Nano-Bioelectronics for the 21st Century 2008 L1

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NanoNano--bioelectronics for the 21stbioelectronics for the 21stcentury:century:

NanostructuredNanostructured chemically modifiedchemically modifiedelectrodes as platforms forelectrodes as platforms forelectrochemical biosensors.electrochemical biosensors.

Mike LyonsMike LyonsSchool of ChemistrySchool of ChemistryUniversity of DublinUniversity of Dublin

Trinity CollegeTrinity CollegeDublin 2Dublin 2Ireland.Ireland.

Email:Email: [email protected]@tcd.ieMEDIS

EU/LA

ALFA

IntroductionIntroduction

IrelandIreland

…….Dublin.Dublin…….Trinity.Trinity…….MEDIS.MEDIS



MEDIS

EU/LA

ALFA



MEDIS

EU/LA

ALFA

MEDIS

EU/LA

ALFA



Course objectivesCourse objectives

Review basic electrochemical , biochemicalReview basic electrochemical , biochemicaland materials science ideas pertinent toand materials science ideas pertinent toelectrochemicalelectrochemical biosensingbiosensing..

Appreciate the potential offered byAppreciate the potential offered bynanostructurednanostructured chemically modifiedchemically modifiedelectrodes (electrodes (NCMENCME’’ss) to function as) to function asamperometricamperometric biosensors.biosensors.

Understand and quantify theUnderstand and quantify the physicophysico--chemicalchemicalbasis underlying the mechanism of operationbasis underlying the mechanism of operationof NCME based electrochemical biosensors.of NCME based electrochemical biosensors.

Detailed analysis of specific examples derivedDetailed analysis of specific examples derivedfrom current literature.from current literature.

Course Material OverviewCourse Material Overview Fundamental concepts:Fundamental concepts:

BiosensorsBiosensors Electrochemical methodsElectrochemical methods Basic enzyme biochemistryBasic enzyme biochemistry

Materials Science :Materials Science : Chemically modified electrodes (CME) as nanostructures for bioseChemically modified electrodes (CME) as nanostructures for biosensornsor

applications.applications. Self assembledSelf assembled monolayersmonolayers CarbonCarbon nanotubenanotube dispersionsdispersions Electronically conducting polymersElectronically conducting polymers

Characterization of material nanostructuresCharacterization of material nanostructures

Specific case studies:Specific case studies: AmperometricAmperometric enzyme biosensors:enzyme biosensors:

Experimental examples & theoreticalExperimental examples & theoretical modellingmodelling:: Direct reaction of electroDirect reaction of electro--enzymesenzymes Homogeneous mediationHomogeneous mediation CarbonCarbon nanotubenanotube //redoxredox enzyme compositesenzyme composites Heterogeneous mediation : electronically conducting polymer/Heterogeneous mediation : electronically conducting polymer/redoxredox enzyme compositesenzyme composites

Electronically conducting polymers (ECP) forElectronically conducting polymers (ECP) for amperometricamperometric enzymeenzymebiosensorsbiosensors Experimental examples and theoreticalExperimental examples and theoretical modellingmodelling Application toApplication to macrosizemacrosize andand ultramicroelectrodeultramicroelectrodesystems.systems.

General conclusions.General conclusions.



Lecture 1Lecture 1Fundamental conceptsFundamental concepts

Biosensors, Electron Transfer, MassBiosensors, Electron Transfer, MassTransport, ElectrochemicalTransport, ElectrochemicalInterfaces & Techniques.Interfaces & Techniques.

What are Biosensors?What are Biosensors? Biosensors combine theBiosensors combine the

exquisite selectivity of biologyexquisite selectivity of biologywith the processing power ofwith the processing power ofmodern microelectronics andmodern microelectronics andoptoelectronics to offeroptoelectronics to offerpowerful new analytical toolspowerful new analytical toolswith major applications inwith major applications inmedicine, environmentalmedicine, environmentaldiagnostics and the food anddiagnostics and the food andprocessing industries.processing industries.

Biosensors consist of bioBiosensors consist of bio--recognition systems, typicallyrecognition systems, typicallyenzymes or binding proteins,enzymes or binding proteins,

such as antibodies,such as antibodies, immobilisedimmobilisedonto the surface ofonto the surface of physicophysico--chemical transducers.chemical transducers.

In addition to enzymes andIn addition to enzymes andantibodies, the bioantibodies, the bio--recognitionrecognitionsystems can also include nucleicsystems can also include nucleicacids, bacteria and single cellacids, bacteria and single cellorganisms and even wholeorganisms and even wholetissues of higher organisms.tissues of higher organisms.

Specific interactions betweenSpecific interactions betweenthe targetthe target analyteanalyte and theand thecomplementarycomplementary biorecognitionbiorecognitionlayer produces alayer produces a physicophysico--chemical change which ischemical change which isdetected and may be measureddetected and may be measuredby the transducer.by the transducer.

