Fluorescence-Lifetime Imaging and Super-Resolution Microscopies Shed Light on the Directed- and Self-Assembly of Functional Porphyrins onto Carbon Nanotubes and Flat Surfaces.

Functional porphyrins have attracted intense attention due to their remarkably high extinction coefficients in the visible region and potential for optical and energy-related applications. Two new routes to functionalised SWNTs have been established using a bulky ZnII -porphyrin featuring thiolate groups at the periphery. We probed the optical properties of this zinc(II)-substituted, bulky aryl porphyrin and those of the corresponding new nano-composites with single walled carbon nanotube (SWNTs) and coronene, as a model for graphene. We report hereby on: i) the supramolecular interactions between the pristine SWNTs and ZnII -porphyrin by virtue of π-π stacking, and ii) a novel covalent binding strategy based on the Bingel reaction. The functional porphyrins used acted as dispersing agent for the SWNTs and the resulting nanohybrids showed improved dispersibility in common organic solvents. The synthesized hybrid materials were probed by various characterisation techniques, leading to the prediction that supramolecular polymerisation and host-guest functionalities control the fluorescence emission intensity and fluorescence lifetime properties. For the first time, XPS studies highlighted the differences in covalent versus non-covalent attachments of functional metalloporphyrins to SWNTs. Gas-phase DFT calculations indicated that the ZnII -porphyrin interacts non-covalently with SWNTs to form a donor-acceptor complex. The covalent attachment of the porphyrin chromophore to the surface of SWNTs affects the absorption and emission properties of the hybrid system to a greater extent than in the case of the supramolecular functionalisation of the SWNTs. This represents a synthetic challenge as well as an opportunity in the design of functional nanohybrids for future sensing and optoelectronic applications.

Enhanced N2 dissociation on Ru-loaded inorganic electride.

Electrides, i.e. salts in which electrons serve as anions, are promising materials for lowering activation energies of chemical reactions. Ab initio simulations are used to investigate the effect of the electron anions in a prototype mayenite-based electride (C12A7:e(-)) on the mechanism of N2 dissociation. It is found that both atomic and molecular nitrogen species chemisorb on the electride surface and become negatively charged due to the electron transfer from the substrate. However, charging alone is not sufficient to promote dissociation of N2 molecules. In the presence of Ru, N2 adsorbs with the formation of a cis-Ru2N2 complex and the N-N bond weakens due to both the electron transfer from the substrate and interaction with Ru. This complex transforms into a more stable trans-Ru2N2 configuration, in which the N2 molecule is dissociated, with the calculated barrier of 116 kJ mol(-1) and the overall energy gain of 72 kJ mol(-1). In contrast, in the case of the stoichiometric mayentie, the cis-Ru2N2 is ~34 kJ mol(-1) more stable than the trans-Ru2N2, while the cis-trans transition has a barrier of 192 kJ mol(-1). Splitting of N2 is promoted by a combination of the strong electron donating power of C12A7:e(-), ability of Ru to capture N2, polarization of Ru clusters, and electrostatic interaction of negatively charged N species with the surface cations.

Fluorescent naphthalimide boronates as theranostics: structural investigations, confocal fluorescence and multiphoton fluorescence lifetime imaging microscopy in living cells.

New design and synthetic strategies were developed to generate functional phenyl boronic acid (BA)-based fluorescent probes incorporating the 1,8-naphthalimide (NI) tag. This fluorescent core was anchored onto the BA unit through small organic linkers consisting of nitrogen groups which can arrest, and internally stabilise the phenyl-boronate units. The newly synthesised fluorophores were characterised spectroscopically by NMR spectroscopy and mass spectrometry and evaluated for their ability to bind to a naturally occurring polysaccharide, ß-d-glucan in DMSO and simultaneously as act as in vitro cell imaging reagents. The uptake of these new NI-boronic acid derivatives was studied living cancer cells (HeLa, PC-3) in the presence, and absence, of ß-d-glucan. Time-correlated single-photon counting (TCSPC) of DMSO solutions and two-photon fluorescence-lifetime imaging microscopy (FLIM) techniques allowed an insight into the probes' interaction with their environment. Their cellular uptake and distributions were imaged using laser scanning confocal fluorescence microscopy under single- and two-photon excitation regimes (λmax 910 nm). FLIM facilitated the estimation of the impact of the probe's cellular surroundings using the fluorophore lifetime. The extent to which this was mediated by the ß-d-glucan was visualised by 2-photon FLIM in living cells. The fluorescence lifetime observed under a range of temperatures varied appreciably, indicating that changes in the environment can be sensed by these probes. In all cases, the cellular membrane penetration of these new probes was remarkable even under variable temperature conditions and localisation was widely concentrated in the cellular cytoplasm, without specific organelle trapping: we conclude that these new probes show promise for cellular imaging in living cancer cells.

