A simple method for preparing gelled alginate beads with a diameter smaller than 5 µm is described. A 1% alginate solution and a medium chain triglyceride (MCT) oil are used to prepare a water-in-oil (w/o) emulsion, stabilized by polyglycerol polyricinoleate. CaCl2 nanoparticles with dimensions in the nano-range (6–400 nm), dispersed in MCT oil, are then added to the emulsion. Energy-dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy (AES) show that these nanoparticles migrate to the emulsion droplet interface, where they dissolve into the aqueous alginate phase and cause gelation, forming beads. Gelation of the beads was confirmed with a novel technique using Congo red as an indicator. A color change occurs upon the addition of CaCl2 to a Congo red solution and we believe this is due to formation of a Congo red–calcium complex. Scanning electron microscopy shows that alginate beads are mostly in a size range around 1 µm, but beads as small as 200 nm and smaller were also found. Extending the size range of alginate beads into the submicron range, while maintaining relatively mild pH conditions in the interior of the bead, will significantly extend the range of applications for this type of beads.
- electrostatic droplet generation
- internal gelation