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.