ZnO Nanorods as an Intracellular Sensor for pH Measurements

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 μm in diameter) demonstrate a remarkable linear response to proton H₃ 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�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.