Entropy-Driven Reversible Agglomeration of Crown Ether Capped Gold Nanoparticles

Effect of Alkali Metal Cations on the Unilamellar Vesicle of a Squalene Bearing 18-Crown-6

Squalene bearing 18-crown-6 was synthesized and formed unilamellar vesicles with a membrane thickness of about 6 nm and a diameter of about 0.32 μm. In the wake of the recognition of alkali metal cations, squalene unilamellar vesicles become larger as multilamellar vesicles or smaller while maintaining unilamellar vesicles depending on cations.

Crown Ether-Capped Gold Nanoclusters as a Multimodal Platform for Bioimaging

The distinct polarity of biomolecule surfaces plays a pivotal role in their biochemistry and functions as it is involved in numerous processes, such as folding, aggregation, or denaturation. Therefore, there is a need to image both hydrophilic and hydrophobic bio-interfaces with markers of distinct responses to hydrophobic and hydrophilic environments. In this work, we present a synthesis, characterization, and application of ultrasmall gold nanoclusters capped with a 12-crown-4 ligand. The nanoclusters present an amphiphilic character and can be successfully transferred between aqueous and organic solvents and have their physicochemical integrity retained. They can serve as probes for multimodal bioimaging with light (as they emit near-infrared luminescence) and electron microscopy (due to the high electron density of gold). In this work, we used protein superstructures, namely, amyloid spherulites, as a hydrophobic surface model and individual amyloid fibrils with a mixed hydrophobicity profile. Our nanoclusters spontaneously stained densely packed amyloid spherulites as observed under fluorescence microscopy, which is limited for hydrophilic markers. Moreover, our clusters revealed structural features of individual amyloid fibrils at a nanoscale as observed under a transmission electron microscope. We show the potential of crown ether-capped gold nanoclusters in multimodal structural characterization of bio-interfaces where the amphiphilic character of the supramolecular ligand is required.

Gold nanoparticles endowed with low-temperature colloidal stability by cyclic polyethylene glycol in ethanol

The colloidal stability of metal nanoparticles is tremendously dependent on the thermal behavior of polymer brushes. Neat polyethylene glycol (PEG) presents an unconventional upper critical solution temperature in ethanol, where phase segregation and crystallization coexist. This thermal behavior translated to a PEG brush has serious consequences on the colloidal stability in ethanol of gold nanoparticles (AuNPs) modified with PEG brushes upon cooling. We observed that AuNPs (13 nm diameter) stabilized with conventional linear PEG brushes (M_n = 6 and 11 kg mol^-1) in ethanol suffer from reversible phase separation upon a temperature drop over the course of a few hours. However, the use of a polymer brush with cyclic topology as a stabilizer prevents sedimentation, ensuring the colloidal stability in ethanol at -25 °C for, at least, four months. We postulate that temperature-driven collapse of chain brushes promotes the interpenetration of linear chains, causing progressive AuNP sedimentation, a process that is unfavorable for cyclic polymer brushes whose topology prevents chain interpenetration. This study reinforces the notion about the importance of polymer topology on the colloidal stability of AuNPs.

Ion shuttling between emulsion droplets by crown ether modified gold nanoparticles

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.