A disposable immunomagnetic electrochemical sensor based on functionalised magnetic beads and carbon-based screen-printed electrodes (SPCEs) for the detection of polychlorinated biphenyls (PCBs)

Towards miniaturized electrochemical sensors for monitoring of polychlorinated biphenyls

Pollution of our environment as a result of industrialization and other human activities is a growing concern due to the harmful effects of most chemicals that are released into the environment. Of particular interest are the persistent organic pollutants (POPs) that are reported to be toxic and build up in the environment due to their persistence. Among the POPs are polychlorinated biphenyls (PCBs), which were widely used in the past in various applications ranging from additives in pesticides to dielectric fluids in electrical equipment. As a way of protecting the one health trilogy (environment, human and animal health), their determination in the environment is a paramount call that has seen researchers continue to provide advanced technologies towards achieving this goal. These technologies involve the conventional gold standard gas chromatography systems coupled to sensitive detectors that can detect trace level concentrations. They have come in handy in monitoring of PCBs but their application for routing monitoring may not be sustainable because of the cost of operation associated with them and the need for experts to run the equipment. As a result, there is need for affordable systems that are still able to achieve the required sensitivity for routine monitoring and real-time data acquisition. Sensor systems fit very well in this category since they can be miniaturized for affordability and portray many other desirable features. PCBs as environmentally relevant environmental pollutants have received minimal attention with regards to sensor development and this review highlights the efforts that have been made so far. It provides in-depth discussions on electrochemical sensors and the various modifications that have been employed to date to achieve detection of PCBs at low concentrations as well as the future prospects in remote and routine monitoring.

A Novel Label-Free Electrochemical Immunosensor Based on a Self-Assembled Monolayer-Modified Electrode for Polychlorinated Biphenyl (PCB) in Environmental Analysis

PCBs (polychlorinated biphenyls) are a very large group of organic compounds that have between two and ten chlorine atoms attached to the biphenyl. These compounds have an acute impact as environmental pollutants, causing cancer and other adverse health effects in humans. It is therefore imperative to develop techniques for the cost-effective detection of PCBs at very low concentrations in ecosystems. In this paper, a novel label-free, indirect, competitive electrochemical immunosensor was first developed with a PCB-BSA conjugate. It is shown herein to compete with free PCBs for binding to the anti-PCB polyclonal primary antibody (IgY). Then, we used a secondary antibody to enhance the sensitivity of the sensor for the detection of PCB in a sample. It has been successfully immobilized on an 11-mercaptoundecanoic acid (11-MUA)-modified gold electrode via a carbodiimide-coupling reaction using cross-linking 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) on the electrode surface. The immunosensor was investigated by cyclic voltammetry and differential pulse voltammetry in a standard solution of [Fe(CN)6]3−/4−. A linear range of 0.011–220 ng/mL−1 and a limit of detection (LOD) of 0.11 ng/mL−1 for PCBs detection were achieved by the developed immunosensor, showing advantages over conventional assays. The novel label-free electrochemical immunosensor discussed in this paper is a solution for simple, rapid, cost-effective sample screening in a portable, disposable format. The proposed immunosensor has good sensitivity, and it can prove to be an adequate real-time monitoring solution for PCBs in soil samples or other samples.

Electrochemical Development of an Immunosensor for Detection Polychlorinated biphenyls (PCBs) for Environmental Analysis

Polychlorinated biphenyls (PCBs) are a highly toxic family of synthetic chemical compounds. PCBs are widely spread in the environment and their toxicity can cause serious ailments to living organisms such as cancer; therefore, developing a device for the detection of PCBs in the environment is significant. In this paper, polyclonal primary anti-PCB antibodies were immobilized onto a gold screen-printed electrode with the purpose of creating an electrochemical immunosensor for the detection of Aroclor 1254. It was modified with 11-mercaptoundecanoic acid (11-MUA) and the activation of the carboxylic acid terminal was performed by cross-linking 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hyrodsuccinmide (NHS) on the electrode surface. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), linear sweep voltammetry, atomic force microscopy (AFM), scanning electron microscopy (SEM), and contact angle measurement were employed to characterize SAM development on the gold electrode. Using a competitive assay, a 0.09 ng/mL−1 limit of detection and a linear range of 0.101–220 ng/mL−1 were determined. The self-assembled monolayers (SAM) were successful in encapsulating the PCBs on the immunosensor. The electrochemical detection showed better resolution when compared to traditional methods such as the ELISA optical technique. The novel electrochemical immunosensor approach that is discussed in this paper has the potential to offer rapid sample screening in a portable, disposable format and could contribute to the effective control and prevention of PCBs in the environment.

