| Summary: | A rapid, sensitive and reliable indicator displacement assay (IDA) for specific detection of 2′- and 3′-deoxyadenosine (2′-dAde and 3′-dAde), the latter is also known as cordycepin, was established. The formation of inclusion complex between protonated acridine orange (AOH<sup>+</sup>) and cucurbit[7]uril (CB7) resulted in the hypochromic shift of fluorescent emission from 530 nm to 512 nm. Addition of cordycepin to the highly fluorescent AOH<sup>+</sup>/CB7 complex resulted in a unique tripartite AOH<sup>+</sup>/CB7/dAde complex with diminished fluorescence, and such reduction in emission intensity serves as the basis for our novel sensing system. The detection limits were 11 and 82 μM for 2′- and 3′-deoxyadenosine, respectively. The proposed method also demonstrated high selectivity toward 2′- and 3′-deoxyadenosine, owing to the inability of other deoxynucleosides, nucleosides and nucleotides commonly found in <i>Cordyceps</i> spp. to displace the AOH<sup>+</sup> from the AOH<sup>+</sup>/CB7 complex, which was confirmed by isothermal titration calorimetry (ITC), UV-Visible and proton nuclear magnetic resonance (<sup>1</sup>H-NMR) spectroscopy. Our method was successfully implemented in the analysis of cordycepin in commercially available <i>Ophiocordyceps</i> and <i>Cordyceps</i> supplements, providing a novel and effective tool for quality assessment of these precious fungi with several health benefits.
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