Interfacial synthesis of covalent organic framework composited woods for ultrafast ion conduction

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Authors Fang, Zhaofeng;Lin, Yuhui;Dong, Zhijun;Xu, Wenxin;Qi, Yiyin;Zeng, Ruiyuan;Yuan, Jinqiu;Song, Zifan;Zhu, Feng;Liu, Haixiong;Cao, Li;Yuan, Jiashu;Huang, Biao;You, Xinda

Citation Fang, Z., Lin, Y., Dong, Z., Xu, W., Qi, Y., Zeng, R., Yuan, J., Song, Z., Zhu, F., Liu, H., Cao, L., Yuan, J., Huang, B., & You, X. (2023).

Interfacial synthesis of covalent organic framework composited woods for ultrafast ion conduction. Cell Reports Physical Science, 4(7), 101477. https://doi.org/10.1016/j.xcrp.2023.101477

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Article

Interfacial synthesis of covalent organic

framework composited woods for ultrafast ion conduction

Interfacial synthesis is developed by Fang et al. to fabricate ionic covalent organic framework composited woods (iCOFWs) with high ion conductivity. The wood skeleton stores aqueous-phase monomer and reacts with organic-phase monomer, confining the growth of iCOFs throughout the wood. This interfacial strategy and the high-performance iCOFWs offer a platform for advanced wood nanomaterials.

Zhaofeng Fang, Yuhui Lin, Zhijun Dong, ..., Jiashu Yuan, Biao Huang, Xinda You

bhuang@fafu.edu.cn (B.H.) youxinda@fafu.edu.cn (X.Y.)

Highlights

Interfacial synthesis of ionic COF composite woods (iCOFWs) Interface confines the growth of high-crystalline COF throughout the woods

iCOFWs achieve superior ion conductivity with robust stability Interfacial strategy demonstrates high scalability, sustainability, and versatility

Fang et al., Cell Reports Physical Science4, 101477

July 19, 2023ª2023 The Author(s).

https://doi.org/10.1016/j.xcrp.2023.101477

Article

Interfacial synthesis of covalent

organic framework composited woods for ultrafast ion conduction

Zhaofeng Fang,

^1,2

Yuhui Lin,

¹

Zhijun Dong,

¹

Wenxin Xu,

¹

Yiyin Qi,

¹

Ruiyuan Zeng,

¹

Jinqiu Yuan,

³

Zifan Song,

^1,2

Feng Zhu,

^1,5

Haixiong Liu,

^1,4

Li Cao,

⁶

Jiashu Yuan,

⁷

Biao Huang,

^1,2,

* and Xinda You

^1,2,8,

*

SUMMARY

Integration of natural woods and functional nanomaterials is emerging as sustainable nanotechnology. Covalent organic frame- works (COFs), crystalline nanomaterials with pre-designable func- tional groups and inherent compatibility with wood, hold great promise in wood nanotechnology but remain challenging in fabrica- tion. Here we report the interfacial synthesis of ionic COF compos- ited woods (iCOFWs), in which the wood skeleton stores aqueous phase with ionic amine monomer and undertakes subsequent reaction with aldehyde monomer in the organic phase. The reactive nucleation is confined in the wood substrate by the aqueous-organic interface and generates high-crystalline ionic COFs (iCOFs) throughout the wood via covalent interaction. As a proof of concept, the iCOFW based on TpPa-SO

3

H displays ultrahigh lithium conductivity, physicochemical stability, and antimicrobial properties, which demonstrates applicability in an ionotronic device.

Besides, this strategy shows scalability for mass production and versatility for different wood species and iCOFs, offering a powerful and sustainable platform for developing COF-based wood nano- materials.

INTRODUCTION

Wood, the most abundant forest biomass with an annual output of more than 3 billion cubic meters, is playing an essential role in promoting the sustainability of our daily life and various industries.¹Because of the low-carbon property of wood resources, material substitution with wood contributes to fewer carbon emissions—each mega- gram of carbon in harvested wood products reduces 0.25–5.6 Mg of carbon emis- sions.²More than that, after multimillion years of optimization, wood has evolved oriented micro-nano pores with high mechanical stability for fast mass transport, thus gathering extensive research interest in wood-derived functional nanomaterials and nanotechnology.^3,4These wood nanomaterials provide a sustainable platform to develop high-performance separators,⁵reactors,⁶sensors,⁷and nanofluidic devices⁸ for energy-, environment-, and biology-related applications.⁹

Nano-composited woods (NCWs) are a class of wood nanomaterial integrated by natural woods and functional nanomaterials, which possess hierarchical pore struc- ture with tailored-made functionality.^10,11 In NCWs, wood can provide robust porous skeleton for loading nanomaterials, while the nanomaterials can offer artifi- cial functions beyond natural wood. To this end, massive efforts have been devoted to the fabrication of NCWs. Direct filling of prefabricated nanomaterials into wood is

1College of Material Engineering, Fujian Agriculture and Forestry University, Fuzhou 350108, China

2National Forestry and Grassland Administration Key Laboratory of Plant Fiber Functional Materials, Fujian Agriculture and Forestry University, Fuzhou 350108, China

3Ningbo Institute of Measurement and Testing (Ningbo Inspection and Testing Center for New Material), Ningbo 315048, China

4State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China

5Fujian Fiber Inspection Center, Fuzhou 350026, China

6Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia

7School of New Energy, Ningbo University of Technology, Ningbo 315336, China

8Lead contact

*Correspondence:bhuang@fafu.edu.cn(B.H.), youxinda@fafu.edu.cn(X.Y.)

