Origin of size-dependent energy transfer from photoexcited CdSe quantum dots to gold nanoparticles

The photoluminescence quenching of a CdSe quantum dot by hexanethiolate-monolayer-protected gold clusters (MPCs) with core diameters of 1.1 -4.9 nm is described. Experimental evidence suggests that the quenching occurs via an energy transfer mechanism. The energy transfer quenching efficiencies of MPCs were compared using Stern-Volmer plots. The quenching constant (KQ) obtained from the slope of the plot shows an enormous increase from 2.5 × 10⁸ to 2.3 × 10⁸ M^-1 by nearly 1000-fold as the core size increases from 1.1 to 4.9 nm. The origin of the size dependence is considered. There is a remarkable linear correlation found between the quenching constant and the MPC core volume. This correlation suggests that the energy transfer quenching efficiency is governed by the absorption cross-section of the MPC quencher that scales as its core volume.