Portable sensor devices based on multifunctional framework materials: Recent advances for food safety assurance by on-site detection

Food contaminants are often present in trace or ultra-trace amounts in complex sample matrices, making it difficult to achieve accurate measurement by conventional instrumental analysis. Based on this, framework materials have been widely used as carriers for the enrichment and identification of targets in sensors due to their ability to interact with food contaminants through the pore confinement effect, which can interact with food contaminants through the pore confinement effect (Li et al., 2024b; Wu et al., 2024a; Zhu et al., 2023). Moreover, the inherent fragility of enzymes restricts their application under complex sensing conditions. Stable framework materials, with highly tunable structures, have emerged as robust carriers for enzyme immobilization. Wang and Liao (2021) summarized the latest advances in the use of MOFs and COFs as enzyme carriers and further emphasized the importance of designing and adjusting the framework pore sizes to better accommodate enzyme encapsulation, which offered new insights into the development of high-quality framework material capsules. Given the mild crystallization conditions of HOFs, Chen et al. (2022) proposed the HOFs biomimetic encapsulation strategy. Cytochrome c conformation was modulated by HOFs to confer peroxidase-like biocatalytic function, which highlighted the advantages of HOFs in regulating and stabilizing enzyme activity through nanotechnology. Additionally, framework materials serve as efficient carriers for encapsulating various guests, such as dye molecules or quantum dots, providing excellent fluorescence platforms for detecting food contaminants (Ruan et al., 2020). Likewise, encapsulation of metal nanoparticles, in the case of gold nanoparticles (AuNPs), silver nanoparticles (AgNPs), and platinum nanoparticles (PtNPs), in framework materials enhanced their electroactivity, which has been widely used in electrochemical sensors (Zhang et al., 2020a).

The ultra-high specific surface area of framework materials can provide extensive catalytic sites for reactions, and multiple catalytic effects can be achieved by adjusting their structures. Therefore, framework materials are widely employed as signal amplification tools in sensors. For example, the unsaturated coordination sites on the metal nodes in MOFs can function as Lewis acid catalytic centers, while the ligands can be modified with different functional groups to endow catalytic properties. Meanwhile, it was found that covalent triazine backbone materials in COFs were also widely used in catalysis due to their high carbon and nitrogen content, which offered a substantial number of active sites (Xu et al., 2024a). Although HOFs are less favorable for enzyme synthesis due to weak hydrogen bonding, it was shown that certain HOFs (such as HOF-100, HOF-101 and HOF-102) with pyrene units could simulate enzyme pockets and exhibited light-triggered oxidase-like activity (Tong et al., 2023). Recently, various nanoenzymes based on framework materials have been developed to detect food contaminants, including oxidase-like, peroxidase-like, catalase-like, and superoxide dismutase-like activities. For instance, Zhang et al. (2020b) presented an electrochemical sensor with Cu²⁺-anchored MOFs as an ideal catalyst for enzyme-free signal amplification to detect Hg²⁺ in dairy products. In contrast to natural enzymes, framework materials with oxidase-like activity could catalyze the oxidation of massive glucose, significantly amplifying the electrochemical signal. In addition, a hybrid nanozyme Fe₃O₄@ZIF-8@COF, with peroxidase-like activity, was applied with the sensitivity detection of phenol in the colorimetric sensor. The core-shell structure of the framework materials with excellent catalytic sites inside and a stable protective layer on the outside resulted in the sensor with good stability and easy recovery properties (Hou et al., 2023).

The tunable environment, pore size, and porosity of framework materials promote efficient electron conduction within the porous structure, which enhances their effectiveness in electrochemical applications (Daniel et al., 2022). However, MOFs generally exhibit poor electrical conductivity due to high charge transfer barriers and limited free carriers, hindering their diverse applications. To improve conductivity, researchers combined conductive nanomaterials, such as carbon nanomaterials, noble metal nanoparticles and conductive polymers, with MOFs to form conductive MOFs with superior electrical conductivity (Zhao, Bai, Bo & Guo, 2019). For example, Fang et al. (2019) incorporated reduced graphene oxide into Zr-based MOFs to enhance electroactivity and electron transfer, achieving sensitive ciprofloxacin (Cip) detection using the complexation reaction between Cu²⁺ and Cip. In comparison, some 2D COFs provide diverse transfer pathways for highspeed charge carriers due to their conjugated frameworks, interlayer π-electron clouds, and open pores. Huang et al. (2022) synthesized COFs exhibiting favorable electrical conductivity by utilizing the electroactive monomer 2,6-diamino anthraquinone as the structural unit for the detection of bisphenol A and bisphenol S. Moreover, porous COF-modified glassy carbon electrodes have been applied in electrochemical sensors due to their high specific capacitance and electrochemically active surface area (Pang et al., 2020). In addition, the abundant hydrogen bond in the structure of HOFs also provides unique proton transport pathways, making them potential candidates for electrochemical sensors. Currently, the high conductivity of HOFs has been leveraged for monitoring various food contaminants, including foodborne pathogens, heavy metal ions, and pesticide residues (Peng et al., 2023).

