Influence of Electronegative Substituents on the Binding Affinity of Catechol-Derived Anchors to Fe₃O₄ Nanoparticles

Successful applications of nanoparticles are often limited by insufficient nanoparticle stability due to low binding affinity of dispersants. However, excellent Fe3O4 nanoparticle stability was reported in a recent study (Nano Lett. 2009, 9,4042-4048) that compared different catechol derivative-anchored low molecular weight dispersants. Here, we investigate mechanistic binding aspects of five different anchors from this study that showed radically different efficiencies as dispersant anchors, namely nitroDOPA, nitrodopamine, DOPA, dopamine, and mimosine, using electron paramagnetic resonance, Fourier transform infrared spectroscopy, and UV-vis spectroscopy. We demonstrate enhanced electron delocalization for nitrocatechols binding to Fe2+ compared to unsubstituted catechols if they are adsorbed on Fe3O4 surfaces. However a too high affinity of mimosine to Fe3+ was shown to lead to gradual dissolution of Fe3O4 nanoparticles through complexation followed by dissociation of the complex. Thus, the binding affinity of anchors should be optimized rather than maximized to achieve nanoparticle stability.