Sandra J. Rosenthal

Femtosecond solvation dynamics in acetonitrile: Observation of the inertial contribution to the solvent response

The solvation dynamics of acetonitrile were characterized by a time resolved fluorescence shift measurement determined via the fluorescence upconversion technique. The solvation response is clearly two part in character. The fast initial relaxation accounts for ∼80% of the amplitude and is well fit by a Gaussian of 120 fs FWHM, giving a decay time of 70 fs. The slower tail is exponential with a decay time of ∼200 fs. Comparison of the results to molecular dynamics simulations performed by Maroncelli [J. Chem. Phys. 94, 2085 (1991)] reveal the fast initial part of the solvent response arises from small amplitude inertial rotational motion of molecules in the first solvation shell. The implications of a large amplitude, rapid inertial Gaussian component in the solvent response for theoretical descriptions of chemical reaction dynamics in solution are discussed.

Biocompatible Quantum Dots for Biological Applications

Semiconductor quantum dots are quickly becoming a critical diagnostic tool for discerning cellular function at the molecular level. Their high brightness, long-lasting, size-tunable, and narrow luminescence set them apart from conventional fluorescence dyes. Quantum dots are being developed for a variety of biologically oriented applications, including fluorescent assays for drug discovery, disease detection, single protein tracking, and intracellular reporting. This review introduces the science behind quantum dots and describes how they are made biologically compatible. Several applications are also included, illustrating strategies toward target specificity, and are followed by a discussion on the limitations of quantum dot approaches. The article is concluded with a look at the future direction of quantum dots.

Ultrafast solvent dynamics: Connection between time resolved fluorescence and optical Kerr measurements

The vibrational characteristics of liquid dynamics are used to describe the ultrafast relaxations observed in time‐dependent fluorescence Stokes shift [J. Chem. Phys. 95, 4715 (1991)] and heterodyne detected optical Kerr effect measurements on acetonitrile, via a Brownian oscillator model. Introducing a frequency distribution of vibrational modes makes it possible to compare the two experiments. The ultrafast decays observed in the fluorescence Stokes shift and optical Kerr signals are produced by destructive superposition of the high frequency, underdamped modes.

Femtosecond to nanosecond solvation dynamics in pure water and inside the γ-cyclodextrin cavity

The dynamics of solvation of an excited chromophore in pure water and in a restricted space with a limited number of water molecules have been studied. The time-dependent Stokes shift of Coumarin 480 (C480) and Coumarin 460 (C460) were measured using femtosecond fluorescence upconversion and time-correlated single-photon-counting techniques. The system with a limited number of water molecules was an inclusion complex of Coumarin dyes with γ-cyclodextrin (γCD). The results of molecular dynamics simulations are compared with the observed solvent response in pure water and in the γCD cavity. The observed relaxation times range from <100 fs to 1.2 ns. Solvation of C480 in pure water is observed to occur with time constants of <50 and 310 fs. In sharp contrast with the solvation response in pure water, in the case of the C480/γCD inclusion complex, additional long solvation time constants of 13, 109 and 1200 ps are observed. The stoichiometry, structure and dynamics of the Coumarin/γCD complexes are also discussed.

White light-emitting diodes based on ultrasmall CdSe nanocrystal electroluminescence.

We report white light-emitting diodes fabricated with ultrasmall CdSe nanocrystals, which demonstrate electroluminescence from a size of nanocrystals (<2 nm) previously thought to be unattainable. These LEDs have excellent color characteristics, defined by their pure white CIE color coordinates (0.333, 0.333), correlated color temperatures of 5461-6007 K, and color rendering indexes as high as 96.6. The effect of high voltage on the trap states responsible for the white emission is also described.