Based on the tunability of framework materials, different wavelengths and intensities of luminescence can be achieved by introducing luminescent centers, making them ideal candidates for light-emitting applications (Xu & Cheng, 2024). Luminescent MOFs, as a branch of MOF materials, have attracted wide attention in fluorescence sensing and photocatalysis. In general, the luminescence mechanism of MOFs can be divided into direct excitation of organic ligands, metal center emission, charge transfer, and loading of a variety of guest fluorescent substances, which offer diverse luminescent properties (Zhou, Han, Cuan & Zhou, 2022). For example, based on the property that organic ligands in MOFs could enhance the fluorescence of lanthanide ions, the Eu@1 with a good luminescence performance was constructed for the sensitive detection of 5-hydroxymethylfurfural in food (Jia & Yan, 2023). Compared to MOFs, the molecular bond rotations/vibrations and π-π stacking effects result in aggregation-induced quenching (ACQ), causing poor photoluminescence performance of most COFs in the solid state. (Haug et al., 2020). Research has shown that spatial separation of conjugated fluorescent chromophores and aggregation-induced emission (AIE) materials were effective strategies for achieving high fluorescence emission in COFs (Wei, Ze & Qiu, 2023). In recent years, HOFs have gained attention due to the flexibility of hydrogen bonds, which allow them to dynamically adjust and switch their structure and optical signals in response to varying environmental conditions. Most HOFs contain conjugated aromatic structures that promote photoluminescence UV excitation, making HOFs a prospective and tunable platform for constructing functional luminescent sensing materials (Xiong et al., 2024).

The porosity of framework materials allows for selective recognition of analytes through size exclusion. The analytes entering the pores can easily change the electronic structure of the framework material and coordinate with the active sites in its structure, thus changing its optical or electrochemical properties and achieving sensitive detection of analytes. For example, Yu et al. (2020) easily detected Hg²⁺ in pollutants by constructing allyl hydroxyl-functionalized COFs, given that functionalized COFs could provide specific receptor sites for Hg²⁺ and form a five-membered ring with Hg²⁺. In a similar study, Li et al. (2024c) prepared a novel hybrid material (COF@Eu) for the determination of tetracycline (TC) by loading Eu³⁺ into COF, where TC could displace the water molecules and coordinate with Eu³⁺ in COF through the antenna effect to emit red fluorescence. Further, integrated with a smartphone, rapid TC detection could be realized with a limit of detection (LOD) of 30 nM. In particular, the effective immobilization of recognition elements (antibodies, aptamers cells, etc.) on the framework materials is also crucial for target recognition in porous frameworks. On this basis, Li et al. (2022) used EDC/NHS to activate carboxyl-functionalized ochratoxin A (OTA) aptamer and further obtained Zr-MOF probes by amidation reaction. Ultimately, the sensitive monitoring of OTA in maize samples was achieved by a dual-channel readout strategy with excellent electrical and optical properties of Zr-MOF.

Test strips utilize impregnation, covalent linkage, or nanocomposite to uniformly distribute framework materials on the surface of the paper or penetrate into the porous structure of the paper substrate for the detection of the targets through the transport of reagents. Due to their portability, ease of operation, and low cost, test strips have emerged as an essential component of portable sensors. The excellent dispersion and tunability of framework materials provide stable catalytic activity and signal amplification in paper-based sensors. By effectively combining with test strips, high sensitivity, rapid response, and multiplex detection of the sensors can be achieved (Wang et al., 2024a). Given the urgent need for intelligent on-site detection of food contaminants, miniaturized test strips based on framework materials have great potential for POCT applications.

Apart from the paper-based devices discussed above, screen-printed electrodes (SPE) were also widely employed in electrochemical sensors by virtue of their low cost, ease of mass production, portability, and miniaturization. To enhance electrode performance and meet the demands of complex analyses, framework materials with good conductivity and flexibility are often employed as surface modifiers for SPE. Firstly, the high specific surface area and abundant functional sites of framework materials can increase the adsorption capacity of analytes. Secondly, the good electrical conductivity and catalytic properties of framework materials promote electron transport and accelerate the rate of electrochemical reactions, improving signal amplification. Hu et al. (2024) immobilized modified MOFs (CoCe-LDH) onto the surface of SPE for adsorption of antibodies and accelerated electron transfer, which endowed the sensors with unparalleled electrochemical performance (Fig. 2C). As the SPE transducer layer, the large specific surface area and good biophilicity of CoCe-LDH caused it to exhibit enhanced electron transfer effect. With the help of Bluetooth system, the portable, visual and efficient analysis of erythromycin in food could be achieved by inserting the test paper into an electronic device with a remarkably low LOD. The loading of COFs onto magnetic nanomaterials has been the topic of intensive research in recent years because it leads to the formation of magnetic probes with high adsorption capacity. For example, Xiao et al. (2022) fabricated a novel magnetic capture probe (m-COF@IgY) immobilized on SPE to detect Escherichia coli in complex food samples. The magnetic COF-based SPE provided a large specific surface area for the sensing system and the target enrichment was achieved by magnetic separation. As shown in Fig. 2D, an obvious potential-enhanced signal was obtained because m-COF@IgY and ferrocene boronic acid could co-recognize Escherichia coli to form a sandwich complex on the SPE. It was worth mentioning that the SPE-based electrochemical immunosensor could detect Escherichia coli in less than 20 min with LOD as low as 3 CFU/mL. Furthermore, molecularly imprinted magnetic COFs were also integrated into SPE for the rapid on-site detection of TC. The incorporation of COFs effectively enhanced the adsorption performance of the sensors, achieving sensitive detection over a wide linear range from 1×10^-10 g/mL to 1×10^-4 g/mL (Yang et al., 2022).

Hydrogels are 3D porous polymer networks formed by physical or chemical cross-linking of hydrophilic molecules. Compared to paper-based devices mentioned above, hydrogels offer significant advantages for creating portable sensors due to their high flexibility, deformability, and excellent stability, allowing easy integration into solid-state reaction systems. By integrating framework materials, hydrogels can be transformed into intelligent materials capable of recognizing external stimuli. Typically, framework material-functionalized hydrogels can bind directly to the target through hydrogen bonding, electrostatic or hydrophobic interactions, which induce visual optical changes, and achieve the qualitative analysis of the target (Xi et al., 2024). In addition, combining the superior optical properties of reticulated hydrogels with the powerful data processing capability of smartphones can realize the quantitative analysis of targets, which presents a promising foundation for building a portable sensing platform for food safety.

