Jose H. Hodak

Size dependent properties of Au particles: Coherent excitation and dephasing of acoustic vibrational modes

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 vR=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 modula...

Electron-phonon coupling dynamics in very small (between 2 and 8 nm diameter) Au nanoparticles

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.

ULTRAFAST STUDY OF ELECTRON-PHONON COUPLING IN COLLOIDAL GOLD PARTICLES

Abstract The hot electron dynamics in 11±2 nm diameter Au particles has been examined by transient absorption and bleach recovery measurements. These experiments show that the timescale for energy relaxation by electron–phonon coupling decreases as the pump laser power decreases, and that the relaxation time is independent of the pump or probe wavelength at low pump power. The electron–phonon relaxation time measured in our experiments (∼650 fs) is very similar to the time constant obtained from experiments with bulk gold samples. These observations are explained using the two-temperature model for electron–phonon coupling.

OBSERVATION OF ACOUSTIC QUANTUM BEATS IN NANOMETER SIZED AU PARTICLES

Quantum beats due to coherently prepared acoustic vibrational modes have been observed for 14 to 17 nm diameter Au particles. The beat frequency is inversely proportional to the particle size. The dephasing time is ca. 15 ps, which is limited by the broad size distribution (±2 nm) of the samples.

Ultrafast study of interfacial electron transfer between 9-anthracene-carboxylate and TiO2 semiconductor particles

The excited state dynamics of 9-anthracene-carboxylic acid adsorbed onto the surface of TiO2 semiconductor particles were examined with ca. 250 fs time resolution. A combination of transient absorption and time-resolved anisotropy measurements show that approximately 76% of the photo-excited dye molecules transfer an electron to the TiO2 particles. The time scale for the forward electron transfer reaction was determined to be ⩽1 ps. The 9-anthracene-carboxylate radical cations produced by this reaction undergo back electron transfer on a 54 ps time scale. A more accurate estimate of the forward electron transfer reaction time is not possible, due to the contribution to the transient absorption signal from adsorbed dye molecules that do not transfer electrons to TiO2. These nonreactive species are deactivated by either nonradiative decay or fluorescence emission. The fluorescence spectrum from the dye molecules bound to the TiO2 particles is very different to that of the free dye in solution. The free dye ...

Tuning the spectral and temporal response in PtAu core–shell nanoparticles

Ultrafast laser spectroscopy has been used to measure the electron–phonon coupling constant in PtAu core–shell bimetallic nanoparticles. A chemical reduction method was used to prepare Pt cores of 12.5 nm diameter and a γ-radiolytic deposition technique was then used to grow Au shells of variable thickness. The resulting nanoparticles have a spectrum that is characteristic of Au. It is found that the electron–phonon coupling time for these hybrid nanoparticles (τe–ph∼200 fs) is over a factor of 3 shorter than that for plain Au nanoparticles (τe–ph∼650 fs). The faster dynamical response is due to the large electron–phonon coupling constant for Pt, which provides efficient scattering of the excited electrons. Platinum dominates the temporal response, even for a 1:6 molar ratio of Pt to Au, because it has a much larger density of states near the Fermi level compared to Au.

Monovalent and divalent promoted GAAA tetraloop-receptor tertiary interactions from freely diffusing single-molecule studies.

Proper assembly of RNA into catalytically active three-dimensional structures requires multiple tertiary binding interactions, individual characterization of which is crucial to a detailed understanding of global RNA folding. This work focuses on single-molecule fluorescence studies of freely diffusing RNA constructs that isolate the GAAA tetraloop-receptor tertiary interaction. Freely diffusing conformational dynamics are explored as a function of Mg(2+) and Na(+) concentration, both of which promote facile docking, but with 500-fold different affinities. Systematic shifts in mean fluorescence resonance energy transfer efficiency values and line widths with increasing [Na(+)] are observed for the undocked species and can be interpreted with a Debye model in terms of electrostatic relaxation and increased flexibility in the RNA. Furthermore, we identify a 34 +/- 2% fraction of freely diffusing RNA constructs remaining undocked even at saturating [Mg(2+)] levels, which agrees quantitatively with the 32 +/- 1% fraction previously reported for immobilized constructs. This verifies that the kinetic heterogeneity observed in the docking rates is not the result of surface tethering. Finally, the K(D) value and Hill coefficient for [Mg(2+)]-dependent docking decrease significantly for [Na(+)] = 25 mM vs. 125 mM, indicating Mg(2+) and Na(+) synergy in the RNA folding process.