Electrochemical (Bio)Sensors for Pesticides Detection Using Screen-Printed Electrodes

Pesticides are among the most important contaminants in food, leading to important global health problems. While conventional techniques such as high-performance liquid chromatography (HPLC) and mass spectrometry (MS) have traditionally been utilized for the detection of such food contaminants, they are relatively expensive, time-consuming and labor intensive, limiting their use for point-of-care (POC) applications. Electrochemical (bio)sensors are emerging devices meeting such expectations, since they represent reliable, simple, cheap, portable, selective and easy to use analytical tools that can be used outside the laboratories by non-specialized personnel. Screen-printed electrodes (SPEs) stand out from the variety of transducers used in electrochemical (bio)sensing because of their small size, high integration, low cost and ability to measure in few microliters of sample. In this context, in this review article, we summarize and discuss about the use of SPEs as analytical tools in the development of (bio)sensors for pesticides of interest for food control. Finally, aspects related to the analytical performance of the developed (bio)sensors together with prospects for future improvements are discussed.

3A-Amino-3A-Deoxy-(2AS, 3AS)-β-Cyclodextrin Hydrate/Tin Disulfide Modified Screen-Printed Carbon Electrode for the Electrochemical Detection of Polychlorinated Biphenyls

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that are widely distributed in the environment. It is noteworthy that the PCBs are endocrine-disrupting substances, and their toxicity induces cancer and damage to the mammalian reproductive system, immune system, stomach, skin, liver, etc. This work aimed to synthesize 3A-amino-3A-deoxy-(2AS, 3AS)-β-cyclodextrin hydrate/tin disulfide composite material and to study its material properties, electrochemical properties, and application to PCB detection. The nanostructured tin disulfide (SnS₂) synthesized by hydrothermal technique and 3A-amino-3A-deoxy-(2AS, 3AS)-β-cyclodextrin hydrate were sequentially modified onto the disposable screen-printed carbon electrode (SPCE) via titration using a micropipette. The 3A-amino-3A-deoxy-(2AS, 3AS)-β-cyclodextrin hydrate (β-CD) improved the selectivity of the modified electrode. The fabricated β-CD/SnS₂/SPCE was employed to determine the presence of PCBs by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The detection range was 0.625-80 μM, with a limit detection of approximately 5 μM. The electrodes were as stable as 88% after 7 days' storage. The results showed that the β-CD successfully encapsulated PCBs to achieve an electrochemical sensor that reduced the time and increased the convenience of PCBs detection.

Nanostructure-based Sensitive Electrochemical Immunosensors

It is well-known that electrochemical immunosensors have many advantages, including but not limited to high sensitivity, simplicity in application, low-cost production, automated control and potential miniaturization. Due to specific antigen–antibody recognition, electrochemical immunosensors also have provided exceptional possibilities for real-time trace detection of analytical biotargets, which consists of small molecules (such as natural toxins and haptens), macromolecules, cells, bacteria, pathogens or viruses. Recently, the advances in the development of electrochemical immunosensors can be classified into the following directions: the first is using electrochemical detection techniques (voltammetric, amperometric, impedance spectroscopic, potentiometric, piezoelectric, conductometric and alternating current voltammetric) to achieve high sensitivity regarding the electrochemical change of electrochemical signal transduction; the second direction is developing sensor configurations (microfluidic and paper-based platforms, microelectrodes and electrode arrays) for simultaneous multiplex high-throughput analyses; and the last is designing nanostructured materials serving as sensing interfaces to improve sensor sensitivity and selectivity. This chapter introduces the working principle and summarizes the state-of-the-art of electrochemical immunosensors during the past few years with practically relevant details for: (a) metal nanoparticle- and quantum dot-labeled immunosensors; (b) enzyme-labeled immunosensors; and (c) magnetoimmunosensors. The importance of various types of nanomaterials is also thoroughly reviewed to obtain an insight into understanding the theoretical basis and practical orientation for the next generation of diagnostic devices.

