Recent advances in functional electronic devices enabled by two-dimensional covalent organic framework films

Abstract

Covalent organic frameworks (COFs) are an emerging class of crystalline organic polymers characterized by highly ordered structures and permanent porosity. Depending on the symmetry and connectivity of their building blocks, COFs can adopt either two-dimensional (2D) or three-dimensional (3D) architectures. Among these, 2D COFs have attracted significant attention due to their remarkable properties, including extended in-plane π-conjugation and topologically ordered columnar π-arrays. These attributes, combined with high crystallinity, large surface area, and tunable porosity, make 2D COFs highly promising candidates for functional electronic devices. This review provides a comprehensive overview of recent advancements in the synthesis of 2D COF thin films and their optical and electrical functionalities, with a focus on their integration into electronic applications. Challenges and future perspectives are discussed to guide further developments in the synthesis and functionalization of 2D COF thin films for next-generation electronic technologies.

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Guinan Chen

Guinan Chen received his PhD from Zhejiang University of Technology in 2022 and is currently a postdoctoral researcher at the same institution. His research focuses on the design and fabrication of functional nanomaterials, including covalent organic frameworks (COFs), metal–organic frameworks (MOFs), silver nanowires, and MXenes, with applications in optoelectronic devices and personal thermal management.

Pengyue Hao

Pengyue Hao is pursuing his MS degree in the College of Materials Science and Engineering at Zhejiang University of Technology. His research focuses on the synthesis of porous crystalline materials and their applications in photocatalysis.

Xiaohui Li

Xiaohui Li is an MS student in the College of Materials Science and Engineering at Wuhan Institute of Technology and is currently conducting research at Zhejiang University of Technology as part of a collaborative exchange program. Her research focuses on the preparation of porous crystalline materials and their applications in photocatalysis.

Liangjun Chen

Liangjun Chen received his PhD from Zhejiang University of Technology in 2023. He is currently a lecturer in the College of Materials Science and Engineering at Wuhan Institute of Technology and collaborates with Zhejiang University of Technology. His research focuses on the design and fabrication of metal–covalent organic frameworks and their applications in catalysis, including photocatalytic hydrogen evolution and acetylene hydrochlorination.

Dawei Gu

Dawei Gu received his PhD from Northeastern University in 2021 and is currently a lecturer in the College of Mechanical Engineering at Zhejiang University of Technology. His research primarily focuses on the design and optimization of smart materials.

Guang Zhang

Guang Zhang received his PhD from Nanjing University of Science and Technology in 2021 and is currently an associate researcher in the College of Mechanical Engineering at Zhejiang University of Technology. His research focuses on the design and optimization of smart materials.

Yongwu Peng

Yongwu Peng received his PhD from Shanghai Jiao Tong University in 2013 and is currently a professor in the College of Materials Science and Engineering at Zhejiang University of Technology. His research focuses on the design and synthesis of porous crystalline materials, such as hydrogen-bonded organic frameworks and covalent organic frameworks, as well as their hybrid composites for catalysis, separation, and smart devices.

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1. Introduction

The rapid advancement of flexible electronic technologies has positioned functional electronic devices at the forefront of attention in both research and industrial domains, owing to their remarkable mechanical flexibility, adaptability, and plasticity.^1–5 The performance of these devices is predominantly governed by the intrinsic physical and chemical properties of the active materials employed.^6–10 Consequently, a broad spectrum of materials, including graphene, MXenes, carbon nanotubes, metallic nanowires, conductive polymers, metal–organic frameworks (MOFs), and covalent organic frameworks (COFs), has been explored to address diverse practical requirements.^11–16 Among these, COFs have emerged as a particularly compelling class of materials for functional electronic applications, attributable to their exceptional crystallinity, high porosity, large specific surface area, and structural tunability.^17–20

COFs are crystalline porous materials constructed from lightweight, sustainable, and non-toxic elements (H, B, C, Si, N, O) and are amenable to low-energy processing.^21,22 The dynamic covalent bonds forming the backbone of COFs facilitate the design of diverse macroscopic architectures through the condensation of a wide array of organic building blocks.^23,24 This versatility allows precise control over the connectivity and macroscopic properties of the resulting frameworks.^25,26 From an electronic perspective, the tunability of COFs enables the modulation of their band gaps and energy levels by incorporating functional groups with tailored properties.^27,28 As a result, COFs have demonstrated broad applicability across fields such as memristors, energy storage and conversion, optoelectronics, and chemical sensing.^29,30 The success of COFs in these multidisciplinary applications underscores their potential to drive the development of advanced functional electronic devices.^31,32 However, their widespread implementation remains constrained by two key challenges: low electrical conductivity and limited processability.³³ Recent progress in manufacturing strategies has facilitated the fabrication of high-quality COF films for functional electronic applications, particularly in energy storage.^34,35 Furthermore, advances in film preparation and device integration continue to highlight COFs as promising candidates for next-generation electronic materials.³⁶

