A decade of heterogeneous photocatalysis in our laboratory: Pure and applied studies in energy production and environmental detoxification

Heterogeneous Photocatalysis (HP) has been examined and explored as a potentially viable alternative technology to classical “best” technologies in both environmental detoxification and in energy production. Both fundamental and applied investigations have been pursued over the last decade (1983–1993). This novel technology employs illuminated semiconductor materials as photocatalysts such as CdS and TiO2 to produce both reducing and highly oxidizing species on the particle surface, poised to unleash redox reactions in aqueous media some of which are not possible by normal chemical means. Our laboratory has been involved in this area and has made a significant contribution in (i) demonstrating the generality of the techniques to mineralize a large variety of environmental organics to carbon dioxide together with disposing of trace and toxic metals from aqueous media, and in (ii) understanding the events taking place initially when the light photons interact with the photocatalysts and later when the mineralization process proceeds to its pathological conclusion(s). In this paper we briefly review some of the salient features of our work in Heterogeneous Photocatalysis over the last decade.     

Effect of charge density fluctuations within a droplet on dielectric polarization of ionic microemulsions

A statistical model of the dielectric polarization of ionic water-in-oil microemulsions is proposed. The model makes it possible to describe the effect of temperature and dispersed phase content on the static dielectric permittivity behavior of the microemulsions at a region far below percolation. With the help of this model, the microemulsions formed with the surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), have been analyzed. The studied systems are considered to consist of nanometer-sized spherical non-interacting water droplets of equal size with negatively charged head groups , staying at the interface and positive counterions Na⁺, distributed in the electrical diffuse double layer of the droplet interior. It can be conjectured that two different mechanisms, that provide an increase of the static dielectric permittivity as a function of temperature, may take place. These may be attributed either to the aggregation of droplets or the temperature growth of polarizability of non-interacting and therefore non-aggregating droplets dispersed in oil. The results support the hypothesis that the experimental temperature behavior of dielectric polarization far below the percolation region is only due to the polarization of a single droplet and not to an aggregation. The droplet polarizability is proportional to the fluctuation mean-square dipole moment of a droplet. It is shown that this mean-square dipole moment and the corresponding value of the dielectric increment, depend upon the equilibrium distribution of counterions within a diffuse double layer. The density distribution of ions is determined by the degree of the dissociation of the ionic surfactant. The dissociation of the ionic surfactant in the system has been analyzed numerically. The relationship between the constant of dissociation and the experimental dielectric permittivity has been ascertained.     

WAVELENGTH DEPENDENCY OF THE RATE OF IODINE CATALYZED PHOTOISOMERIZATION OF RETINOL and RETINAL

In separate experiments, hydrocarbon solutions of the 11-cis isomers of retinol and retinal containing catalytic amounts of iodine were irradiated with monochromatic light. Changes in absorbance were followed spectroscopically for each wavelength of light and measured as a function of time. The curve obtained by plotting the change in absorbance (delta A) vs time (t) is a hyperbola, and thus the plot of 1/delta A vs 1/t forms a straight line. The half reaction time, t1/2, was extracted from this equation, giving a value for each wavelength of light. The parameter 1/t1/2 is adjusted to compensate for variation of light quanta from the xenon source, and it is plotted vs wavelength. A response spectrum is obtained that is a Gaussian. The lambda max is 510 nm for retinol and 519 nm for retinal. This shows that in the photoisomerization iodine is the absorbing species and not the carbocation, which absorbs at 589 nm.     

Efficient photoinduced conversion of an azo dye on hexachloroplatinate(IV)-modified TiO2 surfaces under visible light irradiation-A photosensitization pathway

The titanium dioxide photocatalyst is employed to examine the influence of chemisorbed hexachloroplatinate(IV) anions (PtCl(6) (2-)) on the surface of P-25 TiO(2) particles on the photoinduced conversion of the azo dye Ethyl Orange (EO) in visible light-illuminated Pt(IV)/TiO(2) dispersions. Spin-trap electron spin resonance (ESR) spectral results, measurement of quantities of organoperoxides formed, total organic carbon (TOC) and chemical oxygen demand (COD(Cr)) assays, together with XPS evidence show that the self-sensitized transformation dynamics of the EO dye mediated by Pt(IV)/TiO(2) are much faster than those occurring on naked TiO(2) under otherwise identical conditions of visible light irradiation. X-ray photoelectron spectral data also show that under the experimental conditions used, no Pt(0) formed on the titania particles during visible light irradiation. We propose a reaction mechanism in which the more rapid conversion of EO in the presence of PtCl(6) (2-) is caused principally by photoexcitation of the dye and not by localized excitation of the tetrachloroplatinate(IV)/TiO(2) particles.     

