Silk Bionanocomposites for Organic Dye Absorption and Degradation

Organic dyes are extensively used in the textile, paper and paint industries, among others. However, the lack of efficient treatment of disposals leads to the release of these toxic molecules into the environment, which has an enormous impact on living organisms. Dye absorption is the most common approach used to tackle this problem. However, the ideal solution should include dye degradation and absorbent regeneration, reducing the environmental impact of the procedure. Dye degradation can be achieved by catalysis. Recently, silk fibroin (SF) has been shown to have incredible absorbent properties. Herein, we characterized the capacity of SF hydrogels to absorb methylene blue (MB), an extensively used cationic organic dye. Moreover, the effect of a pretreatment of the SF hydrogel at different pH and ionic environments is also studied. Interestingly, opposite behaviors are observed when absorbing MB or brilliant blue (an anionic dye), suggesting an electrostatic-based interaction. Furthermore, the regeneration of a MB-saturated SF hydrogel by immersion in acidic pH and its further reuse were evaluated. Finally, the SF hydrogel was coupled with a gold nanoparticle catalyst, which resulted in a material able to absorb and catalyze the MB reduction by sodium borohydride in situ, leading to dye degradation. Overall, this work presents a biodegradable reusable material able to absorb and reduce MB in aqueous media.