This paper describes efforts aimed at setting the stage for the application of giant magnetoresistance sensor (GMRs) networks as readers for quantification of biolytes selectively captured and then labeled with superparamagnetic particles on a scanned chip-scale array. The novelty and long-range goal of this research draws from the potential development of a card-swipe instrument through which an array of micrometer-sized, magnetically tagged addresses (i.e., a sample stick) can be interrogated in a manner analogous to a credit card reader. This work describes the construction and testing of a first-generation instrument that uses a GMR sensor network to read the response of a "simulated" sample stick. The glass sample stick is composed of 20-nm-thick films of permalloy that have square or rectangular lateral footprints of up to a few hundred micrometers. Experiments were carried out to gain a fundamental understanding of the dependence of the GMR response on the separation between, and planarity of, the scanned sample stick and sensor. Results showed that the complex interplay between these experimentally controllable variables strongly affect the shape and magnitude of the observed signal and, ultimately, the limit of detection. This study also assessed the merits of using on-sample standards as internal references as a facile means to account for small variations in the gap between the sample stick and sensor. These findings were then analyzed to determine various analytical figures of merit (e.g., limit of detection in terms of the amount of magnetizable material on each address) for this readout strategy. An in-depth description of the first-generation test equipment is presented, along with a brief discussion of the potential widespread applicability of the concept.Magnetoresistance is defined as the change in the resistance of a material in response to an externally applied magnetic field (H app ). All conductors show a small level of magnetoresistance, but normally not more than 1%. However, as discovered in 1988, [1][2][3] some multilayer systems exhibit a change in resistance of up to 80%. 4 This phenomenon is based upon the quantum mechanical exchange coupling between thin layers of magnetic materials that are separated by comparably thin layers of nonmagnetic materials 5 and is known as giant magnetoresistance. With these materials, an increase in H app induces a decrease in resistance. Since 1988, giant magnetoresistance sensors (GMRs) have become one of the most heavily used tools for the detection of magnetic fields. While almost universally employed as compact, high-density, and highspeed read heads in computer hard drives, 6 GMRs have also found specialty applications in several other areas, 4,7,8 including antilock brakes, 9 magnetic imaging, 10 and galvanic isolators. 11Several laboratories have begun to transition GMRs from the data storage domain to that of the bioanalytical sciences.12-24 The goal is to create miniaturized devices based on magnetic labeling concepts by taking advantage o...

Microfabricated devices formed from alternating layers of magnetic and nonmagnetic materials at combined thicknesses of a few hundred nanometers exhibit a phenomenon known as the giant magnetoresistance effect. Devices based on this effect are known as giant magnetoresistive (GMR) sensors. The resistance of a GMR is dependent on the strength of an external magnetic field, which has resulted in the widespread usage of such platforms in high-speed, high-data density storage drives. The same attributes (i.e., sensitivity, small size, and speed) are also important embodiments of many types of bioanalytical sensors, pointing to an intriguing opportunity via an integration of GMR technology, magnetic labeling strategies, and biorecognition elements (e.g., antibodies). This paper describes the utilization of GMRs for the detection of streptavidin-coated magnetic particles that are selectively captured by biotinylated gold addresses on a 2 × 0.3 cm sample stick. A GMR sensor network reads the addresses on a sample stick in a manner that begins to emulate that of a "card-swipe" system. This study also takes advantage of on-sample magnetic addresses that function as references for internal calibration of the GMR response and as a facile means to account for small variations in the gap between the sample stick and sensor. The magnetic particle surface coverage at the limit of detection was determined to be ∼2%, which corresponds to ∼800 binding events over the 200 × 200 µm capture address. These findings, along with the potential use of streptavidin-coated magnetic particles as a universal label for antigen detection in, for example, heterogeneous assays, are discussed.Several laboratories are exploring the utility of giant magnetoresistors (GMRs) as a new chip-scale readout tool in the bioanalytical sciences. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Examples include the application of GMRs and magnetic labeling concepts for the detection of DNA hybridization, biotin-avidin coupling, and antibody-antigen binding. We have also been investigating the integration of immunosorbent assays on a GMR platform, 18 as well as the use of GMRs for the detection of magnetic objects in a microfluidics flow stream as a potential basis for a magnetics flow cytometer. 19 These efforts seek to take advantage of the same set of magnetic detection * To whom correspondence should be addressed.

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