Nature Nanotech. doi:10.1038/nnano.2009.83 (2009)

A tiny machine made from DNA can measure the acidity of living cells from the inside.

Yamuna Krishnan and her colleagues at the Tata Institute of Fundamental Research in Bangalore, India, designed three short pieces of DNA that spontaneously assemble into the nanoscale device when they are in close proximity. The device changes shape when in an acidic environment, thanks to changes in how cytosine binds. Fluorescent tags change colour to reflect the shape change — green for neutral, red for acidic. The sensor could be used to study changes in cell pH that accompany viral invasion.