Poly(ethylene glycol) (PEG) has been widely used for decades as a "gold standard" in bioconjugation, nanomedicine, and antifouling. Although being extensively studied since 1859, PEG remains mysterious, as can be exemplified by the facts that PEG is the only polyether showing excellent water solubility, and the molecular structure of PEG is surprisingly simple if the fantastic properties are considered. Since PEG is usually used in an aqueous medium, the interactions between PEG and water should be the key to understanding the mechanism. Here, we find that by capturing hydronium ions (H 3 O + ) in water, PEG changes from a neutral polymer to a supra-polyelectrolyte, which is a new category of polymer that becomes a polyelectrolyte when an external ion is dynamically bonded to the polymer via intermolecular interactions. This conclusion is supported by multiple experimental methods from the ensemble to single-molecule level. This finding casts new light on the relationship between the simple structure and fantastic functions of PEG. With known species of polymers and ions, numerous novel supra-polyelectrolytes can be prepared, which may present exciting properties in water.

Janus nanoparticles could exhibit a higher interfacial activity and adsorb stronger to fluid interfaces than homogeneous nanoparticles of similar sizes. However, little is known about the interfacial diffusion of Janus nanoparticles and how it compares to that of homogeneous ones. Here, we employed fluorescence correlation spectroscopy to study the lateral diffusion of ligand-grafted Janus nanoparticles adsorbed at water/oil interfaces. We found that the diffusion was significantly slower than that of homogeneous nanoparticles. We carried out dissipative particle dynamic simulations to study the mechanism of interfacial slowdown. Good agreement between experimental and simulation results has been obtained only provided that the flexibility of ligands grafted on the nanoparticle surface was taken into account. The polymeric ligands were deformed and oriented at an interface so that the effective radius of Janus nanoparticles is larger than the nominal one obtained by measuring the diffusion in bulk solution. These findings highlight further the critical importance of the ligands grafted on Janus nanoparticles for applications involving nanoparticle adsorption at an interface, such as oil recovery or two-dimensional self-assembly.