High sensitivity detection of molecular recognition using magnetically labelled biomolecules and magnetoresistive sensors

Gold nanoparticle-decorated fluorine-doped tin oxide substrate for sensitive label-free OIRD microarray chips

Oblique incidence reflectance difference (OIRD) is an emerging technique enabling real-time and label-free detection of bio-affinity binding events on microarrays. The interfacial architecture of the microarray chip is critical to the performance of OIRD detection. In this work, a sensitive label-free OIRD microarray chip was developed by using gold nanoparticle-decorated fluorine-doped tin oxide (AuNPs-FTO) slides as a chip substrate. This AuNPs-FTO chip demonstrates a higher signal-to-noise ratio and improved sensitivity compared to that built on FTO glass, showing a detection limit of as low as 10 ng mL-1 for the model target, HRP-conjugated streptavidin. On-chip ELISA experiments and optical calculations suggest that the enhanced performance is not only due to the higher probe density enabling a high capture efficiency toward the target, but most importantly, the AuNP layer arouses optical interference to improve the intrinsic sensitivity of OIRD. This work provides an effective strategy for constructing OIRD-based microarray chips with enhanced sensitivity, and may help extend their practical applications in various fields.

Real-time temperature correction for magnetoresistive biosensors integrated with temperature modulator

Magnetoresistance-based biosensors utilize changes in electrical resistance upon varying magnetic fields to measure biological molecules or events involved with magnetic tags. However, electrical resistance fluctuates with temperature. To decouple unwanted temperature-dependent signals from the signal of interest, various methods have been proposed to correct signals from magnetoresistance-based biosensors. Yet, there is still a need for a temperature correction method capable of instantaneously correcting signals from all sensors in an array, as multiple biomarkers need to be detected simultaneously with a group of sensors in a central laboratory or point-of-care setting. Here we report a giant magnetoresistive biosensor system that enables real-time temperature correction for individual sensors using temperature correction coefficients obtained through a temperature sweep generated by an integrated temperature modulator. The algorithm with individual temperature correction coefficients obviously outperformed that using the average temperature correction coefficient. Further, temperature regulation did not eliminate temperature-dependent signals completely. To demonstrate that the method can be used in biomedical applications where large temperature variations are involved, binding kinetics experiments and melting curve analysis were conducted with the temperature correction method. The method successfully removed all temperature-dependent artifacts and thus produced more precise kinetic parameters and melting temperatures of DNA hybrids.

Current Progress of Magnetoresistance Sensors

Magnetoresistance (MR) is the variation of a material’s resistivity under the presence of external magnetic fields. Reading heads in hard disk drives (HDDs) are the most common applications of MR sensors. Since the discovery of giant magnetoresistance (GMR) in the 1980s and the application of GMR reading heads in the 1990s, the MR sensors lead to the rapid developments of the HDDs’ storage capacity. Nowadays, MR sensors are employed in magnetic storage, position sensing, current sensing, non-destructive monitoring, and biomedical sensing systems. MR sensors are used to transfer the variation of the target magnetic fields to other signals such as resistance change. This review illustrates the progress of developing nanoconstructed MR materials/structures. Meanwhile, it offers an overview of current trends regarding the applications of MR sensors. In addition, the challenges in designing/developing MR sensors with enhanced performance and cost-efficiency are discussed in this review.

Magneto-Impedance Biosensor Sensitivity: Effect and Enhancement

Biosensors based on magneto-impedance (MI) effect are powerful tools for biomedical applications as they are highly sensitive, stable, exhibit fast response, small in size, and have low hysteresis and power consumption. However, the performance of these biosensors is influenced by a variety of factors, including the design, geometry, materials and fabrication procedures. Other less appreciated factors influencing the MI effect include measuring circuit implementation, the material used for construction, geometry of the thin film sensing element, and patterning shapes compatible with the interface microelectronic circuitry. The type magnetic (ferrofluid, Dynabeads, and nanoparticles) and size of the particles, the magnetic particle concentration, magnetic field strength and stray magnetic fields can also affect the sensor sensitivity. Based on these considerations it is proposed that ideal MI biosensor sensitivity could be achieved when the sensor is constructed in sandwich thick magnetic layers with large sensing area in a meander shape, measured with circuitry that provides the lowest possible external inductance at high frequencies, enclosed by a protective layer between magnetic particles and sensing element, and perpendicularly magnetized when detecting high-concentration of magnetic particles.

