In‐situ controllable synthesis of carbon dots for patterned fluorescent wood films rapid fabrication strategy

Fluorescent‐patterned materials are widely used in information storage and encryption. However, preparing a patterned fluorescent display on a matrix currently requires a time‐consuming (hours or even days) and complex multi‐step process. Herein, a rapid and mild technique developed for the in‐situ controllable synthesis of fluorescent nitrogen‐doped carbon dots (NCDs) on eco‐friendly transparent wood films (TEMPO‐oxidized carboxyl wood film [TOWF]) within a few minutes was developed. A wood skeleton was employed as the carbon precursor for NCD synthesis as well as the matrix for the uniform and controlled distribution of NCDs. Moreover, the in‐situ synthesis mechanism for preparing NCDs in TOWF was proposed. The resulting fluorescent wood films have excellent tensile strength (310.00 ± 15.57 MPa), high transmittance (76.2%), high haze (95.0%), UV‐blocking properties in the full ultraviolet (UV) range, and fluorescent performance that can be modified by changing the heating parameters. Fluorescent patterning was simply achieved by regulating the in‐situ NCD synthesis regions, and the fluorescent patterns were formed within 10 s. These fluorescent‐patterned wood films can effectively store and encrypt information, and they can interact with external information through a transparent matrix. This work provides a green and efficient strategy for fabricating fluorescent information storage and encryption materials.

distribution of CDs in a matrix is almost impossible to regulate.Therefore, the main challenge in the preparation of fluorescent patterned films based on CDs is developing a simple and efficient process for the in-situ and controllable synthesis of CDs in a solid-state matrix.
Polymers that exhibit a degree of excitation light transmission can be used as fluorescent film matrices.[24] However, petroleumbased artificial polymers are typically non-recyclable and difficult to degrade.Meanwhile, renewable natural polymers such as CNF and CNC require energy-intensive preparation processes, and the destruction of naturally ordered cell wall structures to break the strong hydrogen bonds of wood fibers also sacrifices mechanical strength. [25,26]Consequently, the in-situ removal of colored lignin from wood sheets under mild conditions and the subsequent impregnation of these wood sheets with resin is a widely used strategy for reducing the negative impact of color-emitting groups and the porous structure of wood on transmittance.Moreover, this strategy retains the mechanical strength derived from the natural structure of wood. [27,28]Biomass polymer films prepared with this strategy exhibit significantly improved transmittance, allowing treated wood to serve as a fluorescent film matrix.However, this strategy is not very sustainable, and the prepared films are not easily recycled.In addition, the resin and wood exhibit poor interfacial compatibility due to non-covalent bonding and polarity differences.Therefore, an efficient method for synthesizing biomass-based fluorescent film matrices without the addition of any other artificial polymer is necessary.Selective modification or the grafting of delignified wood functional groups (carboxylation, acetylation, etc.) can be used to disentangle the microfibril bundles of wood cell walls.Subsequently, compression or self-densification can be used to obtain wood films with a tightly ordered arrangement of microfibrils.These wood films have specific optical properties such as high transmittance, high haze, and anisotropic light scattering properties, which are highly beneficial for fluorescent anti-counterfeiting and information encryption applications. [29,30]o the best of our knowledge, the in-situ controlled synthesis of CDs in composite matrices has rarely been reported.The in-situ synthesis of fluorescent CDs can be achieved by direct laser writing on composite films composed of PVA and rare metal nanoparticles or cellulose acetate butyrate films that undergo photochemical reactions.However, the preparation processes for obtaining these fluorescent CDs are high-cost and require specialized equipment. [31,32]Fluorescent hydrogels can be obtained by hydrothermally reacting CNF to form CDs in situ, [33] but the water loss during film preparation means that the ACQ effect is unavoidable.In this work, a rapid, facile, and one-pot method for the in-situ formation of fluorescent CDs on transparent TEMPOoxidized carboxyl wood film (TOWF) matrixes is reported.Wood was employed as a carbon precursor for CD synthesis as well as a matrix for the uniform and controlled distribution of CDs.A simple heating process was used to supply energy to the ethylenediamine (EDA) introduced wood matrix.This resulted in the in-situ synthesis of cyan fluorescent emitting nitrogen-doped CDs (NCDs), which were chemically crosslinked with the TOWF to form fluorescent wood films (FWFs).These FWFs exhibit intrinsic mechanical and optical anisotropy, excellent tensile strength, high optical transmittance, high haze, and superb ultraviolet (UV) blocking properties in the full UV spectrum.Moreover, the fluorescence intensity and luminescence color of the FWFs can be adjusted by modifying the heating time and temperature.Two strategies were employed to regulate the synthesis region of the NCDs: (i) using a commercially available handheld electrothermal marking machine to control the heating area and (ii) using a fountain pen to control the regions where EDA was introduced.The obtained fluorescent-patterned wood films show excellent promise for optical information storage and encryption applications, and the low-cost, efficient, and controllable method for achieving the in-situ synthesis of CDs in treated wood film matrices provides inspiration for subsequent research on environmentally friendly biomass-based fluorescent films.

