Tuning Nanoparticle Coating and Size for inductive biodetection

The use of magnetic nanoparticles (MNP) is considered promising in the design of biosensors and immunosensors thanks to two major advantages. First, MNP allow a pre-treatment and pre-concentration of the targeted analyte, 1 enabling a significant reduction of the detection limit by a factor 2 to 1000. Secondly, the reduced magnetic background in biological samples compared to its electrical or electrochemical equivalent makes MNP suitable nanomaterials as labels (or ‘tags’) for detection with improved sensitivity and lower noise levels. 2 However, the functionalization of the MNP surfaces for biological detection alters their initial magnetical properties, making the electromagnetical quantification of MNP within the biosensor system more challenging. 3 Given the pluridisciplinary nature of biosensor development, this issue needs to be addressed in order to facilitate integration of MNP. In this work, we characterize the influence of frequently used surface modifications on magnetite Fe3O4 nanoparticles : (i) the presence of carboxylic acid groups which facilitate covalent binding, (ii) silica coating which improves biocompatibility and (iii) the coating of MNP with gold nanoparticles, which provides functional surface groups for further bioconjugation. The studied parameters are initial susceptibility 𝜒𝑖, saturation magnetisation 𝑀𝑠, coercivity 𝐻𝑐 and remanent magnetisation 𝑀𝑟. Two size ranges of MNP are analysed. The first type MNP-TD (synthesized by thermal decomposition 4) has an average diameter size (19 nm) close to the critical particle diameter that defines the shift from the superparamagnetic to the ferromagnetic regime. The superparamagnetic behavior maximizes susceptibility, which is useful for e.g. inductive transduction 5. The second type MNP-Comm (Sigma-Aldrich, ref 637106) has an average diameter size (90 nm) past the critical diameter for single-to multidomain transition, raising coercivity. Figure 1 summarizes the magnetic properties of the « blank » samples used in our study. We analyse how the aforementioned surface functionalisations interact with the mainly superparamagnetic MNP-TD sample and the MNP-Comm having higher (but still small1) coercivity (Hc) and squareness ratio (Mr :Ms).