Formation of the Self-assembled Multilayers Containing the Temperature/ pH Dual-responsive Microgels

Abstract

Background: Stimuli-responsive microgels have attracted extensive investigations due to their potential applications in drug delivery, catalysis, and sensor technology. The self-assembled mcirogel films can contain different functional groups (e.g., -COOH, -NH2, -C=ONH2) to interact with specific molecules and ions in water, and their study is becoming increasingly important for developing both absorbent materials and sensor coatings. This paper is aimed to obtain a better understanding of the LbL multilayer formation of microgels and the branched PEI using the mass sensitive QCM. Additionally the influence of the temperature and pH on the formation of the microgel films can be achieved. Methods: The temperature and pH sensitive P(NIPAM-co-AA-co-TMSPMA) microgels were prepared by surfactant-free emulsion polymerization and confirmed by FT-IR, laser particle size analysis, and SEM. The obtained microgel and PEI were further used to prepare multilayer thin films by the LbL self-assembly technique monitored by QCM, and their morphology and hydrophilic properties were determined by AFM and water contact angle measurements. Results: The thermosensitive and pH sensitive P(NIPAM-co-AA-co-TMSPMA) microgels were prepared by surfactant-free emulsion polymerization. The size and swelling properties of the microgels prepared are highly dependent on the preparation conditions such as the AA and crosslinker content, and microgels showed good temperature and pH responsive properties. SEM images showed that microgels dispersed evenly on the substrate and had a uniform particle size distribution, which was consistent with the light particle size analysis results. Furthermore, multilayer films composed of the negatively charged microgels and the positively charged PEI have been built up by a facile LbL assembly method and the influence of the deposition conditions on their formation was monitored in real time by QCM. Compared to the temperature of 25 °C, the high temperature of 35°C above the phase transition temperature leads to the more adsorbed mass of microgels on the gold surface of QCM sensors. The absorbed mass values at the deposition pH 7 and 10 are 9.82 and 7.28 µg cm-2, respectively, which are much higher than 1.51 µg cm-2 of the layers deposited at pH 4. The water contact angle and AFM both confirmed the wettability properties and morphology of multilayers on the gold surface of QCM sensors. Conclusion: The formation of the multilayer films on the gold surface by the layer-by-layer deposition technique of the negatively charged microgels and the oppositely charged PEI can be achieved. The controllable multilayer formation can be attributed to the size difference, changes in the hydrophilic property and surface charge density of microgels responsive to the external temperature and pH.