A molecular rotor that can measure changes in the viscosity of the fluid inside a single cell as it dies is reported online in Nature Chemistry this week. Measurements from this rotor show that there is a significant viscosity increase on cell death and this may lead to new opportunities in drug-delivery research and cancer therapy.

Photodynamic therapy involves a chemical sensitizer being introduced into cells that produces a toxic substance on exposure to light. This is a promising cancer treatment because the light can be accurately targeted and only cells in a specific area are killed whereas others are left untouched.

Marina Kuimova, Peter Ogilby and co-workers designed and made a sensitizer molecule that not only promotes the formation of an important cytotoxic agent ? so-called singlet oxygen ? but simultaneously allows the viscosity of the cell contents to be measured by monitoring how much they fluoresce. They found that the increase in viscosity affects both how quickly the toxic agent is formed, and how quickly it can react with other molecules in the cell ? both important factors in the efficiency of photodynamic therapy. This research offers a general strategy for monitoring diffusion dynamics inside cells, which could also lead to a better understanding of other cellular processes such as signalling and mass transport.