Electron-phonon coupling in 11 ( 2 nm diameter Au particles and 10 ( 3 nm and 50 ( 10 nm Ag particles has been examined by ultrafast pump-probe spectroscopy. The observed relaxation times are strongly dependent on the pump laser power. At the lowest pump powers used, the time constants for relaxation are 0.8 ( 0.1 ps for the 11 nm Au particles, 1.1 ( 0.1 ps for the 10 nm Ag particles, and 1.0 ( 0.1 ps for the 50 nm Ag particles. The measured relaxation times are similar to those for bulk metals, which implies that there are no size-dependent effects in the dynamics for particles in this size region. The transient absorption/ bleach recovery signals for the particles were modeled using the theory developed by Rosei et al. (Surf. Sci. 1973, 37, 689). These calculations yield the transient absorption spectrum as a function of the temperature of the electron distribution. The time dependence of the electronic temperature after pump laser excitation was calculated using the two-temperature model for electron-phonon coupling. The experimental signal versus time traces at selected wavelengths were then simulated by combining the two calculations. The results from the simulations are in semiquantitative agreement with the experimental results. In particular, the low-power relaxation times are correctly predicted by the model calculations. At very high pump laser power (>5 mJ/cm 2 ) the transient bleach signal for Ag shows an unusual 10 ps growth. This growth is attributed to either a change in the dielectric constant of the surrounding medium due to heat transfer from the particles or thermally induced dissociation of adsorbed molecules.

Ultrafast laser spectroscopy has been used to characterize the low frequency acoustic breathing modes of Au particles, with diameters between 8 and 120 nm. It is shown that these modes are impulsively excited by the rapid heating of the particle lattice that occurs after laser excitation. This excitation mechanism is a two step process; the pump laser deposits energy into the electron distribution, and this energy is subsequently transferred to the lattice via electron–phonon coupling. The measured frequencies of the acoustic modes are inversely proportional to the particle radius; a fit to the data for the different sized particles yields v̄R=0.47cl/Rc, where R is the particle radius, cl is the longitudinal speed of sound in Au, and c is the speed of light. This functional relationship exactly matches the prediction of classical mechanics calculations for the lowest frequency radial (breathing) mode of a free, spherical particle. The inverse dependence of the frequency on the radius means that the modulations are damped for polydisperse samples. Analysis of our data shows that this inhomogeneous decay dominates the damping, even for our high quality samples (8%–10% dispersion in the size distribution). The size dependence of the electron–phonon coupling constant was also examined for these particles. The results show that, to within the signal to noise of our measurements, the electron–phonon coupling constant does not vary with size for particles with diameters between 4 and 120 nm. Furthermore, the value obtained is the same as that measured for bulk gold.

We report the redox mediation of glucose oxidase (GOx) in a self-assembled structure of cationic poly(allylamine) modified by ferrocene (PAA-Fc) and anionic GOx deposited electrostatically layer-by-layer on negatively charged alkanethiol-modified Au surfaces. Successive PAA-Fc and GOx layers were deposited by alternate immersion of the thiol-modified Au in the respective polyelectrolyte and enzyme solutions. The uptake of thiol, redox polymer, and GOx on the surface was monitored by quartz crystal microbalance. Cyclic voltammetry shows nearly ideal surface waves of ferrocene in the polymer with charge independent of sweep rate; the redox surface concentration was obtained from integration of the ferrocene/ferricinium voltammetric peaks. The redox charge increases in step with the number of PAA-Fc layers deposited. Enzyme catalysis for the oxidation of β-d-glucose was achieved with a multilayer PAA-Fc/GOx assembly, with each GOx layer contributing equally to the catalytic response. Only a small fraction of the active assembled GOx molecules are “electrically wired” by the ferrocene polymer although all of the enzyme could be oxidized by soluble ferrocenesulfonate when added to solution.

The wide variety of applications of metal nanoparticles has motivated many studies of their properties. Some important practical issues are how the size, composition and structure of these materials affect their catalytic and optical properties. In this article we review our recent work on the photophysics of metal nanoparticles. The systems that have been investigated include Au particles with sizes ranging from 2 nm diameter (several hundred atoms) to 120 nm diameter, and bimetallic core-shell particles composed of Au, Ag, Pt and/or Pb. These particles, which have a rather narrow size distribution, are prepared by radiolytic techniques. By performing time-resolved laser measurements we have been able to investigate the coupling between the electrons and phonons in the particles, and their low frequency "breathing" modes. These experiments show that for Au the time scale for electron-phonon coupling does not depend on size, in contrast to metals such as Ga and Ag. On the other hand, the frequency of the acoustic breathing modes strongly depends on the size of the particles, as well as their composition. These modes are impulsively excited by the rapid lattice heating that accompanies ultrafast laser excitation. The subsequent coherent nuclear motion modulates the transmitted probe laser intensity, giving a "beat" signal in our experiments. Unlike quantum-beats in molecules or semiconductors, this signal can be completely understood by classical mechanics.

