Nanoparticle Coating and Size : Key for optimal magnetic Biosensors

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. Results and Discussion 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, saturation magnetisation, coercivity and remanent magnetisation. Two size ranges of MNP are analysed. The first type is ‘homemade’ (thermal decomposition 4) with an average diameter size (10 nm) near the critical particle diameter Dp (Fig 1) that defines the shift from the superparamagnetic to the ferromagnetic regime. The second type of MNP are commercial (Sigma-Aldrich, ref 637106) with a very large size distribution, mainly past the critical diameter Dc for single-to multidomain transition, raising coercivity, as shown in Figure 1. We analyse how the surface functionalisation of both types of MNP interacts with their initial magnetic properties.