Here we report on the effect of DNA modification on individual collisions between Pt nanoparticles (PtNPs) and ultramicroelectrode (UME) surfaces. These results extend recent reports of electrocatalytic amplification (ECA) arising from collisions between naked surfaces, and they are motivated by our interest in using ECA for low-level biosensing applications. In the present case, we studied collisions between naked PtNPs and DNA-modified Au and Hg UMEs and also collisions between DNA-modified PtNPs and naked Au and Hg UMEs. In all cases, the sensing reaction is the catalytic oxidation of N2H4. The presence of ssDNA (5-mer or 25-mer) immobilized on the UME surface has little effect on the magnitude or frequency of ECA signals, regardless of whether the electrode is Au or Hg. In contrast, when DNA is immobilized on the PtNPs and the electrodes are naked, clear trends emerge. Specifically, as the surface concentration of ssDNA on the PtNP surface increases, the magnitude and frequency of the current transients decrease. This trend is most apparent for the longer 25-mer. We interpret these results as follows. When ssDNA is immobilized at high concentration on the PtNPs, the surface sites on the NP required for electrocatalytic N2H4 oxidation are blocked. This leads to lower and fewer ECA signals. In contrast, naked PtNPs are able to transfer electrons to UMEs having sparse coatings of ssDNA.