Structural and Functional Diversity in Rigid Thiosemicarbazones with Extended Aromatic Frameworks: Microwave-Assisted Synthesis and Structural Investigations.

The long-standing interest in thiosemicarbazones (TSCs) has been largely driven by their potential toward theranostic applications including cellular imaging assays and multimodality imaging. We focus herein on the results of our new investigations into: (a) the structural chemistry of a family of rigid mono(thiosemicarbazone) ligands characterized by extended and aromatic backbones and (b) the formation of their corresponding thiosemicarbazonato Zn(II) and Cu(II) metal complexes. The synthesis of new ligands and their Zn(II) complexes was performed using a rapid, efficient and straightforward microwave-assisted method which superseded their preparation by conventional heating. We describe hereby new microwave irradiation protocols that are suitable for both imine bond formation reactions in the thiosemicabazone ligand synthesis and for Zn(II) metalation reactions. The new thiosemicarbazone ligands, denoted HL, mono(4-R-3-thiosemicarbazone)quinone, and their corresponding Zn(II) complexes, denoted ZnL2, mono(4-R-3-thiosemicarbazone)quinone, where R = H, Me, Ethyl, Allyl, and Phenyl, quinone = acenapthnenequinone (AN), aceanthrenequinone (AA), phenanthrenequinone (PH), and pyrene-4,5-dione (PY) were isolated and fully characterized spectroscopically and by mass spectrometry. A plethora of single crystal X-ray diffraction structures were obtained and analyzed and the geometries were also validated by DFT calculations. The Zn(II) complexes presented either distorted octahedral geometry or tetrahedral arrangements of the O/N/S donors around the metal center. The modification of the thiosemicarbazide moiety at the exocyclic N atoms with a range of organic linkers was also explored, opening the way to bioconjugation protocols for these compounds. The radiolabeling of these thiosemicarbazones with 64Cu was achieved under mild conditions for the first time: this cyclotron-available radioisotope of copper (t1/2 = 12.7 h; ß+ 17.8%; ß- 38.4%) is well-known for its proficiency in positron emission tomography (PET) imaging and for its theranostic potential, on the basis of the preclinical and clinical cancer research of established bis(thiosemicarbazones), such as the hypoxia tracer 64Cu-labeled copper(diacetyl-bis(N4-methylthiosemicarbazone)], [64Cu]Cu(ATSM). Our labeling reactions proceeded in high radiochemical incorporation (>80% for the most sterically unencumbered ligands) showing promise of these species as building blocks for theranostics and synthetic scaffolds for multimodality imaging probes. The corresponding "cold" Cu(II) metalations were also performed under the mild conditions mimicking the radiolabeling protocols. Interestingly, room temperature or mild heating led to Cu(II) incorporation in the 1:1, as well as 1:2 metal: ligand ratios in the new complexes, as evident from extensive mass spectrometry investigations backed by EPR measurements, and the formation of Cu(L)2-type species prevails, especially for the AN-Ph thiosemicarbazone ligand (L-). The cytotoxicity levels of a selection of ligands and Zn(II) complexes in this class were further tested in commonly used human cancer cell lines (HeLa, human cervical cancer cells, and PC-3, human prostate cancer cells). Tests showed that their IC50 levels are comparable to that of the clinical drug cis-platin, evaluated under similar conditions. The cellular internalizations of the selected ZnL2-type compounds Zn(AN-Allyl)2, Zn(AA-Allyl)2, Zn(PH-Allyl)2, and Zn(PY-Allyl)2 were evaluated in living PC-3 cells using laser confocal fluorescent spectroscopy and these experiments showed exclusively cytoplasmic distributions.

