Refine
Has Fulltext
- yes (20) (remove)
Year of publication
Document Type
- Postprint (19)
- Habilitation Thesis (1)
Keywords
- Huminstoffe (2)
- lanthanides (2)
- resonance energy transfer (2)
- Absorption (1)
- Absorptionsspektroskopie (1)
- DBD dye (1)
- Europium (1)
- FRET (1)
- Fluorescence (1)
- Fluoreszenz (1)
Institute
Results of an inter-laboratory round-robin study of the application of time-resolved emission spectroscopy (TRES) to the speciation of uranium(VI) in aqueous media are presented. The round-robin study involved 13 independent laboratories, using various instrumentation and data analysis methods. Samples were prepared based on appropriate speciation diagrams and, in general, were found to be chemically stable for at least six months. Four different types of aqueous uranyl solutions were studied: (1) acidic medium where UO22+aq is the single emitting species, (2) uranyl in the presence of fluoride ions, (3) uranyl in the presence of sulfate ions, and (4) uranyl in aqueous solutions at different pH, promoting the formation of hydrolyzed species. Results between the laboratories are compared in terms of the number of decay components, luminescence lifetimes, and spectral band positions. The successes and limitations of TRES in uranyl analysis and speciation in aqueous solutions are discussed.
The hydration process of Portland cement in a cementitious system is crucial for development of the high‐quality cement‐based construction material. Complementary experiments of X‐ray diffraction analysis (XRD), scanning electron microscopy (SEM) and time‐resolved laser fluorescence spectroscopy (TRLFS) using europium (Eu(III)) as an optical probe are used to analyse the hydration process of two cement systems in the absence and presence of different organic admixtures. We show that different analysed admixtures and the used sulphate carriers in each cement system have a significant influence on the hydration process, namely on the time‐dependence in the formation of different hydrate phases of cement. Moreover, the effect of a particular admixture is related to the type of sulphate carrier used. The quantitative information on the amounts of the crystalline cement paste components is accessible via XRD analysis. Distinctly different morphologies of ettringite and calcium−silicate−hydrates (C−S−H) determined by SEM allow visual conclusions about formation of these phases at particular ageing times. The TRLFS data provides information about the admixture influence on the course of the silicate reaction. The dip in the dependence of the luminescence decay times on the hydration time indicates the change in the structure of C−S−H in the early hydration period. Complementary information from XRD, SEM and TRLFS provides detailed information on distinct periods of the cement hydration process.
Vibrationally resolved fluorescence spectra of four angular [N]phenylenes were recorded with laser excited Shpol’skii spectroscopy (LESS) in an n-octane matrix at 10 K. In general, the same vibrational frequencies were observed in the fluorescence excitation and emission spectra, indicating that the geometries of ground and electronically excited state are very similar. Because of intensity borrowing from the S2 state, vibrations of two different symmetries were observed in the fluorescence excitation spectra of angular [3]phenylene and zig-zag[5]phenylene. This finding allowed the location of the S2 state for these compounds. DFT calculations(RB3LYP/6-31G*) of the ground state vibrational frequencies were made. The calculated vibrational modes were in reasonably good agreement with the experimental data. A new very low-frequency vibration of approximately 100 cm-1 was predicted and experimentally confirmed for all [N]phenylenes investigated. This vibration seems to be unique for [N]phenylenes and is attributed to an in-plane movement of the carbon backbone.
Deuteration effects on the vibronic structure of the emission and excitation spectra of triangular [4]phenylene (D3h[4]phenylene) were studied using laser-excited Shpolskii spectroscopy (LESS) in an octane matrix at 4.2 K. For correct assignment of the vibrational modes, the experimental results were compared with calculated frequencies (B3LYP/6-31G*). CH vibrations were identified by their characteristic isotopic shifts in the spectra of deuterated triangular [4]phenylenes. Two CC stretching modes, at 100 cm–1 and 1176 cm–1, suitable as probes for bond strength changes in the excited state, were identified. The isotope effect on the internal conversion rates of triangular [4]phenylene was evaluated from measurements of temperature dependent lifetime. Isotope dependency and the magnitude of the internal conversion rates indicate that internal conversion in triangular [4]phenylene is most likely induced by CH vibrations. The results obtained by LESS and lifetime measurements were compared with PM3 PECI calculations of the excited state structure. The theoretical results and the relation between ground and excited state vibration energies of the 1176 cm–1 probe vibration indicate a reduction of bond alternation of the central cyclohexatriene ring in the excited state.
