Here, we report a potentiometric method for detecting single platinum nanoparticles (Pt NPs) by measuring a change in open-circuit potential (OCP) instead of the current during single Pt NP collisions with the mercury-modified Pt ultramicroelectrode (Hg/Pt UME). Similar to the current-time (i-t) response reported previously at Hg/Pt UMEs, the OCP-time (v-t) response consists of repeated potential transient signals that return to the background level. This is because Hg poisons the Pt NP after collision with the Hg/Pt UME due to amalgamation and results in deactivation of the redox reaction. For individual Pt NP collisions the amplitude of the OCP signal reaches a maximum and decays to the background level at a slower rate compared to the comparable i-t response. Due to this, OCP events are broader and more symmetrical in shape compared to i-t "spikes." The collision frequency of Pt NPs derived from v-t plots (0.007 to 0.020 pM(-1) s(-1)) is in good agreement with the value derived from i-t plots recorded at Hg/Pt UMEs (0.016 to 0.024 pM(-1) s(-1)) under similar conditions and was found to scale linearly with Pt NP concentration. Similar to the current response, the amplitude of the OCP response increased with the NP's size. However, unlike the change in current in a i-t response, the change in OCP in a v-t response observed during single Pt NP collisions with Hg/Pt UME is larger than the estimated change in OCP based on the theory. Therefore, the Pt NP sizes derived from the v-t response did not correlate with the TEM-derived Pt NP sizes. In spite of these results the potentiometric method has great value for electroanalysis because of its significant advantages over the amperometric method such as a simpler apparatus and higher sensitivity.