Hugo Alexandre Ferreira

Biodetection using magnetically labeled biomolecules and arrays of spin valve sensors (invited)

On-chip spin-valve sensors (2×6 μm2) were used to detect the binding of streptavidin-functionalized superparamagnetic labels to sensor surface-bound biotin. Both micron-sized and nanometer-sized labels were studied. The detection of biomolecular recognition was demonstrated using 2 μm Micromer®-M and 250 nm Nanomag®-D labels, with signals ranging from ∼300 μV to ∼2 mV (8 mA sense current; ∼15 Oe in-plane magnetizing fields). For smaller labels, detection of biomolecular recognition was not achieved. The capability of detecting single labels was demonstrated for label moments down to 2×10−12 emu (2 μm labels), corresponding to signals of 100–400 μV. Although, theoretical calculations suggest that the minimum measurable moment is in the order of 6×10−15 emu, due to noise limitations of the present setup, this limit is in the order of 2×10−13 emu, corresponding to a single 250 nm label. On-chip tapered aluminum current line structures were used for placement of magnetic labels at sensor sites.

Single magnetic microsphere placement and detection on-chip using current line designs with integrated spin valve sensors: Biotechnological applications

Superparamagnetic labels, 400 nm dextran iron oxide particles and 2 μm polymer encapsulated iron oxide microspheres, with biomolecules immobilized on the surface, e.g., the enzyme horseradish peroxidase (20–40 molecules per label) were controllably placed on chip sites (5×15 μm2) using tapered Al current lines (10–20 mA current) and moved to and from adjacent spin valve sensors [2×6 μm,2, magnetoresistance (MR) ∼5%]. Average MR signals of 1.2 and 0.6 mV were obtained for the detection of bulk numbers of 400 nm and 2 μm labels respectively using an on-chip field of 15 Oe and a sense current of 5 mA. The moment per label was calculated at 5×10−13 emu for the 400 nm labels and 5×10−12 emu for the 2 μm labels, illustrating the higher density of the 400 nm particles. MR signals of ∼100 μV were obtained for single 2 μm labels positioned over the spin valve sensor using an on-chip field of 15 Oe and 8 mA sense current. The corresponding sensor saturation occurred at ∼1 mV, with a noise level of ∼10 μV. The estim...

Planar Hall effect sensor for magnetic micro- and nanobead detection

Magnetic bead sensors based on the planar Hall effect in thin films of exchange-biased permalloy have been fabricated and characterized. Typical sensitivities are 3 μV/Oe mA. The sensor response to an applied magnetic field has been measured without and with coatings of commercially available 2 μm and 250 nm magnetic beads used for bioapplications (Micromer-M and Nanomag-D, Micromod, Germany). Detection of both types of beads and single bead detection of 2 μm beads is demonstrated, i.e., the technique is feasible for magnetic biosensors. Single 2 μm beads yield 300 nV signals at 10 mA and 15 Oe applied field.

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

Small magnetoresistive spin valve sensors (2 x 6 microm(2)) were used to detect the binding of single streptavidin functionalized 2 microm magnetic microspheres to a biotinylated sensor surface. The sensor signals, using 8 mA sense current, were in the order of 150-400 microV for a single microsphere depending on sensor sensitivity and the thickness of the passivation layer over the sensor surface. Sensor saturation signals were 1-2 mV representing an estimated 6-20 microspheres, with a noise level of approximately 10 microV. The detection of biomolecular recognition for the streptavidin-biotin model was shown using both single and differential sensor architectures. The signal data compares favourably with previously reported signals for high numbers of magnetic microspheres detected using larger multilayered giant magnetoresistance sensors. A wide range of applications is foreseen for this system in the development of biochips, high sensitivity biosensors and the detection of single molecules and single molecule interactions.

On-chip manipulation and magnetization assessment of magnetic bead ensembles by integrated spin-valve sensors

Manipulation and detection of magnetic beads on a semiconductor chip opens up new perspectives for analysis of magnetically labeled specimens in biomechanical micro-electromechanical systems for biological applications. Sensitive spin-valve sensors were integrated with magnetic field generating conductors to assess the behavior of ensembles of superparamagnetic nanoparticles 300 nm in diameter that contain 75%–80% magnetite. The spin-valve multilayer including a nanooxide layer achieves 8% magnetoresistance (MR) for an integrated device of 2×16 μm2. Motion of the magnetic particles towards and across the sensor is achieved by two tapered magnetic field generating current conductors. The spin-valve sensor detects the stray magnetic field that emanates from the ensemble of magnetic particles. We study the transients in the magnetic signal on the order of 1% MR. These results lead to a model that describes magnetization configurations of the cluster of beads.

