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Square-wave voltammetry: the most advanced electroanalytical technique

Square-wave voltammetry: the most advanced electroanalytical technique. Valentin Mir č eski Institute of Chemistry Faculty of Natural Sciences and Mathematics “Ss Cyril and Methodius” University, Skopje Republic of Macedonia. Square-Wave Voltammetry: Potential Modulation. Red  Ox + e.

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Square-wave voltammetry: the most advanced electroanalytical technique

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  1. Square-wave voltammetry: the most advanced electroanalytical technique Valentin Mirčeski Institute of Chemistry Faculty of Natural Sciences and Mathematics “Ss Cyril and Methodius” University, Skopje Republic of Macedonia

  2. Square-Wave Voltammetry: Potential Modulation Red  Ox + e • DE E / V Esw E / V • t Ox + e  Red t / s t / s A single potential cycle consisting of a two equal potential pulses superimposed on a single potential tread in two opposite (anodic and cathodic) directions. The current is measured at the end of each pulse in order to discriminate against the capacitate current and to extract only the faradic response of the electrode reaction. Properties of the potential modulation are: Esw – SW amplitude (pulse height); DE –potential step; t– duration of a single potential cycle; f - frequency of the pulses. Square-wave voltammetry (SWV) is a pulsed voltammetric technique. The potential modulation consists of a train of equal potential pulses superimposed on a staircase potential ramp. f = 1/t v = DE f

  3. Variation of the current with the time in the course of the experiment

  4. Faradaic vs. capacitive current in the course of a single potential pulse Faradaic current I f (due to electrode reaction) If/ Ic >> 1 (sampling point) I Capacitive current, Ic (due to charging - formation of the double layer) t 0

  5. Ynet Ynet = Yf - Yb Yf Yb SW voltammogram Net component, calculated (not measured!) as a difference between the forward and backward components Forward component measured at the end of each pulse with odd serial number (i.e., 1st, 3rd, etc.; Backward component measured at the end of each pulse with even serial number (i.e., 1st, 3rd, etc.;

  6. Time window of the voltammetric experiment • SWV • Scan rate: v = f DE • Example: • DE = 0.1 mV, f = 200 Hz • v = 0.020 V/s t = 1/f • = 5 ms • Example: • DE = 0.1 mV, f = 500 Hz • v = 0.050 V/s t = 2 ms CV For 300 mV potential path v = 60 V/s v = 150 V/s

  7. A technique for mechanistic, kinetic and analytical application An electrode reaction of a dissolved redox couple irrevrersible quasirev. reversible Surface confined electrode reaction irrevrersible quasirev. reversible Y

  8. EC mechanism ECE mechanism

  9. Electrode mechanisms • Electrode reaction of an immobilized redox coupe (surface electrode reaction); • Electrode mechanism involving formation of an insoluble compound with the electrode material;

  10. Reaction scheme for the electrode reaction of an immobilized redox coupe (surface confined electrode reaction)  Oxbulk Ox(ads) Electrode ne- Diffusion mass transport is neglected  Redbulk Red(ads) Ox(ads) + ne- Red(ads)

  11. Application: Protein-film voltammetry; Electrochemicaly active drugs; Simple adsorbates (many organic compounds); Azodies; Metal complexes; Organometalic compounds; Surface modified electrodes; Voltammetry of solid micro- particles etc. Toward electrode kinetic measurements: Modeling and application

  12. w increases Net dimensionless SW voltammograms simulated for different reversibility of the electrode reaction Dimensionless current Y = I/(nFAG*f ) w = ks / f irreversible quasireversible region reversible

  13. DYp log(w) Quasireversible maximum and the SW response at the quasireversible maximum

  14. The origin of the quasireversible maximum: Chronoamperometry of the surface eelectrode reaction f = 250 Hz, a = 0.5 ks = 500 s-1 ks = 375 s-1 dimensionless current ks = f ks = 25 s-1 t Synchronisation of the rate of the redox transformation with the SW frequency!

