A DNA-based nanosensor for in situ precise imaging of lysosomal potassium ions via pH activation

For subcellular imaging, activating probes by specific species in targeted organelles is an attractive strategy to eliminate nonspecific signals during transport. The characteristic pH (∼4.5-5.0) of lysosomes enables selective probe activation within lysosomes. In this work, a pH-activated K⁺ nanosensor for lysosomal K⁺ imaging was designed by capturing Y-typed DNA probe on linker modified gold nanoparticle. The DNA probe could be conveniently prepared by assembling dual fluorophores-labelled G-quadruplex (G4) strand, Sgc8c modified i-motif structure and capturing DNA sequence. The folding capability of G4 in the presence of K⁺ was inhibited at neutral pH, which prevented fluorescence resonance energy transfer (FRET) between two fluorophores labelled at both ends of the G4. Upon targeted delivery into lysosomes, the conformational change of the i-motif structure was activated by acidic pH, leading to G4 folding and thus K⁺-dependent FRET signal. Sgc8c modification enhanced target-cell-specific internalization for nanosensor delivery. Both in vitro experiments and in situ lysosomal K⁺ imaging along with external pharmacological stimuli demonstrated the excellent performance of the proposed K⁺ nanosensor, which expanded the application of DNA-based probes in subcellular imaging field.