https://doi.org/10.1016/j.xcrp.2023.101477

a facile way to fabricate NCWs, during which the nanomaterials can be loaded into wood via a facile impregnation process^12–14but the wood cell wall may block the diffusion of nanomaterials and lead to insufficient and nonuniform loading. By contrast, in situ synthesis of NCWs allows controllable growth of nanomaterials within wood by pre-loading molecular or ionic precursors of nanomaterials because of the unobstructed transport of small precursors into wood cell cavities. For instance, Au³⁺, Pd²⁺, and Ag⁺ions were pre-anchored on wood via coordination with carboxyl groups in wood and further reduced into metal nanoparticles.¹⁵Simi- larly, by anchoring Zn²⁺ion, ZIF-8 nanoparticles were grownin situwithin various species of woods and simultaneously improved the surface area, mechanical strength, and gas adsorption selectivity of wood.^16,17To enhance the penetration of precursors, infiltrating method was developed and promoted thein situgrowth of hydroxyapatite nanocrystals in NCWs.¹⁸Nevertheless, above successes in fabri- cating NCWs are mostly based on inorganic nanomaterials that have disparate na- ture with organic woods, leading to compatibility issue when engineering NCWs.

Covalent organic frameworks (COFs) are crystalline polymeric nanomaterials with predesigned functional groups,¹⁹ high stability,²⁰ and especially, the inherent compatibility with natural wood because of their similar organic property rich in car- bon, oxygen elements, and benzene-ring structure. However, the combination between COFs and woods is challenging and rarely reported.^21,22Conventional syn- thetic method of COFs on the basis of homogeneous reaction (e.g., solvothermal synthesis) will generate a mass of COF products in the solvent rather than on sub- strate because of the thermodynamically-unfavorable heterogeneous nucleation,²³ which requires additional reagent as surface linker for the combination between COF and wood.²¹Moreover, the variety of functional groups of COFs can bring diverse properties into NCWs but remains unexplored, such as ionic COFs (iCOFs) that possess positively or negatively charged groups showing outstanding perfor- mance in adsorption, catalysis, ion conduction,²⁴etc. Recently, interface-assisted method has been developed to fabricate a series of iCOFs under mild condi- tions.^25–27 The interfacial synthesis of iCOFs uses water and aliphatic acid to construct the organic-aqueous interface, where the aldehyde monomers in the organic phase diffuse to the aqueous phase and react with the amino monomers.

Benefiting from the interfacial confinement, the formation of iCOFs takes place mainly in the aqueous phase, providing a facile route to control the growth of iCOFs.

Herein, we propose the interfacial synthesis of iCOF composited woods (iCOFWs).

The wood skeleton is employed as a storage tank for aqueous-phase monomers and then contacted with organic-phase monomers to trigger reaction, which confines the formation of iCOFs within wood substrate by aqueous-organic interface. Thein situ grown iCOFs display high crystallinity and combine tightly with the wood substrate by forming covalent linkage throughout the substrate, leading to superior physico- chemical stability. More important, the abundant charged groups in iCOFWs harvest fast ion transport and antimicrobial property, showing great potential in ionotronic devices. Finally, we further demonstrate the scalability, sustainability, and versatility of the interfacial synthesis method by cyclic batch fabrication as well as material expansion to various wood species and iCOFs.

RESULTS

Interfacial synthesis of iCOFWs

The synthesis procedure of iCOFWs is illustrated inFigure 1A. We first carried out delignification for raw wood (RW) to generate oxygen-containing groups (e.g.,

carboxylate),¹⁶which can attract the amine monomers in aqueous solution and pro- vide reactive nucleation sites for iCOF. Besides, lignin removal can expose cellulose fibrils and increase the growing space for iCOFs.²⁸Then, the delignified wood (DW) was immersed in water to maintain a hydrated state (Figure S1), which is beneficial to the following diffusion of amine monomers into the wood skeleton. After that, the DW was saturated in aqueous phase so that the amine monomers can be homoge- neously distributed throughout the wood substrate (Figures 1B and S2). Subse- quently, the DW was drained and immersed in the organic phase solution (Figure S3), during which the aldehyde monomers diffuse into the wood substrate with aqueous phase solution to star reaction and crystallization. This reaction process leads to an obvious color change of DW from light brown to orange because of the formation of iCOF (the resultant orange wood sample was denoted as iCOFW). Notably, the color of the organic phase solution changes little after the interfacial reaction (Figure S4A), suggesting that the formation of iCOF only occurs in the wood substrate rather than the organic phase. Above inference is confirmed by the absence of the characteristic peak of TpPa-SO3H (450 nm) in ultraviolet-visible (UV-vis) spectra (Figure S4B).

Such a phenomenon is different from conventional synthetic methods for NCWs based on the homogeneous reaction that can generate abundant products in solu- tion. Finally, the iCOFW was heated with steam to construct covalent linkages between iCOF and wood (Figure S5). The color change of wood sample to dark red is probably caused by the chemical reaction between reactive groups at the edge of iCOF and hydroxyl groups from wood (mainly in cellulose) during steam treatment (denoted as iCOFW-H), which will be elucidated by further characteriza- tion. Besides, the cross-section of iCOFW-H is also decorated with uniform red color, indicating the growth of iCOF throughout the wood substrate (Figure S6). Finally, the weakly bound COF nanocrystals were removed by ultrasonication to ensure the stability of iCOFW-H during application.

