Entropy‐Driven Reversible Agglomeration of Crown Ether Capped Gold Nanoparticles

Abstract

It is shown that plasmonic gold nanoparticles functionalised with a thiolated 18-crown-6 ligand shell agglomerate spontaneously from aqueous dispersion at elevated temperatures. This process takes place over a narrow temperature range, is accompanied by a colour change from red to purple-blue and is fully reversible. Moreover, the temperature at which it occurs can be adjusted by the degree of complexation of the crown ether moiety with appropriate cations. More complexation leads to higher transition temperatures. The process has been studied by UV/Vis spectroscopy, electron microscopy, dynamic light scattering and zeta potential measurements. A thermodynamic rationale is provided to suggest an entropy-driven endothermic agglomeration process based on attractive hydrophobic interactions of the complexed crowns that are competing against electrostatic repulsion of the charged ligand shells.

Ion shuttling between emulsion droplets by crown ether modified gold nanoparticles

“…The complexing of the ligand shell with barium ions in the source droplets has two opposing effects. As mentioned before, it increases the hydrophobicity of the crown ether ligand, 34,35,41 but it also increases the net charge of the GNPs making them more dispersible in water. The presence of Fe(II) (samples 4, 5 and 6) can bring down the zeta potential of the particles close to zero (Fig.…”

“…40 nm in size and to modulate their dispersibility and phase transfer between water and oil. [28][29][30][31][32][33][34][35] When shaking the emulsion, the chloroform sublayers immediately adopted a reddish brown (4 nm GNPs) or pink (7 nm GNPs) color, which we attribute to the transfer of some GNPs to the chloroform phase (see Fig. ESI1 †).…”

“…Cations that form no, or weak complexes, with the crown ether such as Li + , K + , Cs + or Mg 2+ , can electrostatically replace the barium ions that have been removed from the ligand shell of the GNPs by precipitation. 34 Signicant co-transport of anionic species to maintain electroneutrality is unlikely since this would lead to the precipitation of barium sulfate also in the source droplets (by gradual ingress of sulfate), which is not observed. A range of different barium salts (Cl À , NO 3 À , ClO 4 À and acetate) had no noticeable effect on the overall process observed (Fig.…”

“…Notably, under the conditions of the emulsion experiments, the GNPs always carry a signicant amount of net charge (positive zeta potential), which helps to retain them within the water droplet phase of the emulsion. In a separate experiment, using a two-phase water/chloroform system, it is shown that complexation of cations, which makes the crown-ether more hydrophobic, 34,35,41 causes the GNPs to partition between both phases and also to settle at the phase boundary (Fig. ESI11 †).…”

Origin and Control of Kinetic and Thermodynamic Hysteresis in Clustering of Gold Nanoparticles

“…Rate-dependent hysteresis has been observed in pH-driven clustering of AuNPs stabilized with carboxylated alkanethiols, [13][14][15][16] poly-L-lysine, 17 and thermoresponsive polymers demonstrating an upper 18 or lower 19 critical solution temperature. Despite a vast body of works devoted to dynamic self-assembly, [20][21][22][23][24][25] the hysteresis, when observed, receives mainly a qualitative description, needless to mention the lack of experimental control over its rate-dependent or rate-independent components. A reason for this is an insufficient time-resolution of data acquisition (e.g., spectroscopy) that could otherwise aid information and understanding of intermediate states in such systems.…”