The transducer can take manyThe transducer can take manyforms depending upon theforms depending upon theparameters being measuredparameters being measured --electrochemical, optical, masselectrochemical, optical, massand thermal changes are theand thermal changes are themost common.most common.



Formal definitionFormal definition Chemical/biologicalChemical/biological receptorreceptor microstructuremicrostructure

(where there is a specific molecular(where there is a specific molecularinteraction between receptor andinteraction between receptor and analyteanalytespecies) coupled to an electronicspecies) coupled to an electronic transducertransducerwhich convertswhich converts chemical/biochemicalchemical/biochemical activityactivityintointo electricalelectrical signals which can be amplified,signals which can be amplified,stored, displayed and manipulated.stored, displayed and manipulated.

A biosensor is a device that recognizes an

analyte in an appropriate sample andinterprets its concentration as an electricalsignal via a suitable combination of a biologicalrecognition system and a suitable transducer.



ElectronicsReceptor

Transducer

Substrate(analyte)

Interferentspecific molecular

recognitionbetween substrate and

receptor site

chemical interactiontranslated into a useful

signal

data transformed

and processedinto useful

format

Chemical/BiologicalSensor Configuration.

Foundations of a viable sensorFoundations of a viable sensortechnologytechnology Chemical/biological sensor technology involves theChemical/biological sensor technology involves the

interplay of many fundamental scientific andinterplay of many fundamental scientific andengineering disciplines.engineering disciplines. Chemistry (Physical, analytical, synthetic)Chemistry (Physical, analytical, synthetic)

Materials scienceMaterials science

Physics (solid state, optical)Physics (solid state, optical)

Biology (biochemistry: enzyme/Biology (biochemistry: enzyme/

substrate, antibody/antigen etc)substrate, antibody/antigen etc)

Engineering (electronics, ICT,Engineering (electronics, ICT,microfabricationmicrofabrication etc.).etc.).



A. Heller, B. Feldman,Electrochemical glucose sensorsand their applications indiabetes management. Chem.Rev. 2008, in press.

J. Wang, Electrochemicalglucose biosensors. Chem. Rev.,108 (2008) 814-825.

J. Wang, Glucose biosensors: 40 years of advances andchallenges. Electroanalysis, 13(2001) 983-988.

J. Wang, In vivo glucosemonitoring: towards ‘sense andact’ feedback loop individualized

medical systems. Talanta, 75(2008) 636-641.

Amperometric Glucose Sensors

A selection of general literature referencesA selection of general literature references

M. Keusgen, Biosensors: newapproaches in drug discovery.Naturwissenschaften , 89 (2002)433-444.

I Willner, B. Willner, E. Katz,Functional biosensor systems viasurface-nanoengineering ofelectronic elements. Rev. Molecular Biotech., 82 (2002) 325-355.

J-M Zen, A.S. Kumar, D-M. Tsai,Recent updates of chemicallymodified electrodes in analyticalchemistry. Electroanalysis. 15(2003) 1073-1087.

I. Willner, B. Willner, Biomaterialsintegrated with electronic elements:en route to bioelectronics. Trends inBiotech., 19 (2001) 222-230.

I. Willner, E. Katz, Integration oflayered redox proteins andconductive supports forbioelectronic applications. Angew.Chem. Int. Ed., 39 (2000) 1180-1218.

J.J. Davis, D.A. Morgan, C.L.Wrathmell, D.N. Axford, J. Zhao, N.Wang, Molecular Bioelectronics. J.Mater. Chem., 15 (2005) 2160-2174.

N.L. Rosi, C.A. Mirkin,Nanostructures in biodiagnostics.Chem. Rev., 105 (2005) 1547-1562.

D. Chen, G. Wang, J. Li, Interfacialbioelectrochemistry: Fabrication,properties and applications of

functional nanostructuredbiointerfaces. J. Phys. Chem. C., 111(2007) 2351-2367.

A.L. Ghindilis, P. Atanasov, E.Wilkins, Enzyme catalysed directelectron transfer: fundamentals andanalytical applications.Electroanalysis, 9 (1997) 661-674.



Electrochemistry :Electrochemistry :

Recap of basic ideas.Recap of basic ideas. Electrochemistry defines a branch of chemistryElectrochemistry defines a branch of chemistry

which deals with the chemical action ofwhich deals with the chemical action ofelectricity and the production of electricity byelectricity and the production of electricity bychemical reactions.chemical reactions.

