The natural occurrence of the human telomeric G-quadruplex or i-motif in vivo has not been demonstrated and the biological effects of the induction of these structures need to be clarified. Intracellular environments are highly crowded with various biomolecules and in vitro studies under molecular-crowding conditions will provide important information on how biomolecules behave in cells. Here we report that cell-mimic crowding can increase i-motif stability at acid pH and cause dehydration. However, crowding can not induce i-motif formation at physiological pH. Intriguingly, single-walled carbon nanotubes (SWNTs) can drive i-motif formation under cell-mimic crowding conditions and cause more water to be released. To our knowledge, there is no report to show how SWNTs can influence DNA under cell-mimic crowding conditions. Our results indicate that SWNTs may have the potential to modulate the structure of human telomeric DNA in vivo, like DNA B–A transitions and B–Z changes on SWNTs in live cells, which demonstrates potential for drug design and cancer therapy.