Current based methods

Amperometric and voltammetric sensors

More significant influence on analytical parameters (sensitivity, selectivity,interferences elimination) – kind of method, potential range, electrode material .

The signal is linearly dependent on analyte concentration.

A greater number of exchanged electrons – higher sensitivity(in potentiometry – lower sensitivity).

Low apparatus costs.

Numerous analytes.

Detection limit to 10-12 mol/dm3.

No need of electrode conditioning.

Electrodes

3-electrode system:

w.e. (working electrode), current electrode, controlled potential vs. reference electrode

r.e. (reference electrode), no current flow

c.e. (counter electrode), e.g. Pt, currentelectrode, the potential is not controlled

Benefits of 3-electrode arrangement:- controlled potential of the working electrode,- no polarization (and potential change) of the reference electrode,- low influence of ohmic potential drop, iR.

Oxygen presence:O2 + 2H+ + 2e → H2O2

Current at constant potential: chronoamperometry

Linear diffusion: toward the flat electrode surface

δ – diffusion layer thickness

Potential, E

Cyclic voltammetry

ab c

d

Non-stationary method

Randles-Sevcik equation

e.g.Fe(CN)6

3-/4-

Ox formpresent in solution

Electrode materials

Mercury

Renewable surface.

High overpotential for hydrogen evolution,Hg oxidation at positive potentials;Applicable in negative potential range.

Typical electrodes: DME, HMDE (elektrodeof Kemula, Kublik), SMDE, MFE.

For dropping Hg electrode – polarography,quasi-stationary state

Toxic metal

Platinum

Noble metal, catalytically active,high rate of charge transfer reactions

Low overpotential for hydrogen evolution;applicable range of positive potentials.

Decreasing electroactivitydue to impurities adsorption;need for careful electrode surfacepretreatment.

Other metals

Gold – noble metal, catalytically active,lower activity than for Pt,hydrogen evolution overpotential higher than for Pt.

Copper, nickel – detection of amino acids andcarbohydrates.

Silver – detection of cyanide and sulphur compounds.

Bismuth – alternative for mercury.

Metal alloys – specific applicationscan have catalytic properties.

Carbon – (glassy carbon, GC)

Very popular electrode material.

Non-porous material.

High overpotential for hydrogen evolution;a wide potential window.

Porous glassy carbon (RVC – reticulatedvitreous carbon) – high surface area(e.g. 66 cm2/cm3, applicationin flow analysis.

RVC

Carbon – carbon paste

Mixture of graphite and oil.

Very popular electrode material.

Relatively low background current,wide potential range,lower rate of charge transfer reaction.

Renewable surface (new portions of paste).

Easy to modify, by addition of enzymesto the paste (biosensors).

Risk of oil lekage into the soloution(if organic solvent is present).

Carbon – other electrodes

Carbon fibre, diameter ∼μm,applied in microelectrodes,mainly to study biological objets – tissues, cells(e.g. for determination of neurotransmitters in brain).

Diamond electrodes – after doping,e.g. by boron; wide potential range up to 3 V,weak adsorption, low background level, usefulfor work under extreme conditions.

Screen printed electrodes (graphite witha polymeric binder) – disposable electrodes.

Electrical double layer

C = Q / ΔE

C – capacitance

Q = C ΔE

Capacitive current iC = C (dE/dt)

Capacitive current

cF iii +=

⎟⎠⎞

⎜⎝⎛ −Δ

=RC

tREic exp

Typical conditions: R = 10 Ω, C = 5⋅10-7 F,

RC = 5⋅10-6 s (5 μs)

After 50 μs ic ∼ 0

For cyclicvoltammetry

q = C E

dtdEC

dtdqic ==

ic ∼ v iF ∼ v1/2

Detection limit, ~ 10-5 mol/dm3

v

ChronoamperometryE = const

ΔE

E

t

Elimination of ic – pulse methods

iC ~ exp(-t / RC)

iF ~ 1/t1/2

Normal pulse voltammetry, NPV)

Time tm adjustedto eliminate the capacitivecomponent

Differential pulse voltammetry, DPV

Pulse voltammetry

DPV (differential)

NPV (normal)

1 mg/dm3 Pb(II) i Cd(II), 0.1 mol/dm3 HNO3

Square wave voltammetry, SWV

High sensitivity. Compensation ofcapacitive currents forreduction and oxidation.High rate: f ΔE,e.g. f = 50 Hz, ΔE = 10mV, rate 0.5 V/s.

A – reductionB- oxidationC - difference

Detection limit lowering

•Preconcentration (cathodic process) ⇒ determination (anodicprocess)Anodic stripping voltammetry (ASV)e.g. Mn+ + ne → M(Hg), M(Hg) → Mn+ + ne

•Preconcentration (cathodic process) ⇒ determination(cathodic process)Cathodic stripping voltammetry (CSV)np. Hg + X- → HgX + e, HgX + e → Hg + X-

•Adsorption (coadsorption) ⇒ determinationAdsorptive Stripping Voltammetry (AdSV)

Mn+

Mn+

Mn+

•Catalytic processesnp. Mn+ + e → M(n-1)+; M(n-1)+ + Ox → Mn+ + Red

↑_______________________________↓

•Joint methods

Example:Fe3+ + e → Fe2+

Fe2+ + H2O2 → Fe3+ + H2OFe3+ + e → Fe2+

Comparison of voltammetric techniques

J. Wang, Analytical Electrochemistry, Wiley-VCh

Stripping voltammetryDetermination of traces of metals

Mixture of cations2 . 10-7 mol/dm3 each

Determination of lead traces

Determination of folic acid in pharmaceuticalsubstances

Determination of metal ions

Stripping voltammetry (cathodic preconcentration)

Voltammetry with adsorptive preconcentration and reduction of complexes

Voltammetry with ligand reduction

Voltammetry with catalytic process

Clark electrode(oxygen determination)

Cathode (Pt or Au): O2 + 2 H2O + 4e → 4 OH-

Electrolyte: aqueous solution of KClAnode (Ag): 4 Ag + 4 Cl- → 4 AgCl + 4eMembrane: polietylene or polytetrafluoroethylene

Biosensors

Electrochemical method + detection+

biological recognition

Biocatalytic sensorsusing enzymes, cells,tissues, immobilized biocomponents

Affinity biosensorsusing antigenes,membrane receptorsnucleic acids

Enzymatic biosensors

Electrode

Biocatalytic layer(enzyme)

Sample solution

S: substrate (analyte)

C: cofactor

30 – 0,3 μl bloodtime 5 – 120 s

Glucose sensor

The first example of enzymatic electrode (with glucose oxidase)

Enzyme entrapped in a polyuretane membrane

Glucose + O2 Gluconic acid + H2O2

Glucose oxidase

electrochemicaldetection

H2O2 → O2 + 2 H+ + 2e

Replace oxygen by another electron acceptor ?...

Difficulty in in direct electron transfer between enzyme centreand the electrode – protein shell