Enhanced sensitivity of self-assembled-monolayer-based SPR immunosensor for detection of benzaldehyde using a single-step multi-sandwich immunoassay

Strategy of Dimercaptothiol as Self-assembled Monolayers Enhance the Sensitivity of SPR Immunosensor for Detection of Salbutamol

The formation of self-assembled monolayers (SAMs) is a normal method for the immobilization of biorecognition elements immobilized on SPR sensors. With this method, mercaptopropionic acid (MPA) with carboxylic and thiol group is the most commonly used. Dimercaptosuccinic acid (DMSA) having two carboxylics and two thiol groups is a classical antidote for heavy metal ions. In this paper, DMSA was first used to form SAMs to connect the antigen on the chip of a surface plasmon resonance (SPR) immunosensor for detection of salbutamol (SAL), and the results were compared with a traditional (MPA)-SPR sensor. Dihydrolipoic acid(DHLA)-SPR showed that the recognition efficiency of antigen and antibody of DMSA-SPR immunosensor was 170.1% at room temperature with the linear range of 5 - 150 ng/mL. The recovery rate of this sensor applied to SAL detection in pork reached 94.9 - 108.0% and the limit of quantification (LOQ) was 5 ng/mL. The results were in good correlation with the analysis results of ultra-high phase liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) analysis. This novel DMSA-SPR immunosensor provides insight into a new idea and method for improving the sensitivity of SPR immunosensors and can be widely used in the detection of other small molecules.

Integrated optical waveguide-based fluorescent immunosensor for fast and sensitive detection of microcystin-LR in lakes: Optimization and Analysis

Nowadays, biosensor technologies which can detect various contaminants in water quickly and cost-effectively are in great demand. Herein, we report an integrated channel waveguide-based fluorescent immunosensor with the ability to detect a maximum of 32 contaminants rapidly and simultaneously. In particular, we use waveguide tapers to improve the efficiency of excitation and collection of fluorescent signals in the presence of fluorophore photobleaching in a solid surface bioassay. Under the optimized waveguide geometry, this is the first demonstration of using such a type of waveguide immunosensor for the detection of microcystin-LR (MC-LR) in lake water. The waveguide chip was activated by (3-Mercaptopropyl) trimethoxysilane/N-(4-maleimidobutyryloxy) succinimide (MTS/GMBS) for immobilization of BSA-MC-LR conjugate, which was confirmed to have uniform monolayer distribution by atomic force microscopy. All real lake samples, even those containing MC-LR in the sub-microgram per liter range (e.g. 0.5 μg/L), could be determined by the immunosensor with recovery rates between 84% and 108%, confirming its application potential in the measurement of MC-LR in real water samples.

Direct Detection of α-1 Antitrypsin in Serum Samples using Surface Plasmon Resonance with a New Aptamer-Antibody Sandwich Assay

The challenges associated with performing surface plasmon resonance (SPR) based measurements in serum and other biofluids have continued to limit the applicability of this valuable sensing technology for sensitive bioaffinity measurements of proteins in clinically relevant samples. In this paper, a new sandwich assay is introduced for the quantitative SPR analysis of α-1 antitrypsin (AAT), which is a recognized biomarker for Alzheimer's disease. Detection was performed via the specific adsorption of AAT onto a gold chip surface modified with a DNA aptamer. The measurement dynamic range and also sensitivity in serum were improved with the subsequent surface binding of antiAAT. A methodology was established to measure the target protein in serum, albumin and immunoglobulin G (IgG) solutions with the results correlated with measurements in buffer only. A comparison between SPR and enzyme-linked immunosorbent assay (ELISA) measurements was also made. The detection of AAT in serum at clinically relevant concentrations was demonstrated with target concentrations as low as 10 fM readily achievable.

Bi-epitope SPR surfaces: a solution to develop robust immunoassays

Surface plasmon resonance (SPR)-based immunoassays have numerous applications and require high affinity reagents for sensitive and reliable measurements. We describe a quick approach to turn low affinity antibodies into appropriate capture reagents. We used antibodies recognizing human ephrin type A receptor 2 (EphA2) and a ProteOn XPR36 as a model system. We generated so-called 'bi-epitope' sensor surfaces by immobilizing various pairs of anti-EphA2 antibodies using standard amine coupling. The apparent binding affinities to EphA2 and EphA2 detection sensitivities of the bi-epitope and 'single-epitope' surfaces were then compared. For all antibody pairs tested, bi-epitope surfaces exhibited an ∼ 10-100-fold improvement in apparent binding affinities when compared with single-epitope ones. When pairing 2 antibodies of low intrinsic binding affinities (∼ 10(-8) M) and fast dissociation rates (∼ 10(-2) s(-1)), the apparent binding affinity and dissociation rate of the bi-epitope surface was improved up to ∼ 10(-10) M and 10(-4) s(-1), respectively. This led to an ∼ 100-200-fold enhancement in EphA2 limit of detection in crude cell supernatants. Our results show that the use of antibody mixtures in SPR applications constitutes a powerful approach to develop sensitive immunoassays, as previously shown for non-SPR formats. As SPR-based assays have significantly expanded their reach in the last decade, such an approach promises to further accelerate their development.

