(93k) Frequency-Domain Approach to Determine the Separation Distance Between Addresses and Magnetic Readout Sensor in a Magnetoresistive Immunoassay Array Platform

Magnetoresistive (MR) sensors have emerged as a heterogeneous microarray-based assay platform for the rapid and sensitive detection of disease biomarkers labeled with superparamagnetic nanoparticles (SMNPs). Our platform consists of a periodic array of gold capture addresses and nickel reference addresses. Readout is performed in a manner similar to a hard disk drive by scanning the MR sensor across the array in a noncontact mode. Control of the separation distance between the MR sensor and capture surface is critical to detection because the magnetic field of the SMNPs decays with the cube root of the separation. That is, the separation distance should be minimized to maximize the readback signal; however, recognizing that the SMNPs have a nominal diameter of ~1 mm, a stand-off must be maintained to avoid contact with the SMNP label. To maintain the required stand-off, we employed the harmonic ratio method (HRM), which is used by the hard disk drive industry to maintain much smaller â?? nanometer â?? separations between a magnetic read head and moving data platter. This method relies on a comparison of the amplitude of the fundamental frequency of the readback response to those of its harmonics. When the signal is derived from a periodic magnetic array (i.e., the Ni addresses of our array) that is scanned at a constant rate, the harmonic ratios contain the information necessary to determine and control the separation distance. In this presentation, we will demonstrate that the tenets governing the HRM used in the hard drive industry can be applied to our MR immunoassay array to determine separation distances with submicron resolution.