Reviewing Magnetic Particle Preparation: Exploring the Viability in Biosensing

In this review article, we conceptually investigated the requirements of magnetic nanoparticles for their application in biosensing and related them to example systems of our thin-film portfolio. Analyzing intrinsic magnetic properties of different magnetic phases, the size range of the magnetic particles was determined, which is of potential interest for biosensor technology. Different e-beam lithography strategies are utilized to identify possible ways to realize small magnetic particles targeting this size range. Three different particle systems from 500 μm to 50 nm are produced for this purpose, aiming at tunable, vertically magnetized synthetic antiferromagnets, martensitic transformation in a single elliptical, disc-shaped Heusler Ni₅₀Mn_32.5Ga_17.5 particle and nanocylinders of Co₂MnSi-Heusler compound. Perspectively, new applications for these particle systems in combination with microfluidics are addressed. Using the concept of a magnetic on–off ratchet, the most suitable particle system of these three materials is validated with respect to magnetically-driven transport in a microfluidic channel. In addition, options are also discussed for improving the magnetic ratchet for larger particles.

Biomarkers detection with magnetoresistance-based sensors

Biosensing platforms for detecting and quantifying biomarkers have played an important role in the past decade. Among them, platforms based on magnetoresistance (MR) sensing technology are attractive. The resistance value of the material changes with the externally applied magnetic field is the core mechanism of MR sensing technology. A typical MR-based sensor has the characteristics of cost-effective, simple operation, high compactness, and high sensitivity. Moreover, using magnetic nanoparticles (MNPs) as labels, MR-based sensors have the ability to overcome the high background noise of complex samples, so they are particularly suitable for point-of-care testing (POCT). However, the problem still exists. How to obtain high-throughput, that is, multiple detections of biomarkers in MR-based sensors, thereby improving detection efficiency and reducing the burden on patients is an important issue in future work. This paper reviews three MR-based detection technologies for the detection of biomarkers, i.e., anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunneling magnetoresistance (TMR). Based on these three common technologies, different typical applications that include biomedical diagnosis, food safety, and environmental monitoring are presented. Furthermore, the existing MR-based detection method is better expanded to make it more in line with present detection needs by combining different advanced technologies including microfluidics, Microelectromechanical systems (MEMS), and Immunochromatographic test strips (ICTS). And then, a brief discussion of current challenges and perspectives of MR-based sensors are pointed out.

A Review on the Role of Nanosensors in Detecting Cellular miRNA Expression in Colorectal Cancer

BACKGROUND

Colorectal cancer (CRC) is one of the leading causes of death across the globe. Early diagnosis with high sensitivity can prevent CRC progression, thereby reducing the condition of metastasis.

OBJECTIVE

The purpose of this review is (i) to discuss miRNA based biomarkers responsible for CRC, (ii) to brief on the different methods used for the detection of miRNA in CRC, (iii) to discuss different nanobiosensors so far found for the accurate detection of miRNAs in CRC using spectrophotometric detection, piezoelectric detection.

METHODS

The keywords for the review like micro RNA detection in inflammation, colorectal cancer, nanotechnology, were searched in PubMed and the relevant papers on the topics of miRNA related to CRC, nanotechnology-based biosensors for miRNA detection were then sorted and used appropriately for writing the review.

RESULTS

The review comprises a general introduction explaining the current scenario of CRC, the biomarkers used for the detection of different cancers, especially CRC and the importance of nanotechnology and a general scheme of a biosensor. The further subsections discuss the mechanism of CRC progression, the role of miRNA in CRC progression and different nanotechnology-based biosensors so far investigated for miRNA detection in other diseases, cancer and CRC. A scheme depicting miRNA detection using gold nanoparticles (AuNPs) is also illustrated.

CONCLUSION

This review may give insight into the different nanostructures, like AuNPs, quantum dots, silver nanoparticles, MoS2derived nanoparticles, etc., based approaches for miRNA detection using biosensors.