Electrical Wiring of the Aldehyde Oxidoreductase PaoABC with a Polymer Containing Osmium Redox Centers: Biosensors for Benzaldehyde and GABA

Biosensors for the detection of benzaldehyde and γ-aminobutyric acid (GABA) are reported using aldehyde oxidoreductase PaoABC from Escherichia coli immobilized in a polymer containing bound low potential osmium redox complexes. The electrically connected enzyme already electrooxidizes benzaldehyde at potentials below -0.15 V (vs. Ag|AgCl, 1 M KCl). The pH-dependence of benzaldehyde oxidation can be strongly influenced by the ionic strength. The effect is similar with the soluble osmium redox complex and therefore indicates a clear electrostatic effect on the bioelectrocatalytic efficiency of PaoABC in the osmium containing redox polymer. At lower ionic strength, the pH-optimum is high and can be switched to low pH-values at high ionic strength. This offers biosensing at high and low pH-values. A "reagentless" biosensor has been formed with enzyme wired onto a screen-printed electrode in a flow cell device. The response time to addition of benzaldehyde is 30 s, and the measuring range is between 10-150 µM and the detection limit of 5 µM (signal to noise ratio 3:1) of benzaldehyde. The relative standard deviation in a series (n = 13) for 200 µM benzaldehyde is 1.9%. For the biosensor, a response to succinic semialdehyde was also identified. Based on this response and the ability to work at high pH a biosensor for GABA is proposed by coimmobilizing GABA-aminotransferase (GABA-T) and PaoABC in the osmium containing redox polymer.

Configuration-controlled Au nanocluster arrays on inverse micelle nano-patterns: versatile platforms for SERS and SPR sensors

Nanopatterned 2-dimensional Au nanocluster arrays with controlled configuration are fabricated onto reconstructed nanoporous poly(styrene-block-vinylpyridine) inverse micelle monolayer films. Near-field coupling of localized surface plasmons is studied and compared for disordered and ordered core-centered Au NC arrays. Differences in evolution of the absorption band and field enhancement upon Au nanoparticle adsorption are shown. The experimental results are found to be in good agreement with theoretical studies based on the finite-difference time-domain method and rigorous coupled-wave analysis. The realized Au nanopatterns are exploited as substrates for surface-enhanced Raman scattering and integrated into Kretschmann-type SPR sensors, based on which unprecedented SPR-coupling-type sensors are demonstrated.

Rapid and sensitive detection of maize chlorotic mottle virus using surface plasmon resonance-based biosensor

We report a biosensor based on surface plasmon resonance (SPR) for the selective detection of maize chlorotic mottle virus (MCMV). 11-Mercaptoundecanoic acid was applied on a gold surface to form a self-assembled monolayer, and a layer of anti-MCMV antibody was crosslinked on the surface for specific recognition of MCMV. The effects of coupling reaction time and antibody concentration on detection sensitivity were studied. The coverage mass change is a function of the concentration of MCMV with a dynamic range from 1 to 1000 ppb. The detection limit is approximately 1 ppb, which is approximately two orders of magnitude higher than that of the existing enzyme-linked immunosorbent assay (ELISA) method. The developed SPR sensor showed highly specific recognition for both purified MCMV and crude extracts from real-world samples.

Portable surface plasmon resonance immunosensor for the detection of fluoroquinolone antibiotic residues in milk

An inexpensive and portable surface plasmon resonance (SPR) sensor, SPReeta Evaluation Kit SPR3, has been used to develop a biosensor for the determination of fluoroquinolone antibiotics (FQs) and to demonstrate its performance analyzing FQ residues in milk samples. The SPReeta three-channel gold chips were activated with a mixed self-assembled monolayer (m-SAM) and functionalized with a FQ haptenized protein. Binding of the antibody produced a concentration-dependent increase of the SPR signal as a result of the change in the refraction index. Similarly, the presence of the FQ produced a dose-dependent decrease of the response, which allowed a good limit of detection (LOD) to be obtained (1.0 ± 0.4 μg L(-1) for enrofloxacin in buffer). The response was reproducible in all three channels, on different injections and days, and also between chips. Milk samples could be analyzed after a simple sample treatment involving fat removal by centrifugation and dilution with water. Under these conditions calibration curves were obtained showing that FQ residues can be analyzed in milk samples with an IC(50) value of 26.4 ± 7.2 μg L(-1) and a LOD of 2.0 ± 0.2 μg L(-1) (for enrofloxacin), far below the European Union regulations for this antibiotic family in this matrix. Finally, the paper also demonstrates that the biosensor is able to selectively detect the presence of FQs in milk samples, even in the presence of other antibiotics. Enrofloxacin, ciprofloxacin, and norfloxacin residues were detected in blind samples supplied by Nestlé Co.

A mediatorless and label-free amperometric immunosensor for detection of h-IgG

A novel experimental methodology for studying a mediatorless and label-free immunosensor is proposed by immobilizing antibody on gold nanoparticle/L-cysteine coated electrode (nano-Au/L-cysteine electrode). Differential pulse voltammograms (DPV) resulting from the assembled immunosensor indicate that the immunosensor shows excellent electrochemical response to dopamine so that the electrochemical response is utilized for the signal generation step of the immunosensor. Therefore, by means of unenzymatic-labeling procedure combined with the amperometric detection using dopamine as substrate, the immunological reaction can be detected. After the immunosensor is incubated with h-IgG solution, the access of electrocatalytic behavior center of the immunosensor to dopamine is partly inhibited, which leads to a linear decrease in amperometric response of the immunosensor with h-IgG concentration over a range 0.82-90 ng mL(-1) by DPV.