Advances in cell-penetrating peptides for cytoplasmic delivery of fluorescent probes

Fluorescent probes, with their superior optical properties and labeling versatility, have greatly advanced the visualization of intracellular molecules and subcellular structures. However, poor cytoplasmic delivery, caused by charge, size, or targeting groups, limits the effective use of many fluorescent probes in live cells. Recently, cell-penetrating peptides (CPPs) have emerged as efficient carriers, offering great potential for the cytoplasmic delivery of fluorescent probes in live cells. This review provides a comprehensive overview of CPPs as vehicles for probe delivery, outlining advances in their development, conjugation chemistries, and intracellular delivery mechanisms. Recent applications in live-cell imaging are highlighted and organized according to major CPP modification strategies, including sequence engineering, cyclization, hybrid design and enhancement by chemical reagents. Finally, the challenges that remain and the future outlook of this rapidly evolving field are discussed.