Editing Biosensor (section)

==Biotransducer==
{{main|Biotransducer}}
[[File:Biosensors based on biotransducers.png|thumb|Classification of biosensors based on type of biotransducer]]
Biosensors can be classified by their [[biotransducer]] type. The most common types of biotransducers used in biosensors are:

* electrochemical biosensors
* optical biosensors
* electronic biosensors
* piezoelectric biosensors
* gravimetric biosensors
* pyroelectric biosensors
* magnetic biosensors

===Electrochemical===
Electrochemical biosensors, based on enzymes, work through the enzymatic catalysis of reactions that directly or indirectly produce or consume electrons (such enzymes are rightly called [[Oxidoreductase|redox enzymes]]). The sensor design usually consists of three [[electrode]]s; a [[reference electrode]], a working electrode, and a counter electrode. The target analyte is involved in the reaction that takes place on the surface of the active working electrode, and the reaction may cause either electron transfer across the [[Double_layer_(surface_science)|double layer]] (producing a current) or can contribute to the double layer potential (producing a voltage). The current (rate of flow of electrons is now proportional to the analyte concentration) can be measured at a fixed potential or the potential can be measured at zero current (this gives a logarithmic response). Note that potential of the working electrode is space charge sensitive and this is often used. Additionally, the label-free and direct electrical detection of small peptides and proteins is possible by their intrinsic charges using [[Bio-FET|biofunctionalized]] [[ISFET|ion-sensitive]] [[field-effect transistors]].<ref>{{cite journal | last1 = Lud | first1 = S.Q. | last2 = Nikolaides | first2 = M.G. | last3 = Haase | first3 = I. | last4 = Fischer | first4 = M. | last5 = Bausch | first5 = A.R. | year = 2006 | title = Field Effect of Screened Charges: Electrical Detection of Peptides and Proteins by a Thin Film Resistor | journal = ChemPhysChem | volume = 7 | issue = 2| pages = 379–384 | doi = 10.1002/cphc.200500484 | pmid = 16404758 }}</ref>

Another example, the potentiometric biosensor, (potential produced at zero current) gives a logarithmic response with a high dynamic range. Such biosensors are often made by screen printing the electrode patterns on a plastic substrate, coated with a conducting polymer and then some protein (enzyme or antibody) is attached. They have only two electrodes and are extremely sensitive and robust. They enable the detection of analytes at levels previously only achievable by HPLC and LC/MS and without rigorous sample preparation. All biosensors usually involve minimal sample preparation as the biological sensing component is highly selective for the analyte concerned. The signal is produced by electrochemical and physical changes in the conducting polymer layer due to changes occurring at the surface of the sensor. Such changes can be attributed to ionic strength, pH, hydration and redox reactions, the latter due to the enzyme label turning over a substrate.<ref>{{Cite web | url=http://www.universalsensors.co.uk/ |archive-url = https://web.archive.org/web/20141218190512/http://universalsensors.co.uk/|archive-date = 18 December 2014|title = Multivitamine Kaufberatung: So finden Sie das beste Präparat}}</ref> Field effect transistors, in which the [[metal gate|gate]] region has been modified with an enzyme or antibody, can also detect very low concentrations of various analytes as the binding of the analyte to the gate region of the FET cause a change in the drain-source current.

Impedance spectroscopy based biosensor development has been gaining traction nowadays and many such devices / developments are found in the academia and industry. One such device, based on a 4-electrode electrochemical cell, using a nanoporous alumina membrane, has been shown to detect low concentrations of human alpha thrombin in presence of high background of serum albumin.<ref>{{Cite journal |doi = 10.1016/j.bios.2018.10.010|pmid = 30396022|pmc = 6383723|title = Label free thrombin detection in presence of high concentration of albumin using an aptamer-functionalized nanoporous membrane|journal = Biosensors and Bioelectronics|volume = 126|pages = 88–95|year = 2019|last1 = Gosai|first1 = Agnivo|last2 = Hau Yeah|first2 = Brendan Shin|last3 = Nilsen-Hamilton|first3 = Marit|last4 = Shrotriya|first4 = Pranav}}</ref> Also interdigitated electrodes have been used for impedance biosensors.<ref>{{cite journal |last1=Sanguino |first1= P.|last2 = Monteiro |first2=T.|last3= Bhattacharyya |first3=S.R.|last4= Dias |first4=C.J.|last5= Igreja |first5=R.|last6= Franco |first6=R. |title = ZnO nanorods as immobilization layers for Interdigitated Capacitive Immunosensors |journal = Sensors and Actuators B-Chemical |volume = 204 |pages = 211–217 |year = 2014|doi= 10.1016/j.snb.2014.06.141|bibcode= 2014SeAcB.204..211S}}</ref>

