining their great commercial potential and revealing how nanotechnology and biotechnology can be strictly connected. In the present scenario, biosensors have increased their detection limit and sensitivity unthinkable until a few years ago. The most widely used biosensors are optical-based devices such as surface plasmon resonance (SPR)-based biosensors and fluorescence-based biosensors. Here, we will review them by highlighting how the progress in their design and development could impact our daily life.

Phosphorescent palladium-tetrabenzoporphyrin indicators for immunosensing of small molecules with a novel optical device

Small-molecule detection is important for many applications including clinical diagnostics, drug discovery, environmental screening, and food technology. Current techniques suffer from various limitations including cost, complex sample processing, massive instrumentation, and need for expertise. To overcome these limitations, a new optical immunosensing assay for the detection of small molecules was developed and assessed with the targets estrone (E1) and estradiol (E2). For this purpose, phosphorescent indicators were designed based on the tetrabenzoporphyrin skeleton directly linked to E1 or E2, or attached through a linker, with phosphorescence lifetimes in the range of ~100-~300 μs. The assay is an indicator displacement assay (IDA). The best performances of our optical immunosensor were obtained with the indicators E1-L-Por and E2-L-Por. As they bound to specific polyclonal antibodies, their phosphorescence (τ ~200 μs) was quenched. When an endogenous competitor was added, the indicator was displaced, and the phosphorescence was immediately recovered. These effects were measured with a new optical device, described here, and able to detect picograms of luminescent molecules emitting in the NIR range, simply by measuring phosphorescence decay. This radical switch-off/switch on process demonstrates that E1-L-Por and E2-L-Por are good candidates for in vivo and in vitro immunosensing of E1 and E2. Importantly, the present immunosensing assay can be easily adapted to other small molecules such as other hormones and drugs.

Fast Detection of 2,4,6-Trinitrotoluene (TNT) at ppt Level by a Laser-Induced Immunofluorometric Biosensor

The illegal use of explosives by terrorists and other criminals is an increasing issue in public spaces, such as airports, railway stations, highways, sports venues, theaters, and other large buildings. Security in these environments can be achieved by different means, including the installation of scanners and other analytical devices to detect ultra-small traces of explosives in a very short time-frame to be able to take action as early as possible to prevent the detonation of such devices. Unfortunately, an ideal explosive detection system still does not exist, which means that a compromise is needed in practice. Most detection devices lack the extreme analytical sensitivity, which is nevertheless necessary due to the low vapor pressure of nearly all explosives. In addition, the rate of false positives needs to be virtually zero, which is also very difficult to achieve. Here we present an immunosensor system based on kinetic competition, which is known to be very fast and may even overcome affinity limitation, which impairs the performance of many traditional competitive assays. This immunosensor consists of a monolithic glass column with a vast excess of immobilized hapten, which traps the fluorescently labeled antibody as long as no explosive is present. In the case of the explosive 2,4,6-trinitrotoluene (TNT), some binding sites of the antibody will be blocked, which leads to an immediate breakthrough of the labeled protein, detectable by highly sensitive laser-induced fluorescence with the help of a Peltier-cooled complementary metal-oxide-semiconductor (CMOS) camera. Liquid handling is performed with high-precision syringe pumps and chip-based mixing-devices and flow-cells. The system achieved limits of detection of 1 pM (1 ppt) of the fluorescent label and around 100 pM (20 ppt) of TNT. The total assay time is less than 8 min. A cross-reactivity test with 5000 pM solutions showed no signal by pentaerythritol tetranitrate (PETN), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). This immunosensor belongs to the most sensitive and fastest detectors for TNT with no significant cross-reactivity by non-related compounds. The consumption of the labeled antibody is surprisingly low: 1 mg of the reagent would be sufficient for more than one year of continuous biosensor operation.

Engineered Biomolecular Recognition of RDX by Using a Thermostable Alcohol Dehydrogenase as a Protein Scaffold

There are many biotechnology applications that would benefit from simple, stable proteins with engineered biomolecular recognition. Here, we explored the hypothesis that a thermostable alcohol dehydrogenase (AdhD from Pyrococcus furiosus) could be engineered to bind a small molecule instead of a cofactor or molecules involved in the catalytic transition state. We chose the explosive molecule 1,3,5-trinitro-1,3,5-triazine (royal demolition explosive, RDX) as a proof-of-concept. Its low solubility in water was exploited for immobilization for biopanning by using ribosome display. Docking simulations were used to identify two potential binding sites in AdhD, and a randomized library focused on tyrosine or serine mutations was used to determine that RDX was binding in the substrate binding pocket of the enzyme. A fully randomized binding pocket library was selected, and affinity maturation by error-prone PCR led to the identification of a mutant (EP-16) that gained the ability to bind RDX with an affinity of (73±11) μm. These results underscore the way in which thermostable enzymes can be useful scaffolds for expanding the biomolecular recognition toolbox.

