Advanced "turn-on" colorimetric uranium platform based on the enhanced nanozyme activity of a donor-acceptor structured covalent organic framework.

BACKGROUND: The increasing uranium containing wastes generated during uranium mining and finishing pose a huge threat to the environment and human health, and thus robust strategies for on-site monitoring of uranium pollutant are of great significance for environmental protection around uranium tailings. RESULTS: Herein, a facile "turn-on" colorimetric platform that can achieve uranium detection by spectrometry and naked eyes was developed based on the uranium-enhanced nanozyme activity of covalent organic framework (JUC-505). Thanks to the extended π-conjugated skeleton and donor-acceptor (D-A) structure, JUC-505 exhibited superior photo-activated nanozyme activity, which would be prohibited when the cyano group in JUC-505 skeleton was transformed to the amidoxime group. Further results elucidated that the coordination of uranium with amidoxime groups led to the electron transfer between uranium and the JUC-505-AO skeleton, and thus significantly restored the nanozymatic activity of JUC-505-AO with the subsequent remarkable color changes. Moreover, the uranium concentrations in uranium tailing wastewater detected by the present "turn-on" colorimetric method were well agreed with those by ICP-MS, demonstrating a high accuracy of the present method in real samples. SIGNIFICANCE: The D-A structured JUC-505 with superior photocatalytic property and nanozymatic activity was applied to facilitate colorimetric detection of uranium, which displays the advantages of low detection limit, excellent selectivity, fast response and simple operation for uranium detection in real samples, and shows a great potential in on-site monitoring of uranium pollutant around uranium tailings as well as nuclear power plant.

Europium(III) Functionalized Covalent Organic Framework as Sensitive and Selective Fluorescent Switch for Detection of Uranium.

Uranium poses severe health risks due to its radioactivity and chemical toxicity if released into the environment. Therefore, there is an urgent demand to develop sensing materials in situ monitoring of uranium with high sensitivity and stability. In this work, a fluorescent Eu3+-TFPB-Bpy is synthesized by grafting Eu3+ cation onto TFPB-Bpy covalent organic framework (COF) synthesized through Schiff base condensation of monomers 1,3,5-tris(4-formylphenyl)benzene (TFPB) and 5,5'-diamino-2,2'-bipyridine (Bpy). The fluorescence of Eu3+-TFPB-Bpy is enhanced compared with that of TFPB-Bpy, which is originated from the intramolecular rotations of building blocks limited by the bipyridine units of TFPB-Bpy coordinated with Eu3+. More significantly, Eu3+-TFPB-Bpy is a highly efficient probe for sensing UO22+ in aqueous solution with the luminescence intensity efficiently amplified by complexation of UO22+ with Eu3+. The turn-on sensing capability was derived from the resonance energy transfer occurring from UO22+ to the Eu3+-TFPB-Bpy. The developed probe displayed desirable linear range from 5 nM to 5 µM with good selectivity and rapid response time (2 s) for UO22+ in mining wastewater. This strategy provides a vivid illustration for designing luminescence lanthanide COF hybrid materials with applications in environmental monitoring.

The construction of a stable hydrogen-bonded organic framework for the photocatalytic reduction and removal of uranium.

An imidazolyl hydrogen-bonded organic framework (HOF-T) with outstanding thermal and water stability was constructed by C-H⋯N hydrogen bonding and C-H⋯π interactions. UO22+ can be selectively captured by the imidazole group of HOF-T and rapidly reduced to UO2 under visible light irradiation, realizing exceptional uranium removal with high capacity and fast kinetics.

σ-Hole Effect-Induced Electroluminescence of Halogen Cocrystals for Determination of Iodide in Seawater.

