Fig. 6.

(A) Model of a typical Type 2 probe, a ratiometric pH sensing nanogel, which is constructed by uploading of the pH probe dye molecules into the gel networks. (B) pH-dependent absorption of bromothymol blue in aqueous solution at pH 5.0, 7.0, and 9.0 (gray curves), and absorption and emission spectra of coumarin 6 (C6) and Nile Red (NR) in ethanol. The picture in (B) shows the green fluorescence of a mixture of C6 and NR in ethanol/water solvent (left), and the same components in the nanogel (NG) in aqueous suspension under 365 nm illumination (right). (C) Fluorescence micrographs of NRK cells incubated with a pH 7.4 buffer and loaded with the pH-responsive nanogel: (a) Fluorescence of the C6 acquired in the green channel (scale bar=20 mm); (b) fluorescence of NR acquired in the red channel; (c) overlay of (a) and (b). Adapted from Reference (60). Reprinted with permission from Wiley-VCH Publishing Group, Copyright (2010).