A sensitive immunoassay based on electropolymerized films by capacitance measurements for direct detection of immunospecies

Design and Preparation of Sensing Surfaces for Capacitive Biodetection

Despite their high sensitivity and their suitability for miniaturization, biosensors are still limited for clinical applications due to the lack of reproducibility and specificity of their detection performance. The design and preparation of sensing surfaces are suspected to be a cause of these limitations. Here, we first present an updated overview of the current state of use of capacitive biosensors in a medical context. Then, we summarize the encountered strategies for the fabrication of capacitive biosensing surfaces. Finally, we describe the characteristics which govern the performance of the sensing surfaces, along with recent developments that were suggested to overcome their main current limitations.

Label-free and reagentless capacitive aptasensor for thrombin

This investigation develops a label-free and reagentless aptasensor, based on a capacitive transducer with simple face-to-face electrode pairs. The electrode pairs of the transducer are composed of a gold electrode and an indium tin oxide film with micrometer separation with a double-side polyethylene terephthalate tape. Aptamers and 1-dodecanethiol are modified to form a self-assembled monolayer (SAM) on the gold electrode surfaces, and function as bio-recognition elements and preventers of non-specific protein binding, respectively. Electrochemical characterization results indicate that the SAM also forms an effective insulating layer, which is sufficient for capacitive sensing. The feasibility of the capacitive biosensor is validated using thrombin as a model analyte. The ultra-small value changes of capacitance originating from thrombin binding with the aptamers modified on the biosensor were measured with a home-made capacitance measuring circuit based on switched capacitor (SC) technology. The developed biosensor has detection limits of 1 pM and 10 pM of thrombin in phosphate buffered saline and mimic serum solution, respectively. The linear range for thrombin detection in human serum solution is from 10 pM to 1 μM, with a regression coefficient of 0.98. Additionally, the proposed aptasensor does not have significant levels of non-specific binding of bovine serum albumin and human serum albumin. Accordingly, the combination of SC and SAM bringing capacitive transduction at the forefront of ultrasensitive label-free and reagentless biosensing devices, particularly for point-of-care clinical analysis, which adopts small numbers of biological samples with low analyte concentrations.

Interfacial capacitance immunosensing using interdigitated electrodes: the effect of insulation/immobilization chemistry

With the aim of improving the reproducibility of capacitive immunosensors, we performed a comparative study of four different insulating/immobilization chemistries.

Preparation and characterization of Fe3O4@Au-C225 composite targeted nanoparticles for MRI of human glioma

Objective

To study the characterization of Fe₃O₄@Au-C225 composite targeted MNPs.

Methods

Fe₃O₄@Au-C225 was prepared by the absorption method. The immunosorbent assay was used to evaluate its absorption efficiency at C225 Fc. ZETA SIZER3000 laser particle size analyzer, ultraviolet photometer and its characteristics were analyzed by VSM. the targeting effect of Fe₃O₄@Au-C225 composite targeted MNPs on U251 cells in vitro were detected by 7.0 Tesla Micro-MR; and subcutaneous transplanted human glioma in nude mice were performed the targeting effect in vivo after tail vein injection of Fe₃O₄@Au-C225 composite targeted MNPs by MRI.

Results

The self-prepared Fe₃O₄@Au composite MNPs can adsorb C225 with high efficiency of adsorption so that Fe₃O₄@Au-C225 composite targeted MNPs were prepared successfully. Fe₃O₄@Au-C225 composite targeted MNPs favorably targeted human glioma cell line U251 in vitro; Fe₃O₄@Au-C225 composite targeted MNPs have good targeting ability to xenografted glioma on nude mice in vivo, and can be traced by MRI.

Conclusion

The Fe₃O₄@Au-C225 composite targeted MNPs have the potential to be used as a tracer for glioma in vivo.

Virus-Enabled Biosensor for Human Serum Albumin

The label-free detection of human serum albumin (HSA) in aqueous buffer is demonstrated using a simple, monolithic, two-electrode electrochemical biosensor. In this device, both millimeter-scale electrodes are coated with a thin layer of a composite containing M13 virus particles and the electronically conductive polymer poly(3,4-ethylenedioxy thiophene) or PEDOT. These virus particles, engineered to selectively bind HSA, serve as receptors in this biosensor. The resistance component of the electrical impedance, Zre, measured between these two electrodes provides electrical transduction of HSA binding to the virus-PEDOT film. The analysis of sample volumes as small as 50 μL is made possible using a microfluidic cell. Upon exposure to HSA, virus-PEDOT films show a prompt increase in Zre within 5 s and a stable Zre signal within 15 min. HSA concentrations in the range from 100 nM to 5 μM are detectable. Sensor-to-sensor reproducibility of the HSA measurement is characterized by a coefficient-of-variance (COV) ranging from 2% to 8% across this entire concentration range. In addition, virus-PEDOT sensors successfully detected HSA in synthetic urine solutions.

