Magnetic Force Microscopy for Nanoparticle Characterization

In Chapter 8 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss Biomagnetism. The 5th edition of IPMB contains a brief new section on magnetic nanoparticles

8.8.4 Magnetic Nanoparticles

Small single-domain nanoparticles (10-70 nm in diameter) are used to treat cancer (Jordan et al. 1999; Pankhurst et al. 2009). The particles are injected into the body intravenously. Then an oscillating magnetic field is applied. It causes the particles to rotate, heating the surrounding tissue. Cancer cells are particularly sensitive to damage by hyperthermia. Often the surface of the nanoparticles can be coated with antibodies that cause the nanoparticle to be selectively taken up by the tumor, providing more localized heating of the cancer.

Suppose you wanted to image the distribution of nanoparticles in tissue. How would you do it? In IPMB we describe several ways to map magnetic field distributions, including using a superconducting quantum interference device (SQUID) magnetometer. Is there a way to get even better spatial resolution than using a SQUID? Gustavo Cordova, Brenda Yasie Lee, and Zoya Leonenko recently published a review in the NanoWorld Journal describing Magnetic Force Microscopy for Nanoparticle Characterization (Volume 2, Pages 10-14, 2016). Their abstract states

“Since the invention of the atomic force microscope (AFM) in 1986, there has been a drive to apply this scanning probe technique or a form of this technique to various disciplines in nanoscale science. Magnetic force microscopy (MFM) is a member of a growing family of scanning probe methods and has been widely used for the study of magnetic materials. In MFM a magnetic probe is used to raster-scan the surface of the sample, of which its magnetic field interacts with the magnetic tip to offer insight into its magnetic properties. This review will focus on the use of MFM in relation to nanoparticle characterization, including superparamagnetic iron oxide nanoparticles, covering MFM imaging in air and in liquid environments.”

Figure 1 from their paper shows how the MFM “two-pass technique” works: a first scan produces a topographical image using an atomic force microscope, and then a second pass creates a magnetic image using a magnetic force microscope. Both the AFM and MFM are based on using a small, nanometer sized scanning tip attached to a cantilever to detect small-scale tip deflections.

Their Figure 2 shows the results of MFM imaging of superparamagnetic iron oxide nanoparticles (SPIONs). The technique has sufficient sensitivity that they can measure how the size distribution of SPIONs depends on the particle coating.

Cordova et al. conclude

“Considering rapid development of novel applications of magnetic nanoparticles in medicine and biomedical nanotechnology, as therapeutic agents, contrast agents in MRI imaging and drug delivery [3] MFM characterization of nanoparticles becomes more valuable and desirable. Overall, MFM has proven itself to be an effective yet underused tool that offers great potential for the localization and characterization of magnetic nanoparticles.”

The magnetic force microscope does not have the exquisite picotesla sensitivity and millisecond time resolution of a SQUID, and it requires access to the tissue surface so you can scan over it. But for static, strong-field systems such as magnetic nanoparticle distributions, the magnetic force microscope provides exceptional spatial resolution, and represents one more tool in the physicists arsenal for imaging magnetic fields in biology.