Hollow Ni3S4@Co3S4 with core-satellite nanostructure derived from metal-organic framework (MOF)-on-MOF hybrids as an electrode material for supercapacitors.

Metal-organic frameworks (MOFs) have found wide applications in the field of supercapacitors due to their highly controllable porous structure, big specific surface area, and abundant chemical functional groups. MOF-on-MOF hybrids not only enhance the composition of MOFs (such as ligands and/or metal centers) but also provide greater structural diversity. By utilizing MOFs as precursors for preparing sulfides, the unique characteristics and inherent structure of MOFs are preserved but their conductivity and capacitance are enhanced. This study successfully synthesized hollow-structured Ni3S4@Co3S4 derived from an Ni-MOF@ZIF-67 hybrid structure, where the Ni-MOF serves as the core and ZIF-67 as the satellite. The Ni3S4@Co3S4 electrode demonstrated a specific capacity as high as 747.3 C g-1 at 1 A g-1, and it could still maintain 77% of its initial capacity at 10 A g-1. Furthermore, the assembled Ni3S4@Co3S4//AC hybrid supercapacitor (HSC) device achieved a maximum energy density of 30.8 W h kg-1 when the power density was 750 W kg-1. The device exhibited remarkable cycling durability, retaining 85.4% of its initial capacitance after 5000 cycles. Therefore, the derived functional materials based on MOF-on-MOF provide a more scalable and promising approach for the preparation of efficient electrode materials.

Polyarylether-based COFs coordinated by Tb3+ for the fluorescent detection of anthrax-biomarker dipicolinic acid.

In this study, a rare-earth hybrid luminescent material (lanthanide@COF) was constructed for the detection of a biomarker for anthrax (dipicolinic acid, DPA). JCU-505-COOH was prepared by the hydrolysis of the cyano group in JCU-505 via a post-synthetic modification strategy, then the carboxyl groups in JCU-505-COOH coordinated with Tb3+ ions, similar to pincer vising nut. The prepared Tb3+@JCU-505-COOH exhibited a turn-on response toward DPA, which allowed the lanthanide@COF to serve as a fluorescence sensor with excellent selectivity and high sensitivity (binding constant Ka = 3.66 × 103). The fluorescent probe showed satisfactory performance for the determination of DPA in saliva and urine with a detection limit of 0.6 µM. Moreover, we established a facile point-of-care testing (POCT) using the Tb3+@JCU-505-COOH-based fluorescent test paper together with a smartphone for the initial diagnosis of anthrax. As expected, Tb3+@JCU-505-COOH showed great potential for the rapid screening of anthrax due to low cost, simple operation, and wide applicability.

Eu3+-Modified Covalent Organic Frameworks for the Detection of a Vinyl Chloride Monomer Exposure Biomarker.

The vinyl chloride monomer (VCM), a common raw material in the plastics industry, is one of the environmental pollutants to which humans are mostly exposed. Thiodiglycolic acid (TDGA) in human urine is a specific biomarker of its exposure. TDGA plays an important role in understanding the relationships between exposure to the VCM and the identification of subgroups that are at increased risk for disease diagnosis. Therefore, its detection is of great significance. Here, we designed and established a ratiometric fluorescent sensor for TDGA by using Eu3+ as a bridge connecting the covalent organic framework (COF) and the energy donor molecule 2,6-dipicolinic acid (DPA) and named it DPA/Eu@PY-DHPB-COF-COOH. The sensor not only possesses the advantages of a ratiometric fluorescent sensor that can provide built-in self-calibration to correct a variety of target-independent factors but also presents high selectivity and high sensitivity. Currently, there are only a few reports on the detection of TDGA, and to the extent of our knowledge, this report is the first work on the detection of TDGA based on a COF system; so, it has an important reference value and lays a solid foundation for designing advanced sensors of TDGA.

A novel electrochemical sensing platform based on double-active-center polyimide covalent organic frameworks for sensitive analysis of levofloxacin.

The development of a simple and sensitive electrochemical sensing platform for levofloxacin (LVF) analysis is of great significance to human health. In this work, a covalent organic framework (TP-COF) was in situ grown on the surface of Sn-MoC nanospheres with nanoflower-like morphology through a one-pot method to obtain the TP-COF@Sn-MoC composite. The prepared composite was used to modify a glassy carbon electrode (GCE) to realize the sensitive detection of levofloxacin. TP-COF was formed by polycondensation of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and pyromellitic dianhydride (PMDA), in which C = O and C = N groups served as double active centers for the recognition and electrocatalytic oxidation of the target molecule. Meanwhile, the introduction of Sn-MoC improved the conductivity of the electrode. The TP-COF@Sn-MoC composite produced a strong synergistic effect and showed a high electrocatalytic ability toward levofloxacin oxidation. The linear range of LVF was 0.6-1000 µM and the limit of detection (LOD) was 0.029 µM (S/N = 3). In addition, the sensor has been successfully applied for the analysis of LVF in human urine and blood serum samples with acceptable recovery rates, demonstrating that the sensor was promising in practical applications.

