The familiar thermometer from a doctor’s office is slightly too big to take the temperature of living cells, considering that the average human skin cell is only 30 millionths of a metre wide. But the capability is significant: researchers at DARPA (Defence Advanced Research Projects Agency) in the US say developing the right technology to gauge and control the internal temperatures of cells and other nanospaces might open the door to a number of defence and medical applications.
It also promises better thermal management of electronics, the monitoring of the structural integrity of high-performance materials, cell-specific treatment of disease, and new tools for medical research.
A team of researchers working on DARPA’s Quantum-Assisted Sensing and Readout (QuASAR) programme recently demonstrated sub-degree temperature measurement and control at the nanometre scale inside living cells. The QuASAR team, led by researchers from Harvard University, described its techniques in a Nature paper titled “Nanometer scale quantum thermometry in a living cell”.
To measure temperature, the researchers used imperfections engineered into diamond, known as nitrogen-vacancy (NV) colour centres, as nanoscale thermometers. Each NV centre can capture an electron, such that the centre behaves like an isolated atom trapped in the solid diamond. Changes in temperature cause the lattice structure of the diamond to expand or contract, similar to the way the surface of a bridge does when exposed to hot or cold weather. These shifts in the lattice induce changes in the spin properties of the trapped atoms, which researchers measure using a laser-based technique. The result is that scientists can now monitor sub-degree variations over a large range of temperatures in both organic and inorganic systems at length scales as low as 200 nanometers.
The QuASAR team also demonstrated control and mapping of temperature gradients at the subcellular level by implanting gold nanoparticles into a human cell alongside the diamond sensors. The 100-nanometer-diameter nanoparticles were then heated using a separate laser. By varying the power of the heating laser and the concentration of gold nanoparticles, the researchers were able to modify and characterize (using the diamond sensors) the local thermal environment around the cell. In particular, they were able to verify that the heating was localized near the gold nanoparticles and that the cell did not experience an overall ambient rise in temperature.