Studies on Energy Transfer Between Gold Nanoparticles and Quantum Dots

Abstract

Introducion Colloidal semiconductor nanoparticles, or quantum dots (QDs), have attracted much attention due to their unique size dependent optical properties. QDs have great potential for many kinds of applications, such as light emitting diodes, displays, and bioimaging etc. These applications all require the physicochemical properties of a single QDs even in highly concentrated conditions. However, the concentration quenching due to the fluorescence resonant energy transfer (FRET) between nanoparticles is likely to happen in such situations. We demonstrate the distance dependence of the FRET by using gold nanoparticles (AuNPs) having a high absorption coefficient as an energy acceptor. As a method to suppress the FRET from the QDs to the AuNPs, we focused on the capping ligands of nanoparticles. Polystyrene having a thiol terminal with well-controlled molecular weight was synthesized and employed as capping ligands of the AuNPs to change the interparticle distance between the QDs and AuNPs. Of course, when the the QDs and AuNPs are separated to the extent that the energy transfer between these species are improbable, FRET between the QDs should be naturally suppressed. Experimental 1-dodecanethiol(DDT)-capped CdSe/CdS core/shell nanoparticles were synthesized following the existing protocols1 and used as QDs. Briefly, CdSe core nanoparticles were synthesized by the hot injection method, and then a CdS shell was formed on the CdSe core using cadmium oleate and 1-octanethiol as precursors. The AuNPs were synthesized in a toluene/water two phase system by using DDT and tetraoctylammonium bromide as capping ligands and surfactant, respectively.2 An aqueous solution containing hydrogen tetrachloroaurate were in contact with the toluene solution, and another aqueous solution containing sodium tetrahydridborate as a reducing agent was added with vigorous stirring. Polystyrenes with two different weight-average molecular weight of 3937 (PS3937) and 5307 (PS5307) were synthesized by the reversible addition-fragmentation chain transfer polymerization,3 which were then used in the synthesis of AuNPs as capping ligands instead of DDT. To compare the energy transfer rate between the nanoparticles, photoluminescence lifetime of the mixture of AuNPs and QDs in chloroform solution was measured. Then, the nanoparticles were coated on a glass substrates to facilitate the interaction between the nanoparticles in a non-solvent, closely packed condition. Results and Discussion Figure 1 shows photoluminescence decay curves of the mixture of DDT-capped CdSe/CdS NPs and AuNPs in chloroform solution (a) and on the glass substrates (b). For comparison, results obtained without AuNPs were also shown. Photoluminescence lifetimes were calculated from the decay curves by using the two- or three-exponential equations. Decrease in the photoluminescence lifetime indicates the quenching by the FRET. In the solution state, although the intense photoluminescence quenching was expected, the average lifetimes was only reduced to 77% (29.9 ns) relative to the original QDs (38.8 ns) when the DDT-capped AuNPs were added. When polystyrene-capped AuNPs were added, photoluminescence lifetime was almost unchanged (Fig. 1a, 41.3ns). On the other hand, the addition of DDT-capped AuNPs decreased the lifetime down to 20% (7.6 ns) of the original QDs (37.1 ns) when the same combination was measured in the solid state (Figure 1b). The photoluminescence quenching by the energy transfer became efficient since the two types of nanoparticles locate in close proximity each other. On the other hand, the use of PS3937-capped AuNPs instead of DDT-capped ones maintained relatively long photoluminescence lifetime (28.4ns, 77% of the original value), and PS5307-capped AuNPs did not reduce the photoluminescence lifetime of DDT-capped CdSe/CdS NPs at all. These results indicated that bulky polystyrenes maintained interparticle distance even in the solid state to the extent that the energy transfer is improbable. Finally, we employed the terminally-thiolated polystyrene as capping ligands of the QDs. The photoluminescence intensity was almost maintained even in the solid state due to the suppression of the energy transfer (date not shown). These results corresponded with the theory that the energy transfer efficiency by the FRET decreases with increase in the distance between the fluorophore and dye molecules or particles. References (1) O. Chen, J. Zhao, V. P. Chauhan, J. Cui, C. Wong, D. K. Harris, H. Wei, H. S. Han, D. Fukumura, R. K. Jain, and M. G. Bawendi, Nat Mater, 12, 445 (2013). (2) M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, and R. Whyman, J. Chem. Soc., Chem. Commun., 801 (1994) (3) S.-K. Bae, S.-Y. Lee, and S. C. Hong, React. Funct. Polym., 71, 187 (2011). . Figure 1