Synthesis and characterization of short peptide-based hydrogel composites for antibacterial and environmental applications

Peptide-based hydrogels have attracted increasing attention for biological applications and diagnostic research due to their impressive features including biocompatibility and biodegradability, injectability, mechanical stability, high water absorption capacity, and tissue-like elasticity. Furthermore, peptide hydrogels can be used as carriers in drug and gene delivery since, given their porous structure, they allow the entrapment of different molecules (such as drugs, nucleic acids and metal nanoparticles) with antibacterial activity. Nanoparticles in the hydrogel are normally used as nanofillers and nano crosslinkers. In order to obtain hydrogel nanocomposites, a variety of organic or inorganic nanoparticles known as nanofillers are mixed in the hydrogel matrix, and this results in very high enhancement of both internal and external properties of the conventional hydrogel matrix. The combination of organic/inorganic components in the hydrogle network lead to extremely modified chemical and physical, biological, swelling, and releasing properties. In this Ph.D. thesis, we developed a hydrogel by the self-assembly of a short peptidic sequence, the hydrogelator Fmoc-Phe3, enzymatically synthesized in water by a lipolytic enzyme, starting from LPhenylalanyl-L-phenylalanine (Phe2) and an N-(9-Fluorenylmethoxycarbonyl)-L-phenylalanine (Fmoc-Phe). The formed tripeptide molecules (Fmoc-Phe3) self-assemble in water into a three-dimensional fibrous structure. We focused our attention on the synthesis and characterization of novel peptide-based hydrogel composites containing metal and metal oxide nanoparticles such as silver (Ag), titania (TiO2), and magnetic (γ-Fe2O3) NPs, due to their potential antibacterial and environmental applications. On this basis we employed silver salts to develop an in situ one-pot approach for preparing AgNPs inside of the peptide hydrogels using a photochemical synthesis, without any toxic reducing agents. In this study the effect of “green” reducing agents as honey on both the mechanical stability of the hydrogel composite and as capping agent for the AgNPs was investigated. The structure and morphology of the nanohybrids were characterized with different techniques such as FESEM, UV-Vis, DLS, SAXS, and XPS. Moreover, the antibacterial properties of these composites were investigated on a laboratory strain and a clinical isolate of Staphylococcus aureus. Results demonstrated that honey increased both the swelling ability and mechanical stability of the hydrogel. In addition, those hydrogel composites containing honey showed monodispersed AgNPs. Finally, a higher antibacterial effect of AgNPs in the hybrid was observed in the presence of honey. Regarding titania nanoparticles (TiO2NPs), we used commercial ones in the anatase phase and impregnated the peptide hydrogels with them. Two different concentrations of NPs were hired to see the effect of concentration on different parameters of the peptide hydrogel such as toughness and swelling abilities. In addition, the effect of ultrasound waves on the dispersion of the NPs inside of the peptide hydrogels, along with its effect on the synthesized composite like mechanical stability and water absorbance was studied. The results demonstrated that increasing the concentration of the NPs along with the sonication can increase the mechanical stability of the hydrogel significantly but the swelling ability obeys the reverse trend, which may be related to the cross-section caused by the NPs. The antibacterial tests against laboratory Staphylococcus aureus strain and two Methicillin-Resistant S. aureus (MRSA) clinical isolates demonstrated that after encapsulation of the NPs inside of the hydrogels, their toxicity increased significantly in the presence and absence of UV light. Maghemite NPs modified with Polyacrylic acid (γ-Fe2O3@PAA), were synthesized successfully and characterized using various techniques such as FESEM, UV-Vis, DLS, TEM, and XPS. The results showed that PAA can increase both the monodispersity of the NPs and their stability. After impregnation of peptide hydrogels with γ-Fe2O3@PAA NPs, their environmental application for the adsorption of heavy metal ions like Cr(III), Ni(II), and Co(II) and some dyes such as methyl orange (MO), methylene blue (MB), and rhodamine 6G (Rh6G) was studied. The results of the removal studies demonstrate that the encapsulation of γ-Fe2O3NPs in peptide hydrogels in the presence of an external magnetic field increases the adsorption efficiency of the hydrogel matrix for the metal ions and desired dyes tested in this study by more than 50% for each.