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Posted on Thu, Jan 03, 2019

Biodetection and biosensing

Biodetection is a general term that encompasses the global strategies in place for the detection of biological threats. Biological threats are pathogens, infectious disease, and biological weapons that can infect significant populations of humans and to which we have little innate immunity or defense against. Wee must improve our ability to detect the infectious pathogens at the earliest sign of an outbreak. This will be accomplished by improving our methods of biodetection by developing more sensitive and portable biosensing devices. The use of bioassays in clinical laboratories are standardized and validated to improve the accuracy and speed of pathogen detection and disease diagnosis. New technologies are being developed to integrate biodetection platforms with smartphone devices and extend the sensing range to the hands of ordinary individuals.

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Biodetection of pathogens and infectious diseases

The most commonly used bioassay for the detection of pathogens is the Enzyme Linked ImmunoSorbant Assay (ELISA). As indicated by the name, this technique takes advantage of the unique biochemical affinity between antibodies and the antigen proteins and molecules that they identify like a lock and key. The antigen can be a lipopolysaccharide on a bacterial cell wall, an endotoxin released by botulism, a glycoprotein on the surface of an Influenza or Ebola virus, or even an increase in circulating tumor cells with identifying surface proteins. These tell-tale markers must first be discovered by the work of basic research scientists before they can be incorporated into bioassays. The ELISA is an example of a labeled assay, which means that a tag is necessary for the identification and quantification of a target antigen. The first tags used in immunoassays were radiometric, but these have largely been replaced by safer fluorescent, chemiluminescent, or colorimetric systems.

Label free assays for biodetection

The ELISA is still a gold standard for clinical biodetection, but newer label-free assays may soon gain traction. Examples of label free assays include Surface Plasmon Resonance (SPR), Interferometric techniques, and ring resonator waveguide platforms. The common thread running through all of these technologies is the use of light to detect changes in the refractive indices of materials. All materials, including proteins and antigens, have a property called refractive index, which is simply a value that indicates the amount that light bends, or refracts, when it encounters the material. When a detection molecule, commonly an antibody, is conjugated to the sensing surface, the device is calibrated to consider the antibody layer as a baseline condition. If a target antigen is present in a biological sample, then it will bind to the antibodies on the sensing layer via non-covalent interactions. The amount of antigen bound to the sensing surface can be quantified by measuring the change in refractive index of the surface as the antigen binds to antibody. This can be measured as a change in the angle of reflected light, or a change in absorbed wavelength, or as an increase or decrease in the intensity of light reflected off of the sensing surface.

The label-free bioassays have the benefit of a simpler workflow, a more direct detection mechanism, and greater sensitivity. Additionally, as the methods for creating waveguides on silicon chips and coupling defined wavelengths of light through fiber optic cables improve, the cost of these bioassays will decrease steadily. Since these devices will be chip-based, it is plausible that easy-to-use, portable, and inexpensive platforms will be easily integrated with smartphones to establish universal biodetection platforms in the hands of ordinary citizens. The ability to detect biological threats will rapidly increase once the technology leaves the confines of the laboratory.

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