Here we report a method to assemble nanoscale DNA structures with single-molecule precision. This assembly is accomplished by performing nanografting in the presence of short, thiolated DNA strands that have been diluted by a positively charged alkanethiol. The expected number of DNA molecules per patch can be modulated by the application of an electric potential to the surface during patterning. Our ability to position individual DNA within a controlled nanoscale environment and observe these molecules in situ will allow us to understand and potentially decouple the heterogeneity caused by the local environment from the intrinsic properties in single-molecule biophysical measurements. Additionally, our approach can potentially be extended to the molecule-by-molecule assembly of larger artificial test structures of nucleic acids or proteins.