RESUMO
Three-dimensional (3D) bulk materials, such as metal-organic frameworks (MOFs) and inorganic phosphors, show the properties of large backscattering and stress concentration, which result in low mechanical and inferior transmittance when these materials are hydridized with a polymer matrix. Inspired by the "reinforcement" effects of two-dimensional (2D) materials, such as grapheme, C3N4, MoS2, and Mxene, it was interesting to examine a 2D lanthanide (Ln)-based MOF-grafted natural polymer (nanocellulose) with the goal of achieving light emission, transparency, and good mechanical properties. A series of near-infrared (NIR) luminescent cellulose nanopapers were prepared via 2D Ln-MOF-grafted (2,2,6,6-tetramethylpiperidin-1-yl)oxyl-oxidized cellulose nanofibrils (tCNFs; Ln = Nd, Yb, or Er). In addition to efficient NIR luminescence, these Ln nanopapers exhibited good flexibility, transparency (>90%), and mechanical properties (>28 MPa). Notably, the haze of these nanopapers was increased by 93-95% from 26% due to compositing with 2D Ln-MOFs, which prevented dense packing among the cellulose and formed air cavities in the nanopaper, inducing internal light scattering and improving optical haze. Moreover, these flexible Ln nanopapers exhibited efficient NIR luminescence, which, together with optical haze and transparency, offered an opportunity for utilization in paper-based anticounterfeiting, NIR-light-emitting diodes, or light softening devices.
RESUMO
A series of lanthanide-based nanopaper (Nd-nanopaper) was synthesized via a neodymium organic framework (Nd-MOFs)-grafted TEMPO-oxidized cellulose nanofibrils (tCNF) using a solvothermal reaction. Not using the traditional down-conversion visible emissions of anti-counterfeiting techniques, this Nd-nanopaper achieved down-conversion near-infrared (NIR) and up-conversion visible emissions. The down-conversion luminescent property of these Nd-nanopapers exhibited characteristic NIR luminescence (λEm = 1080 nm) of Nd3+ ions with 311 nm excitation, undergoing an "antenna" effect. In contrast, the up-conversion visible light emission (λEm =450 nm) of Nd-nanopaper was detected under 580 nm excitation. The mechanism of up-conversion fluorescence was ascribed to excited-state absorption and energy transfer up-conversion. Interestingly, Nd-nanopaper induced both up and down-conversions for visible and NIR emissions that were completely devoid of the interference from fluorescent brighteners and background fluorescence. These switchable up and down-conversion fluorescent Nd-nanopapers with visible and NIR dual emissions or dual channels could be applied in high level anti-counterfeiting applications.