Increased proliferation of infectious diseases stresses the need for immediate development of a state of the art lab-on-a-chip with the capabilities of single biomolecular recognition and parallel processing not only to minimize the death rates but also to enhance the protection from rapid spread of epidemics. Though there are several methods for detection of biomolecules, the magnetic bead sensing technique has been promising and versatile due to its increased ease of fabrication in miniature designs and also its scope for rapid, inexpensive, high sensitive and ultrahigh resolution point of care diagnosis of several human diseases; thus, magnetic biosensors and biochips have become the subject of intense research interest in recent times globally.
In the magnetic bead sensing technique, the detection of biofunctionalized magnetic beads is normally carried out by sensors that are embedded underneath the sensing regions and provide a direct electrical readout proportional to the surface density of immobilized magnetic beads. There are several magnetic sensor principles in operation; namely, anisotropic magnetoresistance (AMR) sensors [1-2], giant magnetoresistance (GMR) and spin valve sensors [3–6], magnetic tunnel junctions (MTJ) [6-7], micro-Hall sensors , and planar Hall effect (PHE) sensors [9–11]. Common procedure employed for all these sensor principles is that the magnetic immunoassay of biological sample is introduced to the biofunctionalized sensor array followed by washing steps. In order to establish reproducible conditions under these various incubation and washing steps, it is desirable to integrate the sensor in a microfluidic system, which further facilitates a study of real time response of the sensor as a function of fluid flow, sensor bias current and bead concentrations. Moreover, multi-analyte biosensors integrated with microfluidic systems can be made to perform numerous tasks automatically by way of sensitively and specifically detecting multiple targets from unprocessed sample material, thus creating a compact instrument in the form of ‘‘lab-on-a-chip’’.
With the phenomenal success of GMR-spin valve sensors and MTJ sensors in hard disc drives and magnetic memories, they have become an inspiration for testing their use in other areas including that of magnetic biodetection. Obviously, GMR and MTJ sensors take pride in finding themselves as one of the most widely investigated magnetic sensors for bioapplications [12-13]. They are also successful biosensors commercially as they offer high sensitivities, flexible sensor geometries and large bead-to-sensor ratio with well established integrated circuit fabrication technology. However, relatively low signal to noise ratio of these sensors may often leave scope for erroneous detection. The AMR sensors, in turn, offer greater ease of fabrication but the sensitivity of the AMR signal measured along the longitudinal direction is, however, limited by Johnson noise originating from thermal fluctuations at high frequencies, and by temperature drift at low frequencies . However, the flaws associated with longitudinal AMR measurements can be greatly improved by measuring the voltage change in the transverse direction instead, a phenomenon known as the planar Hall effect . It has been shown that by using the PHE, the temperature drift was reduced by at least 4 orders of magnitude, and nano-Tesla sensitivity has been exhibited . In addition, compared with longitudinal AMR signals, PHE signals are more sensitive to local spin configuration and have much lower background voltage as well.
We propose here a planar Hall sensor array in exchange biased multilayer structure and demonstrate the performance of the sensor with the capability of detection of a single magnetic bead. Also, the sensor is further shown to be capable of single biomolecule detection. Following a brief introduction on the need for exploring magnetic sensors, the book chapter describes the principle of magnetic sensing and highlights the merits of planar Hall sensor in terms of field sensitivity and resolution in the second section.
In the experimental parts, the details of the general procedure for fabrication sequence of the sensor, its characterization and microarray integration were described. Subsequently, an account on the theory and experiments of bead detection using planar Hall resistance (PHR) sensor in different multilayer structures and geometries leading to a complete evolution of novel PHR sensors is elaborately presented in the fourth section. Nevertheless, a hybrid AMR-PHR sensor in ring geometry has been identified for optimum sensor performance towards the end of this section.
In the fifth section, apart from a brief description on the magnetic beads and their functionalization, a description of sensor performance and its capability for detection of magnetic beads including a single magnetic bead is given. This section also presents an account on the integration of microarray sensors with the aid of microfluidics for performing biomolecule experiments while showing the possibility of the planar Hall sensor for a sensitive detection of even single biomolecule. And, finally, it concludes the processes involved with a specific mention on future trends to cater the needs of the society in general.
