Novel pH-responsive granules with tunable volumes from oxidized corn starches

Novel pH-responsive granules with tunable volume from oxidized starch granules were prepared from waxy and normal corn starches using 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation and pregelatinization. Conditions of both oxidation and pregelatinization were designed on the basis of maintaining intact granules' morphology. Carboxylate groups at C6 in anhydroglucose of the oxidized starch acting as pH-responsivity controllers were introduced to by TEMPO/NaClO/NaClO₂ system at pH 6.8 to achieve degrees of oxidation of 9-10%. Formation of the carboxylate groups was confirmed by Fourier transform infrared spectroscopy and thermogravimetric analysis. The TEMPO-oxidized waxy and normal corn starch granules (TWSG and TNSG) were pregelatinized at 65 °C for 5 and 16 min, respectively, to provide desirable swelling capacity of granules. The anionic pregelatinized TWSG and TNSG show swelling ratios of 15.6 and 11.7 at pH 2 and 31.6 and 25.0 at pH 7, respectively, exhibiting good pH-responsivity and potentials for active compounds delivery applications.