We have synthesized and applied a nanoparticle-based pH sensor for noninvasive monitoring of intracellular pH changes induced by drug stimulation. The pH sensor is a two-fluorophore-doped nanoparticle sensor (2DFNS) that contains a pH-sensitive indicator (fluorescein isothiocyanate, FITC) and a reference dye (tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate, RuBPY). The nanoparticles have an average diameter of 42 +/- 3 nm and can easily be taken up by cells for noninvasive intracellular pH measurement. The 2DFNS exhibited excellent pH sensitivity, reversibility, and a dynamic range of pH 4-7 for biological studies. We have used 2DFNS to monitor pH changes in living cells by drug stimulation. Both lysosomal pH changes in murine macrophages stimulated by chloroquine and intracellular acidification in apoptotic cancer cells were monitored in real time and with high pH sensitivity. Hela cells underwent intracellular acidification with a drop in pH from 7.2 to 6.5 after 8 h of treatment with 2 mumol/L dexamethasone, and this intracellular pH drop in the apoptotic cells was not influenced by the addition of zinc ions. The application of 2DFNS to intracellular pH measurements yields some important advantages: excellent pH sensitivity, little environmental effect on the pH dye, excellent quantification, high stability and excellent reversibility.