Ion Sensitive FET (ISFET) - What and Why?

1. Ion Sensitive FET (ISFET) - What and Why?

2. 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

3. 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.

4. 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.

5. 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.

6. 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)

7. Demand for in vivo biosensing

8. 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)

9. ISFET – smalland reliable ISFET small & reliable for in-vivo biosensing applications Conventional ISE too big, fragile & unstable for in-vivo biosensing applications

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