Advances in Magnetoresistive Biosensors

Magnetoresistance (MR) based biosensors are considered promising candidates for the detection of magnetic nanoparticles (MNPs) as biomarkers and the biomagnetic fields. MR biosensors have been widely used in the detection of proteins, DNAs, as well as the mapping of cardiovascular and brain signals. In this review, we firstly introduce three different MR devices from the fundamental perspectives, followed by the fabrication and surface modification of the MR sensors. The sensitivity of the MR sensors can be improved by optimizing the sensing geometry, engineering the magnetic bioassays on the sensor surface, and integrating the sensors with magnetic flux concentrators and microfluidic channels. Different kinds of MR-based bioassays are also introduced. Subsequently, the research on MR biosensors for the detection of protein biomarkers and genotyping is reviewed. As a more recent application, brain mapping based on MR sensors is summarized in a separate section with the discussion of both the potential benefits and challenges in this new field. Finally, the integration of MR biosensors with flexible substrates is reviewed, with the emphasis on the fabrication techniques to obtain highly shapeable devices while maintaining comparable performance to their rigid counterparts.

Magnetization Manipulation of a Flexible Magnetic Sensor by Controlled Stress Application

Spin-based electronic devices on polymer substrates have been intensively investigated because of several advantages in terms of weight, thickness, and flexibility, compared to rigid substrates. So far, most studies have focused on maintaining the functionality of devices with minimum degradation against mechanical deformation, as induced by stretching and bending of flexible devices. Here, we applied repetitive bending stress on a flexible magnetic layer and a spin-valve structure composed of Ta/NiFe/CoFe/Cu/Ni/IrMn/Ta on a polyimide (PI) substrate. It is found that the anisotropy can be enhanced or weakened depending upon the magnetostrictive properties under stress. In the flat state after bending, due to residual compressive stress, the magnetic anisotropy of the positive magnetostrictive free layer is weakened while that of the pinned layer with negative magnetostriction is enhanced. Thus, the magnetic configuration of the spin-valve is appropriate for use as a sensor. Through the bending process, we design a prototype magnetic sensor cell array and successfully show a sensing capability by detecting magnetic microbeads. This attempt demonstrates that appropriate control of stress, induced by repetitive bending of flexible magnetic layers, can be effectively used to modify the magnetic configurations for the magnetic sensor.

Modelling of magnetoimpedance response of thin film sensitive element in the presence of ferrogel: Next step toward development of biosensor for in-tissue embedded magnetic nanoparticles detection

In-tissue embedded magnetic nanoparticle (MNPs) detection is one of the most interesting cases for cancer research. In order to understand the origin, the limits and the way of improvement of magnetic biosensor sensitivity for the detection of 3D mezoscopic distributions of MNPs, we have developed a magnetoimpedance biosensor prototype with a [Cu (3 nm)/FeNi(100 nm)]₅/Cu(500 nm)/[FeNi(100 nm)/Cu(3 nm)]₅ rectangular sensitive element. Magnetoimpedance (MI) responses were measured with and without polyacrylamide ferrogel layer mimicking natural tissue in order to evaluate stray fields of embedded MNPs of γ-Fe₂O₃ iron oxide. A model for MI response based on a solution of Maxwell equations with Landau-Lifshitz equation was developed in order to understand the origin of the prototype sensitivity which reached 1.3% of ΔZ/Z per 1% of MNPs concentration by weight. To make this promising technique useful for magnetically labeled tissue detection, a synthesis of composite gels with MNPs agglomerates compactly located inside pure gel and their MI testing are still necessary.

Investigation of contactless detection using a giant magnetoresistance sensor for detecting prostate specific antigen

This paper presents a contactless detection method for detecting prostate specific antigen with a giant magnetoresistance sensor. In contactless detection case, the prostate specific antigen sample preparation was separated from the sensor that prevented the sensor from being immersed in chemical solvents, and made the sensor implementing in immediately reuse without wash. Experimental results showed that applied an external magnetic field in a range of 50 Oe to 90 Oe, Dynabeads with a concentration as low as 0.1 μg/mL can be detected by this system and could give an approximate quantitation to the logarithmic of Dynabeads concentration. Sandwich immunoassay was employed for preparing PSA samples. The PSA capture was implemented on a gold film modified with a self-assembled monolayer and using biotinylated secondary antibody against PSA and streptavidinylated Dynabeads. With DC magnetic field in the range of 50 to 90 Oe, PSA can be detected with a detection limit as low as 0.1 ng/mL. Samples spiked with different concentrations of PSA can be distinguished clearly. Due to the contactless detection method, the detection system exhibited advantages such as convenient manipulation, reusable, inexpensive, small weight. So, this detection method was a promising candidate in biomarker detection, especially in point of care detection.

