Synthetic Strategies and Linkage-Dependent Performance in Covalent Organic Frameworks-based Chemoresistive Gas Sensors

1. INTRODUCTION

The development of smart gas sensors has enabled the precise detection of airborne gases, providing alerts to ensure public safety and improve the quality of life. Initially, gas sensors were primarily used to monitor industrial pollutants and toxic gases [1,2]. However, their applications have since expanded to include everyday uses, such as food quality assessment, indoor air quality monitoring, and non-invasive disease diagnosis [3-5]. Chemoresistive gas sensors have been widely adopted for real-time gas monitoring because of their small size, rapid kinetics, and high sensitivity [6-8]. Metaloxide semiconductors (MOSs) are the most widely used materials for chemoresistive gas sensors because of their high thermal stability and catalytic activity for gas oxidation [9-11]. However, they typically require high operating temperatures and often exhibit poor selectivities. In recent years, two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs), carbon-based materials, and metal–organic frameworks (MOFs) have emerged as attractive alternatives because of their large surface areas, mechanical flexibility, and abundant active sites [12-16]. Furthermore, as the demand for highly selective, low-temperature operation, and selective gas sensors has increased, researchers have focused on covalent organic frameworks (COFs) as a new class of crystalline porous materials with tunable chemistry and architecture.

COFs represent an emerging class of crystalline, porous organic polymers made from organic building blocks linked by strong covalent bonds. Since their discovery by Yaghi and colleagues in 2005, a wide range of COFs with tailored structural features and diverse linkage chemistries such as imines, triazines, and boronate esters, have been developed (Fig. 1) [17]. The geometry and symmetry of these building blocks determine the dimensionality and topology of the resulting frameworks, allowing precise control over their structural architecture. COFs offer a unique combination of properties, including high specific surface area, permanent porosity, thermal stability, low density, tunable pore size, and chemical functionality [18-20]. These characteristics make COFs promising materials for various applications, including catalysis [21,22], gas storage [23], batteries [24,25], and gas sensors [26,27]. In particular, 2D π-conjugated COFs have attracted increasing interest as platforms for chemoresistive gas sensing. Their layered structures facilitate both in-plane and interlayer π-electron delocalization, enhancing charge transport across the framework [28,29]. Combined with their porous nature, which allows rapid gas diffusion, conductive COFs hold significant potential as next-generation sensing materials, offering high sensitivity, selectivity, and operational versatility.

Fig. 1.

Representative examples of covalent linkages used in COF construction.

In this review, we comprehensively highlight recent advances in the development of COF-based chemoresistive gas sensors. First, the synthetic strategies for preparing COFs are discussed, covering both powder- and thin-film approaches, which are crucial for tuning their structural and electronic properties. Next, we review recent advances in COF-based chemoresistive gas sensors, highlighting representative studies that showcase exceptional sensitivity, selectivity, and reversible sensing behavior toward various target gases. The content follows a classification based on the types of bonding within the COF building blocks, illustrating their characteristic behaviors that may influence their gas-sensing properties. In these studies, the proposed gas-sensing mechanisms are also discussed. Finally, we conclude with a perspective on COF-based chemoresistive gas sensors, aiming to inspire further research on their practical applications.

2. FABRICATION METHODS OF COFs

2.2 Synthesis of COF thin films

2.2.1. Bottom-up synthesis

Through bottom-up synthesis, COFs can be directly synthesized at an interface or deposited onto a specific substrate with a controllable thickness and surface properties. During the fabrication process, the growth direction of the COF is influenced by the confined physical space in which the synthesis takes place. There are four common methods for preparing COF films: (1) interfacial polymerization, (2) synthesis on a substrate under solvothermal conditions, (3) layer-by-layer (LbL) self-assembly on a substrate, and (4) chemical vapor deposition (Fig. 2 (a)).

Fig. 2.

Schematic illustration of COF powder synthesis via (a) top-down and (b) bottom-up strategies.

