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...

This paper describes the integration of opto-chemosensors in microfluidics networks. Our technique exploits the internal surface of the network as a platform to build a sensing system by coating the surface with a self-assembled monolayer and subsequently binding a fluorescent sensing molecule to the monolayer. Fluorescent molecules were used that can switch between a fluorescent and a non-fluorescent state, depending on the acidity of the surrounding solution. Two systems were investigated. The first employs surface confinement of a Rhodamine B dye in a glass micro channel that serves as a molecular switch in organic solutions. Upon rinsing the micro channels with acidic or basic solutions it was possible to switch between the fluorescent and non-fluorescent forms reversibly. Moreover, this system could be used to monitor the mixing of two solutions of different acidity along the micro channel. To widen the scope of optical sensing in micro channels an Oregon Green dye derivative was immobilized, which functions as a sensing molecule for pH differences in aqueous solutions. In this case, a hybrid system was used consisting of a glass slide and PDMS channels. The fluorescence intensity was found to be directly correlated to the pH of the solution in contact, indicating the possibility of using such a system as a pH sensor. These systems allow real-time measurements and can be easily implemented in micro- and nanofluidics systems thus enabling analysis of extremely small sample volumes in a fast and reproducible manner.

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

Such a self‐assembly on a surface has not before been considered: Atomic force microscopy (AFM) was used as a “molecular ruler” to observe the direct assembly of coordination cages on self‐assembled monolayers (SAMs). A simple process was employed using microcontact‐printed (μCP) modified substrates. Metal‐induced self‐assembly of cavitand‐based cages on SAMs (see picture) was proven by means of electrochemistry, contact‐angle, and X‐ray photoelectron spectroscopy measurements. AFM measurements on individual molecules has enabled the observation of the coordination process on a single‐molecular level.

No abstract