Double Emulsion Droplets as Microreactors for Synthesis of Mesoporous Hydroxyapatite
Original entry: Caspar Floryan, APPHY 225, Fall 2010
"Double Emulsion Droplets as Microreactors for Synthesis of Mesoporous Hydroxyapatite" Ho Cheung Shum, Amit Bandyopadhyay, Susmita Bose and David A. Weitz, Chemistry of Materials 21(22), 5548–5555 (2009).
emulsions, droplets, reactor, synthesis, hydroxyapatite
This article discusses the synthesis of Hydroxyapatite inside double emulsions.
The double emulsions with hydroxyapatite reactants are created in a droplet generating microfluidic device. An injection tube containing an aqueous solution of reactants flows into a junction where it meets an oil phase and another aqueous phase. Here the reactant phase breaks into droplets encapsulated by a shell of oil. The microlfuidic setup is illustrated in figure 1 below.
After the reactants are encapsulated they are deposited in a collection tube. Here the reaction is initiated by increases the pH. This is done by adding ammonium hydroxide to the surrounding fluid. Once the reaction is complete, the hydroxyapatite is removed by increasing the osmolarity of the surrounding fluid, thus swelling the droplet and breaking apart the oil shell. These process steps are shown below in figure 2:
Photos of the emulsions are shown in figure 3. Image (b) shows the reactant phase surrounded by a thick shell of oil. Image (c) shows the droplets after the external osmolarity was increased, swelling the droplets and thinning he oil shell. Images (d) and (e) show close-ups of a swollen droplet.
This method of fabricating hydroxyapatite has several advantages. The reactants are shielded by the oil phase and further reactants can be added by fusing droplets together. The size and geometry of the droplets can be easily tuned and the hydroxyapatite created with this method has exceptionally high porosity and surface area. The surface area can be as high as 162m^2/g.
This method can be used to synthesize particles of other compounds with exceptional control over their nanoscale porosity and morphology.