Concentrated colloids of silica-encapsulated gold nanoparticles: colloidal stability, cytotoxicity, and X-ray absorption.

As an effort to develop a new, effective, nontoxic X-ray contrast agent, the concentrated colloids of silica-encapsulated gold nanoparticles (Au@SiO2 NPs) were fabricated and their colloidal stability, cytotoxicity, and X-ray absorption were investigated. The concentrated colloidal NPs were manufactured by forming a 4 nm-thick silica shell on the surface of each Au NP with 15 nm diameter, followed by enrichment to [Au] = 100 mM. They were very stable in water: the NPs were well separated each other without forming agglomerates and their optical property was very similar to that before enrichment. The colloidal stability of the NPs in biological environment was strongly dependent on their previous morphology in water. The NPs with minor shell damage were stable in phosphate buffered saline (PBS) solution: both in water and in PBS solution, they showed very similar morphology and optical property. However, the NPs with profound shell damage formed big agglomerates in PBS solution, resulting in the red-shift and broadening of the Au surface plasmon resonance peak. Cell viability and proliferation assessments revealed the biocompatibility of the Au@SiO2 NPs: no apparent cytotoxicity was observed even at 100 ppm NPs. The concentrated colloidal NPs showed very strong X-ray absorption. Their relative X-ray transmittance to water was comparable to that of a commercial agent. Considering these, the concentrated colloids of the Au@SiO2 NPs are suitable for an X-ray contrast agent.