Functionalized gold nanoparticle-enhanced competitive assay for sensitive small-molecule metabolite detection using surface plasmon resonance

A functionalized gold nanoparticle-enhanced competitive assay was developed to overcome the sensitivity challenge associated with direct SPR sensing of small-molecule metabolites.

Array-Based Rational Design of Short Peptide Probe-Derived from an Anti-TNT Monoclonal Antibody

Complementarity-determining regions (CDRs) are sites on the variable chains of antibodies responsible for binding to specific antigens. In this study, a short peptide probe for recognition of 2,4,6-trinitrotoluene (TNT), was identified by testing sequences derived from the CDRs of an anti-TNT monoclonal antibody. The major TNT-binding site in this antibody was identified in the heavy chain CDR3 by antigen docking simulation and confirmed by an immunoassay using a spot-synthesis based peptide array comprising amino acid sequences of six CDRs in the variable region. A peptide derived from heavy chain CDR3 (RGYSSFIYWF) bound to TNT with a dissociation constant of 1.3 μM measured by surface plasmon resonance. Substitution of selected amino acids with basic residues increased TNT binding while substitution with acidic amino acids decreased affinity, an isoleucine to arginine change showed the greatest improvement of 1.8-fold. The ability to create simple peptide binders of volatile organic compounds from sequence information provided by the immune system in the creation of an immune response will be beneficial for sensor developments in the future.

High-Throughput, Protein-Targeted Biomolecular Detection Using Frequency-Domain Faraday Rotation Spectroscopy

A clinically relevant magneto-optical technique (fd-FRS, frequency-domain Faraday rotation spectroscopy) for characterizing proteins using antibody-functionalized magnetic nanoparticles (MNPs) is demonstrated. This technique distinguishes between the Faraday rotation of the solvent, iron oxide core, and functionalization layers of polyethylene glycol polymers (spacer) and model antibody-antigen complexes (anti-BSA/BSA, bovine serum albumin). A detection sensitivity of ≈10 pg mL^-1 and broad detection range of 10 pg mL^-1 ≲ c_BSA ≲ 100 µg mL^-1 are observed. Combining this technique with predictive analyte binding models quantifies (within an order of magnitude) the number of active binding sites on functionalized MNPs. Comparative enzyme-linked immunosorbent assay (ELISA) studies are conducted, reproducing the manufacturer advertised BSA ELISA detection limits from 1 ng mL^-1 ≲ c_BSA ≲ 500 ng mL^-1 . In addition to the increased sensitivity, broader detection range, and similar specificity, fd-FRS can be conducted in less than ≈30 min, compared to ≈4 h with ELISA. Thus, fd-FRS is shown to be a sensitive optical technique with potential to become an efficient diagnostic in the chemical and biomolecular sciences.

Multiplex microarray ELISA versus classical ELISA, a comparison study of pollutant sensing for environmental analysis

The present study describes the development, optimization and performance comparison of three ELISAs and one multiplex immunoassay in a microarray format. The developed systems were dedicated to the detection of three different classes of pollutants (pesticide, explosive and toxin) in water. The characteristics and performances of these two types of assays were evaluated and compared, in order to verify that multiplex immunoassays can replace ELISA for multiple analyte sensing. 2,4-Dichlorophenoxyacetic acid, 2,4,6-trinitrotoluene and okadaic acid were chosen as model targets and were immobilized in classical microtiter plate wells or arrayed at the surface of a microarray integrated within a classical 96-well plate. Once optimized, the classical ELISAs and microarray-based ELISA performances were evaluated and compared in terms of limit of detection, IC50, linearity range and reproducibility. Classical ELISAs provided quite good sensitivity (limit of detection down to 10 μg L(-1)), but the multiplex immunoassay was proven to be more sensitive (limit of detection down to 0.01 μg L(-1)), more reproducible and an advantageous tool in terms of cost and time expenses. This multiplex tool was then used for the successful detection of the three target molecules in spiked water samples and achieved very promising recovery rates.

Receptor-based detection of 2,4-dinitrotoluene using modified three-dimensionally ordered macroporous carbon electrodes

Detection of explosives, such as 2,4,6-trinitrotoluene (TNT), is becoming increasingly important. Here, 2,4-dinitrotoluene (DNT, a common analogue of TNT) is detected electrochemically. A receptor based electrode for the detection of DNT was prepared by modifying the surface of the walls of three-dimensionally ordered macroporous (3DOM) carbon. Nitrophenyl groups were first attached by the electrochemical reduction of 4-nitrobenzenediazonium ions, followed by potentiostatic reduction to aminophenyl groups. Chemical functionalization reactions were then performed to synthesize the receptor, which contains two urea groups, and a terminal primary amine. Detection of DNT using cyclic voltammetry was impeded by a large background current that resulted from the capacitance of 3DOM carbon. Detection by square wave voltammetry eliminated the background current and improved the detection limit. Unfunctionalized 3DOM carbon electrodes showed no response to DNT, whereas the receptor-modified electrodes responded to DNT with a detection limit of 10 μM. Detection of DNT was possible even in the presence of interferents such as nitrobenzene.

