  We conducted$^{195}$Pt-nuclear magnetic resonance measurements on
various-diameter Pt nanoparticles coated with polyvinylpyrrolidone in order to
detect the quantum size effect and the discrete energy levels in the electron
density of states, both of which were predicted by Kubo more than 50 years ago.
We succeeded in separating the signals arising from the surface and interior
regions and found that the nuclear spin-lattice relaxation rates in both
regions show the metallic behavior at high temperatures. Surprisingly, the
magnetic fluctuations in both regions exhibited anomalous behavior below the
same temperature $T^*$, which points to a clear size dependence and is well
scaled with $\delta_\mathrm{Kubo}$. These results suggest that a size-tunable
metal-insulator transition occurs in the Pt nanoparticles as a result of the
Kubo effect.