2. Principle of magnetic sensors
The principle of detection employed by the magnetic sensors for magnetic bioassay involves a magnetic transduction mechanism which uses the magnetic micro- or nanoparticles as labels. The biomolecules are commonly detected by attaching them to highly specific magnetic labels that can, upon their binding, produce an observable quantitative electrical signal, as compared to the cumbersome detection of light signal from fluorescent labels. The specificity is traditionally achieved through a biomolecular recognition mechanism, such as antigen-antibody affinity which can be accomplished by label functionalization, as demonstrated in Fig.1.
Magnetoresistance (MR) is the property of magnetic materials that results in a change of resistance with applied magnetic field. The MR materials are being developed for the applications such as hard-disk read heads, magnetic random access memories and magnetic field sensors. The materials that display MR property with desired characteristics can replace the inductive coil sensors in a variety of applications including that of biomolecule recognition.
2.1. Magnetoresistive materials for sensing
Thin films and multilayer structures in different geometries show MR characteristics specific to their geometry and all these structures are found to be suitable for one or the other applications. The ferromagnetic single layer exhibits an AMR which is measured in the current direction, while the planner Hall resistance (PHR) effect can be measured in current perpendicular direction in AMR materials . The AMR and PHR effects are due to the anisotropic magnetoresistivity in ferromagnetic layers. The magnetic thin film multilayer structures can give giant magnetoresistance  and tunneling magnetoresistance [18-19] effects and the MR property of these structures are superior to that of the AMR structure. The GMR structure consists of two layers of ferromagnetic metal separated by ultra-thin non-magnetic metal spacer layers. The TMR structures are similar to GMR except that they utilize an ultra-thin insulating layer to separate two magnetic layers rather than a conductor. The GMR and TMR effects occur mainly due to the spin-dependent scattering as the current passes from one layer to the other through the spacer layer. The usual figure of merit of the MR ratio is traditionally defined as
A majority of the studies in MR effect in thin films are devoted to the research of multilayered structures showing the largest possible sensitivity of the resistivity for the magnetic field, and consequently a large number of transition metal-based multilayered structures exhibiting large MR ratios have been found. In connection with the technological problems to be solved, this book chapter devotes to a number of MR sensor designs using the planar Hall effect and tested to linearize the transducer signal, to enhance the resolution limited by the MR ratio.
2.2. Relevant sensor characteristics
Though there is a wide choice for sensor designs, optimum sensor performance in each design can be ensured only when specific sensor characteristics are satisfactorily addressed. Among these characteristics, the field sensitivity and the sensor resolution are of utmost concern particularly for a PHR sensor and, thus, they are considered to be described here for elucidating their importance.
2.2.1. Field sensitivity
The field sensitivity of PHR sensor,
In a typical curve of PHR signal with applied magnetic field, the maximum field sensitivity of PHR sensor is appeared at low (near zero) magnetic fields. Therefore, the PHR sensor can be used as low magnetic field sensors. Also, the PHR signal does not depend on the sensor size such as the width and length, and therefore, the micro or nano meter order of sensor size is possible by maintaining the same output signal. Thus, the PHR sensor is one of the good candidate bio-sensors for the micro- or nano- bead detector.
2.2.2. Resolution (S/N)
A comparative study of the sensor’s characteristics was made systematically and summarized in Table 1 for some of the sensors in practice . It must be mentioned that all the compared sensors have similar active areas and were normally designed for detection of single or small number of micro-size particles.
In Table 1, the part (A) represents the dimensions and properties of different sensor devices compared. The represented thickness is that of the sensing volume used in 1/f noise calculations. Whereas the part (B) shows calculated signals obtained from a single 2
The comparison results have shown that the PH-AMR or PHE sensor has prominent advantages over others such as very high signal-to-noise ratio (S/Nf) as well as very high resolution (
3. Sensor fabrication and characterization
3.1. Fabrication procedure of a novel planar Hall sensor
Nowadays, with the advancement of the accurate sputtering and lithography technologies, the sensor with desired composition in micro-size can be easily fabricated by using a lift off method. The general fabrication procedure of a novel exchange biased planar Hall sensor, for example in typical spin valve geometry, using the lift off method is shown in a simplified description in the figure 2 below. However, the same procedure is applied for fabrication of other PHR sensors too in different geometries mentioned in this book chapter.