Recent progress in the development of recognition bioelements for polychlorinated biphenyls detection: Antibodies and aptamers

Polychlorinated biphenyls (PCBs) are persistent pollutants, which have expanded in foods and the environment. Detection of PCBs is considered essential due to recognized side-effects of PCBs on health and the public concerns in this regard. On the other hand, due to the trace levels of these organic chlorine compounds, reliable and sensitive assays must be developed. Recognition elements are essential parts of analytical detection assays and sensors of PCBs since these elements are involved in the selective identification of the analytes of interest. Understanding the fundamentals of the recognition elements of PCBs and the benefits of the sensor strategies result in the development of next-generation recognition devices. This review aimed to highlight the recent progress in the recognition elements as key parts of biosensors. We initially, focused on the developed antibody-based biosensors for the detection of PCBs, followed by discussing the aptamers as novel recognition elements. Furthermore, the recent advancement in the development of aptamer-based solid phase extractions has been evaluated. These findings could contribute to improving the design of commercial PCB-kits in the future.

Pushing the limits of electrochemistry toward challenging applications in clinical diagnosis, prognosis, and therapeutic action

Constant progress in the identification of biomarkers at different molecular levels in samples of different natures, and the need to conduct routine analyses, even in limited-resource settings involving simple and short protocols, are examples of the growing current clinical demands not satisfied by conventional available techniques. In this context, the unique features offered by electrochemical biosensors, including affordability, real-time and reagentless monitoring, simple handling and portability, and versatility, make them especially interesting for adaptation to the increasingly challenging requirements of current clinical and point-of-care (POC) diagnostics. This has allowed the continuous development of strategies with improved performance in the clinical field that were unthinkable just a few years ago. After a brief introduction to the types and characteristics of clinically relevant biomarkers/samples, requirements for their analysis, and currently available methodologies, this review article provides a critical discussion of the most important developments and relevant applications involving electrochemical biosensors reported in the last five years in response to the demands of current diagnostic, prognostic, and therapeutic actions related to high prevalence and high mortality diseases and disorders. Special attention is paid to the rational design of surface chemistry and the use/modification of state-of-the-art nanomaterials to construct electrochemical bioscaffolds with antifouling properties that can be applied to the single or multiplex determination of biomarkers of accepted or emerging clinical relevance in particularly complex clinical samples, such as undiluted liquid biopsies, whole cells, and paraffin-embedded tissues, which have scarcely been explored using conventional techniques or electrochemical biosensing. Key points guiding future development, challenges to be addressed to further push the limits of electrochemical biosensors towards new challenging applications, and their introduction to the market are also discussed.

Determination of Cadherin-17 in Tumor Tissues of Different Metastatic Grade Using a Single Incubation-Step Amperometric Immunosensor

This paper reports the development of an amperometric immunosensing platform for the determination of cadherin-17 (CDH-17), an atypical adhesion protein involved in the progression, metastatic potential, and survival of high prevalence gastric, hepatocellular, and colorectal tumors. The methodology developed relies on the efficient capture and enzymatic labeling of the target protein on the magnetic microparticles (MBs) surface using commercial antibodies and amperometric transduction at screen-printed carbon electrodes (SCPEs) through the HRP/H₂O₂/HQ system. The developed immunosensing platform allows the selective determination of the target protein at low ng mL^-1 level (LOD of 1.43 ng mL^-1) in 45 min and using a single incubation step. The electrochemical immunosensor was successfully used for the accurate determination of the target protein in a small amount (0.5 μg) of raw lysates of colon cancer cells with different metastatic potential as well as in extracts from paraffin embedded cancer colon tissues of different metastatic grade.

Single-Step Incubation Determination of miRNAs in Cancer Cells Using an Amperometric Biosensor Based on Competitive Hybridization onto Magnetic Beads