This review provides a comprehensive overview of recent developments in the fabrication of 2D COF films and their applications in functional electronic devices. Initially, the key preparation strategies, including liquid–liquid, liquid–vapor, solid–vapor, and solid–liquid interface polymerization methods, are discussed. Subsequently, the application of 2D COF films in various electronic devices, such as optoelectronic devices, memristors, and biosensors, is summarized. Finally, the challenges related to process optimization, theoretical modeling, and device integration are addressed, alongside perspectives on future research directions.

2. Preparation methods of 2D COF films

The fabrication of 2D COF films is a critical step for their successful integration into functional electronic devices. Among the various methods available, interfacial polymerization has emerged as a predominant approach. This technique involves the linkage of organic monomers through imine, boronate, or other covalent bonds to form two-dimensional networks, enabling the incorporation of COFs into electronic systems.³⁷ Interfacial polymerization typically occurs at the boundary between two immiscible phases, such as liquid–liquid, liquid–vapor, solid–vapor, or solid–liquid interfaces.³⁸

2.1 Liquid–liquid interface polymerization method

The liquid–liquid interface polymerization method employs two immiscible phases, where the reaction monomers are dissolved separately in an aqueous phase and an organic solvent. The reaction occurs at the interface of these two phases, resulting in the formation of a thin COF film.³⁹ This method is widely adopted due to its simplicity, rapid reaction kinetics, and excellent controllability.⁴⁰

Patra et al. demonstrated a distinct dynamic covalent chemistry (DCC)-directed transformation of discrete organic imine cages (C1) into free-standing COF films at the liquid–liquid interface under ambient conditions of atmospheric pressure and room temperature.⁴¹ As shown in Fig. 1(a), the process begins with dissolving the C1 cage in 6 mL of chloroform within a glass container. A spacer is created by carefully layering 3 mL of water over the chloroform. Aromatic diamine or triamine linkers, such as benzidine (BD), 1,3,5-tri(4-aminophenyl)triazine (TAPT), or 1,3,5-tri(4-aminophenyl)benzene (TAPB), are then dissolved in a 3 mL acetic acid solution and introduced into the aqueous layer. Film growth becomes visible at the interface within 5–10 minutes, with the reaction allowed to proceed at room temperature for 24 hours. The resulting COF film is subsequently collected by removing the water layer, gently harvesting the film, and rinsing it with organic solvents.

Fig. 1 (a) Representation of the acid-catalyzed room temperature interfacial transformation of the imine cage to COF films, facilitated by various amine linkers. Reproduced with permission from ref. 41. Copyright 2023 John Wiley & Sons. (b) Schematic depiction of the interface-assisted synthesis of COF nanocapsules. Reproduced with permission from ref. 42. Copyright 2024 John Wiley & Sons.

In contrast to conventional liquid–liquid interfacial methods, which rely on distinct macroscopic boundaries, Liu et al. developed an innovative nanoconfined synthesis technique for 2D COFs at the interface of microdroplets. This approach leverages the high surface-to-volume ratio of emulsified droplets to regulate interfacial polymerization effectively.⁴² Initially, an oil-in-water (O/W) emulsion stabilized by cationic surfactants, such as hexadecyl trimethyl ammonium bromide (CTAB), was prepared. The oil phase contained one monomer (e.g., TAPB, at 15 mmol L⁻¹ in ethyl acetate), and the aqueous phase served as the continuous medium. The second monomer was introduced in a controlled manner using a metering pump, facilitating polymerization at the droplet interface (Fig. 1(b)). This nanoconfined strategy enabled the production of highly uniform COF nanocapsules with hierarchical porosity, achieving yields of approximately 86% within 4.5 hours, without requiring additional catalysts.

2.2 Liquid–vapor interface polymerization

The liquid–vapor interface polymerization method is commonly employed for the fabrication of Langmuir–Blodgett (LB) films.⁴³ In the LB technique, a COF monomer and an amphiphilically modified counterpart are spread on a water surface, and solvent evaporation is utilized to control the surface pressure.⁴⁴ Through sliding, the monomers are brought into close proximity, leading to acidification, polycondensation, and the subsequent transfer of the resulting film onto a substrate, either vertically or horizontally.⁴⁵ COF films fabricated via LB interfacial polymerization exhibit well-ordered 2D crystal networks, large-scale uniformity, stable long-range porosity, and circumvent challenges such as pore activation and membrane pressure deformation. These attributes make them particularly effective for applications such as the selective nanofiltration of organic solvents.⁴⁶

