2. Lanthanide upconversion

Planar optodes for ratiometric measurement of pH were prepared. [6] As an upcoverting phosphor we utilized NaCaY(MoO4)3 host doped with both Ho3+ and Tm3+ which shows dual emission in the red (645 nm) and NIR (797 nm) parts of the spectrum upon NIR excitation (980 nm). The emission of the phosphor was modulated by highly photostable colorimetric pH indicator on the aza-BODIPY basis. The phosphor particles and the pH indicator were dispersed/dissolved in polyurethane hydrogel. In contrast to the reported systems the new pH sensor enabled reliable ratiometric read-out (Fig. 2.1) with both the excitation light and the emission bands located in the red-NIR part of the spectrum thus fully making use of the advantages of upconversion techniques.  
Figure 2.1. Left: pH-dependent emission spectra of the sensor based on new upconverting phosphor and colorimetric NIR pH indicator (λexc 980 nm); insert: emission spectra of the phosphors co-doped with both Tm3+and Ho3+. Right: the corresponding calibration curve for the ratio of luminescence intensities at 797 and 645 nm. Reproduced from ref. [6].

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References:

[6] Strobl, M.; Mayr, T.; Klimant, I.; Borisov, S. M. Photostable Upconverting and Downconverting PH Sensors Based on Combination of a Colorimetric NIR Indicator and Stable Inorganic Phosphors as Secondary Emitters. Sensors and Actuators B: Chemical 2017, 245, 972–979.

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