2.2.1.a. Interfacial synthesis

Interfacial synthesis is a widely used method for preparing COF thin films, in which monomers react at confined interfaces to promote controlled film growth. Various types of interfaces, such as liquid-air, liquid-liquid, and solid-vapor interfaces, can be employed, depending on the desired synthesis conditions [37-39]. For example, in liquid-air interfacial synthesis, Shinde et al. synthesized a crystalline TFP-DHF membrane at the water-air interface [40]. The TFP and DHF solutions were dissolved in toluene and gently spread onto the water surface. After the evaporation of toluene, the resulting layer was compressed, and polymerization was initiated by adding catalytic trifluoroacetic acid. The as-synthesized TFP-DHF thin film was formed at the interface and transferred using the Langmuir-Blodgett method. The volatilization of low-boiling-point organic solvents can create liquid-air interfaces. In liquid-liquid interfacial synthesis, each monomer is dissolved in immiscible solvents such as water-dichloromethane and water-ethyl acetate. Dey et al. reported the synthesis of a TFP-BPy thin film on a water-dichloromethane interface using a three-layered system: TFP dissolved in dichloromethane as the bottom layer, pure water as the middle layer, and BPy-PTSA salt in water as the topmost layer [41]. The intermediate water prevented the premature mixing of the precursors and promoted the slow diffusion of monomers, yielding a highly crystalline COF. In solid-vapor interfacial synthesis, one monomer is immobilized on a solid substrate, while the other is introduced in the vapor phase. This strategy promotes controlled COF growth solely at the confined interface and allows heating without disturbing the interface. For instance, Khan et al. fabricated highly crystalline and ultrathin TFP-PDA membranes by spin coating TFP onto a Si/SiO₂ substrate followed by exposure to PDA vapor at 150°C [42]. The elevated temperature substantially reduced the reaction time and ensured sufficient PDA vapor pressure, thereby enhancing membrane quality.

2.2.1.b. Solvothermal synthesis

The in-situ solvothermal approach facilitated the fabrication of highly crystalline, uniform, and dense COF structures with fewer defects. Colson et al. synthesized 2D COF films on single-layer graphene under simple solvothermal conditions [43]. The as-synthesized COF films were stacked perpendicular to the single-layer graphene surface with a well-oriented structure and exhibited enhanced crystallinity compared to COF powders. Medina et al. fabricated BDT COF on different metal oxide substrates including indium tin oxide (ITO) glass and NiO-modified ITO glass [44]. The as-synthesized BDT COF films exhibited a crack-free and uniform structure with a thickness of approximately 150 nm and good adherence to the ITO surface. Sun et al. synthesized a TFPPy-PDA COF film on single-layer graphene under optimized solvothermal conditions [45]. The resulting TFPPy-PDA COF exhibited a well-defined structure and high crystallinity, making it a suitable semiconducting layer for vertical field-effect transistor devices.

2.2.1.c. Layer-by-layer self-assembly

LbL self-assembly is a convenient method of membrane fabrication that involves alternating reactions between two or more precursors on a substrate surface. Compared to conventional coating methods, LbL self-assembly offers distinct advantages, such as simplicity of the fabrication process and precise control over the film thickness [46]. Selection of an appropriate substrate surface is crucial for initiating and sustaining sequential polymerization reactions. In 2018, Shi et al. reported the fabrication of a crystalline TFP-BD COF on a porous hydrolyzed polyacrylonitrile (HPAN) membrane using the LbL process [47]. The abundant surface carboxyl groups on HPAN facilitate the anchoring of amine monomers, enabling subsequent COF growth by alternating exposure to ethanolic solutions of amine and aldehyde monomers. The thickness of the TFP-BD layer was controlled by simply adjusting the number of LbL cycles.

2.2.1.d. Chemical vapor deposition

Chemical vapor deposition (CVD) offers distinctive advantages, including simplicity of fabrication and a fast reaction rate. Because it does not require organic solvents to dissolve the monomer reactants, it is environmentally friendly. Synthetic conditions such as temperature, reaction time, pore size of the basement membrane, and pressure determine the crystallinity, thickness, and quality of the COF films. In 2021, Hao et al. reported the fabrication of a TFP-PDA-1 COF membrane via chemical vapor deposition [48]. The authors systematically elucidated the effects of the evaporation temperature, evaporation time, amount of Fe antioxidant, and pore size of the basement substrate on the quality of TFP-PDA-1.

