Polymeric fluorescent nanoparticles, R6GDARs, containing rhodamine 6G within 1,3-phenylenediamine resin are prepared using the extensive Stöber method. The R6GDAR is capable of sensing intracellular pH in living cells, with the fluorescence intensity increasing upon decreasing the pH values from 8.0 to 3.0. This fluorescence enhancement at low pH is based on the "retro-self-quenching" mechanism, where the protonation of the R6GDAR backbones expands the particle structure, leading to increase in the separation among concentrated R6G molecules as well as their release. Fluorescence time-course measurement shows that even individual R6GDARs have high sensitivity to report the environmental pH at the single particle level. Compared to other existing pH sensors, R6GDARs offer a wider working pH range (5 pH units), higher sensitivity, and greater photostability. R6GDARs have been demonstrated to be sensitive to map local pH values inside MCF7 and MDA-MB-231 cells, with extremely low cell toxicity. R6GDARs serve as an excellent pH sensing probe for cellular microenvironments.