COMPOSITE HYDROGELS WITH CARBON NANOSTRUCTURES AND A SELF-ASSEMBLING TRIPEPTIDE

Hydrogels are promising materials that can be used in different applications such as tissue engineering and drug delivery. The self-assembling peptide L-Leu-D-Phe-D-Phe (Lff) is a popular gelator due to its low cost and easy preparation, and therefore it can give novel composites a great advantage. However, the fragility of the hydrogel based on this short peptide can limit its further application. The introduction of carbon nanomaterials (CNMs) to the hydrogel can overcome its low mechanical properties. In particular, previous studies found that Lff self-assembly is compatible with oxidized multi-walled carbon nanotubes (MWCNTs), graphene oxide (GO), and oxidized carbon nanohorns (CNHs), although the properties of the nanocomposite hydrogels differed, with only gels with oxidized MWCNTs demonstrating self-healing ability, raising questions as to what are the key parameters that enable the acquisition of this interesting property. This thesis describes the research efforts to develop nanocomposite hydrogels with the self-assembling tripeptide Lff and different CNMs to compare the effects of the different CNM morphology on the properties of the final materials to assist in their future design. To this end, the tripeptide was synthesized in solid phase, purified by HPLC, and characterized by standard spectroscopic techniques. CNMs were oxidized by treatment with strong acids (i.e., sufonitric mixture or nitric acid) and characterized by standard techniques, such as Raman and infrared spectroscopy, transmission electron microscopy (TEM), and thermogravimetric analysis. The two components were combined non-covalently by simple mixing (Chapter 4), or also covalently by firstly functionalizing the CNMs with the peptide (Chapter 3) to promote their coating by the free peptide, improve their dispersibility, and reduce their aggregation. The effect of CNT diameter was assessed by comparing the properties of the peptide-gel nanocomposites with MWCNTs, with those obtained with SWCNTs and DWCNTs (Chapter 4). All the materials were characterized by standard techniques, such as oscillatory rheology, infrared and Raman spectroscopy, and TEM. Furthermore, Chapter 5 describes the design of experimental setups to assess the nanocomposite hydrogels’ antibacterial properties on E.coli, and conductive properties, by using the four-probe method and the electrochemical impedance spectroscopy. This study revealed the importance of CNT elongated morphology to impart self-healing ability to the nanocomposite hydrogels, while the CNT diameter was not critical. In particular DWCNTs demonstrated the highest improvement in the viscoelastic properties of the peptide gels. Furthermore, the covalent anchoring of Lff onto the CNMs proved difficult, and did not lead to notable advantages, expect for the case of CNHs that did not self-segregate from the free peptide when they were first also functionalized with it. In the case of CNTs, however, the simple mixing of the free peptide with the oxidized CNTs proved to be the best strategy towards nanocomposite hydrogels with enhanced viscoelastic properties using a very simple protocol. Conductivity measurements did not reveal major differences between the obtained materials, and they revealed that the inorganic buffer contribution played a major role due to ion mobility. Finally, the testing performed on these materials with E.coli did not reveal antibacterial properties of the CNMs, and it is possible that their embedding within the peptide-gel matrix impeded the required contact between bacterial cells and their bare surface. Overall, this work significantly advanced our understanding of these systems to enable their better future design, and highlighted nanocomposites with oxidized DWCNTs as the hydrogels with the best viscoelastic properties that deserve future development for more advanced applications.