As the saying goes - diamonds may be a girl's best friend, but they could also one day help us understand how the brain processes information, thanks in no part to a novel sensing technique developed at Massachusetts Institute of Technology (MIT), USA.
A team in MIT's Quantum Engineering Group have managed to develop a new method to control nanoscale diamond sensors, which are capable of measuring very weak magnetic fields. Subsequently the MIT researchers presented their work in January 2014 edition of Nature Communications.
The new control technique allows for tiny sensors to monitor how magnetic fields change over time, such as when neurons in the brain transmit electrical signals to each other. Plus, it could also enable researchers to more precisely measure the magnetic fields produced by novel materials such as the metamaterials employed to make super lenses and "invisibility cloaks"
In 2008, a research team consisting of MIT, Harvard University and other institutional scholars first managed to reveal that nanoscale defects inside diamonds could be exploited as magnetic sensors. The naturally occurring defects, typically known as Nitrogen-Vacancy (N-V) centres were sensitive to external magnetic fields, similar to compasses, as stated by Paola Cappellaro, the Esther and Harold Edgerton Associate Professor of Nuclear Science and Engineering (NSE) at MIT. It is known that defects inside diamonds are classed as colour centres, because they give the gem-stones a particular hue - as commented upon by Cappellaro "So if you ever see a nice diamond that is blue or pink, the colour is due to the fact that there are defects in the diamond."
Cappellaro continues on, and confirms that N-V centre defects consists of a Nitrogen (N) atom in place of a Carbon (C) atom and next to a vacancy or hollow within the diamond's lattice structure. Many of the diamond defects would give the gem-stone a pink colour, and when illuminated with light they emit a red light.
To help implement the new method of controlling the sensors, Cappellaro's team firstly probed the diamond with green laser light until they detected a red light being emitted, which assisted the team in informing them where the defect was located. Thereafter a microwave field was applied on to the nanoscale sensor to help manipulate the electron spin of the N-V centre. The microwave field helped alter the intensity of light emitted by the defect, to a degree that depends not only on the microwave field but also on any external magnetic fields present. To measure the external magnetic fields and how they may change over time, the researchers targeted the nanoscale sensor with a microwave pulse, which switched the direction of the N-V centre's electron spin - as stated by a team member and NSE graduate student Alexandre Cooper. Therefore, by applying different series of the microwave pulses, acting as filters the team were able to efficiently collect information about the external magnetic field. An applied signal-processing technique was used to interpret the information as well as reconstruct the entire magnetic field. As, again, commented upon by Cappellaro - "So we can reconstruct the whole dynamics of this external magnetic field, which gives you more information about the underlying phenomena that is creating the magnetic field itself."
The nanoscale diamond sensor consisted of a sample which was a square of a diamond three millimeters in diameter, but it is possible to use sensors that are tens of nanometers in size. Such diamond sensors can be used at Room temperature (RT), and because they consist of entirely of C meaning they can be injected into living cells without causing them any harm.
It is speculated that one application could be to grow neurons on top of the diamond sensor, to enable it to measure the magnetic fields created by the "action potential" or signal which are produced and then transmitted on to other nerves. Also, previously, another research team have used the electrodes inside the brain to "pole" a neuron and measure the electric field produced. However, as stated by Cappellaro, unfortunately, the technique is very invasive - "You don't know if the neuron is still behaving as it would have if you hadn't done anything."
Alternatively, the diamond sensor could measure the magnetic field non-invasively. Commenting, again, Cappellaro - "We could have an array of these defect centres to probe different locations on the neuron, and then you would know how the signal propagates from one position to another one in time."
In their Proof-of-Principle (PoP) experiments to fully demonstrate the sensor, the team used a waveguide as an artificial neuron and applied an external magnetic field. The diamond sensor was then placed on to the waveguide where they were able to accurately reconstruct the magnetic field.
Complementing the study, above - Mikhail Lukin, a professor of Physics at Harvard University, stated that the study demonstrated nicely the ability to reconstruct time-dependent profiles of weak magnetic fields using a novel magnetic sensor based on quantum manipulation of the defects in the diamond sensor. Professor Lukin states - "Someday techniques demonstrated in this work may enable use to do real-time sensing of brain activity and to learn how they work." Also, he continues on - "Potential far-reaching implications may include detection and eventual treatment of brain diseases, although much work remains to be done to show if this actually can be done." Original article available here
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