Application of hydrophobically modified water-soluble polymers for the dispersion of hydrophobic magnetic nanoparticles in aqueous media

Hydrophobically modified water-soluble polymers (HMWSPs), comprised of a poly(sodium methacrylate) (PMANa) or poly(sodium acrylate) (PANa) backbone and pendent dodecyl methacrylate (DMA) or dodecyl acrylamide (DAAm) chains, respectively, were synthesized. The hydrophobic content of the copolymers, P(MANa-co-DMA) and P(ANa-co-DAAm), is in the range of 0 to 25 mol%, while their weight-average molar mass varies from ∼10000 up to ∼75000. Their self-assembly behavior in dilute aqueous solution was followed through Nile Red probing, DLS and TEM measurements. The critical micelle concentration (CMC) is mainly controlled by the hydrophobic content and not the molar mass of the copolymers. Above CMC, spherical and large-compound micelles are identified by DLS and TEM. Moreover, oleylamine coated CoFe₂O₄ nanoparticles (CoFe₂O₄@OAm MNPs) of 9.4 nm with a saturation magnetization M_s = 85 emu g^-1 were solvothermally prepared. The hydrophobic CoFe₂O₄@OAm MNPs were successfully encapsulated into the hydrophobic cores of the structures formed by the copolymers above CMC through a solvent mixing procedure, and in that way hydrophilic CoFe₂O₄@HMWSP nanohybrids resulted. For comparison purposes, two alternate phase transfer approaches were also used to convert CoFe₂O₄@OAm MNPs to hydrophilic ones: (a) addition of a coating layer by cetyltrimethyl ammonium bromide (CTAB) and (b) by the ligand exchange procedure with 2,3-dimercaptosuccinic acid (DMSA). NMR transverse relaxivity measurements of the aqueous suspensions of CoFe ₂O₄@P(ANa-co-DAAm), CoFe₂O₄@CTAB and CoFe₂O₄@DMSA were recorded and the r₂ relaxivity was determined. CoFe₂O₄@CTAB demonstrated the highest r₂ relaxivity of 554.0 mM^-1 s^-1, while CoFe₂O₄@P(ANa-co-DAAm) and CoFe₂O ₄@DMSA showed lower values of 313.6 mM^-1 s^-1 and 76.3 mM^-1 s^-1, respectively.