Exploring relevant properties of superparamagnetic nanoferrites towards inductive sensing
(2026)
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- Lateral flow assays (LFA) provide rapid point-of-care diagnostics, but their standard colourimetric readout with gold nanoparticle labels is only semi-quantitative. Replacing these labels with superparamagnetic nanoferrites (SPNF) and reading the strip with magnetic sensors enables quantification that is insensitive to the optical and electrochemical variability of biological matrices. Leveragingthese advantages requires understanding how SPNF composition, size, coating, substrate and measurement conditions jointly determine the effective magnetic properties. This thesis investigates Fe3O4, MnFe2O4 and CoFe2O4 nanoparticles from synthesis to inductive detection. Their magnetic properties are characterised from DC to the GHz range, and from 2 K to 300 K. Studied configurations are compacted powder, cellulose membranes and aqueous suspensions. A combined Debye fitting procedure applied simultaneously to mobile and immobilised suspensions separates the Néel and Brownian relaxation contributions, resolving the effective anisotropy diameter degeneracy that limits single-configuration analysis. Inductive detection is demonstrated with affordable and portable sensors. Four main findings emerge. First, SPNF dispersed in membranes exhibit initial susceptibilities three to four times higher than in powder, attributed to reduced interactions; characterisation in a configuration representative of the final application is therefore necessary to avoid underestimating the available signal. Second, manganese ferrite consistently yields higher susceptibility than iron oxide across all methods and configurations, a consequence of its lower magnetocrystalline anisotropy. Third, drying nanoparticle-loaded membranes under a 500 mT static field improves the inductive signal by 8 to 16 %, by pre-aligning the easy axes of the nanoparticles with the sensing field before immobilisation upon evaporation; the effect requires no modification to the SPNF or the sensor electronics. Fourth, while the inductive sensor’s lowest sensed mass (7 μg for MnFe2O4) remains at least two orders of magnitude above the levels relevant for low-abundance biomarker detection on lateral flow assays, the sensor still provides genuine insight into the mobility and alignment of the nanoparticles, and classifies the materials by their effective magnetic properties consistently with the advanced laboratory measurements.
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Hauwaert, M. (2026). Exploring relevant properties of superparamagnetic nanoferrites towards inductive sensing. https://hdl.handle.net/2078.5/278257