Tijana Rajh

Hollow iron oxide nanoparticles for application in lithium ion batteries.

Material design in terms of their morphologies other than solid nanoparticles can lead to more advanced properties. At the example of iron oxide, we explored the electrochemical properties of hollow nanoparticles with an application as a cathode and anode. Such nanoparticles contain very high concentration of cation vacancies that can be efficiently utilized for reversible Li ion intercalation without structural change. Cycling in high voltage range results in high capacity (∼132 mAh/g at 2.5 V), 99.7% Coulombic efficiency, superior rate performance (133 mAh/g at 3000 mA/g) and excellent stability (no fading at fast rate during more than 500 cycles). Cation vacancies in hollow iron oxide nanoparticles are also found to be responsible for the enhanced capacity in the conversion reactions. We monitored in situ structural transformation of hollow iron oxide nanoparticles by synchrotron X-ray absorption and diffraction techniques that provided us clear understanding of the lithium intercalation processes during electrochemical cycling.

Biofunctionalized magnetic-vortex microdiscs for targeted cancer-cell destruction

Nanomagnetic materials offer exciting avenues for probing cell mechanics and activating mechanosensitive ion channels, as well as for advancing cancer therapies. Most experimental works so far have used superparamagnetic materials. This report describes a first approach based on interfacing cells with lithographically defined microdiscs that possess a spin-vortex ground state. When an alternating magnetic field is applied the microdisc vortices shift, creating an oscillation, which transmits a mechanical force to the cell. Because reduced sensitivity of cancer cells toward apoptosis leads to inappropriate cell survival and malignant progression, selective induction of apoptosis is of great importance for the anticancer therapeutic strategies. We show that the spin-vortex-mediated stimulus creates two dramatic effects: compromised integrity of the cellular membrane, and initiation of programmed cell death. A low-frequency field of a few tens of hertz applied for only ten minutes was sufficient to achieve approximately 90% cancer-cell destruction in vitro.

Role of Water and Carbonates in Photocatalytic Transformation of CO2 to CH4 on Titania

Using the electron paramagnetic resonance technique, we have elucidated the multiple roles of water and carbonates in the overall photocatalytic reduction of carbon dioxide to methane over titania nanoparticles. The formation of H atoms (reduction product) and (•)OH radicals (oxidation product) from water, and CO(3)(-) radical anions (oxidation product) from carbonates, was detected in CO(2)-saturated titania aqueous dispersion under UV illumination. Additionally, methoxyl, (•)OCH(3), and methyl, (•)CH(3), radicals were identified as reaction intermediates. The two-electron, one-proton reaction proposed as an initial step in the reduction of CO(2) on the surface of TiO(2) is supported by the results of first-principles calculations.

SERS of Semiconducting Nanoparticles (TiO2 Hybrid Composites)

Raman scattering of molecules adsorbed on the surface of TiO(2) nanoparticles was investigated. We find strong enhancement of Raman scattering in hybrid composites that exhibit charge transfer absorption with TiO(2) nanoparticles. An enhancement factor up to approximately 10(3) was observed in the solutions containing TiO(2) nanoparticles and biomolecules, including the important class of neurotransmitters such as dopamine and dopac (3,4-dihydroxy-phenylacetic acid). Only selected vibrations are enhanced, indicating molecular specificity due to distinct binding and orientation of the biomolecules coupled to the TiO(2) surface. All enhanced modes are associated with the asymmetric vibrations of attached molecules that lower the symmetry of the charge transfer complex. The intensity and the energy of selected vibrations are dependent on the size and shape of nanoparticle support. Moreover, we show that localization of the charge in quantized nanoparticles (2 nm), demonstrated as the blue shift of particle absorption, diminishes SERS enhancement. Importantly, the smallest concentration of adsorbed molecules shows the largest Raman enhancements suggesting the possibility for high sensitivity of this system in the detection of biomolecules that form a charge transfer complex with metal oxide nanoparticles. The wavelength-dependent properties of a hybrid composite suggest a Raman resonant state. Adsorbed molecules that do not show a charge transfer complex show weak enhancements probably due to the dielectric cavity effect.