This work reports an amperometric biosensor for the determination of miRNA-21, a relevant oncogene. The methodology involves a competitive DNA-target miRNA hybridization assay performed on the surface of magnetic microbeads (MBs) and amperometric transduction at screen-printed carbon electrodes (SPCEs). The target miRNA competes with a synthetic fluorescein isothiocyanate (FITC)-modified miRNA with an identical sequence for hybridization with a biotinylated and complementary DNA probe (b-Cp) immobilized on the surface of streptavidin-modified MBs (b-Cp-MBs). Upon labeling, the FITC-modified miRNA attached to the MBs with horseradish peroxidase (HRP)-conjugated anti-FITC Fab fragments and magnetic capturing of the MBs onto the working electrode surface of SPCEs. The cathodic current measured at −0.20 V (versus the Ag pseudo-reference electrode) was demonstrated to be inversely proportional to the concentration of the target miRNA. This convenient biosensing method provided a linear range between 0.7 and 10.0 nM and a limit of detection (LOD) of 0.2 nM (5 fmol in 25 μL of sample) for the synthetic target miRNA without any amplification step. An acceptable selectivity towards single-base mismatched oligonucleotides, a high storage stability of the b-Cp-MBs, and usefulness for the accurate determination of miRNA-21 in raw total RNA (RNA_t) extracted from breast cancer cells (MCF-7) were demonstrated.

Ready-to-use protein G-conjugated gold nanorods for biosensing and biomedical applications

BACKGROUND

Gold nanorods (GNRs) display unique capacity to absorb and scatter near infrared light, which arises from their peculiar composition of surface plasmon resonances. For this reason, GNRs have become an innovative material of great hope in nanomedicine, in particular for imaging and therapy of cancer, as well as in photonic sensing of biological agents and toxic compounds for e.g. biomedical diagnostics, forensic analysis and environmental monitoring. As the use of GNRs is becoming more and more popular, in all these contexts, there is emerging a latent need for simple and versatile protocols for their modification with targeting units that may convey high specificity for any analyte of interest of an end-user.

RESULTS

We introduce protein G-coated GNRs as a versatile solution for the oriented immobilization of antibodies in a single step of mixing. We assess this strategy against more standard covalent binding of antibodies, in terms of biocompatibility and efficiency of molecular recognition in buffer, serum and plasma, in the context of the development of a direct immunoenzymatic assay. In both cases, we estimate an average of around 30 events of molecular recognition per particle. In addition, we disclose a convenient protocol to store these particles for months in a freezer, without any detrimental effect.

CONCLUSIONS

The biocompatibility and efficiency of molecular recognition is similar in either case of GNRs that are modified with antibodies by covalent binding or oriented immobilization through protein G. However, protein G-coated GNRs are most attractive for an end-user, owing to their unique versatility and ease of bioconjugation with antibodies of her/his choice.

Universal mobile electrochemical detector designed for use in resource-limited applications

This paper describes an inexpensive, handheld device that couples the most common forms of electrochemical analysis directly to "the cloud" using any mobile phone, for use in resource-limited settings. The device is designed to operate with a wide range of electrode formats, performs on-board mixing of samples by vibration, and transmits data over voice using audio--an approach that guarantees broad compatibility with any available mobile phone (from low-end phones to smartphones) or cellular network (second, third, and fourth generation). The electrochemical methods that we demonstrate enable quantitative, broadly applicable, and inexpensive sensing with flexibility based on a wide variety of important electroanalytical techniques (chronoamperometry, cyclic voltammetry, differential pulse voltammetry, square wave voltammetry, and potentiometry), each with different uses. Four applications demonstrate the analytical performance of the device: these involve the detection of (i) glucose in the blood for personal health, (ii) trace heavy metals (lead, cadmium, and zinc) in water for in-field environmental monitoring, (iii) sodium in urine for clinical analysis, and (iv) a malarial antigen (Plasmodium falciparum histidine-rich protein 2) for clinical research. The combination of these electrochemical capabilities in an affordable, handheld format that is compatible with any mobile phone or network worldwide guarantees that sophisticated diagnostic testing can be performed by users with a broad spectrum of needs, resources, and levels of technical expertise.

Electrochemical bioassay for the detection of TNF-α using magnetic beads and disposable screen-printed array of electrodes

BACKGROUND

In this study we have developed an electrochemical bioassay for the analysis of TNF-α, coupling magnetic beads with disposable electrochemical platforms. TNF-α is a pro inflammatory cytokine that participates in the regulation of immune defense against various pathogens and the recovery from injury. It plays a central role in the development of many inflammatory diseases. The bioassay was based on a sandwich format using alkaline phosphatase as an enzymatic label and an eight-sensor screen-printed array as an electrochemical transducer.

RESULTS

The modified magnetic beads were captured by a magnet on the surface of each graphite working electrode of the array and the electrochemical detection was thus achieved through the addition of the alkaline phosphatase substrate (1-naphthylphosphate); 1-naphthol produced during the enzymatic reaction was detected using differential pulse voltammetry. The parameters influencing the different steps of the assay were optimized in order to reach the best sensitivity and specificity.

