@article{AlrefaiMondalWrucketal.2019,
  author    = {Alrefai, Anas and Mondal, Suvendu Sekhar and Wruck, Alexander and Kelling, Alexandra and Schilde, Uwe and Brandt, Philipp and Janiak, Christoph and Schoenfeld, Sophie and Weber, Birgit and Rybakowski, Lawrence and Herrman, Carmen and Brennenstuhl, Katlen and Eidner, Sascha and Kumke, Michael Uwe and Behrens, Karsten and G{\"u}nter, Christina and M{\"u}ller, Holger and Holdt, Hans-J{\"u}rgen},
  title     = {Hydrogen-bonded supramolecular metal-imidazolate frameworks: gas sorption, magnetic and UV/Vis spectroscopic properties},
  series = {Journal of Inclusion Phenomena and Macrocyclic Chemistry},
  volume    = {94},
  journal   = {Journal of Inclusion Phenomena and Macrocyclic Chemistry},
  number    = {3-4},
  publisher = {Springer},
  address   = {Dordrecht},
  issn      = {1388-3127},
  doi       = {10.1007/s10847-019-00926-6},
  pages     = {155 -- 165},
  year      = {2019},
  abstract  = {By varying reaction parameters for the syntheses of the hydrogen-bonded metal-imidazolate frameworks (HIF) HIF-1 and HIF-2 (featuring 14 Zn and 14 Co atoms, respectively) to increase their yields and crystallinity, we found that HIF-1 is generated in two different frameworks, named as HIF-1a and HIF-1b. HIF-1b is isostructural to HIF-2. We determined the gas sorption and magnetic properties of HIF-2. In comparison to HIF-1a (Brunauer-Emmett-Teller (BET) surface area of 471m(2) g(-1)), HIF-2 possesses overall very low gas sorption uptake capacities [BET(CO2) surface area=85m(2) g(-1)]. Variable temperature magnetic susceptibility measurement of HIF-2 showed antiferromagnetic exchange interactions between the cobalt(II) high-spin centres at lower temperature. Theoretical analysis by density functional theory confirmed this finding. The UV/Vis-reflection spectra of HIF-1 (mixture of HIF-1a and b), HIF-2 and HIF-3 (with 14 Cd atoms) were measured and showed a characteristic absorption band centered at 340nm, which was indicative for differences in the imidazolate framework.},
  language  = {en}
}
@article{AntoniouPashalidisGessneretal.2011,
  author    = {Antoniou, Stella and Pashalidis, I. and Gessner, Andre and Kumke, Michael Uwe},
  title     = {The effect of humic acid on the formation and solubility of secondary solid phases (Nd(OH)CO3 and Sm(OH)CO3)},
  series = {Radiochimica acta : international journal for chemical aspects of nuclear science and technology},
  volume    = {99},
  journal   = {Radiochimica acta : international journal for chemical aspects of nuclear science and technology},
  number    = {4},
  publisher = {De Gruyter},
  address   = {Berlin},
  issn      = {0033-8230},
  doi       = {10.1524/ract.2011.1812},
  pages     = {217 -- 223},
  year      = {2011},
  abstract  = {The formation of secondary Ln(III) solid phases (e.g. Nd(OH)CO3 and Sm(OH)CO3) has been studied as a function of the humic acid (HA) concentration in 0.1 M NaClO4 aqueous solution and their solubility has been investigated in the neutral pH range (6.5-8) under normal atmospheric conditions. Nd(III) and Sm(III) were selected as analogues for trivalent lanthanide and actinide ions. The solid phases under investigation have been prepared by alkaline precipitation and characterized by TGA, ATR-FTIR, XRD, TRLFS, DR-UV-Vis and Raman spectroscopy, and solubility measurements. The spectroscopic data obtained indicate that Nd(OH)CO3 and Sm(OH)CO3 are stable and remain the solubility limiting solid phases even in the presence of increased HA concentration (0.5 g/L) in solution. Upon base addition in the Ln(III)-HA system decomplexation of the previously formed Ln(III)-humate complexes and precipitation of two distinct phases occurs, the inorganic (Ln(OH)CO3) and the organic phase (HA), which is adsorbed on the particle surface of the former. Nevertheless, HA affects the particle size of the solid phases. Increasing HA concentration results in decreasing crystallite size of the Nd(OH)CO3 and increasing crystallite size of the Sm(OH)CO3 solid phase, and affects inversely the solubility of the solid phases. However, this impact on the solid phase properties is expected to be of minor relevance regarding the chemical behavior and migration of trivalent lanthanides and actinides in the geosphere.},
  language  = {en}
}
@article{AntoniouPashalidisGessneretal.2011,
  author    = {Antoniou, Stella and Pashalidis, Ioannis and Gessner, Andre and Kumke, Michael Uwe},
  title     = {Spectroscopic investigations on the effect of humic acid on the formation and solubility of secondary solid phases of Ln(2)(CO3)(3)},
  series = {Journal of rare earths},
  volume    = {29},
  journal   = {Journal of rare earths},
  number    = {6},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {1002-0721},
  doi       = {10.1016/S1002-0721(10)60490-5},
  pages     = {516 -- 521},
  year      = {2011},
