Effect of Dipolar Interactions on the Assembly Process of Iron Oxide Nanoparticles Promoted by the CuAAC “Click” Chemistry Reaction

Abstract

Nanoparticle assemblies are very attractive because they allow the fine-tuning of magnetic properties by taking advantage of collective behavior ruled by interparticle interactions. Nevertheless, great efforts to control the spatial arrangement of nanoparticles still have to be developed in order to integrate such nanomaterials in devices for applications in fields such as sensors and recording media. Herein, we report on the assembly of iron oxide magnetic nanoparticles promoted by copper-catalyzed alkyne–azide cycloaddition (CuAAC) “click” reaction. Azido-terminated nanoparticles were assembled onto alkyne-terminated gold substrates. The assembly mechanism was investigated by monitoring the density of nanoparticles as a function of their size and concentration. The kinetics of the assembly process is exponential and is favored by the increase of the nanoparticle size and of the concentration. We show that the assembly of nanoparticles is controlled by the random sequential adsorption (RSA) mechanism below a critical size. In contrast, large nanoparticles allow strong dipolar interactions which participate actively in the assembly process, thus they avoid the RSA pathway. These two different mechanism pathways have a significant influence on the spatial arrangement of nanoparticles. The RSA results in rather isolated small nanoparticles at the earliest reaction times which become gradually a discontinuous monolayer. In contrast, the dipolar interaction assisted RSA mechanism (DIA-RSA) favors linear or 2D small assemblies which quickly grow in tight-packed monolayers of nanoparticles. Finally, the magnetic collective properties of these assemblies are markedly favored by the increase of the nanoparticle size.