Pulse Voltammetric Techniques The important parameters for pulse techniques are as follows: 1. Pulse amplitude is the height of the potential pulse. This.

Pulse Voltammetric Techniques The important parameters for pulse techniques are as follows:

1. Pulse amplitude is the height of the potential pulse.

This may or may not be constant depending upon the technique.

2. Pulse width is the duration of the potential pulse.

3. Sample period is the time at the end of the pulse during which the current is measured.

4. For some pulse techniques, the pulse period or drop time must also be specified. This parameter defines the time required for one potential cycle, and is particularly significant for polarography

Frederick Gardner Cottrell Born in Jan. 10, 1877, Oakland, California, U.S.A.

Dead in November 16, 1948, Berkeley, Calififornia, U.S.A.

Frederick Gardner Cottrell was U.S. educator, scientist, and inventor of the electrostatic precipitator, a device that removes suspended particles from streams of gases. However, he is the best known for electrochemists because of the "Cottrell equation".

time

i

Potential Step Method

time

i

100 mV -300 mV + 50 mV

Concentration vs. distance above the electrode before voltage step

Concentration vs. distance above the electrode a short time after a voltage step

2

323

x

FeD

t

Feo

t

DFenFAi

3

so,

ti

1

Linear Sweep Voltammetry

• V1•  •  • Ep•  •  • V2

V1

time

V2

100 mV -300 mV + 50 mV

red

oxo

C

C

nF

RTEE ln

E0= 0.771 V vs. NHE (ca. 0.57 V vs. Ag/AgCl)

Nernst equation

Concentration Profile of Potential Sweep

now see instead of a current spike, a curve

100 mV + 50 mV

-300 mV [Fe3+]

Distance from electrode x

constant diffusion limited current

Ep

Effect of Scan Rate

2/12/12/3 OFADni p

Typical cyclic voltammogram

Concentration Profile

Fig. 2. Qualitative diagrams showing concentration--distance profile atvarious stage of the cyclic voltammogram shown in left Fig..

Cyclic Voltammetry – ox + e ⇄ Red

1. the peak potential separation (Epa - Epc) is equal to 57/n mV for all scan rates where n is the number of electron equivalents transferred during the redox process.

2. the peak width is equal to 28.5/n mV for all scan rates.

3. the peak current ratio (ipa/ipc) is equal to 1 for all scan rates.

4. the peak current function increases linearly as a function of the square root of v.

Animated Cyclic Voltammetry Experiment 

E(t) = Ei + v t

Nernst equation;  

E = Eo' + RT/nF ln aO /aR

Fc Fc+ + e-

Example of Nernstian (reversible) Behavior 

Electrode area = 0.1 cm2 ks = 1 cm/s, Eo = 0.3 V DO = DR = 1 x 10-5 cm2/s

ip = 2.69 x 105 n3/2 A DO1/2 v1/2 CO

Determination of the Peak Current

ip = 2.69 x 105 n3/2 A DO1/2 v1/2 CO

 where: A is the electrode area (cm2 ), n is the number of electrons transferred, CO is the concentration (mol.cm-3 ),

v is the scan rate (volt/s.)  

Electrochemical Cell

The Nernst Equation 1. The Nernst equation is named after the German physical chemist

Walther Nernst who first formulated it. 2. The Nernst equation links the actual reversible potential of an

electrode (measured in volts), E, to the standard reversible potential of the electrode couple, E0 which is a thermodynamic value.

3. In its most fundamental form the Nernst equation is written as:

R: the universal gas constant, T: the absolute temperature in degrees Kelvin, z : the charge number of the electrode reaction (which is the number of moles of electrons involved in the reaction as written), F: is the Faraday constant (96,500 C mole-1).

The notation ared represents the chemical activities of all of the species which appear on the reduced side of the electrode reaction and the notation aox represents the chemical activities of all of the species which appear on the oxidized side of the electrode reaction. 

Introduction • The charging current decays exponentially, whereas the

faradaic current (for a diffusion-controlled current) decays as a function of 1/(time)½; that is, the rate of decay of the charging current is considerably faster than the decay of the faradaic current.

• The charging current is negligible at a time of 5RuCdl after

the potential step (RuCdl is the time constant for the

electrochemical cell, and ranges from µs to ms). • Therefore, after this time, the measured current consists

solely of the faradaic current; that is, measuring the current at the end of a potential pulse allows discrimination between the faradaic and charging currents

Cyclic Voltammetry – ox + e ⇄ Red

1. the peak potential separation (Epa - Epc) is equal to 57/n mV for all scan rates where n is the number of electron equivalents transferred during the redox process.

2. the peak width is equal to 28.5/n mV for all scan rates.

3. the peak current ratio (ipa/ipc) is equal to 1 for all scan rates.

