ISFET

ISFET pH electrode also redirects here.

An ISFET is an ion-sensitive field-effect transistor used for measuring ion concentrations in solution; when the ion concentration (such as H⁺, see pH scale) changes, the current through the transistor will change accordingly. Here, the solution is used as the gate electrode. A voltage between substrate and oxide surfaces arises due to an ions sheath.

The surface hydrolysis of Si–OH groups of the gate materials varies in aqueous solutions due to pH value. Typical gate materials are SiO₂, Si₃N₄, Al₂O₃ and Ta₂O₅.

The mechanism responsible for the oxide surface charge can be described by the site binding model, which describes the equilibrium between the Si–OH surface sites and the H⁺ ions in the solution. The hydroxyl groups coating an oxide surface such as that of SiO₂ can donate or accept a proton and thus behave in an amphoteric way as illustrated by the following acid-base reactions occurring at the oxide-electrolyte interface:

—Si–OH + H₂O     ↔   —Si–O^–     + H₃O⁺

—Si–OH + H₃O⁺   ↔   —Si–OH₂⁺ + H₂O

An ISFET's source and drain are constructed as for a MOSFET. The gate electrode is separated from the channel by a barrier which is sensitive to hydrogen ions and a gap to allow the substance under test to come in contact with the sensitive barrier. An ISFET's threshold voltage depends on the pH of the substance in contact with its ion-sensitive barrier.

Practical limitations due to the reference electrode

An ISFET electrode sensitive to H⁺ concentration can be used as a conventional glass electrode to measure the pH of a solution. However, it also requires a reference electrode to operate. If the reference electrode used in contact with the solution is of the AgCl or HgCl₂ classical type, it will suffer the same limitations as conventional pH electrodes (junction potential, KCl leak, and glycerol leak in case of gel electrode). A conventional reference electrode can also be bulky and fragile. A too large volume constrained by a classical reference electrode also precludes the miniaturization of the ISFET electrode, a mandatory feature for some biological or in vivo clinical analyses (disposable mini-catheter pH probe). The breakdown of a conventional reference electrode could also make problem in on-line measurements in the pharmaceutical or food industry if highly valuable products are contaminated by electrode debris or toxic chemical compounds at a late production stage and must be discarded for the sake of safety.

For this reason, since more than 20 years many research efforts have been dedicated to on-chip embarked tiny reference field effect transistors (REFET). Their functioning principle, or operating mode, can vary, depending on the electrode producers and are often proprietary and protected by patents. Semi-conductor modified surfaces required for REFET are also not always in thermodynamical equilibrium with the test solution and can be sensitive to aggressive or interfering dissolved species or not well characterized aging phenomena. This is not a real problem if the electrode can be frequently re-calibrated at regular time interval and is easily maintained during its service life. However, this may be an issue if the electrode has to remain immersed on-line for prolonged period of time, or is inaccessible for particular constrains related to the nature of the measurements itself (geochemical measurements under elevated water pressure in harsh environments or under anoxic or reducing conditions easily disturbed by atmospheric oxygen ingress or pressure changes).

A crucial factor for ISFET electrodes, as for conventional glass electrodes, remains thus the reference electrode. When troubleshooting electrode malfunctions, often, most of the problems have to be searched for from the side of the reference electrode.

See also

• Chemical field-effect transistor
• Ion-selective electrodes
• MISFET: metal–insulator–semiconductor field-effect transistor
• MOSFET: metal–oxide–semiconductor field-effect transistor
• pH
• pH meter
• Potentiometry
• Quinhydrone electrode
• Saturated calomel electrode
• Silver chloride electrode
• Standard hydrogen electrode

References

• Bergveld, P. (2003). "Thirty years of ISFETOLOGY, What happened in the past 30 years and what may happen in the next 30 years". Sensors and Actuators B: Chemical 88: 1–20. doi:10.1016/S0925-4005(02)00301-5. 

• Bergveld, P. (2003). "ISFET, theory and practice". IEEE Sensor Conference, October 2003. Toronto: IEEE. p. 26. 
• ISFET pH Sensors

Further reading

• Rothberg, Johnathan M (2011). "An integrated semiconductor device enabling non-optical genome sequencing". Nature 475 (7356): 348–52. doi:10.1038/nature10242. ISSN 1476-4687. 
• Bergveld, P. (1985). "The impact of MOSFET-based sensors". Sensors and Actuators 8 (2): 109–127. doi:10.1016/0250-6874(85)87009-8. ISSN 0250-6874. 

• Bergveld, P. (1986). "The development and application of FET-based biosensors". Biosensors 2 (1): 15–33. doi:10.1016/0265-928X(86)85010-6. ISSN 0265-928X. 

• Bergveld, P. (1991). "A critical evaluation of direct electrical protein detection methods". Biosensors and Bioelectronics 6 (1): 55–72. doi:10.1016/0956-5663(91)85009-L. ISSN 0956-5663. 

• Bergveld, P. (2003). "Thirty years of ISFETOLOGY: What happened in the past 30 years and what may happen in the next 30 years". Sensors and Actuators B: Chemical 88 (1): 1–20. doi:10.1016/S0925-4005(02)00301-5. ISSN 0925-4005. 

• Bergveld, P. (2003). "ISFET, theory and practice". IEEE Sensor Conference Toronto, October 2003. Toronto: IEEE. pp. 26 pp. 