2.2.2. Top-down synthesis

Thin-film fabrication via a top-down approach relies on the exfoliation of bulk COFs into 2D nanosheets (NSs), which can subsequently be assembled into continuous films. These bulk COFs comprise periodically stacked layers assembled through weak intermolecular forces, such as van der Waals interactions, π–π stacking, and hydrogen bonding. Upon the application of an external force, these layered structures can be delaminated into single- or few-layered NSs (Fig. 2 (b)). This top-down strategy offers an efficient, scalable, and cost-effective route for producing highly crystalline COF films.

2.2.2.a. Mechanical exfoliation

External mechanical forces can be applied by ball milling, grinding, ultrasonication, or tape-assisted exfoliation. In 2013, Banerjee et al. demonstrated mechanical delamination of bulk COFs to produce a series of covalent organic nanosheets (CONs) [49]. They synthesized eight types of COFs (TFP-PDA-1, TFP-PDA-2, TFP-BD, TFP-PDA-NO₂, TFP-PDA-F₄, TFP-BD-(NO₂)₂, TFP-BD-Me₂, and TFP-BD-(OMe)₂) and successfully exfoliated them via a simple and safe mechanical grinding method. Since individual layers are stacked by relatively weak π-π interactions and possess inherent chemical stability, mild mechanical forces are sufficient to exfoliate them. Wang et al. demonstrated the exfoliation of a 2D COF into few-layered NSs with abundant redox acid sites for Li-ion battery applications [50]. To shorten the Li-ion transport pathway, a simple ball milling process was used to reduce the COF thickness. The micron-sized DAAQ-TFP COF was ball-milled at room temperature without the use of additional exfoliating agents, resulting in thinner, sheet-like delaminated CONs.

2.2.2.b. Solvent assisted exfoliation

Solvent molecules can intercalate between COF layers, weakening π-π interactions and facilitating exfoliation. This process is typically enhanced by sonication, which further aids in the separation of individual layers. Berlanga et al. reported the delamination of COF-8 into 10–20 layers by sonication in dichloromethane, which was chosen for its volatility, low water content, and high purity [51]. Similarly, Bunck, et al. emonstrated the solvent-induced exfoliation of COF-43 by immersion in tetrahydrofuran, trichloromethane, toluene, and methanol [52]. Although X-ray diffraction (XRD) analysis revealed a loss of crystallinity, Fourier-transform infrared (FTIR) and in-situ IR spectra confirmed that no new chemical species were formed during soaking, indicating that the observed structural changes originated from physical delamination rather than chemical decomposition. These findings demonstrate that solvent-assisted exfoliation can effectively yield few-layered CONs, while preserving the intrinsic chemical structure of the framework.

2.2.2.c. Chemical exfoliation

Chemical exfoliation utilizes specific chemical reactions, such as oxidation, reduction, or intercalation, to weaken the interlayer interactions and promote delamination. Banerjee et al. demonstrated the chemical exfoliation of an anthracene-based COF (DA-TFP) to yield CONs with tunable thicknesses and high dispersibility [53]. A bulk DA-TFP COF was synthesized via a solvothermal reaction, and exfoliation was then induced by incorporating N-hexylmaleimide molecules into the anthracene moieties of the COF through a Diels–Alder cycloaddition. This incorporation disrupted the π–π interactions between layers, facilitating exfoliation. The chemically exfoliated NSs were subsequently assembled onto silicon substrates via air-water interfacial LbL deposition, enabling the fabrication of uniform, thickness-tunable films. Similarly, Chen et al. chemically exfoliated a 2D TFPB-PDA COF through oxidative intercalation using HClO₄ and KMnO₄ [54]. The in-situ formation of MnO₂ nanoparticles effectively inhibited the restacking of the exfoliated NSs, thereby improving access to redox-active sites and enhancing the Liion storage performance.

