Fig. 3. Proof of microenvironmental pH decrease through PAA confinement.

a Scheme of PCN-222 NPs and PCN-222@PAA NPs. The light absorption and fluorescence of PCN-222 NPs are intrinsically responsive to proton concentrations. b Photos of 0.5 mg/mL PCN-222 NPs through PAA modification or introduction of HCl. c, d SEM images of PCN-222 NPs and PCN-222@PAA NPs, respectively, taken from one batch of NPs. e Ratios of absorbance at 445 nm to absorbance at 420 nm (A_{445 nm}/A_{420 nm}) of 10 µg/mL PCN-222 NPs and PCN-222@PAA NPs in Tris buffer solutions with different concentrations at pH 7.4. f Normalised fluorescence intensities at 660 nm of 20 µg/mL PCN-222 NPs and PCN-222@PAA NPs in Tris buffer solutions with different concentrations at pH 7.4. λ_ex = 420 nm. Data in (e) are expressed as mean values ± SEM, n = 3. Data in (f) are expressed as mean values ± SEM, n = 4. Data are expressed as mean ± standard error of 3 experiments.