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Humic acids (HA) are often used as a stabilizer for magnetite nanoparticles (Fe3O4) and as a functional component for magnetic nanocomposites. Despite the protective shell of HA the oxidative degradation of Fe3O4 to maghemite Fe2O3 nanoparticles is observed. This led to a change in the biological activity of the composite (Terekhova et al., 2018). To improve chemical stability of Fe3O4 the surface charge control was used through pretreatment using silanes. In this study, effect of HA adsorption on pH-dependent surface charging and aggregation of the Fe3O4 functionalized by 3-aminopropyl-triethoxysilane (APTES) was studied. A set of the Fe3O4-APTES samples were formulated: under argon (Ar) or air atmosphere and/or further dialysis (D) and subsequent treatment of the samples: in harsh (US-treatment) and mild (magnetic stirrer, M) conditions. As a total, five samples were obtained: Fe3O4, Fe3O4-APTES (Ar), Fe3O4-APTES (Ar-D-M), Fe3O4APTES (Ar-D-US), and Fe3O4-APTES (air). Magnitude of isoelectric points (iep, the pH of ξ = 0), zeta-potential and hydrodynamic size were measured at 25 ± 0.1 °C in a disposable zeta cell (DTS 1060) of NanoZS (Malvern, UK) apparatus [2]. The range of pH was between ∼3 and ∼10. According to the magnitude of the shift of the pH-dependent zeta potential functions due to the sorption of increasing amount of HA in case of all samples, it is possible to formulate a series of samples in which the sorption capacity decreases: Fe3O4-APTES (Ar-D-M)≥ Fe3O4-APTES(air) > Fe3O4≥Fe3O4-APTES(Ar-D-US) >> Fe3O4-APTES (Ar). Treatment way has a great effect on the sorption capacity of the samples. Dialysis of the sample Fe3O4-APTES (Ar) led to enhance adsorbing of HA. The sample exposed to the magnetic stirrer has a larger sorption capacity than the sample after the US-treatment: Fe3O4-APTES (Ar-D-M)> Fe3O4-APTES (Ar-D-US) >> Fe3O4-APTES (Ar). It is likely that harsh conditions during sonication (extreme high temperature and pressure in cavities forming at interfaces) led to destroy Fe3O4-APTES-HA bonds and reduce the sorption capacity. The hydrodynamic diameter increases near the isoelectric point for the samples Fe3O4-APTES without HA. The hydrodynamic size for the samples Fe3O4-APTES at pH < 5 and pH> 9 varied from 700 nm to 300 nm. In the range 5<pH<9, the hydrodynamic size is about 100-300 nm. But, the hydrodynamic diameter for Fe3O4-APTES the increases near the isoelectric point (from 200 to 750 nm), when not fully coverage with HA. In case of full coverage with HA, the hydrodynamic size remains unchanged at all pH range (around 150 nm for all of samples).

The effect of humic acids adsorption on pH-dependent surface charging and aggregation of Fe3O4-APTES nanoparticles