2.2.2.d. Self-exfoliation

Self-exfoliation refers to the spontaneous delamination of layered materials driven by internal forces or molecular interactions without external energy or chemical treatment. In particular, ionic COFs have shown potential for the direct synthesis of CONs without external treatment. For example, Mitra et al. reported the self-exfoliation of guanidinium halide ionic CONs [55]. The inherent positive charge of guanidinium induced electrostatic repulsion, overcoming the π–π interactions between layers. Similarly, Zhang et al. synthesized an ionic COF via a one-pot method, which subsequently self-exfoliated upon dispersion in polar solvents (dioxane and mesitylene) [56]. In this system, BF₃·OEt₂, a Lewis acid catalyst, not only facilitated the formation of boron-containing COFs but also coordinated with nitrogen atoms to form organic ion pairs. The incorporation of ionic moieties played a crucial role in enabling self-exfoliation, and the resulting nanosheets were successfully assembled into uniform thin films on SiO₂ substrates.

3. COF-BASED CHEMORESISTORS

3.1 Imine-linked COF chemoresistors

Imine-linked COFs are the most extensively explored class of COFs owing to their high crystallinity, well-defined porosity, and superior structural regularity. Compared with boron-containing COFs, imine-linked COFs exhibit superior chemical stability in a wide range of organic solvents and water. In 2009, Yaghi and co-workers first reported a 3D crystalline porous imine-linked COF (COF-300) synthesized via the condensation between tetrahedral tetra-(4-aniyl)-methane and linear terephthaldehyde [57]. Since then, numerous imine-linked COFs have been developed via the condensation of aldehydes with amines or hydrazides, with growing interest in their electrochemical applications, including chemoresistive sensors. For instance, Niu et al. prepared an imine-linked COF by condensing TAPB and PBDA for NH₃ detection (Fig. 3 (a)) [58]. The resulting TAPB-PBDA COF showed a notable response to NH₃ (S_NH3 = 23% to 100 ppm) at room temperature, along with rapid response/recovery times (10/106 s) and ultra-high selectivity (Figs. 3 (b) and 3 (c)). These distinct NH₃ sensing properties were attributed to the strong hydrogen bonding between the imine N and NH₃. The authors also found that structurally analogous imine-linked COFs, such as TFB-DAB COF, TAPB-PDA COF, and TAPB-TPDA COF, exhibited similar NH₃ sensing behavior. Various post-treatments have been investigated to improve the gas-sensing properties. Liu et al. reported the metallization of a TAPP-PCBA COF with Co ions on the porphyrin rings using a heat-assisted reflux method (Fig. 3 (d)) [59]. The introduction of Co substantially enhanced the response to NO₂, as Co-TAPP-PCBA COF exhibited approximately twice the response (S_NO2 = 2,713 to 100 ppm), about 1.5 times shorter response time (5.3 min), and nearly 180 times lower detection limit (6.8 ppb) compared to the pristine TAPP-PCBA COF (Fig. 3 (e)). These improvements were ascribed to the strong interaction between coporphyrin and NO₂ molecules, confirmed by in-situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and density functional theory (DFT) calculations (Fig. 3 (f)). Moreover, Xu and co-workers reported a facile one-pot post-treatment to generate dangling bonds (–NH₂) on a Zn-TAPP-DFP COF for sensitive NO₂ detection [60]. Zn metallization induced the elongation of the imine linkage, which was easily broken by the subsequent hydrolysis reaction, forming dangling bonds (Fig. 3 (g)). The resulting Zn-TAPP-DFP COF exhibited a high NO₂ response (S_NO2 = 543 to 100 ppm) under visible-light irradiation, with an ultra-low detection limit of 7.9 ppb and high selectivity against interfering gases such as NH₃ and SO₂ (Fig. 3 (h)). From the DFT calculations and in-situ FTIR, the authors revealed that dangling –NH₂ groups were the functional sites for NO₂ adsorption, resulting in distinct NO₂ sensing performance (Fig. 3 (i)).

Fig. 3.

