Principles of Electronic Nanobiosensors
Unit 3: Sensitivity
Lecture 3.4: Potentiometric Sensors:
ISFET as a pH-Meter
By Muhammad A. Alam
Professor of Electrical and Computer Engineering Purdue University
alam@purdue.edu
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Outline
•Background: what is pH ?
•pH and surface charge of an ISFET
–Surface binding model
•Nernst Limit of an ISFET
•Conclusions
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Alam, Principles of Nanobiosensors, 2013 |
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MOSFET as a pH sensor
What is pH? pH = - log10 [H+]B Example: [H+]B = 10-7 M → pH = 7
Acid Alkaline
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H+ |
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H+ H+ |
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-OH |
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OH |
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H |
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-OH |
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O ̶ OH2 |
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ox
Si
Human blood: 7.35 < pH < 7.45
pH off the range is fatal to human body!
IDS |
pH pH’ |
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-O- + H+ |
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∆VT |
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-OH + H+ |
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VG
Alam, Principles of Nanobiosensors, 2013 |
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Outline
•Background: what is pH ?
•pH and surface charge of an ISFET
–Surface binding model
•Nernst Limit of an ISFET
•Conclusions
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Alam, Principles of Nanobiosensors, 2013 |
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Basics of pH control: pH of Pure Water
H2O [H + ] +[OH − ]
Kw = H +[ OH] − = 10−14
H2O
[H+ ] =[OH − ] =10−7 M
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= x + 2y |
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= 10−14 |
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x −2y |
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y =10−7
water
pH= −log10[H + ] = 7 pOH = −log10[OH− ] = 7 pH + pOH =14
Alam, Principles of Nanobiosensors, 2013 |
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