# Silk Bionanocomposites for Organic Dye Absorption and Degradation
###### Abstract  
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. s 2022 2022022022022022022022022022022022022022022022022022022022022022202202220222022202220222022202220222022202222022220222202222022222022222202222222  
Extensive studies have been conducted on the development of efficient water degollution methods. The latter include precipitation, flocculation, ion exchange, filtration, adsorption and photodegradation, which have been successfully used [5; 8; 9]. Depollution through adsorption remains, however, the most widely used method due to its cost effectiveness and simplicity [10]. Ideal adsorbent materials should have high adsorption capacities, biodegradability, ease regeneration and recyclability. In this context, silk fibroin (SF)-based materials have been developed in applications dealing with the degollution of organic and inorganic compounds [11; 12; 13; 14; 15; 16; 17]. Silk is a natural biopolymer produced by many members of the arthropod's family. Because of its biocompatibility, biodegradability and tunable mechanical properties [18; 19; 20; 21; 22; 23], silk-based materials have been developed and applications for them have been found in many fields such as medicine, catalysis and depollution. _Bombyx mori_ worm's silk is the most abundant due to worm domestication and its mass production mainly for the textile industry. _B. mori_ silk is composed of two proteins: fibroin and sericin. Fibroin forms the core structure of silk threads and is surrounded by sericin. Traditional procedures, mainly thread production for the textile industry, use a degumming process that results in the removal of the surrounding sericin. Therefore, most of the silk materials are composed only of SF. Recently, many different SF materials have been developed, including electrospun mats, additive manufacturing, hydrogels and microspheres, among others [24]. Interestingly, the absorption capacity of SF materials has been scarcely shown in the literature. [PERSON] et al. showed the potential of silk electrospun membranes for air filtration [25]. The produced filters were able to remove from air up to 99.99% of the particles between 0.3-10 \(\upmu\)m, while showing a decreased pressure drop in comparison with the state-of-the-art materials. Another study proved the combination of SF with hydroxyapatite to be efficient for water filtration and purification [12]. The described material was able to remove dyes and heavy metal ions from solutions, which cannot be achieved with conventional nanofiltration membranes. Finally, silk hydrogels obtained from recombinant bee silks produced in _E.