We present here a p H-responsive activatable aptamer probe for targeted cancer imaging based on i-motif-driven conformation alteration. This p H-responsive activatable aptamer probe is composed of two single-stranded DNA. One was used for target recognition, containing a central, target specific aptamer sequence at the 3′-end and an extension sequence at the 5′-end with 5-carboxytetramethylrhodamine(TAMRA) label(denoted as strand A). The other(strand I), being competent to work on the formation of i-motif structure, contained four stretches of the cytosine(C) rich domain and was labeled with a Black Hole Quencher 2(BHQ2) at the 3′-end. At neutral or slightly alkaline p H, strand I was hybridized to the extension sequence of strand A to form a double-stranded DNA probe, termed i-motif-based activatable aptamer probe(I-AAP). Because of proximityinduced energy transfer, the I-AAP was in a "signal off" state. The slightly acidic p H enforced the strand I to form an intramolecular i-motif and then initiated the dehybridization of I-AAP, leading to fluorescence readout in the target recognition. As a demonstration, AS1411 aptamer was used for MCF-7 cells imaging. It was displayed that the I-AAP could be carried out for target cancer cells imaging after being activated in slightly acidic environment. The applicability of I-AAP for tumor tissues imaging has been also investigated by using the isolated MCF-7 tumor tissues. These results implied the I-AAP strategy is promising as a novel approach for cancer imaging.
