In this work, we fabricated cobalt-doped carbon quantum dots (Co-CQDs) by a one-pot hydrothermal method with cobalt tetraphenylporphyrin and 1,2-ethanediamine as precursors. The morphology and structure of the Co-CQDs were characterized through transmission electron microscopy, X-ray diffraction spectra, Fourier transform infrared, and X-ray photoelectron spectra. The Co-CQDs emitted intense blue luminescence under ultraviolet irradiation and exhibited a typical excitation-dependent emission property. Moreover, they can act as a fluorescent probe for the detection of Fe3+ and ascorbic acid (AA) with high selectivity and sensitivity through an "on-off-on" mode. The limit of detection (LOD) of Fe3+ was measured as 38 μM (S/N = 3). The quenched emission of carbon quantum dots can be recovered with the addition of ascorbic acid (AA) to the Co-CQDs/Fe3+ system. The change of fluorescence was linear with the concentration of AA (0.6-1.6 mM) with the LOD of 18 μM. Furthermore, the Co-CQDs exhibited high oxidase-like catalytic behavior, which could convert transparent 3,3',5,5'-tetramethylbenzidine (TMB) into blue ox-TMB by dissolved oxygen. After adding ascorbic acid to the Co-CQDs/TMB system, the blue color of the solution faded due to the reduction of blue ox-TMB to colorless TMB. Based on this phenomenon, the Co-CQDs were capable of detecting AA (10-400 μM) with the LOD of 0.27 μM. The fluorometric and colorimetric assays based on the Co-CQDs for the AA detection were then successfully applied in fresh fruits. Furthermore, the high biocompatibility of the Co-CQDs against HeLa cells was verified by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. Thus, the Co-CQDs could be used as a powerful tool for the detection of AA in real samples through a dual-mode method.

Cobalt-Doped Carbon Quantum Dots with Peroxidase-Mimetic Activity for Ascorbic Acid Detection through Both Fluorometric and Colorimetric Methods.

The selective detection of ascorbic acid (AA) and glutathione (GSH) is very significant for dietary guidance and health monitoring. Herein, we prepared a metalloporphyrin-containing porous organic framework (Co-POP) via a facile Friedel–Crafts reaction. Co-POP was able to catalyze the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to a blue substrate (ox-TMB) in the role of dissolved oxygen. The best catalytic performance of Co-POP was realized at 25 °C and pH 4 according to the optimized experiments. Because of the high oxidase-like activity of Co-POP, a simple and sensitive colorimetric assay was established for the determination of AA and GSH in a wide range. The detection limit (LOD) of AA and GSH was calculated to be 1.60 and 0.71 μM, respectively. Finally, the application of the colorimetric sensor for AA and GSH detection in real samples (fruits and diluted serum) was also evaluated, suggesting its great potential for biocatalysis and biosensing.

Oxidase Mimetic Activity of a Metalloporphyrin-Containing Porous Organic Polymer and Its Applications for Colorimetric Detection of Both Ascorbic Acid and Glutathione

Multimodal detection is a promising paradigm because of its advantages of expanding usage scenarios and improving reliability. However, it is very challenging to design reasonable strategies to achieve the multimodal sensing of targets. Herein, we developed an unprecedented bimodal ratiometric colorimetric/fluorometric method by exploring a novel bifunctional artificial oxidase mimic, Mn-doped N-rich carbon dots (Mn-CDs), to achieve the high-performance determination of nitrite in complicated matrices. The Mn-CDs exhibited both tunable photoluminescence and high oxidase-like activity, effectively catalyzing the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to generate blue TMB+. When nitrite was introduced, the TMB+ species generated would specifically react with nitrite to produce diazotized TMB+, resulting in a color change from blue to green and finally to yellow. Simultaneously, the fluorescence of Mn-CDs was quenched by the diazotized TMB+ product via the inner filter effect. Hence, the existence of nitrite could lead to the simultaneous variations of visual color and photoluminescence, providing the principal basis for the bimodal ratiometric colorimetric/fluorometric quantification of the target. With the method, excellent sensitivity, selectivity, reliability, and practicability for nitrite detection were verified. Our work proposes a new bimodal strategy for nitrite measurement using bifunctional CDs-based enzyme mimics, which will inspire future effort on the exploration of promising multifunctional nanozymes and their advanced applications in biochemical sensing.

