Complete Exchange of the Hydrophobic Dispersant Shell on Monodisperse Superparamagnetic Iron Oxide Nanoparticles

Abstract

High-temperature synthesized monodisperse superparamagnetic iron oxide nanoparticles are obtained with a strongly bound ligand shell of oleic acid and its decomposition products. Most applications require a stable presentation of a defined surface chemistry; therefore, the native shell has to be completely exchanged for dispersants with irreversible affinity to the nanoparticle surface. We evaluate by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) the limitations of commonly used approaches. A mechanism and multiple exchange scheme that attains the goal of complete and irreversible ligand replacement on monodisperse nanoparticles of various sizes is presented. The obtained hydrophobic nanoparticles are ideally suited for magnetically controlled drug delivery and membrane applications and for the investigation of fundamental interfacial properties of ultrasmall core-shell architectures.

Figure 1

TEM of spherical, monodisperse magnetite nanoparticles with narrow size distributions of (a) 3.5 ± 0.4 nm, (b) 5.0 ± 0.4 nm, and (c) 8.3 ± 0.4 nm and (d) high-resolution (HR)-TEM of the 8.3 ± 0.4 nm SPIONs in panel c.

Figure 2

ATR–FTIR spectra of various 3.5 nm magnetite nanoparticle preparations: (a) as-synthesized SPION containing excess physisorbed OA, (b) purified SPION with a monolayer of chemisorbed oleate, (c–j) mixed dispersant OA/P-NDA–SPION and (l–m) post-coated, pure nitrocatechol-ligand-capped SPIONs. SPIONs were purified by the following methods: (b) pre-extraction in hot MeOH containing 1 mM OA as a stabilizer, (c) hot MeOH extraction, (d) cold MeOH extraction, (e) syringe filtration (PTFE), (f) surfactant addition (CTAB), (g) silica column chromatography (THF/MeOH = 4:1), (h) ligand saturated chromatography, (i) repeated MeOH/n-hexane (1:1) recrystallization, (j) 2,6-lutidine as a solvent, (k) post-coated P-NDA–SPION, (l) post-coated d₃₁P-NDA–SPION, (m) post-coated NDA–SPION, and (n) SPION with combusted shell (post-TGA) with 10× scaled inset of residual absorptions. Peaks corresponding to physisorbed and chemisorbed OA are indicated by circles and asterisks, respectively, while crosses depict bands related to nitrocatechol ligands. The post-coated P-NDA (k), d₃₁P-NDA (l), and NDA particles (m) are the only particles that demonstrate an absence of the characteristic OA bands and, therefore, complete ligand replacement.

Figure 3

TGA (solid lines) and DSC curves (dashed lines) of representative 3.5 nm SPION preparations measured in synthetic air: as-synthesized OA–SPION containing excess physisorbed oleic acid (black, stars), purified OA–SPION with an oleate monolayer (gray, squares), column-chromatographed SPION with physisorbed impurities (magenta, diamonds), cold MeOH-extracted mixed dispersant SPION containing around 5% w/w physisorbed OA (green, triangles), CTAB-treated mixed dispersant SPION free of physisorbed OA (blue, circles), and spectroscopically clean P-NDA post-coated SPION (red, triangles).