Temperature and oxygen-concentration dependence of singlet oxygen production by RuPhen as induced by quasi-continuous excitation

Advanced Search Home > Journals > Photochemical & Photobi... > Temperature and oxygen-... For Authors & Referees | For Librarians | For Members Journal cover: Photochemical & Photobiological Sciences Photochemical & Photobiological Sciences Issue 12, 2014 A society-owned journal publishing high quality research on all aspects of photochemistry and photobiology. Impact Factor 2.939 12 Issues per Year Indexed in Medline Journal Home Previous Article | Next Article Paper Temperature and oxygen-concentration dependence of singlet oxygen production by RuPhen as induced by quasi-continuous excitation Jaroslav Varchola,a Veronika Huntosova,b Daniel Jancura,ab Georges Wagnières,c Pavol Miskovskyab and Gregor Bánó*ab Show Affiliations Photochem. Photobiol. Sci., 2014,13, 1781-1787

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Abstract Cited by Related Content

Assessment of partial pressure of oxygen (pO2) by luminescence lifetime measurements of ruthenium coordination complexes has been studied intensively during the last few decades. RuPhen (dichlorotris(1,10-phenanthroline) ruthenium(II) hydrate) is a water soluble molecule that has been tested previously for in vivo pO2 detection. In this work we intended to shed light on the production of singlet oxygen by RuPhen. The quantum yield of singlet oxygen production by RuPhen dissolved in 0.9% aqueous NaCl solution (pH = 6) was measured at physiological temperatures (285–310 K) and various concentrations of molecular oxygen. In order to minimize the bleaching of RuPhen, the samples were excited with low power (

La diffusion Brillouin dans les fibres optiques

Among all non-linear optical effects observed in single-mode optical fibres, stimulated Brillouin scattering (SBS) is of particular importance since it has numerous practical implications. SBS occurs when laser waves are scattered through the interaction of light with hypersonic acoustic waves in the medium. It manifests through the generation of a backscattered Stokes wave that carries most of the input optical power once a definite intensity level is reached within the fibre core. It limits the maximum optical power that can be transmitted through an optical fibre and therefore can cause a severe penality for fibre optics telecommunications. The backscattered Stokes wave is down shifted in frequency with respect to the incident lightwave frequency. This frequency shift – often called the Brillouin frequency shift – depends on the fibre parameters and on the wavelength of the incident light. It is directly proportional to the acoustic velocity and ranges from 12 to 13 GHz for silica fibres at a 1.3 µm wavelength. The Brillouin frequency shift is also very sensitive to environmental quantities changing the acoustic velocity such as temperature and strain. This feature makes SBS very interesting for temperature and strain monitoring in optical fibres and has been used in the design of fibre optics sensors. In the present work, a novel method to measure SBS characteristics in single-mode optical fibres has been developed. It is based on the so-called pump and probe technique using two counterpropagating optical waves within the test fibre. It basically presents the originality to require only one laser source and relies on an electro-optic modulator to generate the probe signal by microwave modulation of the pump light. The inherent high stability of the experimental set-up has made possible the characterisation of different fibres in terms of SBS parameters with a higher accuracy than the traditional two lasers set-up. The influence of dopant concentration in silica fibres on the SBS properties has been fully characterised. Furthermore it has been determined that the presence of core dopants decreases the acoustic velocity resulting in a smaller Brillouin frequency shift. The temperature and strain effects on the SBS characteristics have been extensively investigated. The problem of the evolution of the polarisation of the interacting lightwaves has been studied and a model clarified the so far unexplained behaviour in low birefringence fibres. A new measurement procedure has been defined to achieve polarisation-independent Brillouin gain determination. The second facet of the present thesis deals with the concept of distributed fibre optics sensors based on SBS. The determination of the Brillouin frequency shift gives access to the temperature or strain experienced by the fibre, while a modified OTDR technique provides the information on the position. Here again the use of a single laser source together with an electro-optic modulator brings several advantages. Besides the convenient flexibility in the generation of the probe signal, it makes possible to pulse the optical signals to localise the interaction within the test fibre. The overall temperature or strain distribution of the test fibre can be carried out by measuring the Brillouin frequency shift at any locations throughout the test fibre. A sensor has been experimentally achieved and its performances can be summarised as follow: spatial resolution less than 10 meters over more than 10 kilometres, with a resolution on the Brillouin frequency determination of 1 MHz, that corresponds to a ±1°C temperature resolution or to a 2x10-5 strain resolution. The dynamic range can be increased up to 30 kilometres to the detriment of the spatial resolution. The ultimate performances of the SBS distributed fibre optics sensors have been investigated in terms of spatial resolution and dynamic range. It turns out that these sensors are eventually dedicated to distributed measurements over several tens of kilometres with a spatial resolution limited to the meter range. Finally two practical applications of such sensors are described: the measurement of the strain distribution in installed fibre optics cables for telecommunications and the temperature monitoring of electrical energy distribution cables. On site measurements using a Brillouin sensor have been performed for the first time thanks to the high stability and reliability of the sensor.

