Incubation of the five tile-forming DNA strands in TANa buffer readily produced DNA nanotubes, which grew in solution for days at room temperature as observed by fluorescence microscopy (Movie S1). Transmission electron microscopy (TEM) qualitatively shows that short nanotubes slowly disappear while longer ones become predominant (Figure ). After 20 days, very long nanotubes (>10 µm) dominate in the sample, with a vanishing number of shorter tubes (1-2 µm), signifying the ongoing growth of the nanotubes over a long period of time. To analyze the effect of ions on nanotubes stability, we used fluorescence microscopy image analysis to estimate the nanotube melting temperature (T m ).