Ultrasensitive Magnetic Nanoparticle Detector for Biosensor Applications

Ta/Ru/Co/Ru/Co/Cu/Co/Ni₈₀Fe₂₀/Ta spin-valve giant magnetoresistive (GMR) multilayers were deposited using UHV magnetron sputtering and optimized to achieve a 13% GMR ratio before patterning. The GMR multilayer was patterned into 12 sensor arrays using a combination of e-beam and optical lithographies. Arrays were constructed with 400 nm × 400 nm and 400 nm × 200 nm sensors for the detection of reporter nanoparticles. Nanoparticle detection was based on measuring the shift in high-to-low resistance switching field of the GMR sensors in the presence of magnetic particle(s). Due to shape anisotropy and the corresponding demag field, the resistance state switching fields were significantly larger and the switching field distribution significantly broader in the 400 nm × 200 nm sensors as compared to the 400 nm × 400 nm sensors. Thus, sensor arrays with 400 nm × 400 nm dimensions were used for the demonstration of particle detection. Detection of a single 225 nm Fe₃O₄ magnetic nanoparticle and a small number (~10) of 100 nm nanoparticles was demonstrated. With appropriate functionalization for biomolecular recognition, submicron GMR sensor arrays can serve as the basis of ultrasensitive chemical and biological sensors.

Magnetic sensing platform technologies for biomedical applications

Detection and quantification of a variety of micro- and nanoscale entities, e.g. molecules, cells, and particles, are crucial components of modern biomedical research, in which biosensing platform technologies play a vital role. Confronted with the drastic global demographic changes, future biomedical research entails continuous development of new-generation biosensing platforms targeting even lower costs, more compactness, and higher throughput, sensitivity and selectivity. Among a wide choice of fundamental biosensing principles, magnetic sensing technologies enabled by magnetic field sensors and magnetic particles offer attractive advantages. The key features of a magnetic sensing format include the use of commercially available magnetic field sensing elements, e.g. magnetoresistive sensors which bear huge potential for compact integration, a magnetic field sensing mechanism which is free from interference by complex biomedical samples, and an additional degree of freedom for the on-chip handling of biochemical species rendered by magnetic labels. In this review, we highlight the historical basis, routes, recent advances and applications of magnetic biosensing platform technologies based on magnetoresistive sensors.

Giant Magnetoresistance: Basic Concepts, Microstructure, Magnetic Interactions and Applications

The giant magnetoresistance (GMR) effect is a very basic phenomenon that occurs in magnetic materials ranging from nanoparticles over multilayered thin films to permanent magnets. In this contribution, we first focus on the links between effect characteristic and underlying microstructure. Thereafter, we discuss design criteria for GMR-sensor applications covering automotive, biosensors as well as nanoparticular sensors.

Frequency-Domain Approach To Determine Magnetic Address-Sensor Separation Distance Using the Harmonic Ratio Method

In this work, we describe an approach to determine the distance separating a magnetic address from a scanning magnetoresistive sensor, a critical adjustable parameter for certain bioassay analyses where magnetic nanoparticles are used as labels. Our approach is leveraged from the harmonic ratio method (HRM), a method used in the hard drive industry to control the distance separating a magnetoresistive read head from its data platter with nanometer resolution. At the heart of the HRM is an amplitude comparison of a signal's fundamental frequency to that of its harmonics. When the signal is derived from the magnetic field pattern of a periodic array of magnetic addresses, the harmonic ratio contains the information necessary to determine the separation between the address array and the read head. The elegance of the HRM is that there is no need of additional components to the detection platform to determine a separation distance; the streaming "bit signal" contains all the information needed. In this work, we demonstrate that the tenets governing HRM used in the hard drive industry can be applied to the bioanalytical arena where submicrometer to 100 μm separations are required.

Tyrosinase-catalyzed polymerization of l-DOPA (versusl-tyrosine and dopamine) to generate melanin-like biomaterials for immobilization of enzymes and amperometric biosensing

The tyrosinase-catalyzed polymerization of l-DOPA (versusl-tyrosine and dopamine) is recommended as an excellent system to immobilize enzymes for amperometric biosensing of catechol and glucose.

