 real - time measurement 
 of intracellular ph in live cells is of great 
 importance for understanding physiological / pathological processes 
 and developing intracellular drug delivery systems . 
 we report here 
 the first use of nanoscale metal  organic frameworks ( nmofs ) 
 for intracellular ph sensing in live cells . 
 fluorescein isothiocyanate 
 ( fitc ) was covalently conjugated to a uio nmof to afford f - uio nmofs 
 with exceptionally high fitc loadings , efficient fluorescence , and 
 excellent ratiometric ph - sensing properties . upon rapid and efficient 
 endocytosis 
 , f - uio remained structurally intact inside endosomes . 
 
 live cell imaging studies revealed endo- and exocytosis of f - uio and 
 endosome acidification in real time . 
 fluorescently labeled nmofs thus 
 represent a new class of nanosensors for intracellular ph sensing 
 and provide an excellent tool for studying nmof  cell interactions . 
Supplementary Material