In the adsorption process of COFs materials for heavy metal ions, the absorption mechanisms include coordination bonding/chelation effect^[26],electrostatic interactions^[27],hydrogen bonding, and ion exchange^[28].Among these, COFs materials modified with heteroatom-containing functional groups such as —SH, —NH₂, and —COOH primarily function through coordination chelation. Specific examples are shown in Table 2.While the adsorption mechanisms do play a certain role, they are also influenced by external environmental conditions. Factors such as solution pH, temperature, and ionic strength can significantly affect the adsorption performance^[21].Additionally, the morphology of COFs materials themselves is another critical factor. Due to the diverse morphologies of COFs materials, including nanoparticles, thin films, and fibers, their selectivity for adsorbing heavy metal ions varies accordingly.

Wang et al.^[38]utilized COFs materials in the form of nano-fiber clusters, which exhibit good crystallinity and thermal stability, to achieve stable adsorption and removal of copper ions. Compared with structures such as thin-layer fibers, COFs materials with larger surface areas and shorter mass transfer distances are of greater significance for effectively adsorbing heavy metal ions. Therefore, Liu et al.^[39]proposed a simple strategy for transitioning from an amorphous to a crystalline state, which effectively enhances conductivity and pore area while preserving the original structure of hollow COFs materials. This approach gives them advantages in large-area contact and rapid adsorption, enabling effective adsorption and detection of Pb²⁺,Cu²⁺,and Hg²⁺. It is precisely the morphological diversity and designability of COFs materials that endow them with unique selective characteristics in the field of heavy metal ion adsorption and removal, offering more possibilities for precise and efficient detection of heavy metal ions.

COFs materials possess a large specific surface area, good conductivity, and abundant active sites. Introducing them into electrochemical sensors opens up new possibilities for enhancing sensor performance. For instance, modifying electrode surfaces with COFs can significantly improve the performance of electrochemical sensors used for detecting heavy metal ions. Mirzaei et al.^[40]developed a selective electrochemical sensor based on glassy carbon electrodes (GCEs) modified with covalent organic frameworks (COFs) and carbon black (CB), achieving simultaneous detection of Zn²⁺,Cd²⁺,Pb²⁺,and Hg²⁺, with detection limits of 0.003, 0.002, 0.001, and 0.0003 nmol/L, respectively. Wang et al.^[41]modified COFs materials with thiol groups and subsequently employed square-wave anodic stripping voltammetry to selectively determine lead(II). This method exhibits excellent precision, a wide linear range (0.05~20 ng/mL, r = 0.991), and a low detection limit (0.015 ng/mL), with the detection limit being lower than the World Health Organization's permissible level of lead(II) in drinking water (10 ng/mL).

Through careful overall design, COFs materials can create multifunctional detection systems in conjunction with fluorescent monomers, meeting diverse detection needs. Chen et al.^[42]utilized a pre-designed O, N, O′-chelating site as a COF-fluorescent sensor, enabling simultaneous detection and adsorption of Fe³⁺(with a detection limit of 3.07 μmol/L). It can also serve as a gas storage adsorbent and a volatile organic compound (VOCs) scavenger. Moreover, in practical applications, using COFs as fluorescent sensing materials does not require complex pretreatment steps or stringent experimental conditions, significantly enhancing the convenience and operability of detection^[43]. Xiu et al.^[44]combined tetraphenylethylene with COF materials featuring a unique pore structure to prepare a covalent organic framework (COF-DHTA), the synthesis scheme of which is shown in Figure 1A. This material exhibits high sensitivity and a good linear relationship in the detection of Al³⁺, with a detection limit as low as 0.93 μmol/L.

Surface-enhanced Raman scattering (SERS) offers advantages such as high sensitivity, non-destructive and in-situ analysis, and ultra-trace analysis at the single-molecule level. Additionally, its rapid and convenient analysis makes SERS highly promising for the sensitive detection of metal ions^[45]. The introduction of COFs further enhances detection efficiency, reduces analytical costs, and facilitates the detection of heavy metal ions that are difficult to detect using conventional methods. The porous structure of COFs can eliminate uneven molecular adsorption, block large molecules from entering, and minimize interference from non-target substances, thereby endowing the SERS substrate with high stability and strong anti-interference capability. Gai et al.^[46]developed an Ag/Ag₂O-COF (covalent organic framework) composite SERS substrate for detecting uranyl ions in environmental aqueous solutions. The schematic diagram of this detection method is shown in Figure 1B. This method achieves a limit of detection (LOD) of 8.9×10^-10 mol/L for trace analysis of uranyl ions, which is lower than all previously reported direct detection methods. Shu et al.^[47]utilized COF materials with high catalytic activity and stability, combined with a novel peptide-mediated nanocatalysis, to achieve ultra-trace SERS detection of Cu²⁺. The detection scheme is illustrated in Figure 1C, and an ideal linear range of 0.005~0.115 nmol/L was obtained.

