Acid-triggered nucleic acid release from gold nanoparticles via Schiff base linkages: In vitro validation of endosomal escape and gene silencing

Abstract

Gold nanoparticles with brush structures of nucleic acid drugs (Nuc–AuNPs) are prepared by mixing thiol-modified nucleic acid drugs and AuNPs due to the strong affinity of the Au–S bond. However, effectively regulating the intracellular kinetics of nucleic acids remains a challenge in achieving highly efficient nucleic-acid delivery. In this study, we designed new DNA-Schiff–AuNPs. The DNA release behaviors of these DNA-modified AuNPs were characterized under acidic conditions analogous to those in intracellular endosomes. Cy5-labeled DNA with an amine terminus (Cy5-DNA-NH₂) reacted with 4-mercaptobenzaldehyde in dimethyl sulfoxide to form a Schiff base. The resulting Cy5-DNA-Schiff-SH was added to the AuNP dispersion system to prepare Cy5-DNA-Schiff–AuNPs. These nanoconjugates were characterized by fluorescence spectroscopy. In the appropriate buffer, the fluorescence intensity of AuNPs remained constant at pH 7.4, while exhibiting an approximately four-fold increase at pH 4.0 and 5.0. In addition, a drastic color change from red to blue was observed after 24 h, suggesting that DNA was released from AuNPs at a pH of 5.5. The DNA-Schiff–AuNPs were effectively incorporated into HeLa cells without any toxicity. Furthermore, siRNA–AuNPs incorporating Schiff base linkages demonstrated significantly enhanced suppression of epidermal growth factor receptor gene expression compared to conventional siRNA–AuNPs. These findings highlight the potential of Schiff base–linked Nuc–AuNPs as a promising platform for intracellular nucleic acid delivery.