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ELECTROCHEMICAL BIOSENSORS
Modern and future approaches to medical diagnostics
James F. Rusling Dept. of Chemistry
Dept. of Cell Biology, Univ. of CT Health Center
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Medical Diagnostics
• Doctors increasingly rely on testing • Needs: rapid, cheap, and “low tech” • Done by technicians or patients • Some needs for in-vivo operation, with
feedback
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electrode
substrate product
Figure 9
Enzyme (label)
Apply voltage Measure current prop. to concentration of substrate
Principle of Electrochemical Biosensors
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E, V
time
E-t waveform
potentiostat
Electrochemical cell
counter working electrode
N2 inlet
Protein film reference
insulator electrode material
Equipment for developing electrochemical biosensors
Cyclic voltammetry
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Electrode
enzyme
A lipid-enzyme film
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-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
-0.8-0.6-0.4-0.200.20.40.60.8
I , µ
A
E, V vs SCE
Cyclic voltammogram (CV) at 100 mV s-1 and 25 oC of Mycobacterium Tuberculosis KatGcatalase-peroxidase in a thin film of dimyristoylphosphatidylcholine on basal plane PG electrode,in anaerobic pH 6.0 buffer.
Oxidation Of FeII
Reduction Of FeIII
Reversible Peaks for
Direct electron Transfer
(not all proteins do this)
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Catalytic enzyme electrochemistry a basis for biosensor - glucose oxidase
oxidation
Fc mediator
Fc + glucose + enzyme
I = f [glucose]
A. Cass, G. Davis, G. D. Francis, H. O. A. Hill, W. J. Aston, I. J. Higgins, E. V. Plotkin, L. D. L.Scott, A. P. F. Turner, Anal. Chem. 56, 667-671 (1984).
Mediator shuttles Electrons between Enzyme and electrode
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Scheme 2
Glucose + GO(FAD) + 2 H+ gluconolactone + GO(FADH2) (1)
GO(FADH2) + 2 Fc+ GO(FAD) + 2 Fc + 2 H+ (4)
Fc Fc+ + 2 e- (at electrode) (5)
Mechanism for catalytic oxidation of glucose With Glucose oxidase (GO) and Fc mediator
Signal can also be measured by amperometry: Hold const. E where oxidation occurs, measure I vs time
Fc = ferrocenecarboxylate
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Commercial Glucose Sensors • Biggest biosensor success story! • Diabetic patients monitor blood glucose
at home • First made by Medisense (early 1990s),
now 5 or more commercial test systems • Rapid analysis from single drop of blood • Enzyme-electrochemical device on a slide
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• Most sensors use enzyme called glucose oxidase (GO) • Most sensors are constructed on electrodes, and use a mediator to carry electrons from enzyme to GO Fc = mediator, ferrocene, an iron complex
These reactions occur in the sensor:
Fc Fc+ + e- (measured) GOR + 2 Fc + --> GOox + 2 Fc
GOox + glucose --> GOR + gluconolactone
Reach and Wilson, Anal. Chem. 64, 381A (1992) G. Ramsay, Commercial Biosensors, J. Wiley, 1998.
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Glucose biosensor test strips (~$0.50-1.00 ea.)
Meter Read glucose
Dry coating of GO + Fc
Patient adds drop of blood, then inserts slide into meter
Output: Amperometry
Constant E
I
t
Patient reads glucose level on meter
e’s electrodes
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Research on glucose sensors
• Non-invasive biosensors - skin, saliva • Implantable glucose sensors to
accompany artificial pancreas - feedback control of insulin supply
• Typical use 3-4 weeks for implantable sensor in humans
• Failure involved fouling and inflammation
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Other biosensors
• Cholesterol - based on cholesterol oxidase • Antigen-antibody sensors - toxic
substances, pathogenic bacteria • Small molecules and ions in living things:
H+, K+, Na+, CO2, H2O2 • DNA hybridization and damage • Micro or nanoarrays, optical abs. or
fluorescence
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Layer by layer Film construction:
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PSS layer
SPAN layer
Enzyme layer
Detection of hydrogen peroxide Conductive polymers efficiently wire
peroxidase enzymes to graphite
Xin Yu, G. A. Sotzing, F. Papadimitrakopoulos, J. F. Rusling, Highly Efficient Wiring of Enzymes to Electrodes by Ultrathin Conductive Polyion Underlayers: Enhanced Catalytic Response to Hydrogen Peroxide, Anal. Chem., 2003, 75, 4565-4571. !