Fang et al. developed a liquid–air interfacial polymerization technique to synthesize large-area, free-standing metalloporphyrin-based COF films, demonstrating their applicability in oxygen electrocatalysis.⁴⁷ This approach utilized meso-benzohydrazide-substituted metal porphyrins and tris-aldehyde linkers, which polymerized at the liquid–air interface (Fig. 2(a)). Unlike traditional imine bonds formed between amines and aldehydes, the acylhydrazone bonds formed between benzohydrazide and aldehydes imparted enhanced hydrolytic stability to the resulting COFs. The defect-free, free-standing films produced by this method spanned areas of up to 3000 cm² and exhibited significant mechanical robustness. Specifically, Co porphyrin and TOB were dissolved in dimethyl sulfoxide (DMSO), where the amphiphilic nature of Co porphyrins caused their preferential accumulation at the DMSO–humid air interface. Polymerization at this interface yielded CoP-TOB films with excellent structural integrity and functional performance.

Fig. 2 (a) Schematic of COF film synthesis at the liquid–air interfaces. Reproduced with permission from ref. 47. Copyright 2023 John Wiley & Sons. (b) Illustration of electric field-assisted interface synthesis of COF films. Reproduced with permission from ref. 48. Copyright 2024 American Chemical Society.

Recent research has focused on controlled nucleation and cross-linking at the liquid–vapor interface to achieve precise control over membrane pore structures. Lai et al. introduced an electric field to guide the preferential alignment of monomers, regulating nucleation and epitaxial growth to produce single-crystalline COF membranes at the air–water interface (Fig. 2(b)).⁴⁸ In this method, positively charged 5,10,15, and 20-tetrakis(4-aminophenyl)-21H,23H-porphyrin (TAPP) monomers were preassembled at the air–water interface under an electric field and subsequently reacted with 2,5-dihydroxyterephthalaldehyde (PDA) to form single-crystalline domains. The applied electric field facilitated horizontal orientation and served as a driving force for epitaxial growth in both in-plane and out-of-plane directions. Over time, the expanding single-crystalline domains merged seamlessly through grain boundaries or amorphous regions, resulting in a continuous, defect-free COF membrane. This innovative process yielded through-pore structures spanning the full membrane thickness, enabling rapid molecular transport and further broadening the applicability of COF membranes in advanced separations and catalysis.

2.3 Solid–vapor interface polymerization

The solid–vapor interface polymerization method represents an innovative approach for the fabrication of two-dimensional COF thin films.⁴⁹ This technique involves the chemical reaction of gaseous precursors to form solid films that are subsequently deposited onto substrate surfaces.⁵⁰ Compared to traditional liquid–liquid and liquid–vapor interface polymerization methods, solid–vapor interface polymerization offers the advantage of operating at elevated temperatures without disrupting the interface, thereby accelerating the reaction rate.⁵¹ Moreover, the static nature of the solid-phase monomer confines the reaction to the substrate interface, enabling precise control over film formation.⁵² COF films directly grown on specific substrates can be seamlessly integrated into functional electronic devices, circumventing the need for additional transfer steps, simplifying operational workflows, and reducing associated costs.^53,54

Kim et al. employed a vapor-assisted method to reduce meso- and macroscale grain boundaries, effectively minimizing contact resistance and facilitating the production of freestanding films.⁵⁵ Phosphorus-based COFs were synthesized using phosphorus-centered monomers—tris(4-formylphenyl)(methyl)phosphonium iodide and tri(4-formylphenyl)phosphine—reacted with p-phenylenediamine via a Schiff base reaction. By fine-tuning the ratio of cationic and neutral monomers, films with tailored properties were achieved through a vapor-assisted planar self-assembly approach (Fig. 3(a)). In this process, solvent vapor containing acid acted as a catalyst, initiating monomer reaction and self-assembly into nanosheets. The resultant films were produced with an efficiency of approximately 90%. After the reaction, the films were detached from the substrate by immersion in a 0.5 M KOH solution (Fig. 3(b)).

Fig. 3 (a) Schematic representation of the vapor-assisted process for COF thin-film fabrication. (b) Photograph of a freestanding macroscale COF film. Reproduced with permission from ref. 55. Copyright 2024 John Wiley & Sons. (c) Schematic of TpPa-1 COF membrane fabrication. Reproduced with permission from ref. 56. Copyright 2024 John Wiley & Sons.

Xu et al. advanced this technique by developing a steam/steam–solid (V/V–S) interface polymerization method for growing COF films specifically on the inner cavity surfaces of alumina hollow fibers.⁵⁶ In this approach (Fig. 3(c)), the 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa) monomers were dissolved in separate beakers containing caprylic acid (OA) as both solvent and catalyst. Upon heating at 170 °C, vaporized monomers and solvent were transported via natural convection to the fiber cavity, where they deposited on the PDA-modified inner surface (S). Within 8 hours, the V/V–S interface facilitated the formation of TFA-1 COF films, as indicated by the reddish-brown coloration of the alumina fiber's inner surface.