MOFs with high fluorescence emission properties have also been employed to prepare intelligent hydrogel probes for rapid visual detection of TC, as demonstrated by Chen et al. (2023). Functionalized MOF nanoprobes were immobilized in background interference-free agarose gels to construct portable hydrogel-responsive materials. The incorporation of TC changed the color of the hydrogel from green to red in less than 10 s, allowing for intelligent analysis through a color-recognition APP (LOD = 5.99 nM). This strategy detected trace TC residues in complex water samples and eggs on-site, showing great potential for monitoring food safety. Additionally, due to the water-holding capacity of hydrogels prolonging the residence time of volatile substances, Fig. 3B (i) demonstrates a color-changing hydrogel based on double-emitting MOFs for monitoring biogenic amines (BAs). The presence of BAs damaged the MOFs and inhibited the ligand-metal charge transfer and excited-state intramolecular proton-transfer processes, resulting in a color shift of the probe from orange-red to bright yellow. Ultimately, this strategy achieved the monitoring of the spoilage level of seafood products such as shrimp and fish. As illustrated in Fig. 3B (ii), the portable hydrogel integrated with a smartphone app by headspace preparation of the sensor sensitively detected trimethylamine over a concentration range of 10 to 50,000 ppb, providing a visual and intelligent assessment of seafood freshness (Jia et al., 2023b). Apart from the fluorescent sensor, a novel spherical colorimetric hydrogel sensor based on Prussian blue@Fe-COF@Au core-shell nanocomposite was constructed for visual monitoring of organophosphorus pesticides (OPs). The synthesis process of core-shell nanocomposite was portrayed in Fig. 3C (i). Due to the triplet-like peroxidase activity of the nanomaterials, the presence of pesticides altered the catalytic activity of the nano-enzymes and caused the reduction of oxidized 3,3′,5,5′-tetramethylbenzidine, with varying degrees of discoloration of the hydrogel spheres. Fig. 3C (ii) illustrates the preparation of the hydrogel sensor in detail and demonstrates its color change at different concentrations of OPs. As the reaction spread from the outer to the inner hydrogel sphere, higher concentrations of OPs resulted in less discoloration on the outside and a larger blue core, which allowed for easy detection with the naked eye, making the sensor highly portable. Remarkably, even after 140 days of storage, the hydrogel sensor maintained good stability, demonstrating its potential for on-site detection of OPs (Li et al., 2023d).

Gas monitoring plays a crucial role in controlling food contaminants. For instance, the release of volatile gases is often associated with food spoilage or deterioration (Andre et al., 2022), while the concentrations of carbon dioxide and oxygen correlate with the freshness of fruits and vegetables. Non-invasive smart tags can capture and enrich gas which serves as a portable on-site monitoring technology. The integration of framework materials into smart tags significantly enhances gas adsorption due to their high surface area and tunable porous structures. Moreover, the framework materials achieve highly selective detection of specific gases through surface functionalization, thereby improving the sensitivity and specificity of the smart tags. Notably, the chemical and thermal stability of framework materials ensures that smart tags remain functional in complex environments such as humidity and temperature, guaranteeing the long-term use of the sensors. In this context, the exceptional optical properties of framework materials facilitate on-site detection without direct contact with food samples, making smart tags a viable POCT platform.

Furthermore, the excellent gas adsorption capacity and affinity of framework materials make them widely used in colorimetric sensors as well through smart tags. Based on this, Kang et al. (2024) blended Cu-MOFs with starch/polyvinyl alcohol (SP), then cast the mixture into the film to create colorimetric smart tags responsive to BAs (Fig. 4B (i)). The composite film exhibited a significant color change from green to deep blue within 1 min upon exposure to ammonia, as the gas formed coordination complexes with unsaturated copper atoms in Cu-MOFs. After that, the SP composite films with different Cu-MOFs contents for real-time monitoring of shrimp freshness at 4 °C were shown in Fig. 4B (ii). The film with added Cu-MOFs changed its color from teal green to brown as the freshness of shrimp decreased, while the film without MOFs had almost no visible color. This indicated that the SP/Cu-MOFs composite film was specific for volatile ammonia, and the higher the Cu-MOFs content, the higher the sensitivity. Remarkably, after 40 days of storage in a constant temperature and humidity environment, the SP/Cu-MOFs smart tag maintained excellent color stability, showcasing its promising application. Considering that porous framework materials could enhance gas adsorption capacity, Wang et al. (2022) found that films loaded with MOFs exhibited stronger, deeper, and more stable colorimetric responses to volatile ammonia compared to films with the same effective dye content, thus providing a more effective strategy for monitoring fish spoilage. This phenomenon was demonstrated in Fig. 4C, where CPB represented the tag without MOFs while CPMB represented the tag loaded with MOFs. In addition to monitoring meat freshness, the freshness of cut fruits and vegetables is crucial due to their susceptibility to bacterial contamination. To enhance the mechanical properties and moisture resistance of smart tags, Zhang et al. (2024a) developed composite films based on Co-based MOFs as CO₂-responsive packaging materials for real-time monitoring of apple freshness. CO₂ formed during the storage phase of fruits and vegetables would further form carbonic acid and decompose into H⁺. The increased H⁺ load raised the concentration of acidic pigments, which led to a change in the color of the smart tags in composite films, permitting the tracking of fruit freshness (Fig. 4D). Notably, the incorporation of MOFs not only filled the voids in the composite films but improved moisture resistance, thermal stability, and pigment migration resistance, providing an effective strategy for food freshness monitoring. A similar study was constructed by Zhao et al. (2024), who proposed a flexible colorimetric sensor based on γ-cyclodextrin metal-organic frameworks (γ-CD-MOFs), achieving highly accurate prediction of fruit ripeness. Since the γ-CD-MOFs improved the loading capacity and sensitivity of the sensor to volatile organic compounds (VOCs), the array demonstrated excellent on-site detection capability and effectively avoided fruit spoilage.