EPFL1997

Study of N2O decomposition over Fe-ZSM-5 with transient methods

Petra Prechtl

Nitrous oxide is the third most important gas contributing to global warming and its concentration in the atmosphere is still on the rise. Nitric acid production represents the largest source of N2O in the chemical industry, with a total annual emission of about 400 kt N2O. An efficient way to reduce the N2O is the catalytic decomposition into N2 and O2. ZSM-5 zeolites containing iron are known to be effective in this reaction involving Fe(II)-extraframework species as active sites. But the nature of these sites and the reaction network is still under debate. Often the N2O is in the exhaust together with other gases like NOx, COx, oxygen, and water vapor, which may influence the catalyst efficiency towards N2O decomposition. The effect of these gases on the reaction mechanism is still not understood yet. The aim of this work is to study the decomposition mechanism of N2O over isomorphously substituted Fe-ZSM-5 (0.02-0.55% Fe) using transient response methods. Kinetic studies under steady-state conditions give overall information about the reaction, whereas transient experiments lead to information on the individual steps and the possibility to suggest a reaction model. Transient response methods are applied in vacuum using a Temporal Analysis of Products (TAP) setup and at ambient pressure using a Micromeritics AutoChem 2910 analyzer. The influence of different gases such as NO, H2O and NO2 on the decomposition of N2O over Fe-ZSM-5 is investigated to gain insight into their effect on this reaction. The TAP multifunctional reactor system is used to perform transient pulse experiments for micro-kinetic analysis of complex heterogeneous catalytic reactions. This setup allows to dose precisely reactants and to monitor product formation with submillisecond time resolution implying the absence of external mass or heat transfer. A reproducible low hydroxylation level of the catalyst can be attained in vacuum. The results from the transient responses with the Micromeritics analyzer are obtained under conditions more relevant to "operando". The mechanism of N2O decomposition over Fe containing ZSM-5 was studied by transient response methods at ambient pressure with the Micromeritics analyzer and under vacuum with the TAP setup. Both approaches showed that the mechanism strongly depends on the reaction temperature, but in all cases involves atomic oxygen loading from N2O with N2 evolution as the first reaction step. The surface oxygen, (O)Fe, formed from N2O possesses very high reactivity and oxidizes CO to CO2 already at 373 K. A complete saturation of the active sites can be reached by the catalyst exposure to N2O at low temperatures (T < 600 K in vacuum, T < 523 K at ambient pressure). Under these conditions oxygen is stored and does not desorb. On that basis, the concentration of sites active for the surface atomic oxygen loading from N2O is determined by pulse experiments using the TAP setup. The concentration of active sites is found to be the same as in a flow experiment with the Micromeritics analyzer. The mechanism of surface atomic oxygen recombination and desorption as O2 depends on the temperature. At temperatures higher than 773 K, the mechanism probably involves direct (O)Fe recombination. The investigation of the response profiles upon N2O pulses at 803 K evidences the O2 formation is