Researchers at the Indian Institute of Science (IISc) have developed a technique to precisely manoeuvre quantum sensors through highly viscous biological environments, such as the interior of living cells, using magnetic microbots. This opens up possibilities for real-time, minimally invasive measurement of parameters like local viscosity and temperature inside cells.
Cells are squishy and soft. Tiny nanometer-sized particles such as quantum sensors cannot move freely inside them due to viscous drag, which makes sensing challenging. “In soft environments, measurements are limited by the probability of the analyte coming close to the sensor. So, the question arises if one can instead bring the sensor [closer] and look around for the analyte,” explains Ambarish Ghosh, Professor at Center for Nanoscience and Engineering (CeNSE), IISc, and corresponding author of the study published in Advanced Functional Materials.
Ghosh’s team combined a nanodiamond quantum sensor containing a nitrogen vacancy (NV) defect with a magnetically controlled microbot. An NV defect is a specific site in the diamond lattice where a carbon atom is replaced by nitrogen, adjacent to an empty site. It has electrons whose quantum spin states depend on the surrounding environment – different physical properties such as temperature and magnetic field disturb the spin states in distinct, measurable ways. When a laser beam excites the nanodiamond, the resulting fluorescence can be used to measure multiple parameters inside the cell. The challenge was getting such a sensor to the right place. Earlier approaches used focused laser beams called optical tweezers to position and steer nanodiamonds, but the intense light could scorch cells.
The IISc team’s solution was to attach the nanodiamond to a magnetically controlled microbot and drive it through the fluid like a corkscrew. The microbot contains iron, so when an external rotating magnetic field is applied, the microbot spins to align with it. Due to its helical shape, rotation is converted into forward linear motion. This allowed the team to steer the sensor precisely in three dimensions without any light-based manipulation. Light is only needed while taking measurements using fluorescence and not to move the sensor, which minimises phototoxicity and heating.
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At the nanoscale, there is another issue to resolve: the random thermal jostling of the surrounding molecules, called brownian motion, can cause the sensor to orient unpredictably, which could increase noise and reduce sensitivity. But because the microbot can be oriented precisely using the external magnetic field, the researchers were able to hold the nanodiamond steady, suppressing noise and recovering a clean signal.
“We are able to counter brownian motion with magnetic manipulation. This makes this platform more promising than optics or any other techniques,” says Ghosh.
To design their sensor, the team had to precisely combine a nanodiamond with a magnetic motor without having one’s properties interfering with the other. “It was not intuitive because the sensor itself may be affected by the magnetic elements,” says Eklavy Vashist, Research Associate at CeNSE and first author of the study. The team resolved this by positioning the nanodiamond approximately one micron away from the microbot’s iron head, where the motor’s magnetic field has negligible effect on the sensor.
“This sensor can also be used to look around inside living cells and measure, for instance, Reactive Oxygen Species (ROS), which is crucial in cancer and ageing,” says Ghosh.
Source: Indian Institute of Science
Published on April 09, 2026