Construction of Donor–Acceptor Heteroporous Covalent Organic Frameworks as Photoregulated Oxidase-like Nanozymes for Sensing Signal Amplification

Abstract

Nanomaterials with enzyme-like characteristics (called nanozymes) show their extreme potentials as alternatives to natural enzymes. Covalent organic frameworks (COFs) as metal-free nanozymes have attracted huge attention for catalytic applications due to their flexible molecular design and synthetic strategies and conjugated, porous, and chemically stable architectures. Designing high-performance two-dimensional (2D) porous COF materials embedded with functional building units for modulating nanozymes' catalytic activity is of immense importance in contemporary research. The proper combination of donor−acceptor (D−A) fragments within a porous COF skeleton is an effective strategy to decrease the band gap and provide a strong charge-transfer pathway for highly effective charge separation. Herein, two donor−acceptor heteroporous COFs using an electron-deficient 4,4′-(thiazolo[5,4-d]thiazole-2,5-diyl)dibenzaldehyde (Tz) unit or 4,4′-(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzaldehyde (Td) unit and electron-rich tetrakis(4-aminophenyl)ethane (ETTA) linkers were presented. The resulting crystalline and heteroporous COFs showed outstanding oxidase-like activity under light irradiation, which can catalyze the oxidation of typical substrates and corresponding evolution in color and absorption. The light-activatable ETTA-Tz COF with prominent oxidase-like activity can serve as a colorimetric probe for quantitative detection of sulfide ions with a linear range of 1−50 μM and a detection limit of 0.27 μM within 3 min. The colorimetric approach could also be used for sulfide ion detection in human serum samples. The research demonstrated the future potential of D−A motifs within fully conjugated COFs to obtain excellent mimic enzyme activity.

Fluorescence/Colorimetry/Smartphone Triple-Mode Sensing of Dopamine by a COF-Based Peroxidase-Mimic Platform

“…Covalent organic frameworks (COFs) are crystalline porous networks linked by covalent bonds, which have developed rapidly since the year 2005 . With designable structures, definite crystalline skeletons, tunable ordered pore channels, easily functionalized sites, and chemical stability, COFs are widely exploited in heterogeneous catalysis, , adsorption, separation, energy storage, and sensing. , Among the above applications, as the intersection of catalysis and sensing, COF-based enzyme mimics , are evolving and show good performances in detections. − For example, AuNPs@Tp-Bpy exhibited peroxidase-mimic activity, which can serve as colorimetric sensors for Hg 2+ ; ETTA-Tz COF was the oxidase mimic, and under light irradiation, it can perform the colorimetric determination of sulfide ions . Nevertheless, COF-based enzyme-mimic systems for multi-mode sensing are still rare.…”

“…22−25 For example, AuNPs@Tp-Bpy exhibited peroxidase-mimic activity, which can serve as colorimetric sensors for Hg 2+ ; 22 ETTA-Tz COF was the oxidase mimic, and under light irradiation, it can perform the colorimetric determination of sulfide ions. 25 Nevertheless, COF-based enzyme-mimic systems for multi-mode sensing are still rare. Dopamine (DA) is a neurotransmitter, which plays a crucial role in human physiological mechanisms, such as human movement, behavior, cognition, and memory.…”

Molecular Recognition-Triggered Aptazyme Sensor Using a Co-MOF@MCA Hybrid Nanostructure as Signal Labels for Adenosine Triphosphate Detection in Food Samples

“…In recent years, to improve the sensitivity of biosensors, many researchers have explored various signal amplification strategies. , Multiple enzymes are studied in electrochemical sensors, such as terminal deoxynucleotidyl transferase (TdT), exonuclease I, exonuclease III, and DNA polymerase . These enzymes can enhance the performance of electrochemical biosensors to a certain extent.…”

Highly Selective Colorimetric Detection of Cu2+ Using EDTA-Complexed Chlorophyll-Copper/ZnO Nanorods with Cavities Specific to Cu2+ as a Light-Activated Nanozyme

“…Compared with other peroxidase mimics, light-activated peroxidase mimics can avoid the use of hydrogen peroxide in colorimetric detection, effectively reducing the risk of secondary pollution. Some photocatalysts, such as TiO 2, MMoO 4 (M = Co, Ni), water-soluble carbon nitride (WSCN), carbon quantum dots, photosensitive metal–organic frameworks, etc., have been studied as light-activated peroxide mimics and applied in colorimetric detection.…”

“…Compared with other peroxidase mimics, light-activated peroxidase mimics can avoid the use of hydrogen peroxide in colorimetric detection, effectively reducing the risk of secondary pollution. Some photocatalysts, such as TiO 2, 13 MMoO 4 (M = Co, Ni), 14 water-soluble carbon nitride (WSCN), 15 carbon quantum dots, 16 photosensitive metal−organic frameworks, 17 been studied as light-activated peroxide mimics and applied in colorimetric detection. As a common photoactive semiconductor, ZnO is widely used in photocatalytic reactions, photodegradation of pollutants, photoelectrochemical detection, etc., because it can be obtained easily and its photoactivity can be improved after being modified or doped.…”