ISFET-Bergveld.pdf, retrieved 2010-10-31 

• Bergveld, P; A Van den Berg, PD Van der Wal, M Skowronska-Ptasinska, EJR Sudhölter, DN Reinhoudt (1989). "How electrical and chemical requirements for REFETs may coincide". Sensors and Actuators 18 (3-4): 309–327. ISSN 0250-6874.  Cite uses deprecated parameters (help)

• Chudy, M; W Wróblewski, Z Brzózka (1999). "Towards REFET". Sensors and Actuators B: Chemical 57 (1-3): 47–50. doi:10.1016/S0925-4005(99)00134-3. ISSN 0925-4005.  Cite uses deprecated parameters (help)

• Chudy, Michal; Wojciech Wr�blewski, Zbigniew Brz�zka (1999). "Towards REFET". Sensors and Actuators B: Chemical 57 (1-3): 47–50. doi:10.1016/S0925-4005(99)00134-3. ISSN 0925-4005.  Cite uses deprecated parameters (help)

• Collins, S.D. (1993). "Practical limits for solid-state reference electrodes". Sensors and Actuators B: Chemical 10 (3): 169–178. doi:10.1016/0925-4005(93)87002-7. ISSN 0925-4005. 

• Duroux, P; C Emde, P Bauerfeind, C Francis, A Grisel, L Thybaud, D Armstrong, C Depeursinge, A L Blum (1991). "The ion sensitive field effect transistor (ISFET) pH electrode: a new sensor for long term ambulatory pH monitoring.". Gut 32 (3): 240–245. ISSN 0017-5749.  Cite uses deprecated parameters (help)

• Errachid, A.; J. Bausells, N. Jaffrezic-Renault (1999). "A simple REFET for pH detection in differential mode". Sensors and Actuators B: Chemical 60 (1): 43–48. doi:10.1016/S0925-4005(99)00242-7. ISSN 0925-4005. Retrieved 2010-11-01.  Cite uses deprecated parameters (help)

• Ghallab, Y.H.; W. Badawy, K.V.I.S. Kaler. "A novel pH sensor using differential ISFET current mode read-out circuit". Proceedings International Conference on MEMS, NANO and Smart Systems. International Conference on MEMS, NANO and Smart Systems. Banff, Alta., Canada. pp. 255–258. doi:10.1109/ICMENS.2003.1222002. Retrieved 2010-10-31.  Cite uses deprecated parameters (help)

• Guth, U; F Gerlach, M Decker, W Oelßner, W Vonau (2009). "Solid-state reference electrodes for potentiometric sensors". Journal of Solid State Electrochemistry 13 (1): 27–39. ISSN 1432-8488.  Cite uses deprecated parameters (help)

• Huang, I-Yu. Chemical sensors research group. Retrieved 2010-11-01. 

• Huang, I-Yu; Ruey-Shing Huang, Lieh-Hsi Lo (2002). "A new structured ISFET with integrated Ti/Pd/Ag/AgCl Electrode and micromachined back-side P+ contacts". Journal of the Chinese Institute of Engineers 25 (3): 327–334. Retrieved 2010-11-01.  Cite uses deprecated parameters (help)

• Kal, S.; P.V. Bhanu (2007). "Design and modeling of ISFET for pH sensing". IEEE. TENCON 2007 - 2007 IEEE Region 10 Conference. Taipei. pp. 1–4. doi:10.1109/TENCON.2007.4428805. ISBN 978-1-4244-1272-3. Retrieved 2010-10-31.  Cite uses deprecated parameters (help)

• Kisiel, Anna; Agata Michalska, Krzysztof Maksymiuk (September 2007). "Plastic reference electrodes and plastic potentiometric cells with dispersion cast poly(3,4-ethylenedioxythiophene) and poly(vinyl chloride) based membranes". Bioelectrochemistry 71 (1): 75–80. doi:10.1016/j.bioelechem.2006.09.006. ISSN 1567-5394.  Cite uses deprecated parameters (help)

• Lee, YC; BK Sohn (2002). "Development of an FET-type reference electrode for pH detection". JOURNAL-KOREAN PHYSICAL SOCIETY 40: 601–604. ISSN 0374-4884.  Cite uses deprecated parameters (help)

• Lisdat, F.; W. Moritz (August 1993). "A reference element based on a solid-state structure". Sensors and Actuators B: Chemical 15 (1-3): 228–232. doi:10.1016/0925-4005(93)85057-H. ISSN 0925-4005.  Cite uses deprecated parameters (help)

• Skowronska-Ptasinska, M; PD Van Der Wal, A Van Den Berg, P Bergveld, EJR Sudhölter, DN Reinhoudt (1990). "Reference field effect transistors based on chemically modified ISFETs". Analytica Chimica Acta 230: 67–73. ISSN 0003-2670.  Cite uses deprecated parameters (help)

• Suzuki, Hiroaki; Taishi Hirakawa, Satoshi Sasaki, Isao Karube (1998-02-15). "Micromachined liquid-junction Ag/AgCl reference electrode". Sensors and Actuators B: Chemical 46 (2): 146–154. doi:10.1016/S0925-4005(98)00110-5. ISSN 0925-4005.  Cite uses deprecated parameters (help)

• van den Berg, A.; A. Grisel, H.H. van den Vlekkert, N.F. de Rooij (January 1990). "A micro-volume open liquid-junction reference electrode for pH-ISFETs". Sensors and Actuators B: Chemical 1 (1-6): 425–432. doi:10.1016/0925-4005(90)80243-S. ISSN 0925-4005.  Cite uses deprecated parameters (help)

• Vonau, W.; W. Oel�ner, U. Guth, J. Henze (2010-02-17). "An all-solid-state reference electrode". Sensors and Actuators B: Chemical 144 (2): 368–373. doi:10.1016/j.snb.2008.12.001. ISSN 0925-4005.  Cite uses deprecated parameters (help)