Developing new electrochemiluminescence (ECL) luminators with high stability, wide applicability, and strong designability is of great strategic significance to promote the ECL field to the frontier. Here, driven by the I···N bond, 1,3,5-trifluoro-2,4,6-triiodobenzene (TFTI) and 2,4,6-trimethyl-1,3,5-triazine (TMT) self-assembled into a novel halogen cocrystal (TFTI-TMT) through slow solution volatilization. Significant difference of charge density existed between the N atoms on TMT and the σ-hole of the I atoms on TFTI. Upon the induction of σ-hole effect, high-speed and spontaneous charge transferring from TMT to the σ-hole of TFTI occurred, stimulating exciting ECL signals. Besides, the σ-hole of the I atoms could capture iodine ions specifically, which blocked the original charge transfer from the N atoms to the σ-hole, causing the ECL signal of TFTI-TMT to undergo a quenching rate as high as 92.9%. Excitingly, the ECL sensing of TFTI-TMT toward I- possessed a wide linear range (10-5000 nM) and ultralow detection limit (3 nM) in a real water sample. The halogen cocrystal strategy makes σ-hole a remarkable new viewpoint of ECL luminator design and enables ECL analysis technology to contribute to addressing the environmental and health threats posed by iodide pollution.

Hydrogen-Bonded Organic Cocrystal-Encapsulated Perovskite Nanocrystals as Coreactant-Free Electrochemiluminescent Luminophore for the Detection of Uranium.

Lead halide perovskite nanocrystals with excellent photophysical properties are promising electrochemiluminescence (ECL) candidates, but their poor stability greatly restricts ECL applications. Herein, hydrogen-bonded cocrystal-encapsulated CsPbBr3 perovskite nanocrystals (PeNCs@NHS-M) were synthesized by using PeNCs as nuclei for inducing the crystallization of melamine (M) and N-hydroxysuccinimide (NHS). The as-synthesized composite exhibits multiplicative ECL efficiencies (up to 24-fold that of PeNCs) without exogenous coreactants and with excellent stability in the aqueous phase. The enhanced stability can be attributed to the well-designed heterostructure of the PeNCs@NHS-M composite, which benefits from both moiety passivation and protection of the peripheral cocrystal matrix. Moreover, the heterostructure with covalent linkage facilitates charge transfer between PeNCs and NHS-M cocrystals, realizing effective ECL emission. Meanwhile, the NHS and M components act as coreactants for PeNCs, shortening the electron-transport distance and resulting in a significant increase in the ECL signal. Furthermore, by taking advantage of the specific binding effect between NHS-M and uranyl (UO22+), an ECL system with both a low detection limit (1 nM) and high selectivity for monitoring UO22+ in mining wastewater is established. The presence of UO22+ disrupted the charge-transfer effect within PeNCs@NHS-M, weakening the ECL signals. This work provides an efficient design strategy for obtaining stable and efficient ECLs from perovskite nanocrystals, offering a new perspective for the discovery and application of perovskite-based ECL systems.

Rationally designed nanotrap structures for efficient separation of rare earth elements over a single step.

Extracting rare earth elements (REEs) from wastewater is essential for the growth and an eco-friendly sustainable economy. However, it is a daunting challenge to separate individual rare earth elements by their subtle differences. To overcome this difficulty, we report a unique REE nanotrap that features dense uncoordinated carboxyl groups and triazole N atoms in a two-fold interpenetrated metal-organic framework (named NCU-1). Notably, the synergistic effect of suitable pore sizes and REE nanotraps in NCU-1 is highly responsive to the size variation of rare-earth ions and shows high selectivity toward light REE. As a proof of concept, Pr/Lu and Nd/Er are used as binary models, which give a high separation factor of SFPr/Lu = 796 and SFNd/Er = 273, demonstrating highly efficient separation over a single step. This ability achieves efficient and selective extraction and separation of REEs from mine tailings, establishing this platform as an important advance for sustainable obtaining high-purity REEs.

Hydrogen-Bonded Cocrystals Encapsulating CsPbBr3 Perovskite Nanocrystals with Enhancement of Charge Transport for Photocatalytic Reduction of Uranium.

