Introduction

Wood, as a major renewable and natural structural material, has wide applications in building, furniture, and functional materials1,2,3,4,5,6,7,8. Thus, the sources of waste wood residues are increasing, with only ~17% being recycled9. Considering more efficient resource utilization and lower environmental impacts (e.g., carbon emissions from wood decay degradation), wood recycling is clearly a priority. Until now, efforts have been made to convert waste wood into particle boards, fuels, and fertilizers10,11,12,13. Recently, wood was discovered to have RTP emission, attributed to the confined lignin in the wood cell walls14,15,16,17. Nevertheless, the RTP lifetime of natural wood is on the order of tens of milliseconds, which is too short for practical applications. To extend the lifetime, two main strategies were employed: 1) promoting the intersystem crossing (ISC) of lignin in wood to obtain more triplet excitons by employing external heavy atoms14,18; 2) suppressing non-radiative decay of triplet excitons, often achieved by rigidifying the incorporated lignin15,16,19,20. Using these strategies, the RTP lifetime of woody RTP materials was enhanced to hundreds of milliseconds. The developed high-performance woody RTP materials exhibited potential in photocatalysis, 3D printing, visual decoration, and anti-counterfeiting applications21,22,23,24,25,26,27. Taking all these points into account, converting waste wood into RTP materials is a promising strategy for recycling waste wood. Nevertheless, most of the reported methods for converting wood to RTP materials are not applicable here14,16,24. This is because only non-bonded interactions, such as hydrogen bonds, were introduced for rigidifying lignin to activate RTP in previous studies. Such weak interactions are easily degraded by external environments, such as chemical reagents, humidity, etc., leading to compromised RTP. Considering the complex chemical reagents involved in waste wood, a new and reliable method should be developed for converting waste wood into RTP materials.

To this end, we focused on melamine-formaldehyde resin (MF). MF has been explored for producing RTP materials since the resin provides a rigid network and stabilizes the triplet state for RTP emission28,29. Moreover, MF is widely employed as an adhesive in the wood industry due to its high performance, safety, and low cost30,31,32.

Surprisingly, here, we found that MF efficiently enhanced the RTP emission of natural wood by both covalently and non-covalently interacting with the components in the wood. Motivated by this, we conceptualized to convert waste wood to RTP materials by introducing MF into waste wood for converting them into RTP materials (Fig. 1). To this end, the recycled woody powders were immersed into MF firstly. The mixture of woody powders and MF was then thermally cured. As-obtained RTP materials exhibited robust green RTP emission.

Fig. 1: Preparation of R-wood@MF.
Fig. 1: Preparation of R-wood@MF.
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The schematic diagram of fabricating RTP materials from waste wood.

Results

RTP emission of wood@MF and mechanism

Wood@MF was prepared via in situ curing of MF in the presence of wood. SEM images showed that the resin thoroughly penetrated the porous structure of wood (Supplementary Fig. 1). Under 365 nm UV light excitation, wood@MF exhibited fluorescence emission centered at 440 nm and significant afterglow emissions at 500 nm and 530 nm, with lifetimes of 358.3 ms and 352.1 ms, respectively (Fig. 2a and Supplementary Fig. 2, Supplementary Table 1, Supplementary Movie 1). Notably, mechnical stiring did not obviously affect the RTP performance of wood@MF during preparation (Supplementary Fig. 3).

Fig. 2: RTP emission of wood@MF and mechanism.
Fig. 2: RTP emission of wood@MF and mechanism.
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a Fluorescence (black dash line) and phosphorescence (green solid line) spectra of wood@MF. Inset: Digital images of wood@MF upon UV excitation and after switching the UV light sources off; b Time-resolved phosphorescence emission of wood@MF, excitation wavelength = 365 nm; c Phosphorescence spectra of D-wood@MF (blue line), wood@MF (green line)and Lig@MF (cyan line), excitation wavelength = 365 nm. Inset: Digital images of wood@MF upon UV excitation and after switching the UV light sources off; d The FT-IR spectra of wood-MF (green line) and wood@MF (blue line); e The high-resolution C 1 s XPS spectra of wood-MF and wood@MF: red dots: the XPS raw data, red solid lines: separated peaks of various chemical bonds; f Independent gradient models of cellulose vs. MF and lignin vs. MF.

