With the increasing demands towards environmental protection and sustainable development, the importance of renewable biomass resource has been recognized. Wood is a renewable biomass material with clear structural hierarchy, complex and orderly structure, and natural porous characteristics, which is widely used in the fields such as decoration, furniture, architecture, etc. Glazier-based materials have been utilized in windows for the transmittance of light, while their brittleness may result in safety problems. In recent years, transparent wood (TW) products draw considerable attentions as energy-efficient substrates due to their good mechanical properties, low thermal conductivity, and good optical transmittance [1]. The preparing strategy of transparent wood was initially proposed by Fink [2], as the innovative technology for achieving energy-saving material, which not only made full use of wood, but also endowed wood with new function or even intelligence.

Photo-responsive luminescent characteristic is a very important feature for smart materials, which have been applied to various scenarios like traffic clothing and signs [3,4]. Therefore, researches come to focused on the functionalization of transparent wood through the introduction of luminescent materials. The luminescent transparent wood could be applied to traffic signs, safety bar codes, ultraviolet blocking, military camouflage, smart window, ultraviolet conversion, outdoor lighting and many other fields [5]. Meantime, combination with transparent wood could effectively avoid the fluorescence quenching caused by the aggregation of luminescent materials, and the reduction of light stability caused by the long-term exposure. Gan et al. [6] prepared the luminescent and magnetic transparent wood by encapsulating the delignified wood framework with polymethyl methacrylate resin (PMMA) and magnetic γ-Fe2O3@YVO4:Eu3+ nanoparticles, which has great application potential in green LED lighting equipment, luminous magnetic switch and anti-counterfeiting facilities. After immobilizing epoxy resin and rare-earth doped aluminate (REDA) phosphor nanoparticles in lignin-modified wood, El-Naggar et al. [7] developed a translucent wood with long-lasting phosphorescence, high optical transmittance, high photostability and durability, ultraviolet protection and superhydrophobicity. Al-Qahtani et al. [5] permeated lignin-modified wooden substrate with the pre-polymerized methyl methacrylate and photoluminescent lanthanide-doped aluminum strontium oxide (SrAl2O4:Eu2+, Dy3+; ASOED) pigment for fabricating a photoluminescent translucent wood, which offered a substantial color shift from colorless under visible light to green under UV irradiation.

The preparation process of transparent wood mainly includes the structural design of wood framework and the resin impregnation with matching refractive index. The wood delignification is an effective strategy for the structural design of wood framework, which involves the nucleophilic reactions (i.e., chemical pulping) based on Na2SO3 or Na2S systems, or the electrophilic, free radical and oxidation reactions (i.e., chemical bleaching) based on H2O2, NaClO2 (ClO2) or NaClO systems [8]. Such procedure certainly will deconstruct, split and promote the hemicellulose/lignin removal of wood, but should maintain the integrity and hierarchical structure of wood cell wall. Fink initially used NaClO to remove the lignin and other colored substances [2]. Berglund's group obtained the wood framework after NaClO2 treatment, the specific surface area of which increased from 1.2 m2·g−1 (original wood) to 20 m2·g−1 [9]. Research group of Prof. Hu boiled the wood in NaOH and Na2SO3 solution, and then treated with H2O2 solution, resulting in the retained lignin of wood below 3 % [10]. However, complete removal of lignin will seriously reduce the wood structure and strength. Therefore, it is necessary to consider the treating procedure and retained lignin content for the performance and structure design of wood framework. Li et al. proved that complete removal of lignin was not a necessary process for preparing transparent wood [11]. The wood treated with alkaline H2O2 showed the same whiteness as delignified wood framework, retained better mechanical property and more lignin content, displaying less harmful to environment. Apparently, partial delignification gives wood more opportunities for advanced and sustainable applications.

In order to obtain high transmittance transparent wood, the refractive index (RI) of polymer should match with the components in cell wall, especially the main component of cellulose (RI: 1.53) in delignified wood framework. Except for refractive index, the resin properties such as viscosity, compatibility with cell walls, and shrinkage during polymerization have significant impact on the transparent wood performances including optical stability and mechanical property. Li et al. [12] fabricated the transparent wood based on wood framework and PMMA resin (RI: 1.49) by acetylation to improve their compatibility, and eliminate the gap due to their interfacial debonding, thus achieving the high light transmittance of 92 %. Epoxy resin (EP, RI: 1.50) is a series of thermosetting polymers with epoxy groups, which can be cured by heating with curing agent. The polar groups (e.g., hydroxyl, ether bond, ester bond) or non-polar groups (e.g., aromatic ring, alkane) can be introduced to EP as required. Such EP exhibits the excellent mechanical properties, low shrinkage and degradation resistance caused by weather or solvent [13]. These merits make EP a common polymer for preparing transparent wood. Interestingly, the cellulose nanofibers in wood (delignified wood framework) can improve the overall performance of cured EP. Zheng et al. [14] prepared a new EP with multiple functions by changing its network structure, which was impregnated into wood framework to achieve the functional wood with fluorescence that could respond to light, heat and pH.

Carbon quantum dots (CQDs) are the new luminescent materials with excellent biocompatibility, high chemical stability, polychromatic optical property, low toxicity, easy modification, good water solubility and environmental friendliness, which have been widely used in chemical and biological sensing, electrocatalysis, biological imaging, photocatalysis, drug transport, ultraviolet conversion and other fields [[15], [16], [17]]. However, the strong π-π stacking or resonance energy among the solid CQDs will result in the fluorescence quenching, which can be prevented by keeping sufficient space between fluorophores through dispersed media for weakening their interactions [18,19]. Embedding CQDs into transparent wood can eliminate the fluorescence quenching caused by their aggregation effectively. This is because delignified wood framework mainly composed of cellulose can provide the dense hydrogen bonding sites and reduce the non-radiative transitions, which will supply convenience for uniform dispersion and adsorption of CQDs. Besides, the introduction of CQDs not only protects the transparent wood from ultraviolet radiation, but also endows the transparent wood with polychromatic fluorescent and other more functions, thereby enhancing the performance of transparent wood. Balsa wood is able to become an ideal wood framework (cellulose skeleton) after treatment for preparing transparent wood. In this study, the CQDs with yellow (YCD) and red (RCD) fluorescence were prepared by using the chitosan and o-phenylenediamine as the carbon sources. Afterwards, Balsa woods were respectively pretreated by sodium chlorite and hydrogen peroxide to obtained the wood frameworks (delignified wood and lignin-retained wood), which were further combined with epoxy resin and CQDs for achieving transparent woods and luminescent transparent woods. Then the retained lignin content of DW and LW as well as the visible transmission, mechanical strength and ultraviolet blocking property of DW-TW, LW-TW, YTW and RTW were investigated in depth. Besides, the water repellency, durability, thermal stability and insulation of RTW and YTW were also discussed. Predictably, such luminescent transparent woods certainly will enhance the adaptability of wood-based material, and greatly broaden its applications in green decoration, lighting setup, sensor and other fields.