Key event is heterogeneous electron transferKey event is heterogeneous electron transfer(HET) at electrode/solution interface.(HET) at electrode/solution interface. HET is driven via application of external electricalHET is driven via application of external electrical

potentialpotential Current flow (proportional to HET reaction rate)Current flow (proportional to HET reaction rate)

across electrode/solution interface reflects chemistryacross electrode/solution interface reflects chemistryoccurring there.occurring there.

Analytical aspect:Analytical aspect: AnyAny redoxredox active species can in principle be detectedactive species can in principle be detected

since current directly proportional tosince current directly proportional to analyteanalyteconcentration (provided diffusion control pertains).concentration (provided diffusion control pertains).

Electrochemical sensors.Electrochemical sensors. PotentiometricPotentiometric devices.devices.

Local equilibrium established at theLocal equilibrium established at thesensor/environment interface.sensor/environment interface.

Electrode or membrane potential measured.Electrode or membrane potential measured. Equilibrium potential proportional to the logarithmEquilibrium potential proportional to the logarithm

of theof the analyteanalyte concentration via theconcentration via the NernstNernstequation.equation.

AmperometricAmperometric devices.devices.

The electrode potential is used to drive anThe electrode potential is used to drive aninterfacial redox reaction and the currentinterfacial redox reaction and the currentresulting from that reaction is measured.resulting from that reaction is measured.

The current flowing is directly proportional to theThe current flowing is directly proportional to theanalyteanalyte concentration.concentration.



Potentiometric vs amperometric sensors

PotentiometricPotentiometric sensors require rapid electrode kinetics.sensors require rapid electrode kinetics. WithWith amperometricamperometric sensors sluggish electrode reactions aresensors sluggish electrode reactions are

switched on via the applied electrode potential.switched on via the applied electrode potential. Typically the rate constant for ET increases by a factor of 10Typically the rate constant for ET increases by a factor of 10

for every 120mV increase in electrode potential.for every 120mV increase in electrode potential. AA potentiometricpotentiometric signal can be corrupted by electronic noise.signal can be corrupted by electronic noise. PotentiometricPotentiometric sensors do not perturb thesensors do not perturb the analyteanalyte concentrationconcentration

at the interface since there is no net consumption of the latterat the interface since there is no net consumption of the latter..Hence mass transfer ofHence mass transfer of analyteanalyte is unimportant.is unimportant.

ForFor amperometricamperometric sensors have significant depletion of thesensors have significant depletion of theanalyteanalyte next to the sensor surface so mass transfer ofnext to the sensor surface so mass transfer of analyteanalyteto the sensor from the bulk solution must be controlled.to the sensor from the bulk solution must be controlled.

AmperometricAmperometric sensors are more versatile thansensors are more versatile than potentiometricpotentiometric

devices.devices. Sensitivity is greater for anSensitivity is greater for an amperometricamperometric sensor .sensor .

Interfacial electron transfer at electrode/solution interfaces:

oxidation and reduction processes.

ne-

P

Q

Electron sink electrode(Anode).

ne-

A

B

Electron source electrode(Cathode).

.

In principle any species which can be oxidised or reduced can be detectedamperometrically.

Oxidation (de-electronation).P = Reductant (electron donor)Q = Oxidant (product)

Reduction (electronation).A = oxidant (electron acceptor)B = reductant (product).



Electrode Electrolyte

Electronically conducting phase :metal, semiconductor,Conducting polymer material etc.

Ionically conductingmedium : electrolytesolution, molten salt,solid electrolyte,polymericelectrolyte, etc.

Conductionoccurs viamigration ofelectrons .Solid statePhysics : energyband theory.

Material transport occursvia migration, diffusion

and convection

ET

The electrode/electrolyte interface.

CDL

RCT

i

iC

iF

RS

Electrode

Solution

SimpleSimple RandlesRandles equivalent circuit representation ofequivalent circuit representation of

electrode/solution interface region.electrode/solution interface region.

Faradaiccurrent

DL chargingcurrent

F C iii +=

Resistance of solution

Double layer chargingcurrent always presentin addition toFaradaic current inelectrochemicalmeasurements.

ET ET

ZW

MT MT



Metallicelectrode

+

EF

LUMO

HOMO

LUMO

HOMO

Redox couplein solution

Electronenergy

n e-

Energy of electronsin metal decreases uponapplication of a potentialmore positive than thethermodynamic equilibriumvalue.

A net anodic (oxidation)current flows from theHOMO level of the redoxspecies in solution to the

metallic electrode.

Pictorial explanation of currentflow due to oxidation.

-

EF

LUMO

HOMO

LUMO

HOMO

Redox couplein solution

Electronenergy

n e-

Metallic electrode

Energy of electronsin metal increases

upon application of apotential more negativethan the thermodynamicequilibrium value.

A net reduction (cathodic)current flows from metal to

LUMO levels of redox activespecies in solution.

Pictorial explanation ofcurrent flow due to reduction.