Guest-framework interaction in type I inorganic clathrates with promising thermoelectric properties: on the ionic versus neutral nature of the alkaline-earth metal guest A in A8Ga16Ge30 (A=Sr, Ba)

Periodic density functional calculations using pseudopotentials and a local basis set were performed on the type I clathrates A(8)Ga(16)Ge(30) (A=Sr, Ba). Both are known to show promising thermoelectric properties. Ab initio wave functions were analyzed within the framework of the quantum theory of atoms in molecules. This enabled us to analyze both the charge transfer and bonding properties of the clathrate from a rigorous quantum mechanical viewpoint. The Ba and Sr centers were found to be largely ionic (charge: ca. +1.7 e) both in the smaller 20-atom and in the larger 24-atom cages, consistent with a Zintl-phase view of these type I clathrates. The assertion that the Sr atoms in the different cages have similar oxidation states is shown to be consistent with multiwavelength diffraction experiments on Sr(8)Ga(16)Ge(30); while the assertion of ionicity of the Sr center is supported by the observation that the adsorption edge lies close to that previously found in the Sr K-edge XANES spectra of Sr(OH)(2).8 H(2)O. As such, this work contradicts previous experimental and theoretical studies that claim that the guest atoms are neutral. We show that the discrepancy is related to the definitions used for electron transfer. Definitions based on electron displacement (rearrangement) in space, as in previous works, do not account for the variation in shape and volume of the atomic catchment regions upon change in the number and average locations of the particles in the system. Eventually, such definitions lead to underestimation of charge transfer. The large binding energy found in earlier work for Ba and Sr in these materials is found to be consistent with a simple picture of charge transfer from the guest to the frame. Preliminary investigations on a clathrate of perfect stoichiometry appear to rule out any important relationship between the observed increase in the thermoelectric figure of merit with increasing external pressure and host-guest charge transfer.     

Interactions of Ni(II) and Cu(II) ions with the hydrolysis products of the C-terminal -ESHH- motif of histone H2A model peptides. Association of the stability of the complexes formed with the cleavage of the -E-S- bond

We studied the interactions of Ni(II) and Cu(II) ions with the synthetic tetrapeptides SHHK- and SAHK-, which were blocked by amidation making them more realistic models of the hydrolysis peptidic products of the hexapeptides models of H2A histone. A combination of potentiometric and spectroscopic techniques (UV/Vis, CD, NMR and EPR) suggested that at pH > 7 both tetrapeptides coordinated equatorially through the imidazole ring of His in position 3, the N-terminal amino group and the two amide nitrogens existing between these groups {NH2, 2N-, NIm} forming 4N square-planar complexes. While in the case of the CuH(-1)L complex with SHHK- a possible axial coordination of the imidazole ring of His in position 2 was suggested, in the case of the analogous NiH(-1)L complex a completely different interaction of the same ring with metal ions was observed. As expected these complexes have the same structures with the hydrolysis products produced from the Ni(II)- or Cu(II)-assisted hydrolysis of previously studied hexapeptide models of the C-terminal of histone H2A, due to their predominance at pH > 7.4. In addition, the competition plots presented herein showed that the synthetic tetrapeptides SHHK- and SAHK- have higher affinity towards Ni(II) and Cu(II) ions than the previously studied hexapeptides, suggesting that metal ions remain bound to the peptidic products during the hydrolysis cleavage. Thus, it can be concluded that the stability of Ni(II) or Cu(II) complexes with the synthetic tetrapeptides and consequently with the real hydrolysis peptidic products is the driving force of the hydrolysis reaction of H2A histone blocked hexapeptide models, presented in previous studies.     

Determination of rhizosphere 13C pulse signals in soil thin sections by laser ablation isotope ratio mass spectrometry

In grassland ecosystems, soil animals act as key soil engineers and architects. The diversity of soil animals is also a regulator of ecosystem carbon flow. However, our understanding of the link between soil animals, carbon fluxes and soil physical organisation remains poor. An integrated approach based on soil micromorphology and laser ablation stable isotope ratio mass spectrometry (LA-IRMS) was developed to provide spatially distributed data of pulse-derived (13)C tracer from roots in the soil environment. This paper describes the development and testing of a LA-IRMS (13)C/(12)C analytical method on soil thin sections as a means to determine the fate of root carbon derived from photosynthesis into soil. Results from this work demonstrated (1) that micro-scale delta(13)C (per thousand) analysis could be made on targeted features located within a soil thin section and (2) that LA-IRMS delta(13)C (per thousand) measurements made on samples obtained from (13)CO(2) pulse labelled plant-soil blocks confirmed the presence of recent photosynthates in the rhizosphere (1 and 4 weeks post-pulse).     

