Articles you may be interested inOne-photon photodetachment of I − in glycerol: Spectra and yield of solvated electrons in the temperature range 329 T 536 K Mechanisms of the ultrafast production and recombination of solvated electrons in weakly polar fluids: Comparison of multiphoton ionization and detachment via the charge-transfer-to-solvent transition of Na − in THF The spectra and the relative yield of solvated electrons produced by resonant photodetachment of iodide anion in ethylene glycol in the temperature range 296T453 K Two-photon dissociation and ionization of liquid water studied by femtosecond transient absorption spectroscopyThe ultrafast dynamics following one-photon UV photodetachment of I Ϫ ions in aqueous solution are compared with those following two-photon ionization of the solvent. Ultrafast pump-probe experiments employing 50 fs ultraviolet pulses reveal similar and very rapid time scales for electron ejection. However, the electron ejection process from water pumped into the conduction band and from iodide ions detached at threshold are readily distinguishable. The observed picosecond timescale geminate recombination and electron escape dynamics are reconstructed using two different models, a diffusion-limited return of the electron from ϳ15 Å to its parent and a competing kinetics model governed by the reverse electron transfer rate. We conclude that the ''ejected'' electron in the halide detachment is merely separated from the halogen atom within the same solvent shell. The assignment of detachment into a contact pair is based on the recombination profile rather than by the postulate of any new spectral absorption due to an electron in a contact pair. The contact pair is surprisingly long-lived and the nonadiabatic recombination is rather slow considering the proximity of the partners. Experiments in mixed solvents confirm our assignment of the two distinct ejection mechanisms. The detachment mechanism is therefore fundamentally different in the resonant ͑one photon͒ charge-transfer-to-solvent ͑CTTS͒ process from the multiphoton detachment of aqueous iodide ions, which bears more similarity to the direct solvent ionization.

The early time dynamics of electron photoejection and relaxation after one-photon UV photodetachment of iodide ions in aqueous solution is compared with that resulting from two-photon ionization of neat water. The effect of solvent composition on the ejection and relaxation is probed via experiments on iodide photodetachment in a water/ethylene glycol mixture. Representation of our pump−multiple wavelength probe experimental data sets as two-dimensional contour plots provides a convenient fingerprint of the electron dynamics. Global fitting of the data to a solvation model for spectral evolution indicates varying time scales for solvation for each of the ejection systems. In all cases, the spectral evolution is complete in the first 10 ps, however electrons ejected via the anion charge-transfer-to-solvent pathway relaxes by a factor of 2 slower. For iodide detachment in the water/glycol mixture, evidence is found for a precursor excess electron state in the infrared that decays on the order of 250 fs. No evidence for an electron precursor state is found for the ionization of water within the 400−1000 nm window studied, and the ground state is apparent within 200 fs. From these results, and from picosecond scale recombination dynamics presented elsewhere (Kloepfer et al. J. Chem. Phys. 2000, 113, 6288−6307), we conclude that the electron production mechanism is distinct for the anion detachment and solvent ionization pathways.

Quantum dots (QDs) rendered water soluble for biological applications are usually passivated by several inorganic and/or organic layers in order to increase fluorescence yield. However, these coatings greatly increase the size of the particle, making uptake by microorganisms impossible. We find that adenine-and AMPconjugated QDs are able to label bacteria only if the particles are <5 nm in diameter. Labeling is dependent upon purine-processing mechanisms, as mutants lacking single enzymes demonstrate a qualitatively different signal than do wild-type strains. This is shown for two example species, one gram negative and one gram positive. Wild-type Bacillus subtilis incubated with QDs conjugated to adenine are strongly fluorescent; very weak signal is seen in mutant cells lacking either adenine deaminase or adenosine phosphoribosyltransferase. Conversely, QD-AMP conjugates label mutant strains more efficiently than the wild type. In Escherichia coli, QD conjugates are taken up most strongly by adenine auxotrophs and are extruded from the cells over a time course of hours. No fluorescent labeling is seen in killed bacteria or in the presence of EDTA or an excess of unlabeled adenine, AMP, or hypoxanthine. Spectroscopy and electron microscopy suggest that QDs of <5 nm can enter the cells whole, probably by means of oxidative damage to the cell membrane which is aided by light.

Biologically conjugated quantum dots (QDs) have shown great promise as multiwavelength fluorescent labels for on-chip bioassays and eukaryotic cells. However, use of these photoluminescent nanocrystals in bacteria has not previously been reported, and their large size (3 to 10 nm) makes it unclear whether they inhibit bacterial recognition of attached molecules and whether they are able to pass through bacterial cell walls. Here we describe the use of conjugated CdSe QDs for strain-and metabolism-specific microbial labeling in a wide variety of bacteria and fungi, and our analysis was geared toward using receptors for a conjugated biomolecule that are present and active on the organism's surface. While cell surface molecules, such as glycoproteins, make excellent targets for conjugated QDs, internal labeling is inconsistent and leads to large spectral shifts compared with the original fluorescence, suggesting that there is breakup or dissolution of the QDs. Transmission electron microscopy of whole mounts and thin sections confirmed that bacteria are able to extract Cd and Se from QDs in a fashion dependent upon the QD surface conjugate.Colloidal quantum dots (QDs) are semiconductor nanocrystals whose photoluminescence emission wavelength is proportional to the size of the crystal. The emission spectra of QDs are narrow, which allows multiwavelength labeling with differ-

The photophysical properties of CdSe and ZnS(CdSe) semiconductor quantum dots in nonpolar and aqueous solutions were examined with steady-state (absorption and emission) and time-resolved (time-correlated single-photon-counting) spectroscopy. The CdSe structures were prepared from a single CdSe synthesis, a portion of which were ZnS-capped, thus any differences observed in the spectral behavior between the two preparations were due to changes in the molecular shell. Quantum dots in nonpolar solvents were surrounded with a trioctylphosphine oxide (TOPO) coating from the initial synthesis solution. ZnS-capped CdSe were initially brighter than bare uncapped CdSe and had overall faster emission decays. The dynamics did not vary when the solvent was changed from hexane to dichloromethane; however, replacement of the TOPO cap by pyridine affected CdSe but not ZnS(CdSe). CdSe was then solubilized in water with mercapto-acetic acid or dihydrolipoic acid, whereas ZnS(CdSe) could be solubilized only with dihydrolipoic acid. Both solubilization agents quenched the nanocrystal emission, though with CdSe the quenching was nearly complete. Additional quenching of the remaining emission was observed when the redox-active molecule adenine was conjugated to the water-soluble CdSe but was not seen with ZnS(CdSe). The emission of aqueous CdSe could be enhanced under prolonged exposure to room light and resulted in a substantial increase of the emission lifetimes; however, the enhancement occurred concurrently with precipitation of the nanocrystals, which was possibly caused by photocatalytic destruction of the mercaptoacetic acid coating. These results are the first presented on aqueous CdSe quantum dot structures and are presented in the context of designing better, more stable biological probes.