Multifunctional silk: from fabrication to application
Abstract: Silk fibers offer untapped internal structures to template the formation of nano-objects and active coatings. So far, access to all or part of the internal and organized structures has been a significant challenge. The aim of the thesis is, therefore, to identify and exploit silk templating ability to create value-added multifunctional hybrid materials with enhanced conductive and catalytic properties. The application of nanotechnology in textiles is limited by the difficulties of loading the textile fibers with nanoparticles (NPs), and by the uncontrolled leakage of the loaded NPs. We first demonstrate using supercritical carbon dioxide (sc-CO2) impregnation that the four major commercially available Indian silk (mulberry, eri, tasar and muga) could be loaded without leakage with standard gold NPs sized between 5-150 nm. Next, we developed a one-step synthesis and impregnation of metal oxides in the silk fibers using mild sonication. Here we sonochemically reduce potassium permanganate (KMnO4)to manganese oxide (MnO2) in silk fibers. The obtained MnO2-Silk hybrid fibers effectively decomposed hydrogen peroxide (H2O2) and oxidized the typical horseradish peroxidase substrates, such as o-phenylenediamine (OPD), and 3,3´,5,5´- tetramethylbenzidine (TMB) in the presence or absence of H2O2. The oxidative properties of MnO2-Silk fiber hybrid showed an enzyme-like behavior for the catalase-like activity,oxidase-like activity, and peroxidase-like activity. The operational stability of the MnO2-Silk fiber hybrid over ten cycles showed a constant residual activity of about 25-30% after 2-3 cycles indicating that MnO2-Silk fiber hybrid could be used as a satisfactory oxidoreductase enzyme mimics. We used potentiometric titration to understand the surface charges on the MnO2-Silk hybrid materials. We identified the reactive species with a pK of approximately 5.2. We further developed an in-situ UV-Visible spectroscopy-based method to study the mechanism of formation of MnO2 on a silk film and its associated enzymatic activity. The results suggested a three components route for sonication and auto-reduction (as control) to form MnO2-Silk from KMnO4. Overall, we found that the smaller size, more mono-dispersed, and deeper buried MnO2 NPs in silk film prepared by sonication, conferred a higher catalytic activity and stability to the hybrid material. The dimensions and oxidation states of the MnO2-Silk hybrid material were determined by the use of X-says structural and spectroscopic methods: a small-angle X-ray scattering (SAXS), anomalous small-angle X-ray scattering (ASAXS), and near-edge X-ray absorption fine structure (NEXAFS). ASAXS allowed us to analyze the MnO2 alone. We found that the MnO2 NP had a size below 20 nm. NEXAFS (pre-peak and main peak) confirms the formation of Mn(IV) oxide. Finally, we demonstrated that the combination of scCO2 impregnation and sonochemistry could yield new or improved multifunctionality. Here we fabricated a soft working electrode for the simultaneous degradation and detection of hydrogen peroxide (H2O2). The multifunctional silk hybrid showed an enzyme-like behavior for the degradation of H2O2 with a Km of about 13 mM. Together these studies suggest that judicious access and use of silk internal structures can enhance silk already remarkable properties.
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