As a proof of concept, the iCOF used here is TpPa-SO3H (Figure 1C), a Schiff-based COF that possesses high-density sulfonate groups. Such charged property of TpPa- SO3H has demonstrated great potential in adsorption,²⁹water desalination,^26,30 Figure 1. Interfacial synthesis of iCOFWs

(A) Synthesis procedure of iCOFWs. Insets: digital photographs of wood samples.

(B) Schematical mechanism of interfacial synthesis for iCOFWs.

(C) Chemical structure of TpPa-SO3H COF.

photocatalysis,³¹ gas separation,^26,32and proton/ion conduction,^33,34thus gath- ering a broad research interest. We envisage that the combination between TpPa- SO3H and woods may generate NCWs with unprecedented functionalities.

Physicochemical structures of iCOFWs

Poplar, a typical hardwood, was employed as an RW substrate. Scanning electron microscopy (SEM) images of RW show a developed microporous structure from the cross-sectional direction (Figure 2A). The relatively smooth internal structure observed from the longitudinal direction is composed of vessels and fibers, which can serve as oriented pathways for mass transport. Such porous structure is well pre- served during the delignification process according to the similar SEM images Figure 2. Characterization of wood samples

(A and B) SEM images of RW (A) and iCOFW-H(TpPa-SO3H) (B).

(C) SEM image and corresponding elemental maps of carbon (red), sulfur (yellow), oxygen (blue), and nitrogen (green) in the tangential section of iCOFW-H(TpPa-SO3H).

(D) Transmission electron microscopy (TEM) image and corresponding elemental maps of carbon (red), sulfur (yellow), oxygen (blue), and nitrogen (green) in cross-section of iCOFW-H(TpPa-SO3H). Inset: the red frame denotes the testing zone for elemental mapping.

(E) FTIR curves of RW, DW, iCOFW (TpPa-SO3H), and iCOFW-H(TpPa-SO3H).

(F) Schematic illustration of chemical reaction between iCOF and wood substrate.

(G) Experimental, Pawley-refined, and simulated PXRD patterns and the difference plots for nanocrystal from iCOFW-H(TpPa-SO3H).

(H) HRTEM image of nanocrystal from iCOFW-H(TpPa-SO3H).

(I) SEAD patten of nanocrystal from iCOFW-H(TpPa-SO3H).

between RW and DW (Figures 2A andS7), providing sufficient space for growing iCOF. As suspected, an interfacial reaction carries out within the micropores of the wood substrate and generates a densely and homogeneously distributed iCOF layer on the lumen surface (Figure S8). Despite all that, the decoration of nanomaterials inside the cell wall is more challenging than in lumen space because of the steric hin- drance.⁴ From the cross-sectional direction, abundant iCOF nanocrystals is also found in iCOFW(TpPa-SO3H), which indicates the penetrating growth of iCOF in wood. This phenomenon is benefited from the confinement by interface and the mo- lecular-level penetration of reactive monomers into wood substrate. As shown in Figure 2B, the iCOF layer on iCOFW-H(TpPa-SO3H) becomes much denser after steam treatment, which is probably caused by the chemical crosslinking. Notably, by sonication, the weakly bound iCOF nanocrystals will detach, and the cellulose fibrils under the iCOF layer can be partially exposed, leading to a rougher lumen surface.

The successful formation and uniform distribution of iCOF nanocrystals are further validated by energy-dispersive spectrometry (EDS) maps of the iCOFW-H(TpPa- SO3H) from both tangential section and cross-section, which clearly show the exis- tence of nitrogen and sulfur elements from TpPa-SO3H (Figures 2C and 2D). Then Fourier transform infrared (FTIR) spectroscopy was employed to monitor the chem- ical changes during iCOF formation. As shown inFigure 2E, compared with pristine DW, the iCOFW(TpPa-SO3H) presents new peaks at1,264 cm ¹,1,446 cm ¹, and 1,700 cm ¹corresponding to C-N, O=S=O, and C=O, respectively, which are characteristic groups from TpPa-SO3H.²⁵And the absence of C=N stretching at 1,620 cm ¹ indicated the presence of keto tautomer. The weak N-H stretching band at3,400 cm ¹and the negligible aldehydic -CHO peak at1,650 cm ¹indi- cated the consumption of amine and aldehyde monomers within the wood, respec- tively.³⁵ Compared with iCOFW (TpPa-SO3H), the peak intensity of C-N at 1,264 cm ¹ and C=C at 1,582 cm ¹ of iCOFW-H(TpPa-SO3H) significantly enhances. The C-N linkage is probably attributed to the reaction between the amino groups at the edge of iCOF and hydroxyl groups from wood (Figure 2F), which can be supported by the enhanced C-N peak of wood sample soaked with amino mono- mer after steam treatment (Figure S9). Additionally, the C=C bond may be caused by the Schiff-base reaction between the amino groups and aldehyde groups at the edge of iCOF followed by enol-keto tautomerism (Figure S10A), which is verified by the enhanced C=C peak of iCOF after steam treatment (Figure S10B). Besides, the crosslinking between iCOFs may also contribute to the formation of C-N bond, thus leading to enhanced C-N peak in both FTIR spectra of iCOFW-H(TpPa- SO3H) (Figure 2E) and iCOF treated by steam (Figure S10B). Above reactions forms a covalently crosslinking network throughout the wood substrate and reinforce the combination between wood and iCOF. More precise surface chemistry of the wood samples was characterized by X-ray photoelectron spectroscopy (XPS). As shown inFigure S11, the existence of carbon, nitrogen, oxygen, and sulfur elements are validated in both iCOFW(TpPa-SO3H) and iCOFW-H(TpPa-SO3H) that consists with the EDS results. The slightly decreased oxygen element content from 22.97%

to 22.13% of iCOFW-H(TpPa-SO3H) compared with iCOFW(TpPa-SO3H) is probably attributed to the consumption of residual aldehyde groups during crosslinking within iCOF and the dehydration reaction between iCOF and cellulose wood.

