Radiative and nonradiative decay rates of chromophores bound to differently sized gold nanoparticles

Abstract

Resonant energy transfer (RET) systems consisting of organic dye molecules bound to gold nanoparticles have recently gained considerable interest in materials science as well as in biophotonics [I]. Gold-nanoparticles are chemically inert, so no photobleaching occurs, they have a high extinction coefficient leading to a larger transfer radius and finally goldnanoparticles do not luminesce, thus crosstalk between donor and acceptor does not exist.'Here, we report results of time-resolved fluorescence experiments on a donor-acceptor system composed of lissamine dye molecules (donor) chemically attached to a gold nanoparticle (acceptor) in aqueous solution. The distance between the lissamine molecule and the surface of the nanoparticle is kept constant at 1 nm, whereas the nanoparticle radius is varied between 1 nm and 30 nm. Figure 2 shows time-resolved fluorescence measurements (streakcamera) of the hybrid-gold-molecule-system for a r =15nm particle. The sample hasbeen excited by a fs-laser-source (12Ofs, frequency-doubled TkSa, 400nm). The composite solution shows an ultrafast peak (Raman signal), an intermediate decay (99ps).of molecules bound to the gold nanoparticles, hence suffering from energy transfer to the nanoparticle. and a long decay (154Ops) of unbound molecules. The results for various nanoparticle radii show time constants for the energy transfer which turn out to decrease with increasing radii [2]. Supplementary to this non-radiative decay process one has to consider the effect the nanoparticle exerts on the radiative rate of the dye molecule. Changes in the radiative lifetime can be calculated by measuring the signal intensity at t=O. The reduced intensity at t=O indicates a strong decrease in the molecules' radiative rate by more than an order of magnitude in the presence of a gold nanoparticle. This drastically decreased emission rate is a consequence of a phase shift between the molecular and the metal dipole leading to a destructive interference effect. I Fig. 1: Fluorescence decay of free Lissamine molecules (Upper curve, 1540~s) and Lissamine-Au composites (lower curve. r = 15 tun. ). The composite solution shows an ultrafast p=ak (Raman signal), an intermediate decay (9%~) of molecules bound to the gold and a long decay (1540~s) of unbound molecules. The shonening in lifetime indicates nonradiative energy tnnsfer whereas the ratio of intensities at t=O is a mmure for the changes in the radiative lifetime. 0 200 400 600 800 Time (ps)We find that both processes rW and rm play a crucial role in fluorescence quenching. Even for very small particles of 1 nm radius the fluorescence yield is reduced by 99.8 % [2]. Qualitative agreement of these drastic changes of the decay rates with the model proposed by Gersten and Nitzan (GN) is found. [I] B. Dubcruet. M. Calame, and A. I. Libchaber. N~N E Biotech. 19.365 (2001) [2] E.