@article{BastianNacakRoddatisetal.2020, author = {Bastian, Philipp U. and Nacak, Selma and Roddatis, Vladimir and Kumke, Michael Uwe}, title = {Tracking the motion of lanthanide ions within core-shell-shell NaYF4 nanocrystals via resonance energy transfer}, series = {The journal of physical chemistry : C}, volume = {124}, journal = {The journal of physical chemistry : C}, number = {20}, publisher = {American Chemical Society}, address = {Washington, DC}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.0c02588}, pages = {11229 -- 11238}, year = {2020}, abstract = {Lanthanide resonance energy transfer (LRET) was used to investigate the motion of dopant ions during the synthesis of core-shell-shell-nanocrystals (NCs) that are frequently used as frequency upconversion materials. Reaction conditions (temperature, solvent) as well as lattice composition and precursors were adapted from a typical hydrothermal synthesis approach used to obtain upconversion nanoparticles (UCNPs). Instead of adding the lanthanide ions Yb3+/Er3+ as the sensitizer/activator couple, Eu3+/Nd3+ as the donor/acceptor were added as the LRET pair to the outer shell (Eu-3) and the core (Nd-3). By tailoring the thickness of the insulation shell ("middle shell"), the expected distance between the donor and the acceptor was increased beyond 2 R-0, a distance for which no LRET is expected. The successful synthesis of core- shell-shell NCs with different thicknesses of the insulation layer was demonstrated by high-resolution transmission electron microscopy measurement. The incorporation of the Eu3+ ions into the NaYF4 lattice was investigated by high-resolution time-resolved luminescence measurements. Two major Eu3+ species (bulk and surface) were found. This was supported by steady-state as well as time-resolved luminescence data. Based on the luminescence decay kinetics, the intermixing of lanthanides during synthesis of core- shell UCNPs was evaluated. The energy transfer between Eu3+ (donor) and Nd3+ (acceptor) ions was exploited to quantify the motion of the dopant ions. This investigation reveals the migration of Ln(3+) ions between different compatiments in core-shell NCs and affects the concept of using core-shell architectures to increase the efficiency of UCNPs. In order to obtain well-separated core and shell structures with different dopants, alternative concepts are needed.}, language = {en} } @article{LopezdeGuerenuKurganovaKlierHaubitzetal.2022, author = {L{\´o}pez de Guere{\~n}u Kurganova, Anna and Klier, Dennis Tobias and Haubitz, Toni and Kumke, Michael Uwe}, title = {Influence of Gd3+ doping concentration on the properties of Na(Y,Gd)F-4}, series = {Photochemical \& photobiological sciences / European Society for Photobiology}, volume = {21}, journal = {Photochemical \& photobiological sciences / European Society for Photobiology}, number = {2}, publisher = {Springer}, address = {Heidelberg}, issn = {1474-905X}, doi = {10.1007/s43630-021-00161-4}, pages = {235 -- 245}, year = {2022}, abstract = {We present a systematic study on the properties of Na(Y,Gd)F-4-based upconverting nanoparticles (UCNP) doped with 18\% Yb3+, 2\% Tm3+, and the influence of Gd3+ (10-50 mol\% Gd3+). UCNP were synthesized via the solvothermal method and had a range of diameters within 13 and 50 nm. Structural and photophysical changes were monitored for the UCNP samples after a 24-month incubation period in dry phase and further redispersion. Structural characterization was performed by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) as well as dynamic light scattering (DLS), and the upconversion luminescence (UCL) studies were executed at various temperatures (from 4 to 295 K) using time-resolved and steady-state spectroscopy. An increase in the hexagonal lattice phase with the increase of Gd3+ content was found, although the cubic phase was prevalent in most samples. The Tm3+-luminescence intensity as well as the Tm3+-luminescence decay times peaked at the Gd3+ concentration of 30 mol\%. Although the general upconverting luminescence properties of the nanoparticles were preserved, the 24-month incubation period lead to irreversible agglomeration of the UCNP and changes in luminescence band ratios and lifetimes.