===Ion channel switch===
[[File:wiki ics-a.jpg|right|thumb|130px|ICS – channel open]] [[File:wiki ics-b.jpg|right|thumb|130px|ICS – channel closed]]

The use of ion channels has been shown to offer highly sensitive detection 
of target biological molecules.<ref name=Vockenroth2005>{{cite book |vauthors=Vockenroth I, Atanasova P, Knoll W, Jenkins A, Köper I |title= IEEE Sensors, 2005 |chapter= Functional tethered bilayer membranes as a biosensor platform |pages=608–610 |year=2005 |doi= 10.1109/icsens.2005.1597772|isbn= 978-0-7803-9056-0 |s2cid= 12490715 }}</ref> By embedding the ion channels in supported or [[Model lipid bilayer|tethered bilayer membranes]] (t-BLM) attached to a gold electrode, an electrical circuit is created. Capture molecules such as antibodies can be bound to the ion channel so that the binding of the target molecule controls the ion flow through the channel. This results in a measurable change in the electrical conduction which is proportional to the concentration of the target.

An ion channel switch (ICS) biosensor can be created using gramicidin, a dimeric peptide channel, in a tethered bilayer membrane.<ref name=Cornell1997>{{cite journal |author=Cornell BA |title= A biosensor that uses ion-channel switches |journal=Nature |volume=387 |issue=6633 |pages=580–583 |year=1997 |doi=10.1038/42432 |pmid=9177344 |author2=BraachMaksvytis VLB |author3=King LG |display-authors=3 |last4=Osman |first4=P. D. J. |last5=Raguse |first5=B. |last6=Wieczorek |first6=L. |last7=Pace |first7=R. J. |bibcode = 1997Natur.387..580C |s2cid= 4348659 }}</ref> One peptide of gramicidin, with attached antibody, is mobile and one is fixed. Breaking the dimer stops the ionic current through the membrane. The magnitude of the change in electrical signal is greatly increased by separating the membrane from the metal surface using a hydrophilic spacer.

Quantitative detection of an extensive class of target species, including proteins, bacteria, drug and toxins has been demonstrated using different membrane and capture configurations.<ref name=Oh2008>{{cite journal |author=Oh S |title=Rapid detection of influenza A virus in clinical samples using an ion channel switch biosensor |journal=Biosensors & Bioelectronics |volume=23 |issue=7 |pages=1161–1165 |year=2008 |doi=10.1016/j.bios.2007.10.011 |pmid=18054481 |author2=Cornell B |author3=Smith D |display-authors=3 |last4=Higgins |first4=G. |last5=Burrell |first5=C.J. |last6=Kok |first6=T.W. }}</ref><ref name=Krishnamurthy2010>{{cite journal |vauthors=Krishnamurthy V, Monfared S, Cornell B |title= Ion Channel Biosensors Part I Construction Operation and Clinical Studies|journal=[[IEEE Transactions on Nanotechnology]] |volume=9 |issue=3 |pages=313–322  |year=2010 |doi=  10.1109/TNANO.2010.2041466|bibcode = 2010ITNan...9..313K |s2cid= 4957312}}</ref> The European research project [https://projects.leitat.org/greensense/ Greensense] develops a biosensor to perform quantitative screening of drug-of-abuse such as THC, morphine, and cocaine <ref>{{Cite web|url=https://www.greensense-project.eu/|title=Greensense Projekt: Cannabis-Tests und Drogen Screening|website=greensense-project.eu}}</ref> in saliva and urine.

===Reagentless fluorescent biosensor===
A reagentless biosensor can monitor a target analyte in a complex biological mixture without additional reagent. Therefore, it can function continuously if immobilized on a solid support. A fluorescent biosensor reacts to the interaction with its target analyte by a change of its fluorescence properties. A Reagentless Fluorescent biosensor (RF biosensor) can be obtained by integrating a biological receptor, which is directed against the target analyte, and a [[solvatochromism|solvatochromic]] fluorophore, whose emission properties are sensitive to the nature of its local environment, in a single macromolecule. The fluorophore transduces the recognition event into a measurable optical signal. The use of extrinsic fluorophores, whose emission properties differ widely from those of the intrinsic fluorophores of proteins, tryptophan and tyrosine, enables one to immediately detect and quantify the analyte in complex biological mixtures. The integration of the fluorophore must be done in a site where it is sensitive to the binding of the analyte without perturbing the affinity of the receptor.