Simple and portable magnetic immunoassay for rapid detection and sensitive quantification of plant viruses

Plant pathogens cause major economic losses in the agricultural industry because late detection delays the implementation of measures that can prevent their dissemination. Sensitive and robust procedures for the rapid detection of plant pathogens are therefore required to reduce yield losses and the use of expensive, environmentally damaging chemicals. Here we describe a simple and portable system for the rapid detection of viral pathogens in infected plants based on immunofiltration, subsequent magnetic detection, and the quantification of magnetically labeled virus particles. Grapevine fanleaf virus (GFLV) was chosen as a model pathogen. Monoclonal antibodies recognizing the GFLV capsid protein were immobilized onto immunofiltration columns, and the same antibodies were linked to magnetic nanoparticles. GFLV was quantified by immunofiltration with magnetic labeling in a double-antibody sandwich configuration. A magnetic frequency mixing technique, in which a two-frequency magnetic excitation field was used to induce a sum frequency signal in the resonant detection coil, corresponding to the virus concentration within the immunofiltration column, was used for high-sensitivity quantification. We were able to measure GFLV concentrations in the range of 6 ng/ml to 20 μg/ml in less than 30 min. The magnetic immunoassay could also be adapted to detect other plant viruses, including Potato virus X and Tobacco mosaic virus, with detection limits of 2 to 60 ng/ml.

A novel HBV genotypes detecting system combined with microfluidic chip, loop-mediated isothermal amplification and GMR sensors

Genotyping of hepatitis B virus (HBV) can be used for clinical effective therapeutic drug-selection. A novel microfluidic biochip for HBV genotyping has been fabricated, for the first time, integrating loop-mediated isothermal amplification (LAMP), line probes assay (LiPA) and giant magnetoresistive (GMR) sensors. Coupling LAMP with LiPA in microfluidic chip shortened reaction time substantially, and combining LAMP with GMR sensor enabled limit of detection to attain 10 copies mL(-1) target HBV DNA molecules in 1 h. Furthermore, the independent designed GMR sensors and microfluidic chip can decrease manufacturing cost and patient's test-cost, and facilitate GMR detector repeating use for signal detection. In addition, the detection system has a lower background signal owing to application of superparamagnetic nanoclusters. And it can be expected to use for multiple target molecules synchronous detection in microfluidic chip based on a characteristic of stationary reaction temperature of LAMP. In conclusion, the neoteric detecting system is well suitable for quick genotyping diagnosis of clinical HBV and other homothetic biomolecule detection in biological and medical fields.

Continuous-flow particle guiding based on dipolar coupled magnetic superstructures in rotating magnetic fields

Under the influence of homogeneous, rotating magnetic fields, superparamagnetic beads can be assembled into one- and two-dimensional superstructures on demand and used as dynamic components in microfluidic systems for colloidal separation. In this paper, the influence of the magnetic field strength and the rotation frequency on the device efficiency is studied. The optimum region is found to be between 100 and 200 rpm for a magnetic field strength of 330 Oe, while the highest value for separated mass per time (28 pg s(-1)) is achieved for a flow velocity of 370 μm s(-1) at a magnetic field strength of 690 Oe. Furthermore, the employment of superparamagnetic beads as a continuous-flow bioseparation device is shown in a proof-of-principle study.

A Synthetic Biology Project - Developing a single-molecule device for screening drug-target interactions

This review describes a European-funded project in the area of Synthetic Biology. The project seeks to demonstrate the application of engineering techniques and methodologies to the design and construction of a biosensor for detecting drug-target interactions at the single-molecule level. Production of the proteins required for the system followed the principle of previously described "bioparts" concepts (a system where a database of biological parts - promoters, genes, terminators, linking tags and cleavage sequences - is used to construct novel gene assemblies) and cassette-type assembly of gene expression systems (the concept of linking different "bioparts" to produce functional "cassettes"), but problems were quickly identified with these approaches. DNA substrates for the device were also constructed using a cassette-system. Finally, micro-engineering was used to build a magnetoresistive Magnetic Tweezer device for detection of single molecule DNA modifying enzymes (motors), while the possibility of constructing a Hall Effect version of this device was explored. The device is currently being used to study helicases from Plasmodium as potential targets for anti-malarial drugs, but we also suggest other potential uses for the device.

