AFM Study of Surface Derivatization for Biosensor Applications
Optical biosensors are typically transducers that detect the presence of molecules at a surface. They have several desirable features, particularly for the detection of biological molecules: (1) they can be extremely sensitive (nanomoles or less) and (2) they are nondestructive to the sample. The optical properties or transduction process of fused silica quartz beads are governed by the surface condition of the beads. The AFM is ideal for direct visualization of quartz surfaces that are chemically treated. This application note demonstrates the usefulness of an AFM for studying chemically treated quartz surfaces.
Study of Derivatized Quartz Slides
In an effort to establish the viability of using an AFM for studying chemically treated quartz, several tests were made on quartz slides. The images below illustrate the results of experiments on streptavidin coated quartz slides.   AFM image of streptavidin on a fused quartz substrate. The right image is a zoomed in view (2.3 µm × 2.3 µm) of the image at the left (10 µm × 10 µm). These images were acquired in ambient air with the "close contact" mode. Study of Quartz Beads
Quartz beads used in this study were prepared at the end of a fused
quartz rod and mounted in the AFM instrument with a magnetic holder.
The configuration of the holder is illustrated below.
After the quartz bead holder is mounted in the AFM microscope, the
AFM probe is then positioned over the top of the glass bead. The
following image displays the cantilever being positioned over the
quartz bead.
Optical microscope view of the AFM cantilever at the top of the
Quartz sphere.
Contamination Study
The spheres were fire-polished but not pre-etched prior to coating. Polished spheres were immersed in 3% mercaptopropyltrimethoxy saline (MPTS) in methanol for 30-40 minutes under a nitrogen atmosphere. They were then removed into room air and washed 3-4 times with methanol. Images were measured directly after preparation and then after the sphere remained in ambient air for 5 hours.  AFM image of fused silica quartz slide directly after cleaning. The line profile shows a defect in the surface that is approximately 10 nm in height. Streaks in the image are a result of vibrations in the quartz rod used for mounting.  AFM image of quartz bead after exposure to air for 5 hours. The areas of contamination have heights of approximately 100 nm. Biotin and Streptavidin Coated Spheres
Quartz spheres are coated with both biotin and streptavidin and AFM images
of the spheres were collected. With the AFM it is possible to estimate the
percentage of coverage of the sphere.
Left - AFM image of streptavidin coated sphere showing approximately 9% coverage.
Right - AFM image of Biotin coated sphere with approx. 8% coverage.
Bacteria Coated Sphere
 This AFM image shows a quartz sphere coated with bacteria. The curvature of the sphere was removed from the image with image processing algorithms. About the Author
Dmitri Kossakovski completed this work as a member of the technical staff at the Jet Propulsion Lab (JPL) in Pasadena, CA. He is presently a senior member of the technical staff at OEWaves. Dr. Kossakovski earned his masters degree in biophysics from the Moscow Institute of Physics and Technology. He later earned a Ph.D. in physical chemistry from the California Institute of Technology. The full account of this research may be found in the SPIE proceedings of Photonics West 2002 conference, the paper titled "High-Q whispering-gallery mode sensor in liquids" by Jay L. Nadeau, Vladimir S. Iltchenko, Dmitri Kossakovski, Gregory H. Bearman, Lute Maleki.
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