Shelled-isolated nanoparticle-enhanced Raman spectroscopy

S1. Supplementary Methods S1.1. Preparation of Au@SiO2 core-shell NPs We first synthesized Au NPs with a diameter of about 55 nm as cores by a standard sodium citrate reduction method. A freshly prepared aqueous solution of 1 mM (3-Aminopropyl) trimethoxysilane (APS) was added to the gold sol under vigorous magnetic stirring in 15 min, ensuring complete complexation of the amine groups with the gold surface. Then a 0.54 wt% sodium silicate solution was added to the sol, again under vigorous magnetic stirring. To accelerate the synthesis procedure and make the ultra-thin silica shell, we elevated the reaction temperature from room temperature to 90 C, so that the original coating time of 2-3 days for coating silica shells of 4-5 nm can be shortened to ca. one hour. More importantly, this method ensures to form the ultra-thin shell thickness of only 2-4 nm that is free of pinholes (see S8) by carefully controlling parameters such as reaction time, temperature, pH, and the concentration.