}, language = {en} } @article{PilarYesteCarlosHernandezGarridoKumkeetal.2022, author = {Pilar Yeste, Maria and Carlos Hernandez-Garrido, Juan and Kumke, Michael Uwe and Alvarado, Sarah and Cauqui, Miguel Angel and Juan Calvino, Jose and Primus, Philipp-Alexander}, title = {Low-temperature growth of reactive pyrochlore nanostructures on Zirconia-supported ceria}, series = {ACS applied nano materials}, volume = {5}, journal = {ACS applied nano materials}, number = {5}, publisher = {American Chemical Society}, address = {Washington}, issn = {2574-0970}, doi = {10.1021/acsanm.2c00416}, pages = {6316 -- 6326}, year = {2022}, abstract = {The use of a catalyst support for the design of nanoscale heterogeneous catalysts based on cerium oxide offers vast possibilities for future catalyst development, particularly with regard to an increased focus on the use of renewable biogas and an emerging hydrogen economy. In this study, zirconia-supported ceria catalysts were synthesized, activated by using different thermochemical treatments, and characterized by way of temperature-programmed reduction (TPR), oxygen storage capacity, Xray diffraction, electron microscopy, and luminescence spectroscopy using Eu3+ as a spectroscopic probe. Through reduction-oxidation pretreatment routines, reactive pyrochlore structures were created at temperatures as low as 600 degrees C and identified through TPR and electron microscopy experiments. A structural relationship and alignment of the crystal planes is revealed in high-resolution scanning transmission electron microscopy experiments through the digital diffraction patterns. Low-temperature pretreatment induces the formation of reactive pyrochlore domains under retention of the surface area of the catalyst system, and no further morphological changes are detected. Furthermore, the formation of pyrochlore domains achieved through severe reduction and mild reoxidation (SRMO) treatments is reversible. Over multiple alternating SRMO and severe reduction and severe reoxidation (SRSO) treatments, europium spectroscopy and TPR results indicate that pyrochlore structures are recreated over consecutive treatments, whenever the mild oxidation step at 500 degrees C is the last treatment (SRMO, SRMO-SRSO-SRMO, etc.).}, language = {en} } @article{ChemuraHaubitzPrimusetal.2020, author = {Chemura, Sitshengisiwe and Haubitz, Toni and Primus, Philipp A. and Underberg, Martin and H{\"u}lser, Tim and Kumke, Michael Uwe}, title = {Europium-doped Ceria-Gadolinium mixed oxides}, series = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, volume = {124}, journal = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, number = {24}, publisher = {American Chemical Society}, address = {Washington}, issn = {1089-5639}, doi = {10.1021/acs.jpca.0c03188}, pages = {4972 -- 4983}, year = {2020}, abstract = {Gadolinium-doped ceria or gadolinium-stabilized ceria (GDC) is an important technical material due to its ability to conduct O2- ions, e.g., used in solid oxide fuel cells operated at intermediate temperature as an electrolyte, diffusion barrier, and electrode component. We have synthesized Ce1-xGdxO2-y:Eu3+ (0 <= x <= 0.4) nanoparticles (11-15 nm) using a scalable spray pyrolysis method, which allows the continuous large-scale technical production of such materials. Introducing Eu3+ ions in small amounts into ceria and GDC as spectroscopic probes can provide detailed information about the atomic structure and local environments and allows us to monitor small structural changes. This study presents a novel approach to structurally elucidate europium-doped Ce1-xGdxO2-y:Eu3+ nanoparticles by way of Eu3+ spectroscopy, processing the spectroscopic data with the multiway decomposition method parallel factor (PARAFAC) analysis. In order to perform the deconvolution of spectra, data sets of excitation wavelength, emission wavelength, and time are required. Room temperature, time-resolved emission spectra recorded at lambda(ex) = 464 nm show that Gd3+ doping results in significantly altered emission spectra compared to pure ceria. The PARAFAC analysis for the pure ceria samples reveals a high-symmetry species (which can also be probed directly via the CeO2 charge transfer band) and a low-symmetry species. The GDC samples yield two low-symmetry spectra in the same experiment. High-resolution emission spectra recorded under cryogenic conditions after probing the D-5(0)-F-7(0) transition at lambda(ex) = 575-583 nm revealed additional variation in the low-symmetry Eu3+ sites in pure ceria and GDC. The total luminescence spectra of