Small molecule immunosensing using surface plasmon resonance

Surface plasmon resonance (SPR) biosensors utilize refractive index changes to sensitively detect mass changes at noble metal sensor surface interfaces. As such, they have been extensively applied to immunoassays of large molecules, where their high mass and use of sandwich immunoassay formats can result in excellent sensitivity. Small molecule immunosensing using SPR is more challenging. It requires antibodies or high-mass or noble metal labels to provide the required signal for ultrasensitive assays. Also, it can suffer from steric hindrance between the small antigen and large antibodies. However, new studies are increasingly meeting these and other challenges to offer highly sensitive small molecule immunosensor technologies through careful consideration of sensor interface design and signal enhancement. This review examines the application of SPR transduction technologies to small molecule immunoassays directed to different classes of small molecule antigens, including the steroid hormones, toxins, drugs and explosives residues. Also considered are the matrix effects resulting from measurement in chemically complex samples, the construction of stable sensor surfaces and the development of multiplexed assays capable of detecting several compounds at once. Assay design approaches are discussed and related to the sensitivities obtained.

Flow immunoassay for nonioinic surfactants based on surface plasmon resonance sensors

A rapid, sensitive immunoassay based on a surface plasmon resonance sensor in a flow system for the determination of alkylphenol polyethoxylate (APEO) is described. The method is based on an indirect competitive reaction between an anti-APEO antibody in the sample solution and APEO immobilized on a sensor chip and APEO in the same sample solution. A sensor chip was prepared by immobilizing an APEO-horseradish peroxidase (APEO-HRP) conjugate on the thin gold film of the sensor chip. The adsorption constants for the APEO-HRP conjugate on the sensor chip and the surface density of the APEO-HRP adsorbed on the sensor chip at the saturated state were estimated to be 4.7 x 10(5) M(-1) and 5.0 x 10(-14) mol/mm(2), respectively, using a Langmuir adsorption isotherm equation and results from the adsorption experiments. The affinity constants for the immunocomplexes of the anti-APEO antibody with the APEO conjugate on the sensor chip and for APEO in the sample solution were estimated to 2.0 x 10(6) and 5.1 x 10(6) M(-1), respectively. A typical sigmoid calibration curve for APEO was obtained in the concentration range from 1 ppb to 1000 ppb. The detection limit, defined as the concentration of APEO, at which 85% of the sensor signal was observed, was ca. 10 ppb. The assay was applied to the determination of APEO in tap water in conjunction with a solid phase extraction pretreatment; APEO levels of approximately 50 ppt were successfully determined.

Rapid method for qualitative detection of in environmental samples

A gel-based immunoassay that can be used for the detection of 2,4,6-trinitrotoluene (TNT) in water samples was developed. Four polyclonal antibodies were generated in chickens using TNT derivatives. The assay was based on the immunoaffinity preconcentration and immuno-enzyme analysis of TNT in the gel. The results of the assay, assessed by color development, were evaluated visually and also by using a flatbed scanner and subsequent digital processing of the scanned gel. The most sensitive color mode, parameter S (saturation, HSB mode), was used for the immunoassay optimization and evaluation of the results. The immunoassays with the best parameters were optimized and characterized. A cut-off level of 5 µg TNT L-1 was reached for water samples. It was shown that tap and environmental water samples could be analyzed directly, without sample preparation and dilution. The developed test is acceptable for use in an on-site field test to provide rapid (about 15 min for six samples), qualitative and reliable results for making environmental decisions such as identifying "hot spots", monitoring of military and terrorist activities, and selecting of site samples for laboratory analysis.

A general electrochemical method for label-free screening of protein-small molecule interactions

Here we report a versatile method by which the interaction between a protein and a small molecule, and the disruption of that interaction by competition with other small molecules, can be monitored electrochemically directly in complex sample matrices.

High sensitivity molecular detection with enzyme-linked immuno-sorbent assay (ELISA)-type immunosensing

Here we describe an immunosensing method, which is designed for high sensitivity sensing of various substances utilizing specificity of antigen-antibody (ELISA-type) interaction. The building up of the nanostructured sensing interface and the immunointeraction at the surface were characterized by atomic force microscopy. The proposed design makes potentially feasible attaining ultimate single-molecule sensitivity upon optimization of the system. The first non-optimized prototype described here has already demonstrated sensitivity to the presence of dinitrophenyl (DNP) in concentrations as low as 10 pM, which is 100 times better than reported limits of detection of DNP with a traditional enzyme-linked immuno-sorbent assay setup.