Optical Properties and Surface Morphologies of Silica–Gold Nanoshells Depending on Buffer Solutions and Reducing Agents

Silica–gold nanoshells (SGNSs) were prepared at different concentrations of reducible gold salts (K + AuCl 4 − ) with a toxic formaldehyde (FAH) reducing agent. FAH-induced SGNSs (F-SGNSs) showed not only a distinct red-shift of the plasmon resonance peak but also an increase in the thickness of the gold shell in proportion to the concentration of gold salts (0.38–1.90 mM). However, the excessive addition of reducible gold salts resulted in agglomeration of the SGNSs. The F-SGNSs showed enhanced colloidal stability after the addition of buffer solutions containing different oxyanions in the following order: K 2 CO 3 > citrate > citrate + PBS > PBS. In addition, biocompatible ascorbic acid (ASA) was used to avoid the problem of toxic FAH agents for in vivo applications. The ASA-induced SGNSs showed a smoother surface morphology and more red-shift of the surface plasmon resonance band in comparison to F-SGNSs, probably because of the coalescence of small gold clusters formed under the mild reducing power of ASA. The surface morphologies of the SGNSs and their optical properties were characterized by transmission electron microscopy, scanning electron microscopy, zeta-potential measurement, and UV–visible spectroscopy. Graphic Abstract