Bead magnetorelaxometry with an on-chip magnetoresistive sensor

Two Orders of Magnitude Boost in the Detection Limit of Droplet-Based Micro-Magnetofluidics with Planar Hall Effect Sensors

Magnetofluidics is a dynamic research field, which requires novel sensor solutions to boost the detection limit of tiny quantities of magnetized objects. Here, we present a sensing strategy relying on planar Hall effect sensors in droplet-based micro-magnetofluidics for the detection of a multiphase liquid flow, i.e., superparamagnetic aqueous droplets in an oil carrier phase. The high resolution of the sensor allows the detection of nanoliter-sized superparamagnetic droplets with a concentration of 0.58 mg/cm³, even when they are biased in a geomagnetic field only. The limit of detection can be boosted another order of magnitude, reaching 0.04 mg/cm³ (1.4 million particles in a single 100 nL droplet) when a magnetic field of 5 mT is applied to bias the droplets. With this performance, our sensing platform outperforms the state-of-the-art solutions in droplet-based micro-magnetofluidics by a factor of 100. This allows us to detect ferrofluid droplets in clinically and biologically relevant concentrations and even below without the need of externally applied magnetic fields. These results open the route for new strategies of the utilization of ferrofluids in microfluidic geometries in, e.g., bio(-chemical) or medical applications.

Characterization of Binding of Magnetic Nanoparticles to Rolling Circle Amplification Products by Turn-On Magnetic Assay

The specific binding of oligonucleotide-tagged 100 nm magnetic nanoparticles (MNPs) to rolling circle products (RCPs) is investigated using our newly developed differential homogenous magnetic assay (DHMA). The DHMA measures ac magnetic susceptibility from a test and a control samples simultaneously and eliminates magnetic background signal. Therefore, the DHMA can reveal details of binding kinetics of magnetic nanoparticles at very low concentrations of RCPs. From the analysis of the imaginary part of the DHMA signal, we find that smaller MNPs in the particle ensemble bind first to the RCPs. When the RCP concentration increases, we observe the formation of agglomerates, which leads to lower number of MNPs per RCP at higher concentrations of RCPs. The results thus indicate that a full frequency range of ac susceptibility observation is necessary to detect low concentrations of target RCPs and a long amplification time is not required as it does not significantly increase the number of MNPs per RCP. The findings are critical for understanding the underlying microscopic binding process for improving the assay performance. They furthermore suggest DHMA is a powerful technique for dynamically characterizing the binding interactions between MNPs and biomolecules in fluid volumes.

Biosensing Using Magnetic Particle Detection Techniques

Magnetic particles are widely used as signal labels in a variety of biological sensing applications, such as molecular detection and related strategies that rely on ligand-receptor binding. In this review, we explore the fundamental concepts involved in designing magnetic particles for biosensing applications and the techniques used to detect them. First, we briefly describe the magnetic properties that are important for bio-sensing applications and highlight the associated key parameters (such as the starting materials, size, functionalization methods, and bio-conjugation strategies). Subsequently, we focus on magnetic sensing applications that utilize several types of magnetic detection techniques: spintronic sensors, nuclear magnetic resonance (NMR) sensors, superconducting quantum interference devices (SQUIDs), sensors based on the atomic magnetometer (AM), and others. From the studies reported, we note that the size of the MPs is one of the most important factors in choosing a sensing technique.

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.

On-chip magnetometer for characterization of superparamagnetic nanoparticles

An on-chip magnetometer was fabricated by integrating a planar Hall magnetoresistive (PHR) sensor with microfluidic channels. The measured in-plane field sensitivities of an integrated PHR sensor with NiFe/Cu/IrMn trilayer structure were extremely high at 8.5 μV Oe(-1). The PHR signals were monitored during the oscillation of 35 pL droplets of magnetic nanoparticles, and reversed profiles for the positive and negative z-fields were measured, where magnitudes increased with the applied z-field strength. The measured PHR signals for 35 pL droplets of magnetic nanoparticles versus applied z-fields showed excellent agreement with magnetization curves measured by a vibrating sample magnetometer (VSM) of 3 μL volume, where a PHR voltage of 1 μV change is equivalent to 0.309 emu cc(-1) of the volume magnetization with a magnetic moment resolution of ~10(-10) emu.

A magnetic nanobead-based bioassay provides sensitive detection of single- and biplex bacterial DNA using a portable AC susceptometer

Bioassays relying on magnetic read-out using probe-tagged magnetic nanobeads are potential platforms for low-cost biodiagnostic devices for pathogen detection. For optimal assay performance it is crucial to apply an easy, efficient and robust bead-probe conjugation protocol. In this paper, sensitive (1.5 pM) singleplex detection of bacterial DNA sequences is demonstrated in a portable AC susceptometer by a magnetic nanobead-based bioassay principle; the volume-amplified magnetic nanobead detection assay (VAM-NDA). Two bead sizes, 100 and 250 nm, are investigated along with a highly efficient, rapid, robust, and stable conjugation chemistry relying on the avidin–biotin interaction for bead-probe attachment. Avidin-biotin conjugation gives easy control of the number of detection probes per bead; thus allowing for systematic investigation of the impact of varying the detection probe surface coverage upon bead immobilization in rolling circle amplified DNA-coils. The existence of an optimal surface coverage is discussed. Biplex VAM-NDA detection is for the first time demonstrated in the susceptometer: Semi-quantitative results are obtained and it is concluded that the concentration of DNA-coils in the incubation volume is of crucial importance for target quantification. The present findings bring the development of commercial biodiagnostic devices relying on the VAM–NDA further towards implementation in point-of-care and outpatient settings.

Integrated capture, transport, and magneto-mechanical resonant sensing of superparamagnetic microbeads using magnetic domain walls

An integrated platform for the capture, transport, and detection of individual superparamagnetic microbeads is described for lab-on-a-chip biomedical applications. Magnetic domain walls in magnetic tracks have previously been shown to be capable of capturing and transporting individual beads through a fluid at high speeds. Here it is shown that the strong magnetostatic interaction between a bead and a domain wall leads to a distinct magneto-mechanical resonance that reflects the susceptibility and hydrodynamic size of the trapped bead. Numerical and analytical modeling is used to quantitatively explain this resonance, and the magneto-mechanical resonant response under sinusoidal drive is experimentally characterized both optically and electrically. The observed bead resonance presents a new mechanism for microbead sensing and metrology. The dual functionality of domain walls as both bead carriers and sensors is a promising platform for the development of lab-on-a-bead technologies.