Nanodiamond quantum sensors mounted on magnetic microbots achieve coherent spin control while moving freely through fluid, a first for untethered quantum sensing.
(Nanowerk Spotlight) The quantum properties that make nitrogen-vacancy (NV) centers in diamond exceptional sensors are also extraordinarily fragile in practice. Each NV center, a nitrogen atom next to a vacant site in the carbon lattice, shifts its fluorescence in response to minute changes in magnetic field or temperature. At room temperature and at the nanoscale, this sensitivity surpasses what classical instruments can achieve, and researchers have already used it for applications such as nanodiamond quantum sensors for real-time intracellular monitoring.
But exploiting that sensitivity in new environments requires placing the sensor precisely where a measurement is needed, and every major method devised to move a nanodiamond through fluid has relied on intense light. Optical tweezers need 30 to 150 mW at the focal point; thermophoretic schemes require over 300 mW; plasmonic traps confine nanodiamonds near metallic surfaces where optical absorption generates additional heat.
In each case, the power that steers the particle also disrupts the NV center’s spin states, heats the surroundings, and damages nearby biological material. The act of positioning the sensor undermines the measurement it was designed to make.
A study published in Advanced Functional Materials (“Magnetically Maneuvered Quantum Sensors”) demonstrates that magnets, rather than light, can solve the mobility problem without compromising quantum performance. The research team mounted NV-center-containing nanodiamonds onto magnetically propelled helical microbots, creating hybrid devices they call Mobile Quantum Sensors (MQS).
Design of Mobile Quantum Sensors (MQS). (A) MQS‑1: Helical, magnetically actuated microbots functionalized with single or multiple nanodiamonds distributed along their surface. Corresponding optical microscopy images show nanodiamond locations on two representative MQS‑1 devices. (B) MQS‑2: Helical microbots engineered for terminal integration of a single nanodiamond at one end. Optical images confirm precise tip localization of nanodiamonds on two distinct MQS‑2 devices. In both configurations, rotating magnetic fields enable independent control of translation and orientation. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The microbots are built at wafer scale through glancing angle deposition, a vacuum technique in which material is evaporated onto a rotating substrate at a steep angle, gradually assembling helical silica structures with embedded iron segments.
In a rotating magnetic field, the iron’s permanent magnetization locks to the field and drives a corkscrew motion, propelling the device through fluid without any optical power. A laser is switched on only briefly to read the NV center’s quantum state through optically detected magnetic resonance (ODMR), a technique that measures how microwave-driven spin transitions change the nanodiamond’s fluorescence. Because the laser serves no propulsion role, local heating remains negligible in any liquid environment.
Two device configurations target different measurement needs. In MQS-1, multiple nanodiamonds coat the helical surface, collectively providing thousands of NV centers that amplify the fluorescence signal for sensitive temperature readings. In MQS-2, a single nanodiamond sits at one tip of the helix, about 1 µm from the iron segments where their stray field is negligible, enabling precise vector magnetometry and finer spatial resolution.
Thermal tests showed that in any liquid, whether glycerol, water, or biological saline, the fluid absorbs and conducts heat away from the laser spot so efficiently that ODMR spectra displayed no measurable frequency shift even at an illumination intensity of 65 µW/µm². Only in air, where the surrounding medium carries heat away far more slowly, did temperature rises appear near the iron. For all practical fluidic settings, the sensor operates free of thermal artifacts.
In suspension, the microbots reached speeds on the order of micrometers per second. They maintained synchronized rotation with the driving field up to a threshold called the step-out frequency, beyond which viscous drag exceeds the magnetic torque and the device begins to slip. This threshold ranged from 7 to 23 Hz depending on field strength and fluid viscosity. Attaching the nanodiamond did not measurably change the swimming behavior.
The suppression of orientational noise was equally important. A free 100 nm nanodiamond tumbling in viscous fluid rotates by roughly 65 degrees over a characteristic timescale. Mounted on an MQS, the same nanodiamond fluctuated by only about 1.2 degrees, a 55-fold reduction, because the microbot’s larger size dramatically slows rotational Brownian motion.
With stable orientation and positioning established, the team demonstrated three sensing capabilities that no mobile, untethered platform had achieved before. ODMR spectra recorded under applied static fields of 1, 2, and 5 G showed progressively wider splitting of the resonance lines, verifying magnetometry while the sensor drifted in suspension.
The researchers then performed Rabi oscillations, a pulsed experiment in which a resonant microwave field coherently flips the NV spin between its quantum states at a known rate. The MQS produced clear oscillations with a characteristic flip time of approximately 150 ns, fast enough that the measurement finishes well before Brownian motion, the random jostling of the sensor by surrounding fluid molecules, can alter its orientation.
The result matched the performance of a stationary nanodiamond fixed to a substrate, making this the first time coherent quantum control has been sustained in a sensor that can be steered through its environment.
The team also mapped a temperature gradient inside a microfluidic chamber heated by a nichrome wire. By navigating the MQS to successive positions and tracking how the NV resonance frequency shifted, they reconstructed spatial and temporal temperature profiles that closely matched finite-element simulations. The sensor registered progressive heating as it approached the wire and captured further increases when the current was raised.
Vector magnetometry required one additional step. Because Brownian motion still nudges the MQS orientation between measurements, the team applied short pulses of a control field to periodically reset the device’s heading. The NV defect in diamond can point along four equivalent crystallographic directions, so a single ODMR spectrum in a known orientation contains eight resonance lines that encode both the magnitude and direction of the surrounding field.
Spectra recorded at different controlled orientations under test fields of 1 and 2 mT allowed full three-dimensional reconstruction of the applied field, a capability that optical tweezers, limited to two-axis orientation control, cannot provide.
Helical magnetic microbots have an established track record of navigating biological tissues and microporous media without causing damage, with demonstrated applications in magnetically actuated microbots for targeted biomedical delivery and minimally invasive diagnostics. The paper’s authors note that this biocompatibility, combined with the decoupled actuation and sensing architecture, positions the MQS platform for non-invasive nanoscale metrology in biological settings.
Future use of isotopically purified ¹²C diamond to extend NV spin coherence times could further sharpen both magnetic and thermal resolution, broadening the range of environments and quantities the system can probe.
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Ambarish Ghosh (Indian Institute of Science, Bangalore)
, 0000-0002-2524-0014 corresponding author
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