Microfluidics is a technology that reacts small amounts of fluid on a chip. The technology has been called lab-on-a-chip because the chip can complete a series of complex experimental operations such as sampling, separation, reaction, and detection in a limited space. On the one hand, as highly ordered nanomaterials, framework materials offer efficient sample capture, signal amplification, and target detection in microfluidic chips, significantly enhancing detection sensitivity. Another, the highly integrated structure of microfluidic chips achieves precise positioning and utilization of framework materials, reducing sample and reagent waste, simplifying operational steps, and improving the efficiency and reliability of portable sensors (Xiang et al., 2024). Overall, the integration of multifunctional framework materials with microfluidic technology offers unparalleled design flexibility for microfluidic chips, paving the way for the development of all-in-one, intelligent portable detection devices.

Fluid transport can be achieved through capillary forces by hydrophobically patterning, stacking, and folding the paper. Based on this, low-cost, environmentally friendly, and convenient paper has been extensively utilized as a substrate material for microfluidic chips. According to the reports, several functional framework materials, have been incorporated into paper-based chips for on-site detection of food contaminants. For instance, a three-dimensional folded paper-based microfluidic analytical device (3D-μPAD) based on MOFs for chlorpyrifos (CPF) detection was developed by Liu et al (2024). The preparation process of the devices was schematically illustrated in Fig. 5B (i) and (ii). As can be seen, catalytic MOFs and upconversion magnetic nanoparticles (UCMPs) were connected by base complementary pairing and loaded on the detection region of the paper. Afterward, the folding of the paper caused the sample to flow from one layer to the next, thus executing the reaction continuously. As shown in Fig. 5B (iii), based on the excellent catalytic properties of MOFs and the fluorescence stability of UCMPs, the detection region changed from colorless to blue with increased fluorescence intensity in the presence of CPF. This dual-signal response exhibited high sensitivity, with a LOD of 0.028 ng/mL in the fluorescent sensor and 0.043 ng/mL in the colorimetric sensor, demonstrating the effectiveness of this portable paper-based microfluidic platform.

Heavy metal contaminations are widespread in ecosystems and human life, posing serious threats to human health and safety (Saidur et al., 2017). Most heavy metals are highly toxic, non-biodegradable, and can be continuously accumulated in organisms through the food chain. Once their concentration exceeds biological safety thresholds, the excess heavy metal ions will bind to protein sites within the body, causing systemic disorders and serious health consequences (Mukherjee et al., 2021). Framework materials are widely applied in monitoring heavy metal ions on portable sensing platforms due to their large specific surface area and high porosity for efficient capture and removal of heavy metal ions. In general, there are two main detection strategies for heavy metals using portable devices based on multifunctional framework materials.

The first strategy utilizes the interaction to trigger changes in the properties of the framework material to detect the target. Examples can be seen in the selective detection of Hg²⁺, Cd²⁺and Cu²⁺ (Diamantis et al., 2022; Zhang et al., 2022c). Inspired by these precedents, Chang and coworkers constructed CMC-MOF membranes to detect hexavalent chromium (Cr⁶⁺) on-site by coordinating zirconium-metal sites in MOFs with carboxyl groups in CMC. The fluorescence intensity of the CMC-MOF membranes gradually decreased with increased Cr⁶⁺ concentration, as shown by portable laser-induced fluorescence spectrometers (LIFs). Subsequently, it was confirmed that the fluorescence quenching was attributed to the energy transfer of the luminescent MOF by Cr⁶⁺ coordinating with the lone pair of electrons of the nitrogen atoms on -NH- and C=N-C. Under optimal conditions, this sensing platform exhibited high selectivity and sensitivity to Cr⁶⁺ through portable LIFs with a LOD as low as 3.72 ppb (Chang et al., 2024). Alternatively, the same strategy had been employed in COF-based test strips for Pb²⁺ detection. Interestingly, when COF test strips were positioned in PdCl₂ solution, a significant burst of fluorescence accompanied by color change from yellow to dark brown could be observed. Due to the combination of Pb²⁺ with the aryl group in COFs, the electrons were transferred from the conjugated skeleton to the empty orbitals. This destroyed the electron supply and acceptance system of the sensing system, leading to fluorescence quench. It was worth mentioning that the fluorescent COFs grown in situ on a paper substrate by the solvothermal method were highly stable in the range of pH 2-10, which conveniently and quickly achieved the sensitive detection of Pb²⁺ with a LOD of 0.29 μM (Lu et al., 2021). Owing to the low melting point and the nature of the weak interatomic force between metal and covalent bonds, bismuthum (Bi) has offered great tendency to interact with heavy metal ions, which is considered a potential electrode material for heavy metal ions detection. To enhance the charge transport capacity of electrodes and the utilization of Bi metal, the strategy of compounding Bi nanomaterials with framework materials has been explored. For example, a new type of BiCu metal-organic frames (MOF) derived carbon film encapsulating BiCu alloy nanoparticles (BiCu-ANPs) was developed and integrated into a potable electrochemical sensing system for real-time on-site detection of Pb²⁺, Cd²⁺ and Zn²⁺. Particularly, the formed BiCu_0.5-ANPs@CF significantly improved the electrocatalytic activity and stability, which was attributed to the strain effect and electronic effect induced by the encapsulation structure of the mixed metal and carbon framework (Yin et al., 2023). By the same strategy, Deng et al. (2024) employed bismuth metal-organic framework (Bi-MOF) nanocomposites to construct a SPE to achieve the sensitive detection of Cd²⁺ in vegetables with a LOD of 0.22 ng/mL.