As a control, neat MF in the absence of wood exhibited lower RTP emission with shorter lifetime than wood@MF, confirming the crucial role of natural wood in the system (Supplementary Fig. 4).

Additionally, natural wood exhibited weak afterglow emission with a relatively short lifetime of 17.7 ms (Supplementary Fig. 5). The emission behavior of wood@MF was investigated at different temperatures. Both the intensity and lifetime decreased with the increased temperature. This result suggests that the delayed emission is RTP rather than thermally activated delayed fluorescence (TADF) (Supplementary Fig. 6 and Supplementary Table 2). To further understand why TADF did not occur in this material, the singlet-triplet energy gap (ΔEST) was calculated. The ΔEST(500 nm) and ΔEST(530 nm) were ~0.34 eV and ~0.47 eV, respectively (Calculation details in the SI). When the ΔEST < 0.3 eV, reversible intersystem crossing (RISC) is possible, inducing TADF33,34,35,36. While RISC does not happen when the value exceeds 0.3 eV, resulting in the absence of TADF. The total quantum yield and phosphorescent quantum yields of wood@MF, wood and neat MF were 7.92%, 0.62% and 3.34% and 4.51%, 0.41% and 2.75%, respectively (Supplementary Fig. 7). Time-resolved spectroscopic analysis confirmed that wood@MF exhibited long-lasting and stable afterglow phosphorescence up to ~1000 ms, with dual afterglow emissions clearly observed (Fig. 2b). To demonstrate the feasibility of using MF resin treatment to stimulate phosphorescence, the afterglow spectra of treated woods were investigated. The RTP lifetimes for MF resin-treated balsa, beech, maple, paulownia, pine, poplar, sycamore, and walnut were 327.6, 260.6, 243.1, 321.1, 306.6, 83.7, 94.7, and 101.1 ms, respectively. Compared to natural wood, the RTP lifetimes of all wood@MFs increased significantly (Supplementary Figs. 8, 9, Supplementary Table 1).

In order to explore the RTP emission of wood@MF, we first removed lignin from the wood to obtain delignified wood (D-wood), which mainly consists of holocellulose. Subsequently, MF resin was introduced into the D-wood to fabricate D-wood@MF. The D-wood@MF displayed a single RTP emission peak centered at 500 nm, which was consistent with the emission of wood@MF. To gain a deeper understanding of the other emission source in wood@MF, pure lignin was combined with MF resin to produce Lig@MF. The as-obtained Lig@MF presented an afterglow emission centered at 530 nm. These findings demonstrated that the RTP emission at 500 nm originated from the holocellulose-MF composites, while the emission at 530 nm came from the lignin-MF composites (Fig. 2c). After clarifying the dual emission phenomenon, our focus shifted to further exploring the RTP emission from the components within wood@MF. For this purpose, a control sample, wood-MF, was prepared by physically mixing wood and cured MF. Intriguingly, the wood-MF exhibited a significantly lower lifetime of 55.1 ms (Supplementary Fig. 10). A comparison of the FT-IR analysis between wood@MF and wood-MF revealed that the signals at 1160 cm−1, corresponding to the ether bond, were notably enhanced. This indicated that MF reacted with wood during the curing process (Fig. 2d).

