Direct cytoplasm delivery of gold nanoparticles for real-time apoptosis detection

Gold nanoparticles (AuNPs) assembled with fluorescent peptides through Au-S bonds (pep-AuNPs) have been widely used in biomolecular detection. However, due to the endo/lysosomal trapping after the nanoprobes enter cells, the direct delivery of AuNP probes into the cytoplasm for real-time imaging remains a difficult barrier for many cytoplasm-targeting agents. Here, we prepare AuNP@gel by wrapping a multi-functional nanogel structure on the surface of a single AuNP probe by in-situ polymerization in order to directly deliver AuNP probes into the cell cytoplasm. Compared with the pep-AuNP probes, which are trapped inside lysosomes for long periods, the AuNP@gel probes use the proton-sponge effect to effectively disrupt endo/lysosomal membranes and remain in the cytoplasm. In addition, the AuNP@gel probes rapidly escape from endo/lysosomes to avoid the complex environment that interferes with the stability of the AuNP probes and the lysosomal-storage trigger the upregulation of oxidative stress into the cells. The nanogel structure enables the AuNP probes to avoid some detrimental effects and to achieve high-fidelity fluorescence signals in the cells. Compared to traditional strategies for lysosomal escape, this one-step in-situ polymerization procedure avoids the complicated modification of additional ligands and is generally applicable to peptide-, DNA-, and polymer-linked AuNP probes.