# Silk Bionanocomposites for Organic Dye Absorption and Degradation
## 3 Results and Discussion
### MB Degradation by a SF-Au NPs Bion nanocomposite  
In order to develop an adsorbent-based approach that can combine a non-pollutant adsorbent with an in situ degradation strategy we evaluate, as a proof-of-concept, the use of a silk fibroin hydrogel containing Au nanoparticles (SF-Au NPs bionanocomposite) for a combined adsorption and degradation of MB. The use of Au NPs as a catalyst has been largely studied in the literature [50, 51, 52]. Au NPs have been proved to catalyze the reduction of MB to leuco methylene blue (LMB, colorless) by sodium borohydride (NaBH\({}_{4}\)) [34, 53]. This reaction was chosen as a model reaction as it can be easily followed by UV-Vis measurements due to its blue color extinction. The combination of the MB adsorbent capacity with the catalytic activity of Au NPs can further be used in a continuous water treatment procedure. Not only Au NPs are able to degrade MB, but the direct contact of both elements is enhanced by the adsorption capacity of the SF material.  
Figure 9B shows that, after only 15 min, all the MB had been reduced to LMB (see video x8 as supporting information). Control experiments were performed with hydrogels without Au NPs and if an adsorption of MB was observed, no degradation could be evidenced. Additionally, reduction of MB to LMB was successfully conducted with a  
Figure 8: MB-saturated SF hydrogel immersion in pH 3 solution results in MB release (**A**). MB adsorption (pH 7) and release (pH 3) cycles by silk hydrogels (**B**).  
solution of Au NPs alone to confirm the catalytic role of Au NPs. In this later case, a small enhancement of the catalytical activity was evidenced (12 min rather than 15 min for a complete degradation in the same conditions). This result is expected because Au NPs are embedded within the hydrogel and the MB has to diffuse through the gel to reach the NPs.  
Not only the reaction occurs in a relatively fast manner with SF-Au NPs bionanocomposite, but also the hydrogel can be reused by adding more MB and NaBH\({}_{4}\) at least up to 15 times resulting in the degradation of 5.9 umol of MB by each umol of Au NPs. Thus, it appears that MB is efficiently adsorbed by the SF-Au NPs composite and transformed into leuc MB by an electron transfer mechanism catalyzed by Au NPs [54, 55]. We can hypothesize that the leuco MB, which is not charged, will have less affinity toward the silk adsorbent and be released then replaced by new MB molecules (see Figure 10).  
In spite of the great reduction of MB observed, the chosen reaction results in the production of hydrogen gas (coming from sodium borohydride) that is entrapped within the hydrogel (Figure 9A) resulting in a partial break down of the material after numerous reutilizations. Nevertheless, it should be noted that the NPs were attached to the remaining  
Figure 10: Scheme of the MB reduction by NaBH\({}_{4}\) catalyzed by AuNPs.  
Figure 9: Silk-Au NPs hydrogel bionanocomposite and MB solution (**A**), image taken at the start (**B**) and during (**C**) MB reduction reaction. The formation of H\({}_{2}\) results in gas bubbles entrapped within the silk-Au NPs hydrogel. MB reduction kinetics monitored by UV-vis spectrophotometry (**D**).  
SF fibers and no release into the solution was observed. These results on the lack of NPs release are in accord with the results that we have previously described [39]. These results are thus encouraging for the development of a non-pollutant and reusable system combining the MB adsorption and in situ degradation.