CeO2-y:Eu3+ showed Eu3+ ions located in at least three slightly different coordination environments with the same fundamental symmetry, whereas the overall hypsochromic shift and increased broadening of the D-5(0)-F-7(0) excitation in the GDC samples, as well as the broadened spectra after deconvolution point to less homogeneous environments. The data of the Gd3+-containing samples indicates that the average charge density around the Eu3+ ions in the lattice is decreased with increasing Gd3+ and oxygen vacancy concentration. For reference, the Judd-Ofelt parameters of all spectra were calculated. PARAFAC proves to be a powerful tool to analyze lanthanide spectra in crystalline solid materials, which are characterized by numerous Stark transitions and where measurements usually yield a superposition of different contributions to any given spectrum.}, language = {en} } @article{BastianYudeGuerenuKurganovaetal.2020, author = {Bastian, Philipp U. and Yu, Leixiao and de Guere{\~n}u Kurganova, Anna Lopez and Haag, Rainer and Kumke, Michael Uwe}, title = {Bioinspired confinement of upconversion nanoparticles for improved performance in aqueous solution}, series = {The journal of physical chemistry : C, Nanomaterials and interfaces}, volume = {124}, journal = {The journal of physical chemistry : C, Nanomaterials and interfaces}, number = {52}, publisher = {American Chemical Society}, address = {Washington, DC}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.0c09798}, pages = {28623 -- 28635}, year = {2020}, abstract = {The resonance energy transfer (RET) from NaYF4:Yb,Er upconverting nanoparticles (UNCPs) to a dye (5-carboxytetramethylrhodamine (TAMRA)) was investigated by photoluminescence experiments and microscale thermophoresis (MST). The dye was excited via RET from the UCNPs which was excited in the near-infrared (NIR). The change of the dye diffusion speed (free vs coupled) was investigated by MST. RET shows significant changes in the decay times of the dye as well as of the UCNPs. MST reveals significant changes in the diffusion speed. A unique amphiphilic coating polymer (customized mussel protein (CMP) polymer) for UCNP surface coating was used, which mimics blood protein adsorption and mussel food protein adhesion to transfer the UCNP into the aqueous phase and to allow surface functionalization. The CMP provides very good water dispersibility to the UCNPs and minimizes ligand exchange and subsequent UCNP aging reactions because of the interlinkage of the CMP on the UCNP surface. Moreover, CMP provides N-3-functional groups for dick chemistry-based functionalization demonstrated with the dye 5-carboxytetramethylrhodamine (TAMRA). This establishes the principle coupling scheme for suitable biomarkers such as antibodies. The CMP provides very stable aqueous UCNP dispersions that are storable up to 3 years in a fridge at 5 degrees C without dissolution or coagulation. The outstanding properties of CMP in shielding the UCNP from unwanted solvent effects is reflected in the distinct increase of the photoluminescence decay times after UCNP functionalization. The UCNP-to-TAMRA energy transfer is also spectroscopically investigated at low temperatures (4-200 K), revealing that one of the two green Er(III) emission bands contributes the major part to the energy transfer. The TAMRA fluorescence decay time increases by a factor of 9500 from 2.28 ns up to 22 mu s due to radiationless energy transfer from the UCNP after NIR excitation of the latter. This underlines the unique properties of CMP as a versatile capping ligand for distinctly improving the UCNPs' performance in aqueous solutions, for coupling of biomolecules, and for applications for in vitro and in vivo experiments using UCNPs as optical probes in life science applications.