A covalent organic framework (COF)-MnO2 based dual signal sensing platform for sensitive alkaline phosphatase activity detection via dynamic regulating the mimicking oxidase content

Au confined covalent organic frameworks nanoenzyme integrated with sodium alginate microsphere for portable colorimetric tannic acid detection

Developing a portable, affordable, and instrument-free colorimetric sensor for tannic acid detection is crucial for food quality assessment and health monitoring. In this work, we designed a rapid and efficient colorimetric platform by integrating Au-modified covalent organic framework (COF) with sodium alginate hydrogel microspheres for tannic acid detection assisted by mobile phone photo technology. The Au NPs with an average size of 4 nm were reduced by the bipyridine nitrogen on the COF layer under light irradiation, which enhanced the peroxidase-like activity of the nanozyme. Furthermore, the adsorption, hydrogen-bond, and covalent interaction between tannic acid and sodium alginate make the functional hydrogel microspheres exhibit enhanced colorimetric performance, with a linear range from 5.0 to 130 μM and a detection limit of 0.091 μM. In addition, the obtained colorimetric sensor was effectively used for tannic acid analysis in grape beverages and model wine samples with good recovery. This work broadens the horizon for the synthesis and design of visual sensors with functional porous COFs-based nanozyme and the development of portable and low-cost sensors by combining hydrogel microspheres, which could be applied for food analysis and quality control in real environments.

Sensitive and rapid fluorescent detection of metronidazole based on stable light-emitting vinylene-linked covalent organic framework

Development of ultra-sensitive and rapid fluorescent nanoprobe for quantitative and targeted monitoring of metronidazole is of crucial practical significance, but is of great challenge. Herein, a vinyl-linked covalent organic frameworks (sp2-BNTP-COF) was fabricated via integrating the 1,3,5-tris-(4-formylphenyl) triazine with 5,5'-bis(cyanomethyl)-2,2'-bipyridine into the skeleton. As-obtained sp2-BNTP-COF exhibited excellent luminescence characteristics with an absolute fluorescence quantum yield of 8 %. However, fluorescent emission of sp2-BNTP-COF could be sharply quenched via metronidazole based on internal filtration effect. A sensitive fluorescent strategy was built for targeted monitoring of metronidazole. Furthermore, the analysis operation could be accomplished within 20 s, which was desirable for point-of-care monitoring of metronidazole. Therefore, this work not only provides a reliable method for the sensitive, rapid, and quantitative detection of metronidazole residues based on the sp2-BNTP-COF, but also paves the way for exploring stable luminescent vinyl-linked COFs materials as promising fluorescent nanoprobes for targeted monitoring of antibiotic residues.

Colorimetric detection of alkaline phosphatase based on the off-on effect of light-responsive oxidase mimicking activity of covalent organic framework (Cu-TpBpy-COF) under near-neutral condition

A colorimetric strategy has been developed for the detection of alkaline phosphatase (ALP) activity based on the off-on effect of the catalytic activity of light-responsive oxidase mimics covalent organic framework (Cu-TpBpy-COF) in near-neutral condition. Cu-TpBpy-COF can effectively catalyze the oxidation of the colorless substrate 3,3',5,5'-tetramethylbenzidine (TMB) by oxygen to form a blue oxidized product (oxTMB) with an absorption peak at 652 nm. Cu2+ is the active center of Cu-TpBpy-COF and pyrophosphate (PPi) can form a complex with Cu2+ to weaken the catalytic activity of Cu-TpBpy-COF. In the presence of ALP, PPi is hydrolyzed into orthophosphates (Pi) with low affinity to Cu2+, thus resulting in absorbance restoration. The absorbance at 652 nm is related to ALP activity in the linear range 10-150 U·L-1 with a detection limit of 7.17 U·L-1. The recoveries of ALP in serum samples are in the range 94.7~107.0% with relative standard deviations (RSD) lower than 5%. The decisive role of Cu2+ on the enhancing catalytic activities of Cu-TpBpy-COF in neutral condition was verified by TpBpy-COF and TpBD-COF as controls, in which the main difference between them is that TpBpy-COF contains pyridine nitrogen. Upon Cu2+ modification, Cu-TpBpy-COF has better catalytic activity than TpBpy-COF in a broader pH range because of the in situ generation of Cu+ under irradiation.

Rational synthesis of carbon dots with phosphate ester group for direct mapping of endogenous alkaline phosphatase and polarity monitoring in living cells

Carbon dots (CDs) have been extensively employed in biomolecule imaging. However, the imaging of biological enzymes with CDs has not been reported, which greatly limits their application in biological imaging. Herein, for the first time, a new type of fluorescent CDs is elaborately designed to realize the direct mapping of alkaline phosphatase (ALP) in cells. The obtained phosphorus and nitrogen co-doped CDs (P, N-CDs) generate specific structures including xanthene oxide and phosphate ester, thereby enabling P, N-CDs to be exclusively cleaved by ALP without auxiliary media. The fluorescence intensity of P, N-CDs can be specifically turned on in the presence of ALP, making them powerful probes for sensitive sensing of ALP activity with a detection limit of 1.27 U·L^-1. Meanwhile, P, N-CDs possessing electron deficiency structure fulfill sensitive responding to polarity variations. The excellent photo-bleaching resistance and biocompatibility of the P, N-CDs are taken for directly mapping the intracellular endogenous ALP via turned-on fluorescence imaging, as well as real-time monitoring the polarity fluctuation in cells through ratiometric fluorescence imaging. The present work offers a new way to design and synthesize functional CDs for direct imaging of intracellular enzymes.