Selective metal ion-capturing by striped nanoparticles

Abstract

Nanomaterials have attracted lots of attention due to their tremendous potential in extensive fields, ranging from biology, physics, and chemistry to electronics, and also already proven their superior functionality to conventional materials. The unique properties of nanomaterials comes from their size, morphology, and structure, and frequently a ligand shell structure plays a key role in their distinct behavior and feature since it determines an interaction with environments. We manipulated gold nanoparticles protected by self-assembled monolayer (SAM) of mixed ligand molecules, which have shown noticeable characteristics. Thiolended molecules adsorb onto gold surface, resulting in the formation of SAM. Particularly, two dissimilar ligand molecules undergo a phase-separation on gold nanoparticle surface, and it leads to an ordered structure, alternating striped-like domains. This so-called "striped" structure is induced by a competition between enthalpy and entropy. The longer ligand molecules surrounded by the shorter molecules gain extra conformation entropy at the expense of enthalpy loss, generated by the presence of two different molecules in the same interface. This unique ligand shell structure, i.e. the striped structure, contributes to the superior properties of our gold nanoparticles. Here we report that the striped gold nanoparticles, consisting of a hydrophilic and a hydrophobic molecule, can selectively capture metal ions, of which sensitivity and selectivity are incomparable to homo-ligand coated nanoparticles and disordered structure nanoparticles. The interaction between striped gold nanoparticles and metal ions was intensively demonstrated with various experimental methods, including UV-vis, FT-IR, ITC, zeta potential, and XPS. As a part of application, a solid-state metal ion sensor using striped gold nanoparticles with a different pair of hydrophilic and hydrophobic ligands was exploited. The change of the conductance was measured upon the interaction with a particular metal ion, and it exhibited a matchless selectivity and sensitivity. For example, the conductance of gold nanoparticle film with a specific pair of ligands is changed remarkably with methymercury ions, but that of other kinds films is not, and also it shows extremely low detection limit, which is in the unit of aM (10-¹⁸ M), corresponding to ~600 methylmercury ions in a 1ml of solution. Furthermore, the removal methods of toxic metal ion contaminants from water were manipulated with striped gold nanoparticles, functionalizing with magnetic particles, and also fabricating them as a metal-ion sponge. Additionally, it was observed that these striped nanoparticles display a different ion-capturing behaviour, depending on temperature, which has rarely shown in other ion-chelating materials. This was induced by the entropy effect of the flexible hydrophilic ligands on the striped nanoparticles. Finally, the same ion-capturing behaviour was found in nanoparticles of a different core material (i.e. silver) with Eu³+ions. This proposed the possibility of the use of other core materials when this striped nanoparticle was applied in a real life.