In situ pore-forming alginate hydrogel beads loaded with in situ formed nano-silver and their catalytic activity

Abstract

An aqueous mixture of sodium carbonate (Na₂CO₃) and sodium alginate (Na-ALG) was added dropwise into an aqueous solution of Ca(NO₃)₂, leading to the formation of calcium alginate (Ca-ALG) hydrogel beads. Meanwhile Na₂CO₃ as a pore-forming precursor was transformed in situ into CaCO₃ nanoparticles (CaCO₃ NPs). SEM images show that CaCO₃ NPs aggregates with a size of ∼10 μm were uniformly distributed in the Ca-ALG hydrogel beads. After subsequent erosion using acetic acid, Ca-ALG hydrogel beads with a uniform microporous structure were obtained. The porosity and specific surface area of such in situ pore-formed hydrogel beads are 16 and 14 times higher than those of the beads prepared in the absence of Na₂CO₃. Additionally, their porous structure can be modulated by varying the amount of Na₂CO₃. The obtained porous Ca-ALG hydrogel beads were further immersed into an aqueous solution of AgNO₃. Under UV irradiation, the Ag⁺ ions adsorbed in the Ca-ALG were in situ reduced to Ag nanoparticles (Ag NPs). SEM and TEM images show that Ag NPs with a size of ∼10 nm were uniformly distributed in the matrix of the hydrogel beads. The loading amount of Ag in the beads can also be modulated by varying the amount of Na₂CO₃. Furthermore, the resultant Ca-ALG beads loaded with Ag NPs (Ag/Ca-ALG) were used as catalysts. Their catalytic activity was evaluated by using the reduction reaction of 4-nitrophenol as a model reaction. The rate constant of the reaction in the presence of dry porous Ag/Ca-ALG beads was found to be 36 times higher than that in the presence of beads prepared in the absence of Na₂CO₃. Such a high catalytic efficiency can be attributed to their porous structure and consequent high Ag-loading capacity.