0

c

∞c

δ

Diffusion layer(stagnant)

Bulk solution

(well stirred)

( )δ

0cc D f

−≅

∞

ΣMaterialflux

E l e c t r o d e

Transport viaMolecular diffusiononly

Double layerregion

Hydrodynamiclayer

A

B

A kDk ET

Transport and kineticsin electrochemical systems.

InterfacialET

Simple diffusion layermodel neglectsconvection effectsand also simplifiesanalysis of Fickdiffusion equations.

ξ = F(E-E0)/RT

-6 -4 -2 0 2 4 6

ψ =

i / i D

0.0

0.5

1.0

0ck f ET =Σ

{ } { }00

0

cck cc D

dx

dc D f D −=−=⎟

⎠

⎞⎜⎝

⎛ = ∞∞Σ

δ

D

k

c

c ET δ +=

∞

10

ET D

D ET

k k

ck k f +

= ∞Σ ∞∞

Σ

+=ck ck f D ET

111

ET

MT

ET & M T

[ ]ξ β ±= exp0k k ET

Normalisedpotential

δ

Dk D =

Transport and kinetics

at electrodes.

Net flux

nF

J

nFA

i f ==Σ

Current density

Potentiometricmeasurement

Amperometricmeasurement



ξ = θ − θ0 = F(E-E0)/RT

-10 -5 0 5 10 15 20

Ψ =

f Σ / f D

0.0

0.2

0.4

0.6

0.8

1.0

1.2

ζ = 0.1ζ = 10−2

ζ = 10−3

ζ = 10−4

Voltammetric profile:Irreversible ET coupled with diffusiveMass transport.

0

decreasing D

k

k ζ =

ξ = F(E-E0)/RT

-10 -5 0 5 10

Ψ

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

u,v

u

v

u

vu

v

u

v

A

B+e-

B+e-

A

A B+e-AB+e-

AB+e-

∞Σ=Ψ

Da

f δ

A∞A0

B0B∞

k D

k 0A∞A0

B0B∞

k D

k 0



ξ

-10 -5 0 5 10

Ψ

-2

-1

0

1

2

ζ = 10−2

ζ = 0.1ζ = 1ζ = 10ζ = 100

[ ] [ ]{ }[ ] [ ]{ }ξ−+βξ+ζ

ξ−κ−βξ=Ψ − exp1exp

exp1exp1

D

k

k

k

a

b

0

D

0 δ==ζ

=κ∞

∞

∞Σ=Ψ

Da

f δ

5.0

1

=β

=κ( )0EE

RT

F−=ξ

large ζ limit ΜΤcontrol

small ζ limit

ET control

A∞A0

B0B∞

k D

k 0A∞A0

B0B∞

k D

k 0

Transient electrochemical techniquesTransient electrochemical techniques



PotentiodynamicPotentiodynamic electrochemistryelectrochemistry

Schematic circuit

Voltammogram

Fe(CN)63-/4- (1 mM) in aqueous KCl (0.1 M)

J. Heinze, Angew. Chem. Int. Ed. Engl.,23 (1984) 831-847.

Cyclic Voltammetry

CyclicCyclic voltammetryvoltammetry (CV) is a form of(CV) is a form ofelectrochemical spectroscopy.electrochemical spectroscopy.

In CV the potential applied to theIn CV the potential applied to theelectrode is swept in a linear mannerelectrode is swept in a linear mannerfrom an initial valuefrom an initial value EEii to a givento a givenintermediate value Eintermediate value El,l, and then theand then thedirection of the potential scan isdirection of the potential scan isreversed to a final valuereversed to a final value EEff. Usually. Usuallythe initial and final potentials arethe initial and final potentials arethe same.the same.

The output obtained is called aThe output obtained is called avoltammogramvoltammogram and is a plot ofand is a plot ofcurrent versus applied potential.current versus applied potential.

A series of peaks appears atA series of peaks appears atdefinite potentials corresponding todefinite potentials corresponding tothe occurrence of oxidation andthe occurrence of oxidation andreduction reactions at the electrodereduction reactions at the electrode

surface.surface. A major experimental variable is theA major experimental variable is the

sweep ratesweep rate dE/dtdE/dt == νν. This defines. This definesthe experimental time scale.the experimental time scale.

Typically the shape of theTypically the shape of thevoltammogramvoltammogram as a function ofas a function ofsweep rate is examined. Peaksweep rate is examined. Peakpotentials Epotentials EPP, peak currents, peak currents iiPP, peak, peakseparationsseparations ΔΔEEPP and peak widths atand peak widths athalf peak heights d are measured ashalf peak heights d are measured asa function of sweep rate.a function of sweep rate.

P o t e n t i a l

timeEi

Eλ

Ef

EiEλ

Ef

Electrode

RED OX

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