1 applied electrochemistry dept. chem. & chem. eng

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Applied Electrochemistry

Dept. Chem. & Chem. Eng.

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Lecture 14Electrochemical sensors

Dept. Chem. & Chem. Eng.

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Outline

Introduction 1

Potentiometric sensors2

Amperometric sensors 3

Voltametric sensors4

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small devices that can be used for direct measurement of the analute in the sample matrix.

characteristics

(1) responding continuously and reversibly

(2) Does not perturb the sample

Chemical sensors:

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Contruction of chemical sensors

A transduction element covered with chemical or biochemical recognition layer

Target analyte interact with the recognition layer and change the resulting from interactions to electrical signals

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An example of biochemical sensor

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Electrochemical sensors

Electrochemical sensors is a subclass of chemical sensors in which electrode is used as a transduction elements

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Outline

Introduction 1

Potentiometric sensors2

Amperometric sensors 3

Voltametric sensors4

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Miniaturization of potentiometric sensors. (A) Conventional ion-selective electrode (ISE) with reference electrode connected to the field-effect transistor amplifier. (B) The electrical connection between ISE and the amplifier is made shorter. (C) Electrical connection is eliminated and the ISE membrane is placed directly at the input of the amplifier, thus forming an ISFET (D)

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Analytical potentiometric signal is equally divided between the ISE and the reference electrode.

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Principle

reference electrode 2 || sample solution | membrane | inner solution || reference electrode 1

Charge unique for the analyte

activity in the sample solution of analyte I

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Schematic view of the equilibrium between sample

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Classification and Mechanism

(1) Phase boundary potential

izi (membrane) ⇌ izi (sample)

Under equilibrium conditions

iM = i

W

which is equivalent to

aiW and ai

M are the ion activities in the sample and membrane phases

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with

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(2) Ion-exchanger-based ISEs(ion-selective electrodes)

Membrane compositions and selectivity coefficients of ion-exchanger-based ISEs

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The salt-extraction process can be defined as

I+(water) + X-(water) I⇌ +(membrane) + X-(membrane)

Also, the equilibrium reaction can be quantified by salt-partitioning constant, Kp, as defined by

Thus, the concentration of the aqueous anion in the cation-selective membrane doped with anionic sites is negligible in the charge balance in the membrane phase

[I+]M = [R-]M + [X-]M ⇌ [R-]M

selectivity of anion-exchanger-based ISEs followsClO4

- > SCN- > I- > NO3- >Br- > Cl- > HCO3

- > SO42- > F-

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(3) Neutral-ionophore-based ISEs

I+(Membrane) + L(membrane) IL⇌ +(membrane)

The formation constant, , is given by

= aILM/(aI

M aLM)

the charge balance in the membrane phase

RT = [I+]M + [IL+]M ⇌ [IL+]M

[L]M = LT - [IL+]M ⇌ LT - RT

aIL = IL

M RT/ LM(LT - RT)

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(4) Charged-ionophore-based ISEs

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Outline

Introduction 1

Potentiometric sensors2

Amperometric sensors 3

Voltametric sensors4

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Conceptual drawing of three electrode amperometric electrochemical sensor and potentiostat

When the information is obtained from measurement of current, that is in amperometric sensors, the role of the Ohm’s Law becomes immediately apparent.

1. Introduction

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2. Amperometric titrations

A. Simple amperometric titration

I

VVequiv

i

ii

iii

iv

Forms of amperometric titration

Two electrodes: a redox indicator electrode & a reference electrode

A fixed potential difference

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B. Biamperometric titration

Two redox electrodesNon-reference electrode

App: a reversible system before or after the endpoint

i

ii

iii

I

VVequiv.

R1 + O2 ⇌ O1 + R2

i Both 1 and 2 are rev.ii only 2 is rev.iii only 1 is rev.

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C. Amperometric titrations with double hydrodynamic electrode

generator A±n1e →Bsolution B+X → productsDetector B±n2e → C (or A)

Igen

Idet N0

N0

M2/3

N0

M2/3

J. Electroanal. Chem., 1983, 144, 211

N’= 0.035

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3. Membrane and membrane-covered electrodes

Enzyme & Microb. Tech. 1998, 23, 10–13

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L. C. Clark Jr., Trans. Am. Soc. Artif. Intern. Organs, 1956, 2, 41

The Clark electrode for determination of dissolved oxygen

Amperometric sensors for dissolved gases

Gases dissolved in aqueous phase: O2, NO, halothane, CO2

Gas phase: H2S, HCN, CO, NO, NO2, Cl2

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4. Modified electrodes

A. Chemical modification {chemical bonding).

B. Adsorption

C. Electroadsorption

D. Plasma( 等离子体 )

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Examples of modifiers for amperometric sensors

Bard and Faulkner, 2001, pp. 584–585

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Processes that can occur at a modified electrode.