2.4 Solid–liquid interface polymerization

In the solid–liquid interface polymerization method, substrates are immersed in a liquid phase containing dissolved COF monomers.⁵⁷ Polymerization occurs at the substrate surface, yielding COF films with robust adhesion.⁵⁸ Unlike solid–vapor interface polymerization, this method leverages the fluidity of organic molecules in the liquid phase, enhancing crystallization and overall film quality.⁵⁹ Furthermore, the equipment requirements are minimal, rendering the method cost-effective for producing large-area COF films.⁶⁰

Meng et al. introduced a novel “scraping-assisted interfacial polymerization (SAIP)” method, enabling eco-friendly and scalable production of COF membranes.⁶¹ Ionic liquids (ILs) replaced traditional organic solvents in the interfacial polymerization process, achieving dual objectives in a single step. The organic phase, comprising Tp in ILs with p-toluenesulfonic acid (PTSA) and acetic acid as catalysts, was deposited onto a hydrolyzed polyacrylonitrile (PAN) substrate immersed in an aqueous phase containing Pa monomers. A motor-driven scraper spread the organic phase uniformly over the substrate, initiating Schiff base polymerization at the IL–water interface. This process yielded TpPa membranes characterized by a continuous crimson-colored layer with enhanced structural stability (Fig. 4(a)). The hydrolysis of the PAN substrate introduced carboxyl groups that electrostatically attracted Pa monomers, improving adhesion and interface compatibility (Fig. 4(b)). The resulting TpPa membranes demonstrated strong substrate adhesion, withstanding fluid flow in crossflow filtration applications.

Fig. 4 (a) Schematic illustration of the scraping-assisted interfacial polymerization technique for TpPa membrane fabrication. (b) Schematic of TpPa COF layer formation via interfacial polymerization at the IL–H₂O interface. Reproduced with permission from ref. 61. Copyright 2024 John Wiley & Sons. (c) Electrochemical interfacial polymerization of an ultrathin TpPa membrane on a PAN substrate. Reproduced with permission from ref. 62. Copyright 2023 John Wiley & Sons.

Recent advancements have also focused on developing COF films with self-healing and self-inhibiting properties. Jiang et al. introduced an electrochemical interfacial polymerization method to fabricate ultra-thin COF membranes.⁶² By exploiting electron concentration at the cathode, deprotonation reactions were promoted at the confined solid–liquid interface, driving interfacial polymerization under favorable kinetics and thermodynamics (Fig. 4(c)). This approach simultaneously imparted self-healing capabilities, addressing structural defects, and self-inhibiting effects, controlling membrane thickness. Regions with incomplete coverage experienced localized polymerization driven by higher current density, ensuring defect-free membrane formation. Conversely, pre-deposited insulating layers reduced current density, halting further growth and establishing a self-inhibiting mechanism. By adjusting the electrochemical parameters, such as time and voltage, membrane thickness and defect distribution could be finely regulated, enabling precise control over film morphology and properties.

3. 2D COF films for functional electronic devices

COFs have emerged as key materials for functional electronic devices, owing to their exceptional characteristics, including abundant π-electron arrays, efficient charge mobility, tunable band gaps, and excellent crystallinity.^63,64 This section examines the primary applications of 2D COF films in functional electronics, with a focus on optoelectronic devices, memristors, and biosensors.

3.1 Optoelectronic devices

Optoelectronic devices, which operate based on light-electron or electron-light conversion mechanisms, constitute a broad category of functional technologies.⁶⁵ Driven by advances in materials, principles, and methodologies, high-performance optoelectronic devices continue to proliferate.^66–68 Notably, 2D COFs, with their ideal π–π stacking arrangements, enable efficient out-of-plane charge transport, rendering them promising candidates for optoelectronic applications.⁶⁹ Based on their working mechanisms and functionalities, optoelectronic devices discussed here are classified into two categories: optoelectronic synaptic devices and electroluminescent devices.