The preparation of gold-silver nanoparticles with a core-shell structure by radiation chemistry is described. The optical properties of particles containing Au cores and Ag shells are compared to those of the reverse system for a variety of overall particle compositions. Nanosecond and picosecond laser-induced heating (at 532 nm) is used to melt the Au core Ag shell particles into homogeneous alloyed nanoparticles. The transition from the kinetically stable core-shell structure to the alloy is demonstrated by TEM and by the spectral changes accompanying melting. It is found that the particles must accumulate many laser pulses to completely mix into the alloy. In the case of nanosecond excitation, alloying and reshaping from faceted and irregular particles into smooth spheres occurs at absorbed energies of 5-6 mJ/pulse, and fragmentation takes place at higher energies, >10 mJ/pulse. In the case of 30 ps laser excitation, the thresholds for alloying/reshaping and fragmentation are lower: 1 and 4 mJ/pulse, respectively. The higher energy threshold for nanosecond excitation compared to the picosecond case is due to dissipation of the absorbed energy to the solvent during excitation, which is estimated to occur on a 100-200 ps time scale. Thus, the temperatures reached in the particles by nanosecond excitation are lower than those achieved by picosecond excitation for equal pulse energies.

Ultrafast laser experiments were used to study electron-phonon coupling in Au nanoparticles in the 2.5 to 8 nm size range in aqueous solution. The electron-phonon coupling constants for these samples were found to be independent of the particle size. This is attributed to a weak interaction between the electron gas and the surface phonon modes in Au. Calculations were performed which show that the coupling between the hot electrons and the surface accounts for less than 10% of the total electron energy losses for these particles. Thus, bulk electron-phonon coupling dominates the relaxation of excited electrons in Au particles, for particles as small as several hundred atoms.

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Fig. 1. Protein-protein interactions between gp32 and gp59 on fDNA. (A) The fluorescence from individual molecules of fDNA with the proteins bound in the order as indicated at the side of each row. The gp32 protein is labeled with A488 (gp32 D ) and the gp59 protein is labeled with A555 (gp59 A ). The filter sets are described in Experimental Methods: F1 is for A488 emission, F2 for FRET between A488 and A555, and F3 for A555 emission. (B) Ensemble FRET studies of Oregon-green-488-maleimide-labeled gp59 titrated into a solution of 400 nM CPM-labeled gp32 and 100 nM fDNA. The fluorescence spectra of 400 nM CPM-gp32 alone (black line), the endpoint of the titration at 1 M Oregon-green-488-maleimide-gp59 (dark gray line), and several intermediate spectra (light gray lines) are shown. (C) Analysis of the donor quenching and acceptor sensitization plotted against the gp59 concentration determines the stoichiometry among gp32, gp59, and fDNA to be 1:1:1 with a calculated binding constant of Ϸ40 nM. The gp32 protein is labeled with A488 (gp32 D ), the gp59 protein is labeled with A555 (gp59 A ), and the gp41 protein is unlabeled. MgATP␥S (500 M) is present for the sample in row 2, and 500 M MgATP is present for the sample in row 3. (B) The gp32 protein is unlabeled, the gp59 protein is labeled with A555 (gp59 A ), and the gp41 is labeled with A488 (gp41 D ). MgATP␥S (500 M) is present for the sample in row 2, and 500 M MgATP is present for the sample in row 3 (5 min after addition of gp41 and nucleotide) and in row 4 (30 min after addition of gp41 and nucleotide). The filter sets are described in the legend to Fig. 1. RNA ͉ tertiary interactions ͉ heterogeneity ͉ time-correlated single photon counting T he discovery that RNAs can catalyze biological reactions has led to intensive effort aimed at identifying additional biological functions for RNA. More than 20 years later, we now know that RNAs play critical functional roles in metabolism, replication, regulation, and development in cells. Extensive biochemical and biophysical studies have led to a better understanding of the molecular mechanisms by which RNAs achieve their biological function, highlighting the important roles of both structure and dynamics. In this regard, single-molecule methods have recently emerged as particularly powerful tools. The folding dynamics of various functional RNAs have been investigated by single-molecule FRET experiments, which probe dynamics under equilibrium conditions via observation of the stochastic fluorescence trajectories as a function of time (1-6). These techniques offer a unique glimpse into subpopulations of a system and, in some cases, have identified conformational heterogeneity or the presence of intermediates that would otherwise be undetectable by ensemble methods (3-5, 7, 8).Unlike the highly cooperative, all-or-none, folding process observed for most protein domains, RNAs generally fold in a noncooperative manner, where the secondary structure forms independently of the tertiary structure. The thermodynamics for f...