Development of a Novel Enhanced Biosensor System for Real-Time Monitoring of Fish Stress Using a Self-Assembled Monolayer

Wireless biosensor systems were developed in our lab for monitoring blood glucose concentrations in fish as an indicator of fish stress. However, uniform immobilization of the enzyme on the surface of the electrode is difficult, so the sensor response is typically reduced at a range of high glucose concentrations during the stress monitoring. In this study, we attempted to enhance sensor response by using a self-assembled monolayer-immobilized enzyme. Glucose oxidase was immobilized on a working electrode modified with a self-assembled monolayer. The proposed biosensor showed a good correlation between the output current and the glucose concentration range of 10–3500 mg dL⁻¹ under an optimized working condition. The dynamic measurement range of this newly developed sensor is significantly improved, especially over a high concentration range, which helps the sensor to achieve better performance in dramatic changes in the stress response of fish. In addition, we used biological samples from test fish and obtained a good correlation coefficient between the sensor output current and the glucose concentration using a conventional method. The proposed wireless biosensor system was also applied to monitor fish stress responses in real time through different stressors and to obtain some precise data that reflect real fish stress responses.

Label-Free Potentiometric Ion Flux Immunosensor for Determination of Recombinant Human Myelin Basic Protein: Application to Downstream Purification from Transgenic Milk

Recombinant human myelin basic protein (rhMBP) produced in the milk of transgenic cows was found exclusively associated with milk caseins. This hindered its direct determination without extensive sample pretreatment. Here, a label-free potentiometric immunosensor was developed and validated for the determination of rhMBP. An ion flux was generated under zero-current based on surface blocking of the polymeric membrane ion-selective electrode by anti-hMBP antibody and tetrabutylammonium bromide as a marker ion. The immunosensor was successfully employed in the quantitative determination of hMBP in the range of 0.10-20.00 μg/mL with a limit of detection of 50.00 ng/mL. The applicability of the passive ion flux immunosensor for determination of target analyte in complex matrices was investigated. Downstream purification of rhMBP from the milk of transgenic cows was achieved using cation exchange chromatography, immobilized metal affinity chromatography, and immunoaffinity chromatography. The specificity of the immunosensor along with matrix effect of milk proteins were demonstrated. Results obtained using the rhMBP immunosensor were further cross-validated using an orthogonal testing protocol assembled of RP-HPLC and SE-HPLC. It should be noted that the proposed ion flux immunosensor provided a feasible and specific tool for monitoring rhMBP concentration/purity, immunogenic activity, and stability. Such approach provides an attractive economic alternative to sophisticated biosensors required for in-process quality control of biopharmaceutical products.

Electrochemical switching with a DNA aptamer-based electrochemical sensor

The present study was focused on the application of NH₂-functionalized Fe₃O₄/gold nanoparticles (Fe₃O₄/AuNPs)-decorated carbon nanotubes (CNTs) in the development of electrochemical sensor for bisphenol A (BPA) detection. After the nanocomposite synthesis and its characterization, the optimization of the measurement conditions and working parameters of sensors were evaluated. Aminated detection probe (DNA aptamer) was surface confined on the NH₂-functionalized Fe₃O₄/AuNPs surface using glutaraldehyde as a linker. The constructed nanoaptasensor incorporated the advantages of the neatly deposited Fe₃O₄/AuNPs and the covalent attachment of the detection probe at the surface of sensing interface. The results revealed that BPA could be detected in a wide linear range from 1 to 600nM with a low detection limit down to 300pM. Moreover, the resultant aptasensor exhibited good specificity, stability and reproducibility, indicating that the present strategy was promising for broad potential application in clinic assay.

Detection of bisphenol A using palm-size NanoAptamer analyzer

We have demonstrated a palm-size NanoAptamer analyzer capable of detecting bisphenol A (BPA) at environmentally relevant concentrations (<1ng/mL or ppb). It is designed for performing reaction and fluorescence measurement on single cuvette sample. Modified NanoGene assay was used as the sensing mechanism where signaling DNA and QD₆₅₅ was tethered to QD₅₆₅ and magnetic bead via the aptamer. Aptamer affinity with BPA resulted in the release of the signaling DNA and QD₆₅₅ from the complex and hence corresponding decrease in QD₆₅₅ fluorescence measurement signal. Baseline characterization was first performed with empty cuvettes, quantum dots and magnetic beads under near-ideal conditions to establish essential functionality of the NanoAptamer analyzer. Duration of incubation time, number of rinse cycles, and necessity of cuvette vibration were also investigated. In order to demonstrate the capability of the NanoAptamer analyzer to detect BPA, samples with BPA concentrations ranging from 0.0005 to 1.0ng/mL (ppb) were used. The performance of the NanoAptamer analyzer was further examined by using laboratory protocol and commercial spectrofluorometer as reference. Correlation between NanoAptamer analyzer and laboratory protocol as well as commercial spectrofluorometer was evaluated via correlation plots and correlation coefficients.

