Hydrophobic up-conversion carboxylated nanocellulose/fluoride phosphor composite films modified with alkyl ketene dimer.

Hydrophobic up-conversion nanocomposite films have been developed based on TEMPO-oxidized cellulose nanofibrils (TOCNF) modified with alkyl ketene dimer (AKD) as a matrix and MF2:Ho (M = Ca, Sr) as a phosphor. Fabrication of homogeneous, strong and translucent TOCNF/MF2:Ho-AKD films with water contact angle of 123 ± 2° was accomplished with mild drying at 110 °C. These hydrophobic nanocomposite films demonstrated stable up-conversion luminescence in the visible spectral range upon excitation of the 5I7 level of Ho3+ ions by laser irradiation at 1912 nm both under ambient conditions and in a humid atmosphere (92 ± 2% humidity). The absence of luminescence quenching in a high humidity atmosphere for TOCNF/MF2:Ho-AKD composite films was considered to be due to the reliable shielding effect of the hydrophobic TOCNF-AKD matrix. The films show promise for visualizing 2 μm laser radiation in medicine and monitoring of the atmosphere.