Molecular structure determination for photogenerated intermediates in photoinduced electron transfer reactions using steady-state and transient XAFS

Many photoinduced electron transfer reactions are accompanied by nuclear rearrangements of the molecules involved. In order to understand the reactivities of the molecules and the reaction mechanisms, precise information on the molecular structural changes accompanying the electron transfer is often required. We present here conventional XAFS and transient energy dispersive XAFS studies on structures of excited and photoinduced charge separated state of porphyrin and porphyrin based supermolecules, and structures of TiO{sub 2} colloid and the heavymetal ions that bind to the colloid surfaces during photocatalytic reductions

New Hybrid Properties of TiO2 Nanoparticles Surface Modified With Catecholate Type Ligands

Surface modification of nanocrystalline TiO2 particles (45 Å) with bidentate benzene derivatives (catechol, pyrogallol, and gallic acid) was found to alter optical properties of nanoparticles. The formation of the inner-sphere charge–transfer complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites. The binding structures were investigated by using FTIR spectroscopy. The investigated ligands have the optimal geometry for chelating surface Ti atoms, resulting in ring coordination complexes (catecholate type of binuclear bidentate binding–bridging) thus restoring in six-coordinated octahedral geometry of surface Ti atoms. From the Benesi–Hildebrand plot, the stability constants at pH 2 of the order 103 M−1 have been determined

Photoluminescence Stokes shift and exciton fine structure in CdTe nanocrystals

The photoluminescence spectra of spherical CdTe nanocrystals with zincblende structure are studied by size-selective spectroscopic techniques. We observe a resonant Stokes shift of 15 meV when the excitation laser energy is tuned to the red side of the absorption band at 2.236 eV. The experimental data are analyzed within a symmetry-based tight-binding theory of the exciton spectrum, which is first shown to account for the size dependence of the fundamental gap reported previously in the literature. The theoretical Stokes shift presented as a function of the gap shows a good agreement with the experimental data, indicating that the measured Stokes shift indeed arises from the electron-hole exchange interaction.Comment: 8 pages, 4 figures, LaTe

Intracellular localization of titanium dioxide-biomolecule nanocomposites.

Emerging areas of nanotechnology hold the promise of overcoming the limitations of existing technology for intracellular manipulation. These new developments include the creation of nanocomposites that can be introduced into the cells, targeted to specific subcellular sites, and subsequently used as platforms for initiation of intracellular processes dependent on or aided by locally high concentrations of specific molecules delivered as components of the nanocomposites. Nanocomposites that combine functional properties of biomolecules with the functional properties of inorganic components could provide new tools for biology, medicine, chemistry and material sciences. Here we describe how we introduced TiO{sub 2}-DNA nanocomposites into cells, and localized titanium in the cells by mapping the Ti K{alpha} X-ray fluorescence induced at the 2-ID-E microprobe of the SRI-CAT at the Advanced Photon Source at Argonne National Laboratory

Conversion of holes into reducing species on surface modified small-particle TiO{sub 2}

Complexation of colloidal titanium dioxide nanoparticles (40 {angstrom}) by cysteine as a surface derivative was investigated by electron paramagnetic resonance (EPR) and infra-red (diffusion reflectance infra-red Fourier Transform DRIFT) spectroscopies. It was found that cysteine strongly binds to the colloid surface. The authors have demonstrated with EPR spectroscopy that cysteine modifies the TiO{sub 2} surface with formation of new trapping sites where photogenerated electrons and holes are localized. Illumination of cysteine modified TiO{sub 2} at 77K resulted in formation of a sulfur centered radical observed by EPR spectroscopy at 200 K. Upon addition of lead ions, a new complex of cysteine that bridges surface titanium atoms and lead ions was detected by IR spectroscopy. Illumination of lead/cysteine modified TiO{sub 2} did not result in the formation of sulfur centered radical, but symmetrical, lattice defect type EPR signal for trapped holes was observed. However, addition of methanol to this system resulted in the formation of {center_dot}CH{sub 2}OH radical following illumination at 8.2 K. After the temperature was raised to 120 K, doubling of the signal associated with electrons trapped at particle surface (Ti(3){sub surf}) was observed. On further increase of the temperature to 200 K the EPR signal for trapped electrons disappeared as a result of the reduction of Pb{sup 2+} ions, and metallic lead was observed to precipitate. Conversion of photogenerated holes into trapped electrons due to the presence of methanol doubles the yield of trapped electrons that can reduce Pb{sup 2+}. Direct reduction of Pb{sup 2+} ions by {center_dot}CH{sub 2}OH radical on TiO{sub 2} was not detected

