TY - JOUR A1 - Christ, S. A1 - Schäferling, Michael T1 - Chemical sensing and imaging based on photon upconverting nano- and microcrystals: A review N2 - The demand for photostable luminescent reporters that absorb and emit light in the red to near-infrared (NIR) spectral region continues in biomedical research and bioanalysis. In recent years, classical organic fluorophores have increasingly been displaced by luminescent nanoparticles. These consist of either polymer or silica based beads that are loaded with luminescent dyes, conjugated polymers, or inorganic nanomaterials such as semiconductor nanocrystals (quantum dots), colloidal clusters of silver and gold, or carbon dots. Among the inorganic materials, photon upconversion nanocrystals exhibit a high potential for application to bioimaging or biomolecular assays. They offer an exceptionally high photostability, can be excited in the NIR, and their anti-Stokes emission enables luminescence detection free of background and perturbing scatter effects even in complex biological samples. These lanthanide doped inorganic crystals have multiple emission lines that can be tuned by the selection of the dopants. This review article is focused on the applications of functionalized photon upconversion nanoparticles (UCNPs) to chemical sensing. This is a comparatively new field of research activity and mainly directed at the sensing and imaging of ubiquitous chemical analytes in biological samples, particularly in living cells. For this purpose, the particles have to be functionalized with suitable indicator dyes or recognition elements, as they do not show an intrinsic or specific luminescence response to most of these analytes (e.g. pH, oxygen, metal ions). We describe the strategies for the design of such responsive nanocomposites utilizing either luminescence resonance energy transfer or emission–reabsorption (inner filter effect) mechanisms and also highlight examples for their use either immobilized in sensor layers or directly as nanoprobes for intracellular sensing and imaging. KW - Photon upconversion KW - Nanoprobes KW - Chemical sensors KW - Imaging PY - 2015 DO - https://doi.org/10.1088/2050-6120/3/3/034004 SN - 2050-6120 VL - 3 IS - 3 SP - 1 EP - 22 PB - IOP Publ. CY - Bristol AN - OPUS4-33857 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hyppänen, I. A1 - Perälä, N. A1 - Arppe, Riika A1 - Schäferling, Michael A1 - Soukka, T. T1 - Environmental and excitation power effects on the ratiometric upconversion luminescence based temperature sensing using nanocrystalline NaYF4:Yb3+,Er3+ N2 - The luminescence intensity ratio (LIR) of the green emissions of the near-infrared excited NaYF4:Yb3+,Er3+ nanocrystals is a promising method for temperature sensing. Here, the influence of excitation power density, excitation pulse length, excitation wavelength, silica shell, and solvent on the LIR and its temperature response is reported. The primary objective is to study the LIR mechanism and the impact of measurement and environmental parameters on the calibration and precision of the LIR. The LIR value is demonstrated to be unaffected by the excitation intensity in the studied range. This result is essential, considering the application feasibility of the LIR method as temperature sensor, where the effective excitation power density depends on the sample matrix and the distance excitation light travels in the sample. The pulsed excitation, however, results in an increase in the LIR value upon short pulse width. Silanization of bare nanocrystals has no effect on the LIR values, but the local warming of H2O samples under laser exposure results in slightly increased LIR values compared to other solvents; D2O, oleic acid, and dimethyl sulfoxide. The thermal quenching of luminescence lifetimes of Er3+ emission is proved to be too weak for sensing applications. KW - Photon upconversion KW - Temperature sensing KW - Luminescence intensity ratio KW - Excitation intensity PY - 2017 UR - http://onlinelibrary.wiley.com/doi/10.1002/cphc.201601355/epdf DO - https://doi.org/10.1002/cphc.201601355 SN - 1439-4235 SN - 1439-7641 VL - 18 IS - 6 SP - 692 EP - 701 PB - Wiley-VCH CY - Weinheim AN - OPUS4-39899 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hyppänen, Iko A1 - Höysniemi, Niina A1 - Arppe, Riika A1 - Schäferling, Michael A1 - Soukka, Tero T1 - Environmental impact on the excitation path of the red upconversion emission of nanocrystalline NaYF4:Yb3+,Er3+ N2 - The mechanism for red upconversion luminescence of Yb−Er codoped materials is not generally agreed on in the literature. Both two-photon and three-photon processes have been suggested as the main path for red upconversion emission. We have studied β-NaYF4:Yb3+,Er3+ nanoparticles in H2O and D2O, and we propose that the nanoparticle environment is a major factor in the selection of the preferred red upconversion excitation pathway. In H2O, efficient multiphonon relaxation (MPR) promotes the two-photon mechanism through green emitting states, while, in D2O, MPR is less effective and the three-photon path involving back energy transfer to Yb3+ is the dominant mechanism. For the green upconversion emission, our results suggest the common two-photon path through the 4F9/2 energy state in both H2O and D2O. KW - Photon upconversion KW - Three photon excitation KW - Multiphonon relaxation KW - Excitation intensity PY - 2017 UR - http://pubs.acs.org/doi/pdf/10.1021/acs.jpcc.7b01019 DO - https://doi.org/10.1021/acs.jpcc.7b01019 SN - 1932-7447 SN - 1932-7455 VL - 121 IS - 12 SP - 6924 EP - 6929 PB - ACS Publications AN - OPUS4-39900 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -