Fluorescence sensing telomerase activity: From extracellular detection to in situ imaging

•This review paper systematically summarized the novel strategies for telomerase assays by fluorescence sensors.•Different fluorescent sensing telomerase activity: from extracellular detection to in situ imaging.•The current challenges and future directions of Intracellular and extracellular telomerase activity were highlighted. Human telomerase is a ribonucleoprotein complex consisting of telomerase RNA and telomerase reverse transcriptase that catalyzes the addition of (TTAGGG)n repeat sequences to the 3′-end of the telomere. As almost all malignant cells overexpress telomerase, and it is seen as a diagnostic and prognostic cancer biomarker. Consequently, accurate and reliable quantification of telomerase activity is essential in medical diagnostics. A series of novel probes of telomerase activity, including quantum dot-based biosensors, gold nanoparticles, and mesoporous silica nanoparticles, and novel fluorescent labels such as aggregation induced emission (AIE) luminogens and conjugated polymer, have recently been developed and tested. This review describes the available fluorescence assays of extracellular and in situ telomerase activity. Current challenges and future research directions are discussed.