The SiO2 wafer is first cleaned in the acetone and methanol solutions while placing in the ultrasonic bath, then the SiO2 wafer is covered by a commercial photoresist such as Az (5214E, 9260,…) or SU8-(2000, 3000,…) by using a spin coating system with a defined thickness. The blank cross-junctions are stenciled out on the photoresist coated on SiO2 wafer
The sensor materials,
After fabricating sensor junctions, the electrodes made by Au are connected with sensor junction to establish the external circuitry and to measure the sensors’ response. Further, the sensor junctions and the electrodes are passivated with a SiO2 or a Si3N4 layer coated on top of the sensor junctions and electrodes to protect them from the corrosion and fluid environment during the experiments. Finally, the sensor is activated by a very thin Au layer for bio-molecule immobilization. All these steps are carried out at the same way for all the sensors as the steps for the sensor junction fabrication.
3.2. Sensor characterization
The quality of the spin-valve structure, Ta(3)/NiFe(10)/Cu(1.5)/NiFe(2)/IrMn(10)/Ta(3) (nm), as observed by a cross sectional transmission electron microscope (TEM) image and also by an energy dispersive X-ray (EDX) spectrum along its thickness, are shown in Fig. 3. The TEM-specimen was prepared by polishing the Si/SiO2 substrate mechanically to a thickness of about 100
It is evident from Fig. 3(a) that the existence of a multilayer structure is clearly revealed as pointed out by an arrow for each layer. The both seed and top layers of Ta have amorphous behavior and the thickness is about 3 nm. However, there is a difference in the color of the two layers; this can be assumed that the Ta top layer is slightly oxidized . The IrMn layer is well defined, and its thickness is about 10 nm as the nominal thickness when the layer is deposited. In the NiFe/Cu/NiFe region, it is clearly seen that the diffusion of Cu takes place into the adjacent NiFe layers. This kind of diffusion is known to influence the anisotropy significantly . There is also an existence of a rumpling or even a rupture of the layers in some parts. This can be explained when considering the roughness of the NiFe layers; the roughness of NiFe layer is normally about 1.5 – 2 nm .
The result of the cross sectional TEM image is supported by the EDX patterns of the same sample shown in Fig 3(b). It can be seen that the peak of Cu is mixing inside the Ni and Fe peaks indicating the diffusion of Cu in NiFe layers. The overlap between the peaks of Ta and Ni (Fe), of Ta and Ir (Mn), and of Ir (Mn) and Ni (Fe) confirms the roughness at the surface of the NiFe and IrMn layers. Moreover, the shadow in Ni and Fe peaks (black arrows in Fig. 3(b)) indicates the separation of the NiFe pinned and NiFe free layers.
The magnetic property of the fabricated spin-valve structure used for sensor material is characterized by a vibrating sample magnetometer (VSM) of the make Lakeshore 7407 series with a sensitivity of 10-6 emu. The external magnetic field is swept in the film plane.