Development of a real-time capacitive biosensor for cyclic cyanotoxic peptides based on Adda-specific antibodies

The harmful effects of cyanotoxins in surface waters have led to increasing demands for accurate early warning methods. This study proposes a capacitive immunosensor for broad-spectrum detection of the group of toxic cyclic peptides called microcystins (∼80 congeners) at very low concentration levels. The novel analytical platform offers significant advances compared to the existing methods. Monoclonal antibodies (mAbs, clone AD4G2) that recognize a common element of microcystins were used to construct the biosensing layer. Initially, a stable insulating anchor layer for the mAbs was made by electropolymerization of tyramine onto a gold electrode surface, with subsequent incorporation of gold nanoparticles (AuNPs) on the glutaraldehyde (5%) activated polytyramine surface. The biosensor responded linearly to microcystin concentrations from 1×10(-13)M to 1×10(-10)M MC-LR standard with a limit of detection of 2.1×10(-14)M. The stability of the biosensor was evaluated by repeated measurements of the antigen and by determining the capacitance change relative to the original response, which decreased below 90% after the 30th cycle.

Novel impedimetric immunosensor for detection of pathogenic bacteria Streptococcus pyogenes in human saliva

Streptococcus pyogenes , also known as group A streptococcus (GAS), is a Gram positive human pathogen responsible for invasive and noninvasive human infections with a high incidence rate. Traditional detection methods involve cell culture and PCR, which are limited by long processing times or the need for high cost equipment. Impedance-based electrochemical immunosensors provide an alternative by which precise and rapid quantitative detection of the organism can help with rapid clinical decisions. To bring a biosensor for point-of-care applications to market, strict optimization of each level of construction and operation is required. In this paper, commercial screen-printed gold electrodes have been used to construct polytyramine (Ptyr)-based immunosensors. Biotin tagged whole antibodies against S. pyogenes were conjugated to Ptyr amine group via biotin-NeutrAvidin coupling. Sensors were optimized at each level of construction, particularly for Ptyr electrodeposition and antibody concentration, to optimize signal and specificity. Scanning electron microscopy, fluorescence microscopy, and on-sensor analysis (HRP conjugated enhanced chemiluminescence-based semiquantitative method) to detect Ptyr surface amine and bound antibody were performed as supporting techniques. Cumulative and single shot incubations had shown detection range of 100 to 10(5) cells per 10 μL and 100 to 10(4) cells per 10 μL of bacteria in PBS, respectively. Sensors were also able to specifically detect S. pyogenes in 50% (v/v) human saliva, with good selectivity and low cross-reactivity.

Highly sensitive electrochemical impedance sensing of PEP gene based on integrated Au-Pt alloy nanoparticles and polytyramine

Fabrication of an electrochemical impedimetric DNA biosensor based on the integration of Au-Pt alloy nanoparticles (Au-Pt(NPs)) and electropolymerized polytyramine (Pty) film for the detection of phosphoenolpyruvate carboxylase (PEP) gene is described in this article, where Pty films acted as an ideal combination platform for Au-Pt(NPs) via electrostatic adsorption. The electrochemical properties of the Au-Pt(NPs)/Pty, the characteristics of the immobilization and hybridization of DNA were investigated by cyclic voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy (EIS), respectively. Primary study indicated that Au-Pt(NPs)/Pty had a synergistic effect on the electrochemical signal of [Fe(CN)(6)](3-/4-), which served as the classic redox probe in the most electrochemical impedimetric sensors. DNA sequence-specific of PEP transgene existed in some transgenic crops was detected by this EIS protocol. The dynamic detection range of this DNA electrochemical biosensor to the DNA target sequence was from 1.0×10(-12)M to 1.0×10(-7)M. The detection limit was measured to be 3.6×10(-13)M. The DNA biosensor also had good selectivity, stability and reproducibility.

Impedimetric immunosensor for human serum albumin detection on a direct aldehyde-functionalized silicon nitride surface

In this work we report the fabrication and characterization of a label-free impedimetric immunosensor based on a silicon nitride (Si(3)N(4)) surface for the specific detection of human serum albumin (HSA) proteins. Silicon nitride provides several advantages compared with other materials commonly used, such as gold, and in particular in solid-state physics for electronic-based biosensors. However, few Si(3)N(4)-based biosensors have been developed; the lack of an efficient and direct protocol for the integration of biological elements with silicon-based substrates is still one of its the main drawbacks. Here, we use a direct functionalization method for the direct covalent binding of monoclonal anti-HSA antibodies on an aldehyde-functionalized Si-p/SiO(2)/Si(3)N(4) structure. This methodology, in contrast with most of the protocols reported in literature, requires less chemical reagents, it is less time-consuming and it does not need any chemical activation. The detection capability of the immunosensor was tested by performing non-faradaic electrochemical impedance spectroscopy (EIS) measurements for the specific detection of HSA proteins. Protein concentrations within the linear range of 10(-13)-10(-7) M were detected, showing a sensitivity of 0.128 Ω μM(-1) and a limit of detection of 10(-14) M. The specificity of the sensor was also addressed by studying the interferences with a similar protein, bovine serum albumin. The results obtained show that the antibodies were efficiently immobilized and the proteins detected specifically, thus, establishing the basis and the potential applicability of the developed silicon nitride-based immunosensor for the detection of proteins in real and more complex samples.

