Using tiny diamonds, or nanodiamonds, scientists have worked out how to measure heat transfer inside living cells – something they say that until now has proved difficult.
“A cell’s thermal conductivity – the rate that heat can flow through an object if one side is hot and another is cold – has remained mysterious,” says Taras Plakhotnik from the University of Queensland, co-author of a study published in the journal Science Advances.
But understanding this is critical to clarify how internal heat is generated and controlled in living cells and organisms, the researchers write.
Not only were they able to determine thermal properties inside cells and different locations within them with an extraordinary level of accuracy (with a spatial resolution of around 200 nanometres), the key finding was also quite unexpected.
“The thermal conductivity inside cells turned out to be five to six times smaller than that of water,” Plakhotnik explains.
He says researchers in recent years have tried to measure temperature in living isolated cells using organic dye molecules of fluorescent proteins: “The results were very puzzling and hard to explain.”
Some reported hot spots with temperatures up to 10ºC higher than the temperature inside cellular environments, producing a discrepancy with theoretical models – and heated discussions between physicists and biologists.
“Physicists have pointed out that an ordinary cell does not have enough energy to support such a high temperature in a hot spot,” says Plakhotnik, “and that the highest increase one can reasonably expect is more than 10,000 times smaller, but typically will be about 0.0001 of a degree Celsius.”
The estimate depended on several parameters, particularly intracellular thermal conductivity, which was assumed to be equal to that of water – not an unreasonable assumption, says Plakhotnik, since living cells are full of water.
To investigate this, the research team coated nanodiamonds measuring about 100 nanometres with a polymer, polydopamine, that absorbs light and generates heat.
After first testing them in water, oil and the air – all of which have well-known thermal conductivity factors – the non-toxic particles were placed inside the cell (“the cells happily continued their life cycle after that,” says Plakhotnik).
When inside, the nanodiamonds were illuminated with laser light, which caused them to emit fluorescent light as well as heat. In an environment with high thermal conductivity, the particles didn’t get very hot because heat escaped quickly. But when thermal conductivity was low, they became hotter.
Because the properties of the emitted light depend on the temperature, the team could calculate the rate of heat diffusion in cells.
The key finding was so unexpected, says Plakhotnik, that the editor of Science Advances asked them to measure a second line of cells to confirm the results were more general than a single cell line.
While the smaller than expected thermal conductivity bridges the discrepancy between experimental and theoretical physics, it doesn’t explain it entirely, so the team is working on numerical modelling to shed more light on intracellular heat transfer.
The discovery could help explain fundamental questions about cellular heat management and the role that heat can play inside cells; it could be used for communication, for example, says Plakhotnik.
These insights should be considered when cells are heated to treat cancer, he adds. “The cancer cells are killed faster but there is a limit to what healthy cells can tolerate,” he explains, “and the temperature should be carefully controlled.”
The technique could also be used for basic cell research, such as monitoring biochemical reactions in real time, and for better understanding of metabolic disorders such as obesity.