In order to achieve the magnetic anisotropy of the free layer in the fabricated spin-valve structure, we measured the magnetization as a function of external magnetic field in the range of ± 80 Oe in both the easy and hard axis, which is presented in Fig. 4. The shift along the external magnetic field axis of the magnetization profile (
3.2.1. Microarray of the magnetic sensors
Fig. 5 shows a complete micro-array of planar Hall resistance (PHR) sensors. In the figure, it was shown that the unidirectional field,
4. Evolution of novel PHR sensors
Among all the developed magnetoresistive sensors for bioapplications, we mainly focus on the development of PHE sensor because it has prominent advantages compared with others such as signal-to-noise ratio, linearity signal
4.1. AMR sensor
The magteoresistive anisotropy in ferromagnetic material depends on the direction of magnetization. The electric field due to the magnetoresistivity is expressed as follows ;
where is magnetization vector in single domain, and is current density direction. The
The AMR properties have been discovered at ferromagnetic material by William Thomson in 1857 . In the AMR response, varying differences between the direction of the magnetizing vector in the ferromagnetic film and the direction of the sensing current passing through the film lead to varying the resistance in the direction of the current. The maximum resistance occurs when the magnetization vector in the film and the current direction are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The resistance change by AMR effect in the patterned film with thickness
where is the anisotropic magnetoresitivity and the is the angle between the magnetization vector and current,
4.2. Planar Hall resistance sensor
The planar Hall resistance (PHR) in ferromagnetic thin films was considered when the resistivity depends on the angle between the direction of the current density
The PHR effect also varies when there is a difference between the direction of the magnetizing vector in the ferromagnetic film and the direction of the sensing current passing through the film; however, it leads to varying the resistance in the perpendicular direction of the current only. The longitudinal component of PHR voltage is related to
In order to analyze the PHR signal with magnetic field, we must know the angle between the magnetization vector and current direction, which depends on the magnetic field. Fig. 6 shows the general coordinates used to describe the rotational magnetization process under the applied magnetic field in ferromagnetic/antiferromagnetic (F/AF) coupled samples.
The applied magnetic field
Fig. 7 shows the calculated
Therefore, the PHR sensor justifies that it can be used as the micro- or nano- sized magnetic field sensor for the detection of the micro- or nano- bead. The PHR signal in ferromagnetic single layer shows large hysteresis behavior. The hysteresis effect is due to the switching of the magnetization in ferromagnetic layer. In order to remove the hysteresis of PHR, the exchange biased F/AF bilayers are considered.
4.3. PHR effect in exchange biased F/AF multilayer structures
The single ferromagnetic layer with high AMR ratio such as NiFe, CoFe and NiCo alloys has the uniaxial anisotropy. The easy axes for stable magnetization direction are 0 and 180 degrees. If one cycle of magnetic field is applied in the perpendicular direction to the easy axis in ferromagnetic films, the magnetization direction changes from 0 to 90 degrees as the magnetic field increases, and 90 to 180 degrees as the magnetic field decreases. And then the direction of the magnetization changes from 180 to 270 and then to 360 degrees as the reversed magnetic field increases and decreases. In that case, the AMR and PHR, which are dependent on the angle between the current and magnetization directions, can show the large hysteresis loop. On the other hand, the exchange biased F/AF bilayers induce the unidirectional anisotropy, which rotates the magnetization direction from 0 to 90 and 90 to 0 degrees as the magnetic field increases and decreases, respectively. It means that the AMR and PHR signal in exchange biased F/AF bilayers show the reversal behavior and the hysteresis can be disappeared.
Fig. 8 shows the calculated PHR signal for the exchange biased F/AF bilayers. By comparing the PHR signal of the exchange biased F/AF bilayers in Fig. 8 with that of the single ferromagnetic layer in Fig. 7(b), we can clearly confirm that no hysteresis behavior of PHR signal takes place in exchange biased F/AF bilayers. The exchange bias field,
There exists an interfacial coupling in F/AF bilayers. The hysteresis loop of the F layer, instead of being centered at zero magnetic field, is now displaced from
In a bilayer structure, the exchange coupling between the F and AF layers can easily induce the unidirectional magnetic anisotropy of the F layer. In addition, the F layer is improved to be constrained to the magnetization in coherent rotation towards the applied fields, so the sensor can prevent Barkhausen noise associated with the magnetization reversal, and improves the thermal stability . Because of these advantages, a bilayer structure is a good candidate for developing sensor materials.
Bilayer has been used as PHE sensor materials by M.F. Hansen
The spin-valve structure, as shown in Fig. 10(a), which was known as a simple embodiment of the GMR effect, typically consists of two F layers separated by a nonmagnetic conductor whose thickness is smaller than the mean-free path of electrons. The magnetic layers are uncoupled or weakly coupled in contrast to the generally strong AF state interaction in Fe-Cr-like multilayer; thus the magnetization of F layer with uniaxial anisotropy can be rotated freely by a small applied magnetic field in the film plane, while the magnetization of other magnetic layer had unidirectional anisotropy and was pinned by exchange bias coupling from AF layer. If the relative angle between the magnetization of the two layers changes, a giant magnetoresistance change occurs.
