Editing Biosensor (section)

=== Optical biosensors ===
Many optical biosensors are based on the phenomenon of [[surface plasmon resonance]] (SPR) techniques.<ref>{{cite journal|author=S.Zeng|title=Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications|journal=Chemical Society Reviews|volume=43|pages=3426–3452|year=2014|doi=10.1039/C3CS60479A|last2=Baillargeat|first2=Dominique|last3=Ho|first3=Ho-Pui|last4=Yong|first4=Ken-Tye|pmid=24549396|issue=10|hdl=10356/102043|display-authors=etal|url=http://dr.ntu.edu.sg/bitstream/handle/10220/18851/Nanomaterials%20enhanced%20surface%20plasmon%20resonance%20for%20biological%20and%20chemical%20sensing%20applications.pdf?sequence=3|access-date=14 September 2015|archive-url=https://web.archive.org/web/20160106172212/https://dr.ntu.edu.sg/bitstream/handle/10220/18851/Nanomaterials%20enhanced%20surface%20plasmon%20resonance%20for%20biological%20and%20chemical%20sensing%20applications.pdf?sequence=3|archive-date=6 January 2016}}</ref><ref>{{cite journal|author=Krupin, O. |author2=Wang, C. |author3=Berini, P.|title=Optical plasmonic biosensor for leukemia detection|journal=SPIE Newsroom|issue=22 January 2016|doi=10.1117/2.1201512.006268|year=2016 }}</ref> This utilises a property of gold and other materials (metals);<ref>{{Cite journal |last1=Damborský |first1=Pavel |last2=Švitel |first2=Juraj |last3=Katrlík |first3=Jaroslav |date=2016-06-30 |title=Optical biosensors |journal=Essays in Biochemistry |volume=60 |issue=1 |pages=91–100 |doi=10.1042/EBC20150010 |issn=0071-1365 |pmc=4986466 |pmid=27365039}}</ref> specifically that a thin layer of gold on a high refractive index glass surface can absorb laser light, producing electron waves (surface plasmons) on the gold surface. This occurs only at a specific angle and wavelength of incident light and is highly dependent on the surface of the gold, such that binding of a target [[analyte]] to a receptor on the gold surface produces a measurable signal.

Surface plasmon resonance sensors operate using a sensor chip consisting of a plastic cassette supporting a glass plate, one side of which is coated with a microscopic layer of gold. This side contacts the optical detection apparatus of the instrument. The opposite side is then contacted with a microfluidic flow system. The contact with the flow system creates channels across which reagents can be passed in solution. This side of the glass sensor chip can be modified in a number of ways, to allow easy attachment of molecules of interest. Normally it is coated in carboxymethyl [[dextran]] or similar compound.

The refractive index at the flow side of the chip surface has a direct influence on the behavior of the light reflected off the gold side. Binding to the flow side of the chip has an effect on the [[refractive]] index and in this way biological interactions can be measured to a high degree of sensitivity with some sort of energy. The refractive index of the medium near the surface changes when biomolecules attach to the surface, and the SPR angle varies as a function of this change.

Light of a fixed wavelength is reflected off the gold side of the chip at the angle of total internal reflection, and detected inside the instrument. The angle of incident light is varied in order to match the evanescent wave propagation rate with the propagation rate of the surface plasmon polaritons.<ref>{{cite journal |author=Homola J |title= Present and future of surface plasmon resonance biosensors |journal= Anal. Bioanal. Chem. |volume=377 |issue=3 |pages=528–539 |year=2003 |doi=10.1007/s00216-003-2101-0|pmid= 12879189 |s2cid= 14370505 }}</ref> This induces the evanescent wave to penetrate through the glass plate and some distance into the liquid flowing over the surface.

Other optical biosensors are mainly based on changes in absorbance or fluorescence of an appropriate indicator compound and do not need a total internal reflection geometry. For example, a fully operational prototype device detecting casein in milk has been fabricated. The device is based on detecting changes in absorption of a gold layer.<ref>{{cite journal | last1 = Hiep | first1 = H. M. | display-authors = etal   | year = 2007 | title = A localized surface plasmon resonance based immunosensor for the detection of casein in milk | doi = 10.1016/j.stam.2006.12.010 | journal = Sci. Technol. Adv. Mater. | volume = 8 | issue = 4| pages = 331–338 |bibcode = 2007STAdM...8..331M | doi-access = free }}</ref> A widely used research tool, the micro-array, can also be considered a biosensor.