i-Motif DNA molecular beacon for microRNA detection.

A molecular beacon (MB) is an oligonucleotide probe with a stem-loop structure designed to fluoresce upon hybridization with a specific target nucleic acid. Herein, we introduce a novel i-motif DNA-based molecular beacon (iMB) designed for nucleic acid detection. DNA chemical sequencing showed that the iMB can be selectively opened when its target is present, thanks to the formation of a duplex between this target and the iMB recognition loop. Phase diagrams were introduced to analyze the binding properties between the iMB and its target, and to optimize the detection conditions with different combinations of pH and temperature. Benefiting from the environment-sensitive stability and special structural feature inherent to the i-motif, this iMB exhibits a controllable sensitivity and specificity through choosing appropriate temperature/pH combination and recognition loop position in the i-motif probe. Leveraging the duplex-specific nuclease (DSN) resistance of the i-motif, we significantly enhanced the sensitivity by more than four orders of magnitude through the implementation of a DSN-assisted amplification strategy. Finally, miRNAs in cell lines and patient plasma were detected by the iMBs, as validated via quantitative reverse transcription polymerase chain reaction (RT-qPCR). This research successfully broadens the structural repertoire of MB stems to include a pH- and temperature-responsive i-motif stem.