Optical magnetic microscope for high-resolution, wide-field cell imaging

Apr 25, 2013 by John Hewitt report
Magnetotactic bacterium. Credit: Imperial College London

(Phys.org) —Nano-sized crystals of magnetic material can be found in a wide variety of organisms. Among the most studied are magnetotactic bacteria, which can orient and navigate using biosynthesized magnetosomes. These organelles are made from iron oxide or iron sulphide particles and may eventually have many therapeutic applications. When used in conjunction with new MRI-based methods, they can potentially be used as contrast agents or magnetic labels in imaging, or for drug delivery and local hyperthermic heating in treatment. A better understanding of how these magnetosomes are biomineralized, and how the three-dimensional fields that originate from them are structured is essential to achieve these goals. A new report this week in Nature describes an optical magnetic imaging technique that can be used to map magnetic field variations on the nanoscale. The author's wide-field microscopy technique allows parallel optical and magnetic imaging of many individual cells across a field of 100 microns or more.

The general principles of the magnetic imaging technique used here have been known for some time. The basic concept is to detect changes in the quantum spin states of crystallographic defects, known as nitrogen-vacancy centers, within a diamond chip. The new Nature study is the first to do this for a living creature. The rig put together by the authors is probably one of the more complicated sets of probing devices you are likely ever to see. The bacteria sit above the diamond surface, where the fields from the magnetosomes affect characteristic signals ( resonances) of the nitrogen-vacancy centers. An optical beam is used to readout those signals, and the vector components in each direction of the magnetic field can later be reconstructed.

The used was Magnetospirillum magneticum AMB-1, which forms cubo-octahedral roughly 50 nm in diameter. The fields produced by these bacteria could be imaged to a resolution of 400nm. The optical images of the field distributions were collected using a sCMOS camera across a field of view 100um by 30um. Bright field images were concurrently acquired using 660nm red LED back-illumination to link cell features with the magnetic patterns. Fluorescence excitation to asses bacterial viability was done through the microscope objective using 470nm illumination.

In addition to the geometrical tangle of all the required optical elements, the setup also needed to incorporate a permanent magnetic field for proper operation. This which applied by a nearby permanent magnet. The final element necessary was the appropriate application of microwave energy. The microwaves were applied to the diamond surface from an overlying wire connected to the output of a synthesizer.

Beyond just taking snapshots of field distributions, the setup also has the potential for dynamic imaging of field changes across the development cycle of the magnetosome crystals. Other magnetic imaging techniques, like SQUID and electron holography, might eventually be similarly adapted to yield even higher spatial resolutions then the techniques used here.

Explore further: Could 'Jedi Putter' be the force golfers need?

More information: Optical magnetic imaging of living cells, Nature 496, 486–489 (25 April 2013) doi:10.1038/nature12072

Magnetic imaging is a powerful tool for probing biological and physical systems. However, existing techniques either have poor spatial resolution compared to optical microscopy and are hence not generally applicable to imaging of sub-cellular structure (for example, magnetic resonance imaging), or entail operating conditions that preclude application to living biological samples while providing submicrometre resolution (for example, scanning superconducting quantum interference device microscopy, electron holography and magnetic resonance force microscopy). Here we demonstrate magnetic imaging of living cells (magnetotactic bacteria) under ambient laboratory conditions and with sub-cellular spatial resolution (400 nanometres), using an optically detected magnetic field imaging array consisting of a nanometre-scale layer of nitrogen–vacancy colour centres implanted at the surface of a diamond chip. With the bacteria placed on the diamond surface, we optically probe the nitrogen–vacancy quantum spin states and rapidly reconstruct images of the vector components of the magnetic field created by chains of magnetic nanoparticles (magnetosomes) produced in the bacteria. We also spatially correlate these magnetic field maps with optical images acquired in the same apparatus. Wide-field microscopy allows parallel optical and magnetic imaging of multiple cells in a population with submicrometre resolution and a field of view in excess of 100 micrometres. Scanning electron microscope images of the bacteria confirm that the correlated optical and magnetic images can be used to locate and characterize the magnetosomes in each bacterium. Our results provide a new capability for imaging bio-magnetic structures in living cells under ambient conditions with high spatial resolution, and will enable the mapping of a wide range of magnetic signals within cells and cellular networks.

Commentary: Magnetic bacteria on a diamond plate, Mihály Pósfai & Rafal E. Dunin-Borkowski, Nature 496, 442–443 (25 April 2013) doi:10.1038/496442a

Related Stories

Nanoscale MRI being developed

Feb 01, 2013

(Phys.org)—Two independent groups of scientists in the U.S. and Germany have reduced magnetic resonance imaging (MRI) down to the nanoscale, which may enable them in the future to non-destructively detect ...

New study may lead to MRIs on a nanoscale

Feb 23, 2012

(PhysOrg.com) -- Magnetic resonance imaging (MRI) on the nanoscale and the ever-elusive quantum computer are among the advancements edging closer toward the realm of possibility, and a new study co-authored ...

Birds migrate using magnetic map

Feb 07, 2013

Migrating birds use magnetic particles within their body to create a 'map' with which to navigate using the earth's magnetic field, according to new research published today in Journal of the Royal Society In ...

Recommended for you

Could 'Jedi Putter' be the force golfers need?

Apr 18, 2014

Putting is arguably the most important skill in golf; in fact, it's been described as a game within a game. Now a team of Rice engineering students has devised a training putter that offers golfers audio, ...

Better thermal-imaging lens from waste sulfur

Apr 17, 2014

Sulfur left over from refining fossil fuels can be transformed into cheap, lightweight, plastic lenses for infrared devices, including night-vision goggles, a University of Arizona-led international team ...

User comments : 0

More news stories

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

Filipino tests negative for Middle East virus

A Filipino nurse who tested positive for the Middle East virus has been found free of infection in a subsequent examination after he returned home, Philippine health officials said Saturday.

Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...