Sequential droplet reactions for surface-bound gold nanocrater array

Femtoliter droplet array exhibit unique stability in contact with a flow. This work demonstrates that reactive femtoliter droplets enables sequential chemical reactions that may be leveraged to simplify the process for producing surface-bound materials. Gold nanocraters (GNCs) are formed on a planar substrate from biphasic reactions between water-insoluble thiol droplets and two aqueous solutions in sequence. The detailed process is that gold precursor solution was injected into a flow chamber hosting a substrate with thiol droplet array in a chamber, followed by injection of a reductant solution. The thiol droplets absorb and weakly bond with gold ions in a precursor solution. Subsequent exposure to a reductant solution accelerates the formation of gold clusters in droplets. The final nanoparticles form GNCs over a large surface area, due to fast formation around the droplet rim. The shape of an individual domain was controlled by the duration of ion absorption in the first step of the sequential reaction. Reacting droplets were followed in time by total internal reflection microscope to understand the reaction process. Morphology and composition of GNCs were characterized by atomic force microscope, SEM, microspectrophotometer, and X-Ray photoelectron spectrometer. We demonstrate that the as-prepared GNCs exhibits stable catalytic activity in degradation of azo dyes for multiple cycles. Compared to many current approaches for producing surface-bound nanomaterials, our approach is based on sequential droplet reactions in a flow-in process. This approach offers unique flexibility in varying independently the reactant concentration and reaction time of each step in the sequential reaction. The synthesized surface-bound catalytic nanomaterials may be applied in water treatment, optical display or fluorescence imaging.