the rate-determining step since the oxygen desorption is found to be slow compared to N2O and N2. Furthermore, a fit of the oxygen desorption curve corresponded to a first order dependence on the oxygen surface coverage. In the range of 523 to 773 K, the O2 formation is facilitated by NO, which is formed on the catalyst surface from N2O. The addition of NO to the gas phase confirmed the accelerating effect of NO on the oxygen desorption. NO and (O)Fe are adsorbed on adjacent, but different sites and are not competing for the same ones. Adsorbed NO accommodates oxygen from N2O forming higher oxidized nitrogen oxide species NO2/NO3 which decompose back to NOads and molecular oxygen. Thus, the recombination of surface oxygen loaded from N2O is accelerated by a NO/NOx redox cycle whereas adsorbed NOx acts as co-catalyst. Adsorbed NO is oxidized by N2O easily as shown also by temperature-programmed desorption (TPD) experiments. Therefore, the interaction of gaseous NO2 with Fe-ZSM-5 at 523 K in vacuum and at ambient pressure is investigated and demonstrates the formation of two different NOx surface species accompanied by the evolution of gaseous NO. These surface species block the sites for oxygen loading from N2O whereas the amount of reversible adsorbed N2O stays constant compared to the standard pre-treated catalyst. The formation of reactive surface oxygen from NO2 could neither be evidenced in TPD nor with CO oxidation experiments. Adsorbed water strongly influences the N2O decomposition inhibiting it completely, at least at temperatures ≤ 673 K. Water pulses at 523 and 593 K result in partial desorption of the loaded atomic surface oxygen as O2, indicating the competition of water for the same adsorption sites. The residual adsorbed oxygen desorbs during TPD experiments at 849 K instead of 715 K for the dry catalyst and it can not oxidize CO to CO2. It means that the active sites are transformed into an inactive form in the presence of water. The oxygen which stays on the catalyst seems to participate in the Fe(II) oxidation to the hydroxylated Fe(III) species. These Fe(III) species are inactive in N2O decomposition and could be activated again by high temperature treatment in He or in vacuum. During TAP experiments only small amounts of water are dosed and the water desorption is facilitated due to vacuum. Therefore, the effect of water could be shown to be reversible at temperatures > 700 K. At 803 K oxygen desorbs significantly faster with increasing water amount suggesting a water assisted O2 desorption. Simultaneously, the catalyst activity decreases to about a half of the initial activity of the dry zeolite. To conclude, from the TAP reactor data obtained under vacuum conditions and the results from the transient flow experiments at ambient pressure it follows that the mechanism of the N2O decomposition over Fe-containing ZSM-5 depends strongly on the reaction temperature and on the presence of NO, H2O and NO2. In general, below 523 K only surface atomic oxygen loading from N2O takes place with simultaneous evolution of N2 in the gas phase. The O2 formation proceeds via direct (O)Fe recombination at temperatures above 773 K. In the range of 523 to 773 K, the recombination of the atomic oxygen involves NOx adsorbed on the catalyst surface.

EPFL2007

Development of Minimally-Invasive Optical Methods to Individualize the Doses Used for Therapeutic Applications of Light

