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Atomic Force Microscopy and Its Application in Biological Research

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What is AFM

Atomic force microscope (AFM) is a kind of scanning probe microscope (SPM) that came out in the early 1980s. In 1986, Dr. Binning won the Nobel Prize in Physics for his invention of the scanning probe microscope. The magnification of this microscope far exceeds any previous microscope: the magnification of optical microscopes generally does not exceed 1000 times; the magnification limit of electron microscopes is 1 million times; and the magnification of atomic force microscopes can be as high as 1 billion times The resolution of the electron microscope is 1000 times higher, and it can directly observe the molecules and atoms of substances, which provides an ideal tool for human beings to further explore the microscopic world.


Advantages of AFM

First, the sample preparation of the atomic force microscope technique is simple and does not require special treatment of the sample, so it is much less destructive than other commonly used techniques in biology (such as electron microscopy); Second, the atomic force microscope can operating in a variety of environments (including air, liquid, and vacuum), biomolecules can be directly imaged under physiological conditions, and real-time dynamic observations of living cells can also be performed; Third, atomic force microscopy (AFM) can provide biomolecules and biosurfaces 3D images with molecular/submolecular resolution; Fourth, atomic force microscopy can observe local charge density and physical properties with nanometer-scale resolution, and measure the interaction force between molecules (such as receptors and ligands) ; Fifth, atomic force microscopy can manipulate individual biomolecules; in addition, the information obtained by atomic force microscopy can also complement other analytical techniques and microscopy techniques.


AFM probe

AFM probes are basically prepared by processing Si or Si3N4 with MEMS technology. The probe tip radius is generally 10 to tens of nm. Cantilevers are usually fabricated from a silicon or silicon nitride wafer typically 100–500 μm long and approximately 500 nm–5 μm thick. A typical silicon microcantilever is about 100 μm long, 10 μm wide, and a few microns thick.


Microscopes for various application fields have been developed using various interaction forces between the probe and the sample, such as AFM (van der Far force), electrostatic force microscope (EFM), magnetic force microscope (MFM), Lateral Force Microscope (LFM), etc., so there are corresponding probes for different types of microscopes.


Common AFM probe types

Non-contact/tapping mode tips and contact mode probes

The most commonly used products with high resolution and average lifetime. The probe is constantly worn during use, and the resolution is easily degraded. Mainly used in surface morphology observation.


Conductive probe

It is obtained by plating 10-50 nanometers thick Pt (and other metals that improve the bonding force of the coating, such as Cr, Ti, Pt and Ir, etc.) on ordinary probes. Conductive probes are used in EFM, KFM, SCM, etc. The resolution of conductive probes is worse than that of tapping and contact mode probes, and the conductive coating is easy to fall off during use, and the conductivity is difficult to maintain for a long time. The new products of conductive tips include carbon nanotube tips, diamond-coated tips, all-diamond tips, and all-wire tips. These new technologies overcome the shortcomings of short life and low resolution of ordinary conductive tips.


Magnetic probe

Applied to MFM, it is prepared by plating Co, Fe and other ferromagnetic layers on the probes of common tapping and contact modes. The resolution is worse than that of common probes, and the conductive coating is easy to fall off when used.


Large aspect ratio probe

The large aspect ratio tip is specially designed and produced for measuring deep grooves and approximately vertical sides. Features: Not commonly used products, high resolution, average service life. Technical parameters: tip height > 9μm; aspect ratio 5:1; tip radius < 10 nm.


Diamond-like carbon AFM probe/all-diamond probe

One is to add a layer of diamond-like carbon film on the tip of the silicon probe, and the other is made of all-diamond material (the price is very high). These two diamond carbon probes have great durability, reducing tip wear and increasing service life.