  15. Simple methodology for using the quasireversible maximum for redox kinetic measurements wmax = ks / fmax wmaxcalculated by the model fmax measured in theexperiment ks = wmaxfmax

  16. Splitting of the net SW response for fast and reversible surface electrode reaction w increases

  17. The Origin of the Splitting log(w) = 0.4 log(w) = 0 log(w) = 0.1

  18. The dependence of the splitting on the SW amplitude • Experimental systems that have been analyzed on the base of quasireversible maximum and the splitting: • Cytochrome C; • Alyzarine red-S; • Probucole; • 2-propylthiouracil; • Fluorouracil; • Molybdenum(VI)-phenantroline-fulvic acid; • Azobenzene; • Methilene blue,….; DEp / mV Esw / mV

  19. Examples of surface confined electrode reactions alizarin vitamin B12 vitamin K2

  20. Comparison of theoretical (□) and experimental (○) net peak currents for alizarin as a function of pH.

  21. Mo(VI)-phenantroline-fulvic acid system ks = 8  2 s-1; a= 0.41  0.05 n = 2

  22. Splitting of the net SW response of methylene blue under the influence of the SW amplitude amplitude increases methylene blue 3,7-bis(Dimethylamino)-phenothiazin-5-ium chloride

  23. Square wave voltammetry of azurin immobilized on 1-decanethiol-modified gold Azurin – a blue copper protein

  24. famotidine Square wave voltammetry of famotidin: catalytic hydrogen evolution reaction from adsorbed state Electrode mechanism Fam(ads) FamH+(ads) FamH+(ads) + e- Fam(ads) + H(aq)

  25. Square wave voltammetry of 2-guanidinobenzimidazole: another example for the catalytic hydrogen evolution reaction from adsorbed state

  26. S S S S S S S S Reaction scheme of an electrode reaction involving formation of chemical bonds with the electrode ne- Application: • Sulfur containing amino acids; • Glutathione and other cysteine containing peptides and proteins; • Mercaptans; • Thyroxin; • Thiourea; • Thioethers; • Phorphyrins; • Flavins; • Sulphide; • Iodide etc. S Electrode

  27. Modeling HgL (s) + 2e- Hg(l) + L2-(aq) HgL2(s) + 2e- Hg(l) + 2L-(aq) HgL (s) + 2e- L2-(ads) + Hg(l)  L2-(aq) HgL2(s) + 2e- 2L-(ads) + Hg(l)  2L-(aq)

  28. Qvazireversible maximum of the cathodic stripping reaction Dimensionless current Y = I / (nFAc*(Df )1/2) ks = kmaxD1/4fmax3/4rs-1/2rs= 1 cm precision ± 10 %

  29. pH = 5.6 ks = 5  0.2 cm s-1 pH = 7.0 Ipf -1 / mA s pH = 8.5 f / Hz Cathodic stripping voltammetry of glutathione -0.35 -0.25 I /m A -0.05 0.05 -0.200 -0.300 -0.400 -0.500 -0.600 -0.700 E / V

  30. Cathodic stripping voltammetry of glutathione in the presence of copper -0.20 -0.23 Without Cu2+ With Cu2+ -0.10 I / mA -0.13 I / mA -0.05 0 -0.03 ks < 0.11 cm s-1 ks = 5.22 cm s-1 0.07 0.10 -0.300 -0.500 -0.700 -0.300 -0.500 -0.700 E / V E / V

  31. Influence of different cations on the SW net peak currents of glutathione Cu Mg Ba Ca Zn

  32. S S S S S S Mx+ Mx+ Mx+ The influence of the metal ions on the morphology of the film deposited on the electrode ne- • Additional Interactions: • attraction • repulsion • complexation Electrode

  33. Cathodic stripping mechanism coupled with a chemical reaction theoretical experimental 6-mercaptopurine-9-D-riboside in the presence of nickel(II) ions A(aq) = L(aq) L(aq) + Hg(l) = HgL (s) + 2e-

  34. Cyclic Square-Wave Voltammetry: a technique of the future

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