TY - JOUR A1 - Wilhelm, Stefan A1 - Kaiser, Martin A1 - Würth, Christian A1 - Heiland, J. A1 - Carrillo-Carrion, C. A1 - Muhr, V. A1 - Wolfbeis, Otto S. A1 - Parak, W.J. A1 - Resch-Genger, Ute A1 - Hirsch, T. T1 - Water dispersible upconverting nanoparticles: effects of surface modification on their luminescence and colloidal stability JF - Nanoscale N2 - We present a systematic study on the effect of surface ligands on the luminescence properties and colloidal stability of β-NaYF4:Yb3+,Er3+ upconversion nanoparticles (UCNPs), comparing nine different surface coatings to render these UCNPs water-dispersible and bioconjugatable. A prerequisite for this study was a large-scale synthetic method that yields ~2 g per batch of monodisperse oleate-capped UCNPs providing identical core particles. These ~23 nm sized UCNPs display an upconversion quantum yield of ~0.35% when dispersed in cyclohexane and excited with a power density of 150 W cm-2, underlining their high quality. A comparison of the colloidal stability and luminescence properties of these UCNPs, subsequently surface modified with ligand exchange or encapsulation protocols, revealed that the ratio of the green (545 nm) and red (658 nm) emission bands determined at a constant excitation power density clearly depends on the surface chemistry. Modifications relying on the deposition of additional (amphiphilic) layer coatings, where the initial oleate coating is retained, show reduced non-radiative quenching by water as compared to UCNPs that are rendered water-dispersible via ligand exchange. Moreover, we could demonstrate that the brightness of the upconversion luminescence of the UCNPs is strongly affected by the type of surface modification, i.e., ligand exchange or encapsulation, yet hardly by the chemical nature of the ligand. KW - upconverting nanoparticles (UCNPs) KW - Luminescence KW - surface modification PY - 2015 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-324071 DO - https://doi.org/10.1039/c4nr05954a SN - 2040-3364 SN - 2040-3372 VL - 7 IS - 4 SP - 1403 EP - 1410 PB - RSC Publ. CY - Cambridge AN - OPUS4-32407 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Würth, Christian A1 - Kaiser, Martin A1 - Wilhelm, Stefan A1 - Grauel, Bettina A1 - Hirsch, Th. A1 - Resch-Genger, Ute T1 - Excitation power dependent population pathways and absolute quantum yields of upconversion nanoparticles in different solvents JF - Nanoscale N2 - The rational design of brighter upconversion nanoparticles (UCNPs) requires a better understanding of the radiationless deactivation pathways in these materials. Here, we demonstrate the potential of excitation power density (P)-dependent studies of upconversion (UC) luminescence intensities, slope factors, and absolute quantum yields (ΦUC) of popular β-NaYF4:20% Yb3+,2% Er3+ UCNPs of different surface chemistries in organic solvents, D2O, and water as a tool to gain deeper insight into the UC mechanism including population and deactivation pathways particularly of the red emission. Our measurements, covering a P regime of three orders of magnitude, reveal a strong difference of the P-dependence of the ratio of the green and red luminescence bands (Ig/r) in water and organic solvents and P-dependent population pathways of the different emissive energy levels of Er3+. In summary, we provide experimental evidence for three photon processes in UCNPs, particularly for the red emission. Moreover, we demonstrate changes in the excited population dynamics via bi- and triphotonic processes dependent on the environment, surface chemistry, and P, and validate our findings theoretically KW - Upconverion KW - Quantum Yield KW - Photo physics PY - 2017 UR - http://pubs.rsc.org/en/content/articlepdf/2017/nr/c7nr00092h DO - https://doi.org/10.1039/c7nr00092h SN - 2040-3364 SN - 2040-3372 VL - 9 IS - 12 SP - 4283 EP - 4294 PB - The Royal Society of Chemistry AN - OPUS4-39849 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -