Temperature and oxidation-responsive ion pair self-assembly composed of hydroxyethyl cellulose ethoxylate and phenylthio acetic acid

Ion pair self-assembly (IPSAM) responsive to temperature and oxidation was prepared using quaternium hydroxyethyl cellulose ethoxylate (QHECE) as a cationic hydrophilic polymer and (phenylthio) acetic acid (PTA) as a hydrophobic counter ion. The IPSAM was spontaneously formed when the quaternium to carboxylic group molar ratio was 1/9-2/8. QHECE/PTA IPSAM was found as sphere-like nanoparticles whose diameter was tens of nanometers on the TEM micrograph. The ion pair showed the upper critical solution temperature (UCST) that increased with increasing the PTA content and decreased when the PTA of the ion pair was oxidized by H2O2. The ion pair was interface-active due to its amphiphilic property and the interface activity was decreased upon the PTA oxidation. The critical micelle concentration of the ion pair was about 10.5 mg/ml, determined by a fluorospectroscopic method, and it decreased upon oxidation. The ionic interaction between QHECE and PTA and the oxidation of PTA were confirmed by FT-IR spectroscopy, and the oxidation was re-confirmed by X-ray photoelectron microscopy. The release of a payload (i.e. nile red) in IPSAM was restrained below the UCST but it was triggered above the phase transition temperature possibly due to the disintegration of the IPSAM. The release was expedited by the PTA oxidation, possibly because the UCST shifted downward below the release medium temperature. The release was somewhat dependent on the pH value but it was not affected by pH value as much as by temperature and oxidation.