The Enhancement of Fluorescence by Surface Plasmons in Gold Core Silica Shell Colloids

2003

Oleg G. Tovmachenko, Christina Graf, Dave van den Heuvel, Alfons van Blaaderen, Hans C. Gerritsen,

The optical properties of a fluorescent molecule located at some distance from a metal surface are altered by electromagnetic interactions with the metal. The interaction can cause quenching of the fluorescence, enhancement of the absorption by the fluorescent molecule due to the increase of the electromagnetic field strength near the metal and the radiative decay rate of the fluorescent molecules can be increased. In the case of fluorescent molecules that are located at a very short distance from a metal core, non-radiative energy transfer takes place to surface plasmons in the metal. Field enhancement and increased radiative rates, however, still occur at longer distances from the metal core. As a result, there exists an optimal region of the fluorescent molecule to metal core distance where fluorescence enhancement takes place. Another important factor that affects the magnitude of the fluorescence enhancement is the quantum yield, Kradiative/(Kradiative + Knon-radiative), of the fluorophore. The fluorescence of a fluorophore with a quantum yield close to 1.0 is (almost) not enhanced by the increase in radiative rate due to the presence of the metal core. In this study the size of the metal core and the dye core distance are calculated in order to study the enhancement effects over a range of distances. The colloids can be used in, for instance, biophysical imaging experiments and screening applications. To estimate the optimal distance between the fluorescent molecules and the gold core, three effects of the surface plasmon resonance of the gold particles on the molecular light scattering processes were examined: the increase of the local excitation field strength due to excitation of electronic plasma resonance; the increase in the radiative emission rate due to the metal surface; the effect of additional nonradiative decay. All three effects were studied using two approaches: simulations were carried out using discrete dipole approximation software (modified DDSCAT software) and direct application of the Mie scattering theory. In both cases data for dielectric functions of bulk metals (Au, Ag) was adapted in order to take into account the effect of the mean free electron’s path length. This has a particularly strong influence at the long-wavelength part of spectra. The curves resulting form the calculations show the fluorescence enhancement as a function of the wavelength, metal core diameter, intensity of incident electromagnetic field and dye core distances. The curves show a clear optimal region of the dye-core distance where fluorescence enhancement takes place. Gold core silica shell colloids with core diameters of 30 nm and 40 nm were synthesized. Two silica shells were grown around the particles. The first shell was used as a spacer between the 5-(and-6)-carboxyfluorescein, succinimidyl ester (5(6)-FAM) dye molecules and the gold core and the second shell contained both silica and 5(6)-FAM. Colloids without a gold core were used as a reference specimen. (5(6)-FAM) was chosen as a dye since this dye allows easy variation of its quantum efficiency (Q) by varying the pH. Fluorescence lifetime measurements yielded an excitation wavelength independent lifetime of 3.1ns for the gold core based colloids compared to 4.3 ns for colloids consisting of dye-doped silica alone. The shortening of the lifetime is consistent with a change of the radiative rate as expected from interaction of the dye with the metal shell. Additional fluorescence measurements revealed that the fluorescence intensity of 5(6)-FAM incorporated in colloids strongly depends both on pH and the excitation wavelength. At high pH (9.5) and 440 nm wavelength excitation, the low quantum efficiency form of the dye is excited and enhancement of the fluorescence intensity of the gold core colloids was observed with respect to the silica only based colloids. At 490 nm excitation wavelengths only the high quantum efficiency form of the dye is excited and the fluorescence intensity of the gold core colloids showed no enhancement. Measurements at low pH (2.4) (low quantum efficiency) showed a wavelength independent enhancement of the fluorescence intensity of the gold core colloids. These results indicate that fluorescence enhancement indeed takes place in colloids with a metal core. The effective quantum yield of a molecule can be substantially increased in the presence of a metal core. Based on the theoretical calculations we conclude that the average distance of the 5(6)-FAM to the gold core was not the optimal distance for enhancement to take place. The results suggest that the fluorescence enhancement is due to an increased radiative rate caused by interaction with surface plasmons.

Mots Clés: enhancement of fluorescence, colloids, effective quantum yield, gold core silica shell nanoparticles, surface plasmons