In-situ preparation of FWFs
The in-situ FWF preparation process is shown in Figure 1A, details of the preparation process are described in the Experimental Section of the Supporting Information.Natural wood (NW) cell walls are composed of bundles of cellulose microfibrils surrounded by hemicellulose and lignin.Cellulose molecular chains are assembled into cellulose nanofibrils by strong intermolecular hydrogen bonding, and these cellulose nanofibrils are hierarchically assembled into cellulose microfibrils and microfibril bundles. [34]The presence of lignin with chromogenic groups is the main factor responsible for the brownish-yellow appearance of NW. [35] To remove the lignin and most of the hemicellulose surrounding the cellulose skeleton structure, a delignification treatment was performed on 1 mm thick balsa wood sheets, allowing better accessibility to the cellulose.The obtained delignified wood has a lower absorbance than NW, providing it with an off-white appearance.Next, a TEMPO oxidation process was performed.During this process, the active hydroxymethyl groups on the delignified wood skeleton are in situ oxidized to carboxyl groups.The electrostatic repulsion between carboxyl groups loosens the cellulose microfibril bundles, which results in enhanced accessibility of the hydroxyl groups.Thus, the TEMPO-oxidized wood (TOW) skeleton becomes more hydrophilic. [29]The fibrillation of the cellulose fibers results causes the in-situ carboxylated TOW to display increased flexibility and significantly enhanced water swelling properties. [36]As shown in Figure 1B, the in-situ carboxylated TOW exhibits significant swelling in the direction perpendicular to the fibers.After delignification and TEMPO oxidation, the dimension of the TOW sheet increases by about 1.6 times (from an untreated dimension of about 48-78 mm).The TOW was then soaked in deionized water to remove residual reagents.Next, TOWF was produced via the self-densification of TOW induced by strong hydrogen bonding at room temperature.A TEMPOoxidized wood film with introduced EDA (TOWF+EDA) was also prepared via solvent exchange by adding EDA to the deionized water during soaking.The loose TOW with introduced EDA was heated to temperatures of 120-160 • C to provide energy to perform a series of chemical reactions, resulting in the in-situ synthesis of fluorescent NCDs in the wood skeleton.Thus, self-densified FWFs were obtained. [33]s shown in Figure 1B, under 365 nm UV light, FWF can emit bright cyan fluorescence.

Morphology and chemical structural analysis of FWFs
Photographs of NW, TOWF, and FWF are shown in Figure 2A, demonstrating their macro-structural differences.NW is a sheet, while TOWF and FWF are films.Unlike TOWF, the FWF has a yellowish appearance due to the loading of NCDs.Scanning electron microscope (SEM) images displaying the variation in the microstructure and surface morphology of NW, TOWF, and FWF are presented in Figure 2B-G.NW exhibits distinct porous characteristics both on its surface (Figure 2B) and in its cross-section (Figure 2C), which is formed by the spindle-shaped wood fibers, vessels, and wood rays of the balsa wood.After delignification, TEMPO oxidation, and self-densification, the cellular structure of TOWF is collapsed into a thin film.The cross-sectional view clearly shows the layered structure of the compressed cell walls, but some pores and cracks with lengths of about 10 μm can still be observed (Figure 2D).TOWF has a smoother, denser surface structure than NW, but the intrinsic parallel-aligned fiber orientation can still be readily observed, as indicated by the arrow in Figure 2E.This indicates that the anisotropic skeleton structure of NW is preserved in TOWF.FWF has a denser cross-sectional morphology than TOWF, and pores and cracks are almost invisible in the cross-sectional SEM image of FWF (Figure 2F).This potentially indicates that the NCDs result in a more tightly crosslinked wood skeleton than hydrogen bonds due to the formation of covalent bonds.NW has a thickness of 1 nm, while TOWF and FWF have significantly lower thicknesses of tens of microns.The average thicknesses of TOWF and FWF are 32.64 and 28.22 μm, respectively, indicating that FWF has a denser structure than TOWF.As shown in Figure 2G, some small particles are attached to the surface of FWF, which can be attributed to the rapid increase in temperature during the heating process leading to the partial aggregation of NCDs into amorphous carbon particles. [37]A higher heating temperature and longer heating time lead to an increase in the size and number of these carbon particles (Figure S1).The interior microstructures of NW and FWF were confirmed with high-resolution non-destructive 3D X-ray microscopy (XRM) imaging, as shown in Figure 2H,I.The porous NW structure and dense FWF structure are demonstrated by the 3D reconstruction XRM images and corresponding 2D slice images from the Z-axis, Y-axis, and X-axis at different depths.The 2D slice image of FWF obtained from the Z-axis shows that some carbon particles are embedded in the wood skeleton with an ordered fiber direction, and no severe cracks are observed in the 2D slice images from the X-axis and Y-axis.
Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) were used to verify the presence of NCDs in FWF (Figure S2).The NCDs in the FWF extract show a uniformly distributed dot-like morphology (Figure S2A), and the NCDs have an average diameter of 2.12 nm (Figure S2B).HRTEM analysis clearly shows the existence of the NCD carbon core lattice (Figure S2C), and the average lattice spacing of 0.2025 nm (Figure S2D) is consistent with that of graphite-like carbon. [38,39]The Fourier-transform infrared (FT-IR) spectrum and X-ray photoelectron spectra (XPS) of NCDs from the FWF extract (Figure S3) show that the NCDs consist of nitrogen-doped carbon cores and surface functional groups rich in nitrogen and oxygen.As shown in Figure S3A, the bands located at 3100-3600 cm −1 are attributed to the stretching vibration of N─H and O─H bonds.The two absorption bands located at about 1650 and 1560 cm −1 correspond to amide I and II bands, respectively. [40]The heteroaromatic ring structure of the NCDs is demonstrated by the presence of heterocyclic N─(C) 3 , C─O─C stretching vibration, and aromatic C─H out-of-plane stretching vibration bands at 1385, 1050, and 608 cm −1 , respectively. [41,42] S3C).The N 1s spectrum of NCDs (Figure S3D) shows three peaks ascribed to amino N (398.3eV), amide N (399.1 eV), and graphite N (400.7 eV).Therefore, the surface functional groups of NCDs are mainly hydroxyl, carbonyl, carboxyl, amine, and amide groups, while the internal structure of the NCDs consists of N-doped graphitic carbon cores. [43]T-IR spectra of NW, TOWF, TOWF+EDA, and FWF were obtained to investigate the chemical changes caused by the delignification, TEMPO oxidization, EDA introduction, and heating processes, as shown in Figure 3A.All four spectra exhibit broad bands at about 3000-3600 cm −1 , which can be attributed to O─H and (or) N─H stretching vibrations.Moreover, all four spectra also show similar fingerprint bands at 800-1200 cm −1 that correspond to the glucopyranose rings of cellulose molecular chains, which are the major components of wood. [33]These results indicate that the mild FWF preparation process does not affect the main chemical structure of the cellulose-supported wood skeleton.The NW spectrum also shows three absorption bands located at 1734, 1655, and 1508 cm −1 that correspond to the C═O stretching vibration of the chromophore groups of xylose in hemicellulose, the C═O stretching vibration of the lignin aliphatic portions, and the stretching vibration of the lignin aromatic ring skeleton structure, respectively. [44] I G U R E 3 Chemical structural characterization of natural wood (NW), wood film samples.(A) Fourier-transform infrared (FT-IR) spectra of NW, TEMPO-oxidized carboxyl wood film (TOWF), TOWF+EDA, and fluorescent wood film (FWF).In-situ temperature-variable FT-IR spectra of TOWF+EDA during heating from room temperature (RT) to 160 • C and holding for 60 min in the regions of (B) 3200-3700 cm −1 and (C) 1520-1700 cm These characteristic absorption bands almost entirely disappear in the TOWF spectrum, indicating that most of the lignin and hemicellulose are removed by delignification and TEMPO oxidation.The TOWF spectrum shows a new C═O stretching band at 1601 cm −1 attributed to hydrogen-bonded carboxyl groups.This band indicates that the hydroxymethyl groups on the cellulose molecular chains are oxidized to carboxyl groups by the TEMPO oxidation step. [45,46]The TOWF+EDA spectrum shows sharp N─H stretching vibration and N─H bending vibration bands of amine at 3308 cm −1 and 1575 cm −1 , respectively.The presence of these bands confirms that EDA is introduced into the wood skeleton.After the heating process, the amino absorption bands disappear and are replaced by amide I and amide II bands located at 1602 cm −1 and 1660 cm −1 , respectively.This suggests that EDA is consumed in the reaction process to synthesize N-doped CDs, which are crosslinked with the wood skeleton via a condensation reaction between amino and carbonyl groups. [47,48]o study the chemical reactions occurring during the transformation of TOWF+EDA into FWF during the heating process, in-situ temperature-variable FT-IR spectra were collected at temperatures ranging from room temperature to 160 • C and holding the sample at 160 • C for 1 h (Figure 3B,C).The sharp N─H stretching vibration band at about 3308 cm −1 and N─H bending vibration band of amino groups at about 1576 cm −1 gradually disappear as TOWF+EDA is heated from room temperature to 160 • C, indicating that EDA is almost completely consumed during the reaction.Simultaneously, bands ascribed to hydrogen-bonded amide I [ν(C═O) bond ] and amide II [δ(N─H) bond ] appear at about 1660 and 1601 cm −1 , respectively.This indicates the formation of amide bonds between the synthesized NCDs and the wood skeleton during the heating process. [49]Some additional changes can also be observed with increasing temperature and reaction time: (i) the stretching vibration bands of O─H and N─H at about 3360 cm −1 and the stretching vibration band of hydrogen-bonded C═O shift to high wavenumbers; (ii) the bending vibration band of hydrogen-bonded N─H shifts to a low wavenumber; (iii) the band intensity significantly decreases.In