Humic acid-stabilized superparamagnetic maghemite nanoparticles: surface charge and embryotoxicity evaluation

Superparamagnetic iron oxide $\gamma$-Fe$_{2}$O$_{3}$ (maghemite) nanoparticles (SPION) encapsulated into water-soluble microspheres of rock salt were synthesized via a new aerosol spray pyrolysis procedure. Humic acids (HA) were employed to stabilize the aqueous suspensions of $\gamma$-Fe$_{2}$O$_{3}$ nanoparticles released upon dissolution of the NaCl matrix. The effect of HA on the surface charge of maghemite-based colloids was studied in pH range of 3–10. Humic polyanions compensate positive charges on a hydrated $\gamma$-Fe$_{2}$O$_{3}$ surface resulting in strongly negative $\zeta$-potential ($<$ -40 mV) of colloid even in acidic environment. In neutral and alkaline environment, $\zeta$-potential of maghemite-based colloid drops below -55 mV; thus, HA should effectively stabilize the nanoparticle colloid over the whole pH range studied. Meanwhile, bare maghemite SPION at pH 3–6 have $\zeta$-potential in the +20 mV to -20 mV range (isoelectric point at pH 4.35), which is insufficient for electrostatic stabilization of the suspensions. The absence of embryotoxicity of HA-stabilized nanoparticles was demonstrated.