e’s
(sulfonated polyaniline)
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Horseradish Peroxidase (HRP)
100nm
50nm
Tapping mode atomic force microscopy (AFM) image of HRP film
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O2
H2O2
PFeIII PFeII PFeII-O22e-, 2H+
H2O2 + PFeII•PFeIV=Oactive oxidant
+e-
-e-
PFeIII + H2O + O2
H2O2
Possible reduced species in red
Electrochemical Response of Peroxidases
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-10
0
10
20
30
40
50
60
-0.8-0.6-0.4-0.200.2
I,µA
E, V vs SCE
with SPAN
a
00.5
2
467.5
µM H20
2
Catalytic reduction of H2O2 by peroxidase films Catalytic cycles increase current �
FeIII/FeII
reduction
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0
0.5
1
0 100 200 300 400
I, µ A
t, s
with PAPSA
without PAPSA
Rotating electrode amperometry at 0 V HRP, 50 nmol H2O2 additions
span
No span
reduction
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0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6
I, µ A
[H2O
2], µM
PAPSA/HRPPAPSA/Mb
Mb
HRP
Rotating electrode amperometry at 0 V
Sensitivity much higher with conductive polymer (SPAN); Electrically wires all the protein to electrode
Span/HRP Span/Mb
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• Single walled (1.4 nm o.d.) and multi-walled
• Highly conductive, flexible, strong, patternable
• Commercially Available
Carbon Nanotubes
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Single-Walled Carbon Nanotube Forests: Antigen-Antibody Sensing
SPAN or Nafion
Chattopadhyay, Galeska, Papadimitrakopoulos, J. Am. Chem. Soc. 2001, 123, 9451. End COOH groups allow chemical attachment to proteins (antibodies)
High conductivity to conduct signal (e’s) from enzyme label to meas. circuit
~1.4 nm diameter, high conductivity
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AFM of SWNT forest with and without anti-HSA attached
SWNT forest on Si wafer SWNT forest with anti-human serum albumin (HSA) attached by amide links
• Also linked enzymes to SWNT forests:X. Yu, D. Chattopadhyay, I. Galeska, F. Papadimitrakopoulos, and J. F. Rusling, “Peroxidase activity of enzymes bound to the ends of single-wall carbon nanotubeforest electrodes”, Electrochem. Commun., 2003, 5, 408-411.
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Sandwich Electrochemical Immunosensor Proteins
H2O2
Ab1
Ag
HRP HRP HRP HRP HRP
Ab1
Ag
HRP
HPR
Ab2
Apply E measure I SWNT forest Conductive polymer
(SPAN)
protein
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Amperometry Detection of Human Serum albumin
-2
0
2
4
6
8
10
12
0 500 1000 1500
I , µA
t, s
1.5
7.5
15
45
600 pmol/mL HSA
300
140
140 300140
controls
a0
bc d
no SWNT bare PG
0.4 mM hydroquinone;0.4 mM H
2O
2
• SWNTs provide 10-20 fold signal enhancement; high surf. area • Nanotubes aged in DMF fewer defects denser forests
. Xin Yu, Sang Nyon Kim, Fotios Papadimitrakopoulos, and James F. Rusling,"Protein Immunosensor Using Single-Wall Carbon Nanotube Forests with ElectrochemicalDetection of Enzyme Labels", Molecular Biosystems, 2005, 1, 70-78.
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Initial Target: Prostate Specific Antigen
PSA - Single chain glycoprotein , MW 33 kDa
Sensitive, specific biomarker for detection of prostate cancer up to 5 years before clinical signs of disease
Detection of PSA in serum: clinical method for detection of prostate cancer
Led to less invasive treatment protocols, avoid surgery
Adapted From Brookhaven Protein Databank
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Nanotube Strategies for PSA detection
~170 labels per PSA
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Washing with 2% BSA/0.05% Tween 20 to control non-specific binding LOD - 4 pg/mL; 100-fold enhancement over HRP-Ab2
Amperometric response at –0.3 V and 3000 rpm for SWNT immunosensors incubated with PSAin 10 µL undiluted newborn calf serum for 1.25 hr using the Ab2-CNT-HRP bioconjugate
Mediator + H2O2
Using HRP-Ab2-nanotube
Xin Yu, Bernard Munge, Vyomesh Patel, Gary Jensen, Ashwin Bhirde, Joseph D. Gong, Sang-Nyon Kim, John Gillespie, J. Silvio Gutkind, Fotios Papadimitrakopoulos and James F. Rusling,"Carbon Nanotube Amplification Strategies for Highly Sensitive Immunosensing of CancerBiomarkers in Serum and Tissue", J. Am. Chem. Soc., 2006, 128, 11199-11205.
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Amperometric current at –0.3 V and 3000 rpm for human serum samples and PSA standards incalf serum
Accurate results obtained for cancer patient serum
Good correlation with ELISA!
Using conventional HRP-Ab2
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PSA in prostate tissue samples indistinguishable by the best staining methods
cell lysates obtained after laser microdissection Using multiply labeled Ab2-nanotube-HRP 4 pg/mL DL (100 x better)
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Future - arrays to detect many biomolecules at once
8-electrode sensor array
Sensing electrodes
electrical contacts
An optical- electrochemical
array
Polymer gives off light when electro-oxidized with proper co-reactant, e.g. DNA
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Biosensors
• Future of medical diagnostics; use by patients or in doctors offices and clinics
• Other important applications: cancer biomarkers, DNA, peroxide, etc.
• Method of choice for blood glucose in diabetics • Rapid diagnostics may lead to more timely and
effective treatment