Spintronic platforms for biomedical applications

Since the fundamental discovery of the giant magnetoresistance many spintronic devices have been developed and implemented in our daily life (e.g. information storage and automotive industry). Lately, advances in the sensors technology (higher sensitivity, smaller size) have potentiated other applications, namely in the biological area, leading to the emergence of novel biomedical platforms. In particular the investigation of spintronics and its application to the development of magnetoresistive (MR) biomolecular and biomedical platforms are giving rise to a new class of biomedical diagnostic devices, suitable for bench top bioassays as well as point-of-care and point-of-use devices. Herein, integrated spintronic biochip platforms for diagnostic and cytometric applications, hybrid systems incorporating magnetoresistive sensors applied to neuroelectronic studies and biomedical imaging, namely magneto-encephalography and magneto-cardiography, are reviewed. Also lab-on-a-chip MR-based platforms to perform biological studies at the single molecule level are discussed. Overall the potential and main characteristics of such MR-based biomedical devices, comparing to the existing technologies while giving particular examples of targeted applications, are addressed.

How to design magneto-based total analysis systems for biomedical applications

This article reviews recent developments on magnetoresistive detection of magnetic beads or nanoparticles by nanoscale sized sensors. Sensors are analyzed from an experimental and a numerical point of view in respect to their capability to either localize the position of a single magnetic particle or to detect the number of particles in a certain range. Guidelines are shown up on how to extend single sensors to sensor arrays with very high spatial resolution and how to modify the sensor shape in order to provide long distance measurements. Further, sensors in biological lab-on-a-chip environments are discussed. The magnetic ratchet and a gravitation based microfluidic component are reviewed as important tools to position and, therefore, detect biological components in continuous-flow devices.

Superparamagnetic bead interactions with functionalized surfaces characterized by an immunomicroarray

Magneto-resistive sensors capable of detecting superparamagnetic micro-/nano-sized beads are promising alternatives to standard diagnostic assays based on absorbance or fluorescence and streptavidin-functionalized beads are widely used as an integral part of these sensors. Here we have developed an immunomicroarray for systematic studies of the binding properties of 10 different micro-/nano-sized streptavidin-functionalized beads to a biotin substrate immobilized on SiO(2) with or without surface modification. SiO(2) surface cleaning, immobilized substrate concentration and surface blocking conditions were optimized. Polyethylene glycol-based surfaces with different end groups on the anchor molecule, 2,4,6-trichloro-1,3,5-triazine (TsT), were synthesized and compared with the standard (3-aminopropyl)triethoxysilane (APTS)/glutaraldehyde chemistry. APTS/glutaraldehyde, directly linked TsT and bare H(2)O(2)-activated SiO(2) performed better than polyethylene glycol-modified surfaces. Two beads, Masterbeads and M-280 beads, were found to give superior results compared with other bead types. Antibody/antigen interactions, illustrated by C-reactive protein, were best performed with Masterbeads. The results provide important information concerning the surface binding properties of streptavidin-functionalized beads and the immunomicroarray can be used when optimizing the performance of bead-based biosensors.

Magnetic nanoparticle biosensors

One of the major challenges in medicine is the rapid and accurate measurement of protein biomarkers, cells, and pathogens in biological samples. A number of new diagnostic platforms have recently been developed to measure biomolecules and cells with high sensitivity that could enable early disease detection or provide valuable insights into biology at the systems level. Most biological samples exhibit negligible magnetic susceptibility; therefore, magnetic nanoparticles have been used for diverse applications including biosensing, magnetic separation, and thermal ablation therapy. This review focuses on the use of magnetic nanoparticles for detection of biomolecules and cells based on magnetic resonance effects using a general detection platform termed diagnostic magnetic resonance (DMR). DMR technology encompasses numerous assay configurations and sensing principles, and to date magnetic nanoparticle biosensors have been designed to detect a wide range of targets including DNA/mRNA, proteins, enzymes, drugs, pathogens, and tumor cells. The core principle behind DMR is the use of magnetic nanoparticles as proximity sensors that modulate the spin-spin relaxation time of neighboring water molecules, which can be quantified using clinical MRI scanners or benchtop nuclear magnetic resonance (NMR) relaxometers. Recently, the capabilities of DMR technology were advanced considerably with the development of miniaturized, chip-based NMR (microNMR) detector systems that are capable of performing highly sensitive measurements on microliter sample volumes and in multiplexed format. With these and future advances in mind, DMR biosensor technology holds considerable promise to provide a high-throughput, low-cost, and portable platform for large scale molecular and cellular screening in clinical and point-of-care settings.