Moreover, in high-resolution XPS C1s spectra, a substantial increase in C-C/C=C ratio from 67.29% to 71.74% was observed when comparing iCOFW(TpPa-SO3H) and iCOFW-H(TpPa-SO3H) (Figure S12), which corresponds well with the FTIR results and thus confirms the covalent crosslinking effect of iCOF after steam treatment.

On the basis of the above results, we speculated that the thermal property of wood may change with chemical structure and thermogravimetric analysis (TGA) was per- formed to clarify this point. As shown inFigure S13A, all wood samples undergo dehydration when the temperature is less than 180C. The lower mass decrease of iCOFWs in this stage is caused by the water-retaining capacity of sulfonate groups within iCOF nanochanel.^25,36When the temperature surpasses 180C, the mass ratio decline of iCOF powder and the iCOFW(TpPa-SO3H) accelerate but iCOFW- H(TpPa-SO3H) does not. To quantify the mass ratio decline rate, derivative thermog- ravimetry (DTG) curves give the maximum decomposition peaks of 340C, 344C, and 348C for RW, iCOFW(TpPa-SO3H), and iCOFW-H(TpPa-SO3H), respectively (Figure S13B). After 500C, aromatization and carbonization will occur in wood sam- ples and start to generate char.³⁷When the temperature reaches 800C, the iCOFW- H(TpPa-SO3H) generates higher char residual content than iCOFW(TpPa-SO3H).

These changes in thermal properties are due to the increased covalent crosslinking as mentioned above.

Maintaining high crystallinity of iCOF is a crucial but challenging task for COF-based composite materials, which can be determined by powder X-ray diffraction (PXRD).

As shown inFigure S14, the crystal structure of cellulose from wood remains intact after delignification and iCOF formation according to the unchanged characteristic peaks at23.7and16.5. However, the characteristic peaks of TpPa-SO3H can hardly be identified from the PXRD pattern of iCOFW-H(TpPa-SO3H) because of the screening effect from the high peak intensity of cellulose fibrils.²¹As an alterna- tive, iCOF nanocrystals detached from iCOFW-H(TpPa-SO3H) during ultrasonication washing were gathered for crystallographic analysis.Figure 2G displays a strong characteristic peak at4.6assigned to (100) lattice. Structural modeling and Paw- ley refinement suggested that TpPa-SO3H possesses an eclipsed AA stacking manner (Figure S15), and the refinement lattice parameters were obtained (a= 22.23188 A˚, b= 22.23188 A˚,c= 5.35247 A˚,a= 90.000,b= 90.000, andg= 120.000,Table S1). The low values ofR^wp(2.41%) andR^p(3.52%) further supported the consistency between refined diffraction and experimental PXRD data. The unit cell parameters and atomistic coordinates were shown inFigure S15andTable S1.

These results demonstrate the high crystallinity of iCOF nanocrystals grown in wood substrate.^25,38The typical lattice fringe images in high-resolution transmission electron microscopy (HRTEM) along with a selected area electron diffraction (SEAD) patterns also indicate the high crystallinity. High-crystalline iCOF can arrange high- density sulfonated groups in the ordered framework structure and harvest high chargeability,²⁶which is supported by the lower zeta potential value of iCOFW- H(TpPa-SO3H) ( 21.37G0.24 mV) than that of RW ( 18.43G0.22 mV,Figure S16).

Application properties of iCOFWs

Mechanical strength is crucial to the structural stability of composite materials during application. As shown inFigure 3A, the DW displays a lower tensile stress (36.4 MPa) than RW (79.9 MPa) because of delignification, which is consistent with the previ- ously reported literature.³⁹The introduction of iCOF into DW has very little influence on the tensile stress (33.0 MPa) but significantly elevates the elasticity modulus from 1,241.7 MPa to 1,544.1 MPa as well as the reduction of elongation at break from 4.3% to 2.3%. This enhancement in rigidity is probably caused by the high-crystalline iCOF that combines tightly with the wood substrate through covalent interaction.

Then we evaluate this combination using a mechanical force field on the basis of ul- trasonication. After ultrasonication, the washing solution of iCOFW-H(TpPa-SO3H) keeps pellucid while the washing solution of iCOFW(TpPa-SO3H) turns orange because of the detachment of iCOF (Figure 3Bi), reflecting the contribution of

chemical crosslinking to the stable combination between iCOF and wood substrate.