At present, poor stability and carrier transfer efficiency are the main problems that limit the development of perovskite-based photoelectric technologies. In this work, hydrogen-bonded cocrystal-coated perovskite composite (PeNCs@NHS-M) is easily obtained by inducing rapid crystallization of melamine (M) and N-hydroxysuccinimide (NHS) with PeNCs as the nuclei. The outer NHS-M cocrystal passivates the undercoordinated lead atoms by forming covalent bonds, thereby greatly reducing the trap density while maintaining good structure stability for perovskite nanocrystals. Moreover, benefiting from the interfacial covalent band linkage and long-range ordered structures of cocrystals, the charge transfer efficiency is effectively enhanced and PeNCs@NHS-M displays superior photoelectric performance. Based on the excellent photoelectric performance and abundant active sites of PeNCs@NHS-M, photocatalytic reduction of uranium is realized. PeNCs@NHS-M exhibits U(VI) reduction removal capability of up to 810.1 mg g-1 in the presence of light. The strategy of cocrystals trapping perovskite nanocrystals provides a simple synthesis method for composites and opens up a new idea for simultaneously improving the stability and photovoltaic performance of perovskite.

Efficient capture of iodine in steam and water media by hydrogen bond-driven charge transfer complexes.

Untreated radioactive iodine (129I and 131I) released from nuclear power plants poses a significant threat to humans and the environment, so the development of materials to capture iodine from water media and steam is critical. Here, we report a charge transfer complex (TCNQ-MA CTC) with abundant nitrogen atoms and π-conjugated system for adsorption of I2 vapor and I3- from aqueous solutions. Due to the synergistic binding mechanism of benzene/triazine rings and N-containing groups with iodine, special I-π and charge transfer interaction can be formed between the guest and the host, and thus efficient removal of I2 and I3- can be realized by TCNQ-MA CTC with the adsorption capacity up to 2.42 g/g and 800 mg/g, respectively. TCNQ-MA CTC can capture 92% of I3- within 2.5 min, showing extremely fast kinetics, excellent selectivity and high affinity (Kd = 5.68 × 106 mL/g). Finally, the TCNQ-MA CTC was successfully applied in the removal of iodine from seawater with the efficiency of 93.71%. This work provides new insights in the construction of charge transfer complexes and lays the foundation for its environmental applications.

Rapid Charge Transfer Enabled by Noncovalent Interaction through Guest Insertion in Supercapacitors based on Covalent Organic Frameworks.

Covalent organic frameworks (COFs) have been proposed for electrochemical energy storage, although the poor conductivity resulted from covalent bonds limits their practical performance. Here, we propose to introduce noncovalent bonds in COFs through a molecular insertion strategy for improving the conductivity of the COFs as supercapacitor. The synthesized COFs (MI-COFs) establish equilibriums between covalent bonds and noncovalent bonds, which construct a continuous charge transfer channel to enhance the conductivity. The rapid charge transfer rate enables the COFs to activate the redox sites, bringing about excellent electrochemical energy storage behavior. The results show that the MI-COFs exhibit much better performance in specific capacitance and capacity retention rate than those of most COFs-based supercapacitors. Moreover, through simply altering inserted guests, the mode and strength of noncovalent bond can be adjusted to obtain different energy storage characteristics. The introduction of noncovalent bonds is an effective and flexible way to enhance and regulate the properties of COFs, providing a valuable direction for the development of novel COFs-based energy storage materials.

Construction of covalent organic framework nanozymes with photo-enhanced hydrolase activities for colorimetric sensing of organophosphorus nerve agents.