Gas sensing performance of imine-linked COFs. (a) Structure of TAPB-BPDA COF. (b) Dynamic sensing transient of TAPB-BPDA COF to 100 ppm NH3. (c) Selectivity of TAPB-BPDA COF over various interfering gases. Reprinted with permission from Ref. [58], Copyright (2021) Royal Society of Chemistry. (d) Structure of H2-TP COF. (e) Dynamic response transient of Co-TP COF toward 5–100 ppm NO2. (f) In-situ DRIFT spectra of Co-TP COF during NO2 adsorption. Reprinted with permission from Ref. [59], Copyright (2022) Wiley-VCH. (g) Schematic illustration of defective COF synthesis. (h) Dynamic response transient of Zn-TD COF to 100 ppm NO2. (i) NO2 adsorption energy of Zn-TD COF on different sites. Reprinted with permission from Ref. [60], Copyright (2023) Royal Society of Chemistry.

3.2 β-ketoenamine-linked COF chemoresistors

The β-ketoenamine-linked COFs are formed via an irreversible enol-to-keto tautomerization from imine-linked COFs. This irreversible tautomerism imparts exceptional chemical and thermal stability to the framework and enhances its electronic conductivity through extended π-electron delocalization [61,62]. These structural and electronic features make β-ketoenamine COFs promising candidates for chemoresistive gas-sensing applications. To investigate the influence of linkage type on sensing performance, Yan et al. prepared two isostructural COFs, β-keto-An COF and imine-An COF, that share identical backbones but differ in their linkages (Fig. 4 (a)) [63]. The I-V curve revealed that β-keto-An COF exhibited higher conductivity than imine-An COF, attributed to the extended intralayer π–π conjugation and reduced interlayer stacking distance due to the carbonyl group (–C=O) within the β-ketoenamine linkage (Fig. 4 (b)). The β-keto-An COF also demonstrated better thermal and chemical robustness, owing to the irreversible nature of its tautomerism. Notably, the β-keto-An COF showed a significantly higher NH₃ response (S_NH3 = 189.4% to 10 ppm), which was approximately 32 times higher than that of imine-An COF, along with high selectivity (response ratio ≥7.33) (Fig. 4 (c)). The superior sensing performance was attributed to strong hydrogen bonding interactions between NH₃ and the carbonyl group within the β-ketoenamine linkage, as supported by DRIFT and DFT calculations. These results demonstrate that β-ketoenamine-linked COFs are favorable sensing materials for NH₃ and NO₂.

Fig. 4.

Gas sensing performance of β-ketoenamine-linked COFs. (a) Synthetic routs for β-keto-An COF and imine-An COF. (b) I-V curves of β-keto-An COF and imine-An COF. (c) Repetitive response curve of β-keto-An COF and imine-An COF to 10 ppm NH3. Reprinted with permission from Ref. [63], Copyright (2021) Royal Society of Chemistry. (d) Structure of H2-TP COF. (e) Dynamic response transients of Co-TP COF toward 5–100 ppm NO2. Reprinted with permission from Ref. [64], Copyright (2022) Wiley-VCH. (f) Theoretical models of the combination of Pd exposed by surface detects of perovskites and N and O in TpPa-1. (g) Gas response of TaPa-1 and TaPa-1/Cs2PdBr6. Reprinted with permission from Ref. [27], Copyright (2023) Springer Nature.

Despite their relatively high intrinsic conductivity compared to other COF linkages β-ketoenamine-linked COFs still suffer from low absolute conductivity compared to traditional semiconducting chemiresistors such as metal oxides or carbon-based materials. This often challenges resistance measurements using conventional and cost-effective electrical circuits. To address this, post-synthetic metalation strategies have been explored, utilizing the functional groups of β-ketoenamine-linkages as coordination sites to simultaneously enhance charge transport and gas adsorption. For example, Mei et al. fabricated a Zn-coordinated TFP-BPy COF via interfacial and photochemical methods, where Zn²⁺ was loaded onto both β-ketoenamine and BPy sites (Fig. 4 (d)) [64]. The resulting Zn-TFP-BPy COF exhibited a substantial decrease in resistance (~6 GΩ), a significantly enhanced NH₃ response (S_NH3 = 1,043% to 50 ppm), and fast response/recovery times (55/111 s), while the pristine TFP-BPy COF showed negligible gas response and extremely high resistance (>100 GΩ) (Fig. 4 (e)). The authors revealed that Zn²⁺ coordination at the BPy site contributes to an electron-donating structure, significantly enhancing electron density in the n-type semiconducting TFP-BPy COFs. Meanwhile, the Zn²⁺ coordination at β-ketoenamine linkage sites leads to stronger and more reversible NH₃ adsorption. Ye et al. reported the integration of a TFP-PDA-1 COF with a halide perovskite (Cs₂PdBr₆) to improve its electrical conductivity and activate NO₂ sensing [27]. The Cs₂PdBr₆ acted as electronic glue, forming Pd⁴⁺ coordination bonds with both carbonyl O and imine N in the β-ketoenamine linkage (Fig. 4 (f)). This coordination significantly reduced the boundary resistance between the COF crystallites, lowering the overall sensor resistance by two orders of magnitude. The TFP-PDA-1/Cs₂PdBr₆ COF exhibited a significantly high NO₂ response (S_NO2 = 110.3 to 2 ppm) and outstanding selectivity over interfering gases such as NO and SO₂ (response ratio >70), whereas pristine TFP-PDA-1 showed negligible gas responses (Fig. 4 (g)).

