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



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.