3.1.1 Optoelectronic synaptic devices. Optoelectronic synaptic devices, inspired by human visual neurons, mimic synaptic behaviors by converting light stimuli into electrical signals, enabling applications in image sensing, encrypted optical communication, and wearable health monitoring.^70–73 Leveraging their unique attributes, such as facile crystallization, tunable band gaps, and broad light absorption, 2D COFs hold immense potential as active layers in these devices.^74,75

Xuan et al. reported an anthracene-based 2D COF (COF-DaTp) synthesized via liquid–liquid interface polymerization under ambient conditions.⁷⁶ As illustrated in Fig. 5(a), the reaction between 2,6-diaminoanthracene (Da) and Tp produced a uniform, stable COF-DaTp film with a high surface area. The photoactive anthracene moiety facilitated reversible transformations under UV light (365 and 254 nm), modulating the film's conductivity. When incorporated into an Al/COF-DaTp/ITO structure, the device exhibited dual optoelectronic modulation, enabling 32 distinct optical conduction states and history-dependent memory behavior. These devices effectively simulated artificial neural networks, achieving noise reduction and accurate recognition of noisy handwritten digits.

Fig. 5 (a) Schematic illustration of the room-temperature interfacial synthesis of COF-DaTp film. Reproduced with permission from ref. 76. Copyright 2024 John Wiley & Sons. (b) Schematic of the transfer dehydrogenation strategy for fabricating a BICuPc-COF-based optoelectronic synaptic device. Reproduced with permission from ref. 77. Copyright 2024 John Wiley & Sons. (c) Schematic of PyTTA-TPA COF film synthesis on a silicon wafer. (d) Structural illustration of the human brain and the corresponding artificial optoelectronic synapse. (e) ΔPSC response of the synaptic device under white light illumination. Reproduced with permission from ref. 78. Copyright 2024 John Wiley & Sons.

While the liquid–liquid interface method offers simplicity, practical applications require an additional transfer step to solid substrates. To address this, Liu et al. developed a transfer-dehydrogenation approach to synthesize highly crystalline BICuPc-COF films with notable electrical conductivity (0.022–0.218 S m⁻¹) (Fig. 5(b)).⁷⁷ These films, characterized by dense π–π conjugated structures and broad light absorption spectra, enabled optoelectronic synaptic devices to operate under low source–drain voltages (VDS = 1 V) and simulate key synaptic behaviors such as short-term plasticity (STP) and long-term plasticity.

To further streamline the fabrication process, Chen et al. demonstrated a patterned growth strategy for COF films on selectively treated substrates via Schiff base polymerization of (4,4′,4′′,4′′′-(1,3,6,8-tetrakis(4-aminophenyl)pyrene)) PyTTA and (terephthalaldehyde) TPA monomers (Fig. 5(c)).⁷⁸ These patterned films facilitated the construction of optoelectronic synaptic devices, capable of emulating synaptic functions, such as postsynaptic current alterations (ΔPSC) and paired-pulse facilitation (PPF) index modulation (Fig. 5(d)–(f)). Notably, the observed PPF index of 113.5% at a Δt of 0.5 s decreased with increasing Δt, illustrating a direct correlation between memory strength and repetitive learning frequency—key insights for time-sensitive information processing.

3.1.2 Electroluminescent device. Electrochemical luminescence (ECL), a phenomenon driven by electrochemical reactions rather than photon excitation, has evolved from an academic curiosity into a robust analytical tool.⁷⁹ This technique offers several advantages, including low background noise, high sensitivity, and precise spatiotemporal control, enabling applications in environmental monitoring, clinical diagnostics, and biological analysis.^80,81 COFs, as metal-free materials featuring ordered covalent networks, exhibit remarkable versatility owing to their tunable structures.⁸² Through pre-design strategies and post-synthetic modifications (PSM), COFs can be tailored to meet the specific requirements of ECL applications.^83,84 As a result, COFs represent a promising platform for advancing the capabilities of electroluminescent devices.

Lei et al. presented a novel donor–acceptor (D–A) COF featuring triphenylamine and triazine units, showcasing high efficiency as an ECL emitter with adjustable intramolecular charge transfer (IRCT).⁸⁵ By leveraging the complementary characteristics of donor–acceptor pairs, a luminescent COF was developed with triazine and triphenylamine as the acceptor and donor units, respectively. As illustrated in Fig. 6(a), the authors synthesized a series of tris(4-formylphenyl)amine (TFPA)-based COFs using TAPT, TAPB, and tris(4-aminophenyl)amine (TAPA) as components, denoted as t-COF, b-COF, and a-COF, respectively. Through modulation of the conjugated D–A framework, the t-COF displayed an ECL intensity approximately 123 times greater than b-COF, while a-COF demonstrated negligible ECL activity (Fig. 6(b)), underscoring the pivotal role of D–A architecture in enhancing ECL emission.

Fig. 6 (a) Schematic illustration of the synthesis of three TFPA-based COFs. (b) ECL curves of COF-modified GCEs, with inset showing magnified ECL curves for b-COF and a-COF. Reproduced with permission from ref. 85. Copyright 2021 Springer Nature. (c) Post-synthetic modification of COFs with co-reactant DEDA to generate C-COF nano-emitters. (d) Schematic representation of the bimodal oxidation ECL mechanism of C-COF. Reproduced with permission from ref. 86. Copyright 2024 John Wiley & Sons.

