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Ion Sensitive FET (ISFET) - What and Why?

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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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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


    • 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)


      • CO(NH2)2(aq) + 2H2O(l) ⇋

2NH4+(aq) + CO32−(aq)

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 is essentially a type of MOSFET
  • The metal gate is replaced with electrolytes of interest