Oriented Immobilization of Antibodies
Antibodies are the most commonly used biological capture agents for biosensing. Most common method for attaching antibodies is either through adsorption to the surface or by covalent attachment using lysines at the protein surface. Both these methods result in randomly attached antibodies with suboptimal functionality. Oriented attachment of the antibodies can significantly increase the sensitivity of the biosensor as reported in my previous post. Now a recent report from National Cancer Institute uses molecular biology approach to express site specifically modified antibodies in mammalian cell expression system. These antibodies are modified with selenocysteine at C-terminus of heavy chain. Selenocysteine modified antibodies will react with electrophilic reactive groups like iodoacetamide and maleimide. Any surface presenting these electrophilic reactice groups can be used for oriented capture of antibodies. Biosensors designed using these molecularly defined reactive antibodies should significantly enhance biosensor performance.
In addition to protein orientation, the amount of protein at the surface is also critical for design of sensitive biosensors. Hydrogels, 3D polymer matrix and nanostructured surfaces have been shown to increase the biosensor sensitivity and dynamic range. Now a new method for creating hyperbranched polymer opens up another avenue to increase the protein density at the surface.
The new method termed Thiol-Yne chemistry is easy to implement since it occurs at room temperature and is rapid. In addition various functionalities can be added to the final product for attaching proteins. It is easy to foresee a hyperbranched polymer attached to a biosensor surface that can attach multiple biological molecules for high sensitive detection of analytes.
Paper Sensor as Biosensors
Whitesides and his group at Harvard are pioneering the paper diagnostics that combines microfluidics and lateral flow immunoassay approaches for designing cheap POC devices for the masses. An alternate approach uses SWNTs (Single walled cacrbon nanotubes) coated paper to create conducting biosensors. The conducting paper sensor utilizes dependence of electrical conductivity through nanotube coated paper on the width of nanotube-nanotube tunneling gap. Paper-biosensor was created by mixing the antibodies, SWNTs, a negatively charged polymer (poly(sodium 4-styrene-sulfonate)) and water then dip-coating a regular analytical filtration paper. Paper-biosensor was freeze dried for stability. In the presence of analyte the immune complex of antibody and analyte spreads apart the SWNTs hence reducing the current passing through the material. Decrease in current is proportional to the analyte in the sample. Sensor was able to detect microcystins toxins produced by cyanobacteria in lake water. Paper sensors are certainly on roll!