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Applied Electrochemistry. Dept. Chem. & Chem. Eng. Lecture 14 Electrochemical sensors. Dept. Chem. & Chem. Eng. 1. Introduction. 2. 4. Potentiometric sensors. Voltametric sensors. 3. Amperometric sensors. Outline. Chemical sensors :.

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Applied Electrochemistry

Dept. Chem. & Chem. Eng.


Lecture 14

Electrochemical sensors

Dept. Chem. & Chem. Eng.


1

Introduction

2

4

Potentiometric sensors

Voltametric sensors

3

Amperometric sensors

Outline


Chemical sensors:

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


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


An example of biochemical sensor


Electrochemical sensors

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


1

Introduction

2

4

Potentiometric sensors

Voltametric sensors

3

Amperometric sensors

Outline


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)


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


Principle

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

activity in the sample solution of analyte I

unique for the analyte

Charge


Schematic view of the equilibrium between sample


Classification and Mechanism

(1) Phase boundary potential

izi (membrane) ⇌ izi (sample)

Under equilibrium conditions

iM = iW

which is equivalent to

aiW and aiM are the ion activities in the sample and membrane phases


with


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

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


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 follows

ClO4- > SCN- > I- > NO3- >Br- > Cl- > HCO3- > SO42- > F-


(3) Neutral-ionophore-based ISEs

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

The formation constant, , is given by

 = aILM/(aIMaLM)

the charge balance in the membrane phase

RT =[I+]M + [IL+]M⇌ [IL+]M

[L]M =LT - [IL+]M⇌ LT- RT

aIL = ILM RT/LM(LT - RT)


(4) Charged-ionophore-based ISEs


1

Introduction

2

4

Potentiometric sensors

Voltametric sensors

3

Amperometric sensors

Outline


1. Introduction

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

Conceptual drawing of three electrode amperometric electrochemical sensor and potentiostat


2. Amperometric titrations

A. Simple amperometric titration

Two electrodes:

a redox indicator electrode &

a reference electrode

i

I

ii

iii

iv

V

Vequiv

A fixed potential difference

Forms of amperometric titration


B. Biamperometric titration

R1 + O2⇌ O1 + R2

Two redox electrodes

Non-reference electrode

I

i

ii

App: a reversible system before or after the endpoint

iii

i Both 1 and 2 are rev.

ii only 2 is rev.

iii only 1 is rev.

V

Vequiv.


C. Amperometric titrations with double hydrodynamic electrode

generator A±n1e →B

solution B+X → products

Detector B±n2e → C (or A)

Idet

N0

N’= 0.035

M2/3

N0

M2/3

Igen

N0

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


3. Membrane and membrane-covered electrodes

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


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

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


4. Modified electrodes

A. Chemical modification {chemical bonding).

B. Adsorption

C. Electroadsorption

D. Plasma(等离子体)


Examples of modifiers for amperometric sensors

Bard and Faulkner, 2001, pp. 584–585


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 substrate

electrode


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

i = nFAvA

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


Talanta, 2006, 69, 259–266


Principles of pre-concentration techniques


e.g. Determination step in stripping techniques

A(anodic)

B(cathodic)

I

I

Ip c

Ip c

Ep→species

Ep→species

E

E


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


Example 1. Direct Oxidation of Glucose Oxidase

reactions take place in the enzyme layer

Ag anode:  4Ag + 4Cl- ⇌ 4AgCl + 4e-

Pt cathode:    O2 + 4H+ + 4e- ⇌ 2H2O

Schematic diagram of a simple amperometric biosensor


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


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

(a) biocatalyst

FADH2-oxidase + O2 ⇌ FAD-oxidase + H2O2

electrode

H2O2 ⇌ O2 + 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-


Example 2. Ethanol Electrodes


Example 3. Urea Electrodes


Some of Common Enzyme Electrodes


Gas Sensors

Some potentiometric gas sensors


Example 4. CO2 Sensors


Example 5. O2 Sensors


1

Introduction

2

4

Potentiometric sensors

Voltametric sensors

3

Amperometric sensors

Outline


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


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


Typical example of real-time measurement using the system

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


voltammetric determination of acetaminophen, aspirin and caffeine

Electroch. Acta, 2010, 55, 8638–8648


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.


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