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Posted: May 26, 2011

Nanodiamonds allow quantum control and measurement inside a living cell

(Nanowerk Spotlight) Fluorescent nanoparticles have many potential applications in areas ranging from medicine and biology to material science and even environmental protection. Organic dyes and fluorescent proteins are two types of molecules often used as fluorescent probes; however, the detrimental photophysical properties of these molecules, such as photobleaching and blinking, inevitably restrict their applications for long-term in vitro or in vivo observations. Fluorescent semiconductor nanocrystals (or quantum dots), on the other hand, hold a number of advantageous features including high photobleaching thresholds and broad excitation but narrow emission spectra well suited for multicolor labeling and detection. Unfortunately, most quantum dots are toxic, and hence reduction of cytotoxicity and human toxicity through surface modification plays a pivotal role in their successful application to in vivo labeling, imaging, and diagnosis.
Several years ago, we reported on research that showed that nanodiamond particles possess several unique features, including facile surface modification, long-term photostability, and no fluorescence blinking, that makes their detection and long-term tracking in living cells not only possible but practical ("Single fluorescent nanodiamonds as cellular biomarkers "). However, in this and similar work, the in situ monitoring of the nanodiamonds' quantum properties have not been demonstrated.
"At the confluence of quantum metrology and biology, the nitrogen-vacancy (NV) centre in diamond has emerged as a leading contender for quantum sensing applications" Lloyd Hollenberg, professor of physics at the University of Melbourne, explains to Nanowerk. "These atomic 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. Furthermore, diamond nanocrystals have proven biocompatibility."
According to Hollenberg, this challenge raises several questions: Is it possible to perform a quantum measurement on nanodiamonds in living cells, requiring non-invasive measurement and control of the probe's quantum state? Is it possible to measure an effect of the intracellular medium on the nitrogen-vacancy system? Is quantum measurement sensitive to the motion of the probe in the cell?
Working with collaborators from various groups at the University of Melbourne, Hollenberg and his team conducted studies that confirm that non-invasive quantum measurement is possible on nanodiamonds containing a single NV spin moving within living cells.
Studying the quantum properties of a single NV defect within a diamond nanocrystal, the researchers demonstrate a new technique which enables the orientation of a nanoparticle to be determined to an accuracy of less than one degree in an acquisition time of 89 milliseconds.
As reported in the May 8, 2011 advance online edition of Nature Nanotechnology ("Quantum measurement and orientation tracking of fluorescent nanodiamonds inside living cells"), this new technique offers biologists an extra degree of freedom when studying the translational motion of nanoparticles. Monitoring the coherence from a single electron spin paves the ways for nanoscale bio-magnetometry allowing scientists to probe changes in the cell's electromagnetic environment.
>Quantum measurement of single spins in living cells
Quantum measurement of single spins in living cells. a) Experimental setup, including microwave (MW) control of the NV spin levels and confocal fluorescence readout. b) Overlay of bright-field and confocal fluorescence images of HeLa cells, showing uptake of nanodiamonds. NV fluorescence is shown in red and the nucleus is stained with Hoechst 33342 (blue). Images were obtained on a Leica TCS SP2 confocal microscope. c) Atomic lattice structure of the NV centre. (Reprinted with permission from Nature Publishing Group
The team conducted their investigation over a 16-hour period at 30 second intervals with no sign of signal degradation offering unprecedented long term tracking capability. Their demonstration of quantum coherence measurements in a living cell is also a first.
Defect centers in diamond have been shown to be one of the most sensitive nanoscale electromagnetic detectors. Quantum control of nitrogen-vacancy defects in diamond is conducted using a weak microwave field with the spin state of the defect readout optically using an ordinary light microscope. As well as the electromagnetic sensing and orientation reporting techniques, nanodiamonds are also found to possess a unique spin signature which allows a user to study and return to a specific nanodiamond over infinitely long time scales compared to the life of the cell. Commercial nanodiamond is estimated to posses more than 10,000 unique signatures with the potential for more in 13C enriched nanodiamond.
"We are looking to expand our technique to tackle interesting electromagnetic problems in biology such as detecting action potentials in neural networks in real time and with high resolution and understanding the role free radicals play in intracellular process" says Hollenberg. "We could envisage our quantum-based orientation tracking technology to be used to probe molecular rotation in real time such as in ATPase activity or for local viscosity measurements and cell membrane nanomechanics."
"Quantum systems have an enormous role to play in assisting us understand the complex behavior of nature" he concludes. "By integrating such sensitive quantum probes into relatively harsh environments whilst maintaining their fidelity and readout opens up new possibilities in nanoscale bio-magnetometry. As techniques to manipulate and control the electron spin develop, this technique is expected to be able to detect single electron and even single nuclear spins with nanoscale spatial resolution. We are now in the beginning of an exciting phase of quantum biology!"
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