Bioinspired Stabilization of Fluorescent Au@SiO ₂ Tracers for Multimodal Biological Imaging

ABSTRACT Silica‐coated gold nanoparticles (Au@SiO ₂ NPs) are versatile multimodal imaging probes, yet their application in biological environments is limited by rapid silica shell dissolution in cell culture media. Paralleling the natural “maturation” of biosilica, where silanol‐rich silica is condensed into a dense siloxane network for long‐term durability, a solid‐phase calcination strategy enhances silica shell stability without sacrificing optical performance. Fluorescent Au@SiO ₂ NPs are synthesized using PEGylated gold cores and sequential Stöber silica coating, then subjected to either conventional liquid‐phase calcination or solid‐phase calcination followed by post‐calcination dye conjugation. Systematic stability studies reveal that pH, protein interactions, and nanoparticle concentration are critical drivers of dissolution. Liquid‐phase calcination preserves fluorescence but fails to prevent degradation in complete cell culture medium. In contrast, solid‐phase calcination at 800°C removes silanol groups completely, producing dense silica shells that maintain structural integrity and fluorescence over 24 h in medium and after macrophage uptake. Fluorescence and electron microscopy reveal that only solid–phase–treated shells retain both architecture and optical functionality, allowing for accurate correlative imaging over extended timescales. This approach resolves the long‐standing trade‐off between chemical robustness and fluorescence retention, yielding multimodal tracers with exceptional chemical stability in complex biological environments.