Combined photooxidation/photoreduction using TiO{sub 2} photocatalysts to treat organic/inorganic metal-laden wastewaters

Titanium dioxide (TiO{sub 2}) colloids prepared from titanium chloride (TiCl{sub 4}) were used as photocatalysts in the experiments. Cysteine was selected as the model derivative for the surface modification of the TiO{sub 2} nanoparticles. Testing of TiO{sub 2} photocatalysts conducted in the absence of organic compounds demonstrated that adsorption of lead ions occurred in the system with either untreated (virgin) or treated (modified using cysteine) TiO{sub 2} photocatalysts. Adsorption rates of the metal ions in the systems that used treated TiO{sub 2} photocatalysts were about three times faster than those where untreated TiO{sub 2} photocatalysts were used. Lead ion concentrations in the irradiated solutions decreased as the UV irradiation time increased; greater metal removals were achieved at longer irradiation times. The rate of decreasing lead ion concentrations in the system that used treated TiO{sub 2} photocatalysts was about two to three times faster than that in the system using untreated TiO{sub 2} photocatalysts. Experiments were also performed in which organic compounds (naphthalene or phenol) and heavy metals (Pb{sup 2+}) were simultaneously treated with TiO{sub 2} photocatalysts. The presence of lead ions did not interfere with the photo-degradation of the selected organic compounds from solution. The presence of phenol compound appeared not to affect the photoreduction of lead ions, while addition of naphthalene compound resulted in a lag-time effect on the photoreduction of lead ions from solution. All of the experimental results showed that the cysteine-modified TiO{sub 2} resulted in faster and more effective removal than that for the untreated TiO{sub 2} system. This technique has resulted in the simultaneous photocatalytic removal/recovery of organic and inorganic compounds in the system

Nanoparticles for Applications in Cellular Imaging

In the following review we discuss several types of nanoparticles (such as TiO2, quantum dots, and gold nanoparticles) and their impact on the ability to image biological components in fixed cells. The review also discusses factors influencing nanoparticle imaging and uptake in live cells in vitro. Due to their unique size-dependent properties nanoparticles offer numerous advantages over traditional dyes and proteins. For example, the photostability, narrow emission peak, and ability to rationally modify both the size and surface chemistry of Quantum Dots allow for simultaneous analyses of multiple targets within the same cell. On the other hand, the surface characteristics of nanometer sized TiO2allow efficient conjugation to nucleic acids which enables their retention in specific subcellular compartments. We discuss cellular uptake mechanisms for the internalization of nanoparticles and studies showing the influence of nanoparticle size and charge and the cell type targeted on nanoparticle uptake. The predominant nanoparticle uptake mechanisms include clathrin-dependent mechanisms, macropinocytosis, and phagocytosis

Influence of elevated radiative lifetime on efficiency of CdSe/CdTe Type II colloidal quantum dot based solar cells

Colloidal quantum dots (CQDs) are promising materials for solar cells because their optoelectronic properties are easily adjusted by control of their size, structure and composition. We present calculations of the band gap and radiative lifetime for varying core diameter and shell thickness of CdSe/CdTe core/shell Type II CQDs using a combination of single particle (2,6)-band k·pk·p and many-electron configuration interaction (CI) Hamiltonians. These calculations are validated by comparison with experimental absorption spectra and photoluminescence decay data. The results are then incorporated into a model of photovoltaic efficiency which demonstrates how the overall performance of a solar cell based on Type II CQDs is affected by changes in the core/shell geometry. The largest effect on photovoltaic efficiency is found to be due to the longer radiative lifetime produced by increasing the shell thickness