Inorganic colloidal nanoparticles (NPs) stabilized by a layer of hydrophobic surfactant on their surfaces have poor solubility in the aqueous phase, thus limiting their application as biosensors under physiological conditions. Here we report a simple model to ionize various types of hydrophobic colloidal NPs, including FePt, cubic Fe3O4, Pd, CdSe, and NaYF4 (Yb 30%, Er 2%, Nd 1%) NPs, to multicharged (positive and negative) NPs via ligand exchange. Surfaces of neutral hydrophobic NPs were converted to multicharged ions, thus making them soluble in water. Furthermore, peroxidase-like activity was observed for ionic FePt, Fe3O4, Pd, and CdSe NPs, of which FePt and CdSe catalyzed the oxidation of the colorless substrate 3,3′,5,5′-tetramethylbenzidine (TMB) to the blue-colored product in the absence of H2O2, while Pd and Fe3O4 catalyzed the oxidization of TMB in the presence of H2O2. With the benefit of the ionic functionalization protocols described herein, colloidal NPs should gain wider use as biomarkers, nanozymes, and biosensors.

A facile strategy has been developed to fabricate Cu(OH)2 supercages (SCs) as an artificial enzyme system with intrinsic peroxidase-mimic activities (PMA). SCs with high catalytic activity and excellent recyclability were generated via direct conversion of amorphous Cu(OH)2 nanoparticles (NPs) at room temperature. More specifically, the process that takes a single nanoparticle to a 3D supercage involves two basic steps. First, with addition of a copper–ammonia complex, the Cu2+ ions that are located on the surface of amorphous Cu(OH)2 NPs would evolve into a fine lamellar structure by coordination and migration and eventually convert to 1D nanoribbons around the NPs. Second, accompanied by the migration of Cu2+, a hollow cavity is generated in the inner NPs, such that a single nanoparticle eventually becomes a nanoribbon-assembled 3D hollow cage. These Cu(OH)2 SCs were then engineered as an artificial enzymatic system with higher efficiency for intrinsic PMA than the peroxidase activity of a natural enzyme, horseradish peroxidase.