Construction of covalent organic frameworks (COFs)-based nanozymes is of great importance for the extensive applications in catalysis and sensing fields. In this work, a two-dimensional COF (DAFB-DCTP COF) was fabricated via Knoevenagel condensation reaction. The integration of catalytically active sites of pyridine groups into the donor-acceptor (D-A) conjugated skeleton endows DAFB-DCTP COF with both hydrolytic and photosensitive properties. The DAFB-DCTP COF can be utilized as an artificial enzyme with selective and photo-enhanced catalytic efficiency, facilitating its application in photocatalytic degradation of hydrolase substrates (p-nitrophenyl acetate, pNPA) by nucleophilic reaction and further realizing colorimetric detection of the nanozyme inhibitor of organophosphorus nerve agent (diethyl cyanophosphonate, DCNP). The distinct color changes could be distinguished by naked eyes even at a low DCNP concentration, and the versatile smartphone analysis featured with reliability and simplicity. For the first time, the COFs' intrinsic hydrolase activity depending on their structural characteristics was investigated in synergy with the photosensitive performance originating from their photoelectric features. The present contribution provides a promising direction towards construction of colorimetric sensing platform based on the regulation of COFs' non-oxidoreductase activity under visible light irradiation.

First Report of Cladosporium scabrellum Causing Stipe Spot Disease of Morchella importuna in China.

Morchella is a genus of edible and medicinal fungi with a global distribution and economic value. Artificial cultivation of Morchella has risen to 8,000 hectares in China in last 5 years (Zhao et al. 2020). In the early March of 2021, a peculiar stipe spot disease was first detected on Morchella importuna at a manufacturing base in Yangxian county (33°02' N, 108°11' E), Shaanxi, China. Several dark brown patches were observed on the surface of M. importuna fruiting bodies, with approximately 40% of the fruiting bodies infected at a late stage of production. This symptom typically manifests in March to May each year during the growing season of M. importuna. Following infection of the fruiting body, a light brown spot (2 to 4 mm) appears at the base of the stipe when the fruiting body is about 3 cm long. The spot progressively darkens and proceeds to cover the large stipe of the fruiting body. At 5 to 7 days after the appearance of the first spot, infected young fruiting bodies soften and decay. Temperature (>20℃) or humidity (80 to 90%) increases the likelihood of disease. To isolate the causal agent, the edge of a lesion was carefully scraped, inoculated into potato dextrose agar (PDA), and then incubated in the dark at 24±2°C (Bensch et al. 2010). The colonies were grayish-green in front, green to black on the reverse, and grew as concentric rings after 14 days of culture. The conidiophore was thin, branched with warty ramoconidia, and 2.5 to 4.7 µm long by 1.5 to 2.0 µm wide (length×width, mean=0.72, n=150). The ramoconidia were cylindrical and long at 14.3 to 28.7 µm by 6.4 to 10.7 µm (length×width, mean=2.52, n=150); the secondary ramoconidia were narrower and rod-shaped at 3.8 to 4.9 µm by 2.4 to 3.6 µm (length×width, mean=0.19, n=150). These morphological features were consistent with Cladosporium scabrellum (Bensch et al. 2012). Genomic DNA was extracted from mycelia of the pathogen using an Ezup column fungal genomic DNA extraction kit (Sangon Biotech, Shanghai, China). To confirm the identity of the pathogen (C. scabrellum), the genomic fragments for the internal transcribed spacer (ITS), translation elongation factor 1 alpha (EF-1α) and actin (ACT) gene of the isolate were amplified by PCR (Carbone et al. 1999; Walker et al. 2016; Xu et al. 2021). The resultant sequence was uploaded to the GenBank database with accession number (GeneBank ID of gene ITS, EF-1α, ACT are MZ736660, OQ982380, OQ982381, respectively). BLAST results showed >99% identity with those of C. scabrellum(GeneBank ID of gene ITS, EF-1α, ACT are HM148195, HM148440, HM148685, respectively). Phylogenetic analysis was performed using the maximum-likelihood (ML) method and Bayesian phylogenetic inference (BI) analyses in MEGA7 software (Kumar et al., 2018). For pathogenicity testing, 20 naturally occurring M. importuna fruiting bodies in an artificial climate chamber were randomly selected, inoculated by spraying with a conidial suspension of the pathogen (1×106 conidia/ml sterile water), and incubated at 20°C and 90 to 95% humidity. On the third day after inoculation, a yellow-brown spot appeared around the grafting sites, the spot gradually darkened and began to cover the stipe; the infection rate was 80 to 85%. Ten control fruiting bodies of M. importuna sprayed with sterile water grew normally and were symptomless. The pathogen was reisolated from infected fruiting bodies and confirmed as C. scabrellum by morphology and ITS, EF-1α, ACT sequence. To our knowledge, this is the first report of M. importuna stipe spot disease caused by C. scabrellum. According to a farmer in Yangxian, these spots rapidly spread every year, drastically reducing the production and market value of Morchella. This is a major problem for producers of this edible mushroom.