addition, free C═O stretching vibration and N─H bending vibration bands appear at 1678 and 1583 cm −1 , respectively. [50]The appearance of these bands indicates the existence of a large number of hydrogen bonds between hydroxyl, amino, and amide groups in the FWF reaction system, and these hydrogen bonds are dissociated at high temperatures. [50]Synchronous and asynchronous two-dimensional correlation spectra (2DCOS) were generated from the temperature-variable FT-IR spectra, as shown in Figure 3D,E.Two correlation cross-peaks at Φ (3360, 3360), and Φ (1660, 3360) can be observed in the synchronous spectra, but these signals are absent in the asynchronous spectra.This corroborates the above analysis. [51,52]he X-ray diffraction (XRD) patterns of NW, TOWF, and FWF are shown in Figure 3F.All the diffraction patterns exhibit peaks ascribed to the (101), (002), and (004) crystal planes of the natural cellulose I crystal structure of wood. [53]his demonstrates that the original crystal structure of NW is not destroyed by the mild delignification, TEMPO oxidation, and heating process used to prepare FWF.Moreover, FWF and TOWF are more crystalline than NW.This is because delignification removes the amorphous lignin, and some of the amorphous cellulose regions of the wood skeleton are carbon precursors that are hydrolyzed to synthesize NCDs.Therefore, these processes increase the proportion of crystalline cellulose regions in FWF.
XPS spectra (Figure 3G-I) were obtained to further reveal the surface chemical structures of FWF and TOWF.The FWF survey spectrum (Figure 3G) shows the addition of an N 1s peak, indicating the introduction of the N element.The N 1s spectrum of FWF (Figure 3H) consists of amino N (399.8eV), amide N (400.7 eV), and graphite N (401.9eV) peaks.As shown in Figure 3I The probable mechanism for the in-situ synthesis of fluorescent NCDs and their crosslinking with the wood skeleton during the heating process was proposed, as shown in Figure 3J.First, the sharp increase in temperature partially hydrolyzed the amorphous regions of the TEMPO-oxidized cellulose chains in the wood skeleton, causing the formation of carboxylated derivatives of small-molecule monosaccharides and polysaccharides. [54]Next, the wood undergoes dehydration, ring opening, and polymerization with EDA to form N-doped furfural derivatives. [37]Further reactions lead to the aromatization and reorganization of the carbon chains, resulting in condensation to form NCDs with N, O co-doped carbon cores that retain a surface structure rich in amino, amide, hydroxyl, and carbonyl groups. [32,33]he active groups of NCDs are covalently crosslinked and hydrogen-bonded with wood skeletons, making FWF denser than TOWF while preventing the loss of NCDs.

Mechanical and optical performances of FWFs
The stress-strain curves, tensile strengths, and Young's moduli of NW, TOWF, and FW were obtained to characterize their mechanical performance, as shown in Figure 4A-D.The intrinsic structure of the balsa wood with its ordered parallel fibers arrangement is preserved by the mild treatment.Therefore, TOWF and FWF retain the same anisotropic mechanical properties as NW, which are determined by the material structure.Due to the porous and thin cell wall structure of natural balsa wood, NW exhibits low tensile strengths of 25.77 ± 2.80 and 0.79 ± 0.08 MPa parallel and perpendicular to the fiber direction, respectively.With tensile strengths of 218.56 ± 13.82 and 30.25 ± 4.11 MPa parallel and perpendicular to the fiber direction, TOWF shows a significant increase in tensile strength in both directions.This is mainly due to the self-densification and stronger intermolecular hydrogen bonding of TOWF. [29]The higher tensile strength parallel to the fiber direction can be attributed to the synergistic effect of glucoside bonds on the cellulose chain with intramolecular and intermolecular hydrogen bonds. [55]fter the heating process, the mechanical properties of FWF are further improved, with tensile strengths of 310.00 ± 15.57 and 52.58 ± 5.25 MPa parallel and perpendicular to the fiber direction, respectively.Compared to TOWF, this represents an enhancement of 41.8% and 73.8%, respectively, which is attributed to chemical crosslinking between the in-situ synthesized NCDs and wood fibers. [33]In particular, the more significant improvement in tensile strength perpendicular to the fiber direction of FWF can be attributed to the isotropic chemical crosslinking between the NCDs and the fibers, which compensates for the strength loss of the delignified TOWF in the absence of wrapping and hydrogen bonding of the rigid lignin.This crosslinking also helps improve the tensile strength of FWF parallel to the fiber direction.The trend in Young's moduli of the samples is similar to that of the tensile strength.The Young's moduli of NW, TOWF, and FWF parallel to the fiber direction are 1.64 ± 0.18, 14.91 ± 1.30, and 20.75 ± 2.21 GPa, respectively.The Young's moduli of NW, TOWF, and FWF perpendicular to the fiber direction are 0.02 ± 0.04, 4.94 ± 0.82, and 6.22 ± 0.97 GPa, respectively.FWF exhibits much better mechanical performance than that of NW, polymers, resin-immersed transparent wood, and  S2).