3.3 Triazine-linked COF chemoresistors

Covalent triazine-based frameworks (CTFs), first reported by Kuhn et al. in 2008, feature a well-defined architecture composed of aromatic C=N linkages (triazine units) and offer excellent chemical stability and high nitrogen content [32]. Tao et al. demonstrated the room-temperature NH₃ sensing capability of CTFs (CTF-A-1) synthesized via the ionothermal trimerization of 1,4-dicyanobenzene (Fig. 5 (a)) [65]. The developed sensor exhibited high sensitivity (S_NH3 = ~4.17% to 1 ppm) and reversible behavior, with response/recovery times of ~100/~420 s (Figs. 5 (b) and 5 (c)). The sensing ability was attributed to strong n-π interactions between the lone pair electrons of NH₃ and the π-conjugated, electron-deficient triazine rings, which promote partial charge transfer to the framework. Similarly, Niu et al. prepared a series of P₂O₅-catalyzed CTFs (pCTFs) and evaluated their NH₃ sensing performance at room temperature [66]. As the condensation temperature of 1,4-dicyanobenzene increased, the nitrogen content and triazine ring density decreased, resulting in a diminished NH₃ response, thereby highlighting the crucial role of triazine units in NH₃ detection (Fig. 5 (d)). This trend was consistently observed when the monomers 1,4-dicyanobenzene (for CTF and pCTF) and 1,3,5-benzene tricarboxamide (for pCTF-3) were varied to modulate the triazine ring density (Figs. 5 (e) and 5 (f)). DFT calculations further supported this mechanism by confirming that NH₃ molecules preferentially adsorb onto triazine rings, donating electrons and narrowing the bandgap, which explains the observed decrease in resistance. Triazine-based frameworks have also shown selective detection of NO₂. Yang et al. exfoliated a CTF synthesized via the triflic-acid-catalyzed cyclotrimerization of 1,4-dicyanobenzene into CONs (Fig. 5 (g)) [67]. The sensor exhibited the highest response toward NO₂ (S_NO2 = 505% to 1 ppm), while the responses to other gases were quite low (S_NH3 = 32% and S_H2S= 9.8% to 1 ppm) or negligible (Figs. 5 (h) and 5 (i)). The high sensitivity and selectivity toward NO₂ are attributed to the intrinsic periodic pore structure of the 2D NSs, which facilitates gas diffusion and interaction with the active sites. Similarly, Niu et al. reported a viologen-based CTF that exhibited excellent NO₂ sensing performance, with a 43% response to 50 ppm NO₂ [68]. This superior sensing capability was attributed to the presence of abundant active sites within the viologen units, which promoted the effective adsorption of NO₂ molecules.

Fig. 5.

Gas sensing performance of triazine-linked COFs. (a) Structure of CTF-1-A COF. (b) Response of CTF-1-A COF toward varying concentrations of NH3. (c) Dynamic sensing response of CTF-1-A COF toward 100 ppm NH3. Reprinted with permission from Ref. [65], Copyright (2014) Royal Society of Chemistry. (d) Responses of various pCTFs sensors synthesized at different condensation temperatures to 50 ppm NH3. (e) Dynamic sensing responses toward 50 ppm NH3 and (f) calculated N contents of CTF, pCTF, and pCTF-3. Represented with permission from Ref. [66], Copyright (2020), Elsevier. (g) Structure of T-2DP COF. (h) Repetitive sensing response of T-2DP toward 1 ppm NO2. (i) Response of T-2DP toward 1 ppm of various analyte gases. Represented with permission from Ref. [67], Copyright (2019), American Chemical Society.