In an illustrative demonstration of the operation of the spin-valve, the applied magnetic field is directed parallel to the exchange biased field and cycled in magnitude. The
The sharp magnetization reversal near zero magnetic field is due to the switching of the free magnetic layer in the presence of its weak coupling to pinned magnetic layer. The more rounded magnetization reversal at higher magnetic field is due to the switching of the pinned magnetic layer, which overcomes its exchange biased coupling to an AF layer for these fields. Therefore, it was emphasized that a spin-valve here makes use of two different exchange couplings; exchange biased coupling from pinned layer to AF layer and interlayer exchange coupling between two magnetic layers, which in origin, was tentatively assigned to a Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction. The relative orientations of two magnetic layers were indicated by the pairs of arrows in each region of the
In order to optimize the spin-valve structure for high sensitivity PHE sensor, Kim's group has investigated systematically the effect of the thickness of F-pined and F-free layers (
The origin of interlayer coupling in F/spacer/AF trilayer structure is totally different from interlayer coupling induced in F/spacer/F multilayer thin films. The observation of F/AF exchange coupling across a nonmagnetic layer by Gökemeijer
In our experiment, we choose Cu as spacer layer in the trilayer structure, Ta(3)/NiFe(10)/ Cu(0.12)/IrMn(10) (nm), because it gives a small exchange coupling with a thin Cu layer. In the sensor application, it can reduce the shunt current resulting in enhanced sensitivity. The exchange coupling of the trilayer structure, determined by the shift of the hysteresis loop in the magnetic field direction and is measured in the order of few tens of Oe, is one order smaller compared with the exchange coupling in a typical bilayer structure (in order of hundred Oe) as shown in Fig. 9. A comparison of the PHE voltages generated by the bilayer, spin-valve and trilayer structures and their corresponding sensitivities are shown in Fig. 11. Thus, it can be easily seen from the figure that the trilayer structure can improve the field sensitivity of a sensor better than those of the bilayer and spin-valve structures .
4.4. Sensor geometry
The performance of the sensor depends largely on its physical geometry. There were several geometries reported in the literature in the design of planar Hall sensor. Among these geometries, the cross-junction and circular geometries need special mention as they result better performance of the sensor. Thus, it is intended to present the results of the sensor for better understanding of the sensor performance when the geometries are explored in the form of cross-junction, tilted cross-junction and circular ring junction.
In this part, we discuss the effect of the sensor size on the output voltage of a cross-junction PHE sensor.
Fig. 12 (left) shows the illustration of a fabricated PHR sensor and the Fig.12 (right) shows the SEM image of the passivated single sensor cross-junction of the size 50
For studying the size effect in planar Hall sensor, cross-junctions with various sizes of
The PHE voltage profiles of the fabricated sensors with various junction sizes are given in Fig. 13. Analogous to the other PHE results, the PHE voltage in all the sensor junctions initially changes very fast and appears linear at low fields, reaches a maximal value at
It is noteworthy that the maximum value of the PHE voltage profile is obtained at the field close to the effective uniaxial anisotropy field,
The theoretical voltage profile of the fabricated PHE sensor was also calculated with a set of following parameters:
This result is important for the bio-applications because the sensitive detection of low bimolecular concentration is proportional to the junction size.
4.4.2. Tilted cross-junction
The idea behind the study of the tilted cross-junction is to combine some of the magnetoresistive effects, such as GMR, AMR and PHE and to explore how beneficial the sensor could be in its performance . Therefore, the spin-valve structure which has GMR effect causing by spin scattering of electron between two F layers through a spacer layer, AMR and PHE effects causing by the spin-orbit coupling in the F layer are the best candidates for a sensor material.
To study the tilted cross-junction bars, 100
In general, the GMR and AMR effects could be obtained from the parallel direction to the current bar or longitudinal part while the PHE can be obtained from the transverse part of the sensor junction. Therefore, in a novel design of the sensor based on the tilted cross-junction the longitudinal and transverse contributions could be combined together in one sensor. In this tilted junction, we observed that there is an enhancement of PHE sensitivity and better linearlity of MR longitudinal component.
