Water-Redispersible and Highly Stable Gold Nanoparticles Permanently Capped by Charge-Controllable Surfactants for Potential Medical Applications

A nanosized gold particle (AuNP) can increase the active surface area available for a chemical reaction to maximum; it can also provide enhanced reaction activity in catalysis or present a variety of optical properties. However, in aqueous solution, AuNPs without additional surface functionalization are rather unstable; this causes many limitations on further processing. Moreover, it is still challenging to achieve stable AuNPs with a narrow size distribution in aqueous solution. Herein, we report an approach to fabricating highly stable AuNPs that are water-redispersible and possess a narrow size distribution in aqueous solution. This is achieved by using AuNPs that are prepared in an organic solvent and provide excellent monodispersity (which is the Brust method). The fabrication of stable AuNP in aqueous solution is a very simple three-step procedure: formation of an oil/water microemulsion by ultrasonication, evaporation of organic solvent, and in situ free-radical polymerization of surfactants. Therefore, this process renders it possible to continue the processing in water. The fabricated AuNPs [p-AuNP/cetyltrimethylammonium 4-vinylbenzoate (CTVB)] were individually encapsulated using a stable surfactant layer that was well-dispersed and highly stable even in harsh processes such as freeze-drying and under the action of a strong acid or base. Moreover, these particles also persist for at least 3 months in water; this can provide a wide range of potential applications including the contrast agents or biosensors in medical diagnostics and the optical sensors or optoelectronics in optical devices. Furthermore, the surface charge density of p-AuNP/CTVBs was also controlled by copolymerizing them with an anionic hydrotropic salt; the range of control was +18.3 to-41.9 mV.