Editing Nanowire (section)

=== Sensing of proteins and chemicals using semiconductor nanowires ===

In an analogous way to FET devices in which the modulation of conductance (flow of electrons/holes) in the semiconductor, between the input (source) and the output (drain) terminals, is controlled by electrostatic potential variation (gate-electrode) of the charge carriers in the device conduction channel, the methodology of a Bio/Chem-FET is based on the detection of the local change in charge density, or so-called "field effect", that characterizes the recognition event between a target molecule and the surface receptor.

This change in the surface potential influences the Chem-FET device exactly as a 'gate' voltage does, leading to a detectable and measurable change in the device conduction. When these devices are fabricated using semiconductor nanowires as the transistor element the binding of a chemical or biological species to the surface of the sensor can lead to the depletion or accumulation of charge carriers in the "bulk" of the nanometer diameter nanowire i.e. (small cross section available for conduction channels). Moreover, the wire, which serves as a tunable conducting channel, is in close contact with the sensing environment of the target, leading to a short response time, along with orders of magnitude increase in the sensitivity of the device as a result of the huge S/V ratio of the nanowires.

While several inorganic semiconducting materials such as Si, Ge, and metal oxides (e.g. In<sub>2</sub>O<sub>3</sub>, SnO<sub>2</sub>, ZnO, etc.) have been used for the preparation of nanowires, Si is usually the material of choice when fabricating nanowire FET-based chemo/biosensors.<ref>{{cite book |title=Semiconductor Nanowires |series=Smart Materials Series |editor-first=Wei |editor-last=Lu |editor2-first=Jie |editor2-last=Xiang |publisher=Royal Society of Chemistry |location=Cambridge |year=2015 |doi=10.1039/9781782626947 |isbn=978-1-84973-826-2 |url=https://pubs.rsc.org/en/content/ebook/978-1-78262-694-7 }}</ref>

Several examples of the use of [[Silicon Nanowire|silicon nanowire]](SiNW) sensing devices include the ultra sensitive, real-time sensing of biomarker proteins for cancer, detection of single virus particles, and the detection of nitro-aromatic explosive materials such as 2,4,6-tri-nitrotoluene (TNT) in sensitives superior to these of canines.<ref>{{cite journal |last1=Engel |first1=Yoni |last2=Elnathan |first2=Roey |last3=Pevzner |first3=Alexander |last4=Davidi |first4=Guy |last5=Flaxer |first5=Eli |last6=Patolsky |first6=Fernando |title=Supersensitive Detection of Explosives by Silicon Nanowire Arrays |journal=Angewandte Chemie International Edition |date=2010 |volume=49 |issue=38 |pages=6830–6835 |doi=10.1002/anie.201000847 |pmid=20715224|doi-access=free }}</ref>
Silicon nanowires could also be used in their twisted form, as electromechanical devices, to measure intermolecular forces with great precision.<ref>{{cite journal|last1=Garcia|first1=J. C.|last2=Justo|first2=J. F.|s2cid=118792981|title=Twisted ultrathin silicon nanowires: A possible torsion electromechanical nanodevice|journal=Europhys. Lett.|date=2014|volume=108|issue=3|page=36006|doi=10.1209/0295-5075/108/36006|bibcode=2014EL....10836006G|arxiv=1411.0375}}</ref>

==== Limitations of sensing with [[silicon nanowire]] FET devices ====
Generally, the charges on dissolved molecules and macromolecules are screened by dissolved counterions, since in most cases molecules bound to the devices are separated from the sensor surface by approximately 2–12&nbsp;nm (the size of the receptor proteins or DNA linkers bound to the sensor surface). As a result of the screening, the electrostatic potential that arises from charges on the analyte molecule decays exponentially toward zero with distance. Thus, for optimal sensing, the [[Debye length]] must be carefully selected for nanowire FET measurements.
One approach of overcoming this limitation employs fragmentation of the antibody-capturing units and control over surface receptor density, allowing more intimate binding to the nanowire of the target protein. This approach proved useful for dramatically enhancing the sensitivity of [[Cardiac marker|cardiac biomarkers]] (e.g. [[Troponin]]) detection directly from serum for the diagnosis of acute myocardial infarction.<ref>{{cite journal|last=Elnathan|first=Roey |author2=Kwiat, M. |author3=Pevzner, A. |author4=Engel, Y. |author5=Burstein, L. |author6=Khatchtourints, A. |author7=Lichtenstein, A. |author8=Kantaev, R. |author9=Patolsky, F.|title=Biorecognition Layer Engineering: Overcoming Screening Limitations of Nanowire-Based FET Devices|journal=Nano Letters|date=10 September 2012|volume=12|issue=10|pages=5245–5254|doi=10.1021/nl302434w|pmid=22963381|bibcode=2012NanoL..12.5245E }}</ref>