Colorimetry is a method based on the selective absorption of light by substances, causing solutions to exhibit specific colors. It determines the content of the component being measured by comparing or measuring the depth of color in colored solutions. This method offers advantages such as good selectivity, relatively high sensitivity, visual observation capability, and applicability to various matrices^[48]. In analytical studies combining COFs materials, their roles in metal ion detection mainly include the following three aspects: (1) serving as color-developing reagents; (2) acting as enzyme-mimetic catalysts; and (3) functioning as supports for metal nanoparticle catalysts^[6,49]. These three approaches fully leverage the advantageous characteristics of COFs materials. When used as color-developing reagents, they can enhance the color intensity or changes in absorbance of chromophores. As enzyme-mimetic catalysts, they can overcome the limitations of natural enzymes and improve detection sensitivity. Zhang et al.^[50]designed a colorimetric strategy based on two-dimensional covalent organic frameworks, achieving excellent photocatalytic performance and nanozyme activity, thus providing a new approach for the sensitive detection of uranium ions. When used as catalyst supports, they can significantly enhance catalytic activity and stability, reduce detection costs, and ensure long-term stable operation of colorimetric assays. Additionally, through rational structural design and functional group modification, COFs materials can achieve selective binding to heavy metal ions. For instance, Xiu et al.^[51]prepared a covalent organic framework (TPDQ-COF) modified with diaminanthraquinone, which, as a colorimetric sensor, achieved satisfactory results in the quantitative detection of Fe²⁺ and Fe³⁺, with detection limits of 6.91×10^-7 mol/L and 7.14×10^-7 mol/L, respectively, and demonstrated good recovery rates in real samples.

Organic pollutants in the environment are often present at low concentrations. The designed COF materials possess outstanding adsorption and treatment capabilities, effectively enriching low-concentration organic pollutants in the environment and creating favorable conditions for subsequent precise detection using modern instruments, thereby enhancing detection sensitivity. Meanwhile, using COF materials for enrichment can concentrate target pollutants, achieving adsorption and removal of organic contaminants. The adsorption mechanism is similar to that for heavy metal ions, involving electrostatic interactions, hydrophobic interactions, and hydrogen bonding^[56].For instance, Zhang et al.^[59] utilized the synergistic effects of electrostatic and hydrophobic interactions of their designed COF material to achieve efficient removal of per- and polyfluoroalkyl substances (PFAAs), which are persistent and bioaccumulative. In terms of supporting instrumental detection, Zhang et al.^[65] employed an in-situ method to prepare a novel paper-based material based on covalent organic frameworks (COFs). This material, characterized by its flexibility and stability, enabled effective detection and identification of polycyclic aromatic hydrocarbons in real corn oil samples using paper spray ionization mass spectrometry (PSI-MS), with a limit of detection (LOD) for phenanthrene of 0.50 ng/μL.