Fluorescence upconversion study of cis‐stilbene isomerization

The isomerization dynamics of cis‐stilbene in the first excited singlet state were studied by the technique of fluorescence upconversion. Lifetime measurements were made with subpicosecond resolution in 2‐propanol, decanol, n‐hexane, and tetradecane. The cis‐stilbene fluorescence decay curves are single exponential in all solvents except for decanol, where they are adequately described by a double exponential. A weak viscosity dependence of the decay times is observed in both alcohols and alkanes. These results are discussed in terms of the Bagchi, Fleming, Oxtoby theory [J. Chem. Phys. 78, 7375 (1983)] for activationless electronic relaxation in solution, and the limitations of hydrodynamic models of microscopic friction. The fluorescence decay times of α,α’ dideutero cis‐stilbene (cis‐stilbene‐D2) in tetradecane are found to be approximately 20% greater than those of cis‐stilbene. A picosecond component in the fluorescence anisotropy decay measurements made in 2‐propanol suggest that we are directly mea...

Solvation dynamics in methanol: Experimental and molecular dynamics simulation studies

Abstract We have investigated the ultrafast dynamics of methanol by time dependent fluorescent shift experiments and molecular dynamic simulations. The experiments were performed with two different probe molecules, 1-aminonaphthalene and coumarin 153. The molecular dynamic simulations employed these probes as well as small atomic and diatomic solutes. We find a previously unobserved fast decay component in the solvation response of methanol. The molecular dynamics results are in good agreement with this experimental result. The origin of this fast response and the linearity of the solvent response are discussed.

Bar-coding biomolecules with fluorescent nanocrystals

Microbeads embedded with quantum dots show promise as a multiplex coding technology for the analysis of nucleic acids and proteins.

Bright White Light Emission from Ultrasmall Cadmium Selenide Nanocrystals

A simple treatment method using formic acid has been found to increase the fluorescence quantum yield of ultrasmall white light-emitting CdSe nanocrystals from 8% to 45%. Brighter white-light emission occurs with other carboxylic acids as well, and the magnitude of the quantum yield enhancement is shown to be dependent on the alkyl chain length. Additionally, the nanocrystal luminescence remains enhanced relative to the untreated nanocrystals over several days. This brightened emission opens the possibility for even further quantum yield improvement and potential for use of these white-light nanocrystals in solid-state lighting applications.

Novel fluorescence-based approaches for the study of biogenic amine transporter localization, activity, and regulation.

Pre-synaptic norepinephrine (NE) and dopamine (DA) transporters (NET and DAT) terminate catecholamine synaptic transmission through reuptake of released neurotransmitter. Recent studies reveal that NET and DAT are tightly regulated by receptor and second messenger-linked signaling pathways. Common approaches for studying these transporters involve use of radiolabeled substrates or antagonists, methods possessing limited spatial resolution and that bear limited opportunities for repeated monitoring of living preparations. To circumvent these issues, we have explored two novel assay platforms that permit temporally resolved quantitation of transport activity and transporter protein localization. To monitor the binding and transport function of NET and DAT in real-time, we have investigated the uptake of the fluorescent organic compound 4-(4-diethylaminostyryl)-N-methylpyridinium iodide (ASP+). We have extended our previous single cell level application of this substrate to monitor transport activity via high-throughput assay platforms. Compared to radiotracer uptake methods, acquisition of ASP+ fluorescence is non-isotopic and allows for continuous, repeated transport measurements on both transfected and native preparations. Secondly, we have extended our application of small-molecule-conjugated fluorescent CdSe/ZnS nanocrystals, or quantum dots (Qdots), to utilize antibody and peptide ligands that can identify surface expressed transporters, receptors and other membrane proteins in living cell systems. Unlike typical organic fluorophores, Qdots are highly resistant to bleaching and can be conjugated to multiple ligands. They can also be illuminated by conventional light sources, yet produce narrow, gaussian emission spectra compatible with multiple target visualization (multiplexing). Together, these approaches offer novel opportunities to investigate changes in transporter function and distribution in real-time with superior spatial and temporal resolution.