Telomeres are guanosine-rich nucleic-acid chains that fold, in the presence of K(+) ions and hemin, into the telomeric hemin/G-quadruplex structure, exhibiting horseradish peroxidase mimicking functions. The telomeric hemin/G-quadruplex structures catalyze the oxidation of thiols (e.g., l-cysteine) into disulfides (e.g., cystine). As l-cysteine stimulates the aggregation of Au nanoparticles (NPs), accompanied by absorbance changes from red (individual Au NPs) to blue (aggregated Au NPs), the process is implemented to quantitatively analyze the activity (content) of telomerase, a versatile biomarker for cancer cells. Telomerase extracted from 293T cancer cells catalyzes, in the presence of a dNTPs mixture and an appropriate primer probe, the telomerization process, leading to the generation of catalytic telomeric hemin/G-quadruplex chains that control the l-cysteine-mediated aggregation of Au NPs. The extent of aggregation is thus controlled by the concentration of telomerase. The method enabled the detection of telomerase with a detection limit of 27 cells/μL. The spectral changes accompanying the aggregation of Au NPs are further supported by transmission electron microscopy imaging.