Ion Sensitive FET (ISFET)
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Ion Sensitive FET (ISFET) - What and Why?. Indicator electrode: ISEs. Ion selective electrodes (ISEs) Fritz Haber discovered (1901) that there is a change in potential across a glass membrane when its two sides are in solutions of different acidity.

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Ion Sensitive FET (ISFET) - What and Why?

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Ion sensitive fet isfet what and why

Ion Sensitive FET (ISFET)

- What and Why?


Indicator electrode ises

Indicator electrode: ISEs

  • Ion selective electrodes (ISEs)

    • Fritz Haber discovered (1901) that there is a change in potential across a glass membrane when its two sides are in solutions of different acidity.

    • This led to the development of a new class of indicator electrodes called ISEs.

    • In addition to the glass pH electrode, ISEs are available for a wide range of ions. The development of new ISEs continues to be an active area of research.

    • The ISE’s membrane separates the sample,

      which contains the analyte at an activity

      of (aA)sample, from an internal solution

      containing the analyte with an activity

      of (aA)int.

    • Because the junction potential and the

      potential of the two reference electrodes

      are constant, any change in Ecellis a result

      of a change in the membrane’s potential.

Ecell = Eref(int) − Eref(sample)+ Emem + Ej


By the way what is ph again

By the way, what is pH again?

  • pH is the measurement of acidity (hydrogen ion concentration)

    • pH = -log10[H+], for [H+] =1.8 x 10-5 M, pH = 4.745

    • An important example of pH is that of the blood. Its nominal value of

      pH = 7.4 is regulated very accurately by the body. If the pH of the blood

      gets outside the range 7.35 to 7.45 the results can be fatal.


Ise glass ph electrode

ISE: glass pH electrode

  • Glass pH electrode

    • pH sensitive glasses are manufactured typically with a composition ~22% Na2O, ~6% CaO and ~72% SiO2.

    • Oxygen atoms within the lattice that are not bound to two Si atoms possess a negative charge. Cations(primarily Na+) pair with these oxygen atoms and are able to diffuse slowly in the lattice, moving from one charge pair site to another. This movement of cations within the glass allows a potential to be measured across it.

  • Advantages

    • Its potential is essentially

      not affected by the

      presence of oxidizing or

      reducing agents.

    • Operates over a wide pH

      range

    • Fast

    • Functions well in physiological systems.


Gas sensing electrodes

Gas-sensing electrodes

  • Gas-sensing electrodes

    • A thin membrane that separates the sample from an inner solution containing an ISE.

    • The membrane is permeable to the gaseous analyte, but impermeable to other components. The gaseous analyte passes through the membrane where it reacts with the inner solution, producing a species whose concentration is monitored by the ISE.

      • For example, CO2 diffuses across the membrane where it reacts in the inner solution to produce H3O+. The change in the activity of H3O+ in the inner solution is monitored with a pH electrode

      • CO2(aq) + 2H2O(l) ⇋ HCO3−(aq) + H3O+(aq)

    • CO2, HCN, HF, H2S, NH3, NO2, SO2are commonly measured in this manner.


Potentiometric biosensors

Potentiometric biosensors

  • Potentiometric biosensors

    • Gas-sensing electrodes are modified to create potentiometric electrodes that respond to a biochemically important species.

    • Potentiometric biosensors have been designed around other biologically active species, including enzymes, antibodies, bacterial particles, tissues, and hormone receptors.

    • Ex: enzyme (urease) electrode

      • Urease catalyzes the hydrolysis ofurea

        (CO(NH2)2) to produce NH3 (ammonia)

        andCO2

      • CO(NH2)2(aq) + 2H2O(l) ⇋

        2NH4+(aq) + CO32−(aq)


Demand for in vivo biosensing

Demand for in vivo biosensing


Why isfet

Why ISFET?

  • Why do we need ISFET?

    • Strong demand in biomedical sensing (H, K, Na ions)

    • Miniaturized versions of the glass-membrane ion-selective electrode (ISE) appear to be less stable

    • For in vivo monitoring, glass-membrane ISE is fragile and cannot be used

  • Goals

    • Reduced dimensions to probe biology

    • Fast response

    • Simple integration with measurement electronics

  • Solution

    • Modify a common electrical engineering device (MOSFET) and use as a sensor (ISFET)


Isfet small and reliable

ISFET – smalland reliable

ISFET small & reliable for in-vivo biosensing applications

Conventional ISE too big, fragile & unstable for in-vivo biosensing applications


Isfet

ISFET

  • ISFET is essentially a type of MOSFET

  • The metal gate is replaced with electrolytes of interest

MOSFET

ISFET


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