}, language = {en} } @article{HaubitzJohnFreyseetal.2020, author = {Haubitz, Toni and John, Leonard and Freyse, Daniel and Wessig, Pablo and Kumke, Michael Uwe}, title = {Investigating the Sulfur "Twist" on the Photophysics of DBD Dyes}, series = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, volume = {124}, journal = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, number = {22}, publisher = {American Chemical Society}, address = {Washington}, issn = {1089-5639}, doi = {10.1021/acs.jpca.0c01880}, pages = {4345 -- 4353}, year = {2020}, abstract = {The so-called DBD ([1,3]dioxolo[4,5-f][1,3]benzodioxole) dyes are a new class of fluorescent dyes, with tunable photophysical properties like absorption, fluorescence lifetime, and Stokes shift. With the development of sulfur based DBDs, this dye class is extended even further for possible applications in spectroscopy and microscopy. In this paper we are investigating the basic photophysical properties and their implications for future applications for S-4-DBD as well as O-4-DBD. On the basis of time-resolved laser fluorescence spectroscopy, transient absorption spectroscopy, and UV/vis-spectroscopy, we determined the rate constants of the radiative and nonradiative deactivation processes as well as the energy of respective electronic states involved in the electronic deactivation of S-4-DBD and of O-4-DBD. For S-4-DBD we unraveled the triplet formation with intersystem crossing quantum yields of up to 80\%. By TD-DFT calculations we estimated a triplet energy of around 13500-14700 cm(-1) depending on the DBD dye and solvent. Through solvent dependent measurements, we found quadrupole moments in the range of 2 B.}, language = {en} } @article{HaubitzFudickarLinkeretal.2020, author = {Haubitz, Toni and Fudickar, Werner and Linker, Torsten and Kumke, Michael Uwe}, title = {pH-sensitive fluorescence switching of pyridylanthracenes}, series = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, volume = {124}, journal = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, number = {52}, publisher = {American Chemical Society}, address = {Washington}, issn = {1089-5639}, doi = {10.1021/acs.jpca.0c09911}, pages = {11017 -- 11024}, year = {2020}, abstract = {9,10-substituted anthracenes are known for their useful optical properties like fluorescence, which makes them frequently used probes in sensing applications. In this article, we investigate the fundamental photophysical properties of three pyridyl-substituted variants. The nitrogen atoms in the pyridinium six-membered rings are located in the ortho-, meta-, and para-positions in relation to the anthracene core. Absorption, fluorescence, and transient absorption measurements were carried out and were complemented by theoretical calculations. We monitored the photophysics of the anthracene derivatives in chloroform and water investigating the protonated as well as their nonprotonated forms. We found that the optical properties of the nonprotonated forms are strongly determined by the anthracene chromophore, with only small differences to other 9,10-substituted anthracenes, for example diphenyl anthracene. In contrast, protonation leads to a strong decrease in fluorescence intensity and lifetime. Transient absorption measurements and theoretical calculations revealed the formation of a charge-transfer state in the protonated chromophores, where electron density is shifted from the anthracene moiety toward the protonated pyridyl substituents. While the para- and ortho-derivatives' charge transfer is still moderately fluorescent, the meta-derivative is affected much stronger and shows nearly no fluorescence. This nitrogen-atom-position-dependent sensitivity to hydronium activity makes a combination of these fluorophores very attractive for pH-sensing applications covering a broadened pH range.}, language = {en} } @article{LippoldEidnerKumkeetal.2017, author = {Lippold, Holger and Eidner, Sascha and Kumke, Michael Uwe and Lippmann-Pipke, Johanna}, title = {Dynamics of metal-humate complexation equilibria as revealed by isotope exchange studies - a matter of concentration and time}, series = {Geochimica et cosmochimica acta : journal of the Geochemical Society and the Meteoritical Society}, volume = {197}, journal = {Geochimica et cosmochimica acta : journal of the Geochemical Society and the Meteoritical Society}, publisher = {Elsevier}, address = {Oxford}, issn = {0016-7037}, doi = {10.1016/j.gca.2016.10.019}, pages = {62 -- 70}, year = {2017}, abstract = {Complexation with dissolved humic matter can be crucial in controlling the mobility of toxic or radioactive contaminant metals. For speciation and transport modelling, a dynamic equilibrium process is commonly assumed, where association and dissociation run permanently. This is, however, questionable in view of reported observations of a growing resistance to dissociation over time. In this study, the isotope exchange principle was employed to gain direct insight into the dynamics of the complexation