nanocellulose composite films (Figure 4F and Table S1), and the mechanical performance is comparable to that of densified wood obtained by compression.When density is taken into account, the specific strength of FWF as a lightweight, high-strength material (about 287.04 MPa cm 3 g −1 ) is significantly higher than that of steel (about 66.3 MPa cm 3 g −1 ). [56]Moreover, FWF has superior flexibility and can be bent, rolled, and twisted without visible breakage (Figure 4E).An FWF ribbon with a width of about 2 mm can easily lift a 200 g load.Additionally, wood films have excellent environmental friendliness and are biodegradable.As shown in Figure S4, when buried in moist soil, TOWF and FWF naturally and completely degrade in 15 days.In contrast, the NW is only partially degraded, and PE remains intact.This clearly demonstrates the superiority of wood films compared to artificial polymer films and resin-infused transparent wood in terms of environmental protection.
The self-densification of the prepared wood films and the presence of carbon particles formed by the aggregation of NCDs on the surface of FWF significantly affected the water contact angle (WCA) of these wood materials (Figure 4G and Figure S5).The WCA of NW rapidly decreases from 17.5 • to 0 • in 10 s (Figure 4G).In contrast, TOWF and TOWF+EDA have higher initial WCAs of 21.8 • and 20.4 • , respectively.After 120 s, TOWF and TOWF+EDA still retain WCAs of 9.8 • and 6.9 • , respectively (Figure 4G).This is attributed to the collapse and densification of the wood cell walls, which impede the flow of water within the films.The introduction of EDA slightly increases the hydrophilicity of the wood films, which is likely due to the presence of more hydrophilic amino groups.However, after heating at 160 • C for 30 min, the initial WCA of the obtained FWF significantly increases to 48.1 • .Moreover, FWF still maintains a high WCA of 36.3 • after 120 s, demonstrating its excellent water resistance.Increasing the heating temperature and time improves the initial WCA of FWF, as shown in Figure S5.This is mainly attributed to the consumption of a large number of free hydrogen bonds and the synthesis of more large carbon particles during the heating process, which enhances the surface roughness of the FWFs and improves their hydrophobicity. [57]he dense structures of TOWF and FWF mean that the refractive index difference between the wood skeleton (refractive index ≈ 1.53) and the air in cell cavities (refractive index = 1) is eliminated.Thus, the prepared wood films are optically transparent and exhibit almost uniform refraction. [35]NW has a transmittance of only 2.3% in the visible-light region.In comparison, TOWF, TOWF+EDA, and FWF exhibit significantly improved visible-light transmittance values of 83.2%, 82.5%, and 76.2% (Figure 5B), respectively.In addition, these wood films exhibit high haze properties.TOWF and TOWF+EDA have a calculated haze of about 90%, while that of FWF is 95.0% because its greater surface roughness leads to more light scattering (Figure 5C).The left-hand photographs in Figure 5F demonstrate that text printed on paper placed underneath these wood films is still clearly visible.The paper placed underneath TOWF+EDA and FWF is shaded yellow due to the oxidation of EDA [58] and the loading of NCDs, respectively.The right-hand photographs in Figure 5F show the appearance of the same text underneath wood films raised to a height of 10 mm.This demonstrates the high transparency and haze of the wood films.Notably, TOWF, TOWF+EDA, and FWF exhibit significant differences in transmittance in the UV region (200-400 nm, Figure 5A and Figure S6).TOWF+EDA exhibits a decrease in transmittance throughout the UV range, especially in the UVA range, which can be attributed to the absorption of UV by EDA and its oxides. [58]The transmittance of FWF in the UVA, UVB, and UVC ranges is 5.6%, 3.4%, and 2.6%, respectively.Thus, FWF exhibits excellent UV shielding performance, which is attributed to the strong UV absorption properties of NCDs (Figure S6). [43]The transmittance of the FWFs is affected by the heating time and temperature because these parameters influence the synthesis and aggregation of NCDs (Figure S7).Increasing either the heating temperature or time reduces the transmittance of FWF in the visible region and the UV region.Thus, the UV shielding performance is improved.When the heating temperature is increased from 120 to 160 • C with a heating