High-density ZnO nanorods of 60-80 nm in diameter and 500-700 nm in length grown on the silver-coated tip of a borosilicate glass capillary (0.7 mum in diameter) demonstrate a remarkable linear response to proton H(3)O(+) concentrations in solution. These nanorods were used to create a highly sensitive pH sensor for monitoring in vivo biological process within single cells. The ZnO nanorods exhibit a pH-dependent electrochemical potential difference versus an Ag/AgCl microelectrode. The potential difference was linear over a large dynamic range (pH, 4-11) and had a sensitivity equal to 51.88 mV/pH at 22 degrees C, which could be understood in terms of changes in surface charge during protonation and deprotonation. Vertically grown nanoelectrodes of this type can be smoothly and gently applied to penetrate a single living cell without causing cell apoptosis.