Interactions and reactions of transition metal clusters with the interior of single-walled carbon nanotubes imaged at the atomic scale.

Clusters of transition metals, W, Re, and Os, upon encapsulation within a single-walled carbon nanotube (SWNT) exhibit marked differences in their affinity and reactivity with the SWNT, as revealed by low-voltage aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM). Activated by an 80 keV electron beam, W reacts only weakly with the SWNT, Re creates localized defects on the sidewall, and Os reacts readily causing extensive defect formation and constriction of the SWNT sidewall followed by total rupture of the tubular structure. AC-HRTEM imaging at the atomic level of structural transformations caused by metal-carbon bonding of π- and σ-character demonstrates what a crucial role these types of bonds have in governing the interactions between the transition metal clusters and the SWNT. The observed order of reactivity W < Re < Os is independent of the metal cluster size, shape, or orientation, and is related to the metal to nanotube bonding energy and the amount of electronic density transferred between metal and SWNT, both of which increase along the triad W, Re, Os, as predicted by first-principles density functional theory calculations. By selecting the appropriate energy of the electron beam, the metal-nanotube interactions can be controlled (activated or precluded). At an electron energy as low as 20 keV, no visible transformations in the nanotube in the vicinity of Os-clusters are observed.

Structure and lithium transport pathways in Li2FeSiO4 cathodes for lithium batteries.

The importance of exploring new low-cost and safe cathodes for large-scale lithium batteries has led to increasing interest in Li(2)FeSiO(4). The structure of Li(2)FeSiO(4) undergoes significant change on cycling, from the as-prepared γ(s) form to an inverse ß(II) polymorph; therefore it is important to establish the structure of the cycled material. In γ(s) half the LiO(4), FeO(4), and SiO(4) tetrahedra point in opposite directions in an ordered manner and exhibit extensive edge sharing. Transformation to the inverse ß(II) polymorph on cycling involves inversion of half the SiO(4), FeO(4), and LiO(4) tetrahedra, such that they all now point in the same direction, eliminating edge sharing between cation sites and flattening the oxygen layers. As a result of the structural changes, Li(+) transport paths and corresponding Li-Li separations in the cycled structure are quite different from the as-prepared material, as revealed here by computer modeling, and involve distinct zigzag paths between both Li sites and through intervening unoccupied octahedral sites that share faces with the LiO(4) tetrahedra.

Defects, diffusion and dopants in Li8SnO6.

Octalithium tin (IV) oxide (Li8SnO6) is an important electrode material considered for lithium ion batteries (LIBs) because of its high lithium content. We employed atomistic simulations to examine the intrinsic defects, diffusion of Li-ions together with their migration energies and solution of potential dopants in Li8SnO6. The most thermodynamically favourable intrinsic defect is the Li Frenkel which increases the concentration of Li vacancies needed for the vacancy mediated diffusion of Li-ions in Li8SnO6. The calculated activation energy of migration of Li-ions (0.21eV) shows that the Li-ion conductivity in this material can be very fast. Promising isovalent dopants on the Li and Sn sites are Na and Ti, respectively. Doping of Ga on the Sn site can facilitate the formation of Li interstitials as well as oxygen vacancies in Li8SnO6. While the concentration of Li interstitials can enhance the capacity of this material, oxygen vacancies together with Li interstitials can lead to the loss of Li2O in Li8SnO6.

Self-diffusion in garnet-type Li7La3Zr2O12 solid electrolytes.

