Advancements in magnetic detection and imaging have contributed greatly to a wide range of scientific areas from fundamental physics and chemistry to practical applications such as magnetic storage and medical science. From the brain to the heart, the bodies of humans and animals generate weak magnetic fields that techniques such as magnetic resonance imaging (MRI) measure to aid in illness diagnosis and treatment. These techniques have transformed medical imaging, but they are limited in spatial resolution. Techniques in image processes on the cellular level may enable a revolution in this area.
The ability to image magnetic fields on the nanoscale requires a magnetometer that can measure 3 nT at a distance of 10 nm. This can be used for the study of protein structure or for detailed imaging of a living cell, for example. Over the last few years, using quantum assisted techniques to manipulate a specific defect in synthetic diamond material, substantial progress towards this goal has been made.
Diamond synthesis using chemical vapor deposition (CVD) techniques allows the production of tailored synthetic diamond by manipulating the defects incorporated during growth. Researchers have used this CVD diamond and gained control over quantum behavior in the solid state, in particular the quantum property called spin, which is responsible for magnetism, such that the spin of a single defect in the crystal lattice can be manipulated. Spins of single atoms associated a nitrogen-vacancy (NV) defect, can be isolated, initialized, manipulated, and read using optical techniques. The NV electron spin has a well-defined electronic energy structure and is very sensitive to magnetic fields, electric fields, and light. This can easily be controlled and read out by lasers.
Fig. 1: The layout of a wide-field diamond-based magnetometer.Green excitation light from a laser is used to initialize and read out the spin in the NV defects in the diamond with a microwave pulse (MW) used for control.
Recently researchers demonstrated a milestone result — by using a near-surface NV center in synthetic diamond they were able to detect the presence of a proton located external to the diamond. Using a combination of electron spin echoes and proton spin manipulation, they showed that the NV center senses the nT field fluctuations from the protons, enabling both time-domain and spectroscopic magnetic resonance imaging (MRI) with sufficient resolution to measure as few as 10,000 protons in a volume of only 125 nm3 , which approaches the level of individual protein molecules.
Daniel Twitchen, Chief Technical Officer at Element Six, a maker of synthetic diamonds (www.e6.com), said, “It can be used to measure the magnetic field at a single point on a structure, or scan across the surface to image the structure.” Mathew Markham, Element Six Senior Scientist, added that, “in contrast to traditional MRI that requires expensive and bulky cryogenic equipment, this nano-MRI technology works at room temperatures.”
A second U.S. group, using a different geometry, is using the technology to locate and image the magnetic fields from living magnetotactic bacteria — organisms that contain magnetic nanoparticles. In principle, this technique would enable detailed,
real-time observation of internal cellular processes, such as cell death, evolution, and division, and how cells are affected by disease.
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