@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}
}