We present here a pH-responsive activatable aptamer probe for targeted cancer imaging based on i-motif-driven conformation alteration. This pH-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 ｜）, 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 pH, strand ｜ 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 proximity- induced energy transfer, the I-AAP was in a ＂signal off＇ state. The slightly acidic pH enforced the strand I to form an intramo- lecular 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.