Additionally, FT-IR microscopic analysis was used to compare the content of C-O-C in wood-MF and wood@MF. To this end, a sample was prepared by integrating wood@MF with wood-MF. The analytic result indicated that the signal intensity of C–O–C (located at 1160 cm−1) in wood@MF was stronger than the signals of the signal for wood-MF (Supplementary Fig. 11)37. These results confirmed that ether bonds were formed due to the reaction between MF and wood. Meanwhile, the C 1 s X-ray Photoelectron Spectroscopy (XPS) spectrum showed that C–O–C in wood@MF accounted for 20.5% of all chemical bonds, a value higher than that of wood-MF (10.8%) (Fig. 2e). Moreover, Raman spectra also confirmed that the signal intensity of –C–O–C– in wood@MF (located at 1080 cm−1) was stronger than the signals obtained for wood-MF (Supplementary Fig. 12)37. These results were in line with the FT-IR analysis, further validating the formation of an ether bond between MF and wood. The formed covalent bond was conducive to rigidifying the chromophores and, consequently, promoted the RTP emission of wood@MF. In addition, an independent gradient model demonstrated that both cellulose and lignin had strong non-bonding interactions with the MF molecule. The interaction force between cellulose, the main component of holocellulose, and the MF molecule was −47.1 kcal/mol. Such a strong interaction facilitated the generation of emissive clusters between oxygen—and nitrogen-incorporated moieties. The interaction between lignin and the MF molecule was −34.6 kcal/mol. In contrast, the interaction between lignin and the cellulose molecule, the primary interaction in the wood matrix, was relatively weak, with a value of − 22.2 kcal/mol (Fig. 2f and Supplementary Figs. 13–15). This result indicated that the interactions between cellulose and the MF molecule, as well as between lignin and the MF molecule, were stronger. These stronger interactions were beneficial for restricting the molecular vibrations of natural chromophores and promoting the afterglow emission. Overall, both the covalent and non-covalent interactions between wood and MF contributed to the promotion of the RTP emission of wood@MF.

Preparation and RTP performance of R-wood@MF

Inspired by the previous results, recycled woody powders from waste panels were used to produce RTP materials. To this end, waste panels were crushed into powder. Subsequently, MF resin was mixed with these recycled woody powders (R-wood) to produce R-wood@MF via a thermal curing process. SEM images of R-wood and R-wood@MF confirmed that the MF resin infiltrated the lumina of the R-wood during thermocuring (Supplementary Fig. 16). Mercury intrusion porosimetry tests showed that R-wood@MF exhibited lower porosity (70.1%) than R-wood (80.4%) (Supplementary Fig. 17). XRD analysis indicated that the diffraction peaks of R-wood are 16.2°, 22.6° and 34.5°, which were attributed to the (101), (002) and (004) crystal planes of cellulose, respectively (Supplementary Fig. 18). MF resin has a typical amorphous diffraction peak at 22.9°. Generally, R-wood@MF inherits typical signals from R-wood. Notably, the diffraction peak at about 22.5° of R-wood@MF becomes broader than R-wood, which was attributed to the overlap with the amorphous characteristic diffraction peak of the MF resin.

After that, the optical performance of R-wood@MF was investigated. R-wood@MF showed bright green RTP emission after 365 nm UV light was turned off (Fig. 3a). Specifically, under 365 nm UV light excitation, R-wood@MF exhibited fluorescence emission centered at 440 nm and RTP emissions at 500 nm and 530 nm, with lifetimes of 337.1 ms and 332.5 ms, respectively (Fig. 3b, Supplementary Fig. 19, Supplementary Table 3 and Supplementary Movie 2).

Fig. 3: RTP Emission of recycled wood.
Fig. 3: RTP Emission of recycled wood.
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a Digital images of waste wood board, R-wood powders (upper); Digital images of R-wood@MF in daylight, upon and after turning off UV lamp (365 nm) (down); b Fluorescence (black dash line) and phosphorescence(red solid line) spectra of R-wood@MF, excitation wavelength = 365 nm; c The RTP lifetimes at 530 nm of R-wood and R-wood@MF made from different R-woods, I: finger-joint board, II: decorative board, III: plywood, IV: medium-density wood, V: chipboard; d Time-dependent RTP lifetimes at 530 nm of R-wood@MF in water.