Structural Isomerism of Covalent Organic Frameworks Causing Different Electrochemiluminescence Effects and Its Application for the Detection of Arsenic.

The structural isomerism of the covalent organic framework (COF) has a significant effect on the electrochemiluminescence (ECL) performance. Herein, we report a pair of isomeric COFs, (TFPB-BD(OMe)2-H and TAPB-BD(OMe)2-H), based on the different directions of imine linkages and further conversion of the imine to the quinoline structure. The obtained two isomeric COFs with the same composition and similar structures exhibit dramatic differences in the photoelectrochemical and ECL fields. Indeed, TFPB-BD(OMe)2-H demonstrates robust ECL emission superior to that of TAPB-BD(OMe)2-H. The difference in ECL performance is due to the stronger polar interaction of TFPB-BD(OMe)2-H than that of TAPB-BD(OMe)2-H. The polarity is derived from the uneven charge distribution within the framework and enhances the electron interactions. In addition, the ordered conjugate skeleton provides high-speed charge transport channels for carrier transport. Therefore, the TFPB-BD(OMe)2-H presents a smaller band gap energy and stronger polarization interaction, which are more favorable to charge migration to achieve stronger ECL signals. Furthermore, we describe a convenient ECL sensor for detecting toxic As(V) with an outstanding detection property and ultralow detection limit. This work provides a guiding principle for the design and development of ECL organic luminophores.

Synthesis of propenone-linked covalent organic frameworks via Claisen-Schmidt reaction for photocatalytic removal of uranium.

The type of reactions and the availability of monomers for the synthesis of sp2-c linked covalent organic frameworks (COFs) are considerably limited by the irreversibility of the C=C bond. Herein, inspired by the Claisen-Schmidt condensation reaction, two propenone-linked (C=C-C=O) COFs (named Py-DAB and PyN-DAB) are developed based on the base-catalyzed nucleophilic addition reaction of ketone-activated α-H with aromatic aldehydes. The introduction of propenone structure endows COFs with high crystallinity, excellent physicochemical stability, and intriguing optoelectronic properties. Benefitting from the rational design on the COFs skeleton, Py-DAB and PyN-DAB are applied to the extraction of radionuclide uranium. In particular, PyN-DAB shows excellent removal rates (>98%) in four uranium mine wastewater samples. We highlight that such a general strategy can provide a valuable avenue toward various functional porous crystalline materials.

Ionic covalent organic framework for selective detection and adsorption of TcO4-/ReO4.

Herein, a novel fluorescent ionic covalent organic framework (BTTA-BDNP) based on a linked carbazole unit was constructed for the synchronous monitoring and capture of TcO4-/ReO4-. BTTA-BDNP has a fast fluorescence response time with a low detection limit (66.7 nM) for ReO4- (a non-radioactive substitute for TcO4-). Meanwhile, the high charge density and hydrophobic skeleton of BTTA-BDNP enable it to exhibit rapid and selective trapping of ReO4- in complex environments.

Synergistic effect of double Schottky potential well and oxygen vacancy for enhanced plasmonic photocatalytic U(VI) reduction.