Beyond the pre-synthetic design of COF architectures, PSMs offer a viable strategy to further refine their ECL properties. Lei et al. introduced a coreactant-embedded COF nanoemitter by covalently attaching N,N-diethyl ethylenediamine (DEDA) to a residualized COF via PSM.⁸⁶ Using the ligands 4′,4′′′,4′′′′′,4′′′′′′′-(1,2-ethenediylidene)tetrakis[1,1′-biphenyl]-4-carboxaldehyde (ETBC) and TAPT, a COF with aldehyde residues was synthesized, providing reactive sites for subsequent co-reactant incorporation. The amide reaction between the aldehyde groups and DEDA amino groups yielded a highly ordered staggered A–B stacking structure referred to as C-COF (Fig. 6(c)). This structure exhibited a remarkable 1008-fold enhancement in ECL intensity compared to the unmodified COF when the same amount of DEDA was introduced. As shown in Fig. 6(d), the ECL mechanism of C-COF involves an electron release from DEDA at the electrode to form DEDA⁺·, followed by deprotonation to produce DEDA·. The COF· species, formed via IRCT between DEDA· and COF, subsequently reacts with DEDA^+· in a secondary IRCT process to generate the excited-state COF* responsible for ECL emission. At higher applied potentials, direct oxidation of COF to COF^+· at the electrode surface, combined with DEDA oxidation and subsequent electron transfer, supports an oxidative-reduction ECL mechanism.

COFs, as a class of porous crystalline polymers with high crystallinity and ordered architectures, provide exceptional stability and reliability for applications in electroluminescent devices. The modular nature of COFs enables independent tuning of pore structures and functional groups, establishing them as promising candidates for advanced optoelectronic devices.

3.2 Memristors

Memristors, akin to biological synapses, possess the ability to detect changes in applied voltage and current flow through their resistance states.⁸⁷ This intrinsic property grants memristors memory and logic-processing capabilities, making them a potential alternative to traditional digital logic circuits.⁸⁸ Memristors offer notable advantages, including simple architectures, rapid switching, and low power consumption, and are crucial for developing high-density storage, memory-driven computing, and sensing technologies.^89,90 The unique characteristics of 2D COFs, such as their impressive intrinsic carrier mobility, tunable porous structures, and uniform ultra-thin morphologies, render them ideal for memristor applications.⁹¹

Chen et al. synthesized a two-dimensional conjugated COF (COF-Azu) via liquid–liquid interface polymerization using 1,3,5-tri(4-aminophenyl)benzene and azene-1,3-diacetaldehyde as precursors.⁹² The resulting Al/COF-Azu/ITO memristor exhibited non-volatile resistive switching behavior (Fig. 7(a)). Switch-on and switch-off voltages of −0.50 V and +1.95 V, respectively, were observed, with an on/off current ratio of 50. Both states remained stable throughout the measurement process (Fig. 7(b)). Furthermore, the device's 16 non-overlapping resistance states were utilized as weight parameters in a memristor-based convolutional neural network for image recognition, achieving an accuracy of 80% after eight training epochs (Fig. 7(c)). This highlights the potential of COF-based memristors in neuromorphic computing and advanced data-processing applications.

Fig. 7 (a) Current–voltage characteristics of the Al/COF-Azu/ITO device. (b) On/off state current variation under a constant 0.1 V stress. (c) Recognition accuracy as a function of training epochs. Reproduced with permission from ref. 92. Copyright 2023 John Wiley & Sons. (d) Representative images of campus landmarks and their conversion into RGB format. (e) Schematic of a CNN-based image classification process. (f) Illustration of typical image feature extraction. (g) Confusion matrix comparing actual and predicted classifications for landmark recognition. (h) Recognition accuracy as a function of conductance states in the device. Reproduced with permission from ref. 93. Copyright 2024 John Wiley & Sons.

Beyond liquid–liquid interface polymerization, Zhang et al. synthesized Ta-Cu₃COF thin films through acid exfoliation followed by spin-coating.⁹³ The resulting Al/Ta-Cu₃COF/ITO memristor demonstrated efficient, non-volatile modulation of 128 distinct conductance states in pulse mode, with prolonged retention times exceeding 2000 s and rapid relaxation times of less than 0.5 μs. Leveraging these unique memristive properties, a simple convolutional neural network (CNN) was developed for recognizing ten representative campus landmarks. As depicted in Fig. 7(d) and (e), a 64 × 64-pixel digital photograph of these landmarks was converted into three separate images in RGB mode. Fig. 7(f) illustrates a representative feature extraction process, highlighting the distinction of snails from their background. Notably, the newly added images exhibited the highest similarity to themselves after training (Fig. 7(g)). With increased training epochs, the CNN's recognition accuracy improved, achieving 95.13% accuracy after 25 iterations (Fig. 8(h)).