In Fig. 15 we demonstrated the output voltage profiles of the sensor junctions with different tilted angles. It clearly shows an increase in amplitude of the output voltage profile with increasing tilted angle
In order to understand the voltage contribution from each effect in a titled cross-junction quantitatively, we have performed systematic investigations on the role of the MR and PHE in the tilted cross-junction. In such case, it was noticed that the active PHE region and active MR region are from the transverse part and longitudinal part of the sensor, respectively. When the tilt angle of cross-junction increases, the length of the transverse part (
It is observed that the PHE voltage is independent of the junction size irrespective of its change in the length or the width in previous part. Therefore, the PHE voltage component in the tilted cross-junction is always a constant. Then the transverse PHE component (corresponding to
Applying the above mentioned decomposition procedure for all investigated sensor junctions, one can derive the values of the drift (minimal) voltage (
The results are listed in Table 2. Note that,
In this equation, is the angle between the magnetization direction of the F-pinned layer and the easy axis of F-free layer, and the drift voltage term () was modified from Ref.  in accordance with the investigated sensor junctions, because it depends on the length of the sensor junction. The increased length of the active region of the MR components depends on the sinusoidal function of tilt angle
The decomposed MR voltage profiles can be described with values of the sheet resistance
Finally, the total output voltage profiles of the tilted junctions are calculated by combining both the PHE and MR components represented in Eq. (8). The results are shown by solid lines in Fig. 16, where the calculated drift voltages are adjusted to the experimental drift voltages. It is clearly evident that a rather good consistence between the experimental and the calculated data is obtained. Thus, the tilted cross-junction exhibited not only a better sensitivity in comparison with individual PHR sensor but also a better linearity compared with individual MR sensor.
4.4.3. Ring junction
The idea to develop the sensor based on a ring is to combine both the PHE and AMR components in one ring junction ; thus, the output voltage of the sensor can be enhanced. In the following, the role of the output signal as well as the optimization results will be discussed.
Firstly, for studying the role of the signal in the ring junction, we design the ring with different configurations. These rings have the same diameter of 300
It is evident from Fig. 17 that the signal change in the case of a full ring (350
4.5. Hybrid AMR and PHR ring sensor – Optimized performance
In order to optimize the performance of the sensor using a ring junction, efforts were made to design an hybrid AMR and PHR ring sensor. It is known that the maximum voltage of the AMR and PHR voltages in the ring can be calculated using:
It is clear from the above that the PHR component is always constant while the AMR component increases linearity with the increase in
The results in Fig. 18 ensure that the higher the
The results of the sensitivity versus
It can be summarized from the above that the systematic investigations on the ring junctions revealed that there exist both PHR and AMR voltages contribute to the output voltage profile. The PHR voltage component is always kept constant when varying the size of the ring, while the AMR voltage component linearity increase due to the increasing the
5. Biofunctionalized magnetic bead detection for state of the art lab-on-a-chip
Ever since the report of Baselt
5.1. Magnetic beads
Superparamagnetic nanoparticles coated with Streptavidin make ideal labels in bio-applications using magnetic sensors, because they can be readily magnetized to large magnetic moments. Most of our experiments were carried out with Dynabeads®M-280, which are composed of ultra small Fe2O3 nanopaticles embedded in a polymer matrix and the Streptavidin was conjugateed with the surface of the beads. The magnetization curve of the magnetic beads is shown in Fig. 20 [27, 39].
Assuming that the applied field is in
with and when converting from polar coordinate system to spherical coordinate system.
Substituting and into Eq. (11),
Following the Eq. (12), stray field of magnetic bead reaches maximum when
If the sensor geometry is considered as a circle, the effective field of a bead can be calculated from Eq. (13) as
And if the sensor geometry is quadrate the effective field is given by
It is revealed from Eq. (14) and Eq. (15) that the field effect to the sensor is very much depending on the size of the sensor, , it is proportional to the invert cube of radius of circular sensor or of the width of a quadratic sensor (or ).