In addition, the selective enrichment capability of COFs materials can effectively reduce interference from various non-target substances that may be present in complex environmental samples, thereby enhancing the accuracy and reliability of detection. Yang et al.^[66]also achieved satisfactory results in detecting polycyclic aromatic hydrocarbons using COFs materials. They synthesized a novel COFs material via amide coupling reaction, which was used as a coating for steel fibers in solid-phase microextraction of PAHs. The final detection results showed a wide linear range (0.2 ng/L~2 μg/L), low limits of detection (0.29~0.94 ng/L, S/N = 3), and high enrichment factors (EFs, 819~2420). Yan et al.^[67]prepared porous carbon based on COFs materials, which enhanced the extraction capability for polycyclic aromatic hydrocarbons. Combined with gas chromatography-mass spectrometry (GC-MS) technology, the LOD was as low as 3.12~8.55 μg/kg, and excellent reliability was demonstrated in real samples. Wang et al.^[69]A new type of triazine-based COFs material, named SCAU-1, was prepared by hydrothermal method, and SNW-1 was also synthesized as a comparative material. To investigate their adsorption properties, five sulfonamides (SAs) and four tetracyclines (TCs) antibiotics were selected for detection. Using UPLC-MS/MS analysis, the proposed method exhibited low LOD, a wide linear range, and good repeatability. Lu et al.^[71]A functionalized magnetic covalent organic framework (nitro-Fe₃O₄@COF-(NO₂)₂) was employed for solid-phase extraction (MSPE) of six neonicotinoid pesticide residues in vegetable samples, achieving high enrichment factors of 170~250. Simultaneously, HPLC detection yielded a good linear range (0.02~0.05 ng/mL), with detection limits (S/N = 3) ranging from 0.02 to 0.05 ng/mL. The detection procedure is illustrated in Figure 2.

In this process, both the morphology and functional groups of COF materials play an indispensable and critical role. From a morphological perspective, structures such as high-surface-area nanosheets and porous spheres provide more adsorption sites; for instance, the two-dimensional structure of nanosheets increases the contact area with pollutants. An appropriate pore size distribution and well-organized pore channels create a molecular sieving effect, offering smooth diffusion pathways for pollutants and enhancing adsorption selectivity and kinetic performance. From the perspective of functional groups, their chemical properties determine the interactions with pollutants. For example, amino and carboxyl groups can achieve specific adsorption through hydrogen bonding, electrostatic interactions, and ion exchange; aromatic functional groups can engage in π-π stacking with pollutants possessing conjugated structures; and hydrophilic/hydrophobic groups regulate the adsorption capacity for pollutants of varying polarity. These factors synergistically influence the treatment and enrichment efficiency of COF materials for organic pollutants.

Electrochemical sensors face several challenges when detecting organic pollutants. Different organic pollutants exhibit varying electroactivity, and some may be difficult to detect directly via electrochemical methods, requiring complex pre-treatment or indirect detection approaches. In practical applications, certain natural organic substances can even interfere with detection. Moreover, for emerging organic pollutants with unique structures, existing electrode materials may not effectively facilitate their electrochemical oxidation or reduction. Therefore, combining COFs materials with electrochemical sensors is an ingenious approach to enhance their overall performance in detecting organic pollutants^[76]. Wang et al.^[77]employed an electroactive COF Tab-Dva nanofiber rich in vinyl groups, which enables accurate and reproducible detection of O, O-dimethyl-O-2,2-dichlorovinyl phosphate (DDVP). This pesticide sensor based on COFs not only operates at a low working potential but also effectively improves selectivity, avoiding the influence of large background currents, thus providing a new method for accurate pesticide detection with high accuracy, high reproducibility, and strong stability. To further enhance the stability of electrochemical sensors and fully leverage material advantages, Sun et al.^[78]synthesized COFs materials using a "one-pot" method with 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthalaldehyde (TPA) as monomers, followed by self-assembly of the COFs onto carbon nanotubes. The resulting COF@NH₂-CNT hybrid material boasts a large surface area and excellent conductivity. Subsequently, 8 μL of COF@NH₂-CNT dispersion was coated onto a polished glassy carbon electrode (GCE), dried at room temperature to obtain the COF@NH₂-GCE, which demonstrated outstanding analytical performance in the detection of the antibiotic furazolidone, featuring a wide detection range and a low detection limit (approximately 77.5 nmol/L). Li et al.^[79]first synthesized a stable covalent organic framework (COFs)-based material with strong electrochemiluminescence (ECL) through the condensation reaction between perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and melamine (MA), coupled with a CRISPR/Cas12a-mediated electrochemical-biosensor (PTCA-COF). Its 3D surface image is shown in Figure 3A, demonstrating satisfactory performance in detecting neonicotinoid insecticide dinotefuran. The detection schematic is illustrated in Figure 3B, with a LOD of 2.7 pmol/L (S/N = 3) and a linear correlation coefficient (R²) reaching 0.9994.

