Tunable Loading of Oligonucleotides with Secondary Structure on Gold Nanoparticles through a pH-Driven Method

This paper describes how pH can be used to control covalent attachment of oligonucleotides with secondary structure on gold nanoparticles (AuNPs). The highest loading of thiolated nucleic acids occurred at low pH (pH = 1.7) due to reduced repulsion between the negatively charged oligonucleotides and the AuNP surface. The packing of oligonucleotides at low pH decreased (single-stranded >> duplex > quadruplex) as the spatial footprint of secondary structure increased. As the pH increased, a decrease in the number of DNA strands grafted to the AuNPs was observed. Notably, the loading density depended on the flexibility and spatial organization of the secondary structures at all pH conditions. At the lowest pH tested, circular dichroism analysis revealed that G-quadruplex aptamers underwent a structural change (from parallel to antiparallel or vice versa), although the biological activity of the aptamer-loaded AuNPs was still maintained. We anticipate that pH-tuning can result in quantitative loading of oligonucleotides on various types of AuNPs with different shapes and surface capping layers.