Real-Time Volume Changes in Individual Hydrogel Microspheres Observed By High-Speed AFM

Abstract

Many thermo-responsive polymers show a lower critical solution temperature (LCST), and thus they display a coil-globule transition across the LCST by changing temperature. Hydrogel microspheres (microgels) composed of thermo-responsive polymer are taken over the property from the polymer, as a result, they show a volume phase transition temperature around the LCST. Owing to their rapid temperature-responsiveness, microgels are expected the various application such as sensors and separation technology [1]. Thermo-responsive properties of the microgels have been mainly evaluating by light scattering so far because typical size range of the microgels are tens nanometer to sub-micron order, which can not be clearly observed by optical microscopy. Against this background, the real-time volume transition of individual microgels during heating was visualized for the first time using high-speed atomic force microscopy (HS-AFM) [2]. This method allows the direct visualization of the structural dynamics of biological molecules in action, without significantly disturbing the physiological function of the molecules [3-4], and allows the monitoring dynamic adsorption behavior of microgels onto solid substrate in aqueous solution [5]. The microgels were synthesized from N-isopropyl acrylamide and cross-linker N,N'-methylenebis(acrylamide) with different content by aqueous free-radical precipitation polymerization. In the present study, we will discuss the real-time changes in morphology of the microgels. Partially reproduced with permission from [5] Copyright (2017) John Wiley and Sons.