time of 120 min, the visible region transmittance of FWF decreases from 74.1% to 29.1%, while the UV transmittance decreases from 12.1% to 0.3% (Figure S7A).When the heating time is extended from 10 to 120 min at a heating temperature of 160 • C, the visible region transmittance of FWF decreases from 80.5% to 29.1%, while the UV transmittance decreases from 9.4% to 0.3% (Figure S7B).Both TOWF and FWF exhibit elliptical scattering patterns under vertically incident green laser light, which is due to their anisotropic skeletal structures (Figure 5D,E).Interestingly, compared to the scattering pattern of TOWF, that of FWF has a larger diameter in both the perpendicular and parallel directions to the fiber and the variability between these two directions is reduced.This can potentially be attributed to enhanced light scattering and weaker light scattering anisotropy of FWF, which are caused by the homogeneous distribution of NCDs and aggregates in the FWF skeleton. [29,59]he in-situ loading of NCDs in the wood skeleton provides FWF with excellent fluorescence-emitting properties.The photoluminescence (PL) maximum excitation and emission spectra of TOWF, TOWF+EDA, and FWF are shown in Figure 6A.FWF exhibits strong fluorescence emission, while TOWF and TOWF+EDA have relatively low-intensity fluorescence emissions.The fluorescence emission properties of TOWF and TOWF+EDA are attributed to the small amount of residual lignin chromophores in TOWF and the fluorescence emission of oxidized EDA, respectively. [58,60]WF has a maximum emission peak at 550 nm and dual excitation peaks at 310 and 431 nm.These excitation peaks represent the π-π* transition of the NCD carbon cores and the n-π* transition of the N-and O-containing groups on the NCD surface, respectively. [61,62]As shown in Figure 6B, FWF emits bright blue-green fluorescence.When placed on a sheet of paper with printed text, FWF is capable of illuminating the text.In contrast, TOWF+EDA and TOWF only emit faint fluorescence that is difficult to see with the naked eye.The fluorescence spectrum of TOWF-H (directly heated without the introduction of EDA) is shown in Figure S8.The fluorescence properties of TOWF-H are potentially caused by the formation of fluorescent CDs via the pyrolysispolymerization-carbonation of TEMPO-oxidized cellulose during the heating process. [37]The weaker fluorescence intensity of TOWF-H compared to FWF is due to the absence of nitrogen doping and the mild heating conditions. [33]Due to the presence of NCDs, the fluorescence emission of FWF is wavelength-dependent.As the excitation wavelength is increased from 350 to 490 nm, the PL emission peak of the FWF red-shifts from 557 to 574 nm (Figure 6C).Because the maximum excitation wavelength is 430 nm, the fluorescence intensity of FWF under UV excitation at 365 nm is not as strong as that at 385 nm, as shown in Figure 6C inset.The fluorescence emission intensity of FWF is mainly affected by the heating temperature and time (Figure 6D,E).Controllable fluorescence intensity and color can be achieved by varying the heating time and temperature within a certain range.With a fixed heating time of 30 min, the fluorescence emission intensity of FWF is enhanced as the heating temperature is increased from 120 to 160 • C (Figure 6D).With a fixed heating temperature of 160 • C, the fluorescence emission intensity first increases and then decreases as the heating time is extended from 10 to 120 min.The maximum fluorescence emission intensity is achieved after a heating time of 20 min (Figure 6D).In addition, increasing the heating time or temperature leads to a significant red-shift in the emission peak to higher wavelengths.The fluorescence emission color exhibits a blue-green-yellow shift, which is corroborated by the Commission International De L'Eclairage (CIE) chromaticity coordinates (Figure S9).The FWF obtained by heating at 160 • C for 20 min exhibits a higher fluorescence intensity than the FWFs prepared at 120 or 140 • C for 120 min, as shown in Figure S10.Thus, increasing the heating temperature effectively improves the efficiency of fluorescent NCD synthesis.Compared to the hours or even days required to synthesize and purify conventional CDs via "bottom-up" and "top-down" preparation methods, the in-situ synthesis of CDs on a wood skeleton in this work requires less than 20 min to obtain fluorescent wood films without any other purification and loading steps (Figure 6F and Table S2).