Plasmonic photocatalysis is an effective strategy to solve radioactive uranium hazards in wastewater. A plasmonic photocatalyst Bi/Bi2O3-x@COFs was synthesized by in-situ growth of covalent organic frameworks (COFs) on Bi/Bi2O3-x surface for the U(VI) adsorption and plasmonic photoreduction in rare earth tailings wastewater. The presence of oxygen vacancy in Bi/Bi2O3-x and Schottky potential well formed by Bi and Bi2O3-x interface increased the number of free electrons, which induced localized surface plasmon resonance (LSPR) and enhanced the light absorption performance of composites. In addition, oxygen vacancy improved the Fermi level of Bi/Bi2O3-x, leading to another potential well between Bi2O3-x and COFs interface. The electron transport direction was reversed, thus increasing the electron density of COFs layer. COFs was an N-type semiconductor with specific binding U(VI) groups and suitable band structure, which could be used as an active reaction site. Bi/Bi2O3-x@COFs had 1411.5 mg g-1 removal capacity and high separation coefficient for U(VI) due to the synergistic action of photogenerated electrons and hot electrons. Moreover, the removal rate of uranium from rare earth tailings wastewater by regenerated Bi/Bi2O3-x@COFs was over 93.9%. The scheme of introducing LSPR and Schottky potential well provides another way to improve the photocatalytic effect.

Molecular Insertion: A Master Key to Unlock Smart Photoelectric Responses of Covalent Organic Frameworks.

Covalent organic frameworks (COFs) show potentials in prominent photoelectric responses by judicious structural design. However, from the selections of monomers and condensation reactions to the synthesis procedures, the acquisition of photoelectric COFs has to meet overmuch high conditions, limiting the breakthrough and modulation in photoelectric responses. Herein, the study reports a creative "lock-key model" based on molecular insertion strategy. A COF with suitable cavity size, TP-TBDA, is used as the host to load guests. Merely through the volatilization of mixed solution, TP-TBDA and guests can be spontaneously assembled via non-covalent interactions (NCIs) to produce molecular-inserted COFs (MI-COFs). The NCIs between TP-TBDA and guests acted as a bridge to facilitate charge transfer in MI-COFs, unlocking the photoelectric responses of TP-TBDA. By exploiting the controllability of NCIs, the MI-COFs can realize the smart modulation of photoelectric responses by simply changing the guest molecule, thus avoiding the arduous selection of monomers and condensation reactions required by conventional COFs. The construction of molecular-inserted COFs circumvents complicated procedures for achieving performance improvement and modulation, providing a promising direction to construct late-model photoelectric responsive materials.

Guest Molecular Assembly Strategy in Covalent Organic Frameworks for Electrochemiluminescence Sensing of Uranyl.

The application of covalent organic frameworks (COFs) in electrochemiluminescence (ECL) is promising in environmental monitoring. Developing an emerging design strategy to expand the class of COF-based ECL luminophores is highly desirable. Here, a COF-based host-guest system was constructed through guest molecular assembly to deal with nuclear contamination analysis. The efficient charge transport network was formed by inserting an electron-withdrawing guest tetracyanoquinodimethane (TCNQ) into the open space of the COF host (TP-TBDA; TP = 2,4,6-trihydroxy-1,3,5-benzenetricarbaldehyde and TBDA = 2,5-di(thiophen-2-yl)benzene-1,4-diamine) with an electron-donating property; the construction of the COF-based host-guest system (TP-TBDA@TCNQ) triggered the ECL emission of non-emitting TP-TBDA. Furthermore, the dense active sites in TP-TBDA were utilized to capture the target substance UO22+. The presence of UO22+ broke the charge-transfer effect in TP-TBDA@TCNQ, resulting in the weakening of the ECL signal, thus the established ECL system integrating the low detection limit with high selectivity monitors UO22+. This COF-based host-guest system provides a novel material platform for constructing late-model ECL luminophores and creates an opportunity for the vigorous ECL technology.

Reversed Regulation Effects of ssDNA on the Mimetic Oxidase and Peroxidase Activities of Covalent Organic Frameworks.

