Luminescent probes excitation, emission spectra

The first sensor of that kind was presented by Sun et al. [48] who reported on a pH sensor based on the upconversion luminescence of NaYF4 Yb,Er nanorods (see inset Fig. 6b) that were embedded in a matrix of hydrogel along with the longwave absorbing pH probe (bromothymol blue BTB) that causes a pH dependent inner filter effect (Fig. 6a). The emission spectrum of the nanorods at NIR excitation and the pH dependent absorption spectrum of BTB for three pH values are shown in Fig. 6b. 

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