Filippo Piffaretti

Despite the experience gained over several decades in various types of light-based medical treatments, the optimization of the corresponding therapeutic protocols and accurate forecasting of their outcome have not yet been achieved in many cases. The difficulty often arises from the heterogeneity of living tissues, their variable optical properties, and from the heterogeneous distribution of the photoactive or photosensitive substances – whether naturally present in the tissue or artificially adde. Our work focuses on the individualization and control of irradiation parameters, in order for the physician to be able to elicit a predictable clinical response in the irradiated tissues. In this thesis, we present three separate studies in which we tried to evaluate the possibility of individualizing and optimizing the corresponding clinical outcomes by measuring or monitoring certain, particular parameters. In a first clinical study, performed at the medical practice of Dr Vezzola, MD, in Saló, Italy, the human eye's retinal reflectance was measured and mapped, in the framework of subthreshold thermal laser therapy, using an excitation wavelength identical to that of the treatment laser, i.e. at 810 nm. The specific goal of this study was to correlate the occurrence of retinal burns with the measured infrared retinal reflectance. This study was performed using a modified fundus camera to record infrared reflectance images of the retina, and by recording the slit-lamp based laser therapy parameters (irradiation parameters and spot location) in such a way so as to overlay the map of the laser treatment spots on the corresponding reflectance fundus image. The clinical study demonstrated the expected existence of spatial variations in light reflectance at 810 nm (probably due to changes of the tissue absorption), which we then tried to relate to the occurrences of retinal burns observed during the laser treatment. The analysis of the results obtained with the applied conditions, did not however show a clear correlation between the local retina reflectance, the laser beam parameters, and the occurrence of retinal burns. Therefore we postulate that either the absorbing structures of the retina cannot be seen with the imaging device we used, possibly due to its limited resolution, or other important elements play a role in the laser-tissue interactions during this type of lasers-light interaction with the local retinal tissue. The second study was pre-clinical, performed at EPFL, and aimed at monitoring in real-time the tissular oxygen concentration during photodynamic therapy (PDT). It was performed in vivo on the chicken embryo's chorio-allantoic membrane (CAM) model, which was submitted to aminolevulinic acid (ALA)-based PDT. The molecular oxygen, which is thought to be an essential actor in the cascade of reactions leading to the tissular PDT effect, is actually the main molecule responsible for the photosensitizer's (PS) triplet state quenching. Therefore, the delayed fluorescence lifetime of the photosensitizer, protoporphyrin IX (PpIX), was measured with a specially designed and assembled, optical fiber-based, time-resolved spectrofluorometer, and used as a proxy for tissular pO2. Simultaneously, vascular damages caused by PDT were characterized and quantified, to check for correlation between the two parameters. Using the PS's delayed fluorescence lifetime to evaluate tissular pO2 proved to be a quite reasonable strategy, due to the fact that it is possible to measure the pO2 at the location of the PS molecule. The study's results demonstrate a robust, linear correlation between tissular pO2 reduction and vascular damage extent. They also suggest that the amount of oxygen consumed during PDT could be a useful, measurable parameter for assessing and/or controlling the PDT's therapeutic effect. The third, clinical, study aimed at measuring the fluorescence photobleaching of the PpIX photosensitizer due to PDT. This study was performed at the "Hôpitaux Universitaires de Genève – HUG", in Geneva, Switzerland, in collaboration with Dr Denis Salomon, MD, and Dr Behrooz Kasraee, MD, in the framework of a series of standard clinical PDT treatments of aktinic keratoses (AK), which is a pre-cancerous skin lesion. The lesions' fluorescence intensity was quantified with a specially adapted quantitative imaging device, using a homogeneous and constant intensity fluorescence excitation light. The specific goal was to check for a relation between the extent of photobleaching of the PS (in this case PpIX, which was induced by the administration of Metvix®) and the clinical outcome (disappearance of AK) evaluated several months after the treatment. The study's results show that the amount of photobleached PS is strongly and linearly correlated to the fluorescence measured before the treatment. Likewise, the preliminary assessment of the clinical outcomes confirms the existence of a correlation of these outcomes with the PS's fluorescence bleaching, thus making it possible, in principle, to select and optimize the PDT's irradiation parameters before starting the treatment. The results of this study demonstrate the relevance of measuring the PS's photo-bleaching for optimizing PDT and forecasting of its outcome.

EPFL2010