Tetragonal garnet-type Li7La3Zr2O12 is an important candidate solid electrolyte for all-solid-state lithium ion batteries because of its high ionic conductivity and large electrochemical potential window. Here we employ atomistic simulation methods to show that the most favourable disorder process in Li7La3Zr2O12 involves loss of Li2O resulting in lithium and oxygen vacancies, which promote vacancy mediated self-diffusion. The activation energy for lithium migration (0.45 eV) is much lower than that for oxygen (1.65 eV). Furthermore, the oxygen migration activation energy reveals that the oxygen diffusion in this material can be facilitated at higher temperatures once oxygen vacancies form.

Electronegativity and doping in Si1-xGex alloys.

Silicon germanium alloys are technologically important in microelectronics but also they are an important paradigm and model system to study the intricacies of the defect processes on random alloys. The key in semiconductors is that dopants and defects can tune their electronic properties and although their impact is well established in elemental semiconductors such as silicon they are not well characterized in random semiconductor alloys such as silicon germanium. In particular the impact of electronegativity of the local environment on the electronic properties of the dopant atom needs to be clarified. Here we employ density functional theory in conjunction with special quasirandom structures model to show that the Bader charge of the dopant atoms is strongly dependent upon the nearest neighbor environment. This in turn implies that the dopants will behave differently is silicon-rich and germanium-rich regions of the silicon germanium alloy.

Defect Chemistry and Na-Ion Diffusion in Na3Fe2(PO4)3 Cathode Material.

In this work, we employ computational modeling techniques to study the defect chemistry, Na ion diffusion paths, and dopant properties in sodium iron phosphate [Na3Fe2(PO4)3] cathode material. The lowest intrinsic defect energy process (0.45 eV/defect) is calculated to be the Na Frenkel, which ensures the formation of Na vacancies required for the vacancy-assisted Na ion diffusion. A small percentage of Na-Fe anti-site defects would be expected in Na3Fe2(PO4)3 at high temperatures. Long-range diffusion of Na is found to be low and its activation energy is calculated to be 0.45 eV. Isovalent dopants Sc, La, Gd, and Y on the Fe site are exoergic, meaning that they can be substituted experimentally and should be examined further. The formation of Na vacancies and Na interstitials in this material can be facilitated by doping with Zr on the Fe site and Si on the P site, respectively.

Defects and dopant properties of Li3V2(PO4)3.

Polyanion phosphate based Li3V2(PO4)3 material has attracted considerable attention as a novel cathode material for potential use in rechargeable lithium ion batteries. The defect chemistry and dopant properties of this material are studied using well-established atomistic scale simulation techniques. The most favourable intrinsic defect process is the Li Frenkel (0.45 eV/defect) ensuring the formation of Li vacancies required for Li diffusion via the vacancy mechanism. Long range lithium paths via the vacancy mechanism were constructed and it is confirmed that the lowest activation energy of migration (0.60 eV) path is three dimensional with curved trajectory. The second most stable defect energy process is calculated to be the anti-site defect, in which Li and V ions exchange their positions (0.91 eV/defect). Tetravalent dopants were considered on both V and P sites in order to form Li vacancies needed for Li diffusion and the Li interstitials to increase the capacity respectively. Doping by Zr on the V site and Si on the P site are calculated to be energetically favourable.

Technetium Encapsulation by A Nanoporous Complex Oxide 12CaO•7Al2O3 (C12A7).

Technetium (99Tc) is an important long-lived radionuclide released from various activities including nuclear waste processing, nuclear accidents and atmospheric nuclear weapon testing. The removal of 99Tc from the environment is a challenging task, and chemical capture by stable ceramic host systems is an efficient strategy to minimise the hazard. Here we use density functional theory with dispersion correction (DFT+D) to examine the capability of the porous inorganic framework material C12A7 that can be used as a filter material in different places such as industries and nuclear power stations to encapsulate Tc in the form of atoms and dimers. The present study shows that both the stoichiometric and electride forms of C12A7 strongly encapsulate a single Tc atom. The electride form exhibits a significant enhancement in the encapsulation. Although the second Tc encapsulation is also energetically favourable in both forms, the two Tc atoms prefer to aggregate, forming a dimer.

The encapsulation selectivity for anionic fission products imparted by an electride.

