Colour-optimized quantum yields of Yb, Tm Co-doped upconversion nanocrystals.

We present here a systematic analysis of the influence of Tm3+ doping concentrations (x Tm ) on the excitation power (P)-dependent upconversion luminescence and -performance of hexagonal-phase NaYF4: 20% Yb3+, x Tm % Tm3+ upconversion nanoparticles (UCNPs) for x Tm of 0.2, 0.5, 0.8, 1.2, and 2.0, respectively. Our results reveal the influence of these different Tm3+ doping concentrations with respect to optimized upconversion quantum yield (Φ UC ) values of the various Tm3+ upconversion emission bands, with the highest Φ UC values of the Tm3+ emission bands above 700 nm resulting for different x Tm values as the Tm3+ emission bands below 700 nm. This underlines the potential of Tm3+ dopant concentration for colour tuning. Special emphasis was dedicated to the spectroscopic parameters that can be linked to the (de)population pathways of the various Tm3+ energy levels, like the P- and x Tm -dependent slope factors and the intensity ratios of selected emission bands. The evaluation of all parameters indicates that not only energy transfer upconversion-, but also cross-relaxation processes between neighbouring Tm3+ ions play a vital role in the (de)population of the excited energy levels of Yb3+, Tm3+ codoped nanocrystals.

A protected excitation-energy reservoir for efficient upconversion luminescence.

Lanthanide-doped upconversion nanoparticles (UCNPs) are of great interest for biomedical applications. Currently, the applicability of UCNP bionanotechnology is hampered by the generally low luminescence intensity of UCNPs and inefficient energy transfer from UCNPs to surface-bound chromophores used e.g. for photodynamic therapy or analyte sensing. In this work, we address the low-efficiency issue by developing versatile core-shell nanostructures, where high-concentration sensitizers and activators are confined in the core and shell region of representative hexagonal NaYF4:Yb,Er UCNPs. After doping concentration optimization, the sensitizer-rich core is able to harvest/accumulate more excitation energy and generate almost one order of magnitude higher luminescence intensity than conventional homogeneously doped nanostructures. At the same time, the activator ions located in the shell enable a ∼6 times more efficient resonant energy transfer from UCNPs to surface-bound acceptor dye molecules due to the short distance between donor-acceptor pairs. Our work provides new insights into the rational design of UCNPs and will greatly increase the general applicability of upconversion nanotechnologies.

Beam-profile-compensated quantum yield measurements of upconverting nanoparticles.

The quantum yield is a critically important parameter in the development of lanthanide-based upconverting nanoparticles (UCNPs) for use as novel contrast agents in biological imaging and optical reporters in assays. The present work focuses on the influence of the beam profile in measuring the quantum yield (ϕ) of nonscattering dispersions of nonlinear upconverting probes, by establishing a relation between ϕ and excitation light power density from a rate equation analysis. A resulting 60% correction in the measured ϕ due to the beam profile utilized for excitation underlines the significance of the beam profile in such measurements, and its impact when comparing results from different setups and groups across the world.

Power-dependent upconversion quantum yield of NaYF4:Yb3+,Er3+ nano- and micrometer-sized particles - measurements and simulations.

Photophysical studies of nonlinear lanthanide-doped photon upconverting nanoparticles (UCNPs) increasingly used in biophotonics and photovoltaics require absolute measurements of the excitation power density (P)-dependent upconversion luminescence (UCL) and luminescence quantum yields (ΦUC) for quantifying the material performance, UCL deactivation pathways, and possible enhancement factors. We present here the P-dependence of the UCL spectra, ΦUC, and slope factors of the different emission bands of representative 25 nm-sized oleate-capped ß-NaYF4:17% Yb3+, 3% Er3+ UCNPs dispersed in toluene and as powder as well as ΦUC of 3 µm-sized upconversion particles (UCµP), all measured with a newly designed integrating sphere setup, enabling controlled variation of P over four orders of magnitude. This includes quantifying the influence of the beam shape on the measured ΦUC and comparison of experimental ΦUC with simulations utilizing the balancing power density model of the Andersson-Engels group and the simulated ΦUC of UCµP from the Berry group, underpinned by closely matching decay kinetics of our UC material. We obtained a maximum ΦUC of 10.5% for UCµP and a ΦUC of 0.6% and 2.1% for solid and dispersed UCNPs, respectively. Our results suggest an overestimation of the contribution of the purple and an underestimation of that of the red emission of ß-NaYF4:Yb3+,Er3+: microparticles by the simulations of the Berry group. Moreover, our measurements can be used as a guideline to the absolute determination of UCL and ΦUC.

Particle-Size-Dependent Förster Resonance Energy Transfer from Upconversion Nanoparticles to Organic Dyes.

Upconversion nanoparticles (UCNPs) are attractive candidates for energy transfer-based analytical applications. In contrast to classical donor-acceptor pairs, these particles contain many emitting lanthanide ions together with numerous acceptor dye molecules at different distances to each other, strongly depending on the particle diameter. UCNPs with precisely controlled sizes between 10 and 43 nm were prepared and functionalized with rose bengal and sulforhodamine B by a ligand-exchange procedure. Time-resolved studies of the upconversion luminescence of the UCNP donor revealed a considerable shortening of the donor lifetime as a clear hint for Förster resonance energy transfer (FRET). FRET was most pronounced for 21 nm-sized UCNPs, yielding a FRET efficiency of 60%. At larger surface-to-volume ratios, the FRET efficiency decreased by an increasing competition of nonradiative surface deactivation. Such dye-UCNP architectures can also provide an elegant way to shift the UCNP emission color, since the fluorescence intensity of the organic dyes excited by FRET was comparable to that of the upconversion emission of smaller particles.

Optically Detected Degradation of NaYF4:Yb,Tm-Based Upconversion Nanoparticles in Phosphate Buffered Saline Solution.

In a proof-of-concept study, we assessed different analytical and spectroscopic parameters for stability screening of differently sized ß-NaYF4:20 mol % Yb3+, 2 mol % Tm3+ upconversion nanoparticles (UCNPs) exemplarily in the bioanalytically relevant buffer phosphate buffered saline (PBS; pH 7.4) at 37 and 50 °C. This included the potentiometric determination of the amount of released fluoride ions, surface analysis with X-ray photoelectron spectroscopy (XPS), and steady-state and time-resolved fluorescence measurements. Based on these results, the luminescence lifetime of the 800 nm upconversion emission was identified as an optimum parameter for stability screening of UCNPs and changes in particle surface chemistry.