Some substances in the environment may alter fluorescence intensity through light absorption, energy transfer, and other mechanisms. Additionally, changes in environmental factors such as pH and temperature can affect the dissociation state or molecular structure of fluorescent materials, thereby altering their fluorescence properties and impacting the accurate detection of target organic pollutants. To overcome these limitations, researchers have continuously explored and improved the performance of fluorescent sensors. By combining with COF materials, the sensor's structure and design have been optimized to a certain extent, enhancing its sensitivity, stability, and applicability in detecting organic pollutants^[80].Yu et al.^[81]functionalized COFs containing bipyridine (Dpy-TFPB-COF) with lanthanide europium (III). When tetracycline (TC) is present, TC displaces water molecules and coordinates with Eu³⁺, generating a significant Eu³⁺ fluorescence signal, while the original fluorescence signal of Dpy-TFPB-COF remains. The schematic diagram of material synthesis and detection is shown in Figure 3C.Yang et al.^[82]covalently linked hydrazone-linked fluorescent covalent organic framework (BATHz-COF) with N-acetyl-L-cysteine (NALC) via a "thiol-ene" click reaction and introduced carboxyl groups to enhance fluorescence intensity, thus fabricating a highly selective and reusable COF-NALC fluorescent sensor. This sensor achieved satisfactory results in the quantitative detection of p-nitrophenol (p-NP), with a linear range of 5–50 μmol/L and a detection limit of 1.46 μmol/L.

The reproducibility of SERS signals may be affected by various factors. In particular, when analyzing organic pollutants, certain organic contaminants may exhibit weak interactions with the substrate or have small Raman scattering cross-sections, limiting detection sensitivity. Additionally, traditional SERS substrates may suffer from issues such as uneven nanoparticle distribution and aggregation during preparation, leading to significant variations in enhancement effects at different locations and thus affecting signal reproducibility. Although some novel substrates have improved stability to a certain extent, they can still be influenced by physicochemical factors in complex detection environments, resulting in reduced SERS activity. For instance, noble metal particles may detach from flexible substrates during the detection process, compromising the stability of the detection signal^[83-84]. Therefore, introducing COF materials to address the shortcomings of SERS substrates has become one of the widely focused approaches among researchers. Xie et al.^[85]employed a novel AuNPs/COFs composite material induced by photo-reduction deposition as a highly sensitive and reproducible SERS-active substrate, enabling clear SERS signals for low-concentration antibiotics that were previously difficult to detect. This substrate demonstrated excellent sensitivity and reproducibility in the analysis and detection of four macrolide antibiotics, with LODs of 3.30×10^-11, 3.43×10^-10, 1.10×10^-10, and 5.78×10^-11 mol/L, respectively, and exhibited a good linear relationship within the range of 10^-10~10^-3 mol/L.

COFs materials possess a regular structure and uniform pores, providing more homogeneous adsorption sites and environments. This allows for a more even distribution of organic pollutants to be measured on the substrate, increasing the number of molecules available for effective detection, thereby reducing signal fluctuations and enhancing the SERS signal, ultimately improving the reproducibility of SERS detection. Additionally, they can effectively prevent oxidation and corrosion of the substrate in complex environments, maintaining the structural and performance stability of the substrate^[86].Niu et al.^[87] coated the surface of silver nanowires (AgNW) with COFs materials to fabricate AgNW@COF hybrids. By utilizing the scouring effect induced by fluid flow and the regular pores of COFs, they eliminated interference from large molecules, achieving efficient purification of organic pollutants as well as real-time identification and detection of pollutant types and concentrations.

Colorimetric detection of organic pollutants struggles to meet the requirements for trace-level analysis, and environmental changes such as temperature and pH can affect color development, reducing the accuracy of results^[88]. However, functionalized COFs materials used for colorimetric detection can effectively adsorb organic pollutants, producing distinct color changes that enhance accuracy and sensitivity. Their excellent chemical stability also ensures reliable detection results under various environmental conditions. Afshari et al.^[89] synthesized a nitrogen-rich triazine-based COF material via hydrothermal method, which exhibits exceptionally strong adsorption capacity. By leveraging hydrogen bonding and π-π stacking interactions, this material has achieved remarkable success in adsorbing and removing the fast scarlet 4BS textile dye. Detection using a smartphone-based colorimetric analyzer yielded results consistent with those obtained from UV-Vis spectrophotometry, providing a new approach for on-site colorimetric analysis of textile wastewater and other samples. Zhou et al.^[90] developed a dual-functional smart material based on a methoxy-containing covalent organic framework (COF-OMe), fully exploiting the advantages of COFs materials. On one hand, the light absorption capability of COFs materials enables excellent colorimetric performance; on the other hand, COF-OMe possesses a porous structure that mimics photocatalytic peroxidase activity, allowing it to remove organic pollutants through the synergistic effects of physical adsorption and photodegradation. The detection scheme is illustrated in Figure 4.