5.2. Biofunctionalization of the beads
It is known that the biotin-streptavidin is one of the strongest non-covalent biological interaction systems having a dissociation constant, ‘
To demonstrate the translocation of streptavidin-biotin magnetic labels using the micro system, we have chosen the commercially available streptavidin coated magnetic beads (Dynabead® M-280) of 2.8
Fig. 22 provides the direct evidence of protein immobilization which was obtained by immobilizing green fluorescent protein (GFP) and observed the fluorescence through confocal laser microscopy.
5.3. Sensor size and bead detection capability
For the micro-bead detection using a PHE sensor, it is noted that the magnetization of the magnetic sphere is purely a dipole at the center of the sphere with a magnetic field at a distance identified by the dipole field from Eq. (15). The stray field of a single bead on the sensor surface could be crudely calculated by 
where is the volume of magnetic bead, is the volume susceptibility of magnetic beads. This stray field is in the opposite direction to the applied field, thus it reduces the effective field on the sensor surface. Under the experiment conditions, the stray field of N beads on the sensor surface reduced the sensor output signal as follows:
where is the effective field on the sensor surface,
By substituting the value  and μm (the distance including the radius of Dynabeads® M-280 and the thickness of passivated SiO and Ta layers) into Eq. (17), the stray field of single bead is estimated to be
Fig. 23 shows the
In the PHR sensor, the
The pure bead signal is small compared with
5.3.1. Multi-bead detection
We performed the magnetic bead detection using PHR sensor using Ta(3)/NiFe(16)/Cu(1.2)/NiFe(2)/IrMn(10)/Ta(3) (nm) to demonstrate the feasibility of magnetic bead detection for bio applications. The diluted 0.1 % magnetic bead solution streptavidin coated Dynabeads® M-280 is used for bead drop and wash experiments on the sensor surface. The real-time profile measurements of the PHE voltage for magnetic bead detection are carried out in the optimum conditions, that is, in an applied magnetic field of 7 Oe and with a sensing current of 1 mA. The results are illustrated in Fig. 25 for three consecutive cycles. The lower state represents the signal change in sensor output voltage after dropping the magnetic bead solution on the sensor surface whereas the higher state represents the sensor output voltage after washing magnetic beads from the sensor surface. Total output signal annuls in three consecutive cycles were found to be about 7.1
This two step-type profile is due to the aggregation process of the magnetic beads on the sensor surface. The aggregation of the magnetic beads occurs at the drying stage. That is, after dropping the bead solution on the sensor surface, it needs some time to dry. The first step changes of the signals are assumed to be due to the viscous flow motion for stabilization as well as the Brownian motion of the beads. When the solution dries, the beads rearrange. During this time, some beads aggregate and become clusters on the sensor surface. This lessens the total stray field on the sensor surface and hence, the second step in the second and third cycles was observed in the real-time profile.
In the process of analyzing the micro-bead detection using PHE sensor, it is noted that the direction of magnetic field
Thus, a rough estimate of the number of magnetic particles on the sensor surface in this identical experiment based on the reduced stray field and sensor output signal can be expressed from Eq. (18) by rephrasing it again here for better clarity:
By substituting the value
These estimated results strengthen our explanation. It is clearly shown in the first cycle, the number of beads on the sensor surface is estimated to be small, and the distance among beads on the sensor junction is far enough to avoid the effect from the rearrangement of beads during the drying stage. In the second and third cycles, the number of magnetic beads on the sensor junction is larger; they easily aggregate to become clusters under applied magnetic field due to short bead-bead distance
5.3.2. Single bead detection
We performed single magnetic bead detection experiments on several kinds of sensor structures such as spin-valve and bilayer exchange biased thin films [27, 42 - 44], and the representative results are being presented here. For the purpose of performing single micro-bead detection, the PHR sensor with the junction size of 3
Since the magnetic properties of the MR as well as the PHE response to the magnetic field are described in the previous section, the results of single bead detection using 3
The SEM image of a single bead on the center of the sensor junction is shown in the Fig. 26(a). The voltage profiles of the PHE sensor in the absence and presence of a single micro-bead are presented in Fig. 26(b) by black circle and red rectangle ones, respectively. It is shown from the figure that in the increasing region of the PHE voltage profile (in the field ranging from 0 Oe to 10.6 Oe), the
