pH-responsive chitosan nanofilms crosslinked with genipin

Crosslinked chitosan-based nanofilms (thickness approximately 40 nm) were prepared using a naturally occurring and nontoxic reagent, genipin, and the surface properties were studied as a function of crosslinking degree and pH, by combining three techniques: dynamic contact angle of sessile drops, quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). The results showed a pH-response of these crosslinked chitosan nanofilms, which swelled at pH 3 and shrank at pH 6. This transition was facilitated by chitosan crosslinking, although the wettability, studied by contact angle measurements, decreased with crosslinking. QCM-D and AFM were used to study the swelling capacity and elastic properties of crosslinked chitosan nanofilms, demonstrating that the response of the nanofilms was fully reversible between pH 3 and 6, but irreversible changes occurred at pH 9. QCM-D showed variations in frequency (Δf) generated by the processes of swelling/shrinkage and variations in dissipation (ΔD), related to the changes in the structure of chitosan nanofilms under different pH values. Under various pH conditions, the AFM results also showed swelling at pH 3 and shrinking at pH 6, observing an increase in the elastic modulus from ≈500 MPa at pH 3 to ≈700 MPa at pH 6. These results allowed us to understand the pH-sensitive properties of genipin-crosslinked chitosan nanofilms, which can be very useful in smart biomaterial-based textiles.