In another strategy, porous framework materials can be used as carriers to improve sensor performance and achieve intelligent detection of heavy metal ions in optical and electrochemical sensors. For example, Zhang et al. (2022c) encapsulated blue fluorescent carbon dots (CDs) and red fluorescent quantum dots (InPQDs) in porous MOFs to construct ratiometric sensors for the visual detection of Hg²⁺. Specifically, Hg²⁺ could be strongly chelated with -COOH in InPQDs, which changed the fluorescence intensity using the electron transfer effect. Subsequently, CDs/InPQDs@MOF were immobilized on a paper base, and the color change could perform semi-quantitative detection of Hg²⁺. Furthermore, quantitative analysis of Hg²⁺ could be realized by smartphone image acquisition and RGB analysis, with a LOD of 7.22 nM. Alternatively, the same strategy had been employed by Zhang and his coworkers in SPE sensor. Specifically, the porous MOFs permitted large amounts of Cd²⁺, Cu²⁺, Pb²⁺ and Hg²⁺ to be adsorbed. Afterward, simultaneous detection of these four heavy metal ions through electrochemical deposition was carried out by recording the current values at the respective peak positions. The SPE sensor was successfully applied to detect Cd²⁺, Cu²⁺, Pb²⁺ and Hg²⁺ in complex food samples, providing a powerful tool for the portable detection of heavy metal ions (Zhang et al., 2024b).

The application of pesticides has significantly increased crop yields, but it has also raised considerable concerns about pesticide residues. In particular, highly toxic pesticides such as organophosphorus and organochlorine (even in trace amounts) can negatively affect the human nervous and immune system. Therefore, accurately detecting pesticide residues in food is essential to safeguarding public health. More recently, advanced nanobiosensors with their unique physicochemical properties have opened up new possibilities for the convenient, sensitive and rapid detection of pesticide residues. In this context, portable sensing devices based on porous framework materials have been widely employed for the detection of pesticides through fluorescence, colorimetric, and electrochemical signal outputs (Guo et al., 2022; Rana et al., 2024; Wu et al., 2024b).

In the aforementioned example, a functionalized HOF-on-HOF (Eu@PFC-1@MA-TPA) hydrogel film was constructed for the simultaneous recognition of 2,6-dichloro-4-nitroaniline (DCN) and thiabendazole (TBZ) by fluorescence images. Upon interaction with DCN and TBZ, the high fluorescent hydrogel underwent significant fluorescence quenching due to PET and the inner filter effect, respectively. Notably, by integrating fluorescence fingerprints generated from distinct fluorescent images with machine learning algorithms, the HOF-on-HOF hydrogel film exhibited simultaneous identification and quantification of pesticides in food within 1 s, achieving an impressive 98% accuracy (Hu & Yan, 2024). To cite another similar example, a luminescent MOF-based hydrogel microsphere (kgd-M1@ACPs) was reported to detect DCN in fruits and vegetables. In particular, apart from detection, the porous character of the framework materials provided sufficient channels for capturing DCN. The target could be adsorbed inside the framework material through π-π stacking, making it a portable adsorbent for pesticide wastewater treatment (Jia et al., 2022). In addition to recognition using framework materials themselves, molecular imprinted polymer (MIP) is another effective strategy for encoding specific interactions between framework materials and analytes. For instance, Yang et al. (2024) based on N-doped carbon dots (N-CDs) as a fluorescent probe, developed a ratiometric fluorescent sensor N-CDs@Eu-MOFs@MIP. In the presence of malathion (Mal), the fluorescence intensity of N-CDs was significantly enhanced due to non-radiative recombination, whereas the fluorescence of Eu-MOFs remained unchanged, resulting in a sensitive detection of Mal in the range of 1-10 μM with a LOD of 0.05 μM. Furthermore, combining smartphones with 3D printing technology provided a portable strategy for the intelligent detection of Mal through the variation of N-CDs@Eu-MOFs@MIP fluorescence images.

Furthermore, framework materials have gained increasing attention in colorimetric sensors for pesticide residues. The sensing strategy typically employs pesticide residues as enzyme activity inhibitors and utilizes the color change during the reaction process to realize low-cost and rapid visual detection. For example, Li and coworkers constructed novel core-shell nanocomposites of Prussian blue@Fe-COF@Au with triple peroxidase-like activity. Since the OPs inhibited the activity of acetylcholinesterase (AChE), which in turn affected the production of the peroxidase-like substrate, ultimately the colorimetric detection of OPs was achieved. Interestingly, the detection principle was applied to a portable hydrogel, thus providing an on-site detection device that satisfied the demand for POCT detection of pesticide residues (Li et al., 2023d). Similarly, Zhang et al. (2023) developed a paper-based colorimetric sensor for the portable detection of thiamethoxam using MOFs as a peroxidase-like enzyme. Given these outstanding advantages, the paper-based device demonstrated a continuous linear relationship between 0.1-1.2 μM and 1.2-10 μM, with the LOD as low as 0.04 μM. In order to avoid false positive defects, Shen & Yan (2024) prepared a multifunctional ionic covalent organic framework (TfaTta-MB) via the Menshutkin reaction and ion-exchange for the dual-mode detection of organochlorine pesticides. On the one hand, dicamba (DMA) provided a fluorescence quench for COF hydrogels through internal filtering effects. Meanwhile, the color change due to different DMA concentrations inspired the development of a smartphone-based colorimetric platform. Through the dual portable sensing of fluorescence and colorimetry, the accurate detection of pesticide residues on vegetable surfaces was achieved.

