Overcoming the detection limit of GMR sensors using disk-shaped nanostructures

Superparamagnetic nanoparticles (SPNPs) and magnetoresistive (MR) detection have gained significant interest due to their potential in personalized medicine; therefore, they have been investigated for decades in medical diagnosis. However, they present certain limitations due to their superparamagnetic state. This study presents novel magnetic nanostructures (MNSs) with a specifically designed spin configuration to overcome these limitations. The MNSs were used to monitor infliximab, a reference anti-tumor necrosis factor biological drug employed in the treatment of inflammation, using specific functionalization protocols and giant magnetoresistance (GMR) sensors. Disk-shaped MNSs, fabricated by interference lithography and electron beam evaporation, 300 and 700 nm Permalloy nanodisks, were selected for protein detection. Direct- and indirect-immunoassays were developed to detect infliximab employing a MR platform by functionalizing GMR sensors and MNSs. These nanodisks significantly increased the platform sensitivity to infliximab compared to reference commercial bead-shaped SPNPs, enabling detection at physiological levels. Additionally, MNSs reached saturation levels faster than SPNPs, reducing signal acquisition time. Micromagnetic simulations were performed to analyze the spin configuration and stray-field distribution of the nanodisks, providing physical insight into their enhanced coupling with the GMR sensors. These findings demonstrate the potential of this technology for point-of-care systems, facilitating self-administration of biological drugs and advancing personalized medicine.