Peptide-Based Hydrogels as a Scaffold for In Situ Synthesis of Metal Nanoparticles: Catalytic Activity of the Nanohybrid System

A tripeptide containing an oligo-methylene group was found to form hydrogels at the two pH extremes (in presence of an acid or a base); however, it does not form a hydrogel at neutral pH. We use this property to obtain a hydrogel of the peptide in the presence of an organic acid, citric acid. The redox potential of citric acid is ideal to reduce Ag⁺ and Au³⁺ ions to form the corresponding nanoparticles. Keeping this in mind the hydrogel with citric acid was applied to fabricate silver and gold nanoparticles within the gel phase without the aid of external reducing and stabilising agents. The hydrogels and the peptide nanofiber-metal nanoparticle based hybrid system were thoroughly characterized by UV-visible spectroscopy, FEG-TEM microscopy, FT-IR spectroscopy, and powder X-ray diffraction studies. From the FEG-TEM images it is evident that a considerable amount of Au and Ag nanoparticles are found above the gel nanofibers. Interestingly, the hybrid hydrogel containing silver nanoparticles could be successfully employed as a catalyst in the transformation of p-nitrophenol to p-aminophenol and p-nitroaniline to p-phenylenediamine. It is also interesting to note that the native hydrogel (without Ag nanoparticles) is unable to act as a catalyst. Moreover, this hybrid hydrogel has been recycled a few times without significant loss of activity, showing the potential of this new nanohybrid system for catalysis.