Combination of covalent organic frameworks (COFs) and polyoxometalates (POMs): the preparation strategy and potential application of COF-POM hybrids.

Both covalent organic frameworks (COFs) and polyoxometalates (POMs) show excellent properties and application potential in many fields, thus receiving widespread attention. In recent years, COF-POM hybrid materials were prepared by combining COFs and POMs through physical or chemical methods. COF-POM hybrids have shown high performance in many fields, such as catalysis, sensing, energy storage, and biomedicine. In this review, we introduced the preparation strategy and application of COF-POM hybrids in detail. We believe that the combination of COFs and POMs will provide more abundant functions and broad application prospects.

Recent Progress of Covalent Organic Frameworks Applied in Electrochemical Sensors.

As an emerging porous crystalline organic material, the covalent organic frameworks (COFs) are given more and more attention in many fields, such as gas storage and separation, catalysis, energy storage and conversion, luminescent devices, drug delivery, pollutant adsorption and removal, analysis and detection due to their special advantages of high crystallinity, flexible designability, controllable porosities and topologies, intrinsic chemical and thermal stability. In recent years, the COFs are applied in analytical chemistry, for instance, chromatography, solid-phase microextraction, luminescent and colorimetric sensing, surface-enhanced Raman scattering and electroanalytical chemistry. The COFs decorated electrodes show high performance for detecting trace substances with remarkable selectivity and sensitivity, such as heavy metal ions, glucose, hydrogen peroxide, drugs, antibiotics, explosives, phenolic compounds, pesticides, disease metabolites and so on. This review mainly summarized the application of COF based electrochemical sensor according to different target analytes.

Ratiometric fluorescent sensor based on metal-organic framework for selective and sensitive detection of CO32.

Carbonate ion (CO32-) is an anion essential for the maintenance of life activities and is of great importance to human health. Here, a novel ratiometric fluorescent probe Eu/CDs@UiO-66-(COOH)2 (ECU) was prepared by introducing europium ions (Eu3+) and carbon dots (CDs) into the UiO-66-(COOH)2 parent framework under the guidance of a post-synthetic modification strategy and used for the detection of CO32- ion in the aqueous environment. Interestingly, when CO32- ions were added to the ECU suspension, the characteristic emission of carbon dots at 439 nm was significantly enhanced, while the characteristic emission of Eu3+ ions at 613 nm was reduced. Therefore, the detection of CO32- ions can be realized through the peak height ratio of the two emissions. The probe had a low detection limit (about 1.08 µM) and a wide linear range (0-350 µM) for the detection of carbonate. In addition, the presence of CO32- ions can cause a significant ratiometric luminescence response and resulted obvious red-to-blue color shift of the ECU under UV light, which will facilitate visual analysis by the naked eye.

A novel ratiometric fluorescence sensor based on lanthanide-functionalized MOF for Hg2+ detection.

Post-synthesis modification is an effective strategy for the preparation of rare earth organic framework materials and the derivation of high-performance functional materials. Here, we report the preparation of a dual emission Ln-MOF material (Eu-Ca-MOF) using Ca-MOF as the parent framework and introducing Eu3+ ions into its channels through post-synthesis modification. Eu-Ca-MOF has good photoluminescence properties and can be used as a ratiometric fluorescence sensor (I381/I590) to detect Hg2+ ions in water sensitively. The characteristic of Eu-Ca-MOF obtained is that when the material is dispersed in an aqueous solution containing Hg2+ ions, the characteristic emission of the ligand at 381 nm is enhanced, while the characteristic emission of Eu3+ at 590 nm is quenched. The peak-to-height ratio of the two emissions can be used to achieve highly sensitive detection of Hg2+ ions even in the presence of other potentially competing analytes. In addition, Hg2+ induces Eu-Ca-MOF to produce a significant ratiometric luminescence response, which changes its luminescence color from red to blue, which is beneficial to visual analysis of naked eyes. At the same time, Eu-Ca-MOF has a wider detection range (0.02-200 µM), and a lower limit detection (2.6 nM) for Hg2+ ions. The lanthanide compounds prepared by post-synthetic modification provide an effective synthesis strategy for photoluminescent materials.