Antibodies and artificial families of Antigen Binding Proteins (AgBP) are well suited to provide the recognition module of RF biosensors since they can be directed against any antigen (see the paragraph on bioreceptors). A general approach to integrate a solvatochromic fluorophore in an AgBP when the atomic structure of the complex with its antigen is known, and thus transform it into a RF biosensor, has been described.<ref name="pmid19945965"/> A residue of the AgBP is identified in the neighborhood of the antigen in their complex. This residue is changed into a cysteine by site-directed mutagenesis. The fluorophore is chemically coupled to the mutant cysteine. When the design is successful, the coupled fluorophore does not prevent the binding of the antigen, this binding shields the fluorophore from the solvent, and it can be detected by a change of fluorescence. This strategy is also valid for antibody fragments.<ref>{{cite journal|last1=Renard|first1=M|last2=Belkadi|first2=L|last3=Hugo|first3=N|last4=England|first4=P|last5=Altschuh|first5=D|last6=Bedouelle|first6=H|title=Knowledge-based design of reagentless fluorescent biosensors from recombinant antibodies|journal=J Mol Biol|date=Apr 2002|volume=318|issue=2|pages=429–442|doi=10.1016/S0022-2836(02)00023-2|pmid=12051849}}</ref><ref>{{cite journal|last1=Renard|first1=M|last2=Bedouelle|first2=H|title=Improving the sensitivity and dynamic range of reagentless fluorescent immunosensors by knowledge-based design|journal=Biochemistry|date=Dec 2004|volume=43|issue=49|pages=15453–15462|doi=10.1021/bi048922s|pmid=15581357|citeseerx=10.1.1.622.3557|s2cid=25795463}}</ref>

However, in the absence of specific structural data, other strategies must be applied. Antibodies and artificial families of AgBPs are constituted by a set of hypervariable (or randomized) residue positions, located in a unique sub-region of the protein, and supported by a constant polypeptide scaffold. The residues that form the binding site for a given antigen, are selected among the hypervariable residues. It is possible to transform any AgBP of these families into a RF biosensor, specific of the target antigen, simply by coupling a solvatochromic fluorophore to one of the hypervariable residues that have little or no importance for the interaction with the antigen, after changing this residue into cysteine by mutagenesis. More specifically, the strategy consists in individually changing the residues of the hypervariable positions into cysteine at the genetic level, in chemically coupling a solvatochromic fluorophore with the mutant cysteine, and then in keeping the resulting conjugates that have the highest sensitivity (a parameter that involves both affinity and variation of fluorescence signal).<ref name="pmid21565483"/> This approach is also valid for families of antibody fragments.<ref>{{cite journal|last1=Renard|first1=M|last2=Belkadi|first2=L|last3=Bedouelle|first3=H|title=Deriving topological constraints from functional data for the design of reagentless fluorescent immunosensors|journal=J. Mol. Biol.|date=Feb 2003|volume=326|issue=1|pages=167–175|doi=10.1016/S0022-2836(02)01334-7|pmid=12547199}}</ref>

A posteriori studies have shown that the best reagentless fluorescent biosensors are obtained when the fluorophore does not make non-covalent interactions with the surface of the bioreceptor, which would increase the background signal, and when it interacts with a binding pocket at the surface of the target antigen.<ref>{{cite journal|last1=de Picciotto|first1=S|last2=Dickson|first2=PM|last3=Traxlmayr|first3=MW|last4=Marques|first4=BS|last5=Socher|first5=E|last6=Zhao|first6=S|last7=Cheung|first7=S|last8=Kiefer|first8=JD|last9=Wand|first9=AJ|last10=Griffith|first10=LG|last11=Imperiali|first11=B|last12=Wittrup|first12=KD|title=Design Principles for {{not a typo|SuCESsFul}} Biosensors: Specific Fluorophore/Analyte Binding and Minimization of Fluorophore/Scaffold Interactions|journal=J Mol Biol|date=Jul 2016|doi=10.1016/j.jmb.2016.07.004|pmid=27448945|pmc=5048519|volume=428|issue=20|pages=4228–4241}}</ref> The RF biosensors that are obtained by the above methods, can function and detect target analytes inside living cells.<ref>{{cite journal|last1=Kummer|first1=L|last2=Hsu|first2=CW|last3=Dagliyan|first3=O|last4=MacNevin|first4=C|last5=Kaufholz|first5=M|last6=Zimmermann|first6=B|last7=Dokholyan|first7=NV|last8=Hahn|first8=KM|last9=Plückthun|first9=A|title=Knowledge-based design of a biosensor to quantify localized ERK activation in living cells|journal=Chem Biol|date=Jun 2013|volume=20|issue=6|pages=847–856|doi=10.1016/j.chembiol.2013.04.016|pmid=23790495|pmc=4154710}}</ref>

