Amino acid based smart hydrogel: formation, characterization and fluorescence properties of silver nanoclusters within the hydrogel matrix

Abstract

N-Terminally Fmoc protected amino acid, Fmoc-Phe-OH (Fmoc-L-Phenylalanine-OH), forms an efficient, stable and transparent hydrogel with a minimum gelation concentration of 0.1% w/v. This hydrogel has been nicely utilized to prepare and stabilize fluorescent few-atom silver nanoclusters. Interestingly, in absence of any toxic reducing agents in a water medium, silver ions are complexed with the carboxylate group of the Fmoc-Phe-OH gelator, and they are reduced spontaneously in the presence of diffused sunlight at physiological pH (7.46) and room temperature to form silver nanoclusters. The three dimensional structure provided by the hydrogel helps to stabilize newly formed silver nanoclusters within the hydrogel matrix. These clusters have been examined using UV-Vis, photoluminescence spectroscopy, high resolution transmission electron microscopy (HR-TEM), X-ray powder diffraction (XRPD) and matrix-assisted laser-desorption ionization (MALDI) mass spectrometric analysis. MALDI mass spectrometric analysis shows the presence of just a few atoms within the silver cluster as Ag4. These silver nanoclusters exhibit interesting fluorescent properties including large Stokes shift (more than 110 nm), narrow emission band width (36 ± 1 nm) and a quantum yield of 3.76%. These silver nanoclusters are stable for up to 4 months of storage at 4 °C in the dark. The morphology of the hydrogel changes after the encapsulation of silver ions within the gel and this altered morphology is retained after the formation of silver nanoclusters within the gel. Interestingly, the rheological properties of the hydrogel alone are different from that of the silver nanocluster-containing hydrogel.