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 HO as an intermediate. The HO 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.