Advanced cortisol detection: A cMWCNTs-enhanced MB@Zr-MOF ratiometric electrochemical aptasensor.

A ratiometric electrochemical aptasensor was developed for ultra-sensitive detection of cortisol using aptamer (Apt) as recognition element, methylene blue (MB) as signal probe, and zirconium metal-organic framework (Zr-MOF) as carrier loaded with abundant MB for signal amplification. The carboxylated multi-walled carbon nanotubes (cMWCNTs)-modified Au electrode showed excellent electrochemical performance to immobilize complementary DNA (cDNA) for hybridizing with MB@Zr-MOF-Apt via amide bonds. In the presence of cortisol, it would compete with cDNA for binding the Apt, resulting in the detachment of MB@Zr-MOF-Apt complex from the electrode surface, and the electrochemical signal of MB was decreased, while that of [Fe(CN)₆]^3-/4- was basically unchanged. The ratio of the electrochemical signals of [Fe(CN)₆]^3-/4- to MB was proportional to the cortisol concentration. Due to the greatly enhanced conductivity of the cMWCNTs-decorated Au electrode and the largely improved EC signals of Zr-MOF encapsulated MB probes, this ratiometric electrochemical aptasensor offered high sensitivity with an ultra-low detection limit of 0.0046 nM and a wide linearity of 0.01-1000 nM, as well as satisfactory accuracy with recoveries of 93.79-106.76 % in artificial sweat samples, providing a potential strategy for the detection of more trace hormones in different clinical samples by simply replacing the corresponding aptamers.