Apart from optical sensors, framework materials have also been employed in electrochemical sensors for pesticide residue analysis. For example, the SPE based on MIL-88B(Fe) MOFs was constructed for Mal ultra-trace detection. The highly conductive MOFs increased the modification sites on the electrode surface and accelerated the electron transfer efficiency, which realized on-site monitoring of Mal in vegetables in the concentration range of 1×10^-12 M to 1×10^-6 M with a LOD of 0.79 pM (Janjani et al., 2024). Remarkably, a sandwich electrochemical biosensor based on ionic metal-organic framework (IMOF) was constructed for the sensitive detection of OPs. IMOF enhanced the AChE-based electrochemical sensing due to its unique charge characteristics and abundant enzyme binding sites. Furthermore, by integrating near-field communication technology, a contactless electrochemical biosensor without battery was explored with a low detection limit of 1.24×10^-13 M for glyphosate in the practical detection of OPs (Wu et al., 2024). To avoid the false-positive drawbacks caused by single-mode sensing strategies, Wen et al. (2023) detected OPs by electrochemical and thermal sensing dual-mode platform by preparing COF@MB@MnO₂ with oxidase-like activity. The porosity of the COFs provided a high loading capacity for methylene blue (MB) molecules, while MnO₂ nanosheets served as a blocking layer, forming a composite probe. The activity of AChE was inhibited in the presence of OPs, thus hindering the decomposition of MnO₂ which promoted the ability of MnO₂ nanosheets to catalyze tetramethylbenzidine (TMB). On the one hand, the sensing strategy generated a large amount of oxidized TMB with strong near-infrared absorption, which could be conveniently measured using a portable thermometer. On the other hand, the current of the electroactive MB molecules captured in the framework materials was significantly reduced. This complementary dual-mode sensing strategy provided a broad platform for on-site, portable detection of OPs.

Antibiotics are a class of drugs that can inhibit the growth of bacteria, commonly used in animal farming to prevent and treat disease. However, the misuse and overuse of antibiotics may result in antibiotic residues and resistance, posing potential risks to human health through accumulation in the food chain (Zhang et al., 2024c). Therefore, there is an urgent need to develop rapid and portable analytical methods for detecting antibiotics in agricultural products. The inherent luminescent properties and diverse functional sites of novel framework materials provide excellent opportunities to recognize antibiotics.

Recently, smartphone-integrated optical sensing platforms based on framework materials have gained significant attention for the portable detection of antibiotics. For example, Hu et al. (2023b) encapsulated Mg²⁺ and N-CDs in MOFs (MIL-101) to provide efficient fluorescent signals, and the sensor was endowed with specific recognition capability by MIP to construct fluorescent nano-sensing probes Mg, N-CDs@MIL101@MIP. Further, by integrating fluorescent probes with smart devices, RGB values from the captured fluorescence images were instantly extracted and converted to I_mean values for on-site and accurate detection of oxytetracycline (OTC). Ultimately, this sensing strategy showed a good linear range (0.05-40 μg/mL) and low LOD (16.8 ng/mL) for OTC detection in milk. Notably, the team constructed a ratiometric fluorescent sensor by introducing lanthanide fluorescent groups into porous framework materials in 2022. In the presence of OTC, the fluorescence of the N-CDs at 445 nm was effectively quenched due to the inner filter effect, while the red fluorescence of Eu³⁺ at 620 nm significantly increased due to the antenna effect. By integrating with a portable smartphone, the sensor exhibited a robust linear relationship between the R/B value and OTC concentration within the range of 0.08-50 μg/mL, achieving a low LOD of 25.8 ng/mL (Hu et al., 2022). Another interesting study was mentioned by Liu et al. (2023), who proposed a handheld luminescent sensor array based on lanthanide MOFs to achieve instrument-free on-site detection of multi-target antibiotics. In this study, three Tb/Eu-MOFs with different luminescence intensities were synthesized, and the sensor array platform was constructed by the fluorescence changes triggered by different antibiotics. Further, by integrating the 3D-printed devices with a smartphone, a portable handheld sensor for simultaneous detection of six antibiotics was provided.

Based on the exceptional mechanical stability of the hydrogel, the on-site detection of antibiotics has been accomplished by using the principle that antibiotics can trigger color changes in responsive luminescent hydrogels. For example, a luminescent film was prepared by combining HOFs with sodium alginate using hydrogen bonding and electrostatic interactions. Subsequently, portable HOF hydrogel membranes with high stability were constructed for the sensitive detection of TC by cross-linking of Ca²⁺. The interaction between TC and Ca²⁺ triggered a FRET process, shifting the emission of the luminescent HOFs from blue to green. Due to the interaction between TC and Ca²⁺, the fluorescence of the luminescent HOFs changed from blue to green via FRET. This strategy realized the visible detection of TC, OTC, and chlortetracycline (CTC) in less than 6 min with LODs of 5.1, 7.7, and 32.7 ng/mL, respectively (Li et al., 2024e). In addition, the construction of ratiometric hydrogel sensor by immobilizing luminescent MOFs in gels has also been investigated for TC monitoring. Upon exposure to TC, the coordination and energy transfer between Eu³⁺ and TC caused the red fluorescence of Eu³⁺, whereas the fluorescence of fluorescein adsorbed in the MOFs remained unchanged, resulting in the ratiometric fluorescence detection of TC. In particular, after the dropwise addition of a certain amount of TC on the functionalized hydrogel, a significant fluorescence color change could be observed by the naked eye within 10 s (ultra-low LOD = 5.99 nM) (Chen et al., 2023).