(1) heterogeneous reduction process; (2) successive transfer of electron between reduced molecules Q (5), until the transfer to A at the surface (3); (4) diffusion of A into the film and its reaction with Q; (6) direct penetration of A through the pinhole to the substrateelectrode

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5. Increase in sensitivity: pre-concentration techniques

principle

a. application of a pulse waveform and a.c. voltammetry

b. utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface

process

a. Deposition or adsorption of the species on the electrode

b. Change to an inert electrolyte medium

c. Reduction/oxidation of the species that was accumulated at the electrode

 i = nFAvA 

33Talanta, 2006, 69, 259–266

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Principles of pre-concentration techniques

Method Preconcentration step

Determination

step

Measure-ment

A Stripping

voltammetry

Potential

control

Potential

control

I vs. t

(or I vs. E)

B Adsorptive

stripping

voltammetry

Adsorption (with

or without

applied potential)

Potential

control

I vs. t

(or I vs. E)

C Potentiometric

stripping

analysis

Potential

control

Reaction with

Oxidant or reductant in solution

E vs t

D Stripping Chron-opotentiometry

Potential

control

Current control E vs t

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e.g. Determination step in stripping techniques

I

E

Ip c

Ep → species

A(anodic)I

E

Ip cEp → species

B(cathodic)

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the choice of which technique and experimental protocol to use depends on various factors

• The concentration range of the species to be determined• Possible interferences to its exact determination, i.e. matrix composition• The accuracy and precision necessary• The quantity of sample• The required speed with which an answer is required

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Example 1. Direct Oxidation of Glucose Oxidase

reactions take place in the enzyme layer

Schematic diagram of a simple amperometric biosensor

Ag anode:  4Ag + 4Cl- 4AgCl + 4e⇌ -

Pt cathode:    O2 + 4H+ + 4e- 2H⇌ 2O

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NAD+ nicotinamide adenine dinucleotide

(a) Ferrocene (e5-bis-cyclopentadienyl iron), the parent compound of a number of mediators. (b) TMP+, the cationic part of conducting organic crystals. (c) TCNQ.-, the anionic part of conducting organic crystals. It is a resonance-stabilised radical formed by the one-electron oxidation of TCNQH2

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Substrate(2H) + FAD-oxidase ⇌Product + FADH2-oxidase

(a) biocatalyst FADH2-oxidase + O2 ⇌FAD-oxidase + H2O2

electrode

H2O2 O⇌ 2 + 2H+ + 2e-

(b) biocatalyst

FADH2-oxidase + 2 Ferricinium+ ⇌FAD-oxidase + 2 Ferrocene + 2H+ electrode

2 Ferrocene 2 Ferricinium⇌ + + 2e-

(c) FADH2-oxidase FAD-oxidase + 2H⇌ + + 2e-

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Example 2. Ethanol Electrodes

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Example 3. Urea Electrodes

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Some of Common Enzyme Electrodes

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Gas Sensors

Some potentiometric gas sensors

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Example 4. CO2 Sensors

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Example 5. O2 Sensors

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Outline

Introduction 1

Potentiometric sensors2

Amperometric sensors 3

Voltametric sensors4

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Def. voltammetric systems are produced commercially for the determination of specific species that are of interest in industry and research. These devices are sometimes called electrodes but are, in fact, complete voltammetric cells and are better referred to as sensors. These sensors can be employed for the analysis of various organic and inorganic analytes in various matrices

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Scheme of voltametric electrochemical sensor

PLM (Permeation Liquid Membrane) and voltammetric detector (WE: working electrode, MAE: micro auxiliary electrode).

 Novel PLM-voltammetric Coupling Devices for Trace Metal Speciation   Proc. ECS 2003, Paris

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Typical example of real-time measurement using the system

 Novel PLM-voltammetric Coupling Devices for Trace Metal Speciation   Proc. ECS 2003, Paris

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voltammetric determination of acetaminophen, aspirin and caffeine

Electroch. Acta, 2010, 55, 8638–8648

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AdSDPV curves obtained for the oxidation of ACOP, ASA and CF at equal concentrations of each: (1) blank, (2) 2.91 × 10−7, (3) 2.89 × 10−6, (4) 7.62 × 10−6, (5) 1.78 × 10−5, (6) 2.56 × 10−5, (7) 3.10 × 10−5, (8) 4.08 × 10−5, (9) 5.32 × 10−5 and (10) 6.27 × 10−5 M.