Magnetic Nanotechnology for Biodetection

A highly sensitive biodetection technology using nanomagnetic sensors and magnetic nanoparticles (NPs) was developed. Absorption of magnetic NPs by the hybridized DNA alters the sensor resistance and generated electrical signals that can be directly measured with the off-die or on-die circuitry. Assays with DNA concentration down to sub-10 pM with a dynamic range of three orders of magnitude were demonstrated. The proposed biochip can be applied to other bioreaction detections, for example, protein assay, through different surface modifications.

Magneto-optical biosensing platform based on light scattering from self-assembled chains of functionalized rotating magnetic beads

We describe a simple protocol for the rapid, highly sensitive, and quantitative measurement of the concentration of biomolecules in a solution by monitoring light scattered by self-assembled chains of functionalized superparamagnetic beads (SBs) rotating in the solution. A rotating external field (H(ex)) applied to an aqueous solution containing 250 nm diameter biotinylated SBs produced linear chains of SBs rotating in phase with Hex due to magnetically induced self-assembly. At constant Hex, the addition of avidin to the solution led to the formation of longer SB-chains than without the presence of avidin. The generation of longer SB-chains was revealed by increases in the amplitude of the oscillating optical transmittance signal of the magnetic colloid solution. Monitoring changes in the amplitude of the optical transmittance of the solution enabled quantitative determination of the concentration of avidin added to the solution with a sensitivity of 100 pM (6.7 ng/mL) and a dynamic range of at least 3 orders of magnitude. The rotating chains acted as biomolecule probes and micromagnetic mixers, enabling detection of biomolecular recognition in less than 30 s. This approach offers a rapid, highly sensitive, inexpensive, and homogeneous means for detecting biorecognition processes.

Magnetic nanoparticles for biomedical NMR-based diagnostics

Rapid and accurate measurements of protein biomarkers, pathogens and cells in biological samples could provide useful information for early disease diagnosis, treatment monitoring, and design of personalized medicine. In general, biological samples have only negligible magnetic susceptibility. Thus, using magnetic nanoparticles for biosensing not only enhances sensitivity but also effectively reduces sample preparation needs. This review focuses on the use of magnetic nanoparticles for in vitro detection of biomolecules and cells based on magnetic resonance effects. This detection platform, termed diagnostic magnetic resonance (DMR), exploits magnetic nanoparticles as proximity sensors, which modulate the spin-spin relaxation time of water molecules surrounding molecularly-targeted nanoparticles. By developing more effective magnetic nanoparticle biosensors, DMR detection limits for various target moieties have been considerably improved over the last few years. Already, a library of magnetic nanoparticles has been developed, in which a wide range of targets, including DNA/mRNA, proteins, small molecules/drugs, bacteria, and tumor cells, have been quantified. More recently, the capabilities of DMR technology have been further advanced with new developments such as miniaturized nuclear magnetic resonance detectors, better magnetic nanoparticles and novel conjugational methods. These developments have enabled parallel and sensitive measurements to be made from small volume samples. Thus, the DMR technology is a highly attractive platform for portable, low-cost, and efficient biomolecular detection within a biomedical setting.

