External stimuli responsive syneresis of amino acid-based bioactive hydrogels: a sustainable platform for environmental remediation

Abstract

Syneresis of hydrogels in the presence of external stimuli has potential applications in waste-water treatment and water purification. Fmoc-3-(1-naphthyl)-L-alanine (Fmoc-¹Nap-A) and Fmoc-3-(2-naphthyl)-L-alanine (Fmoc-²Nap-A) form stable hydrogels in 50 mM phosphate buffer of pH 7.4 with minimum gelation concentrations (MGCs) 0.0455% (w/v) and 0.0375% (w/v) respectively. This is the first report on hydrogelation of Fmoc-¹Nap-A and Fmoc-²Nap-A in phosphate buffer at physiological pH and their external stimuli-responsive syneresis. Fmoc-¹Nap-A and Fmoc-²Nap-A are found to be biocompatible and exhibit potent anti-bacterial activity. Fmoc-¹Nap-A shows anticancer activity towards the pancreatic cancer cell line, PANC-1 cells. Syneresis of Fmoc-¹Nap-A and Fmoc-²Nap-A hydrogels has been studied in the presence of various toxic metal ions and the thioflavin T dye. The hydrogel obtained from Fmoc-¹Nap-A shows syneresis in the presence of Hg^II and Ni^II ions selectively and also in the presence of the thioflavin T (TT) dye; however, the hydrogel obtained from Fmoc-²Nap-A shows syneresis only in the presence of the thioflavin T (TT) dye. Structural, mechanical, and morphological characterization have been performed by using X-ray powder diffraction, FT-IR, rheology, and FE-SEM analyses. Interestingly, the absorption of the TT dye from water is more efficient for Fmoc-²Nap-A compared to Fmoc-¹Nap-A. Various spectroscopic experiments have been carried out to estimate the absorption capacity for toxic metal ions and also the TT dye. Minimum amounts of 3.14 mM of 0.6 mL of Hg^II ions, 4.21 mM of 0.3 mL Ni^II ions and 3.14 mM of 0.1 mL of the TT dye for Fmoc-¹Nap-A and 3.1361 mM of 0.2 mL of the TT dye for Fmoc-²Nap-A are at least required to be added on top of 2 mL of 0.08% w/v concentration of hydrogels to exhibit syneresis. The absorption capacities for the TT dye are estimated to be 912.51 μg and 981.92 μg from 1000 μg of TT dye solution by 2 mL of Fmoc-¹Nap-A and Fmoc-²Nap-A hydrogels respectively at a concentration of 0.08% w/v (1600 μg gelator molecules) each by using UV-Vis spectroscopy. ICP-MS data also reveal that 2 mL of the Fmoc-¹Nap-A gel is capable of removing 208.29 μg of Hg^II ions from a total of 440.61 μg of Hg^II contaminated water after syneresis. These hydrogels are capable of removing toxic heavy metal ions (Hg^II) and toxic dye (TT) selectively from waste-water sustainably, showing applicability in the field of water research for environmental remediation.