Spin coating cellulose derivatives on quartz crystal microbalance plates to obtain hydrogel-based fast sensors and actuators

Cellulose based superabsorbent hydrogels were spin coated and chemically crosslinked on Quartz Crystal Microbalance plates to obtain a thin superabsorbent layer. The hydrogel network was obtained by crosslinking hydroxiethylcellulose (HEC) and Carboxymethylcellulose (CMC) by the difunctional Divinylsulfone (DVS). The presence of the polyelectrolyte CMC was found to be responsible for the material high sorption sensitivity to changes in ionic strength and pH of the external solution, due to the Donnan type equilibrium established between the internal of the gel and the external solution. Since the hydrogel was synthesized in the form of a thin layer, there is negligible barrier to water diffusion through the material, which results in a fast swelling response to changes in the external solution properties. The device was immersed in a flux of water solution, where the ionic strength was continuously changed. The fast change in the equilibrium sorption capacity of the spin coated hydrogel were converted to an electrical signal by the apparatus. The time lag of the response was comparable to the inverse of the quartz oscillation frequency (approximately 107 s), and the accuracy in the weight change measurement was of the order of the nanogram. FTIR microscope analysis was used to determine the presence of hydrogel and its distribution on the QCM plate.