A novel and label-free immunosensor for bisphenol A using rutin as the redox probe

In this work, a new and label-free electrochemical immunosensor for sensitive detection of bisphenol A was reported. MWCNTs and gold nanoparticles (AuNPs) were modified on glassy carbon electrode surface to enhance current response. The Anti-BPA was immobilized on the modified electrode through AuNPs. Rutin was used for the first time as the redox probe to construct electrochemical immunosensor of bisphenol A. The peak current change due to the specific immuno-interaction between anti-BPA and BPA on the modified electrode surface was utilized to detect bisphenol A. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) were employed to trace the assembly process of the electrochemical immunosensor. Experimental factors affecting the sensitivity of the immunosensor were examined in terms of incubation time and pH of phosphate buffer solution (PBS). Under optimized conditions, the linear range of calibration curve based on the relationship between current response and BPA concentration was from 1.0×10(-8)-1.0×10(-6)M with detection limit of 8.7×10(-9)M (S/N=3). The proposed immunosensor showed good reproducibility, selectivity, stability and was successfully applied to the determination of BPA in real sample.

Sensitive detection of bisphenol A based on a ratiometric electrochemical aptasensor

A new ratiometric electrochemical aptasensor has been developed for highly sensitive and selective detection of bisphenol A (BPA). The double-stranded DNA (dsDNA), consisting of the BPA aptamer (DNA1) and methylene blue (MB)-labeled complementary DNA (cDNA), was immobilized on a gold nanoparticle (AuNP) modified glassy carbon (GC) electrode. In the presence of BPA, the specific BPA–aptamer interaction drove the release of the MB-labeled cDNA from the electrode surface. As a result, the oxidation peak current of MB (IMB) decreased and that of BPA (IBPA) increased. The peak current ratio (IBPA/IMB) of BPA and MB was linear with the concentration of BPA in the range from 1 to 100 pmol/L with a detection limit of 0.6 pmol/L. The detection limit is much lower than that obtained by most of the reported electrochemical methods. On the other hand, the developed aptasensor possesses good selectivity, reproducibility, and stability, and the related sensing structure is very simple, showing promising practical applications in BPA assays.

Comparison of Electrochemical Immunosensors and Aptasensors for Detection of Small Organic Molecules in Environment, Food Safety, Clinical and Public Security

We review here the most frequently reported targets among the electrochemical immunosensors and aptasensors: antibiotics, bisphenol A, cocaine, ochratoxin A and estradiol. In each case, the immobilization procedures are described as well as the transduction schemes and the limits of detection. It is shown that limits of detections are generally two to three orders of magnitude lower for immunosensors than for aptasensors, due to the highest affinities of antibodies. No significant progresses have been made to improve these affinities, but transduction schemes were improved instead, which lead to a regular improvement of the limit of detections corresponding to ca. five orders of magnitude over these last 10 years. These progresses depend on the target, however.

Development of a label-free immunosensor system for detecting plasma cortisol levels in fish

Fishes display a wide variation in their physiological responses to stress, which is clearly evident in the plasma corticosteroid changes, chiefly cortisol levels in fish. In the present study, we describe a novel label-free immunosensor for detecting plasma cortisol levels. The method is based on immunologic reactions and amperometric measurement using cyclic voltammetry. For the immobilization of the antibody on the surface of sensing electrode, we used a self-assembled monolayer of thiol-containing compounds. Using this electrode, we detect the CV signal change caused by the generation of antigen-antibody complex. The immunosensor showed a response to cortisol levels, and the anodic peak value linearly decreased with a correlation coefficient of 0.990 in diluted plasma. The specificity of the label-free immunosensor system was investigated using other steroid hormones, such as 17α, 20β-dihydroxy-4-pregnen-3-one, progesterone, estriol, estradiol, and testosterone. The specific detection of cortisol was suggested by a minimal change from -0.32 to 0.51 μA in the anodic peak value of the other steroid hormones. The sensor system was used to determine the plasma cortisol levels in Nile tilapia (Oreochromis niloticus), and the results were compared with those of the same samples determined using the conventional method (ELISA). A good correlation was obtained between values determined using both methods (correlation coefficient 0.993). These findings suggest that the proposed label-free immunosensor could be useful for rapid and convenient analysis of cortisol levels in fish plasma samples.

DNA Nanostructure-Based Magnetic Beads for Potentiometric Aptasensing

In this work, a simple, general, and sensitive potentiometric platform is presented, which allows potentiometric sensing to be applied to any class of molecule irrespective of the analyte charge. DNA nanostructures are self-assembled on magnetic beads via the incorporation of an aptamer into a hybridization chain reaction. The aptamer-target binding event leads to the disassembly of the DNA nanostructures, which results in a dramatic change in the surface charge of the magnetic beads. Such a surface charge change can be sensitively detected by a polycation-sensitive membrane electrode using protamine as an indicator. With an endocrine disruptor bisphenol A as a model, the proposed potentiometric method shows a wide linear range from 0.1 to 100 nM with a low detection limit of 80 pM (3σ). The proposed sensing strategy will lay a foundation for the development of potentiometric sensors for highly sensitive and selective detection of various targets.