CONCLUSION

The proposed strategy offers great promise for analysis of clinical diagnostics, considering also that arrays allow the simultaneous analysis of different samples.

Rapid flow injection electrochemical detection of 3,3′,4,4′ tetrachlorobiphenyl using stabilized lipid membranes with incorporated sheep antibody

An electrochemical biosensor based on a supported polymerized lipid film with incorporated sheep anti-3,3′,4,4′ tetrachlorobiphenyl (PCB congener 77) antibody using flow injection analysis was developed. The polymerized lipid film contained 85% (w/w) dipalmitoylphosphatidylcholine (DPPC) and 15% (w/w) dipalmitoylphosphatidic acid (DPPA), methacrylic acid, ethylene glycol dimethacrylate, AIBN and sheep anti-congener 77 antiserum. Congener 77 was injected into flowing carrier electrolyte and the flow stopped to detect the antigen. These membranes gave only a single transient proportional to log [congener 77] from 10−8 to 10−5 M, with a detection limit of ca. 10−8 M. A membrane containing 35% (w/w) DPPA was used to examine regeneration. The maximum number of cycles was about 5.

Magnetic Beads-Based Electrochemical Sensors Applied to the Detection and Quantification of Bioterrorism/Biohazard Agents

Nowadays, detecting the presence of bioterrorism and biohazard agents in environmental and food samples is of great concern, due to their toxicity, and because many of them are prone to be used in terrorism attacks. The use of functionalized magnetic beads (MBs) in the development of electrochemical immuno- and genosensors has resulted in innovative and powerful detection strategies that may be applied to environmental, food and clinical analysis. This review describes current research on the combination of functionalized MBs with electrochemical detection for the development of magnetobiosensors applied to rapid, sensitive and specific detection of bioterrorism and biohazard agents.

Strategies for electrochemical detection in immunochemistry

In this review, the current status of research in electrochemical immunosensors is considered. We primarily focus on label-free and enzyme-labeled immunosensors, and the analytical capabilities of these devices are discussed. Moreover, the use of magnetic beads as new materials for immunosensors coupled with electrochemical sensing is also described, together with the application of new molecules such as aptamers as specific biorecognition elements. Examples of the applicability of these devices in solving various analytical problems in clinical, environmental and food fields are reported. Finally, the prospects for the further development of immunosensor technologies are shown.

Fluorescently imaged particle counting immunoassay for sensitive detection of DNA modifications

Modifications of genomic DNA may change gene expression and cause adverse health effects. Here we for the first time demonstrate a particle counting immunoassay for rapid and sensitive detection of DNA modifications using benzo[a]pyrenediol epoxide (BPDE)-DNA adducts as an example. The BPDE-adducted DNA is specifically captured by immunomagnetic particles and then isolated from unmodified DNA by applying an external magnetic field. By taking advantage of the fluorescence signal amplification through multiple labeling of captured DNA by OliGreen dye, the captured BPDE-DNA adducts can be quantified by particle counting from fluorescence imaging. This clearly demonstrates that the number of fluorescently countable particles is proportional to the modification content in genomic DNA. It is interesting to note that the background fluorescence signal caused by nonspecific adsorption of OliGreen dye can be more effectively quenched than that induced by the binding of OliGreen dye to ssDNA, allowing for significant reduction in the background fluorescence and further enhancing the detection sensitivity. The developed method can detect trace BPDE-DNA adducts as low as 180 fM in the presence of 1 billion times more normal nucleotides in genomic DNA and has a dynamic range over 4 orders of magnitude. By using anti-5-methylcytosine antibody, the method is extended to the detection of global DNA methylation. With high sensitivity and specificity, this rapid and easy-to-perform analytical method for DNA modifications shows a broad spectrum of potential applications in genotoxical and epigenetic analysis.