Luminescent framework materials combined with flexible test strips were also a perfect strategy for constructing portable sensors. In this regard, a test strip was prepared by immobilizing lanthanide MOFs (TDA-Tb) onto carbon paper for metronidazole (MNZ) detection. Notably, the yellow fluorescence intensity of the TDA-Tb test strip decreased progressively with increasing MNZ concentration. It was known that the absorbance of TDA-Tb at 320 nm highly overlapped with MNZ, suggesting that PET was the primary mechanism responsible for the fluorescence quenching of TDA-Tb. By integrating a smartphone, this flexible paper-based sensor showed significant fluorescence changes in the concentration range of 0-0.03 mmol/L with a LOD of 4.1×10^-7 mol/L (Li et al., 2024f). Since the uniform distribution of the porous framework material in the test strip is more conducive to the adequate contact between the analyte and the material, Xu et al. (2023) encapsulated the dye molecule rhodamine B (RhB) in ZIF-8 to construct a dual-emission flexible composite material (RhB@ZIF-8@PVDF-MMM) for the detection of antibiotic nitrofurantoin (NFT) and OTC. Based on the FRET and intramolecular motions, the luminescence color of RhB@ZIF-8@ PVDF-MMM showed a change from pink to green and yellow after immersion in the OTC and NFT solutions, respectively. The excellent stability and portability of polymer films demonstrated their superior practical value in antibiotic detection.

Biogenic amines (BAs) are the general term for nitrogen-containing organic compounds with biological activity, widely found in organisms and food, mainly formed by the decarboxylation and deamidation of amino acids under the action of microorganisms. Normal dietary intake of BAs is not harmful, while excessive consumption of BAs may cause harm to human health, and in severe cases can even be life-threatening (Orouji et al., 2021). To reduce the damage of BAs to human health and to provide reliable indicators for food freshness assessment, it is indispensable to carry out intelligent and convenient detection of BAs. The customizable structure of porous framework materials can provide both specific host-guest interactions for target molecules and efficient adsorption and enrichment of BAs. Based on this, extensive research has integrated framework materials into non-invasive smart films to assess food freshness by monitoring the release of volatile BAs. These intelligent films, based on framework materials, provide visualization, real-time monitoring, and signal amplification, which have been widely investigated as portable non-invasive techniques for BAs detection.

Based on the property of MOFs with multiple fluorescence emission centers, a ratiometric fluorescent MOFs probe was constructed by covalently coupling fluorescein 5-FITC with NH₂-rich lanthanide MOFs. Further, portable smart labels were designed by doping the probe onto a fiber film. In this smart tag, the imidazole ring of the target histamine restricted the rotation of the aromatic ring in the 5-FITC, leading to an increase in green fluorescence. Meanwhile, hydrogen bonding interaction was formed between the H atom on the amino group of histamine and the O atom on the carboxyl group of the ligand, blocking the energy transfer from the ligand to Eu³⁺, causing a reduction in the red fluorescence emitted by Eu³⁺. Consequently, the sensor formed a high-resolution color recognition system when the target gas existed, along with a clear shift in color from orange-red to green. Delightfully, by converting the signals into G/R values, there was a clear linear relationship in the concentration range of 0 to 27.79 mg/L with LOD as low as 2.17 mg/L, providing a novel fluorescence sensing strategy for on-site food freshness monitoring (Wang et al., 2021). Not coincidentally, Li et al. (2024d) used the same sensing strategy to construct a ratiometric smart tag based on lanthanide HOFs for portable monitoring of the freshness of samples, which further broadened the application of framework materials for amine gas detection. As 2D Sp² carbon-conjugated COFs have robust backbones and strong π-π interactions, the de-excitation pathway of their photoexcited states is susceptible to exogenous molecules, therefore a large number of luminescent COFs have been reported to be adopted for the recognition of BAs through the collection of fluorescent signals (Gong et al., 2024). For example, Li et al. (2024g) achieved on-site, visual monitoring of volatile amines by constructing cationic pyridinium sites in COFs (TPCH-mOBPy COFs). In this detection system, TPCH-mOBPy COFs acted as recognition elements and simultaneously provided fluorescence signals. Ultimately, the intelligent detection of volatile amines in fish and shrimp was achieved by homemade COF membranes.

In the detection strategy of BAs, framework materials serve in smart tags not only as luminescent components but also as carriers in colorimetric sensors. Recently, an increasing number of smart films for meat freshness detection have been constructed by integrating pH-responsive dyes. However, natural dyes are prone to environmental oxidation, which compromises their stability. To address this challenge, encapsulating dyes within chemically stable framework materials has emerged as an effective approach. For example, Zhang et al. (2024d) adopted ZIF-8 as an ideal carrier for alizarin (AL) to develop a smart film for ammonia response. Since the volatile BAs produced during beef deterioration altered the pH of the environment, AL as a pH indicator showed a visible color change to the naked eye (from yellow to violet). Notably, the incorporation of AL into the protective framework of ZIF-8 significantly enhanced its stability, allowing the intelligent film to maintain color integrity even after 14 days of storage. Ultimately, the correlation between total volatile basic nitrogen (TVB-N) and ΔE values was evaluated by a linear model. The results revealed a high correlation coefficient (R² = 0.9067), which provided an early warning signal for food safety detection. Further, in order to detect the freshness of food while prolonging its shelf-life, a portable, low-cost, visualized paper sensor (PEu@ZMC) based on the type I heterostructure (Eu@ZMBA) was developed. The production of BAs and changes in pH during food spoilage induced a fluorescence/colorimetric dual-mode response by Eu@ZMBA, forming a portable platform for food freshness monitoring. Interestingly, covering PEu@ZMC on the food surface would release zinc ions and form antibacterial zones, effectively inhibiting bacterial contamination on the food surface (Zhao et al., 2025).

