Current based methods

Transcription

1 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 mol/dm 3. No need of electrode conditioning.

2 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, current electrode, the potential is not controlled Oxygen presence: O 2 + 2H + + 2e H 2 O 2 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.

3 Current at constant potential: chronoamperometry Potential, E δ diffusion layer thickness Linear diffusion: toward the flat electrode surface

4 Randles-Sevcik equation Cyclic voltammetry e.g. Fe(CN) 6 3-/4- a b c Ox form present in solution d Non-stationary method

5 Electrode materials

6 Mercury Renewable surface. High overpotential for hydrogen evolution, Hg oxidation at positive potentials; Applicable in negative potential range. Typical electrodes: DME, HMDE (elektrode of Kemula, Kublik), SMDE, MFE. For dropping Hg electrode polarography, quasi-stationary state Toxic metal

7 Platinum Noble metal, catalytically active, high rate of charge transfer reactions Low overpotential for hydrogen evolution; applicable range of positive potentials. Decreasing electroactivity due to impurities adsorption; need for careful electrode surface pretreatment.

8 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 and carbohydrates. Silver detection of cyanide and sulphur compounds. Bismuth alternative for mercury. Metal alloys specific applications can have catalytic properties.

9 Carbon (glassy carbon, GC) Very popular electrode material. Non-porous material. High overpotential for hydrogen evolution; a wide potential window. Porous glassy carbon (RVC reticulated vitreous carbon) high surface area (e.g. 66 cm 2 /cm 3, application in flow analysis. RVC

10 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 enzymes to the paste (biosensors). Risk of oil lekage into the soloution (if organic solvent is present).

11 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, useful for work under extreme conditions. Screen printed electrodes (graphite with a polymeric binder) disposable electrodes.

12 Electrical double layer C = Q / ΔE C capacitance Q = C ΔE Capacitive current i C = C (de/dt)

13 Capacitive current i = i F + i c For cyclic voltammetry q = C E v Chronoamperometry E = const i c = ΔE R t exp RC Typical conditions: R = 10 Ω, C = F, E ΔE t dq i c = = dt C de dt i c v i F v 1/2 RC = s (5 μs) Detection limit, ~ 10-5 mol/dm 3 After 50 μs i c 0

14 Elimination of i c pulse methods i C ~ exp(-t / RC) i F ~ 1/t 1/2

15 Normal pulse voltammetry, NPV) Time t m adjusted to eliminate the capacitive component

16 Differential pulse voltammetry, DPV

17 Pulse voltammetry DPV (differential) NPV (normal) 1 mg/dm 3 Pb(II) i Cd(II), 0.1 mol/dm 3 HNO 3

18 Square wave voltammetry, SWV A reduction B- oxidation C - difference High sensitivity. Compensation of capacitive currents for reduction and oxidation. High rate: f ΔE, e.g. f = 50 Hz, ΔE = 10 mv, rate 0.5 V/s.

19 Detection limit lowering Preconcentration (cathodic process) determination (anodic process) Anodic stripping voltammetry (ASV) e.g. M n+ + ne M(Hg), M(Hg) M n+ + ne Preconcentration (cathodic process) determination (cathodic process) Cathodic stripping voltammetry (CSV) np. Hg + X - HgX+ e, HgX+ e Hg + X - Adsorption (coadsorption) determination Adsorptive Stripping Voltammetry (AdSV) M n+ M n+ M n+ Catalytic processes np. M n+ + e M (n-1)+ ; M (n-1)+ + Ox M n+ + Red Example: Fe 3+ + e Fe 2+ Fe 2+ + H 2 O 2 Fe 3+ + H 2 O Fe 3+ + e Fe 2+ Joint methods

20 Comparison of voltammetric techniques J. Wang, Analytical Electrochemistry, Wiley-VCh

21 Stripping voltammetry Determination of traces of metals Mixture of cations mol/dm 3 each Determination of folic acid in pharmaceutical substances Determination of lead traces

22 Determination of metal ions Stripping voltammetry (cathodic preconcentration) Voltammetry with adsorptive preconcentration and reduction of complexes Voltammetry with ligand reduction Voltammetry with catalytic process

23 Clark electrode (oxygen determination) Cathode (Pt or Au): O H 2 O + 4e 4 OH - Electrolyte: aqueous solution of KCl Anode (Ag): 4 Ag + 4 Cl - 4 AgCl + 4e Membrane: polietylene or polytetrafluoroethylene

24 Biosensors Electrochemical method + detection + biological recognition Biocatalytic sensors using enzymes, cells, tissues, immobilized biocomponents Affinity biosensors using antigenes, membrane receptors nucleic acids

25 Enzymatic biosensors Electrode 30 0,3 μl blood time s S: substrate (analyte) C: cofactor Biocatalytic layer (enzyme) Sample solution

26 Glucose sensor The first example of enzymatic electrode (with glucose oxidase) Enzyme entrapped in a polyuretane membrane Glucose oxidase Glucose + O 2 Gluconic acid + H 2 O 2 H 2 O 2 O H + + 2e electrochemical detection Replace oxygen by another electron acceptor?... Difficulty in in direct electron transfer between enzyme centre and the electrode protein shell