Bifunctional Mn-Doped N-Rich Carbon Dots with Tunable Photoluminescence and Oxidase-Mimetic Activity Enabling Bimodal Ratiometric Colorimetric/Fluorometric Detection of Nitrite.

Covalent Organic Frameworks (COFs) for heterogeneous catalysis: Recent trends in design and synthesis with structure-activity relationship

In the context of carbon neutrality, significant progress has been made in the exploration of advanced porous organic polymers (POPs) as a promising and emerging platform for heterogeneous CO2 catalysis in the past few decades owing to their structural designability and functional diversity, high thermal and chemical stability, and ability to accelerate the mass-transfer rate. Inspired by natural biological systems, incorporation of metalloporphyrin moieties into the porous skeletons via covalent linkages for affording metalloporphyrin-based POPs can not only facilitate strong interactions with Lewis acidic CO2 molecules, but also can establish highly efficient biomimetic catalytic systems. In this review, we aim to summarise the recent advances made in the task-specific design and synthesis of metalloporphyrin-based POPs and their functionalization for conversion of CO2 into cyclic carbonates. The focus mainly lies on the rational construction of targeted building blocks and the diverse synthetic strategies for preparing metalloporphyrin-based POPs with different functionalities and compositions. Single/multi-active-site synergistic catalytic models relevant to epoxide activation provide feasible theoretical guidance to the multifunctionalization of metalloporphyrin-based POPs. Finally, we discuss the key opportunities and challenges in this field and also highlight possible avenues for future directions to help researchers to fabricate ideal catalysts for industrial applications.

Synthesis of metalloporphyrin-based porous organic polymers and their functionalization for conversion of CO2 into cyclic carbonates: recent advances, opportunities and challenges

Metalloporphyrin-based porous organic polymer as an efficient catalyst for cycloaddition of epoxides and CO2

The chemical fixation of CO2 into epoxides to afford cyclic carbonates has received considerable attention due to the 100% atom economy, which is in accordance with the concept of green chemistry and sustainable development. Here, we develop a facile and high-output strategy to fabricate cobalt phthalocyanine-based nanodots (CoPc-NDs) by the carbonization of citric acid and cobalt phthalocyanine complex. Due to the high thermal stability of phthalocyanine, the macrocycle is well preserved in the core of nanodots under the controlled hydrothermal temperature. The as-prepared CoPc-NDs disperse well in water with a nanoscale size about 70 nm. The morphology and chemical composition of CoPc-NDs are characterized by transmission electron microscopy, dynamic laser scattering, X-ray powder diffraction, X-ray energy spectrum, and absorption spectrum. Compared with the organometallic complex, such as cobalt tetra-nitrophthalocyanine and cobalt tetra-aminophthalocyanine, CoPc-NDs show better catalytic performance for the conversion of epoxides into carbonates. The kinetic experiments of three catalysts are investigated and compared under the same conditions using styrene oxide as substrate. Finally, a tentative mechanism for the catalysis was proposed and described.

Cobalt phthalocyanine-based nanodots as efficient catalysts for chemical conversion of CO2 under ambient conditions

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Papers

Cobalt-Doped Carbon Quantum Dots with Peroxidase-Mimetic Activity for Ascorbic Acid Detection through Both Fluorometric and Colorimetric Methods.

Oxidase Mimetic Activity of a Metalloporphyrin-Containing Porous Organic Polymer and Its Applications for Colorimetric Detection of Both Ascorbic Acid and Glutathione

Metalloporphyrin-based porous organic polymer as an efficient catalyst for cycloaddition of epoxides and CO2

Cobalt phthalocyanine-based nanodots as efficient catalysts for chemical conversion of CO2 under ambient conditions