Analytische Chemie
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- 2017 (3)
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- Englisch (3) (entfernen)
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- Excitation intensity (2)
- Photon upconversion (2)
- Fluorescecne imaging (1)
- Luminescence intensity ratio (1)
- Multiphonon relaxation (1)
- Nanoparticles (1)
- PH sensing (1)
- Temperature sensing (1)
- Three photon excitation (1)
- Upconversion (1)
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.
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.
Measurement of changes of pH at various intracellular compartments has potential to solve questions concerning the processing of endocytosed material, regulation of the acidification process, and also acidification of vesicles destined for exocytosis. To monitor these events, the nanosized optical pH probes need to provide ratiometric signals in the optically transparent biological window, target to all relevant intracellular compartments, and to facilitate imaging at subcellular resolution without interference from the biological matrix. To meet these criteria we sensitize the surface conjugated pH sensitive indicator via an upconversion process utilizing an energy transfer from the nanoparticle to the indicator. Live cells were imaged with a scanning confocal microscope equipped with a low-energy 980 nm laser excitation, which facilitated high resolution and penetration depth into the specimen, and low phototoxicity needed for long-term imaging. Our upconversion nanoparticle resonance energy transfer based sensor with polyethylenimine-coating provides high colloidal stability, enhanced cellular uptake, and distribution across cellular compartments. This distribution was modulated with membrane integrity perturbing treatment that resulted into total loss of lysosomal compartments and a dramatic pH shift of endosomal compartments. These nanoprobes are well suited for detection of pH changes in in vitro models with high biological background fluorescence and in in vivo applications, e.g., for the bioimaging of small animal models.