A ratiometric sensor for selective detection of Hg2+ ions by combining second-order scattering and fluorescence signals of MIL-68(In)-NH2.

Ratio fluorescence has attracted much attention because of its self-calibration properties. However, it is difficult to obtain suitable fluorescent materials with well-resolved signals simultaneously under one excitation. In this work, we report a different strategy, using MIL-68(In)-NH2 as both the fluorescence element and the scattered light unit, and coupling the fluorescence and the scattered light to construct the fluorescence and scattered light ratio system. Based on the optical properties and the second-order scattering (SOS) of the material nanoparticles, the synthesized MIL-68(In)-NH2 can be used to realize the ratio detection of Hg2+. Because the scattering intensity of small particle MIL-68(In)-NH2 is weak, SOS is not obvious. When Hg2+ is introduced the coordination reaction between the amino nitrogen atoms of MIL-68(In)-NH2 and Hg2+ make the particles larger, resulting in the decrease of fluorescence and the enhancement of SOS. As a result, a novel Hg2+ ratiometric detection method is developed by using the dual signal responses of the fluorescence and scattering. Under the optimal conditions (pH = 6, reaction time 5 min, room temperature, and the maximum excitation wavelength 365 nm), the linear range of the method is 0-100 µM, and the detection limit is 5.8 nM (Ksv = 9.89 × 109 M-1). In addition, the probe is successfully used to evaluate Hg2+ in actual water samples. Compared with the traditional method of recording only the fluorescence signal, the proposed fluorescence-scattering method provides a new strategy for the design of ratiometric sensors.

A novel core-shell coordination assembled hybrid via postsynthetic metal exchange for simultaneous detection and removal of tetracycline.

Metal-organic frameworks (MOFs) can perform later transformation without compromising the integrity of the overall framework, and a variety of chemical reactions can be used to modify framework components. Postsynthetic modification (PSM) of MOFs has been developed as an alternative strategy that can expand the range of MOF functional groups. Considering the p-type semiconducting visible light active performance of CuBi2O4 (CBO) and the unique porous nanostructure and stability of zeolitic imidazolate framework-8 (ZIF-8), in this work, a novel core-shell coordination assembled hybrid based on p-type semiconductor@MOFs (Eu-CBO@ZIF-8) is prepared for the first time via in-situ growth and postsynthetic metal exchange. A series of detailed characterizations were conducted to confirm the successful synthesis of the material. Moreover, we are focusing on using this material as a new dual-functional sensing material for simultaneous detection and removal of tetracycline (TC), which shows an outstanding analytical performance with a low detection limit of 17 nM, relatively broad linear range (0-70 µM), fast response of less than 120 s, and excellent adsorption performance (377.07 mg g-1) toward TC. In addition, the sensitive luminescence response caused by TC makes Eu-CBO@ZIF-8 undergo a significant color transition from dark to red under UV-lamps, which is beneficial for visual analysis with the naked eye. The possible detection and adsorption mechanisms, including coordination between Eu3+ and the detected substance, the hydrogen bond between ligand and the detected substance, were further discussed. In addition, the practical feasibility of Eu-CBO@ZIF-8 for TC sensing was also studied, with a satisfactory recovery rate of 96.9%-104.6% and RSD ≤3.32%. These results indicate that this material can be used for the detection and adsorption of TC in actual samples.

High School Students' Online Learning Ineffectiveness in Experimental Courses During the COVID-19 Pandemic.

Due to the COVID-19 pandemic, online learning has been adopted in all stages of education. This sudden change from traditional learning to 100% online learning may affect students' learning effectiveness, especially in experimental courses. However, there has been little discussion of experimental courses conducted entirely through online learning. To address this gap, the present study investigated factors affecting high school students' online learning ineffectiveness (OLI) in online experimental courses, particularly online science experimental courses. The role of gender was also explored to understand whether it affects participants' OLI. An ANOVA was conducted to analyze the data from a survey of 347 online learners in high schools. The results indicated that the number of online experimental courses and the duration of online hands-on learning were negatively related to the high school students' OLI. Meanwhile, the study found that the high school participants' OLI differed by gender, with female students more likely than males to have OLI in the context of online learning. The results of this study can provide a reference for teachers who conduct online experimental courses and wish to improve their online teaching, not only during the COVID-19 lockdown, but also in other pandemic periods.