4. the peak current function increases linearly as a function of the square root of v.

Cyclic Voltammetry

• ipc and ipa refer to the peak

current of the cathodic and anodic sweeps.

• Epa and Epc refer to the

potential (voltage) of the anodic and cathodic current peaks.

A CV of a fully reversible system will display the following characteristics:

1) The voltage separation between the current peaks is

2) The positions of peak voltage do not alter as a function of voltage scan rate

3) The ratio of the peak currents is equal to one

4) The peak currents are proportional to the square root of the scan rate

The influence of the voltage scan rate on the current for a reversible electron transfer can be seen below:

• The effect of differences in the kinetic rate (kred) constant for electron

transfer on the LSV of a system are illustrated below: (Left)

• Both Epc and ipc shift to different values as electron transfer becomes

more difficult. In cyclic voltammetry, a system can have different cathodic (Kred) and anodic (Kox) kinetic rate constant. Such systems are

known as quasi-irreversible. The CV is illustrated below: (Right)

Different pulse techniques• A number of different pulse techniques are available on the epsilon,

which differ in their potential pulse wave forms, the number of sampling points, and whether a solid electrode (voltammetry) or a mercury drop electrode (polarography) is used.

• The discrimination against the charging current that is inherent in these techniques leads to lower detection limits (when compared to linear sweep techniques), which makes these techniques suitable for quantitative analysis.

• Sampled Current Polarography• Normal Pulse Voltammetry/Polarography (NPV)• Differential Pulse Voltammetry/Polarography (DPV)• Square Wave Voltammetry (SWV)

Potential wave form for sampled current polarography

Potential is varied in a series of steps, with the current sampled at the end of each step

Change Parameters dialog box for sampled current polarography

Range of allowed parameter values: Potential = -3275 - +3275 mV Step E = 1 - 40 mV Quiet Time = 0 - 100 s Step Width = 100 - 6550 ms (Polarography); 4 - 6550 ms (Voltammetry)

A typical sampled current polarogram

id = 708 n C D1/2 m2/3 t1/6

where: n = number of electrons transferred/moleculeC = concentration (mol cm-3) D = diffusion coefficient (cm2 s-1)m = mercury flow rate (mg s-1) t = sampling interval (s)

The limiting current (id) is given by the Ilkovic equation:

Potential wave form for normal pulse voltammetry

(NPV)

The potential wave form consists of a series of pulses of increasing amplitude, with the potential returning to the initial value after each pulse

ss ss

Change Parameters dialog box for normal pulse voltammetry (NPV)

• Range of allowed parameter values: • Potential = -3000 - +3000 mV • Step E = 1 - 40 mV • Pulse Width = 3 - 2000 ms • Step Width = 100 - 6550 ms (Polarography);

4 - 6550 ms (Voltammetry) • Quiet Time = 0 - 100 s

A typical normal pulse voltammogram (NPV)

Potential wave form for differential pulse voltammetry (DPV)

The potential wave form consists of small pulses (of constant amplitude) superimposed upon a staircase wave form. Unlike NPV, the current is sampled twice in each Pulse Period (once before the pulse, and at the end of the pulse), and the difference between these two current values is recorded and displayed

t t

ss

(t)

ss

Change Parameters dialog box for differential pulse voltammetry

(DPV)

A typical differential pulse voltammogram (DPV)

Potential = -3000 - +3000 mV ; Step E = 1 - 40 mV; Quiet Time = 0 - 100 s Pulse Amplitude = 5 - 250 mV.; Pulse Width = 3 - 1000 ms Step Width = 100 - 6550 ms (Polarography); 4 - 6550 ms (Voltammetry)

Simultaneous Determination of Dopamine and Ascorbic Acid at an in-site Functionalized Self-Assembled Monolayer on Gold Electrode

L. Zhang, J. Jia, X. Zou, S. Dong*, Electroanalysis, 16(2004)1-6

1. The in-site functionalization of 4-aminothiophenol (4-ATP) self-assembled monolayer on gold electrode at physiological pH yields a redox active monolayer of 4’-mercapto-N-phenylquinone diimine (MNPD). Th functionalized electrode exhibits excellent electrocatalytic responses towards dopamine (DA) and ascorbic acid (AA), reducing the overpotentials by about 0.22 V and 0.34 V, respectively, with greatly enhanced current responses.

2. Due to its different catalytic activities toward DA and AA, the modified electrode resolves the overlapping voltammetric responses of DA and AA into two well-defined voltammetric peaks by differential pulse voltammetry (DPV), which can be used for the simultaneous determination of these species in a mixture.

3. The catalytic peak current obtained from DPV was linearly related to DA and AA concentration in the ranges of 5.0x10-6 -1.25 x10-4 M and 8.0 x10-6

-1.3 x10-4 M with correlation coefficient of 0.999 and 0.998, respectively. The detective limits (3s) for DA and AA were found to be 1.2x10-6 M and 2.4 x10-6 M, respectively.

