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The pH sensor exploits the phenomenon of upconversion luminescence and is based on a hydrogel matrix containing (a) nanorods of the NaYF(4):Er,Yb type that can be excited with 980-nm laser light to give a green and red (dual) emission, and (b) a longwave absorbing pH probe that causes a pH-dependent inner filter effect.
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Sun et al. (2009) studied this question.
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