Label-free technologies for quantitative multiparameter biological analysis

In the postgenomic era, information is king and information-rich technologies are critically important drivers in both fundamental biology and medicine. It is now known that single-parameter measurements provide only limited detail and that quantitation of multiple biomolecular signatures can more fully illuminate complex biological function. Label-free technologies have recently attracted significant interest for sensitive and quantitative multiparameter analysis of biological systems. There are several different classes of label-free sensors that are currently being developed both in academia and in industry. In this critical review, we highlight, compare, and contrast some of the more promising approaches. We describe the fundamental principles of these different methods and discuss advantages and disadvantages that might potentially help one in selecting the appropriate technology for a given bioanalytical application.

Detecting penicillin G in milk with impedimetric label-free immunosensor

A label-free impedimetric flow injection immunosensor for the direct detection of penicillin G has been developed. Anti-penicillin G was immobilized on a gold working electrode modified with a self-assembled monolayer of thioctic acid. Real time monitoring of impedance was carried out at the optimum frequency of 160 Hz. Under optimum operating conditions the system provided a wide linear range between 1.0 x 10(-13) and 1.0 x 10(-8) M with a very low detection limit of 3.0 x 10(-15) M, much lower than the MRL of penicillin G in milk (1.2 x 10(-8) M). The immobilized anti-penicillin G on self-assembled thioctic acid monolayer gold electrode was very stable and provided good reproducible signal after regeneration up to 45 times with a relative standard deviation (R.S.D.) lower than 4%. Good recoveries and precisions were obtained when spiked raw milk samples were analyzed.

Capacitive biosensor for detection of endotoxin

A capacitive biosensor for the detection of bacterial endotoxin has been developed. Endotoxin-neutralizing protein derived from American horseshoe crab was immobilized to a self-assembled thiol layer on a biosensor transducer (Au). Upon injection of a sample containing endotoxin, a decrease in the observed capacitive signal was registered. Endotoxin could be determined under optimum conditions with a detection limit of 1.0 x 10(-13) M and linearity ranging from 1.0 x 10(-13) to 1.0 x 10(-10) M. Good agreement was achieved when applying endotoxin preparations purified from an Escherichia coli cultivation to the capacitive biosensor system, utilizing the conventional method for quantitative endotoxin determination, the Limulus amebocyte lysate test as a reference. The capacitive biosensor method was statistically tested with the Wilcoxon signed rank test, which proved the system is acceptable for the quantitative analysis of bacterial endotoxin (P<0.05).

Label-Free Impedance Biosensors: Opportunities and Challenges

Impedance biosensors are a class of electrical biosensors that show promise for point-of-care and other applications due to low cost, ease of miniaturization, and label-free operation. Unlabeled DNA and protein targets can be detected by monitoring changes in surface impedance when a target molecule binds to an immobilized probe. The affinity capture step leads to challenges shared by all label-free affinity biosensors; these challenges are discussed along with others unique to impedance readout. Various possible mechanisms for impedance change upon target binding are discussed. We critically summarize accomplishments of past label-free impedance biosensors and identify areas for future research.

Protein-sensing assay formats and devices

Proteins are used as biocatalysts, therapeutic or diagnostic agents, and as such they are biotechnological products. Moreover, they are biomarkers for health states, diseases or toxic or other adverse effects, and the intracellular protein network is essential for the adaptation of an organism to its environment. Thus, there is a strong need for analytical methods for protein determination, which allow not only to indicate the presence of a protein, but also its concentration, covalent modification and activity, and corresponding developments of new methods experienced strong support. Among those methods only those were considered here, which are based on affinity reactions between an immobilized capture agent, such as an antibody or a receptor, and the target protein. Immobilization methods range from adsorption on hydrophobic materials, in membranes or gels to covalent binding and bioaffinity reactions, such as the oriented immobilization of antibodies on protein A/G layers. The applicability of the various methods is dependent on physical and chemical properties of the immobilization substrate and of the capture agent, i.e. the presence of surface charges, hydrophobic areas or functional groups for chemical coupling. The choice of the immobilization substrate is influenced by the combination of the assay and detection principle, which meets best the practical requirements. Assay formats range from direct, label-free one-step detection of the affinity reaction between the capture agent and the target protein to multi-step procedures, such as an enzyme-tracer-based sandwich assays. Each approach has its particular advantages and disadvantages with respect to the complexity of the assay, i.e. number of required reagents and of incubation steps, the possible degree of automation, assay time, availability of suitable reagents, required sample volume, sensitivity and specificity, including the possibility to determine several proteins simultaneously. No general recommendation for the "best choice" was given in this contribution, but examples were chosen, which illustrate the potential of the different systems.