Ion shuttling between emulsion droplets by crown ether modified gold nanoparticles

Abstract

Selective unidirectional transport of barium ions between droplets in a water-in-chloroform emulsion is demonstrated. Gold nanoparticles (GNPs) modified with a thiolated crown ether act as barium ion complexing shuttles that carry the ions from one population of droplets (source) to another (target). This process is driven by a steep barium ion concentration gradient between source and target droplets. The concentration of barium ions in the target droplets is kept low at all times by the precipitation of insoluble barium sulfate. A potential role of electrostatically coupled secondary processes that maintain the electroneutrality of the emulsion droplets is discussed. Charging of the GNP metal cores by electron transfer in the presence of the Fe(ii)/Fe(iii) redox couple appears to affect the partitioning of the GNPs between the water droplets and the chloroform phase. Processes have been monitored and studied by optical microscopy, Raman spectroscopy, cryogenic scanning electron microscopy (cryo-SEM) and zeta potential. The shuttle action of the GNPs has further been demonstrated electrochemically in a model system.

Fig. 1. (a) Optical micrographs of the emulsion in the absence of GNPs (left) and in their presence (right). White tags point at barium sulfate precipitates. Droplets containing sulfate are fluorescently labelled (green). Images were obtained within 10 minutes of mixing. (b) Raman microscopy confirms that the precipitate is BaSO₄ and that no detectable amount of it is formed in the absence of GNPs. (c) Cryo-ESEM image of BaSO₄ crystals at the surface of a shock-frozen emulsion droplet (see ESI6 for details and more images). (d) Schematic of GNP-mediated barium transport from a source droplet (blue) to a sulfate containing target droplet (green).

Fig. 2. Cyclic voltammetry of a phospholipid bilayer membrane separating two aqueous compartments both containing a 45 mM aqueous solution of BaCl₂ in the presence of barium-complexing GNPs (black line) and in their absence (red line). The near symmetric current increase in the presence of the GNPs indicates membrane transport of charged carrier particles as illustrated in the image. These experiments confirm the ability of the GNPs to mediate barium ion transfer across surfactant boundaries, either by crossing, or by flipping between the two sides of the membrane.

Fig. 3. Proposed mechanism of electroneutral transport of barium ions across the surfactant boundary from source to target droplets. Note that the replacement of barium ions may occur both by complexation and by electrostatic association of alternative cations (here potassium) within the ligand shell of the GNPs. The zeta-potential values given refer to separate measurements that emulate the conditions in the emulsion droplets (see text).

Fig. 4. Discussion of key results from Table 1. All microscope images were obtained after 10 minutes of mixing, and no further precipitation was observed. The presence of a redox couple influences the degree of barium ion transport by regulating the zeta-potential of the GNPs. Zeta potentials were separately determined in aqueous dispersions emulating the conditions of the source (d) and the target (e) droplets by sequential additions of salt (as indicated). Fe(ii) in the source droplets leads to virtually uncharged particles (a and f) which reside preferentially in the chloroform phase and contribute less to the process. Hence, no or only very little, precipitation of barium sulfate is observed. Conversely, Fe(ii) in the target droplets (b, g, sample 2) charges the particles negatively and reduces their tendency to escape into the chloroform phase. Finally, Fe(iii) in the source droplets (c, g, sample 8) helps to remove negative electronic charge from the particles, which then become positively charged through exchange of potassium against barium ions and are therefore again prevented from escaping into the chloroform phase.