The development of COF-based memristors has seen significant progress, presenting transformative opportunities in data storage, synaptic emulation, and neuromorphic computing. The intrinsic advantages of COFs, including their structural versatility, ease of synthesis, and precise functional tunability, position them as leading candidates for next-generation high-performance memristors.

3.3 Biosensors

Abnormal concentrations of biological species are closely associated with various physiological disorders, underscoring the critical importance of their detection for clinical diagnosis and therapeutic intervention.⁹⁴ The past decade has witnessed significant advancements in the development of high-performance biosensors.⁹⁵ Among these, electrochemical (EC) and photoelectrochemical (PEC) sensors have emerged as advanced platforms for the rapid and accurate detection of target analytes, offering distinct advantages such as low background interference, high signal-to-noise ratios, and rapid response times.⁹⁶ COFs, a class of porous crystalline materials, exhibit greater structural flexibility than traditional inorganic materials, enabling the rational design and optimization of their periodic frameworks and chemical functionalities.^97,98 With enhanced charge mobility and light absorption capabilities, COFs represent promising materials for EC and PEC sensing applications.⁹⁹

Zhu et al. developed an ultra-thin COF-Bta-NS structure with excellent crystallinity and a thickness of approximately 1.95 nm by employing an interfacial disturbance strategy during COF growth (Fig. 8(a)).¹⁰⁰ The synthesized COF-Bta-NSs were combined with acetylcholinesterase (AChE) enzymes to construct an EC biosensor for detecting organophosphorus pesticides (Fig. 8(b)). The remarkable porosity and enhanced substrate enrichment capacity of COF-BTA-NSs coupled with their efficient electron transfer properties, facilitated the effective conversion and output of signal molecules (Fig. 8(c)). Infrared spectroscopic analyses of AChE and AChE/COF-BTA-NSs revealed hydrogen bond interactions between amino acid residues on the protein surface and the organic linker of the COF, leading to the proposal of a hydrogen bond assembly model (Fig. 8(d)).

Fig. 8 (a) Schematic representation of the synthesis and structural features of COF-Bta-NS. (b) Illustration of a portable biosensor for organophosphorus pesticide (Ops) detection. (c) Differential pulse voltammetry (DPV) curves of various modified electrodes. (d) Schematic of hydrogen-bonding interactions between protein surface groups and COF organic linkers. Reproduced with permission from ref. 100. Copyright 2024 John Wiley & Sons. (e) Cyclic voltammetry (CV) curves of p-bpy-COF/GCE at different NO concentrations. (f) Photocurrent response of p-bpy-COF/GCE to NO concentration variations. (g) Calibration curve of photocurrent change versus the logarithm of NO concentration. Reproduced with permission from ref. 101. Copyright 2022 Elsevier.

While EC biosensing systems demonstrate high sensitivity, reliance on a single detection modality can be limiting. Du et al. addressed this limitation by synthesizing p-bpy-COF using 4,4′,4′′-(21H, 23H-porphyrin-5,10,15,20-tetramethylene) tetraaldehyde (p-Por-CHO) and 2,2′-bipyridine-5,5′-diamine (bpy) as precursors through a condensation reaction.¹⁰¹ This novel COF was employed to develop a dual-functional EC and PEC sensor platform. The oxidation peak current of the p-bpy-COF-based sensor exhibited a proportional increase with nitric oxide (NO) concentration, demonstrating high electrocatalytic efficiency for NO oxidation (Fig. 8(e)). Furthermore, the PEC sensor utilizing p-bpy-COF displayed a robust photocurrent response that increased with NO concentrations ranging from 5 μM to 660 μM. Within this range, the photocurrent variation (ΔI) exhibited a strong linear correlation with NO concentration (R² = 0.9914), as shown in Fig. 8(f) and (g).

The intrinsic band structure and electronic properties of COFs can be precisely tailored by modulating the coordination environment, organic ligands, and reaction conditions. Additionally, the formation of heterostructures has proven to be an effective strategy for enhancing carrier separation and transport in photoactive COF-based materials, thereby advancing the development of innovative, high-performance EC and PEC biosensors.

4. Summary and outlook

This review highlights the specialized preparation methods for 2D COF films, covering techniques at liquid–liquid, liquid–vapor, solid–vapor, and solid–liquid interfaces. Furthermore, the application of ultra-thin 2D COF films in functional electronic devices, such as optoelectronic devices, memristors, and biosensors, is thoroughly examined. Despite significant progress in both the synthesis and application of 2D COF films, challenges remain in optimizing production methodologies, enhancing the structural integrity of 2D COF films, elucidating the structure–activity relationships, and improving the performance of functional electronic devices.