Disposable immunosensor for cortisol using functionalized magnetic particles

A novel electrochemical immunosensor using screen-printed carbon electrodes and functionalized magnetic particles was developed for the determination of cortisol. Anti-cortisol antibody was immobilized onto protein A-modified magnetic particles and a direct competitive immunoassay involving cortisol antigen labeled with alkaline phosphatase (AP) was employed. The resulting conjugate was trapped on the surface of the screen-printed electrode with a small magnet. Cortisol quantification was accomplished by using 1-naphthyl phosphate as AP substrate and differential pulse voltammetry for the detection of the formed 1-naphthol. The variables involved in the preparation of the immunosensor and in the electrochemical detection reaction were optimized. The calibration plot obtained for cortisol exhibited a linear range between 5.0 x 10(-3) and 150 ng mL(-1) (r = 0.993). The limit of detection was 3.5 pg mL(-1), and the EC(50) was 0.19 ng mL(-1). These results demonstrate the high sensitivity achieved with this immunosensor design which compares favourably with other previous reports. The immunosensor also exhibited a high selectivity with respect to other corticosteroid compounds closely related to cortisol. The usefulness of the immunosensor for the analysis of real samples was demonstrated by analyzing certified human sera containing cortisol at two different concentration levels.

Single bead-based electrochemical biosensor

A simple, robust, single bead-based electrochemical biosensor was fabricated and characterized. The sensor's working electrode consists of an electrochemically etched platinum wire, with a nominal diameter of 25 microm, hermetically heat-fusion sealed in a pulled glass capillary (micropipette). The sealing process does not require any epoxy or glue. A commercially available, densely functionalized agarose bead was mounted on the tip of the etched platinum wire. The use of a pre-functionalized bead eliminates the tedious and complicated surface functionalization process that is often the bottleneck in the development of electrochemical biosensors. We report on the use of a biotin agarose bead-based, micropipette, electrochemical (Bio-BMP) biosensor to monitor H(2)O(2) concentration and the use of a streptavidin bead-based, micropipette, electrochemical (SA-BMP) biosensor to detect DNA amplicons. The Bio-BMP biosensor's response increased linearly as the H(2)O(2) concentration increased in the range from 1 x 10(-6) to 1.2 x10(-4)M with a detection limit of 5 x 10(-7)M. The SA-BMP was able to detect the amplicons of 1pg DNA template of B. Cereus bacteria, thus providing better detection sensitivity than conventional gel-based electropherograms.

Rapid and dynamic intracellular patterning of cell-internalized magnetic fluorescent nanoparticles

Conjugated magnetic nanoparticles have recently demonstrated potential in activating unique and specific activity within cells. Leveraging microfabrication, we have developed a technique of localizing nanoparticles to specific, subcellular locations by a micropatterned ferromagnetic substrate. Controlled patterns of nanoparticles were assembled and dynamically controlled with submicrometer precision within live cells. We anticipate that the technique will be useful as a compact, simple method of generating localizable, subcellular chemical and mechanical signals, compatible with standard microscopy.

Magnetic particle-based hybrid platforms for bioanalytical sensors

Biomagnetic nano and microparticles platforms have attracted considerable interest in the field of biological sensors due to their interesting physico-chemical properties, high specific surface area, good mechanical stability and opportunities for generating magneto-switchable devices. This review discusses recent advances in the development and characterization of active biomagnetic nanoassemblies, their interaction with biological molecules and their use in bioanalytical sensors.

Magnetic microbead-based electrochemical immunoassays

This review provides a summary of recent works concerning electrochemical immunoassays using magnetic microbeads as a solid phase. Recent research activity has led to innovative and powerful detection strategies that have been resulted in sensitive electrochemical detection. Coupling of magnetic microbeads with highly sensitive electrochemical detection provides a useful analytical method for environmental evaluation and clinical diagnostics, etc. The huge surface area and high dispersion capability of magnetic microbeads strongly contributes towards the development of new sensitive, rapid, user-friendly, and miniaturized electrochemical immunoassay systems. Moreover, the immunocomplexes formed on the magnetic microbead surface can be easily detected without pretreatment steps such as preconcentration or purification, which are normally required for standard methods. The discussion in this review is organized in two main subjects that include magnetic-microbead-based assays using enzyme labels and nanoparticle tags.