Magnetoresistive immunosensor for the detection of Escherichia coli O157:H7 including a microfluidic network

A hand held device has been designed for the immunomagnetic detection and quantification of the pathogen Escherichia coli O157:H7 in food and clinical samples. In this work, a technology to manufacture a Lab on a Chip that integrates a 3D microfluidic network with a microfabricated biosensor has been developed. With this aim, the sensing film optimization, the design of the microfluidic circuitry, the development of the biological protocols involved in the measurements and, finally, the packaging needed to carry out the assays in a safe and straightforward way have been completed. The biosensor is designed to be capable to detect and quantify small magnetic field variations caused by the presence of superparamagnetic beads bound to the antigens previously immobilized on the sensor surface via an antibody-antigen reaction. The giant magnetoresistive multilayer structure implemented as sensing film consists of 20[Cu(5.10nm)/Co(2.47 nm)] with a magnetoresistance of 3.20% at 235Oe and a sensitivity up to 0.06 Omega/Oe between 150Oe and 230Oe. Silicon nitride has been selected as optimum sensor surface coating due to its suitability for antibody immobilization. In order to guide the biological samples towards the sensing area, a microfluidic network made of SU-8 photoresist has been included. Finally, a novel packaging design has been fabricated employing 3D stereolithographic techniques. The microchannels are connected to the outside using standard tubing. Hence, this packaging allows an easy replacement of the used devices.

The Optimization of Magnetosandwich Assays for the Sensitive and Specific Detection of Proteins in Serum

Over the past decade, the use of magnetic particles (MPs) as labels in magnetic biosensors has attracted increasing interest because it provides a highly sensitive platform that can meet the diagnostic needs that are currently not met by existing technologies. However, preparing magnetic biosensors for a specific diagnostic application is a challenging task, and the (bio)chemical aspects are often neglected. Hence, one of the major remaining bottlenecks in the development of magnetic biosensors is the lack of an optimized magnetosandwich assay for the highly sensitive and specific detection of proteins in complex sample matrices. Therefore, in this article, we report on the impact of several different aspects of magnetosandwich assay development, that is, surface chemistry, MP size, rinsing procedure, sample matrix, and blocking procedure on the total-assay performance using quartz crystal microbalance and optical microscopy analysis. The optimization focused on the diagnostically relevant protein S100betabeta, a marker for stroke and minor head injury. It was observed that small MPs in combination with a strong rinsing and a BSA/Tween-20 blocking allows for the most specific and sensitive detection of S100betabeta in serum over a wide concentration range.

CRP determination based on a novel magnetic biosensor

The c-reactive protein (CRP) is a very significant human blood marker for inflammatory processes and is routinely determined for many clinical purposes. The widespread and well established detection method for this approximately 115 kDa hepatic protein is the high-sensitivity ELISA assay (hsCRP-ELISA) in blood serum. New approaches in medical CRP diagnosis (e.g. for CVD, inflammatory bowel disease) require rapid quantification in native matrices. A novel CRP determination method based on magnetic detection is described and tested for human blood serum, saliva and urine. The detection principle is based on two different anti-CRP antibodies (monoclonal, IgG) for CRP trapment and labelling. The linear detection range of this immunosensor ranged from 25 ng/ml to 2.5 microg/ml and is therefore much more sensitive than typical hsCRP-ELISA-assays.

1H NMR Detection of superparamagnetic nanoparticles at 1T using a microcoil and novel tuning circuit

Magnetic beads containing superparamagnetic iron oxide nanoparticles (SPIONs) have been shown to measurably change the nuclear magnetic resonance (NMR) relaxation properties of nearby protons in aqueous solution at distances up to approximately 50 microm. Therefore, the NMR sensitivity for the in vitro detection of single cells or biomolecules labeled with magnetic beads will be maximized with microcoils of this dimension. We have constructed a prototype 550 microm diameter solenoidal microcoil using focused gallium ion milling of a gold/chromium layer. The NMR coil was brought to resonance by means of a novel auxiliary tuning circuit, and used to detect water with a spectral resolution of 2.5 Hz in a 1.04 T (44.2MHz) permanent magnet. The single-scan SNR for water was 137, for a 200 micros pi/2 pulse produced with an RF power of 0.25 mW. The nutation performance of the microcoil was sufficiently good so that the effects of magnetic beads on the relaxation characteristics of the surrounding water could be accurately measured. A solution of magnetic beads (Dynabeads MyOne Streptavidin) in deionized water at a concentration of 1000 beads per nL lowered the T(1) from 1.0 to 0.64 s and the T2 * from 110 to 0.91 ms. Lower concentrations (100 and 10 beads/nL) also resulted in measurable reductions in T2 *, suggesting that low-field, microcoil NMR detection using permanent magnets can serve as a high-sensitivity, miniaturizable detection mechanism for very low concentrations of magnetic beads in biological fluids.