Immobilization of Methyl Orange and Methylene Blue within the Matrix of Charge-Imbalanced Amphoteric Nanogels and Study of Dye Release Kinetics as a Function of Temperature and Ionic Strength

Abstract

Cross-linked polyampholyte nanogels consisting of neutral N-isopropylacrylamide (NIPAM), negatively charged sodium salt of 2-acrylamido-2-methylpropanesulfonate (AMPS), and positively charged (3-acrylamidopropyltrimethylammonium chloride (APTAC) monomers were synthesized via conventional redox initiated free radical copolymerization using N,N-methylenebis(acrylamide) (MBAA) as a cross-linking agent. The resulting nanogels were characterized by means of FTIR and 1H NMR spectroscopy, dynamic light scattering (DLS) and zeta-potential measurements. Surface morphology was analyzed using scanning electron microscopy. Due to the presence of thermally responsive NIPAM units and varying molar ratios of anionic (AMPS) and cationic (APTAC) units, the resulting nanogels were responsive to multiple stimuli in aqueous media and can be used for controlled delivery of dyes. Thus, the NIPAM90-APTAC7.5-AMPS2.5 nanogel with an excess of the cationic units was chosen for immobilization of the anionic dye, methyl orange (MO), whereas the NIPAM90-APTAC2.5-AMPS7.5 nanogel with an excess of the anionic units was chosen for immobilization of the cationic dye, methylene blue (MB). The release kinetics of the dyes from the nanogel was studied depending on the phase transition temperature and the salt content. Mechanism of the dye release from the nanogel matrix was determined using the Ritger-Peppas equation. Disappearance of the ionic contacts between the charged groups of the nanogels and the ionic dyes was suggested to be the main reason for the diffusion of the dyes through the dialysis membrane into the external solution.