Instability of Cationic Gold Nanoparticle Bioconjugates: The Role of Citrate Ions

Gold nanoparticles of 6, 8, and 16 nm, synthesized with HAuCl4 and sodium citrate, were derived with biomolecules based on the peptide CIPGNVG and possessing different terminal charges. We have studied the stability of these conjugates as a function of ionic strength, pH, and the presence of other species in solution. It was observed that multiple electrostatic interactions between the conjugates mediated by cross-linking species led to an effective strong bond and consequently to irreversible aggregation and precipitation. In the presence of citrate or diamine ions, nanoparticles precipitated when two-headed ions had charges opposite (and therefore attractive) to the conjugate, thus acting as bridging molecules. This effect depends on the pH, the concentration of particles, and their size, and it is relevant to designing bioconjugates for biomedical applications.