Considering the application scenarios under a liquid environment, the iCOFW- H(TpPa-SO3H) was immersed in 25 mL solutions of 1 mol L ¹sulfuric acid (H2SO4), 98 wt % ethanol (EtOH), and heptane to evaluate its chemical stability. After 15 days, the immersion solutions of H2SO4(Figure 3Biii), EtOH (Figure 3Biv), and heptane (Figure 3Bv) all keep pellucid, and no characteristic peaks of TpPa-SO3H (450 nm) were found in UV-vis spectra (Figure S17), indicating the hard detachment of iCOF from the wood. Even for the strong polar solvent like N,N-dimethylforma- mide (DMF), the iCOFW-H(TpPa-SO3H) can maintain a stable structure after 6-day immersion (Figure S18).

Antimicrobial property is a fundamental requirement for advanced NCWs from the perspectives of storage and application, which is usually based on contact-killing or release-killing mechanism.^40,41Here,Escherichia coliandStaphylococcus aureus, two representative Gram-positive and Gram-negative bacteria, respectively, were used to evaluate the antibacterial property of wood samples via the spread plate method. The antibacterial rate of iCOFW-H(TpPa-SO3H) reaches 98.74% and Figure 3. Application properties of iCOFWs

(A) Tensile stress-strain curves of RW and iCOFW-H(TpPa-SO₃H).

(B) Digital photographs of iCOFW-H(TpPa-SO₃H) treated by different condition. (i) Ultrasonic treatment at 40 kHz for 90 min, where the left sample is iCOFW(TpPa-SO3H) and the right sample is iCOFW-H(TpPa-SO3H). (ii) Ultrasonic treatment at 40 kHz for 300 min. (iii) Immersion in 1 mol L ¹H2SO4for 15 days. (iv) Immersion in 98 wt % EtOH for 15 days. (v) Immersion in DMF for 15 days.

(C) Photographs of agar plates ofE. coliandS. aureusafter contact with RW and iCOFW-H(TpPa-SO3H).

(D) Ion conductivity of RW and iCOFW-H(TpPa-SO3H) with LiCl as electrolyte. Inset: error bars represent SDs for 3 measurements.

(E) Schematic illustration of ion transport in the charged nanochannels of iCOFW-H(TpPa-SO3H).

(F) A series circuit comprising LED indicators and iCOFW-H(TpPa-SO3H) saturated with LiCl solution (10 ⁶M) (top) and artificial circuit (bottom). Inset:

the term ‘‘FAFU’’ assembled by arranged LED indicators.

91.72% forE. coliandS. aureus, respectively, which is substantially higher than pris- tine RW that could hardly resist both E. coli and S. aureus (Figure 3C). Such outstanding antibacterial performance may arise from the surface property of iCOFW-H(TpPa-SO3H). The abundant sulfonate groups from iCOF render wood with a strong negative charge to repel the cell membrane of bacterium that usually possesses negative charge.^41,42Besides, the locally high charge density in the nano- channels of iCOF may disturb the physiological function of the cell membrane and thus hinder the growth of bacterium.⁴³Therefore, we speculated that the dominant role of contact-killing antibacterial mechanism for iCOFW-H(TpPa-SO3H). In another bactericidal test, the absent inhibition zones around iCOFW-H(TpPa-SO3H) on bacterially inoculated excludes the release-killing mechanism and further validated the key role of the contact-killing mechanism (Figure S19). Such contact-killing feature can persistently resist bacteria without releasing toxic bactericides, thus bringing merits of long-term antibacterial stability and environmental friendliness.

Besides, when placed in a wet environment for one month, the RW undertakes se- vere fungal infection and decay while the iCOFW-H(TpPa-SO3H) maintains clean and intact structure (Figure S20).

The porous structure of wood provides abundant channels to undertake efficient mass transport. Inspired by such biological function, wood nanomaterials have been extensively designed as efficient ion conductors in the forms of ion-conductive membranes, ionic cables, and ionotronic devices.^44–46Here, considering that iCOF can form strong electrostatic interaction with ions, we attempt to evaluate the con- ductivity of iCOFW-H(TpPa-SO3H) to lithium ion (Li⁺), which is an essential monova- lent cation for energy conversion devices.⁴⁷At high LiCl concentration (1.0–10 ²M), the ion conductivity was controlled by the transport behavior of bulk LiCl solution, and the iCOFW-H(TpPa-SO3H) exhibits higher ion conductivity than RW because the exposed cellulose fibrils by delignification can provide additional space for ion transport (Figure 3D). Notably, at low LiCl concentration (10 ⁴ 10 ⁶M), the ion con- ductivity of both the RW and the iCOFW-H(TpPa-SO3H) deviates from the bulk behavior and reaches platform values of 1.1 310 ⁴and2.93 10 ⁴S cm ¹, respectively, which are much higher than the bulk value under the same concentra- tion (from3.0310 ⁶to1.8310 ⁵S cm ¹). This phenomenon can be explained by the surface charge-governed ion transport mechanism in nanochannels and relies heavily on the surface charge of nanochannel.^26,44,48As illustrated inFigure 3E, the abundant negatively charged sulfonated groups on the nanochannels of iCOFW- H(TpPa-SO3H) can exert electrostatic attraction to positively charged lithium ions and facilitate their transport along the growing direction (denoted as parallel) of the wood substrate, thus resulting in higher ion conductivity than RW at low concen- tration. Additionally, the ion conduction of wood samples is strongly influenced by the transport direction. In RW, the ion conductivity vertical to the growing direction (denoted as vertical) is an order of magnitude lower than the parallel, which is caused by the structural anisotropy of wood and may limit certain applications (Figure 3D).