Optical methods play an important role in process analytical technologies (PAT). Four examples of optical process and quality sensing (OPQS) are presented, which are based on three important experimental techniques: near-infrared absorption, luminescence quenching, and a novel method, photon density wave (PDW) spectroscopy. These are used to evaluate four process and quality parameters related to beer brewing and polyurethane (PU) foaming processes: the ethanol content and the oxygen (O2) content in beer, the biomass in a bioreactor, and the cellular structures of PU foam produced in a pilot production plant.
Die Anwendung von optischen Parametern zur Stoffcharakterisierung wird diskutiert. Dabei ist der Schwerpunkt der Diskussion auf absorptions- und fluoreszenzspektroskopische Methoden gesetzt. Beide Methoden können schnell und zuverlässig – auch im on-line Betrieb – eingesetzt werden. Der Beitrag soll einen Überblick über die grundlegenden Möglichkeiten der Anwendung beider Methoden geben.
Rapid synthesis of sub-10 nm hexagonal NaYF4-based upconverting nanoparticles using Therminol® 66
(2018)
We report a simple one-pot method for the rapid preparation of sub-10nm pure hexagonal (-phase) NaYF4-based upconverting nanoparticles (UCNPs). Using Therminol((R))66 as a co-solvent, monodisperse UCNPs could be obtained in unusually short reaction times. By varying the reaction time and reaction temperature, it was possible to control precisely the particle size and crystalline phase of the UCNPs. The upconversion (UC) luminescence properties of the nanocrystals were tuned by varying the concentrations of the dopants (Nd3+ and Yb3+ sensitizer ions and Er3+ activator ions). The size and phase-purity of the as-synthesized core and core-shell nanocrystals were assessed by using complementary transmission electron microscopy, dynamic light scattering, X-ray diffraction, and small-angle X-ray scattering studies. In-depth photophysical evaluation of the UCNPs was pursued by using steady-state and time-resolved luminescence spectroscopy. An enhancement in the UC intensity was observed if the nanocrystals, doped with optimized concentrations of lanthanide sensitizer/activator ions, were further coated with an inert/active shell. This was attributed to the suppression of surface-related luminescence quenching effects.
Upconversion NaYF4:Yb:Er nanoparticles co-doped with Gd3+ and Nd3+ for thermometry on the nanoscale
(2015)
In the present work, the upconversion luminescence properties of oleic acid capped NaYF4:Gd3+:Yb3+:Er3+ upconversion nanoparticles (UCNP) with pure β crystal phase and Nd3+ ions as an additional sensitizer were studied in the temperature range of 288 K < T < 328 K. The results of this study showed that the complex interplay of different mechanisms and effects, causing the special temperature behavior of the UCNP can be developed into thermometry on the nanoscale, e.g. to be applied in biological systems on a cellular level. The performance was improved by the use of Nd3+ as an additional dopant utilizing the cascade sensitization mechanism in tri-doped UCNP.
NaYF4:Yb:Er nanoparticles (UCNP) were synthesized under mild experimental conditions to obtain a pure cubic lattice. Upon annealing at different temperatures up to Tan = 700 °C phase transitions to the hexagonal phase and back to the cubic phase were induced. The UCNP materials obtained for different Tan were characterized with respect to the lattice phase using standard XRD and Raman spectroscopy as well as steady state and time resolved upconversion luminescence. The standard techniques showed that for the annealing temperature range 300 °C < Tan < 600 °C the hexagonal lattice phase was dominant. For Tan < 300 °C hardly any change in the lattice phase could be deduced, whereas for Tan > 600 °C a back transfer to the α-phase was observed. Complementarily, the luminescence upconversion properties of the annealed UCNP materials were characterized in steady state and time resolved luminescence measurements. Distinct differences in the upconversion luminescence intensity, the spectral intensity distribution and the luminescence decay kinetics were found for the cubic and hexagonal lattice phases, respectively, corroborating the results of the standard analytical techniques used. In laser power dependent measurements of the upconversion luminescence intensity it was found that the green (G1, G2) and red (R) emission of Er3+ showed different effects of Tan on the number of required photons reflecting the differences in the population routes of different energy levels involved. Furthermore, the intensity ratio of Gfull/R is highly effected by the laser power only when the β-phase is present, whereas the G1/G2 intensity ratio is only slightly effected regardless of the crystal phase. Moreover, based on different upconversion luminescence kinetics characteristics of the cubic and hexagonal phase time-resolved area normalized emission spectra (TRANES) proved to be a very sensitive tool to monitor the phase transition between cubic and hexagonal phases. Based on the TRANES analysis it was possible to resolve the lattice phase transition in more detail for 200 °C < Tan < 300 °C, which was not possible with the standard techniques.