Intracellular patterning of internalized magnetic fluorescent nanoparticles

We have designed, simulated, and fabricated micro-magnetic substrates for the reversible self-assembly of cell-internalized magnetic fluorescent nanoparticles according to lithographically defined patterns within live cells. Magnetic nanoparticles have recently demonstrated potential in activating highly specific activity within single cells. Using microfabrication, we have developed a technique of localizing both particles and large magnetic fields to highly specific, engineered, sub-cellular locations with various modes of operation. The substrates were simulated in 3 dimensions with ANSYS FEA, and consist of micro-patterned, electroplated permalloy elements planarized with SU-8. Various modes of magnet-orientation dependent patterns of nanoparticles were generated and verified within live cells, with their precise location verified under separate blue and green (absorption and emission wavelengths of the particles) filters using a fluorescent microscope. Results correspond well with modeled positions and response time. We anticipate using the tool as a compact, simple method of generating highly localized, easily distinguishable, sub-cellular chemical and mechanical signals that is compatible with standard biological fluorescence setups.

Immunoassay for folic acid detection in vitamin-fortified milk based on electrochemical magneto sensors

An immunoassay-based strategy for folic acid in vitamin-fortified milk with electrochemical detection using magneto sensors is described for the first time. Among direct and indirect competitive formats, best performance was achieved with an indirect competitive immunoassay. The immunological reaction for folic acid (FA) detection was performed, for the first time on the magnetic bead as solid support by the covalent immobilization of a protein conjugate BSA-FA on tosyl-activated magnetic bead. Further competition for the specific antibody between FA in the food sample and FA immobilized on the magnetic bead was achieved, followed by the reaction with a secondary antibody conjugated with HRP (AntiIgG-HRP). Then, the modified magnetic beads were easily captured by a magneto sensor made of graphite-epoxy composite (m-GEC) which was also used as the transducer for the electrochemical detection. The performance of the immunoassay-based strategy with electrochemical detection using magneto sensors was successfully evaluated using spiked-milk samples and compared with a novel magneto-ELISA based on optical detection. The detection limit was found to be of the order of microgl(-1) (13.1 nmoll(-1), 5.8 microgl(-1)) for skimmed milk. Commercial vitamin-fortified milk samples were also evaluated obtaining good accuracy in the results. This novel strategy offers great promise for rapid, simple, cost-effective and on-site analysis of biological and food samples.

Different approaches for the detection of thrombin by an electrochemical aptamer-based assay coupled to magnetic beads

Different assay formats based on the coupling of magnetic beads with electrochemical transduction were compared here for the detection of thrombin by using a thrombin specific aptamer. By using the thrombin-binding aptamer, a direct and an indirect competitive assay for thrombin have been developed by immobilising the aptamer or the protein, respectively. Moreover, another strategy was based on the direct measurement of the enzymatic product of thrombin captured by the immobilised aptamer. All the assays were developed by coupling the electrochemical transduction with the innovative and advantageous use of magnetic beads. The assays based on the immobilisation of the protein were not successful since no binding was recorded between thrombin and its aptamer. With the direct competitive assay, when the aptamer was immobilised onto the magnetic beads, a detection limit of 430nM for thrombin was achieved. A lower detection limit for the protein (175nM) was instead obtained by detecting the product of the enzymatic reaction catalysed by thrombin. All these assays were finally compared with a sandwich assay which reached a detection limit of 0.45nM of thrombin demonstrating the best analytical performances. With this comparison the importance of a deep study on the different analytical approaches for thrombin detection to reach the performances of the best assay configuration has been demonstrated.

Polychlorinated biphenyls (PCBs) detection in milk samples by an electrochemical magneto-immunosensor (EMI) coupled to solid-phase extraction (SPE) and disposable low-density arrays

An electrochemical immunosensor for polychlorinated biphenyl (PCB) detection based on graphite screen-printed low-density arrays and on magnetic beads is reported. The immunological reaction for the detection of PCBs is based on a direct competitive assay using alkaline phosphatase (AP) as enzymatic label. After the immunochemical recognition, the modified magnetic beads are captured by a magnet on the surface of the graphite working electrode. The electrochemical detection is thus achieved through the addition of the AP substrate (alpha-naphthyl-phosphate). Two different antibodies (sIgG anti-PCB28 and rIgG anti-PCB77) were tested and compared in terms of sensitivity and ability to recognise different congeners. The developed electrochemical magneto-immunosensor (EMI) was successfully combined with solid-phase extraction (SPE) for the analysis of PCBs in milk samples. In spiked samples a recovery of 80% was obtained. The proposed strategy offers great promise for rapid, simple, cost-effective, and on-site analysis of clinical, food and environmental samples, considering also that low-density arrays allow the simultaneous analysis of different processed samples.