Layer-by-layer coating strategy to functionalize the magnetic nanoparticles for their multi-functionalization.

Magnetic nanoparticles (MNPs) hold significant potential for a wide range of applications, however, surface modification or bio-conjugation of MNPs often leads to their aggregation and instability. To address this, we proposed a facile method using a layer-by-layer (LbL) coating technique with polyallylamine hydrochloride (PAH) and poly(styrene sulfonic acid) sodium salt (PSS), so as to maintain the dispersion stability and functionality of MNPs. This method enabled us to develop the powerful MNPs towards to their use in the electrochemical biosensor, by combining both the redox probes (ferrocene (Fc), anthraquinone (AQ), or monocarboxymethylene blue (MB)) and bio-probes (IgG). The redox molecules were effectively anchored to the MNPs under the organic solvents, while such functionalized MNPs surface were subsequently protected by the LbL coating process prior to dispersing in high ionic strength solutions (e.g. Phosphate-buffered saline). And the out-layer of polyelectrolyte shell allowed biomolecules to attach to the MNP surface without chemical cross-linking. Our results demonstrated that the TEM size of MNPs@Fc, MNP@AQ and MNP@MB after LbL coating were characterized as 11.0 ± 2.0 nm, 10.5 ± 2.1 nm and 12.4 ± 2.2 nm and these developed redox MNPs of MNPs@Fc, MNPs@AQ and MNPs@MB were characterized by square wave voltammetry (SWV) with their redox intensity of 0.64 ± 0.10 µA, 23.25 ± 0.73 µA and 0.48 ± 0.13 µA, respectively. In addition, the binding efficiency of adsorption between the MNPs and IgG was up to 78%, evidenced by SDS-PAGE gel analysis. This facile method offered a versatile and effective way to functionalize MNPs, combining redox and biological properties for potential applications in disease diagnosis and point-of-care diagnostics.