Polydopamine-Assisted Nano-Functionalization Of Silk And Its Applications

Silk from Bombyx mori cocoons is a natural fiber consisting of two main proteins: fibroin and sericin. Silk has many superior properties such as its luxury sheen and excellent skin affinity. However, the protein nature makes it a matrix for bacterial adhesion and thriving, further resulting in its deformation and even degradation as well as photo-induced aging and yellowish. In recent years tremendous efforts have been dedicated to the surface functionalization and modification of silk. Modification of silk with multifunctional nanomaterials has been regarded as an effective and safe method to endow silk with other properties. Silver nanoparticles（Ag NPs） are the most attractive nanomaterials that has been utilized to functionalize silk due to their broad spectrum of antibacterial effects on both Gram-negative and Gram-positive bacteria. However, reduction reagents or γ-radiation has to be employed for the reduction of Ag+ in the synthesis process. The use of hazard chemicals or expensive irradiation instruments greatly hinders the applications of AgNPs-modified silk. In addition, the capacity of silk surface-immobilized Ag NPs is another critical issue that needs to be considered during the preparation of Ag NPs-functionalized silk.In this thesis Ag and Au nanomaterials were in-situ grown on the polydopamine（PDA）-functionalized silk without the use of additional reductants for the applications of antibacterial, surface-enhanced Raman scattering（SERS） and catalytic properties, respectively. The thesis consists of the following research works: 1. In situ synthesis of silver nanoparticles uniformly distributed on polydopamine-coated silk fibersA facile, efficient and green approach was developed to in-situ grow Ag NPs on the polydopamine（PDA）-functionalized silk. Ag NPs with the size of 30-90 nm were uniformly deposited on the silk fiber surface with the immobilized PDA as the reduction reagent. The AgNPs exhibit excellent face-centered cubic crystalline structures. The bacterial growth curve and inhibition zone assays clearly demonstrate the antibacterial properties of the functionalized silk. Compared with the UV synthetic method, both a higher Ag+ release level and a longer-time release profile were observed for the asprepared AgNPs-PDA-coated silk, indicating the high-density loading of AgNPs and the possible long-term antibacterial effects. 2. Ferric ion-assisted in situ synthesis of silver nanoplates on polydopamine-coated silkA ferric ion(Fe³⁺)-assisted in situ synthesis approach was developed to grow Ag nanoplates on the PDA-coated silk without the use of additional reductants. The essential role of Fe³⁺ in the formation of Ag nanoplates is revealed by comparing the morphologies of Ag nanostructures prepared on the silk-coated PDA film with/without Fe³⁺ doping. SEM images show that high-density Ag nanoplates could be synthesized in the reaction system containing 50 μg/mL FeCl3 and 50 mM AgNO3. The size of the Ag nanoplates could be tuned by adjusting the reaction duration. Based on the data, a mechanism, involving the doping of Fe³⁺ in the PDA film, nucleation of Ag seeds, and Fe³⁺-selected growth of Ag atoms along the certain crystal faces, was proposed to explain the in situ fabrication process. TEM and XRD results indicate that the Ag nanoplates possess good crystalline structures. The Raman spectra demonstrate that the nanoplates could strongly enhance the Raman scattering of the PDA molecules. The Ag nanoplates-FDPDA-silk could be utilized as a flexible substrate for the development of SERS-based biosensors. 3. In situ synthesis of gold nanoparticles on polydopamine-coated silk fibersAu NPs were in-situ grown on the polydopamine（PDA）-functionalized silk without the use of additional reductants. Results show that Au NPs with excellent face-centered cubic crystalline structures are uniformly deposited on the PDA-modified silk fiber surface. The best catalytic performance with the reaction rate constant of 0.087 min-1 could be achieved with the Au NPs-PDA-silk prepared in a 5 mg/m L HAuCl4 solution for 10 min. After 6 rounds of reaction, the catalytic activity of the silk could be maintained without significant alteration, suggesting the strong adhesion of AuNPs on the PDAcoated silk. This work may provide a new approach to developing stable nanocatalysts for the 4-NPh catalytic reduction.In summary, a PDA-assisted approach was developed for the in-situ synthesis of high density Ag or Au nanomaterials on silk. The nanomaterials-functionalized silk was further employed in the antibacterial, surface-enhanced Raman scattering（SERS） and catalytic applications. This project may not only provide new approach for silk surface nano-functionalization, but also extend the applications of silk in SERS-based biosensors and metal-catalyzed reactions.