Synthesis and characterization of carboxylate complexes of Sn(IV) porphyrin monomers and oligomers

Most of the porphyrin-recognition chemistry we have investigated previously has centred on kinetically labile metal-ligand interactions, such as Z-N and Ru-N. Our interest in the broader scope of molecular recognition required a metal with the ability to specifically recognise non-nitrogen-based ligands, with a significantly different binding interaction to distinguish it from nitrogen-based analogues. In this report we describe interactions of Sn(IV) porphyrins that bind oxygen-based ligands and for which the Sn(IV)bond;O bond is in slow exchange on the NMR timescale. A series of carboxylate complexes is employed to highlight the structural/geometric features of porphyrin monomers and cyclic oligomers. Where more than one porphyrin unit is present in a molecular scaffold, we report the effect of carboxylate binding on the complex when the two porphyrins contain different metals (typically Sn(IV) and Zn(II)). The unexpected spectroscopic and structural properties of the Sn(2)(9-anthroic acid)porphyrin dimer are also reported.     

Infrared fingerprint spectroscopy and theoretical studies of potassium ion tagged amino acids and peptides in the gas phase

Infrared multiple-photon dissociation spectroscopy is effected on the K(+) tagged aromatic amino acids tyrosine and phenylalanine, as well as the K(+) tagged peptides bradykinin fragment 1-5 and [Leu]-enkephalin. The fingerprint (800-1800 cm(-1)) infrared spectra of these species are compared to density-functional theory (DFT) calculated spectra to determine whether the complex is in the charge solvation (CS) or salt bridge (SB) (i.e. zwitterionic) configuration. For the aromatic amino acids the CS structure is favored and the tridentate N/O/ring structure is found to be the preferred binding geometry for K(+). The experimental and theoretical evidence for bradykinin fragment 1-5 tagged with K(+) suggests that the SB structure is favored; the calculations indicate a head-to-tail looped structure stabilized by a salt bridge between the protonated guanidine group and the deprotonated C-terminus, which allows K(+) to sit in a binding pocket with five C=O electrostatic interactions. For K(+) tagged [Leu]-enkephalin the spectroscopic evidence is not as clear. While the calculations clearly favor a CS structure and the observation of a weak carboxylic acid C=O stretching band in the infrared spectrum matches this finding, the prominence of a band at 1600 cm(-1) renders the analysis more ambiguous, and hence the presence of some salt bridge ions cannot be excluded. Another striking feature in the [Leu]-enkephalin spectrum is the high infrared activity of the tyrosine side-chain modes, which can be clearly identified from comparison to the [Tyr + K](+) experimental spectrum, but which is not reproduced by the DFT calculations.     

Diagnostic prospects and preclinical development of optical technologies using gold nanostructure contrast agents to boost endogenous tissue contrast

Numerous developments in optical biomedical imaging research utilizing gold nanostructures as contrast agents have advanced beyond basic research towards demonstrating potential as diagnostic tools; some of which are translating into clinical applications. Recent advances in optics, lasers and detection instrumentation along with the extensive, yet developing, knowledge-base in tailoring the optical properties of gold nanostructures has significantly improved the prospect of near-infrared (NIR) optical detection technologies. Of particular interest are optical coherence tomography (OCT), photoacoustic imaging (PAI), multispectral optoacoustic tomography (MSOT), Raman spectroscopy (RS) and surface enhanced spatially offset Raman spectroscopy (SESORS), due to their respective advancements. Here we discuss recent technological developments, as well as provide a prediction of their potential to impact on clinical diagnostics. A brief summary of each techniques'' capability to distinguish abnormal (disease sites) from normal tissues, using endogenous signals alone is presented. We then elaborate on the use of exogenous gold nanostructures as contrast agents providing enhanced performance in the above-mentioned techniques. Finally, we consider the potential of these approaches to further catalyse advances in pre-clinical and clinical optical diagnostic technologies.

Optical biomedical imaging research utilising gold nanostructures as contrast agents has advanced beyond basic science, demonstrating potential in various optical diagnostic tools; some of which are currently translating into clinical applications.