The Role of Counterion Valence and Size in GAAA Tetraloop–Receptor Docking/Undocking Kinetics

For RNA to fold into compact, ordered structures, it must overcome electrostatic repulsion between negatively charged phosphate groups by counterion recruitment. A physical understanding of the counterion-assisted folding process requires addressing how cations kinetically and thermodynamically control the folding equilibrium for each tertiary interaction in a full-length RNA. In this work, single-molecule FRET (fluorescence resonance energy transfer) techniques are exploited to isolate and explore the cation-concentration-dependent kinetics for formation of a ubiquitous RNA tertiary interaction, that is, the docking/undocking of a GAAA tetraloop with its 11-nt receptor. Rate constants for docking (k(dock)) and undocking (k(undock)) are obtained as a function of cation concentration, size, and valence, specifically for the series Na(+), K(+), Mg(2+), Ca(2+), Co(NH(3))(6)(3+), and spermidine(3+). Increasing cation concentration acceleratesk(dock)dramatically but achieves only a slight decrease in k(undock). These results can be kinetically modeled using parallel cation-dependent and cation-independent docking pathways, which allows for isolation of the folding kinetics from the interaction energetics of the cations with the undocked and docked states, respectively. This analysis reveals a preferential interaction of the cations with the transition state and docked state as compared to the undocked RNA, with the ion-RNA interaction strength growing with cation valence. However, the corresponding number of cations that are taken up by the RNA upon folding decreases with charge density of the cation. The only exception to these behaviors is spermidine(3+), whose weaker influence on the docking equilibria with respect to Co(NH(3))(6)(3+) can be ascribed to steric effects preventing complete neutralization of the RNA phosphate groups.

Multiphoton Excitation of Upconverting Nanoparticles in Pulsed Regime

Upconverting nanoparticles (UCNPs) present emission in the visible region upon irradiation with NIR light through a multiphoton mechanism. However, the long characteristic time of their emission has prevented the use of this kind of entities as multiphoton probes. We present a study on the use of erbium-containing UCNPs under pulsed excitation, showing that both the power density and the duration of the excitation pulse are key factors to understand the emission behavior. By adjusting power and excitation rate, we can obtain typical multiphoton z-axis focal exclusive excitation. These findings open the possibility of using UCNPs as probes for controlled localization of uncaging and imaging with multiphoton z-axis sectioning. We show that this can be achieved even at power densities several orders of magnitude lower than traditional multiphoton microscopies.

Probing photoinduced electron transfer reactions at semiconductor-liquid interfaces

The electron transfer dynamics of 9-anthracene carboxylic acid bound to TiO2 nanoparticles in ethanol has been examined by a combination of transient absorption and time- resolved anistropy measurements. The results from these experiments show that the forward electron transfer reaction is very fast, <EQ 360 fs for anatase TiO2 and <EQ 1 ps for rutile TiO2. The back electron transfer reaction occurs on a slower time scale: 33 +/- 1 ps for the anatase particles and 54 +/- 1 ps for the rutile particles. The rate of the back electron transfer reaction for anatase TiO2 is also very sensitive to the addition of small amounts of water to the ethanolic solutions. For example, adding 1 percent water by volume decreases the average back electron transfer time from 33 ps to 20 ps. The water also produces a red-shift in the absorption spectra of the TiO2 particles. These observations show that the back electron transfer reaction is in the Marcus inverted region.