To confirm the fluorescence origin of FWF, PL spectra of an NCD solution extracted from FWF were obtained (Figure S11).This solution exhibits a maximum emission peak at 485 nm and dual excitation peaks at 325 and 404 nm (Figure S11A).Moreover, wavelength-dependent emission can be observed under excitation light wavelengths from 340 to 440 nm (Figure S11B), and the fluorescence excitation and emission peaks of the solution are similar to those of FWF.However, both the excitation and emission peaks of FWF are significantly red-shifted compared to that of the NCD solution.This is mainly due to the interactions between the NCDs and the TEMPO-oxidized cellulose in the FWF skeleton as well as the aggregation of NCDs.These interactions include crosslinking, physical entanglement, and electrostatic interactions.The surface state of the NCDs is changed and many more energy levels are generated.These are the main factors responsible for the fluorescent excitation of the NCDs, resulting in a red shift in the fluorescent emission wavelength. [63]

Fabrication strategies and demonstration of patterned FWFs
The key to the practical and successful fabrication of fluorescent patterns lies in the controllable synthesis of fluorescent NCDs in wood films.This work demonstrates that the two main factors enabling the in-situ synthesis of NCDs on wood films are the introduction of EDA and heating.Patterned FWFs can be fabricated by controlling the areas in which EDA is introduced or by controlling the heating regions.These two strategies for preparing patterned FWFs are illustrated in Figure 7A, and the preparation methods are described in detail in the Experimental Section. Figure 7C demonstrates the feasibility of both strategies.Fluorescent text messages that are only visible under UV light can be imprinted on the wood films, and these messages can be prepared by either text mold heating or plate heating after handwriting with EDA solution as the ink.Mold heating can even be used to print a recognizable fluorescent QR code on the wood film, and this QR code can be scanned by a smartphone to obtain a web address for information storage purposes (Figure 7B).The patterned FWF preparation process is much more efficient compared to the FWFs described above because a large amount of water has been removed from the wood films before the heating process, thus a fuzzy fluorescent pattern can be formed in 10 s (Figure 7D).A sufficiently clear pattern can be formed in 2 min, and a bright pattern can be formed in 5 min.The high transparency of the wood film allows for the written fluorescent messages to be combined with transmitted messages through the wood film, enabling the possibility of more complex fluorescent encryption and information interactivity.As shown in Figure 7E, the wood film printed with a combinatorial squares fluorescent pattern can be used to play Tetris on paper.Figure 7F shows an SEM image of a localized fluorescent pattern obtained by mold heating for 5 min, and the difference in surface morphology between the two regions separated by the dashed lines can be clearly observed in Figure 7G,H.The surface of the film in the mold-heated region is rough and contains a large number of particles, as shown in Figure 7G.These particles are the NCD aggregations formed on the wood film by violent heating from room temperature to 160 • C. In contrast, the surface of the wood film in the unheated region remains smooth (like the original TOWF), as shown in Figure 7H.This is consistent with the morphological characterization of the FWF preparation process (Figure 2E,G and Figure S1).The significantly different microstructures of the fluorescent pattern region and the unheated TOWF region further enhance the information encryption performance of the patterned FWF with the tunable fluorescent region and emission intensity.Thus, a dual encryption mechanism involving microstructural variation and UV excitation visibility is realized.

CONCLUSION
In conclusion, a rapid and facile strategy for the in-situ formation of fluorescent NCDs on a transparent TOWF matrix was explored.In this process, wood was employed as both a carbon precursor for NCD synthesis and as a matrix for controlling the uniform distribution of NCDs.A heating process was used to supply energy for the in-situ synthesis of fluorescent NCDs and to chemically crosslink these NCDs with TOWF, forming FWF.The mechanism of the in-situ synthesis of NCDs in TOWF was summarized as a "hydrolysispolymerization-condensation-aromatization" process.FWF has excellent tensile strength (310.00 ± 15.57MPa), high optical transmittance (76.2%), high haze (95.0%), and good UV blocking properties in the full UV range (200-400 nm).Moreover, the fluorescence performance of FWF can be controlled by varying the heating time and temperature.Two strategies for preparing patterned FWFs were designed by regulating the NCDs synthesis regions.In the first strategy, the heating region is controlled, and in the second strategy, the areas of the wood film where EDA is introduced are controlled.Using these strategies, fluorescent patterns can be easily and rapidly formed in 10 s.These environmentally friendly patterned fluorescent wood films have promising prospects in optical information storage and encryption applications.

F
I G U R E 1 (A) Schematic illustration for the in-situ preparation process of fluorescent wood film (FWF).(B) Digital photographs of natural wood (NW), TEMPO-oxidized wood (TOW), and FWF under visible light and 365 nm ultraviolet (UV) light.

F I G U R E 2
Morphological characterization of natural wood (NW), TEMPO-oxidized carboxyl wood film (TOWF), and fluorescent wood film (FWF).(A) Digital photographs of NW, TOWF, and FWF.SEM cross-sectional images of (B) NW, (D) TOWF, and (F) FWF (insets: partially enlarged views).Scanning electron microscope (SEM) surface images of (C) NW, (E) TOWF, and (G) FWF (insets: partially enlarged views).(H) High-resolution 3D X-ray microscopy (XRM) images of NW and corresponding 2D slice images from the X-axis, Y-axis, and Z-axis at a depth of 50 μm.(I) High-resolution 3D XRM images of FWF and corresponding 2D slice images from the X-axis, Y-axis, and Z-axis at a depth of 50 μm.