equilibrium, including kinetic inertisation phenomena. Terbium(III), an analogue of trivalent actinides, was used as a representative of higher-valent metals. Isotherms of binding to (flocculated) humic acid, determined by means of Tb-160 as a radiotracer, were found to be identical regardless of whether the radioisotope was introduced together with the bulk of stable Tb-159 or subsequently after pre-equilibration for up to 3 months. Consequently, there is a permanent exchange of free and humic-bound Tb since all available binding sites are occupied in the plateau region of the isotherm. The existence of a dynamic equilibrium was thus evidenced. There was no indication of an inertisation under these experimental conditions. If the small amount of Tb-160 was introduced prior to saturation with Tb-159, the expected partial desorption of Tb-160 occurred at much lower rates than observed for the equilibration process in the reverse procedure. In addition, the rates decreased with time of pre-equilibration. Inertisation phenomena are thus confined to the stronger sites of humic molecules (occupied at low metal concentrations). Analysing the time-dependent course of isotope exchange according to first-order kinetics indicated that up to 3 years are needed to attain equilibrium. Since, however, metal-humic interaction remains reversible, exchange of metals between humic carriers and mineral surfaces cannot be neglected on the long time scale to be considered in predictive transport models.}, language = {en} } @article{HesseKlierSgarzietal.2018, author = {Hesse, Julia and Klier, Dennis Tobias and Sgarzi, Massimo and Nsubuga, Anne and Bauer, Christoph and Grenzer, Joerg and H{\"u}bner, Rene and Wislicenus, Marcus and Joshi, Tanmaya and Kumke, Michael Uwe and Stephan, Holger}, title = {Rapid Synthesis of Sub-10nm Hexagonal NaYF4-Based Upconverting Nanoparticles using Therminol((R))66}, series = {ChemistryOpen : including thesis treasury}, volume = {7}, journal = {ChemistryOpen : including thesis treasury}, number = {2}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {2191-1363}, doi = {10.1002/open.201700186}, pages = {159 -- 168}, year = {2018}, abstract = {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.}, language = {en} } @article{HoangHaubitzKumke2018, author = {Hoang, Hoa T. and Haubitz, Toni and Kumke, Michael Uwe}, title = {Photophysics of "Floppy" Dyadsas Potential Biomembrane Probes}, series = {Journal of fluorescence}, volume = {28}, journal = {Journal of fluorescence}, number = {5}, publisher = {Springer}, address = {New York}, issn = {1053-0509}, doi = {10.1007/s10895-018-2286-4}, pages = {1225 -- 1237}, year = {2018}, abstract = {In the study a dyad (C6 probe), constructed of two dyes with highly different hydrophobicities, was investigated by steady-state and time-resolved spectroscopic techniques in chloroform, methanol, and in phospholipid vesicles, respectively. The dyad was built on two dyes: the lipophilic benzo[a]pyrene (BaP) and the hydrophilic sulforhodamine B (SRB). The dyes were linked via a short, but flexible alkyl chain (six C-atoms). Based on their spectroscopic properties, BaP and SRB showed a very efficient non-radiative resonance energy transfer in solution. Incorporation into a lipid bilayer limited the relative flexibility (degree of freedom) between donor and acceptor and was used for the investigation of fundamental photophysical aspects (especially of FRET) as well as to elucidate the potential of the dyad to probe the interface of vesicles (or cells). The location of the two dyes in vesicles and their respective accessibility for interactions with dye-specific antibodies was investigated. Based on the alteration of the anisotropy, on the rotational correlation time as well as on the diffusion coefficient the incorporation of the C6 probe into the vesicles was evaluated. Especially the limitation in the relative movements of the two dyes was considered and used to differentiate between potential parameters, that influence the energy transfer in the dyad. Transient absorption spectroscopy (TAS) and pulsed-interleave single molecule fluorescence experiments were performed to better understand the intramolecular interactions in the dyad. Finally, in a showcase for a biosensing application of the dyads, the binding of an SRB-specific antibody was investigated when the dyad was incorporated in vesicles.}, language = {en} }