**1. Introduction**

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196 State of the Art in Biosensors - General Aspects

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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 biomole‐ cules, the magnetic bead sensing technique has been promising and versatile due to its in‐ creased 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 mag‐ netic 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 [8], and planar Hall effect (PHE) sensors [9–11]. Common procedure employed for all these sensor princi‐ ples is that the magnetic immunoassay of biological sample is introduced to the biofunc‐ tionalized 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

© 2013 Hung et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Hung et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 es‐ tablished 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 [1]. However, the flaws associated with longitudinal AMR measurements can be greatly im‐ proved by measuring the voltage change in the transverse direction instead, a phenomen‐ on known as the planar Hall effect [14]. It has been shown that by using the PHE, the temperature drift was reduced by at least 4 orders of magnitude, and nano-Tesla sensitivi‐ ty has been exhibited [15]. 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 mag‐ netic 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 ac‐ count 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 to‐ wards the end of this section.

In the fifth section, apart from a brief description on the magnetic beads and their function‐ alization, 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.