Notably, the anisotropy of ion conduction is significantly weakened in iCOFW- H(TpPa-SO3H) benefited from the penetration of iCOF throughout the wood substrate, which constructs intercellular ion transport pathway vertical to the growing direction. Then we employed the iCOFW-H(TpPa-SO3H) as an ion conductor in a series circuit comprising light-emitting diode (LED) indicators, which demonstrates its potential in ionotronic devices that enabled by a hybrid circuit of mobile ions and electrons (Figure 3F). Additionally, the LED indicators also reflect the much higher ion conductivity of iCOFW-H(TpPa-SO3H) than that of RW by showing higher brightness from the former than the latter (Figure S21).

Scalability and sustainability of interfacial synthesis

Mass production with high sustainability is crucial for the practical application of NCWs by reducing material costs and unfavorable environmental impact. Here, we up-scaled the sample size of iCOFW-H(TpPa-SO3H) up to 100310032 mm (Figure 4A), which can be adjusted on demand if the container size is suitable. The uniform dark red color of iCOFW-H(TpPa-SO3H) indicates the successful formation of iCOF on the large-size wood substrate. Moreover, we carried out batch fabrica- tion of iCOFW (TpPa-SO3H) on the hundred-gram scale using the same glass bottle by only changing the volume of phase solution (Figures 4B andS22). To regulate the formation of iCOF on wood substrate, iCOFW(TpPa-SO3H) can be further immersed in a fresh organic phase solution to undertake second-round synthesis. The increased loading of iCOFW-H(TpPa-SO3H) after second-round synthesis is re- flected by the darker orange color from the digital photograph and the thicker iCOF layer from the SEM image (Figures 4C, 4D, andS23). However, overloading of iCOF layer may occupy the space for ion transport and thus sacrifice the Li⁺con- ductivity of iCOFW-H(TpPa-SO3H) (Table S2). Notably, the interfacial synthesis generates no products in the phase solution and thus will not lead to solvent contam- ination, which enables the recycling of the used aqueous phase and organic phase solution for preparing the next batch of iCOFW-H(TpPa-SO3H) (Figure 4E). The re- sulting second-batch samples have little difference with the first-batch ones while the phase solution also change little after the twice reaction. This phenomenon in- dicates that the phase solution with unconsumed monomers can be repeatedly used for synthesis by replenishing monomers, thus avoiding the discharge of waste Figure 4. Scalability and sustainability of interfacial synthesis

(A) Large-sized RW (left) and iCOFW-H(TpPa-SO3H) (right).

(B) Batch-fabricated iCOFW-H(TpPa-SO3H) on the hundred-gram scale.

(C) iCOFW-H(TpPa-SO3H) fabricated by twice synthetic rounds.

(D) SEM image of iCOFW-H(TpPa-SO3H) fabricated by a second-round synthesis.

(E) Sustainable fabrication of iCOFW-H(TpPa-SO3H) with recycled aqueous and organic phase solutions. Insets: digital photographs of recycled aqueous phase solution (top) and organic phase solution (middle) and resultant iCOFW-H(TpPa-SO3H) (bottom).

solvents. The ease in scale-up and high solvent sustainability of interfacial synthesis brings possibility for mass production.

Technical versatility of interfacial synthesis

We attempt to extend this method to different wood species and iCOFs. As shown in Figures 5A–5D, four kinds of representative wood species (hardwood, softwood, poaceae, and bamboo) were selected to fabricate corresponding iCOFW-H(TpPa- SO3H). After the interfacial reaction, all wood substrates undertake significant color change from light brown into orange because of the formation of iCOF within these wood substrates, which is also verified by the SEM images (Figures 5E–5H). The diverse morphologies of iCOF nanocrystals on wood substrates are probably caused by the different chemical compositions of wood species. Then we also evaluated the potential of iCOFW-H(TpPa-SO3H) as ion conductors. Surprisingly, the Li⁺conduc- tivity of iCOFW-H(TpPa-SO3H) on the basis of balsa (LiCl concentration of 10 ⁶M) reaches4.0310 ⁴S cm ¹(parallel) and 3.2310 ⁴S cm ¹(vertical), outperform- ing the values of iCOFW-H(TpPa-SO3H) based on poplar. This result reveals the possibility of regulating ion conduction by changing wood substrates.

The ionic groups of iCOFs can be predesigned by monomer substitution, leading to diverse types and functionalities of iCOF. As shown inFigures 6A and 6B, we chose Figure 5. iCOFW-H(TpPa-SO3H) based on different wood species

(A–D) Digital photographs of RW from Chinese fir (A), kenaf stalk (B), balsa wood (C), and bamboo (D).

(E–H) Digital photographs and corresponding SEM images of iCOFW-H(TpPa-SO₃H) based on different wood species, including Chinese fir (E), kenaf stalk (F), balsa wood (G), and bamboo (H).

two typical iCOFs as examples (TpPa-2SO3H and TpEB:Br), which possess charge density and opposite charge property different from TpPa-SO3H, respectively, to verify the applicability of our method for other iCOFs. After interfacial synthesis, the color change was also observed from iCOFW-H(TpPa-2SO3H) and iCOFW- H(TpEB:Br) (Figures 6C and 6D), which consists well with the iCOFW-H(TpPa- SO3H). The nanocrystals of TpPa-2SO3H and TpEB:Br can be observed from SEM images (Figures 6C and 6D, insets), while the characteristic elements of both iCOFs are detected by EDS maps (Figures 6E and 6F), which is further verified by the FTIR spectrum showing characteristic peaks of -SO3H and C-N/C=C from iCOFs (Fig- ure S24). Besides, the PXRD patterns of nanocrystals from both iCOFW-H(TpPa- 2SO3H) and iCOFW-H(TpEB:Br) show sharp peaks at 4.4 and 3.5 assigned to Figure 6. iCOFWs based on different iCOFs

(A and B) Chemical structure of TpPa-2SO3H (A) and TpEB:Br (B).

