<p>High-quality silica core and gold shell spherical nanoparticles were synthesized with precise control over their dimensions and well-defined plasmonic resonances. We demonstrate that the simultaneous synthesis and adsorption of small gold seeds onto functionalized SiO₂ spheres, combined with a suitable reducing agent and optimized reaction conditions, are crucial for the growth and coalescence of the gold seeds into uniform and closed layers. This improvement enables the production of core–shell nanostructures whose extinction spectra exhibit excellent agreement with predictions from Mie theory.</p> Graphical Abstract <p></p>