Here, we aim to study the impact of polymer mass on the fundamental breathing-mode frequency of [email protected] core-shell nanoparticles (CS-NPs) suspended in water. The tunable absorption and scattering properties of CS-NPs make them well suited for photothermal-therapeutics to eradicate cancer cells in specific areas. The surface of our [email protected] CS-NPs in this work were decorated with polymers of different molar mass; methoxy polyethylene glycol (mPEG, 5 kDa) and polyvinylpyrrolidone (PVP, 40 kDa). Transient absorption spectroscopy was utilized to monitor the oscillatory behavior of the breathing mode following ultrafast laser heating. It is shown that the frequency of the breathing mode decreases as the molar mass of the polymer increases, highlighting the important influence of mass-loading at the solid-liquid interface in nanostructures.