by Josh Green
Many of you Dr Who aficionados will be very familiar with the most famous of the Doctor’s tools; the sonic screwdriver. Its ‘do it all’ abilities often solve many issues much in the same way the ‘tricorders’ of Star Trek are pointed at someone and, almost by magic, it detects the ills or properties of an object or person. Using these objects as the conceptual ideas that they are a team of researchers have come up with a device that is, in effect, a handheld sensor. This sensor has been talked up as something that can be used as a mass-spectrometer (something that tells you what’s something is made of or contains) or even as something to obtain an MRI of a single molecule. The device itself has not had a prototype made yet but the researchers, headed by a group in the Australian National University’s Research School of Physics and Engineering, have published a paper on the concept’s validity.
So, how does this device actually work? The researchers aim to ultimately use diamond, which contains crystal defects on purpose, to measure matter that attaches to a moving diamond cantilever. The defects are able to detect the changes in movement as their behaviour changes when the cantilever changes its movement. You might be very keen to understand how this works and how it is indeed possible. There are a lot of things to explain and only a page to explain but have no fear! A diamond scientist is your author here!
Firstly, there exists the ability to grow synthetic diamond samples inside complicated systems that labs (like my own one) have. When these diamond samples are grown to what the grower desires there exists the ability to add impurities. Since diamond is made from carbon, which forms in a particular way and ideally forms a repetitive atomic arrangement (called a crystal lattice), impurities mean anything that isn’t carbon and which affects this arrangement. When your diamond sample is irradiated with nitrogen something interesting can happen. A particular defect can form which is called the NV center. This on the atomic scale is formed by a nitrogen atom replacing a carbon atom, in the crystal lattice structure, which neighbours an ‘empty space’ in where another carbon atom should be. This defect, in the negatively charged state (which is caused by having an important extra electron), is the defect that is vital to this particular piece of new research. Indeed, the defect in question is very interesting to the scientific community as it has very unique properties that have enabled discussion and research aiming to realise the NV center’s potential applications that range from quantum computing to acting as a biosensor.
Of course, the next question would be how this can be used to create this ‘sonic screwdriver’ device! Well the NV center helps the system to act as a very precise sensor thanks to its spare electron and how that behaves in certain scenarios. This NV centre has been able to measure thermometry in living cells and also has been shown to enable room temperature nanosized MRI measurements. This electron’s spin (that is which direction the particle is spinning) can change due to external influencing forces. This electron spin that belongs to that NV center can be influenced by a number of external forces, however, what is relevant to this research is that the spin of the electron interacts with mechanical stress. One can use this analogy of a ruler being bent with a spinning top on the end of it. As you stress the ruler by bending you are changing how the spinning top on the end is spinning. What happens in this conceptual device is that if something affects the diamond pillar’s movement then the stress will be ‘sensed’ by the NV center. As the pillar or cantilever is moving a biological agent can attach onto the diamond device. What occurs then, if the device is driven/made to move mechanically, is that the diamond device starts to move slower and this change can be detected due to the changes in stress by the negatively charged NV center.
Now this is most of what’s going on in a nutshell. The concept of a defect and how it can change according to something else around it has been explained. However, these systems which are called nanospin-mechanical sensors (NSMS) are incredibly small so you cannot detect changes in the nanopillars by using, say, laser light. How do you pick up the changes occurring with this tiny system or an array of these NV systems? This is possible due to how the NV center works again. The NV center in question, when you have the negative charge state one that is, has what is called a bright fluorescence. Fluorescence is when something absorbs light and emits a different wavelength of light as a result. This fluorescence can be measured from the NV defects and then can be interpreted.
Is it hoped that the research conducted here, by lead researcher Dr Marcus Doherty, can be used to make a successful handheld prototype. It is hoped that this will enable laboratories and hospitals to have the power of using more portable and affordable MRI machines and mass spectrometers. This will, hopefully, lead to easier and more numerous measurements made on complex proteins and other biological agents often associated with things like cancer. This research goes to show about how diamond is not merely a gemstone but also a material capable of saving lives. Who knows? Maybe in a few decades you can accurately cosplay as the Doctor or Captain Jean-Luc Picard (sorry Captain Kirk fans).
Picture: Diamond pillars with negative NV centers implanted near the bottom with electron spin in red. Biological agent (green) attaches to the pillar which now moves more slowly. Fluorescence (bright green) is emitted and detected which enables a read out of the change in spin state of the center due to change in mechanical stress of the pillar.