Versatile surface modification of millimeter‐scale “aqueous pearls” with nanoparticles via self‐polymerization of dopamine

The millimeter-scale calcium alginate aqueous core capsules (mm-CaSA-Caps), which own high entrapment efficiency and excellent biocompatibility, have broad applications in food additives and drug delivery. However, due to pH sensitivity and thermal instability of Ca-alginate, many chemical modifications with intense conditions are hardly conducted on the mm-CaSA-Caps. In this work, layer-by-layer assembly coating on the surface of the mm-CaSA-Caps was proceeded with in situ self-polymerization of dopamine (DA) and nanoparticles under mild conditions. The colorless and transparent mm-CaSA-Caps became beautiful dark "pearls" of polydopamine (PDA) modified mm-CaSA-Caps (mm-PDA@CaSA-Caps), which had uniform coating of PDA. Obviously, deposition of PDA on mm-CaSA-Caps accelerated with higher concentration of DA, weak alkali condition of pH = 8.5, and oxidant of sodium periodate. Water retention ratio of mm-PDA@CaSA-Caps also increased because of the PDA coating. The nanoparticles were easily coated on the mm-PDA@CaSA-Caps due to the strong adhesive property of PDA. Two-dimensional laminar montmorillonites were adhered to mm-PDA@CaSA-Caps, which lead to better barrier property of capsules than that of the one-dimensional linear multiwalled carbon nanotubes modified capsules. It was ascribed to increasing of the path length of water by sheet-like morphology of montmorillonites. This work provided a versatile path for enhancement of barrier property of millimeter-scale hydrogel capsules.