Dual pH/thermal-dependent coloring polymeric dye through Mannich reaction of chitosan: Synthesis and characterization

A novel polymeric dye was synthesized by one-pot Mannich reaction of chitosan (CS) and phenolphthalein (PHP). The grafting onto side chain of CS derivatives was confirmed by UV-vis, FT-IR, H-1 NMR techniques. The degree of substitution (DS) calculated by H-1 NMR and elemental analysis was revealed to increase with increasing mole ratio of formaldehyde and PHP. XRD analysis showed that the grafting through Mannich reaction was caused to amorphous structure in the derivatives. Covalent grafting of PHP onto CS made the grafted products showing pink color in basic conditions without leaching of dye and color fading after several weeks. Moreover, the derivatives dissolved in LiOH/urea systems could showed darker pink after heating. The results suggested that the novel CS derivatives with dual pH/thermal-dependent coloring property could potentially be prepared as pH/thermal-responsive biomaterial in a wide range of applications.