The in-site functionalization of the 4-ATP SAM yields a redox active monolayer of 4’-mercapto-N-phenylquinone diimine (MNPD)

Fig. 5. Differential pulse voltammograms of 1.25x104 M DA and 1.30 x104 MAA mixture at clear Au (A) and MNPD/Au (B) in 0.1 M PBS (pH 7.2).

Fig. 4. Cyclic voltammograms of 1.0x104 M DA (A) and 1.0x 104 M AA (B) at clear Au (dashed line) and MNPD/Au (solid line) in 0.1 M PBS (pH 7.2). Scan rate: 50 mV s1.

Fig. 7. Differential pulse voltammograms of DA and AA mixtures at MNPD/Au in 0.1 M PBS (pH 7.2). DA contents from a) to g) are 5, 25, 45, 65, 85, 105, 125 mM, respectively. AA contents from a) to g) are 8, 25, 50, 70, 90, 110, 130 mM, respectively.

Fig. 6. A) DPVs of DA (40 mM) at MNPD/Au in 0.1 M PBS (pH 7.2) in the different concentrations of AA (a ± f: 60, 75, 90, 105, 120, 135 mM). B) DPVs of AA (120 mM) at MNPD/Au in 0.1 M PBS (pH 7.2) in the different concentrations of DA (a ± g: 5, 20, 35, 50, 65, 80, 95 mM).

Square Wave Voltammetry (SWV)

• The potential wave form consists of a square wave of constant amplitude superimposed on a staircase wave form. The current is measured at the end of each half-cycle, and the current measured on the reverse half-cycle (ir) is subtracted from the current measured on the forward half-cycle (if). This difference current (if - ir) is displayed as a function of the applied potential.

Potential wave form for square wave voltammetry

(SWV)

Change Parameters dialog box for square wave voltammetry (SWV)

Range of allowed parameter values: Quiet Time = 0 - 100 s Potential = -3000 - +3000 mV; Step E = 1 - 40 mV S.W. Amplitude = 1 - 250 mV; S.W. Frequency = 1 - 2000 Hz

A typical square wave voltammogram

Experiment for SWV or DPV5 mM ferrocyanide and 10 mM ferrocyanide in 0.1 M KCl

Concentration Peak Height Peak Area

10 mM 46.3 µA 34.8 µC

5 mM 23.7 µA 17.2 µC

Initial Potential -100 mV

Final Potential 650 mV

Pulse Amplitude 50 mV

Step Height 2 mV

Period 20 ms

Pulse Amplitude 50 mV

Step Height 2 mV

Period 20 ms

1 mm glassy carbon electrode, Ag/AgCl

Picomolar Peroxide Detection Using a ChemicallyActivated Redox Mediator and Square Wave Voltammetry

Jennifer L. Lyon and Keith J. Stevenson* Anal. Chem. 2006, 78, 8518-8525

Scheme 1. Depiction of Enzymatically Generated H2O2 Detection Using the Amplex Red Redox Mediatora

1. A method for low-level, low-potential electrochemical detection of H2O2 using a chemically activated redox mediator is presented. This method is unique in that it utilizes a mediator, Amplex Red, which is only redox-active when chemically oxidized by H2O2 in the presence of the enzyme horseradish peroxidase (HRP).

Abstract2. Microelectrode square wave voltammetry was used to optimize

sensing at ultralow concentrations (<1 mM), this method exhibits marked improvements in analytical sensitivity and detection limits (limit of detection as low as 8 pM) over existing protocols.

3. Sensing schemes incorporating both freely diffusing and immobilized HRP are evaluated, and the resulting analytical sensitivities are 1.22±0.04 and (2.1± 0.6) 10-1 mA/(mMmm2), respectively, for peroxide concentrations in the high picomolar to low micromolar range. A second linear region exists for lower peroxide concentrations. Furthermore, quantitative enzyme kinetics analysis using Michaelis-Menten parameters is possible through interpretation of data collected in this scheme. Km values for soluble and immobilized HRP were 84±13 and 504±19 mM, respectively.