=== Magnetic biosensors ===
Magnetic biosensors utilize paramagnetic or supra-paramagnetic particles, or crystals, to detect biological interactions. Examples could be coil-inductance, resistance, or other magnetic properties. It is common to use magnetic nano or microparticles. In the surface of such particles are the bioreceptors, that can be DNA (complementary to a sequence or aptamers) antibodies, or others. The binding of the bioreceptor will affect some of the magnetic particle properties that can be measured by AC susceptometry,<ref>{{Cite journal|last1=Strömberg|first1=Mattias|last2=Zardán Gómez de la Torre|first2=Teresa|last3=Nilsson|first3=Mats|last4=Svedlindh|first4=Peter|last5=Strømme|first5=Maria|date=January 2014|title=A magnetic nanobead-based bioassay provides sensitive detection of single- and biplex bacterial DNA using a portable AC susceptometer|url= |journal=Biotechnology Journal|language=en|volume=9|issue=1|pages=137–145|doi=10.1002/biot.201300348|issn=1860-6768|pmc=3910167|pmid=24174315}}</ref> a Hall Effect sensor,<ref>{{Cite journal|last1=Liu|first1=Paul|last2=Skucha|first2=Karl|last3=Megens|first3=Mischa|last4=Boser|first4=Bernhard|date=October 2011|title=A CMOS Hall-Effect Sensor for the Characterization and Detection of Magnetic Nanoparticles for Biomedical Applications|journal=IEEE Transactions on Magnetics|volume=47|issue=10|pages=3449–3451|doi=10.1109/TMAG.2011.2158600|issn=0018-9464|pmc=4190849|pmid=25308989|bibcode=2011ITM....47.3449L}}</ref> a giant magnetoresistance device,<ref>{{Cite journal|last1=Huang|first1=Chih-Cheng|last2=Zhou|first2=Xiahan|last3=Hall|first3=Drew A.|date=2017-04-04|title=Giant Magnetoresistive Biosensors for Time-Domain Magnetorelaxometry: A Theoretical Investigation and Progress Toward an Immunoassay|url= |journal=Scientific Reports|language=en|volume=7|issue=1|pages=45493|doi=10.1038/srep45493|issn=2045-2322|pmc=5379630|pmid=28374833|bibcode=2017NatSR...745493H}}</ref> or others.

===Others===
[[Piezoelectric]] sensors utilise crystals which undergo an elastic deformation when an electrical potential is applied to them. An alternating potential (A.C.) produces a standing wave in the crystal at a characteristic frequency. This frequency is highly dependent on the elastic properties of the crystal, such that if a crystal is coated with a biological recognition element the binding of a (large) target analyte to a receptor will produce a change in the resonance frequency, which gives a binding signal. In a mode that uses surface acoustic waves (SAW), the sensitivity is greatly increased. This is a specialised application of the [[quartz crystal microbalance]] as a biosensor

[[Electrochemiluminescence]] (ECL) is nowadays a leading technique in biosensors.<ref>{{cite journal |url= |title= Insights into the mechanism of coreactant electrochemiluminescence facilitating enhanced bioanalytical performance. |year=2020 |last1= Zanut |first1=A. |last2= Fiorani |first2=A. |last3= Canola |first3=S. |last4= Saito |first4=T. |last5= Ziebart |first5=N. |last6=Rapino |first6=S. |last7= Rebeccani |first7=S. |last8= Barbon |first8=A. |last9= Irie |first9=T. |last10= Josel |first10=H. |last11= Negri |first11=F. |last12= Marcaccio |first12=M. |last13= Windfuhr |first13=M. |last14= Imai |first14=K. |last15= Valenti |first15=G. |last16= Paolucci |first16=F.  |journal= Nat. Commun. |volume=11 |issue= 1 |pages=2668 |doi= 10.1038/s41467-020-16476-2|pmid= 32472057 |pmc= 7260178 |bibcode= 2020NatCo..11.2668Z |s2cid= 218977697 }}</ref><ref name="Forster">{{cite journal |vauthors=Forster RJ, Bertoncello P, Keyes TE | title=Electrogenerated Chemiluminescence | journal=Annual Review of Analytical Chemistry | year=2009 | pages=359–85| volume=2 | pmid=20636067 | doi=10.1146/annurev-anchem-060908-155305|bibcode = 2009ARAC....2..359F }}</ref><ref name="Valenti">{{cite journal |vauthors=Valenti G, Fiorani A, Li H, Sojic N, Paolucci F | title=Essential Role of Electrode Materials in Electrochemiluminescence Applications | journal=ChemElectroChem | year=2016 | pages=1990–1997| volume=3 | issue=12 | doi= 10.1002/celc.201600602 | hdl=11585/591485 | hdl-access=free }}</ref> Since the excited species are produced with an electrochemical stimulus rather than with a light excitation source, ECL displays improved signal-to-noise ratio compared to photoluminescence, with minimized effects due to light scattering and luminescence background. In particular, coreactant ECL operating in buffered aqueous solution in the region of positive potentials (oxidative-reduction mechanism) definitively boosted ECL for immunoassay, as confirmed by many research applications and, even more, by the presence of important companies which developed commercial hardware for high throughput immunoassays analysis in a market worth billions of dollars each year.

Thermometric biosensors are rare.