A Mimosa-Inspired Cell-Surface-Anchored Ratiometric DNA Nanosensor for High-Resolution and Sensitive Response of Target Tumor Extracellular pH

Mimosa, a peculiar plant, can close immediately in response to external stimuli. Inspired by the stimuli-responsive behavior of mimosa, we designed a Y-shaped DNA nanosensor (regarded as DNA nanomimosa, DNM) for target tumor extracellular pH (pHe) sensing. The DNM consisted of four single-strand DNA strands, where A-strand contained an aptamer fragment and labeled with Cy5 at the 5'-end, I-strand contained an i-motif fragment, and the 3'-ends of L-strand and B-strand were labeled with Rox and BHQ2, respectively. Initially, the DNM was in an "open" state, Cy5 was separated from Rox and performed fluorescence resonance energy transfer (FRET) with neighboring BHQ2, and only Rox emitted fluorescence. When the DNM was anchored onto the cell surface through the aptamer fragment, the i-motif fragment tended to form a quadruple-helix structure due to low pH stimuli, releasing B-strand and bringing the DNM into a "close" state like stimulated mimosa. At this time, Cy5 was separated from BHQ2 but close to Rox, which led to the FRET signal generation between Rox and Cy5. The FRET ratio (Cy5/Rox) could be used as a signal for pHe sensing. Using the aptamer as an anchoring element, the DNM exhibited high cell-membrane-anchoring efficiency and excellent specificity. Additionally, relying on the pH-sensitive i-motif and twice FRET signaling mechanism, the DNM possessed a narrow pH response range (0.50 units) and performed imaging of pHe with high resolution. With these advantages, the DNM is expected to be a useful tool for the investigation of the tumor extracellular pH-related physiological processes.