Target-controlled in situ formation of G-quadruplex DNAzyme for a sensitive visual assay of telomerase activity

It is of great importance to achieve facile and reliable detection of telomerase because it is an important cancer biomarker. The complex components of cell extracts and ultra-low concentration of telomerase makes it more difficult to realize a simple and sensitive visual detection of telomerase activity. Herein, a facile and sensitive visual strategy was developed for the detection of telomerase based on the telomerase-controlled in situ formation of a G-quadruplex-hemin DNAzyme. To avoid the influence of the complex components of cell extracts, a telomerase substrate (TS) was immobilized onto the surface of magnetic beads (MBs) to form a MB/TS complex. MB/TS incubated with telomerase can add several TTAGGG repeat units to the 3' terminal of TS. After magnetic separation and washing, these G-rich elongated DNA folded into numerous G-quadruplex-hemin DNAzymes under the aid of K+ and hemin, which efficiently catalysed the TMB/H2O2 reaction. Magnetic separation basically eliminated the non-specific background interference from other cell extracts and redundant hemin. Taking full advantage of the in situ formation of multiple catalysts, the telomerase activity could be sensitively evaluated by a color change of the TMB/H2O2 solution. The telomerase activity down to 1 HeLa cell per μL and 0.5 HeLa cell per μL can be measured by the naked eye and UV-vis spectroscopy, respectively. Due to the magnetic separation and enrichment, the sensitivity was obviously improved compared with the previous colorimetric assay. Meanwhile, the telomerase activity of 5 HeLa cells per μL in human serum can be visually detected. Therefore, this study provides a facile, cost-effective and robust colorimetric assay for the visual detection of telomerase activity, which holds great potential in telomerase-based cancer clinical diagnostics.