Nanostructured Bilayered Vanadium Oxide Electrodes for Rechargeable Sodium-Ion Batteries

Tailoring nanoarchitecture of materials offers unprecedented opportunities in utilization of their functional properties. Nanostructures of vanadium oxide, synthesized by electrochemical deposition, are studied as a cathode material for rechargeable Na-ion batteries. Ex situ and in situ synchrotron characterizations revealed the presence of an electrochemically responsive bilayered structure with adjustable intralayer spacing that accommodates intercalation of Na(+) ions. Sodium intake induces organization of overall structure with appearance of both long- and short-range order, while deintercalation is accompanied with the loss of long-range order, whereas short-range order is preserved. Nanostructured electrodes achieve theoretical reversible capacity for Na(2)V(2)O(5) stochiometry of 250 mAh/g. The stability evaluation during charge-discharge cycles at room temperature revealed an efficient 3 V cathode material with superb performance: energy density of ~760 Wh/kg and power density of 1200 W/kg. These results demonstrate feasibility of development of the ambient temperature Na-ion rechargeable batteries by employment of electrodes with tailored nanoarchitectures.

A High-Performance Nanobio Photocatalyst for Targeted Brain Cancer Therapy

We report pronounced and specific antiglioblastoma cell phototoxicity of 5 nm TiO(2) particles covalently tethered to an antibody via a dihydroxybenzene bivalent linker. The linker application enables absorption of a visible part of the solar spectrum by the nanobio hybrid. The phototoxicity is mediated by reactive oxygen species (ROS) that initiate programmed death of the cancer cell. Synchrotron X-ray fluorescence microscopy (XFM) was applied for direct visualization of the nanobioconjugate distribution through a single brain cancer cell at the submicrometer scale.

A Bioinspired Construct That Mimics the Proton Coupled Electron Transfer between P680*† and the Tyrz-His190 Pair of Photosystem II

A bioinspired hybrid system, composed of colloidal TiO2 nanoparticles surface modified with a photochemically active mimic of the PSII chlorophyll-Tyr-His complex, undergoes photoinduced stepwise electron transfer coupled to proton motion at the phenolic site. Low temperature electron paramagnetic resonance studies reveal that injected electrons are localized on TiO2 nanoparticles following photoexcitation. At 80 K, 95% of the resulting holes are localized on the phenol moiety and 5% are localized on the porphyrin. At 4.2 K, 52% of the holes remain trapped on the porphyrin. The anisotropic coupling tensors of the phenoxyl radical are resolved in the photoinduced D-band EPR spectra and are in good agreement with previously reported g-tensors of tyrosine radicals in photosystem II. The observed temperature dependence of the charge shift is attributed to restricted nuclear motion at low temperature and is reminiscent of the observation of a trapped high-energy state in the natural system. Electrochemical studies show that the phenoxyl/phenol couple of the model system is chemically reversible and thermodynamically capable of water oxidation.

The important role of tetrahedral Ti4+ sites in the phase transformation and photocatalytic activity of TiO2 nanocomposites.

Highly photoactive, tetrahedral Ti4+ sites can be created, other than in zeolite cavities and on silica substrate, in mixed-phase TiO2 nanocomposites. The tetrahedral Ti4+ species was shown to be an intermediate formed during the thermally driven phase transformation from anatase to rutile.

In situ visualization of self-assembly of charged gold nanoparticles.

Self-assembly of Au nanoparticles (NPs) coated with positively charged cetyltrimethylammonium ions (CTA(+)) and negatively charged citrate ions in aqueous liquid cell was investigated by in situ transmission electron microscopy (TEM). Under electron illumination in TEM, the hydrated electrons will reduce the overall positive charges of the CTA(+) covered Au NPs and decrease the repulsive electrostatic forces among NPs, leading to assembly of individual NPs into one-dimensional structures. On the contrary, the negatively charged Au NPs coated with citrate ions are steady in liquid cell regardless of electron beam intensity.

Dynamics of Localized Charges in Dopamine-Modified TiO2 and their Effect on the Formation of Reactive Oxygen Species

Modification of TiO(2) nanoparticles with dopamine enables harvesting of visible light and promotes spatial separation of charges. The formation of reactive oxygen species (OH, (1)O(2), O(2)(-), HO(2), H(2)O(2)) upon illumination of TiO(2)/dopamine was studied using complementary spin-trap EPR and radical-induced fluorescence techniques. The localization of holes on dopamine suppresses oxidation of adsorbed water molecules at the surface of nanoparticles, and thus formation of OH radicals. At the same time, dopamine does not affect electronic properties of photogenerated electrons and their reaction with dissolved oxygen to produce superoxide anions. Superoxide anions are proposed to generate singlet oxygen through dismutation reaction, resulting in a low yield of (1)O(2) detected.