To futher illustrate the advantages of the method, RTP materials were produced by introducing sucrose into R-wood16. Both the RTP intensity and lifetime of R-wood@sucrose was inferior to that of R-wood@MF. This result confirms the advantages of the current as-developed strategy (Supplementary Fig. 20). Also, the R-wood@MF exhibited stable optical properties. The lifetime only exhibited a small decrease after ~ 9 months of storage (Supplementary Fig. 21). Notably, phenol-formaldehyde (PF) resin, urea-formaldehyde (UF) resin, furan resin, and polyurethane (PU) resin were also used for producing R-wood@resin. All of them exhibited enhanced RTP emission when compared with untreated natural wood (Supplementary Fig. 22). Moreover, the RTP lifetime of R-wood@MF could be tuned by varying the mass ratios between R-wood and MF resin in R-wood@MF (Supplementary Fig. 23). 20 wt% R-wood@MF exhibited the best RTP properties. Interestingly, the RTP performance of R-wood@MF was closely related to the size of R-wood. When the size of R-wood was reduced from 30 mesh to 150 mesh, both the RTP intensity and lifetime increased. This was because the reduced size was beneficial for the interaction between R-wood and MF, thus promoting the RTP emission (Supplementary Fig. 24). To verify the generality, recycled panels, including decorative board, plywood, density board, and chipboard, were also used to produce R-wood@MF. As expected, all of them could be used as sources for producing R-wood@MF with RTP emission (Fig. 3c and Supplementary Figs. 25, 26). Moreover, R-wood@MF exhibited waterproof RTP emission (Supplementary Fig. 27). After immersing R-wood@MF in water for 5 h, ~171.3 ms lifetime was maintained (Fig. 3d). A lifetime of 196.0 ms was maintained after immersing R-wood@MF in water for 36 h, indicating its excellent optical robustness (Supplementary Fig. 28). This result was much better than previously reported woody RTP materials, whose lifetime were immediately quenched by humidity or water. Additionally, the RTP performance of R-wood@MF was remained observable when immersed in normal organic solvents (Supplementary Figs. 29, 30). Notably, the cost of R-wood@MF preparation was 0.90 US dollar per kg, which is lower than most similar materials (Supplementary Table 4)17. Nevertheless, the RTP lifetime of R-wood@MF is better than most of the other sustainable RTP materials (Supplementary Fig. 31)14,16,24,38,39,40,41,42,43,44,45,46,47,48.

Potential applications and displays

A series of potential applications was demonstrated using R-wood@MF. First, considering that MF resin is a traditional polymer adhesive, R-wood@MF with bright RTP emission was first used as an anti-counterfeiting glue. The broken china glued by R-wood@MF exhibited green afterglow emission in the repaired place after stopping UV excitation, which could be used for identifying the china (Fig. 4a). Utilizing the thermosetting properties of MF, R-wood@MF could be processed into 3D shapes with RTP emission assisted by a template (Fig. 4b–d). The tensile strengthen of materials made from R-wood@MF is 11.58 MPa (Supplementary Fig. 32). R-wood@MF was also used to produce flexible functional films together with epoxy resin. The film exhibited potential as an afterglow display screen (Fig. 4e and Supplementary Fig. 33). By programming the afterglow display control device, the light paths from A to B can be captured under afterglow emission guidance upon ceasing 365 nm UV light excitation (Fig. 4f and Supplementary Figs. 34, 35 and Supplementary Movie 3). The light path direction was identified within 0.5 s, demonstrating the potential of the afterglow display film for fast positioning and accuracy tracking. Besides displaying, R-wood@MF film can also be used as an anti-counterfeiting label for medicine (Fig. 4g and Supplementary Fig. 36). After turning off the UV light, a snowflake-like label with bright green afterglow was observed. In addition, R-wood@MF film can be used to produce a quick-response (QR) code. When the 365 nm UV light was turned on, no obvious information was observed. After the UV light was turned off, a clear QR code was observed, and the QR code could be recognized using a smartphone (Supplementary Fig. 37). In order to explore more potential applications of R-wood@MF resin, we prepared R-wood@MF foam by using the characteristics of the resin that can be foamed. As-obtained foam exhibited a hard porous structure and emitted green afterglow emission after the irradiation of a 365 nm UV lamp (Fig. 4h). Such RTP foams could be used for sensing, light-emitting devices and anti-counterfeiting packaging materials on a large scale, considering of their low price and easy preparation40,49,50,51.