The nanoporous oxide 12CaO·7Al2O3 (C12A7) can capture large concentrations of extra-framework species inside its nanopores, while maintaining its thermodynamical stability. Here we use atomistic simulation to predict the efficacy of C12A7 to encapsulate volatile fission products, in its stoichiometric and much more effective electride forms. In the stoichiometric form, while Xe, Kr and Cs are not captured, Br, I and Te exhibit strong encapsulation energies while Rb is only weakly encapsulated from atoms. The high electronegativities of Br, I and Te stabilize their encapsulation as anions. The electride form of C12A7 shows a significant enhancement in the encapsulation of Br, I and Te with all three stable as anions from their atom and dimer reference states. Successive encapsulation of multiple Br, I and Te as single anions in adjacent cages is also energetically favourable. Conversely, Xe, Kr, Rb and Cs are unbound. Encapsulation of homonuclear dimers (Br2, I2 and Te2) and heteronuclear dimers (CsBr and CsI) in a single cage is also unfavourable. Thus, C12A7 offers the desirable prospect of species selectivity.

Defects, Diffusion, and Dopants in Li2Ti6O13: Atomistic Simulation Study.

In this study, force field-based simulations are employed to examine the defects in Li-ion diffusion pathways together with activation energies and a solution of dopants in Li2Ti6O13. The lowest defect energy process is found to be the Li Frenkel (0.66 eV/defect), inferring that this defect process is most likely to occur. This study further identifies that cation exchange (Li-Ti) disorder is the second lowest defect energy process. Long-range diffusion of Li-ion is observed in the bc-plane with activation energy of 0.25 eV, inferring that Li ions move fast in this material. The most promising trivalent dopant at the Ti site is Co3+, which would create more Li interstitials in the lattice required for high capacity. The favorable isovalent dopant is the Ge4+ at the Ti site, which may alter the mechanical property of this material. The electronic structures of the favorable dopants are analyzed using density functional theory (DFT) calculations.

Defect Chemistry, Sodium Diffusion and Doping Behaviour in NaFeO2 Polymorphs as Cathode Materials for Na-Ion Batteries: A Computational Study.

Minor metal-free sodium iron dioxide, NaFeO2, is a promising cathode material in sodium-ion batteries. Computational simulations based on the classical potentials were used to study the defects, sodium diffusion paths and cation doping behaviour in the α- and ß-NaFeO2 polymorphs. The present simulations show good reproduction of both α- and ß-NaFeO2. The most thermodynamically favourable defect is Na Frenkel, whereas the second most favourable defect is the cation antisite, in which Na and Fe exchange their positions. The migration energies suggest that there is a very small difference in intrinsic Na mobility between the two polymorphs but their migration paths are completely different. A variety of aliovalent and isovalent dopants were examined. Subvalent doping by Co and Zn on the Fe site is calculated to be energetically favourable in α- and ß-NaFeO2, respectively, suggesting the interstitial Na concentration can be increased by using this defect engineering strategy. Conversely, doping by Ge on Fe in α-NaFeO2 and Si (or Ge) on Fe in ß-NaFeO2 is energetically favourable to introduce a high concentration of Na vacancies that act as vehicles for the vacancy-assisted Na diffusion in NaFeO2. Electronic structure calculations by using density functional theory (DFT) reveal that favourable dopants lead to a reduction in the band gap.

Stability of Coinage Metals Interacting with C60.

Buckminsterfullerene (C60) has been advocated as a perfect candidate material for the encapsulation and adsorption of a variety of metals and the resultant metallofullerenes have been considered for the use in different scientific, technological and medical areas. Using spin-polarized density functional theory together with dispersion correction, we examine the stability and electronic structures of endohedral and exohedral complexes formed between coinage metals (Cu, Ag and Au) and both non-defective and defective C60. Encapsulation is exoergic in both forms of C60 and their encapsulation energies are almost the same. Exohedral adsorption of all three metals is stronger than that of endohedral encapsulation in the non-defective C60. Structures and the stability of atoms interacting with an outer surface of a defective C60 are also discussed. As the atoms are stable both inside and outside the C60, the resultant complexes can be of interest in different scientific and medical fields. Furthermore, all complexes exhibit magnetic moments, inferring that they can be used as spintronic materials.