Nanomaterials with enzyme mimetic activity have attracted extensive attention, especially in the regulation of their catalytic activities by biomolecules or other polymers. Here, a covalent organic framework (Tph-BT COF) with excellent photocatalytic activity is constructed by Schiff base reaction, and its mimetic oxidase activity and peroxidase activity is inversely regulated via single-stranded DNA (ssDNA). Under light-emitting diode (LED) light irradiation, Tph-BT exhibited outstanding oxidase activity, which efficiently catalyzed oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to produce blue oxTMB, and ssDNA, especially those with poly-thymidine (T) sequences, can significantly inhibit its oxidase activity. On the contrary, Tph-BT showed weak peroxidase activity, and the presence of ssDNA, particularly poly-cytosine (C) sequences, can remarkably enhance the peroxidase activity. The influence of base type, base length, and other factors on the activities of two enzymes is also studied, and the results reveal that the adsorption of ssDNA on the surface of Tph-BT prevented intersystem crossing (ISC) and energy transfer processes to reduce 1 O2 generation, while the electrostatic interaction between ssDNA and TMB enhanced Tph-BT's affinity for TMB to facilitate the electron transfer from TMB to • OH. This study investigates multitype mimetic enzyme activities of nonmetallic D-A conjugated COFs and demonstrates their feasibility of regulation by ssDNA.

DNAzyme-Derived Aptamer Reversely Regulates the Two Types of Enzymatic Activities of Covalent-Organic Frameworks for the Colorimetric Analysis of Uranium.

Nanozymes are nanomaterials with enzyme-mimetic activity. It is known that DNA can interact with various nanozymes in different ways, enhancing or inhibiting the activity of nanozymes, which can be used to develop various biosensors. In this work, we synthesized a photosensitive covalent-organic framework (Tph-BT) as a nanozyme, and its oxidase and peroxidase activities could be reversely regulated by surface modification of single-stranded DNA (ssDNA) for the colorimetric detection of UO22+. Tph-BT exhibits excellent oxidase activity and weak peroxidase activity, and it is surprising to find that the UO22+-specific DNA aptamer can significantly inhibit the oxidase activity while greatly enhancing the peroxidase activity. The present UO22+ interacts with the DNA aptamer to form secondary structures and detaches from the surface of Tph-BT, thereby restoring the enzymatic activity of Tph-BT. Based on the reversed regulation effects of the DNA aptamer on the two types of enzymatic activities of Tph-BT, a novel "off-on" and "on-off" sensing platform can be constructed for the colorimetric analysis of UO22+. This research demonstrates that ssDNA can effectively regulate the different types of enzymatic activities of single COFs and achieve the sensitive and selective colorimetric analysis of radionuclides by the naked eye.

An in-situ strategy to construct uracil-conjugated covalent organic frameworks with tunable fluorescence/recognition characteristics for sensitive and selective Mercury(II) detection.

Previous researches of covalent organic frameworks (COFs) have shown their potential as fluorescent probes, but the regulation of their optical properties and recognition characteristics still remains a challenge, and most of reports required complicated post-decoration to improve the sensing performance. In this context, we propose a novel in-situ strategy to construct uracil-conjugated COFs and modulate their fluorescence properties for sensitive and selective mercury(II) detection. By using 1,3,6,8-tetrakis(4-formylphenyl)pyrene (TFPPy) and 1,3,6,8-tetrakis(4-aminophenyl)pyrene (TAPPy) as fundamental blocks and 5-aminouraci (5-AU) as the functional monomer, a series of COFs (Py-COFs and Py-U-COFs-1 to Py-U-COFs-5) with tunable fluorescence were solvothermally synthesized through an in-situ Schiff base reaction. The π-conjugated framework serves as a signal reporter, the evenly and densely distributed uracil acts as a mercury(II) receptor, and the regular pores (channels) make the rapid and sensitive detection of the mercury(II) possible. In this research, we manage to regulate the crystalline structure, the fluorescence properties, and the sensing performance of COFs by simply changing the molar ratio of precursors. We expect this research to open up a new strategy for effective and controllable construction of functionalized COFs for environmental analysis.