3.4 Others

Imide bond (O=C–N–C=O)-based COF, as demonstrated by Yan and co-workers, exhibited excellent thermal stability along with a high surface area and ordered porosity, making it a promising candidate for chemoresistive gas sensing [69]. Thokala et al. synthesized a redox-active COF via solvothermal imidization of TAPA and NTCDA, followed by exfoliation into CONs (Fig. 6 (a)) [70]. While the bulk NTCDA-TAPA COF showed a negligible response to H₂, the NTCDA-TAPA CON exhibited a significant response of 30.7% (10,000 ppm H₂), with response/recovery times of 4.5 and 3.9 s, respectively (Fig. 6 (b)). DFT calculations indicated that the carbonyl oxygens of the NDI moiety serve as favorable adsorption sites for H₂ (binding energy = –2.2 kcal mol^–1), while exfoliation improves gas accessibility and facilitates interaction with the active sites. Both factors synergistically contribute to the enhanced sensing performance (Fig. 6 (c)). Similarly, Gao et al. fabricated a freestanding TPTCDA-THSTZ membrane for the selective detection of trimethylamine (N(CH₃)₃, TMA). The sensor exhibited “a response of 3.47 to” 40 ppm TMA at room temperature, with response and recovery times of 96 and 63 s, respectively [71]. The sensor exhibited marked selectivity toward TMA, with a response 5.5–22 times higher than that of the other interfering gases. Insitu FTIR spectroscopy revealed that the sensing reaction relied on adsorbed oxygen species-mediated charge transfer facilitated by hydrogen bonding between the carbonyl oxygen of TPTCDA-THSTZ and the amino hydrogen of TMA.

Fig. 6.

(a) Schematic illustration of the structure and fabrication process of NT-COF and NT-CON. (b) Dynamic H2 sensing transient of the NT-CON sensor. (c) Molecular electrostatic potential plot of the optimized binding site in NT-COF. Represented with permission from Ref. [71], Copyright (2023), Royal Society of Chemistry. (d) Response transients of COF-5 sensors with different morphologies toward various concentrations of NH3. (e) Proposed NH3 sensing mechanism of COF-5 NSs. Represented with permission from Ref. [73], Copyright (2023), Elsevier. (f) Response and recovery times of the HMP-TAPB-1 sensor exposed to 50 ppm NH3 at 47% RH. (g) Cyclic sensing transient of HMP-TAPB-1 sensor. (h) Response of HMP-TAPB-1 sensor to various analytes. Represented with permission from Ref. [74], Copyright (2018), Royal Society of Chemistry.

Boron-based linkages such as boronate esters and boroxines were among the first to be employed for constructing crystalline COFs because of their dynamic reversibility, enabling the formation of well-ordered structures despite their limited hydrolytic stability [17,72]. For instance, Yang et al. fabricated a series of COF-5-based NH₃ sensors by modulating the nanostructure, nanoparticles, nanorods, and NSs via controlling the HHTP concentration, which governed the relative rates of lateral and longitudinal growth [73]. The nanosheets exhibited the highest sensitivity (S_NH3 = 3.13 to 10 ppm) at 60°C, which attributed to their enlarged surface area and efficient gas accessibility (Fig. 6 (d)). The strong Lewis basicity of NH₃ enables selective interactions with the boronate ester linkages, which serve as adsorption sites (Fig. 6 (e)).