Immobilisierung bzw. Mobilisierung und Transport von Schadstoffen in der Umwelt, besonders in den Kompartimenten Boden und Wasser, sind von fundamentaler Bedeutung für unser (Über)Leben auf der Erde. Einer der Hauptreaktionspartner für organische und anorganische Schadstoffe (Xenobiotika) in der Umwelt sind Huminstoffe (HS). HS sind Abbauprodukte pflanzlichen und tierischen Gewebes, die durch eine Kombination von chemischen und biologischen Ab- und Umbauprozessen entstehen. Bedingt durch ihre Genese stellen HS außerordentlich heterogene Stoffsysteme dar, die eine Palette von verschiedenartigen Wechselwirkungen mit Schadstoffen zeigen. Die Untersuchung der fundamentalen Wechselwirkungsmechanismen stellt ebenso wie deren quantitative Beschreibung höchste Anforderungen an die Untersuchungsmethoden. Zur qualitativen und quantitativen Charakterisierung der Wechselwirkungen zwischen HS und Xenobiotika werden demnach analytische Methoden benötigt, die bei der Untersuchung von extrem heterogenen Systemen aussagekräftige Daten zu liefern vermögen. Besonders spektroskopische Verfahren, wie z.B. lumineszenz-basierte Verfahren, besitzen neben der hervorragenden Selektivität und Sensitivität, auch eine Multidimensionalität (bei der Lumineszenz sind es die Beobachtungsgrößen Intensität IF, Anregungswellenlänge lex, Emissionswellenlänge lem und Fluoreszenzabklingzeit tF), die es gestattet, auch heterogene Systeme wie HS direkt zu untersuchen. Zur Charakterisierung können sowohl die intrinsischen Fluoreszenzeigenschaften der HS als auch die von speziell eingeführten Lumineszenzsonden verwendet werden. In beiden Fällen werden die zu Grunde liegenden fundamentalen Konzepte der Wechselwirkungen von HS mit Xenobiotika untersucht und charakterisiert. Für die intrinsische Fluoreszenz der HS konnte gezeigt werden, dass neben molekularen Strukturen besonders die Verknüpfung der Fluorophore im Gesamt-HS-Molekül von Bedeutung ist. Konformative Freiheit und die Nachbarschaft zu als Energieakzeptor fungierenden HS-eigenen Gruppen sind wichtige Komponenten für die Charakteristik der HS-Fluoreszenz. Die Löschung der intrinsischen Fluoreszenz durch Metallkomplexierung ist demnach auch das Resultat der veränderten konformativen Freiheit der HS durch die gebundenen Metallionen. Es zeigte sich, dass abhängig vom Metallion sowohl Löschung als auch Verstärkung der intrinsischen HS-Fluoreszenz beobachtet werden kann. Als extrinsische Lumineszenzsonden mit wohl-charakterisierten photophysikalischen Eigenschaften wurden polyzyklische aromatische Kohlenwasserstoffe und Lanthanoid-Ionen eingesetzt. Durch Untersuchungen bei sehr niedrigen Temperaturen (10 K) konnte erstmals die Mikroumgebung von an HS gebundenen hydrophoben Xenobiotika untersucht werden. Im Vergleich mit Raumtemperaturexperimenten konnte gezeigt werden, dass hydrophobe Xenobiotika an HS-gebunden in einer Mikroumgebung, die in ihrer Polarität analog zu kurzkettigen Alkoholen ist, vorliegen. Für den Fall der Metallkomplexierung wurden Energietransferprozesse zwischen HS und Lanthanoidionen bzw. zwischen verschiedenen, gebundenen Lanthanoidionen untersucht. Basierend auf diesen Messungen können Aussagen über die beteiligten elektronischen Zustände der HS einerseits und Entfernungen von Metallbindungsstellen in HS selbst angeben werden. Es ist dabei zu beachten, dass die Experimente in Lösung bei realen Konzentrationen durchgeführt wurden. Aus Messung der Energietransferraten können direkte Aussagen über Konformationsänderungen bzw. Aggregationsprozesse von HS abgeleitet werden.