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Self-propelled swimming nanodiamonds for biological applications

(Nanowerk Spotlight) Sometimes nanoscale diamonds contain a specific type of impurity: a single nitrogen atom where a carbon atom should be, with an empty space right next to it, resulting from a second missing carbon atom. This nitrogen-vacancy (NV) impurity gives each nanodiamond special optical and electromagnetic properties.
At the confluence of quantum metrology and biology, the nitrogen-vacancy center in nanoscale diamonds has emerged as a leading contender for quantum sensing applications.
NV centers display a remarkable range of properties, including sustained fluorescence, allowing detection at the single molecule level, and long quantum coherence times under ambient conditions.
"Nitrogen vacancy centers in nanodiamonds require a method to manipulate their electron spin orientations physically," Ji Tae Kim, an assistant professor at The University of Hong Kong's Department of Mechanical Engineering, explains to Nanowerk. "However, current methods are limited to 'passive approaches' requiring high energy field across the entire liquid environment. Our recent work provides a general way to create nanosensors that self-propel for programmed locomotion."
Reporting their findings in Advanced Materials ("Nanodiamonds That Swim"), Kim, together with Udit Choudhury, Hyeon-Ho Jeong, and Prof. Peer Fischer, the team from the Max Planck Institute for Intelligent Systems demonstrates a general active NV system: Nanodiamond swimmers that self-propel.
Since the readout of NV centers in nanodiamonds requires light, the researchers show that the same excitation source can also be used to power the swimming motion of nanodiamonds.
"By combining the field of micro- and nanomotors with the field of nanodiamond sensors, we show that how quantum sensors can be manipulated and ultimately made autonomous," Fischer points out. "We foresee that the quantum sensors could be integrated into future sensing therapy systems."
In their paper, the scientists describe their simple procedure to obtain billions of nanodiamond-containing colloids and demonstrated the first self-phoretic nanodiamond swimmers.
"By precisely controlling their geometries using a shadow growth physical vapor deposition method we could show that the dynamics of their active self-thermophoretic motion and sensing can be decoupled," notes Kim. " While the same illumination source is used, the different power levels mean that they are independently addressable."
Design of self-thermophoretic active nanondiamond swimmers
Design of self-thermophoretic active nanodiamond (ND) swimmers. a,b) The swimmers consist of three parts: (head) an ND with nitrogen vacancy (NV) centers attached to a microparticle (body) with programmed shape (tail) that contains platinum (Pt) pad at a location. Illumination by a (532 nm) laser excitation creates a local temperature gradient ∇T near the Pt pad, which causes the swimmer to self-propel by thermophoresis. The same laser is used to simultaneously excite and observe the NV electron spin resonance by its fluorescence. Each swimmer body is designed for (a) translational (ν) or (b) rotational (ω) motion. Inset: Fluorescence images of ND swimmers where the NV center is clearly visible (scale bar: 2 µm). (Reprinted with permission by Wiley-VCH Verlag)
To that end, the team applied a previously reported (Nature Materials, "Hybrid nanocolloids with programmed three-dimensional shape and material composition"), scalable physical vapor shadow growth method to fabricate them.
"No additional external forces or torques are applied to the swimmer," says Kim. "Precise locomotion patterns can be used to control the spatial position of the NV fluorescence in a fluidic medium. Furthermore, we demonstrate that the complex hybrid nanodiamond-colloids can be used to perform self-driven vector magnetometry."
He adds that nanodiamond swimmers enable new possibilities for nanoscale metrology in biological systems, including parallel vector magnetometry and temperature sensing, and integrated sensing therapy systems combined with cargo delivery.
Going forward, the team's next steps will involve coupling their nanosensors to chemical mechanisms (both propulsion and second sensing). Potential future applications include new targeted therapy systems consisting of nanoscale sensing (magnetic field, temperature) and drug delivery and therapy.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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