Tracking the Endosomal Escape of Nanoparticles in Live Cells Using a Triplex‐Forming Oligonucleotide

Abstract Nanoparticle‐mediated intracellular delivery of oligonucleotides is a complex phenomenon that depends on the architecture and the intracellular trafficking of the engineered nanoparticles. Unravelling the molecular arrangements of oligonucleotides within the nanoparticles as well as their intracellular behavior are essential for designing effective nucleic acid delivery systems. Herein, a simple and general strategy for probing the endosomal escape of nanoparticles carrying oligonucleotides in live cells is reported. A triplex‐forming oligonucleotide probe is designed to target the transcription factor, kappa‐light‐chain‐enhancer of activated B cells (NF‐κB), in the cytosol of cells and to transduce the binding into a fluorescent Förster resonance energy transfer (FRET) signal. The combined use of the triplex‐forming oligonucleotide probe and super‐resolution microscopy enables the elucidation of the morphology, intracellular localization, and endosomal escape of the oligonucleotide‐loaded nanoparticles on a molecular level and with nanoscale resolution. The co‐delivery of the FRET probe and mRNA in cells via lipid‐ and polymer‐ based nanoparticles allow simultaneous correlation of the endosomal escape properties of nanoparticles and gene expression efficiency.