(C and D) Digital photographs of iCOFW-H(TpPa-2SO3H) (A) and iCOFW-H(TpEB:Br) (B). Insets: tangential-sectional and cross-sectional SEM images of iCOFW(TpPa-2SO3H) and iCOFW-H(TpEB:Br).

(E and F) SEM images and corresponding elemental maps of carbon (red), sulfur (yellow), oxygen (blue), and nitrogen (green) in iCOFW-H(TpPa-2SO3H) (G) and iCOFW-H(TpEB:Br) (H).

(G and H) Experimental, Pawley-refined, and simulated PXRD patterns and the difference plots for nanocrystal from iCOFW-H(TpPa-2SO3H) (G) and iCOFW-H(TpEB:Br) (H).

(I and J) HRTEM image (I) and SEAD pattern (J) of nanocrystal from iCOFW-H(TpPa-2SO₃H).

(K and L) HRTEM image (K) and SEAD pattern (L) of nanocrystal from iCOFW-H(TpEB:Br).

(100) lattice, respectively (Figures 6G and 6H), suggesting the high crystallinity of iCOFs from iCOFW-H(TpPa-2SO3H) and iCOFW-H(TpEB:Br). The Pawley refinement matches the experimental PXRD data with acceptable values of Rwp(13.21% for TpPa-2SO3H and 6.63% for TpEB:Br) andRp(9.59% for TpPa-2SO3H and 10.34%

for TpEB:Br), and unit cell parameters and atomistic coordinates were shown in Figures S25andS26,Tables S3, andS4.⁴⁹The high crystallinity is further supported by the clear lattice fringe images in HRTEM and SEAD patterns of iCOFs (Figures 6I–6L).

In terms of chargeability, iCOFW-H(TpPa-2SO3H) and iCOFW-H(TpEB:Br) display zeta potential lower ( 22.9G0.2 mV) and higher ( 13.1G0.5 mV) than that of iCOFW-H(TpPa-SO3H), respectively, because of the higher negative charge density from the former and the stronger positive charge from the latter (Figure S27).

Benefited from the chargeability of iCOF, both iCOFW-H(TpPa-2SO3H) and iCOFW-H(TpEB:Br) display bacterial resistance, while the former has a stronger anti- bacterial effect than the latter (Figure S28), which further supports the dominant role of surface charge for iCOFWs when resisting bacterial. The enhanced chargeability also leads to faster ion transport: the Li⁺conductivity of iCOFW-H(TpPa-2SO3H) on the basis of balsa wood reaches 7.0310 ⁴S cm ¹(parallel) and 6.1310 ⁴S cm ¹ (vertical), which is substantially higher than that of iCOFW-H(TpPa-SO3H) and out- performs state-of-the-art wood-based ion conductors (Table S2). We envisage that such a rational combination of iCOF and wood substrates can generate a power- ful platform for developing advanced wood nanomaterials as efficient ion conductors.

DISCUSSION

A facile interfacial synthesis is developed to fabricate iCOFWs in which the formation of iCOF is confined in the wood substrates by the interface of water and aliphatic acid. As a proof-of-concept, the iCOFW based on TpPa-SO3H and poplar wood display fast conduction for lithium ion that displays several times higher platform conductivity compared with RW because of the abundant sulfonate groups from TpPa-SO3H. The formation of covalent linkages renders high physicochemical stabil- ity under various harsh conditions. Besides, the high chargeability of TpPa-SO3H can disturb the growth of bacteria and render iCOFWs with outstanding antibacterial performance (98.74% forE. coliand91.72% forS. aureus) without releasing toxic substances, which is favorable for the storage and application iCOFWs. The pro- posed interface-confined synthesis features ease of mass production to offer large-sized or batched iCOFWs, which also displays high sustainability by recycling the aqueous and organic phase solutions. This strategy is extended to other kinds of woods (Chinese fir, balsa wood, kenaf stalk, and bamboo) as substrates and iCOFs (TpPa-2SO3H and TpEB:Br) as loading nanomaterials, demonstrating great potential in ionotronics, separation, catalysis, energy conversion, and other sustainable applications.

EXPERIMENTAL PROCEDURES Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to the lead contact, Xinda You (youxinda@fafu.edu.cn).

Materials availability

All experiment data are available upon reasonable request to thelead contact.

Data and code availability

This paper does not report original code. Any additional information required to re- analyze the data reported in this paper is available from the lead contact upon request.

Materials and reagents

Poplar wood, and balsa wood were cut into pieces with the dimensions of 203203 2 mm (tangential3longitudinal3radial [T3L3R]). Chinese fir was cut into pieces with the dimensions of 2032032 mm (T3L3R). Kenaf stalk was cut into circle pieces with dimensions of 2320 mm (T3R). LED was brought from Risym (China).

