Water-insoluble cyclodextrin membranes for humidity detection: green synthesis, characterization and sensing performances

Herein, we reported the first highly stable β-CD-based humidity sensor without the need of pre-functionalization of the QCM transducer surface. Two partially benzylated β-cyclodextrins (β-CDBn_x, x = 7 and 14) were prepared via a green and one-pot synthesis in aqueous media. The macromolecular structures were methodically investigated by means of NMR, FT-IR and MALDI-TOF spectroscopies and studied with DFT calculations. XRD analysis and thermal analysis revealed an amorphous morphology with better thermal stability in comparison with pristine β-CD. AFM characterization proved that the obtained water-insoluble cyclodextrin thin films exhibited good solution-processing characteristics and film-forming quality. Contact angle measurements evidenced a systematic increase in the surface hydrophobicity with the benzylation rate. These organic materials were investigated as sensitive membranes for humidity detection based on quartz crystal microbalance technique. The experimental investigation showed that the QCM coated with β-CDBn₇ sensitive layer exhibited good sensing performances within a wide relative humidity range of 11–98% including high sensitivity, good linearity, fast response (15 s), desirable reproducibility and high stability even at high humidity level.