Rapid DNA hybridization based on ac field focusing of magnetically labeled target DNA

Rapid DNA-DNA hybridization between surface-bound probe DNA and magnetically labeled complementary target DNA was achieved using current carrying line structures and oscillating external magnetic fields. Magnetic particles of 250 nm in diameter were focused and manipulated over on-chip U-shaped current lines using dc currents of 40 mA and oscillating magnetic fields of 1.4kA∕mrms with frequencies ranging from 0.1 to 20 Hz. The focusing process was both time and frequency dependent and, consequently, hybridization degree varied with focusing efficiency. Extensive label binding was observed in 5–25 min at 0.1–20 Hz. This technique has strong potential in commercial DNA chip development.

Autonomous bathymetry for risk assessment with ROAZ robotic surface vehicle

The use of unmanned marine robotic vehicles in bathymetric surveys is discussed. This paper presents recent results in autonomous bathymetric missions with the ROAZ autonomous surface vehicle. In particular, robotic surface vehicles such as ROAZ provide an efficient tool in risk assessment for shallow water environments and water land interface zones as the near surf zone in marine coast. ROAZ is an ocean capable catamaran for distinct oceanographic missions, and with the goal to fill the gap were other hydrographic surveys vehicles/systems are not compiled to operate, like very shallow water rivers and marine coastline surf zones. Therefore, the use of robotic systems for risk assessment is validated through several missions performed either in river scenario (in a very shallow water conditions) and in marine coastlines.

A New Hand-Held Microsystem Architecture for Biological Analysis

This paper presents a hand-held microsystem based on new fully integrated magnetoresistive biochips for biomolecular recognition (DNA hybridization, antibody antigen interaction, etc.). Magnetoresistive chip surfaces are chemically treated, enabling the immobilization of probe biomolecules such as DNA or antibodies. Fluid handling is also integrated in the biochip. The proposed microsystem not only integrates the biochip, which is an array of 16times16 magnetoresistive sensors, but it also provides all the electronic circuitry for addressing and reading out each transducer. The proposed architecture and circuits were specifically designed for achieving a compact, programmable and portable microsystem. The microsystem also integrates a hand-held analyzer connected through a wireless channel. A prototype of the system was already developed and detection of magnetic nanoparticles was obtained. This indicates that the system may be used for magnetic label based bioassays

Magnetoresistive DNA chips

Publisher Summary Magnetoresistance (MR) technology is being successfully applied to biomolecular recognition in different biological contexts. Micron-sized magnetic labels are already successfully used in biomolecular recognition experiments, but smaller magnetic labels that are non-remanent, non-clustering, with low anisotropy and high susceptibility are required. The existing magnetoresistive sensing technology allows the successful detection of single nanometer-sized magnetic labels. However, real biological recognition results with MR biochip prototypes done at INESC and elsewhere are successful only with micron-sized labels (INESC, NRL, U. Bielefeld) and with 250 nm labels (INESC). An important figure of merit when comparing biomolecular recognition detection platforms is the amount of target material that can be detected. The minimum target concentration that can be detected by MR biochip platforms depends intrinsically on label dimension, and the number of target biomolecules attached to the label that can hybridize. MR technology has shown the potential for single molecule process detection, a target not usually within the reach of most of the competing technologies.

Detection of cystic fibrosis related DNA targets using AC field focusing of magnetic labels and spin-valve sensors

Over the past few years the concept of using magnetic field sensors for biological applications in particular, the development of magnetoresistive biochips and biosensors, has generated increasing interest from laboratories and companies. A spin-valve sensor based biochip was used to detect cystic fibrosis related DNA targets for the purpose of developing an affordable diagnostic chip and detection system. The strategy is based on the AC magnetic field focusing technique. This method consists of the attraction, concentration and manipulation of magnetically-labelled target DNA within on-chip u-shaped current line regions surface functionalized with a cystic fibrosis-related DNA probe. Cystic fibrosis related probes were immobilized on the oxide surface and 250 nm diameter non-remanent magnetic particles were functionalized with cystic fibrosis related DNA targets complementary or non-complementary to the immobilized probes. The hybridization of the target is detected using a u-shaped spin-valve sensor fabricated within the line structure. The proximity of probe and target at the spin-valve sensor surface promotes the hybridization of complementary DNA strands. In this way, hybridization occurs in relatively short times, (5-25 minutes), in comparison with conventional hybridization approaches (3 to 12 hours), as limited by diffusion of the target DNA in solution. Magnetic labels bound to the sensor surface through the hybridization of complementary DNA strands have a magnetic stray field that changes the resistance of sensors enabling detection of the hybridization in real-time. Results show a discernable difference in sensor response after washing when using complementary or non-complementary DNA targets. The use of complementary target DNA resulted in distinct hybridization signals and the binding of the particles in the sensor area was verified by visual inspection. In addition, it was observed that hybridization signals decreased slightly after the more stringent wash indicating that non-specifically or weakly bound labels were washed away. The use of non-complementary target DNA resulted in negligible sensor response after washing and no particles were observed in the sensor area.