The authors describe cobalt phosphide (CoP) nanowires for use in sensitive fluorometric determination of the activity of the enzyme telomerase. A hybridization chain reaction (HCR) is applied to amplify the signal and carboxyfluorescein (FAM)-labelled hairpin probes (H1 and H2) are applied to match the telomeric DNA sequence. The CoP nanowires act as both the photoinduced electron transfer (PET) acceptor to induce fluorescence quenching, and also as an efficient probe carrier to facilitate telomerase imaging in living cells. The telomerase-triggered primer extension initiates an alternating hybridization reaction between H1 and H2. These result in the dissociation of FAM-labelled probes from CoP nanowires and thus an enhancement of the green fluorescence. The method is fairly simple and was applied to the detection of three types of cancer cells. The detection limit is as low as 7 cells (in case of HeLa cells). Conceivably, the method has a large potential in terms of inhibitor drug screening. Graphical abstract Schematic presentation of telomerase detection based on cobalt phosphide (CoP) nanowires and hybridization chain reaction (HCR). The telomerase-triggered primer extension can initiate the alternating hybridization reaction between carboxyfluorescein (FAM)-labelled hairpin probes (H1 and H2), and the generated long DNA duplex cannot be adsorbed on the CoP nanowires. This prevents the photoinduced electron transfer (PET) from FAM to CoP nanowires.