The unique advantages of 2D COF films, including high surface area, tunable structures, and excellent crystallinity, enhance electron transfer efficiency and maximize active site exposure, making them highly promising for functional electronic devices. However, challenges such as low yield, complex fabrication processes, non-uniformity, and high production costs continue to limit their scalability. Additionally, current strategies for fabricating high-quality COF films remain constrained by size limitations, restricting their production to the laboratory scale. Overcoming these barriers to achieve large-scale, high-quality COF film fabrication is critical for advancing their practical applications, representing a key challenge for future research.

Recent advances in 2D COF film fabrication have broadened their applications in functional electronic devices. However, integrating COFs into devices remains challenging. A key limitation is their poor electron delocalization and low conductivity, which lead to unstable data readout and reduced cycle life, posing a significant bottleneck for their practical deployment. Enhancing COF conductivity while maintaining structural integrity and porosity is therefore a crucial research focus. Additionally, a deeper understanding of their physicochemical properties is essential to establish a theoretical foundation for optimizing their performance in emerging applications, such as photovoltaic conversion and energy transfer. Furthermore, integrating COF-based functional devices into multi-functional systems while preserving their intrinsic properties requires advancements in materials design, interface engineering, and device integration strategies.

In conclusion, COF-based functional electronic devices represent a dynamic and promising frontier in the field of materials science, characterized by complex scientific challenges and substantial application potential. With continued research and interdisciplinary collaboration, we anticipate substantial advancements in the field, paving the way for groundbreaking innovations in COF-enabled functional electronics.

Abbreviations

AChE	Acetylcholinesterase
Al	Aluminum
BD	Benzidine
Co	Cobalt
COF	Covalent organic framework
CTAB	Hexadecyl trimethyl ammonium bromide
CNN	Convolutional neural network
CV	Cyclic voltammetry
Da	2,6-Diaminoanthracene
D–A	Donor–acceptor
DCC	Dynamic covalent chemistry
DEDA	N,N-Diethyl ethylenediamine
DMSO	Dimethyl sulfoxide
DPV	Differential pulse voltammetry
EC	Electrochemical
ECL	Electrochemical luminescence
ILs	Ionic liquids
IRCT	Intramolecular charge transfer
ITO	Indium tin oxide
LB	Langmuir–Blodgett
MOF	Metal–organic frameworks
OA	Caprylic acid
Ops	Organophosphorus pesticide
O/W	Oil-in-water
Pa	P-phenylenediamine
PAN	Polyacrylonitrile
PDA	2,5-Dihydroxyterephthalaldehyde
PEC	Photoelectrochemical
TAPA	Tris(4-aminophenyl)amine
TFPA	Tris(4-formylphenyl)amine
TOB	(1,3,5-Triazine-2,4,6-triyl)tris(oxy)tribenzaldehyde
PPF	Paired-pulse facilitation
PSM	Post-synthetic modifications
PTSA	p-Toluenesulfonic acid
PyTTA	4,4′,4′′,4′′′-(1,3,6,8-Tetrakis(4-aminophenyl))pyrene
SAIP	Scraping-assisted interfacial polymerization
STP	Short-term plasticity
TAPB	1,3,5-Tri(4-aminophenyl)benzene
TPA	Terephthalaldehyde
TAPP	5,10,15, and 20-Tetrakis(4-aminophenyl)-21H,23H-porphyrin
TAPT	1,3,5-Tri(4-aminophenyl)triazine
Tp	1,3,5-Triformylphloroglucinol

Symbols

UV	Ultraviolet
V/V–S	Steam/steam–solid
VDS	Source–drain voltages
R ^2	Coefficient of determination
ΔPSC	Postsynaptic current alterations
Δt	Time interval
ΔI	Photocurrent variation

Author contributions

Guinan Chen: Pengyue Hao: investigation, formal analysis, data curation, methodology, writing – original draft. Xiaohui Li: Liangjun Chen: investigation, methodology, data curation, formal analysis. Dawei Gu: Guang Zhang: data curation, methodology. Yongwu Peng: conceptualization, formal analysis, supervision, funding acquisition, writing – review & editing.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Y. P. thanks the financial support from the National Natural Science Foundation of China (22375179). This work was also supported by the National Natural Science Foundation of China (52405137), the Zhejiang Provincial Natural Science Foundation of China (LQ22E050013, LQ23E050018), Taizhou Natural Science Foundation Project (23gya23), and Open Foundation of the State Key Laboratory of Fluid Power and Mechatronic Systems (GZKF-202418).

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