New transparent flexible nanopaper as ultraviolet filter based on red emissive Eu(III) nanofibrillated cellulose Zhao Zhang a, b, * , Hui Chang c , Bailiang Xue a , Qing Han a , Xingqiang Lü c , Sufeng Zhang a , Xinping Li a, ** , Xunjin Zhu b, *** , Wai-kwok Wong b , Kecheng Li d a Shaanxi Provincial Key Laboratory of Papermaking Technology and Specialty Paper Development, College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi'an, 710021, Shaanxi, China b Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, KowloonTong, Hong Kong c School of Chemical Engineering, Shaanxi Key Laboratory of Degradable Medical Material, Northwest University, Xi'an, 710069, Shaanxi, China d Chemical and Paper Engineering, Western Michigan University, Kalamazoo, MI, 49008-5200, United States article info Article history: Received 5 June 2017 Received in revised form 5 September 2017 Accepted 21 September 2017 Keywords: Transparent nanopapers b-diketones type UVA filter Photo-stability Lanthanide Pure red emission abstract A new kind of highly red emissive and transparent nanopapers as ultraviolet filter are produced from lanthanide complex Eu(TTA) 3 (H 2 O) 2 grafted nanofibrillated cellulose (NFC) by a filtration process using a Buchner funnel. The nanopapers Eu-NFC 1e4 with different thickness (0.023 mm, 1; 0.04 mm, 2; 0.081 mm, 3 and 0.1 mm, 4) possess a fibres with dimensions of approximately 50 nm in diameter and several micrometres in length. Those nanopapers exhibit excellent ultraviolet A (UVA; 320e400 nm) filter property and high optical transmittance (>73% at wavelength of 600 nm). The presence of Eu(TTA) 3 (H 2 O) 2 in Eu-NFC nanopapers can block 97% UVA (at 348 nm) light and convert it into pure red emission (CIE: x ¼ 0.663, y ¼ 0.333) through the efficient triplet-triplet energy transfer process. The efficient red emission can significantly improve the photo-stability of b-diketones type UVA filter. It can sustain for 10 h without decomposition under UV irradiation at 365 nm, which makes it possible to be applied in UVA filters. Moreover, its low coefficient of thermal expansion (CTE: 6.39 ppm K 1 of nano- cellulose), is superior to petroleum-based materials for red organic light-emitting devices. © 2017 Published by Elsevier B.V. 1. Introduction Although sun exposure provides many health benefits to humans, such as an improvement in the endogenous prevention of some diseases [1,2], overexposure of ultraviolet (UV) might cause adverse effects, such as skin cancer or cutaneous photo-aging [3]. For this reason, sunscreen with UV blocking property have been developed to prevent the harmful effects of UV radiation [4,5]. According to the wavelength of the light, the UV sunlight at the Earth's surface can be classified as UVA (320e400 nm) and UVB (290e320 nm), and the UVA is approximately 98% [6]. A avobenzone is one of the most popular UVA absorbing molecules used in the cosmetic industry, which belongs to the large family of the b-diketones [7]. However, those of b-diketone type ultraviolet filters can yield potentially toxic photo-adducts when exposed to ultraviolet radiation into reactive species [8]. An added triplete- triplet quenchers (TeT), also termed as excited-state quenchers (ESQ), is attributed to the photo-stability of b-diketone type ultra- violet filters [9]. The common quenchers for UV filters are some organic compounds such as diethylhexyl naphthalate, polysilicone- 15 and octocrylene [10,11]. Recently, Chisholm et al. reported that Mo 2þ complex of b-diketone is a new class of quenchers. The absorbed high energetic UVA undergoes a rapid internal conversion as 1 LLCT/ 1 MLCT and the b-diketone is stable without decompo- sition up to 25 h [12]. On the other side, the coordination of b- diketone with lanthanide ions can also effectively improve its photo-stability. Carlos tested a series of lanthanide b-diketone complexes under UV irradiation and found there is no severe photo degradation of the b-diketone ligands within 10 h [13]. The absorptive high energetic ultraviolet spectrum of b-diketone may * Corresponding author. Shaanxi Provincial Key Laboratory of Papermaking Technology and Specialty Paper Development, College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi'an, 710021, Shaanxi, China. ** Corresponding author. *** Corresponding author. E-mail addresses: zhangzhaoqg@sust.edu.cn (Z. Zhang), Lixp@sust.edu.cn (X. Li), xjzhu@hkbu.edu.hk (X. Zhu). Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2017.09.039 0925-3467/© 2017 Published by Elsevier B.V. Optical Materials 73 (2017) 747e753