COFs materials exhibit significant advantages in the adsorption and removal of gaseous pollutants. With their high specific surface area and pore structures that are abundant and precisely tunable, COFs can achieve a specific surface area of 1000 to 5000 m²/g, providing numerous adsorption sites. By matching pore sizes to gas molecule dimensions, they can achieve high adsorption capacities and efficient adsorption of specific gases. Their chemical structures are designable and easily functionalizable; by introducing specific functional groups, they can selectively adsorb target gases and even undergo chemical reactions with certain gases for deep purification.

Meanwhile, COFs materials exhibit excellent chemical and thermal stability, with strong covalent bonds ensuring structural integrity across diverse chemical environments and high temperatures, allowing them to stably perform adsorption functions under complex and harsh conditions. Additionally, they possess good processability, enabling fabrication into various shapes to suit different scenarios, and can be easily regenerated and reused through physical or chemical methods once saturated, effectively reducing treatment costs^[104]. In terms of adsorbing certain gases, COFs demonstrate high efficiency and selectivity. For instance, Li et al.^[105]designed a porous framework NAT-COF, achieving highly efficient and specific adsorption of hydrocarbons and carbon dioxide. However, currently, in the field of adsorption and separation of gaseous pollutants, applications of COFs materials are relatively limited compared to research on heavy metal ions and organic pollutants. The adsorption mechanisms of COFs are primarily based on interactions between specific functional groups on their surfaces and gaseous analytes. Beyond this, there are several other notable characteristics. The high fluidity of gas molecules means that the pore structure and porosity of COFs significantly influence their diffusion rates and adsorption kinetics; three-dimensional interconnected pores and high porosity facilitate rapid gas diffusion and adsorption. Moreover, the selective adsorption of different gaseous pollutants relies on more complex mechanisms, involving not only conventional factors but also physical properties such as polarizability and dipole moment of gas molecules—COFs exhibit stronger adsorption capacity for VOCs with higher polarizability. When treating gases, COFs often require good reversible adsorption performance, allowing desorption and regeneration under mild conditions for repeated use. Furthermore, the charge distribution on the surface of COFs materials also affects the adsorption of gaseous pollutants and can be adjusted by modifying or altering synthesis conditions.

Gas sensors detect gas pollutants by inducing changes in their electrical, optical, or acoustic properties through physical or chemical interactions with the target gases, and then converting these changes into interpretable and analyzable signal outputs. Gas sensors offer numerous advantages, such as real-time, continuous monitoring of gas pollutant concentration changes and rapid responsiveness^[106]. They typically exhibit high sensitivity, capable of detecting extremely low concentrations of gas pollutants, accurately capturing even minor concentration variations. Additionally, gas sensors are compact in size, making them easy to install and carry, suitable for various complex detection scenarios. Moreover, they have the advantages of relatively low cost, ease of maintenance, and simple operation. Due to these benefits, gas sensors are widely used by researchers for detecting gaseous pollutants in the environment. When combined with COFs materials, these advantages are further enhanced. Choi et al.^[107] integrated COFs materials with graphene oxide to form a composite material, which demonstrated superior gas adsorption performance compared to pristine graphene oxide, with sensitivity improved by 2.7 times and response time significantly reduced from 234 s to 32 s. The high selectivity of COFs materials enables more accurate identification of specific gas pollutants, reducing false positives; their strong adsorption capacity also enhances sensor sensitivity, allowing detection of lower pollutant concentrations. Meanwhile, the excellent stability of COFs materials extends the sensor's service life and reduces usage costs.

Li et al^[108]have designed a fluorochromic COFs material, as shown in Figure 5A, where pyridine units are modified into larger pyridine frameworks, creating a visual fluorescence-colorimetric molecular "trigger" for the online and visual monitoring of volatile amines, while also enabling regeneration. As illustrated in Figure 5B, this effectively prevents porosity loss and achieves higher sensor-molecule recognition conversion efficiency, resulting in a millisecond response time (0.65 s).