Fig. 4: Applications of R-wood@MF.
Fig. 4: Applications of R-wood@MF.
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a Digital images of china cup repaired using R-wood@MF in the bright field, upon UV irradiation and after switching off the UV sources; b–d Digital images of 3D materials made from R-wood@MF in the bright filed, upon UV irradiation and after switching off the UV light source; e Digital images of film made from R-wood@MF in the bright field, upon UV irradiation and after switching off the UV light source; f The light path tracking with bright green afterglow from A to B is realized by controlling the UV lamp through the program. The tracks with green emission can be captured. g Digital images of logo made from R-wood@MF on a medicine bottle in the bright filed, upon UV irradiation and after switching off the UV light source; h Digital images of rigid porous foam made from R-wood@MF in the bright filed, upon UV irradiation and after switching off the UV light source.

Discussion

In summary, MF was discovered to be an efficient reagent for enhancing the RTP performance of natural wood. Specifically, the MF resin had a rigid and robust chemical network, protecting the triplet excitons from being quenched by humidity and oxygen in the environment. Additionally, the MF resin reacted with the surface hydroxyl moieties and formed covalent bonds. The covalent bonds were beneficial for stabilizing the interfacial interactions between MF and the wood. As a result, the mechanism for such enhancement is ether bonds with MF formed and intensive non-bonded interactions with the components in natural wood, which promoted RTP emission. Motivated by this fundamental discovery, we converted waste wood into RTP materials (R-wood@MF) via thermally curing MF with recycled wood. The obtained R-wood@MF exhibited water and organic solvent-tolerant green RTP emission. As a demonstration of the application, R-wood@MF was employed as an anti-counterfeiting glue. Also, R-wood@MF was used to produce 3D materials for visual decoration. Moreover, R-wood@MF was successfully used to produce display and 2D code films together with epoxy resin. Considering the convenient and low-cost process for converting waste wood into RTP materials and the potential applications, this work is expected to provide a practical way for recycling waste wood sources.

Methods

Preparation of wood@MF

A mixture of polyformaldehyde (9.009 g, 0.3 mol), triethanolamine (1 mL, ~7.5 mmol), and deionized water (25 mL) was heated at 55 °C until the mixture became a homogeneous solution. After that, melamine (12.612 g, 0.1 mol) was added to the mixture and stirred at 80 °C until the solution became transparent. After keeping the reaction system for 150 min at 80 °C, the precursor solution of melamine formaldehyde (MF) resin was obtained. Then, the wood samples (10.0 g) were immersed in the precursor solution under ~1.0 MPa vacuum conditions for 8 h. The obtained sample was cured at 105 °C for 2 h to give wood@MF.

Preparation of D-wood@MF

The wood samples were delignified using a peracetic acid solution (synthesized from a mixture of 30% hydrogen peroxide and acetic acid at a volume-to-volume ratio of 1:1) at 120 °C for 2 h, followed by washing in deionized water several times to remove the chemicals. The delignification wood samples (D-wood) were dried by freeze-drying to maintain their internal pore structure. Then D-wood samples (10.0 g) were immersed in MF resin under ~1.0 MPa vacuum conditions for 8 h. After that, the wood sample was removed from the MF resin and dried at 105 °C for 2 h to obtain D-wood@MF.

Preparation of R-wood@MF

All waste boards were crushed into powders and passed through a 30-mesh sample sieve. 2.0 g of recycled wood powder (R-wood) was immersed into MF precursor solution (8.0 g) under ~1.0 MPa vacuum conditions for 8 h. After that, the samples were cured at 105 °C for 2 h to give R-wood@MF.