Laser damage threshold of hydrophobic up-conversion carboxylated nanocellulose/SrF2:Ho composite films functionalized with 3-aminopropyltriethoxysilane

Luminescent nanocomposite films, containing SrF 2 :Ho up-conversion particles, were prepared by two different protocols from aqueous dispersions of TEMPO-oxidized cellulose nanofibrils (TOCNF) functionalized with 3-aminopropyltriethoxysilane (APS) without the use of organic solvents at pH = 4.0–4.5 and 9.0–9.5, respectively. Proposed synthetic protocols included the formation of the films by drying the dispersions containing pre-hydrolyzed APS adsorbed onto TOCNF and SrF 2 :Ho particles followed by heating at 105 °C. Hydrophobic (water contact angle 101 ± 2°), strong, and translucent TOCNF/SrF 2 :Ho-APS films were prepared by casting from a solution at pH = 4.0–4.5. Scanning electron microscopy, energy-dispersive X-ray spectroscopy with element mapping, Fourier-transform infrared spectroscopy, X-ray diffraction methods confirmed homogeneous distribution of up-conversion particles in TOCNF matrices as well as the grafting of linear polysiloxanes via the condensation of silanol groups and OH-groups on the surface of TOCNF. Differential scanning calorimetry and thermogravimetry data confirmed an increase in thermal stability of the APS modified nanocomposite films obtained at pH = 4.0–4.5. Hydrophobic TOCNF/SrF 2 :Ho-APS nanocomposite films exhibited an intense red luminescence in the visible spectrum range ( 5 I 7 level excitation of Ho 3+ ions with 1912 nm laser irradiation) as well as two-times higher laser damage threshold compared to unmodified TOCNF/SrF 2 :Ho films. TOCNF/SrF 2 :Ho-APS films can be used for visualization 2 μm laser radiation in medicine and long-distance atmosphere monitoring. Graphic abstract