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Bio-sample analysis using AFM

(Photo courtesy:Catholic University of Chile)
Biological samples
AFM can be used for taking images of non-conducting surfaces of biological systems such as for analyzing the crystals of amino acids and organic monolayers, DNA and RNA analysis, protein-nucleic acid complexes, chromosomes, cellular membranes, proteins and peptides, molecular crystals, polymers and biomaterials and ligand-receptor binding. For the analysis bio-samples are investigated on lysine-coated glass and mica substrate, and in buffer solution. By using phase imaging technique one can distinguish the different components of the cell membranes.
Sample treatment
For bioimaging using AFM, sample preparation is simple like spotting a few micro liters of solution on mica or glass by avoiding surface contaminations. Initially the substrate-adsorbate is rinsed with a large excess of buffer followed by dialysis, centrifugation and homogenization. The samples are stabilized by adding covalent cross-linking agents or certain cations capable of linking the constituents of the sample to each other or to the substrate to get good contrast and to reduce mechanical damage.
Imaging of nucleic acids
Nanometer-resolved images of unmodified nucleic acids, chromosome mapping, transcription, translation and small molecule-DNA interactions such as intercalating mutagens need high-resolution studies and for such purposes highly reproducible images can be obtained using AFM. Very clear resolution of nucleic acids are needed to control of the local imaging environment including sample modification, to adopt tapping mode scanning techniques, to develop improved AFM probes such as standard silicon nitride probes modified by electron beam deposition and oxide sharpened nanoprobes and compatible substrates such as salinized mica and carbon coated mica.
Cell studies
AFM can be used to study the dynamic behavior of living and fixed cells such as red and white blood cells, bacteria, platelets, cardiac myocytes, living renal epithelial cells, and glial cells up to a resolution of only 20-50 nm, but not sufficient for resolving membrane proteins but still suitable for imaging other surface features, such as rearrangements of plasma membrane or movement of sub membrane filament bundles.
Study of small molecules
There has been recent success imaging individual proteins and other small molecules with the AFM such as collogen. Smaller molecules that do not have a high affinity for common AFM substrates have been successfully imaged by employing selective affinity binding procedures.
Nano range force measurement
A variety of biological processes such as DNA replication, protein synthesis, drug interaction, and many others are largely governed by intermolecular forces.
These forces are in the nanonewton range. AFM has the ability to measure such forces. This force measurement makes it possible to quantify the molecular interaction in biological systems such as a variety of important ligand-receptor interactions. Using AFM force measurements, electrical surface charge can be imaged and quantified as the dynamics of many biological systems depends on the electrical properties of the sample surface. The binding forces, electrostatic forces and the micromechanical properties specifically, elasticity and viscosity of biological samples ranging from live cells and membranes to bone and cartilage can be analyzed.

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