Nowadays, microbial visual sensing platforms based on aptamers and metal-organic frameworks (MOFs) have been reported, but most of them require complex modification processes, which undoubtedly increases the difficulty of detection. Here, we constructed modified-free magnetic-aptamer MOF (Fe 3 O 4 [email protected] ) and signal-aptamer MOF ( [email protected] ), which effectively avoided complex biological modification. The rapid sensing strategy was adopted in this method to avoid the complicated washing steps and greatly shorten the detection time. The performance of this method was verified by taking Vibrio parahaemolyticus (VP) as the mode target. The limit of detection (LOD) of VP is 10 2 CFU/mL. Furthermore, this strategy can achieve rapid visual analysis of other target objects only by changing different aptamers, and has broad application prospects in the fields of rapid detection of food pollutants, rapid medical diagnosis, environmental monitoring, etc.
Ratiometric electrochemical assays have been demonstrated to be more sensitive and selective in various sensing events, mainly due to their affordable built-in correction and good self-reference capability. But it is known that complicated modification and labeling operations usually are necessary for the construction of ratiometric electrochemical assays, therefore is a hot and important issue needing consideration carefully. We herein report a new yet simple bare electrode-based ratiometric electrochemical bioassay to achieve sensitive and selective analysis of alkaline phosphatase (ALP), using a liquid phase system that contains CoOOH nanozymes and commercially available indicator substrate. This proposed bioassay works based on the ratiometric change of dual electrochemical signals, arising from an exclusive target ALP-triggered hydrolysis of electrochemical substrate p-nitrophenyl phosphate (PNPP). In this design, the two hydrolyzed products of electrochemically active p-nitrophenol (PNP) and electrochemically inactive phosphate anion (PO 4 3− ) are responsible together for the ratiometric electrochemical analysis of ALP. PNP exhibits a straightforward current response toward ALP content; however, PO 4 3− cannot show a direct electrochemical signal thus is rationally designed to offer an alternative response by linking it with the specific CoOOH nanozyme-catalyzed reaction of 3,3′,5,5′-tetramethylbenzidine (TMB) and H 2 O 2 , in which the nanozyme-catalyzed product oxTMB shows a direct reduction current at the GCE, and significantly decreases with increasing PO 4 3− species due to the good inhibition of PO 4 3− toward CoOOH nanozyme activity. As a result, a ratiometric electrochemical strategy for ALP analysis with a low limit of detection of 0.366 U/L (S/N = 3) was successfully achieved by integrating the above direct and indirect dual electrochemical responses. This developed bioassay can allow the quantitative diagnosis of ALP activity especially with a label-free and modification-free merit, therefore paving the way for simple, convenient, and portable electroanalytical tools in biosensing design and application. Graphical abstract
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