Nitrogen-rich heterocyclic linkages other than triazine, such as heptazine (C₆H₃N₇) and phenazine (C₁₂H₈N₂), have also been incorporated into COFs, offering distinct electron-deficient frameworks favorable for chemoresistive gas sensing. Sharma et al. developed a heptazine-based COF (TCH-TAPB-1) for room-temperature NH₃ sensing, achieving a high response of 16.64 to 50 ppm NH₃ with fast response/recovery times (65/9 s) and selectivity exceeding 5.36 (Figs. 6 (f) and 6 (g)) [74]. The strong affinity toward NH₃ was attributed to donor–acceptor interactions between the electron-rich NH₃ and the electron-deficient heptazine moiety, along with the small molecular size of NH₃, which enabled efficient diffusion into the porous structure of the COF. To enhance sensing performance, metal centers, such as Ni have been incorporated into COFs, enabling both redox activity and selective analyte coordination. Mirica and co-workers synthesized phenazine-linked COF-DC-8 from NiOAPc and TOPyr building blocks, which exhibited an outstanding chemoresistive performance toward various gases [26]. The sensor showed responses of S_NH3 = 39%, S_H2S = 62%, S_NO = 3,939%, and S_NO2 = −6,338% to 40 ppm, with corresponding limits of detection as low as 70 ppb (NH₃), 204 ppb (H₂S), 5 ppb (NO), and 16 ppb (NO₂) (Fig. 6 (h)) Electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) revealed that the sensing behavior originated from charge transfer between the analytes and Ni-phthalocyanine units within COF-DC-8, involving weak and reversible interactions for NH₃ and redox-driven processes in the case of H₂S, NO, and NO₂.

4. SUMMARY AND OUTLOOK

Recent advances in the development of robust, stable, and highly porous COFs, coupled with progress in chemical modification and a fundamental understanding of their physical and chemical properties, have paved the way for their application in chemoresistive gas sensing. The distinct characteristics of COFs, including their highly ordered porous structures and large surface areas, facilitate efficient surface reactions with the analyte molecules, making them promising candidates.

Candidates as gas-sensing materials. In this review, we provide an overview of synthetic strategies for COF powders and thin films and summarize the recent progress in COF-based chemoresistors with various types of linkages. The representative gas-sensing performances of COF-based sensors are listed in Table 1. Although significant achievements have been made in this research field, several key challenges remain to be addressed to realize the practical and reliable use of COF-based gas sensors.

Table 1.

The summary of the sensing properties of COF-based chemoresistive gas sensors (*: calculated detection limit, **: experimental detection limit).

First, the inherently insulating nature of covalent bonds within 2D COFs restricts charge transport primarily to π-π stacking between adjacent layers, limiting the effectiveness of signal measurements (e.g., resistance, current, and conductivity) through conventional and cost-effective electronic circuits. Second, the overall gas-sensing performance of COFs, particularly in terms of gas response magnitude, sensitivity, stability, and signal reproducibility, requires further improvement compared to conventional sensing materials. Moreover, the selectivity of current COF-based sensors is restricted to a limited range of analytes, primarily NO₂ and NH₃. Therefore, expanding the selectivity to a broader range of gases is critical. Third, challenges remain in the scalable synthesis of COFs and their integration into practical device platforms, as most reported COFs have been synthesized under solvothermal conditions, which are time-consuming and unsuitable for large-scale fabrication.

To overcome these challenges, several promising strategies have been proposed: (1) Developing synthetic methods that reduce defects and improve framework crystallinity; (2) Incorporating electron-linking units to enhance intrinsic electrical conductivity; (3) Controlling COF morphology and film thickness to improve analyte accessibility and diffusion; (4) Introducing post-synthetic modifications or combining COFs with catalysts to enhance analyte adsorption and promote selective surface interactions; (5) Exploring scalable fabrication techniques, such as CVD. COFs are expected to evolve into promising platforms for next-generation gas sensors due to their outstanding structural tunability, chemical diversity, and potential for functionalization. We hope this review provides valuable guidance and insights for advancing the practical development of high-performance COF-based chemoresistive sensors.

Acknowledgments

This study was financially supported by the Cooperative Research Program for Agriculture Science and Technology Development (Project No. PJ01706703) of the Rural Development Administration, Republic of Korea. This work was also financially supported by the Nano & Material Technology Development Program (RS-2024-00405016) through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT, Republic of Korea. The Inter-University Semiconductor Research Center and Institute of Engineering Research at Seoul National University provided research facilities for this work.

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