The radiative and nonradiative decay rates of lissamine dye molecules, chemically attached to differently sized gold nanoparticles, are investigated by means of time-resolved fluorescence experiments. A pronounced fluorescence quenching is observed already for the smallest nanoparticles of 1 nm radius. The quenching is caused not only by an increased nonradiative rate but, equally important, by a drastic decrease in the dye's radiative rate. Assuming resonant energy transfer to be responsible for the nonradiative decay channel, we compare our experimental findings with theoretical results derived from the Gersten-Nitzan model. DOI: 10.1103/PhysRevLett.89.203002 PACS numbers: 33.50.-j, 81.07.Pr Resonant energy transfer (RET) systems consisting of organic dye molecules and noble metal nanoparticles have recently gained considerable interest in biophotonics [1][2][3][4] as well as in materials science [5,6]. Closely related are donor-acceptor pairs of organic dye molecules forming Förster resonant energy transfer (FRET) systems. They have been theoretically modeled [7] and applied in biophysics extensively during the past decade (see, e.g., [8]). Yet these classical purely dye-based systems show disadvantages regarding quenching efficiency [4] and photostability [9].If the donor molecule is placed in the vicinity of a metal surface instead of an organic acceptor, not only resonant energy transfer takes place but also the radiative lifetime of the donor molecule changes. For metal films this has been investigated extensively [10 -13]. Much less is known about donor molecules in the vicinity of metal nanoparticles. Theoretical treatments of the moleculenanoparticle problem [14 -17] predict energy transfer rates and radiative decay rates that deviate substantially from what has been found for dye molecules in front of a metal film. Both radiative and nonradiative rates are expected to depend critically on size and shape of the nanoparticle, the distance between the dye molecule and the nanoparticle, the orientation of the molecular dipole with respect to the dye-nanoparticle axis, and the overlap of the molecule's emission with the nanoparticle's absorption spectrum. Recent experimental investigations deal with metal island films or rough surfaces only (see [18,19] and references in [20]), where the above mentioned parameters are undefined.Here we report results of time-resolved fluorescence experiments on a donor-acceptor system composed of lissamine molecules (donor) chemically attached to a gold nanoparticle (acceptor). 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 and 30 nm. We find time constants for the energy transfer on a picosecond time scale which turn out to decrease with increasing nanoparticle size. In addition, the dye's radiative rate is reduced by more than an order of magnitude. Both effects are responsible for the drastic quenching of the fluorescence yield as predicted by the so-called...

Light emission at the particle plasmon frequency is observed in optically excited spherical gold nanoparticles. We find a photoluminescence efficiency of 10 −6 , which is essentially independent of particle size and four orders of magnitude higher than the efficiencies determined from metal films. Our experimental findings are explained with a process in which excited d-band holes recombine nonradiatively with sp electrons, emitting particle plasmons. These plasmons subsequently radiate, giving rise to the photoluminescence observed in the experiment. We determine the quantum efficiencies involved in this process.

Lissamine molecules attached to differently sized gold-nanoparticles have been studied using time-resolved spectroscopy. We show that resonant energy transfer and a strong reduction in the molecules' radiative lifetime contribute equally to fluorescence quenching. 02002 Optical Society of America OCIS codes: (260.2160) Energy Transfer 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 [l]. Gold-nanoparticles are chemically inert, i.e. no photobleaching occurs, they have a high extinction coefficient leading to a larger transfer radius and fmally gold-nanoparticles 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 (Fig. 1).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. The binding is accomplished via a thioether group. For all particle radii the concentration of lissamine is chosen in order to achieve 50% surface coverage. Acceptor Donor-. Energy Transfer r = 1,10,15,30 nm Fig. 1: Lissamine dye molecules are attached to gold nanoparticles. The radii of the nanoparticles are chosen between 1 nm and 30 nm. The optical dipole moment pm of the n-conjugated part of the molecule is oriented perpendicular to the dye-particle axis and situated approximately 1 nm from the particles' surface.Time-resolved fluorescence measurements of the hybrid-gold-molecule-system with different particle sizes have been performed by exciting the sample with a fs-laser-source (120fs, frequency-doubled Ti:Sa, 400nni) and using a streak-camera [2]. In Figs. 2(a) and 2(b) the experimentally determined radiative and nonradiative rates, k d (r) and &,".rad (r) are plotted versus the radius of the gold nanoparticles. The nonradiative rate of lissamine dye molecules increases by more than an order of magnitude when attached to gold nanopalticles whereas the radiative rate drops

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.