Novel magnetic beads with improved performance for Alzheimer’s disease biomarker detection

Abstract

Magnetic beads (MBs) have been notably used as platforms in biosensing thanks to their magnetic behavior as they allow to simplify purification and separation by preconcentrating the sample and also to minimize matrix effects, what facilitates the analysis of real samples. Even though it exists a variety of commercially available ones, there is still great interest to develop alternative MBs with improved performance. In this work, we propose the synthesis of novel, reliable and low-cost MBs by colloidal assembly of zinc doped magnetite for their use as electrochemical immunosensing platforms. First, zinc doped magnetite ZnxFe3-xO4 nanoparticles (ZnFeNPs) of a diameter of 13 ± 3 nm and a saturation magnetization of 81 emu/g were synthesized and encapsulated in a polymeric matrix of poly(lactic-co-glycolic) acid (PLGA), generating polymeric MBs that were covered with polyethyleneimine (PEI) (MB@PEI), obtaining particles of 96 ± 16 nm. The PEI external layer provides MBs with a higher degree of encapsulation and stability and with functional groups that convert MB@PEI particles in versatile tools for their use as immunosensing platforms. In order to compare the suitability of the obtained MBs with commercially available ones, the affinity protein neutravidin (NAV) was linked to the MB@PEI surface through glutaraldehyde crosslinking. The obtained MB@NAV exhibited a significantly higher saturation magnetization than commercially available NAV-modified MBs, and also a better reproducibility (RSD of 4% for MB@NAV and 12% for commercial MBs) and enhanced surface functionalization ability when used as immunosensing platforms in a model assay using gold nanoparticle tags. As proof-of-concept of application in real samples, MB@NAV were finally applied for the detection of Tau protein, a well-known Alzheimer’s Disease (AD) biomarker, with a detection limit (LOD) of 63 ng/mL and an excellent performance in human serum samples.