Defect Chemistry and Li-ion Diffusion in Li2RuO3.

Layered Li2RuO3 is an important candidate cathode material in rechargeable lithium ion batteries because of its novel anionic redox process and high reversible capacity. Atomistic scale simulations are used to calculate the intrinsic defect process, favourable dopants and migration energies of lithium ion diffusions together with migration paths in Li2RuO3. The Li Frenkel is calculated to be the most favourable intrinsic defect type. The cation anti-site defect, in which Li and Ru ions exchange their positions is 1.89 eV/defect suggesting that this defect would be observed at high temperatures. Long range vacancy assisted lithium diffusion paths were calculated and it is confirmed that the lowest overall activation energy (0.73 eV) migration path is along the ab plane. Trivalent dopants (Al3+, Co3+, Sc3+, In3+, Y3+, Gd3+ and La3+) were considered to create additional Li in Li2RuO3. Here we show that Al3+ or Co3+ are the ideal dopants and this is in agreement with the experimental studies reported on Co3+ doping in Li2RuO3.

Impact of local composition on the energetics of E-centres in Si1-xGex alloys.

The energetics of the defect chemistry and processes in semiconducting alloys is both technologically and theoretically significant. This is because defects in semiconductors are critical to tune their electronic properties. These processes are less well understood in random semiconductor alloys such as silicon germanium as compared to elementary semiconductors (for example silicon). To model the random silicon germanium alloy we have employed density functional theory calculations in conjunction with the special quasirandom structures model for different compositions. Here we show that, the energetics of substitutional phosphorous-vacancy pairs (E-centres) in Si1-xGex alloys vary greatly with respect to the local Ge concentration and the composition of the alloy. The most energetically favourable E-centres have a Ge atom as a nearest neighbour, whereas the dependence of the binding energy of the E-centres with respect to alloy composition is non-linear.

Defects, Lithium Mobility and Tetravalent Dopants in the Li3NbO4 Cathode Material.

The defect processes of oxides such as self-diffusion impact their performance in electrochemical devices such as batteries and solid oxide fuel cells. The performance of lithium ion batteries can be improved by increasing the Li-ion diffusion. In that respect Li3NbO4 is identified as a positive electrode material for rechargeable lithium ion batteries. Here, we employ static atomistic scale simulations to examine the defect properties, doping behaviour and lithium ion migration paths in Li3NbO4. The present calculations show a correct reproduction of experimentally observed crystal structure of Li3NbO4. The Li-Nb anti-site defect is found to be the dominant intrinsic defect process suggesting that a small concentration of Li on Nb sites and Nb on Li sites is present. Vacancy assisted long range lithium diffusion paths were examined and our calculations reveal that the lowest activation energy (1.13 eV) migration path is two dimensional forming a zig-zag shape. Subvalent doping by Ge on the Nb site is thermodynamically favourable process and a potential strategy to incorporate extra Li in the form of Li interstitial in Li3NbO4. The results presented herein can motivate further experimental work for the development of Li3NbO4 based batteries.

Defects, dopants and Mg diffusion in MgTiO3.

Magnesium titanate is technologically important due to its excellent dielectric properties required in wireless communication system. Using atomistic simulation based on the classical pair potentials we study the defect chemistry, Mg and O diffusion and a variety of dopant incorporation at Mg and Ti sites. The defect calculations suggest that cation anti-site defect is the most favourable defect process. The Mg Frenkel is the second most favourable intrinsic defect though the formation energy is highly endoergic. Higher overall activation energies (>3 eV) are observed for oxygen migration compared to those observed for magnesium (0.88 eV). Dopant substitution energies for a range of cations with charges varying from +2 to +4 were examined. Divalent dopants (Mn2+, Fe2+, Co2+, Ca2+ and Zn2+) on the Mg site exhibit low solution energies. Trivalent dopants prefer to occupy Mg site though their solution energies are high. Exothermic solution energies calculated for tetravalent dopants (Ge4+ and Si4+) on the Ti site suggest the necessity of experimental verification.