Parallel [TG(GA)₃]_n-homoduplexes/thioflavin T: an intense and stable fluorescent indicator for label-free biosensing

Different from the classical antiparallel DNA double-stranded structure, parallel DNA duplexes possess unique structures and potential biological functions. In this work, we found that the parallel DNA homoduplex from the [TG(GA)3]n sequence ([TG(GA)3]n-dsDNA) can dramatically enhance the fluorescence of thioflavin T (ThT), and the fluorescence enhancement is proportional to the number (n) of TG(GA)3 units in [TG(GA)3]n. Compared with the traditional G-quadruplex/ThT system, [TG(GA)3]n/ThT showed more stable and stronger fluorescence emission. In addition, coupled with an isothermal exponential amplification reaction, [TG(GA)3]3/ThT was used as a label-free fluorescent probe to detect microRNA, and the [TG(GA)3]3/ThT probe exhibited higher sensitivity than the G-quadruplex/ThT probe. This work provides a new paradigm to design label-free fluorescent biosensing/imaging systems.