Magnetic Resonance Force Microscopy
Encyclopedia
Magnetic resonance force microscopy (MRFM) is an imaging technique that acquires magnetic resonance images (MRI) at nanometer scales, and possibly at atomic scales in the future. MRFM is potentially able to observe protein
structures which cannot be seen using X-ray crystallography
and protein nuclear magnetic resonance spectroscopy
. Detection of the magnetic spin
of a single electron
has been demonstrated using this technique. The sensitivity
of a current MRFM microscope is 10 billion times better than a medical MRI used in hospitals.
(MRI) and atomic force microscopy (AFM). Conventional MRI employs an inductive coil
as an antenna
to sense resonant nuclear or electronic spins in a magnetic field
gradient. MRFM uses a cantilever
tipped with a ferromagnetic (iron cobalt) particle to directly detect a modulated spin gradient force between sample spins and the tip. As the ferromagnetic tip moves close to the sample, the atoms’ nuclear spins become attracted to it and generate a small force on the cantilever. The spins are then repeatedly flipped, causing the cantilever to gently sway back and forth in a synchronous motion. That displacement is measured with an interferometer (laser beam) to create a series of 2-D images of the sample, which are combined to generate a 3-D image. The interferometer measures resonant frequency of the cantilever. Smaller ferromagnetic particles and softer cantilevers increase the signal to noise ratio. Unlike the inductive coil approach, MRFM sensitivity scales favorably as device and sample dimensions are reduced.
Because the signal to noise ratio is inversely proportional to the sample size, Brownian motion
is the primary source of noise at the scale in which MRFM is useful. Accordingly, MRFM devices are cryogenically cooled. MRFM was specifically devised to determine the structure of proteins in situ.
Almaden Research Center
with 1-μm vertical resolution and 5-μm lateral resolution using a bulk sample of the paramagnetic substance diphenyipicrylhydrazil. The spatial resolution reached nanometer-scale in 2003. Detection of the magnetic spin of a single electron was achieved in 2004. In 2009 researchers at IBM and Stanford announced that they had achieved resolution of better than 10 nanometers, imaging tobacco mosaic virus particles on a nanometer-thick layer of adsorbed
hydrocarbons.
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
structures which cannot be seen using X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
and protein nuclear magnetic resonance spectroscopy
Protein nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance spectroscopy of proteins is a field of structural biology in which NMR spectroscopy is used to obtain information about the structure and dynamics of proteins. The field was pioneered by Richard R. Ernst and Kurt Wüthrich, among others...
. Detection of the magnetic spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
of a single electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
has been demonstrated using this technique. The sensitivity
Sensitivity (electronics)
The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.Sensitivity is...
of a current MRFM microscope is 10 billion times better than a medical MRI used in hospitals.
Basic principle
The MRFM concept combines the ideas of magnetic resonance imagingMagnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
(MRI) and atomic force microscopy (AFM). Conventional MRI employs an inductive coil
Coil
A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
as an antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
to sense resonant nuclear or electronic spins in a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
gradient. MRFM uses a cantilever
Cantilever
A cantilever is a beam anchored at only one end. The beam carries the load to the support where it is resisted by moment and shear stress. Cantilever construction allows for overhanging structures without external bracing. Cantilevers can also be constructed with trusses or slabs.This is in...
tipped with a ferromagnetic (iron cobalt) particle to directly detect a modulated spin gradient force between sample spins and the tip. As the ferromagnetic tip moves close to the sample, the atoms’ nuclear spins become attracted to it and generate a small force on the cantilever. The spins are then repeatedly flipped, causing the cantilever to gently sway back and forth in a synchronous motion. That displacement is measured with an interferometer (laser beam) to create a series of 2-D images of the sample, which are combined to generate a 3-D image. The interferometer measures resonant frequency of the cantilever. Smaller ferromagnetic particles and softer cantilevers increase the signal to noise ratio. Unlike the inductive coil approach, MRFM sensitivity scales favorably as device and sample dimensions are reduced.
Because the signal to noise ratio is inversely proportional to the sample size, Brownian motion
Brownian motion
Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...
is the primary source of noise at the scale in which MRFM is useful. Accordingly, MRFM devices are cryogenically cooled. MRFM was specifically devised to determine the structure of proteins in situ.
Milestones
The basic principles of MRFM imaging and the theoretical possibility of this technology were first described in 1991. The first MRFM image was obtained in 1993 at the IBMIBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
Almaden Research Center
Almaden Research Center
The IBM Almaden Research Center is in San Jose, California, and is one of IBM's nine worldwide research labs. Its scientists perform basic and applied research in computer science, services, storage systems, physical sciences, and materials science and technology. The center opened in 1986, and...
with 1-μm vertical resolution and 5-μm lateral resolution using a bulk sample of the paramagnetic substance diphenyipicrylhydrazil. The spatial resolution reached nanometer-scale in 2003. Detection of the magnetic spin of a single electron was achieved in 2004. In 2009 researchers at IBM and Stanford announced that they had achieved resolution of better than 10 nanometers, imaging tobacco mosaic virus particles on a nanometer-thick layer of adsorbed
hydrocarbons.
External links
- University of Washington Quantum System Engineering and MRFM Home Page, http://courses.washington.edu/goodall/MRFM/.
- Magnetic-Resonance Force Microscopy, http://www.medgadget.com/archives/2005/04/magneticresonan.html.
- Review Article: M. Poggio and C. L. Degen, Nanotechnology 21, 342001 (2010),