Enhanced Photoluminescence from Micellar Assemblies of Cadmium Sulfide Quantum Dots and Gold Nanoparticles

Abstract

We report the preparation and characterization of well-defined hierarchically structured colloidal assemblies, consisting of a block copolymer micelle with a single photoluminescent cadmium sulfide (CdS) quantum dot (QD) in the core and various numbers of gold nanoparticles (GNPs, between 0.001 and 0.077 GNPs per micelle) chemically bonded to thiol groups at the terminus of the micelle coronal chains. These assemblies were prepared from tetrablock copolymer chains, poly(acrylic acid)-b-polystyrene-TTC-polystyrene-b-poly(acrylic acid) (PAA-b-PS-TTC-PS-b-PAA), where TTC represents a single trithiocarbonate group, synthesized using reversible addition–fragmentation chain transfer polymerization. The block copolymer chains were first self-assembled in 1,4-dioxane by adding cadmium acetate to form flower-like micelles, followed by templated synthesis of CdS QDs in the micelle cores. Then, the TTC groups in the micelle coronae were reductively transformed into thiol groups, which bonded to added GNPs. In the final structures, each GNP was anchored at a distance of a single solubilized PS block (∼20 nm) away from a CdS QDs. This precise spacing between QDs and GNPs prevented any quenching of QD photoluminescence (PL) by the colloidal metal and instead led to an apparent enhancement of QD PL emission relative to QD emission from micelles without GNPs. The observed enhancement increased as the number of GNPs per micelle increased, with a maximum observed emission amplification (EA) of ∼8 times the PL intensity in the absence of GNPs. The EA was also found to increase as the excitation wavelength increased toward the spectral region of the surface plasmon resonance of the GNPs. Lifetime measurements revealed no significant difference between QD emission lifetimes in the absence and presence of GNPs. These results suggest a mechanism of PL enhancement via an excitation antenna effect associated with proximal GNP surface plasmons.