2, Universidad Autónoma de Madrid, Madrid, , Spain
3, Universidad de Zaragoza, Zaragoza, , Spain
Atomic Force Microscopy (AFM) is a powerful technique in biophysics, nanomedicine since it allows imaging and manipulating nanostructures in physiological conditions on a single molecule level .
Magnetic Force Microscopy (MFM) is an Atomic Force-based technique where a nanometric magnetic probe is raster –scanned in close proximity to a surface detecting the local magnetic fields near the surface. MFM has been applied to the study of a variety of magnetic systems, including magnetic nanoparticles (MNPs)  but always in vacuum or atmospheric conditions.
Nowadays, magnetic nanostructures play important roles in different fields such as medicine, biology or catalysis. For example, the use of MNPs is growing a lot of attention for its potential applications that include therapeutic drug, contrast enhancement agents or methods for the catabolism of tumours via hyperthermia. Many studies as well are focused on the encapsulation of magnetic particles in different biological entities as bacteria or virus like particles .
Despite the importance of measuring magnetic nanostructures with biological applications in physiological conditions, the applicability of MFM to these systems was limited up to now because of the difficulty in developing MFM for detecting magnetic interactions in liquids. This is a consequence of the high damping forces of the cantilever, which are several times greater than in air. These damping forces are the origin of the low-quality factor (Q) of the cantilever resonance characteristic of liquid measurements, which results in a significant loss of sensitivity in the MFM signal. The work presented here introduces the development of MFM imaging in liquid media and discusses its potential for detecting and imaging nanoscale magnetic domains in biological samples .
MFM studies in liquids have been carried out using commercial probes, being able to detect magnetic signals even from a single 30 nm Fe3O4 nanoparticle. To optimize the magnetic contrast in liquid, 3D modes analysis was done for different dynamic modes. By working in Drive Amplitude Modulation DAM-AFM  the tip is able to detect the magnetic interaction closer to the sample, which translates into a higher magnetic signal.
A study of fundamental MFM noise shows that further improvement on the performance could be gained using specially designed cantilevers for liquid media. Here we present the development of new magnetic probes by following different routes. These special magnetic cantilevers allow imaging magnetic signals in liquid media with similar quality of images as in air ambient conditions.
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