A bifunctional metal organic framework of type Fe(III)-BTC for cascade (enzymatic and enzyme-mimicking) colorimetric determination of glucose.

A metal organic framework (MOF) of type Fe(III)-BTC (where BTC is 1,3,5-benzenetricarboxylic acid) was utilized to construct an integrated system for cascade colorimetric determination of glucose. The MOF performs a dual function in acting (a) as a peroxidase (POx) mimic, and (b) as a solid support for immobilization of glucose oxidase (GOx). The MOF was prepared by a one-pot method. Glucose is consumed while H2O2 is produced during the enzymatic oxidation by GOx. In the presence of H2O2, the POx mimic catalytically oxidizes 3,3',5,5'-tetramethylbenzidine (TMB) to form a blue-green product. The absorbance of oxidized TMB (measured at 652 nm) increases linearly in the 5.0-100 µM glucose concentration range, and the detection limit is 2.4 µM. The GOx@Fe-BTC MOF was successfully applied to the determination of glucose in serum. Graphical abstract Schematic presentation of a bifunctional metal organic framework of type Fe-BTC for cascade (enzymatic and enzyme-mimicking) colorimetric determination of glucose. The Fe-BTC performs a dual function in acting as both a peroxidase mimic and support for immobilizing glucose oxidase. Using the integrated enzyme, a colorimetric method was successfully applied to one-step detection of glucose in human serum.

Synchronous Construction of Hierarchical Porosity and Thiol Functionalization in COFs for Selective Extraction of Cationic Dyes in Water Samples.

Pore size and functionalization are two critical factors for covalent organic frameworks (COFs) as effective adsorbents. However, due to the low crystallinity of COFs, it is a grand challenge to accomplish pore diameter adjustment and functionalization at the same time. In this work, we developed a simple and ingenious strategy, cutting off linkage, to synchronously construct hierarchical porosity and modify thiol groups in COFs under mild conditions. The hybrid COFs containing disulfide bonds were designed and synthesized, and then the disulfide bonds were cleaved by glutathione, resulting in the formation of thiol groups as well as the increase in pore size caused by skeleton defects. The pore diameter of thiol-functionalized hierarchical porous COFs (denoted as HP-TpEDA-SH) was concentrated at 2.6 and 3.5 nm. Thanks to the electrostatic attraction of thiol groups to cationic dyes and the higher number of available adsorption sites, the maximum extraction amounts of methylene blue (MB), malachite green (MG), and crystal violet (CV) by HP-TpEDA-SH were 2.6, 2.1, and 3.3 times those of microporous COFs under optimal extraction conditions, respectively. The proposed analytical method (solid-phase extraction-high-performance liquid chromatography/ultraviolet (SPE-HPLC/UV)) with HP-TpEDA-SH as the adsorbent showed low detection limits of 1.3, 0.13, and 0.12 µg·L-1 for MB, MG, and CV, respectively. The recoveries of three spiked water samples ranged from 81.5 to 113.8%, with relative standard deviations (RSDs) less than 9.7%. This work not only opened a new avenue for the preparation of functionalized hierarchical porous COFs but also established an effective method for detecting trace cationic dyes in fishery water.

Colorimetric Detection of Salicylic Acid in Aspirin Using MIL-53(Fe) Nanozyme.

The impurity of salicylic acid (SA) in aspirin is a required inspection item for drug quality control. Since free SA is significantly toxic for humans, the content determination of free SA is absolutely necessary to ensure people's health. In this work, a facile colorimetric method was developed for the detection of SA in aspirin by utilizing the MIL-53(Fe) nanozyme. As MIL-53(Fe) possesses enzyme mimicking catalytic activity, 3,3,5,5-tetramethylbenzidine (TMB) can be easily oxidized to blue-oxidized TMB (oxTMB) with the existence of H2O2. Moreover, an inhibition effect on the catalytic activity of the MIL-53(Fe) nanozyme is induced due to the specific complexation between SA and Fe3+ in the center of MIL-53(Fe), which results in a lighter color in the oxTMB. The color change of oxTMB can be seen easily by the naked eye with the addition of different concentrations of SA. Thus, a simple colorimetric platform was established for effectively monitoring SA. A good linear relationship (R 2 = 0.9990) was obtained in the concentration range of 0.4-28 µmol L-1, and the detection limit was 0.26 µmol L-1. In particular, the rationally designed system has been well-applied to the detection of SA impurity in aspirin. Satisfyingly, the detection results are highly in accord with those of HPLC. This novel colorimetric platform broadens the application prospects of nanozymes in the field of pharmaceutical analysis.

Colorimetric detection of blood glucose based on GOx@ZIF-8@Fe-polydopamine cascade reaction.

Zeolitic imidazolate framework-8 (ZIF-8) has become one of the most typical examples of nanostructures for multi-enzyme immobilization due to its economical, mild and easy synthesis process. However, ZIF-8 nanocrystals are easily decomposed under acidic conditions. To solve this problem, the Fe-polydopamine (Fe-PDA) was bonded with ZIF-8 surface to form ZIF-8@Fe-PDA hybrid shell with good stability. Based on this, we developed glucose oxidase@ZIF-8@Fe-PDA (GOx@ZIF-8@Fe-PDA) integrated nanozymes (INAzymes) with cascade reactions via a mild and environmentally friendly method. In order to synthesize the INAzymes, GOx was first embedded in ZIF-8 by coprecipitation (GOx@ZIF-8), and then GOx@ZIF-8 was bonded with Fe-PDA, which acted as a peroxidase mimic. The ZIF-8@Fe-PDA hybrid shell protected the INAzymes nanostructure from degradation under acidic conditions, which results in good chemical stability of the GOx@ZIF-8@Fe-PDA. In the INAzymes system, glucose is converted to gluconic acid by GOx in the presence of oxygen to produce H2O2 as an intermediate. The H2O2 reacts rapidly with Fe-PDA to generate OH, which oxidizes 3,3',5,5'-tetramethylbenzidine (TMB). The UV absorbance of oxidized TMB is directly proportional to the glucose concentration, and has a good linear relationship in the range of 5.0-100.0 µM glucose with detection limit of 1.1 µM. The INAzymes system has been successfully applied to rapid colorimetric detection of blood glucose levels. The INAzymes system exhibits high catalytic activity, excellent sensitivity, and enhanced chemical stability, playing great promise in clinical diagnosis and biosensing.