Principles of Electronic Nanobiosensors
Unit 3: Sensitivity
Lecture 3.5: Potentiometric Sensors:
Why are Biomolecules Charged?
By Muhammad A. Alam
Professor of Electrical and Computer Engineering Purdue University
alam@purdue.edu
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Outline
•Theory of DNA charges
–DNA is an acid and acids are charged in solution
•Protein involves charged residues
–Protein charge can be negative or positive
•Biomolecules and surface charges
•Conclusions
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Alam, Principles of Nanobiosensors, 2013 |
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Transistors and Biomolecules
Q1 C G Q1*
A T
G C
A T
T A
Capture Probe
H |
OH |
PO4 |
5C |
N |
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Sugar |
Base |
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Alam, Principles of Nanobiosensors, 2013 |
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Charge States of DNA controlled by pH
pH
[H3PO4 ]0 =[H2 PO4− ]+[HPO42− ]+[PO43− ]
Ka1 |
= |
[H + ][H2 PO4 |
− ] |
Ka2 |
= |
[H + ][HPO4 |
2− ] |
K |
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= |
[H + ][PO43− ] |
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[H3PO4 |
] |
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[H2 PO4 |
− ] |
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a3 |
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[HPO 2− ] |
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7.5*10−3 |
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6.2*10−8 |
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2.14*10−13 |
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This is not H3PO4 originally we put in, but H3PO4 that is still undissociated.
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Average Charge per Base at pH=7
pH=7=log[H+], [H+]=10-7 Molar
Q=1
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[H + ][H2 PO4 |
− ] |
7.5*10−3 |
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[H2 PO4 |
− ] |
7.5*104 |
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[H PO ] |
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[H PO ] |
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Q=2
[H + ][HPO |
2− ] |
6.2*10 |
−8 |
[HPO |
2− ] |
0.62 |
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4 |
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[H |
4 |
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[H PO −] |
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PO − ] |
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2 |
4 |
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2 |
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4 |
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Q=3
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[H + ][PO43− ] |
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2.14*10−13 |
[PO43− ] |
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2.14*10−6 |
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[HPO 2− |
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[HPO 2− |
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Fully ionized
S+D=1 & D/S=0.62
S=0.61q D=0.39q
Q=1S+2D=1.38q
Negligible
Alam, Principles of Nanobiosensors, 2013 |
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