  abstract  = {The formation of secondary Ln(III) solid phases (e.g., Nd-2(CO3)(3) and Sm-2(CO3)(3)) was studied as a function of the humic acid concentration in 0.1 mol/L NaClO4 aqueous solution in the neutral pH range (5-6.5). The solid phases under investigation were prepared by alkaline precipitation under 100\% CO2 atmosphere and characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), time-resolved laser fluorescence spectroscopy (TRLFS), diffuse reflectance ultraviolet-visible (DR-UV-Vis), Raman spectroscopy, and solubility measurements. The spectroscopic data obtained indicated that Nd-2(CO3)(3) and Sm-2(CO3)(3) were stable and remained the solubility limiting solid phases even in the presence of increased humic acid concentration (0.5 g/L) in solution. Upon base addition in the Ln(III)-HA system, decomplexation of the previously formed Ln(III)-humate complexes and precipitation of two distinct phases occurred, the inorganic (Ln(2)(CO3)(3)) and the organic phase (HA), which was adsorbed on the particle surface of the former. Nevertheless, humic acid affected the particle size of the solid phases. Increasing humic acid concentration resulted in decreasing crystallite size of the Nd-2(CO3)(3) and increasing crystallite size of the Sm-2(CO3)(3) solid phase, and affected inversely the solubility of the solid phases. However, this impact on the solid phase properties was expected to be of minor relevance regarding the chemical behavior and migration of trivalent lanthanides and actinides in the geosphere.},
  language  = {en}
}
@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{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{BurekDenglerEmmerlingetal.2019,
  author    = {Burek, Katja and Dengler, Joachim and Emmerling, Franziska and Feldmann, Ines and Kumke, Michael Uwe and Stroh, Julia},
  title     = {Lanthanide Luminescence Revealing the Phase Composition in Hydrating Cementitious Systems},
  series = {ChemistryOpen},
  volume    = {8},
  journal   = {ChemistryOpen},
  number    = {12},
  publisher = {Wiley-VCH-Verl.},
  address   = {Weinheim},
  issn      = {2191-1363},
  doi       = {10.1002/open.201900249},
  pages     = {1441 -- 1452},
  year      = {2019},
  abstract  = {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.},
  language  = {en}
}
@article{BurekKrauseSchwotzeretal.2018,
  author    = {Burek, Katja and Krause, Felix and Schwotzer, Matthias and Nefedov, Alexei and S{\"u}ssmuth, Julia and Haubitz, Toni and Kumke, Michael Uwe and Thissen, Peter},
  title     = {Hydrophobic Properties of Calcium-Silicate Hydrates Doped with Rare-Earth Elements},
  series = {ACS sustainable chemistry \& engineering},
  volume    = {6},
  journal   = {ACS sustainable chemistry \& engineering},
  number    = {11},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {2168-0485},
  doi       = {10.1021/acssuschemeng.8b03244},
  pages     = {14669 -- 14678},
  year      = {2018},
  abstract  = {In this study, the apparent relationship between the transport process and the surface chemistry of the Calcium-Silicate Hydrate (CSH) phases was investigated. For this purpose, a method was developed to synthesize ultrathin CSH phases to be used as a model substrate with the specific modification of their structure by introducing europium (Eu(III)). The structural and chemical changes during this Eu(III)-doping were observed by means of infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and time-resolved laser fluorescence spectroscopy (TRLFS). These alterations of the CSH phases led to significant changes in the surface chemistry and consequently to considerable variations in the interaction with water, as evidenced by measurements of the contact angles on the modified model substrates. Our results provide the basis for a more profound molecular understanding of reactive transport processes in cement-based systems. Furthermore, these results broaden the perspective of improving the stability of cement-based materials, which are subjected to the impact of aggressive aqueous environments through targeted modifications of the CSH phases.},
  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{DemetriouPashalidisNicolaidesetal.2013,
  author    = {Demetriou, Antri and Pashalidis, Ioannis and Nicolaides, Athanassios V. and Kumke, Michael Uwe},
  title     = {Surface mechanism of the boron adsorption on alumina in aqueous solutions},
  series = {Desalination and water treatment : science and engineering},
  volume    = {51},
  journal   = {Desalination and water treatment : science and engineering},
  number    = {31-33},
  publisher = {Taylor \& Francis Group},
  address   = {Philadelphia},
  issn      = {1944-3994},
  doi       = {10.1080/19443994.2013.764354},
  pages     = {6130 -- 6136},
  year      = {2013},
  abstract  = {The adsorption of boron (boric acid) from aqueous solutions on alumina has been investigated at pH 8.0, I=0.1M NaClO4, T=22 +/- 3 degrees C, and under normal atmospheric conditions. The characterization of the adsorbed species was performed by Raman spectroscopy and the spectroscopic speciation was assisted by theoretical DFT calculations. Evaluation of the spectroscopic data points to the formation of inner-sphere surface complexes and indicates the formation of two different types of adsorbed boron species. The theoretical calculations corroborate the spectroscopic data and indicate that at low boron concentration the monodentate surface species dominates, whereas increased boron concentration favors the formation of a bidentate surface species. Assuming low coverage, the conditional formation constant for the monodentate surface species has been evaluated to be log=4.1 +/- 0.1.},
  language  = {en}
}
@article{DoscheKumkeArieseetal.2003,
  author    = {Dosche, Carsten and Kumke, Michael Uwe and Ariese, Freek and Bader, Arjen N. and Gooijer, Cees and Dosa, P. I. and Han, S. and Miljanic, Ognjen S. and Vollhardt, K. Peter C. and Puchta, Ralph and Hommes, N. J. R. V.},
  title     = {Shpol'skii spectroscopy and vibrational analysis of [N]phenylenes},
  year      = {2003},
  language  = {en}
}