You would not expect diamonds to have anything to do with cancer treatment. But they can actually they improve patients’ chances of survival.
“Using traditional techniques, it’s almost impossible to determine whether a cancer treatment has been 100 per cent or 99.9 per cent effective. The difference can have major consequences, because there is a risk that even a small number of surviving cancer cells can spread in the body. A quantum mechanical diamond sensor will be so sensitive that it can detect even very few cancer cells among a large number of healthy cells,” says Alexander Huck, Associate Professor at DTU Physics.
The diamonds have not been purchased from a jeweller. They are made artificially, and the surface has a micrometre thin layer with the special properties needed to make the measurements.
The method is based on magnetic biomarkers, which can bind to the cancer cells but not to healthy cells. This part is a known method. Then comes the difficult part: Although you have created a magnetic difference between the two types of cells, it requires an extremely sensitive sensor to detect magnetic fields as weak as those around each marker. Alexander Huck illustrates:
“The best known example of a magnetic field sensor is a compass. A compass is sensitive to the Earth’s magnetic field. It reacts to differences in the order of 30-40 microteslas. The differences that interest us in connection with cancer diagnosis are in the order of a few dozen nanoteslas.”
A microtesla (10-6 tesla) is one thousand nanotesla (10-9 tesla). In other words, the new sensors have to be at least one thousand times as sensitive as a normal compass.
Green light in, red light out
The diamond sensor has a resolution of about 1 micrometre.
“This is enough for us to distinguish between biological cells. We hope to be able to detect and isolate individual cancer cells among a million healthy cells. Perhaps among 10 million healthy cells,” says Alexander Huck.
Why use diamonds to investigate a magnetic field?
Diamonds consist of carbon atoms held in a fixed three-dimensional lattice. A completely pure diamond does not interact with magnetic fields and would therefore not be suitable as a sensor.
However, Alexander Huck and researchers from the universities in Ulm and Leipzig have incorporated defects into the diamonds. In each defect, a carbon atom is replaced by a nitrogen atom. The change also creates holes in the lattice of carbon atoms, such that individual atoms are missing. The diamond is then heated to approx. 800 °C. This causes the holes to move through the structure until they are adjacent to a nitrogen atom. The nitrogen atom and hole then bind to form a single structure—an NV (nitrogen-vacancy) centre. The effect of the centre is that light from the green part of the spectrum is absorbed. The centre emits red light instead.
Two kinds of spin at the same time
Each NV centre in the diamond also has electron spin (see the article, ‘Vanguard of quantum society’). The spin, which can be either up or down, determines how much red light will be emitted. In addition to the spin being either up or down, it is subject to periodic changes—in the same way as the Earth not only rotates on its axis but also displaces the axis of its rotation. The speed and size of these periodic changes are sensitive to the surrounding magnetic field. In other words, it is possible to investigate the magnetic field by measuring the periodic changes in the spin. This is done by shining green light on the defect centres and detecting how much red light is emitted.
The method exploits superposition—the quantum mechanical phenomenon whereby a particle can be in two states at the same time. “More specifically, we make sure that the electron’s spin is up and down at the same time. The two types of spin in the centre behave differently in a magnetic field. By detecting how the up and down components of the spin have changed, we can determine the magnetic field,” explains Alexander Huck. He notes that it is ideal to measure magnetic fields in connection with medical scans.
“Unlike electric fields, which are significantly affected when they encounter tissue, blood, and bone, magnetic fields are largely unaffected.”
From the lab to the hospital
Diamonds also have a number of advantages in medical contexts. They are a very robust material, and there is no health risk associated with bringing them into contact with the body.
The group at DTU Physics cannot take credit for the idea of using diamonds as magnetic field sensors. This arose in the 1990s, and many international groups have since worked on making it a reality. However, interdisciplinary collaboration with partners from DTU Electrical Engineering, Hvidovre Hospital, and Philips Biocell is giving Alexander Huck and his colleagues a lead in relation to possible medical treatments in particular:
“We can already say that the method is relevant for phase-two cancer patients—who have undergone cancer treatment, where the doctors want to establish how effective the treatment has been. But there is long way from having a relevant method to having a device you can use in a hospital. The next step is to try to find the right design for the equipment, in cooperation with colleagues at DTU Nanotech.”
A wealth of medical applications
Alexander Huck expects a first version of the equipment to be ready in two years. From there, they can choose various paths:
“If we decide that further development can best be done in a new company, the best time will probably be when we have the first prototype ready. In around two years. But there are of course other options, such us partnering with an existing medico-technical company.”
One of the things to consider is how broadly the equipment is to be used.
“The possible applications are in no way limited to the field of cancer. Every time an electron moves from A to B, a small local magnetic field arises. If you have a magnetic field sensor that is sensitive enough, you can detect many processes in the body. It is also a non-invasive technique requiring no surgery or probes. It would therefore be an obvious choice in critical areas of the body such as the brain or heart.”