Regiospecific Grafting of Chitosan Oligomers Brushes onto Silicon Wafers

The functionalization of surfaces using chitosan oligomers is of great interest for a wide range of applications in biomaterial and biomedical fields, as chitosan oligomers can provide various functional properties including biocompatibility, wetting, adhesion, and antibacterial activity. In this study, an innovative process for the regiospecific chemical grafting of reducing-end-modified chitosan oligomers brushes onto silicon wafers is described. Chitosan oligomers (COS) with well-defined structural parameters (average DP ~19 and DA ~0%) and bearing a 2,5-anhydro-d-mannofuranose (amf) unit at the reducing end were obtained via nitrous acid depolymerization of chitosan. After a silanization step where silicon wafers were modified with aromatic amine derivatives, grafting conditions were studied to optimize the reductive amination between aldehydes of amf-terminated COS and aromatic amines of silicon wafers. Functionalized surfaces were fully characterized by AFM, ATR-FTIR, ellipsometry, contact angle measurement, and ToF-SIMS techniques. Smooth surfaces were obtained with a COS layer about 3 nm thick and contact angle values between 72° and 76°. Furthermore, it was shown that the addition of the reducing agent NaBH₃CN could positively improve the COS grafting density and/or led to a better stability of the covalent grafting to hydrolysis. Finally, this study also showed that this grafting process is also efficient for chitosan oligomers of higher DA (i.e., ~21%).