4. This method is amenable to any biological detection scheme that generates hydrogen peroxide as a reactive product.

Several drawbacks when using Pt asa working electrode for H2O2 oxidation

1. Poor selectivity, low sensitivity and high susceptibility to electrode fouling

2. The electrochemical oxidation of generated H2O2 at Pt at physiological pH values (6.5-7.5) occurs at potentials (+0.4 to +0.7 V vs Ag/AgCl) where other electroactive species (ascorbic or uric acid) typically found in biological samples interfere

3. Because the electrochemical oxidation of H2O2 produces protons, a pH gradient is generated at the surface of the Pt electrode, resulting in perturbation of biological samples that are sensitive to changes in pH

4. Quantitative measurements become complicated as the oxidation of H2O2 at physiological pH is mechanistically complex, involving the formation of several reactive intermediates (superoxide, hydroperoxide, and hydroxyl radicals) whose stability is also pH dependent

Amplex Red

Fig. 2. Electrochemistry of Amplex Red and resorufin in pH 5 McIlvaine buffer. Cyclic voltam- mogram: starting potential þ0.1 V (vs. Ag/AgCl), scan rate 0.01 V s1, arrow follows the initial course of CV of Amplex Red, before the occurence of electrochemically generated resorufin.

Reactions of Amplex Red (N-acetyl-3,7-dihydroxyphenoxazine) and of resorufin (7-hydroxy-3H-phenoxazin-3-one).

Electroanalysis, 17(2005)1043

Electrochemical Performance of Amplex Red

Fig. 5. Amperometric flow-through detection of resorufin. Concentration dependence of peak current; glassy carbon electrode (diam. 3 mm) at 0.17 V (vs. Ag/AgCl), buffer flow rate 0.2 mL min-1, sample volume 0.1 mL, buffer deoxygenated.

Fig. 4. H2O2 assay with 0.2 U mL1 HRP and 50 mmol L-1 Amplex Red performed in McIlvaine buffer pH 5 with CV detection. Only the first half-cycle of each CV is plotted: þ0.1 V to 0.7 V (vs. Ag/AgCl), scan rate 0.01 V s-1. Electroanalysis, 17(2005)1043

Comparison of the Behaviors of SWV with CV

Fig. 1. (a) Comparison of square wave (heavy trace) and cyclic(light trace) voltammograms for a 300 nM H2O2 standard, demonstrating improved S/N attainable with SWV. SWV parameters: amplitude 25 mV, step height 5 mV, frequency 25 Hz. CV parameter: scan rate 125 mV/s. Experimental conditions: 10 mm diameter GC working electrode, Ag/AgCl reference electrode, Au wire counter electrode, 0.1 M K2HPO4/0.05 M citric acid buffer, pH 5.0, 0.2 U/mL HRP, 10 mM Amplex Red. (b) Comparison of square wave voltammograms for 300 pM H2O2 collected using the same electrode as in (a) (heavy trace) and a 2 mm diameter GC disk electrode (light trace). All other experimental conditions are the same as those for (a).

Square Wave Voltammetry

The dimensionless kinetic parameter K =ks/f is related to DEp values between 10 and 40 mV

Fig. 2. Peak height vs. frequency data for a 1 nM H2O2 standard detected via Amplex Red-mediated SWV (R2 ) 0.994, SD=0.028 nA). Experimental conditions are the same as those listed for Fig. 1.

Overall rate constant of ks = 40.03 ( 4.78 s-1 was determined for the resorufin-dihydroresorufin couple. This value is in accord with other reports for strongly adsorbing redox- active species studied via SWV.

H2O2 Calibration Curves

Fig. 3. Calibration curves for H2O2 standards detected via Amplex Red-mediated electrochemical sensing, utilizing either soluble (open squares) or immobilized (closed squares) HRP. Peak currents were obtained using SWV experiments outlined in the text and normalized to the working electrode area. SWV parameters: amplitude 25 mV, step height 5 mV, frequency 25 Hz. Experimental conditions: 10 mM diameter GC working electrode, Ag/AgCl reference electrode, Au wire counter electrode, 0.1 M K2HPO4/0.05 M citric acid buffer, pH 5.0, 10 mM Amplex Red. For soluble HRP, [HRP] ) 0.2 U/mL. For immobilized HRP, an equivalent amount of HRP was adsorbed to a glass slide placed in the electrochemical cell. (a) Linear current response in the picomolar H2O2 range for soluble HRP. (b) Linear current response in the pico- to nanomolar range for immobilized HRP. (c) Wide-range linear responses for both soluble and immobilized HRP observed between 300 pM and low micromolar H2O2 concentrations. Sensitivities of these calibration curves are 3 orders of magnitude less than those in (a) and (b).

Quantitative Enzyme Kinetics Measurements

Fig. 4. Enzymatic activity data for Amplex Red-mediated electrochemical sensing. SWV parameters and experimental conditions are the same as those for Figure 3, except that the Amplex Red concentration was increased to 1 mM. (a) Full dynamic range for 15 pMto 1 mM H2O2, reflective of Michaelis-Menten enzymatic activity. (b, c) Lineweaver-Burke plots of current response data shown in Figure 3b for (b) soluble and (c) immobilized HRP. Statistics for (b): R2 ) 0.986, SD ) 0.595 nA-1. Statistics for (c): R2 ) 0.996, SD ) 0.279 nA-1.