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.
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