Electrochemical study on swelling change of poly(N-isopropyl acrylamide) gels in response to additives by using gel-modified microdisk electrodes and a redox probe

Abstract

Poly(N-isopropyl acrylamide) gel (NIPA) undergoes a volume-phase transition in water from a swollen phase to a collapsed phase at 33°C by increasing temperature. The phase transition temperature of NIPA was changed by addition of various additives to the solvent. Typically, LiCl lowered and 1,3-dimethylurea raised the phase transition temperature of NIPA. NIPA was chemically modified onto platinum microdisk electrodes, and change in electrochemical response of a redox probe ([Ru(NH3)6]Cl3) for the additives was explored in water by using the gel-modified microelectrodes. NIPA changed from a swollen phase to a collapsed phase at 1 M LiCl by increasing the concentration at 25°C. The limiting current of the redox probe measured at a NIPA-modified microdisk electrode decreased in accordance with the shrinkage of NIPA. NIPA underwent a phase transition from a collapsed phase to a swollen phase at 0.5 M 1, 3-dimethylurea by increasing the concentration at 35°C. With the addition of 1,3-dimethylurea, the limiting current first decreased, passed through a minimum at 0.5 M, and then sharply increased again. The changes in the limiting current for both of the additives strongly depended on partition and diffusivity of the redox probe in the gels, which was affected by swelling and shrinking of the gel network.