A phosphorescent supramolecular foldamer is conveniently constructed by the 1:1 host–guest complexation with cucurbit[8]uril and 1,2-diaminocyclohexane-bridged 4-(4-bromophenyl)-pyridinium salt. The tightly compact host–guest complexation in molecular foldamer can greatly suppress the fluorescence emissive channel and promote the intersystem crossing from singlet to triplet states, thus leading to the green phosphorescence at ambient temperature in aqueous solution. More intriguingly, the phosphorescence emission shows very rapid and sensitive responsiveness to different antibiotics in both inanimate milieu and living cells. Remarkably, the limit of detection of such binary inclusion complex toward sulfamethazine can reach as low as 1.86 × 10⁻⁷ mol/L. Thus, it is envisaged that this supramolecular nanoplatform featuring unique complexation-enhanced phosphorescence emission may hold great promise in sensing and detecting many other biological targets under physiological environment.