A Mimosa-Inspired Cell-Surface-Anchored Ratiometric DNA Nanosensor for High-Resolution and Sensitive Response of...

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.