Building an aptamer/graphene oxide FRET biosensor for one-step detection of bisphenol A

Bisphenol A (BPA) is an important industrial chemical for polycarbonate (PC) and epoxy resins in paper and plastic industries. In our work, a kind of new method for detection of BPA was designed based on graphene oxide and anti-BPA aptamer. The graphene oxide can specifically adsorb and quench the fluorescence of fluorescently modified ssDNA probes. Meanwhile, the BPA can combine with anti-BPA optamer and switch its configuration to prevent the aptamer from adsorbing on the surface of graphene oxide (GO). Under different concentrations of BPA, based on the target-induced conformational change of anti-BPA aptamer and the interactions between the fluorescently modified anti-BPA aptamer (FAM-ssDNA) and GO, the experimental results show that the intensity of the fluorescence signal was changed. A low limit of detection of 0.05 ng/mL was obtained in the range 0.1-10 ng/mL. In addition, the specificity was outstanding among analogues of BPA. The recovery rate in actual water samples spiked with BPA can be 96.0% to 104.5%. The developed method was successfully used to determine BPA in actual water samples.

E-assay concept: detection of bisphenol A with a label-free electrochemical competitive immunoassay

A label-free electrochemical immunosensor was developed by electropolymerization of N-(3-(4-(2-(4-hydroxyphenyl)propan-2-yl)phenoxy)propyl) 3-(5-hydroxy-1,4-dihydro-1,4-dioxonaphthalen-2(3)-yl)propionamide (JugBPA). By combination with an antibody directed to bisphenol A (αBPA), this conducting polymer-based biosensor can detect BPA directly with a limit of detection of 2pgmL(-1). Square wave voltammetry shows that the polymer film presents a current decrease upon anti-BPA binding and an opposite current increase upon BPA addition in solution. This electrochemical immunosensor (E-assay) also shows high selectivity towards closely related compounds (bisphenol A dimethacrylate, and dibutyl phthalate). The E-assay concept described here could be a promising tool for simple, low-cost and reagentless on-site environmental monitoring.

Immunosensors for detection of Annexin II and MUC5AC for early diagnosis of lung cancer

Amperometric immunosensors were developed to diagnose lung cancer through the detection of Annexin II and MUC5AC. To fabricate the sensor probe, a conducting polymer (poly-terthiophene carboxylic acid; poly-TTCA) was electropolymerized onto a gold nanoparticle/glassy carbon electrode (AuNP/GCE) and a dendrimer (Den) was covalently bonded to the poly-TTCA through amide bond formation, where AuNPs were doped onto the dendrimer. To obtain the final sensor probe, an antibody (anti-Annexin II) and hydrazine (Hyd), which is a catalyst for the reduction of H(2)O(2) generated by glucose oxidase (GOx), were covalently attached onto the Den/AuNP-modified surface. Each surface was then characterized by SEM, impedance spectroscopy and XPS. The final sensor probe was examined before and after interaction with Annexin II and MUC5AC using impedance-spectroscopic, quartz crystal microbalance and amperometric methods. The performance of the immunosensor for the Annexin II was evaluated for the apical surface fluid labeled with GOx by the standard addition method. In this case, the detection limit of the proposed method was 0.051 ng/mL (k=3, n=5). The Annexin II concentration in the secretions collected from squamous metaplastic cells was determined to be 280+/-8.0 pg/mL (n=5).

Monitoring the activation of neuronal nitric oxide synthase in brain tissue and cells with a potentiometric immunosensor

An all solid state potentiometric immunosensor (ASPI) has been developed to study the activation process of neuronal nitric oxide synthase (nNOS), the enzyme involved in the synthesis of nitric oxide generated under physiological conditions. At first, an all solid state H(+)-selective ISE was fabricated with the carboxylated poly(vinyl chloride) (PVC-COOH) film containing H(+) ionophore, antibody was then immobilized on the polymer layer. The immunocomplex formation was detected by monitoring pH change due to interaction between urease labeled secondary antibody and antigen. Experimental parameters such as the amount of phosphorylated nNOS immobilized on the electrode surface and pH responses due to the antibody-antigen reaction were studied in detail. The calibration plot of the potentiometric potential vs. phosphorylated nNOS concentration exhibited a linear relationship in the range of 3.4-340.0 microg/ml. The calibration sensitivity of the phosphorylated nNOS immunosensor was -0.073+/-0.002 mV/microg ml(-1). The detection limit of nNOS was determined to be 0.2 microg/ml based on five-time measurements (95% confidence level, k=3, n=5). The reliability of the immunosensor was examined with rat brain tissues as well as neuronal cells, and the results shown were good, implying a promising approach for a novel electrochemical immunosensor platform with potential applications to clinical diagnosis.