Label-free detection of exosomes based on ssDNA-modulated oxidase-mimicking activity of CuCo₂O₄ nanorods

A label-free method for exosome detection was proposed. It is based on the target-responsive controllability of oxidase-like activity of Cu/Co bimetallic metal-organic frameworks (CuCo₂O₄ nanorods). In the absence of exosomes, the oxidase-like activity was inhibited due to the adsorption of CD63 aptamer onto nanorods' surface. In the presence of exosomes, CD63 aptamer was disassembled from CuCo₂O₄ nanorods by virtue of CD63 aptamer-exosome recognition, which resulted in the recovery of oxidase-like activity. The activity inhibition is attributed to the fact that the ssDNA adsorption hindered the electron transfer between CuCo₂O₄ nanorods and colorimetric substrates. Under optimal conditions, a sensitive colorimetric method for detecting exosomes was established over a range of 5.6 × 10⁴ to 8.9 × 10⁵ particles μL^-1 with a detection limit of 4.5 × 10³ particles μL^-1. The method was further applied in distinguishing healthy people and breast cancer patients by testing exosomes in the serum samples and showed satisfying differentiation ability.