Single amino acid based thixotropic hydrogel formation and pH-dependent morphological change of gel nanofibers

Abstract

A single amino acid (phenylalanine) based pyrene conjugated low molecular weight hydrogelator has been discovered. This amino acid derivative has been found to form hydrogels in a wide range of aqueous solutions of pH 7.46–14 and the estimated minimum gelation concentration (MGC) in phosphate buffer solution at physiological pH 7.46 is 0.037% (w/v) i.e. 0.85 mM. A transparent hydrogel has been produced in the pH range (7.46–10.5), while a translucent hydrogel has been formed at higher pH. The hydrogel has been characterized by field emission scanning electron microscopy (FE-SEM), fluorescence microscopy, differential scanning calorimetry (DSC), circular dichroism (CD), X-ray diffraction (XRD) and rheological studies. A distinct morphological change of the gel nanofibers from helical to tape-like morphology has been noticed in the FE-SEM studies with an increase in pH. These observations have been supported by a significant decrease of the CD signals and different XRD patterns found with the increase in pH. Interestingly, this hydrogel shows a pH dependent thixotropic property. The thixotropic property of the hydrogel has been examined using rheological experiments. This thixotropic property of the hydrogel has been utilized for the encapsulation of vitamin B₁₂ and an anticancer drug, doxorubicin, in the hydrogel matrix without any heating–cooling cycle and sustained release of these bioactive molecules has been studied at physiological temperature (37 °C) and pH (7.46) over a period of about 3 days.