For example, Chen et al. (2022) employed the classical SPE sensing strategy to construct an electrochemical portable sensor based on COFs for the sensitive detection of zearalenone (ZEN). The porous structure of COFs allowed the spatial separation of aptamers immobilized on their surface, which increased the interaction opportunities between reactants and active sites. Integrating the signal output from the smartphone, in the range of 0.001 ng/mL-10.0 ng/mL, the rapid on-site detection of ZEN in real samples could be accomplished. In another example, a novel polydopamine-coated Cu-based metal-organic framework (HKUST@PDA) strip lateral flow immunoassay (SLFIA) was developed for on-site detection of AFB1. Based on the large specific surface area and multifunctional sites of HKUST@PDA, abundant anti-AFB1 monoclonal antibodies could be loaded onto its surface quickly and directly to form probes. Ultimately, the sensitivity of HKUST@PDA-SLFIA compared with AuNPs-based SLFIA method was significantly enhanced with an admirable LOD (0.01 ng/mL) in the detection of AFB1 (Wu et al., 2023). However, the accuracy of the analysis results of single-signal output modes is inevitably affected by various factors. To address this challenge, dual-signal sensing strategies with self-correcting capabilities have garnered significant attention in recent years. Combining the unique properties of MOFs, g-C₃N₄ nanosheets, and CuO@CuPt nanocomposites, a colorimetric-ECL dual-mode portable sensor was built for the ultrasensitive detection of aflatoxin B1 (AFB1). In this strategy, g-C₃N₄ nanosheets functioned as ECL signal emitters, while MOF composites were employed to accelerate the electron transfer rate and enhance the ECL intensity. The CuO@CuPt nanocomposites served as both ECL quenching agents and catalysts. In the presence of AFB1, the CuO@CuPt nanoprobes modified on the aptamer detached from the electrode surface, leading to the recovery of the ECL signal. Simultaneously, due to the superior catalytic activity of CuO@CuPt, the solution color changed significantly from colorless to blue with the increase of AFB1 concentration, which was easily observed in smartphone imaging. Ultimately, a strong linear relationship between AFB1 concentration and the ECL intensity was established with a LOD of 0.971 fg/mL, while the platform realized portable detection in the concentration range of 10 fg/mL to 10 μg/mL with a LOD of 3.26 fg/mL by storing the colorimetric images AFB1 on a smartphone. The dual-mode sensing platform eliminated environmental interferences and avoided sensor false-positive errors, opening up new avenues for mycotoxin detection (Xu et al., 2024b). Interestingly, this dual-mode sensing strategy in test strips was also exploited in the rapid detection of AFB1. In this sensor, porous MOF was synthesized as functional carrier for loading TMB signal molecules and then gate-controlled by a DNA switch comprised of CdS quantum dots-modified aptamer. Upon exposure to AFB1, the binding of the aptamer to the target resulted in the opening of the signal gate and the release of TMB, which triggered both colorimetric and ECL signals. Further, using microfluidic chip technology, the patterned paper electrode was separated from the colorimetric region treated with PtNPs, accomplishing the on-site detection of AFB1 in dual modes paper platform simultaneously (Huang et al., 2024b).

For example, Wei et al. (2023) developed core-shell magnetic framework materials (MCOF-AuNPs) with high fluorescence properties using in situ synthesis strategy for monitoring Salmonella typhimurium. The aptamer-MCOF-AuNPs fluorescent probe specifically bound to the target shielded the chelation site of the quencher Fe³⁺, thus leading to the suppression of the fluorescence quench effect. Ultimately, this sensor successfully achieved the on-site detection of Salmonella typhimurium by integrating the HSV color model in the smartphone app with a LOD of 17 CFU/mL. Similarly, due to the fluorescence of HOFs changing at different PH, it conferred its potential as the sensor array element. Based on this, Jiang et al. (2023) achieved sensitivity monitoring of Staphylococcus aureus and Escherichia coli by fingerprinting signals generated by the interaction of HOFs with their targets. In addition, a colorimetric sensor based on the framework materials was developed for the monitoring of Salmonella by Qi et al (2021). Due to the excellent catalytic properties of the MOFs, colorless o-phenylenediamine and H₂O₂ were catalyzed to produce yellow 2,3-diaminophenazine when the target was present. It was worth mentioning that the sensor integrated solution mixing, separation and detection on a microfluidic chip, which quickly achieved on-site detection of Salmonella, providing a promising application in screening foodborne pathogens. Based on the same strategy, Xing et al. (2023) designed a microfluidic colorimetric sensing platform using MOFs as the peroxidase-like enzyme for the detection of Escherichia coli. When Escherichia coli appeared, TMB was rapidly oxidized to blue ox-TMB under the catalysis of MOFs. The microfluidic chip integrated the entire detection process including injection, immune incubation, separation, enrichment, and signal output. Importantly, this microfluidic biosensor could detect the water, milk, and cabbage samples within 1 h, demonstrating great potential for application. Another noteworthy result was obtained by Feng et al. (2024), who constructed an ECL sensor based on framework materials and disposable electrodes for the non-destructive detection of Staphylococcus aureus. To enhance the ECL performance of the test strips, functionalized MOF nanoflowers (Ru-MOF-5 NFs) were adsorbed at the ITO electrode through electrostatic interactions to form ECL signal molecules. When the complementary probe modified with glucose oxidase (GOD) was placed on the electrode surface, GOD catalyzed the substrate conversion to H₂O₂, significantly suppressing the ECL signal. However, the presence of Staphylococcus aureus forced the separation of the complementary probe from the ITO electrode, permitting a significantly elevated ECL signal. Interestingly, by depositing flexible test strips on the food surface, on-site monitoring of pathogenic bacteria was accomplished in less than 10 min without damaging the sample.