Chemically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 91

OOH

OOH

O

O

NNH HHN

O

O

OOH

HHO HHO

HHO O

NNH HHN

O

OOH HHO

OOH HHO

OOH

Cylam

O

O

OOH HHO

OOH

HHO

O

O OOH HHO

HHO O

OOH

O O

HHO HHO HHO

OOH

O

O

O

O

O O OOH

O

HHO

(A) Dibenzo crown-6 ether Calix [4]arene β-Cyclodextrin

R R R

R

R R

R

OR ´ OR ´

OR ´ OR ´

OR ´ OR ´ OR ´ OR ´

(B)

FIGURE 4.9 Examples.of.macrocyclic.compounds.used.to.modify.CPEs.(A).and.an.example.highlighting.
the.chemical.structure.of.a.simple.calixarene.from.which.it.is.easy.to.see.the.cup.shape.of.the.molecule.likely.
to.act.as.a.host.for.guest.species.(B).. Notes :.The.symbol.R.is.not.a.single.element.but.a.“group”.of.atoms..The.
structure.of.the.R.group.can.be.varied.to.give.the.basic.calixarene.structure.a.more.selective.action.rather.than.
simply.working.on.the.basis.of.size.

4.3.2.2 Organic Catalysts
Charge.transfer.mediators.are.extremely.important.in.electroanalysis.because.they.enable.order.to.
reduce.the.overpotential.usually.observed.for.numerous.electroactive.analytes..This.contributes.to.
increasing.the.sensitivity.of.the.electrochemical.detection.as.well.as.to.enhance.the.selectivity.by.
lowering.the.working.potential.at.values.below.those.characteristic.of.interferences.(with.eminent.inter-
est.in.the.ield.of.amperometric.biosensors,.see.Chapter.5)..The.general.principle.of.the.so-called.
“mediated.electrocatalysis”.is.illustrated.in.Figure.4.10.and.described.by.Equations.4.4.through.4.6,.
for.example,.of.the.speciic.oxidation.of.a.substrate. S Red .into.a.product. P Ox .
Assuming.a.high.overpotential.for.the.electrochemical.oxidation.of. S Red .( E

O
′ ,.Equation.4.4;.
the.subscript. S .indicates.the.substrate,. M .the.mediator;.the.prime.symbol.designates.the.observed.
signal.with.the.corresponding.overpotential),.the.addition.of.a.mediator. M Red .(from.a.fast. M Ox / M Red .
redox.couple).enables.lowering.the.overpotential.barrier.by.electrogeneration.of. M Ox .at. E

O

E

S S

0 0
′ = .
(Equation.4.6),.which.then.reacts.chemically.with. S Red .to.produce. P Ox .with.concomitant.regeneration.
of.the.catalyst. M Red .(Equation.4.5).
The. overall. process. results. therefore. in. the. electrochemical. transformation. of. S Red . at. a. much.
lower.overpotential.than.the.initial.substrate.in.the.absence.of.mediator.( E

E

M M

O

′

E

O

′

):

M S

− O

′

O

S − → ne P ( E E ) .

(4.4)

.

Red Ox S S