For understanding the role of a single micro-bead detection using a PHE sensor, we consider the voltage drop by stray field of a single magnetic bead. The calculation method is the same as deduced for Eq. (18). And when considering that the magnetic bead is located on the center of sensor junction, the stray field affects the PHE voltage as follows:
By comparing the PHE voltage profiles in the absence and presence of a micro-bead, one can find that (
In particular, at low field range, a very good linear and large change of the PHE voltage always occur, so this field range is usually chosen to demonstrate the feasibility of the digital detection of the magnetic beads [10-14]. In our approach for this sensor, the signal change versus the applied field is extracted from PHE voltage curves in the presence and absence of magnetic bead, the result is drawn in Fig. 27, the maximum change of
This simple calculation is suitable for the effect of a single bead on the center of small size sensor junction. When the area of the sensor junction is larger than the area of magnetic beads, the calculation must be considered the effect of the magnetic bead from different positions of sensor junction and the contribution of nearby beads or chains of beads on the sensor. In such a case the output signal changes negative for the bead inside of the sensor junction and changes positive for the beads outside of the junction. Moreover, the signal change does not depend on the number of magnetic beads proportionally. This was studied systematically and was reported by P.P. Freitas
5.4. Integration of magnetic sensors/microfluidic channels
In this part, we design and optimize the planar Hall ring sensor for detecting the hydrodynamic magnetic labels. Once the magnetic labels appear on one arm of the ring sensor, the resistance of the sensor will be changed, the role of resistance change obey the Wheatstone bridge circuit geometry hence the sensor is very sensitive to detect the magnetic labels.
Planar Hall ring sensor was fabricated by photolithography technique. Sensor material Ta(3)/NiFe(10)/IrMn(10)/Ta(3) (nm) was fabricated by using a DC sputtering system with the based pressure of 7×10-8 Torr. The field sensitivity of the ring sensor based on the bilayer thin film was found to be about 0.3 mV.Oe-1. The sensor was integrated with a microfluidic channel, which can produce the laminar flow of the magnetic labels (beads and/or tags) in the specific arms of the ring sensor by hydrodynamic flow focusing technique. This magnetic platform can detect even a single magnetic bead of 2.8
The schematic representing the integrated magnetic platform is shown in Fig. 28. In magnetic bead separation experiments initially the magnetic beads with different sizes are injected into the main stream of the microfluidic channel with certain fluidic flow rate. Then the beads are gathered at the weir in the fluid channel and then sorted according to the attractive force exerted on the magnetic bead by the magnetic elements/magnetic pathways. Therefore, the labeled magnetic beads of same kind will attract to one of the magnetic pathways in the sub channel. The weir at the entrance of the sub-channels opposes the beads temporarily for magnetic beads whose magnetization is insufficient to be attracted by the magnetic elements. But, the beads whose magnetization is sufficient to be attracted by the poles of the saturated ellipses due to the external rotating magnetic field can overcome the weir.
After successful separation of the magnetic beads of different sizes we wish to adopt two types of different sensing techniques such as an array of PHR biosensors and multi-segmented nanowires. The planar array of PHR sensor can detect magnetic beads with micron size only. But in case of nanometer size magnetic beads, we wish to use simple read out technique of multi-segmented nanowires. We are also planning to combine magnetic pathway method with the microwire and coil method.
The underlying principle for magnetic biosensing has been elaborately described at first with the examples of different magnetoresistive sensing techniques. Then, the planar Hall resistance sensor has been shown as one of the best sensors for conducting magnetic bead detection experiments. While making an in depth study on the capabilities of a PHR sensor in different configurations and geometries, the sequence of narration ultimately has lead towards describing the evolution of hybrid AMR and PHR ring sensor in spin-valve configuration with optimized performance for precise detection of even single magnetic bead. Biofunctionalization experiments were also conducted to ensure that our PHR sensor is capable of biomolecule recognition. Therefore, our present sensor can be used to promote for the biomolecular recognition and other molecular interaction detection. This novel planar Hall effect based sensor has been further demonstrated that it can be easily integrated into a lab-on-a-chip and is feasible for bead detection in the sensing current generated magnetic field (without the external applied magnetic field) so as to ensure it an efficient tool for high sensitive biomolecules recognition.