Preparation of Yellow Sodium Alginate Capable of Cross-Linking with Mg²⁺ to Form the Hydrogel of Magnesium Alginate Quickly and Easily

The bright yellow sodium alginate (YSA) powders were successfully prepared by a solid-phase reaction. The mechanism of the reaction and the structure of products have been investigated by Fourier transform infrared spectroscopy, Raman spectroscopy, and ¹H and ¹³C NMR spectroscopy. The terminal groups of the SA chains undergo ring-opening reactions under alkaline conditions, leading to the formation of an α-dicarbonyl group at the chain ends. Notably, for the first time, we discovered that YSA solution (M/G = 1:2) can rapidly and easily cross-link with Mg²⁺ to form magnesium alginate (MgA) hydrogel within 1 s and does not depend on the concentrations of SA or Mg²⁺. The MgA hydrogel exhibits stable, porous, and uniform structures. The YSA has the potential to replace existing applications of SA in alginate materials, enabling biocompatibility and other properties for applications in the medical, textile, and food industries.