Complex Formation of Gold Nanoparticles with Collagen in Aqueous Media Studied by X-ray Scattering and Absorption Spectroscopy

Small-angle X-ray scattering and UV-vis absorption measurements were performed on mixed solutions of gold nanoparticles (AuNPs) and atelocollagen (AC), triple-helical collagen without telopeptide, in acetate buffer at pH 4 under different temperature conditions, i.e., preparation temperature <i>T</i>_prep and measurement temperature <i>T</i>_meas. Due to the significantly higher electron density of gold than that of AC, the structure factor <i>S</i>(<i>q</i>) of AuNPs is readily estimated from the scattering intensities of AuNP-only and mixed solutions. The resulting <i>S</i>(<i>q</i>) profile for the mixed solution indicated significant attractive interactions, especially for the smaller AuNPs. Therefore, the sticky sphere model was applied to analyze <i>S</i>(<i>q</i>) to determine the interaction parameters at different <i>T</i>_prep and <i>T</i>_meas. The attractive interactions between AuNPs were higher at higher <i>T</i>_prep, suggesting that single-chain AC tends to make the interactions between AuNPs more attractive than those for triple-helical AC. Complex formation was also detected by the aggregation-induced surface plasmon absorption shift. More densely packed AuNPs were detected from the absorption spectra for higher AuNP content at which the ζ potential disappeared, while a split absorption band was also found, indicating that not all AuNPs can form a complex with AC at ζ = 0.