XPS spectroscopy further corroborates this FT-IR analysis.The XPS survey spectrum of NCDs (Figure S3B) shows the presence of C, O, and N elements.The C 1s and N 1s spectra of NCDs are shown in Figure S3C,D.The C 1s spectrum of NCDs can be deconvoluted into C─C/C═C (284.8 eV), C─O/C─N (286.2 eV), C═O/C═N (287.8 eV), and ─COOH (288.5 eV) peaks (Figure −1 .(D, E) 2D synchronous and asynchronous spectra generated from (B, C). (F) X-ray diffraction (XRD) patterns of NW, TOWF, and FWF.(G) X-ray photoelectron spectroscopy (XPS) survey spectra of TOWF and FWF.(H) XPS N 1s spectrum of FWF.(I) XPS C 1s spectra of TOWF and FWF.(J) Schematic diagram of the in-situ synthesis of nitrogen-doped carbon dots (NCDs) in wood skeleton.

F I G U R E 4
Mechanical properties and water contact angles of natural wood (NW) and wood film samples.Typical stress-strain curves of NW, TEMPOoxidized carboxyl wood film (TOWF), and fluorescent wood film (FWF) (A) in the longitudinal direction and (C) in the tangential direction.Tensile strength and Young's modulus values of NW, TOWF, and FWF (B) in the longitudinal direction, and (D) in the tangential direction.(E) Photographs of an FWF film bent, rolled, twisted, and used to lift a 200 g weight.(F) Ashby chart of tensile strength versus Young's modulus for natural wood, polymers, wood-based composites, and FWF prepared in this work.(G) Water contact angle photographs of NW, TOWF, TOWF+EDA, and FWF at different times.
, the C 1s spectrum of FWF contains C─C/C═C (284.8 eV), C─O/C─N (286.9 eV), C═O/C═N (287.6 eV), and ─O─C═O─/─N─C═O─ (289.5 eV) peaks.Compared with the C 1s spectrum of TOWF, the intensity of the C─C/C═C peak of FWF increases due to the synthesis of NCDs.Moreover, the C═O and ─COOH peaks of TOWF are shifted from 288.3 eV and 289.5 eV to 287.6 eV and 288.7 eV, respectively, which can be attributed to the appearance of C═N and ─N─C═O─ peaks.The intensity of the C═O/C═N and ─O─C═O─/─N─C═O─ peaks of FWF is ascribed to the synthesis of NCDs and crosslinking between the wood skeleton and NCDs forming amide bonds.

F I G U R E 5
Optical properties of wood film samples.(A) Ultraviolet (UV) transmittance spectra of natural wood (NW) and wood films.(B) Total visible transmittance spectra of NW and wood films.(C) Optical haze spectra of wood films.Normalized anisotropic light scattering intensity and light scattering pattern of light transmitted through (D) TEMPO-oxidized carboxyl wood film (TOWF) and (E) fluorescent wood film (FWF).(F) Appearance of text on printed paper placed directly underneath wood films (left) and wood films raised up 10 mm (right).

F I G U R E 6
Fluorescent properties of wood film samples.(A) Photoluminescence (PL) maximum excitation and emission spectra of TEMPO-oxidized carboxyl wood film (TOWF), TOWF+EDA, and fluorescent wood film (FWF).(B) Photographs of TOWF, TOWF+EDA, and FWF under 365 nm UV light.(C) PL emission spectra of FWF at different excitation wavelengths.Insets: photographs of FWF under 365 and 385 nm UV light.(D) Maximum emission spectra of FWFs prepared with (D) different heating temperatures (120, 140, and 160 • C) and (E) different heating times (10, 20, 30, 60, and 120 min).Insets: corresponding photographs of FWF specimens.(F) Gantt chart of various carbon dots preparation methods (The detailed data are reported in Table

F I G U R E 7
Fabrication and demonstration of patterned fluorescent wood films (FWFs).(A) Schematic illustration of the fabrication strategies for obtaining patterned FWF.(B) Recognizable fluorescent QR code on a wood film under ultraviolet (UV) light and the corresponding URL when viewed by a smartphone.(C) Patterned FWFs prepared by "mold heating" and "handwriting-heating" strategies under visible light (left column) and 365 nm UV light (right column).(D) Photographs of patterned FWFs prepared with different heating times (10 s, 30 s, 1 min, 2 min, and 5 min).(E) Combinatorial squares fluorescent patterned FWF interacting with the game of Tetris on paper.(F) SEM image of patterned FWF.Inset: the corresponding "mold heating" patterned FWF.(G) Heated region and (H) unheated region on the "mold heating" patterned FWF.