NaOH, NaSO3, n-octanoic acid, ethidium bromide (EB:Br), DMF, heptane, and so- dium benzoate were supplied by Alladin. EtOH and sulfuric acid (H2SO4, 98%) were purchased from Sinopharm Chemical Reagent. 1,4-Phenylenediamine-2-sul- fonic acid (Pa-SO3H) was obtained from Tokyo Chemical Industry. 1,3,5-Triformyl- phloroglucinol (Tp) and 2,5-diaminobenzene-1,4-disulfonic acid (Pa-2SO3H) were brought from Jilin Chinese Academy of Sciences-Yanshen Technology. Lithium chloride (LiCl) was purchased from Heowns. All chemicals were used as received.

Deionized (DI) water was manufactured by an Ultrapure DI system. Seesupplemental experimental proceduresfor further details on characterization and experimental protocols.

Interfacial synthesis of iCOFWs

RW samples were pretreated by immersion in a solution containing NaOH (2.5 M) and Na2SO3(0.4 M) at 100C for 4 h. Then, the samples were washed in DI water at 100C until the pH value of the washing solution reached 7.0. The resulting wood samples were called DW. The DW was soaked in DI water waiting for the reac- tion. 0.3 mmol of diamine monomer (Pa-SO3H or Pa-2SO3H or EB:Br) and 0.6 mmol sodium benzoate were dissolved in 30 mL DI water to prepare an aqueous phase so- lution. Tp (0.2 mmol) was dissolved in 20 mLn-octanoic acid to prepare an organic phase solution. The DW samples were immersed in 30 mL of aqueous solution for 24 h. Subsequently, the samples were immersed in the organic solution for 72 h of reaction and obtained iCOFW. The reaction temperature for iCOFW made from Pa-SO3H, Pa-2SO3H, and EB:Br monomers were 20C, 60C, and 20C, respectively.

After that, the iCOFWs were heated under a steam atmosphere for 24 h to promote the covalent linkage between iCOF and wood and the resultant sample was denoted as iCOFW-H. Finally, the iCOFW-H samples were ultrasonicated (40 kHz) in DI water to remove the unreacted monomers, organic solvent, and weakly bound COF nano- crystals. The COF nanocrystals in the washing solution were then recycled, washed, and freeze-dried into powder for further characterization.

Mechanical strength measurements

Different wood samples with the dimension of 803 203 2 mm were prepared, washed with DI water, and cut into a spindle shape. All samples were hydrated in DI water for at least a week to ensure complete hydration of the wood channels.

The tensile tests of the samples were performed with a universal testing system (Ins- tron 3365) with a strain rate of 10 mm min ¹.

Antibacterial experiments

The wood samples were placed in Petri dishes, immersed in 3 mL E. coli and S. aureus suspension, respectively, and incubated at 37C for 1 h subsequently, the samples were removed and washed with sterile PBS solution to remove non- adherent bacteria. After that, the samples were transferred to bacterial culture tubes with 5mL sterile PBS solution and placed in a 37C incubator for 7 h. After the

incubation, the samples were sonicated at 4 kHz for 5 min to detach the bacteria into the PBS solution. The eluent was continuously diluted 10-fold with a sterile PBS so- lution. 100 mL of the bacterial solution was spread on the Luria-Bertani solid nutrient agar medium. The media were cultured at 37C for 18 h, and the number of colony- forming units (CFUs) in each plate was counted to evaluate the antibacterial performance.

Ion conductivity measurements

Before testing, the wood samples were immersed in DI water or electrolyte for at least one week to ensure complete hydration of the wood channels. The wood samples were linked with two electrodes, which was glass slide covered in turn with insulating tape and copper tape, then the impedance of samples was measured by an electric bridge (TH2832 LCR Meter, Tonghui Electronics). Subsequently, the geometric parameters were measured by a vernier caliper. All measurements were measured at least three points and took the average. The ionic conductivity (l) was calculated according to the equationl=l/SRin whichl,S, andRare the length (m), the area of the cross-section (m²), and the measured impedance (U), respectively.

SUPPLEMENTAL INFORMATION

Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.

2023.101477.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (grant 22208052), the Natural Science Foundation of Fujian Province (grant 2022J01145), Innovation Research Program of Fujian Province (grant 2022G02004), Scientific Research Program of Fujian Province (grant 2022N5014), and Fujian Agriculture and Forestry University Outstanding Young Scientific Research Talent Program Proj- ect (grant Kxjq21014).

AUTHOR CONTRIBUTIONS

X.Y., B.H., Z.F., and Y.L. conceived the idea and designed the research. Z.F., Y.L., Z.D., Y.Q., W.X., and R.Z. carried out the experiment. Z.F. and Y.L. co-designed and proceed the ion conduct experiment. L.C. and Jiashu Yuan provided construc- tive suggestions for data visualization. Jinqiu Yuan helped analyze the structures of iCOFs. Z.S., F.Z., H.L., and B.H. provided constructive suggestions for results and discussion. All authors participated in the discussion. Z.F., Y.L., and X.Y. co-wrote the manuscript. All authors contributed to reviewing and editing of the manuscript.

DECLARATION OF INTERESTS

X.Y., Z.F., Y.L., and B.H. are inventors on Chinese patent application 202211253352.9 submitted by Fuzhou Yuanchuang Patent and Trademark Agency, covering the iCOFWs presented in this article.

INCLUSION AND DIVERSITY

We support inclusive, diverse, and equitable conduct of research.

Received: February 6, 2023 Revised: May 19, 2023 Accepted: June 8, 2023 Published: June 29, 2023

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