Direct Ordering of Anchoring Events at the Surface of Iron Oxide Nanoparticles Enabled by A Stepwise Phase‐Transfer Strategy

Abstract

For most nanoparticle applications, understanding the "strength" of anchoring events at the surface is necessary to design effective ligand exchange processes. The present report demonstrates the ability to direct the ordering of phosphonic acid, catechol, and carboxylic acid as anchoring groups for iron oxide nanoparticles, enabled by a stepwise phase-transfer strategy. A key feature was the use of a cyclohexane-based thermomorphic system that provided a unique homogeneous field for ligand-exchange processes, where hydrophobic nanoparticles and a hydrophilic ligand, or hydrophilic nanoparticles and a hydrophobic ligand, effectively mixed together within a moderate temperature range.Iron oxide nanoparticles are powerful platforms capable of producing novel functional materials, and can impart useful magnetic responses to molecules anchored onto their surface. [1] These responses can be exploited to develop contrast agents for magnetic resonance imaging (MRI), probes for in vitro diagnostics, and devices to induce hyperthermia. The low toxicity and low cost of iron oxide nanoparticles have resulted in the development of numerous creative applications, especially in biomedicine as magnetic carriers. Although a one-step process to produce iron oxide nanoparticles capped with organic molecules is possible, [2] biomolecules should be designed and pre-synthesized prior to anchoring onto the surface of the nanoparticles because nanoparticle production conditions generally are too harsh for biomolecules and can damage them. Therefore, the iron oxide nanoparticles and biomolecules should be prepared separately and attached during the final step. Separate preparation of the biomolecules is also important for their characterization, because one-dimensional solution 1 H nuclear magnetic resonance (NMR) spectroscopy, a fundamental organic analytical technique, is much less effective after anchoring due to the magnetic response.To obtain stable colloidal iron oxide nanoparticles, the surface must be capped with suitable ligands to prevent nanoparticle aggregation. In general, the ligands are designed with an anchoring group of specific organic chain(s) selected for affinity toward the solvent. Oleic acid (1) is a standard ligand and one of the most useful for iron oxide nanoparticles because it promotes nanoparticle stability in low-polarity solvents. [3] A carboxylic acid is used as an anchoring group for various oxide surfaces, [4] because it forms relatively weak reversible bonds with the surface. This weak and reversible bonding is important for ligand exchange processes, [5] through which biomolecules equipped with another anchoring group can be loaded onto the oxide surface. The driving force for these ligand exchange processes is based on equilibrium; the ligand anchored onto the surface can be exchanged by the addition of a large excess of another ligand. In addition, anchoring reversibility is key to understanding the driving force for the ligand exchange processes. A ligand forming a reversible bond, such as a c...

Induction heating induced self-healing of nanocomposites based on surface-functionalized cationic iron oxide particles and polyelectrolytes

“…The intensity of the signal increases with progressive functionalization. [47][48][49] In addition the IR spectra reveal the The C=O stretching vibration at 1637 cm -1 , which is present in all samples, shows a nonquantitative exchange of the oleic acid. The signal intensity suggests a significantly larger amount of remaining oleic acid compared to similar exchange protocols of DA and PEG phosphonic acid derivatives reported in literature.…”

Ligand Exchange Reactions between Phosphonic Acids at TiO2 Nanoparticle Surfaces

“…Although carboxylates are categorized as tightly bound ligands by the use of colloidal NMR spectroscopy, they can quantitatively be exchanged by phosphonic acids, using CdSe, [11] PbSe, [12] and ZrO 2 [13] nanoparticles. Previously, we qualitatively demonstrated that carboxylates can be exchanged by phosphonic acids at Fe 3 O 4 nanoparticle surfaces by using a stepwise phase‐transfer strategy [14] . We questioned whether phosphonates tightly bound to nanoparticle surfaces can be exchanged by other phosphonic acids.…”

“…Previously, we qualitatively demonstrated that carboxylates can be exchanged by phosphonic acids at Fe 3 O 4 nanoparticle surfaces by using a stepwise phase-transfer strategy. [14] We questioned whether phosphonates tightly bound to nanoparticle surfaces can be exchanged by other phosphonic acids. Most colloidal NMR studies have used quantum dot nanoparticles as platforms, but the surface chemistry for metal oxide nanoparticles is somewhat less understood.…”

Mapping out the aqueous surface chemistry of metal oxide nanocrystals; carboxylate, phosphonate and catecholate ligands

“…Such a low binding affinity was highly unexpected, given many reports of successful surface functionalization with catechols. 39, 51, 54-58 However, in these reports water or biological buffers were used as the solvent. Therefore, we designed a competitive binding experiment directly in water.…”

“…38 Okada et al came to the same conclusion on 9 nm iron oxide NCs by performing phase-transfer experiments using polar and nonpolar phosphonic acids and catechols. 39 It is clear that despite the common usage of phosphonic acids and catechols in metal oxide NC functionalization, there is no clear consensus on the relative binding affinity. Furthermore, a direct link between ligand binding equilibria and the final colloidal stability of the NCs is usually not made.…”