Attempts to monitor current signals in CV for very dilute concentrations of electroac- tive species in solution can be limited by background current. At low concentrations, the Faradaic current can be very small compared to the background current. Amplifying the signal is not helpful because the background is also amplified in the process. However, understanding its source has led to several strategies for circumventing background cur- rent, and in some cases, these methods enable voltammetric determinations down to the nanomolar level.
A large fraction of the background current is associated with charging the double layer.
The double layer charging occurs quickly and generally decays to zero after the first few milliseconds. The current response to a 10 mV potential step at a carbon disk electrode in a clean electrolyte solution is shown in Figure 5.15.
Because the biggest contribution to the background current is usually associated with charging the double layer, it is the key to lowering the detection limits in various forms of voltammetry discussed in the following.
Whenever a voltage is applied to an electrode surface, an outside power source is being used to impose a difference in potential energy for an electron to move across the interface between the electrode material and some electroactive species in solution. For example, in order to make the electrode more negative, more electrons are pushed from the outside circuit to the electrode/solution interface raising the free energy on the electrons there. On the solution side of the interface, cations in the supporting electrolyte respond
5 ms 60 μA
FIGURE 5.15 Double-layer charging current response of a 3 mm diameter carbon disk working elec- trode in 0.1 M KCl solution to a 10 mV step in applied voltage with no electroactive species present.
k k by moving from the bulk solution toward the OHP in order to balance the charge at the
surface. The double layer is represented by the charge of electrons on the inside of the electrode surface versus the cations in the OHP (plus a thin neighboring “diffuse” region of solution where the cations outnumber the anions). The net excess of cations balances the charge on the electrode side. (For a positive electrode voltage, the electrode side has a deficiency of electrons while the solution side has an excess of anions in the OHP and diffuse charge region.)
For every applied potential, there is a different arrangement of charge. Consequently, when the applied voltage is changed, the charge must rearrange. Electrons move toward or away from the surface on the electrode side, and ions move into or out of the OHP on the solution side. Recall that the movement of charge constitutes a current. Hence, this is current that is required to charge the double layer in order to establish the new poten- tial energy difference across the interface. Whenever the voltage changes, there will be an associated background current for charging the double layer.
Scanning the applied voltage linearly with time, as is done in a CV experiment, gener- ates a continuous background current as shown in Figure 5.16. Effective ways of isolating the Faradaic component have been built around stepping or pulsing the voltage instead.
An early strategy was a program to step the voltage in a staircase manner as shown in Figure 5.17a. There is an immediate surge in the current at the beginning of the step, but it dies out quickly in a few milliseconds. Measuring the current 10–15 ms after the step yields mostly faradaic current. If, instead of scanning the applied voltage continuously, the voltage is stepped in a staircase pattern, the current can be sampled after a short delay following each step and plotted versus the applied voltage to produce a voltammogram as in Figure 5.17a.
Some of the other popular voltage programs are shown in Figure 5.17. As with the staircase waveform, these techniques take advantage of the fact that the interesting (faradaic) current persists after the double layer charging current dies out following a step in the potential. These voltage “pulse” strategies circumvent charging current by sampling a few milliseconds after the voltage is stepped.
One of the most widely used methods for avoiding background current is called square-wave voltammetry (SWV). It was first introduced in the 1950s by Geoffrey Barker in England. It became more popular after innovations in semiconductor manufacturing
E
Voltage waveform
Time
Current
E Voltammogram Double layer charging current
FIGURE 5.16 Scanning the voltage in a linear manner introduces charging current that can obscure the Faradaic current at low analyte concentrations.
k k
Voltage waveform
Current response
Time
E i
i1
i2
Sampled current
Sampled current
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waveform Current
response
Voltammogram
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i ib if
Sampled current
Voltammogram
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(b) (a)
Eapplied
0.45 0.5 0.55 Potential (V) versus Ag/AgCl
0.6 0.65 0.7 0.75 0.4
0.35 0.3 0 0.5 1.5 2.5 3.5 4.5 4 3 2
Current (μA)
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–69 61 R
x10
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–4.00–2.000.002.004.00–6.00 –200
Voltage waveform
Current response
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i ib if
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amplitude, Esw
Step period (τ) (c)
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–200 100 0 –100 –200 –300 –400 –500 n(E–E1/2) mV
B A C
ψ
FIGURE 5.17 “Pulse voltammetry” techniques for overcoming background currents. (a) Staircase voltammetry. The current is sampled after a delay following each step in potential. Source: Adapted with permission from Miaw et al. 1978 [17]. Copyright 1978, American Chemical Society. (b) In dif- ferential pulse voltammetry, the applied potential is a linear ramp plus a periodic pulse in voltage.
The current is measured immediately before the voltage pulse and again at the end of the pulse. The voltammogram is plotted as the difference in current measured before and after the pulse versus the applied potential at the top of the pulse. Source: Adapted with permission from Fan et al. 2012 [18].
Copyright 2012, American Chemical Society. (c) In Square wave voltammetry the potential waveform consists of a staircase with a potential pulse added to the beginning of each step. In the theoretical voltammogram on the right,Ψstands for the current function. In curveA, the vertical axis represents sampled current,if, for the forward step. CurveB shows the reverse current, ib, measured after the voltage is stepped back. For quantitative analysis, the difference current,Δi =if − ib, as shown by curveC is normally used. Source: Adapted with permission from O’Dea et al. 1981 [19]. Copyright
k k made digital electronics common in the 1980s [20] and thanks to developments by Janet
and Robert Osteryoung [19, 20].
In SWV, an additional enhancement of the faradaic signal is realized by recycling some of the electroactive material at the surface. Here is how that works. The SWV-applied volt- age pattern appears in Figure 5.17c. It is helpful to think of the pattern as a pulse sitting on the rising edge of each step of a voltage staircase. The current is sampled twice dur- ing each step, once near the end of the voltage pulse and again after the voltage comes down off of the pulse (shortly before stepping up again). Imagine an experiment scanning from+1 V to−1 V using a staircase of 2 mV steps with a 20 mV pulse at the beginning of each step in a solution of the complex, Ru(NH3)63+. At voltages near the formal poten- tial, the current sampled at the end of the negative going pulse captures current for the reduction of the Ru(NH3)63+. At the end of the pulse, the voltage becomes more positive by 20 mV, and the current is sampled again. Because this experiment is usually applied in an unstirred solution, some of the ruthenium(II) complex produced by the reduction process is available to be converted back to ruthenium(III) (producing some oxidation cur- rent). Consequently, there is more Ru(NH3)63+available near the electrode to be reduced in the next negative-going part of the next step, and a bigger surge in reduction current is obtained than that would have been observed in a simple staircase voltammogram. This recycling of material produces a reduction current in the negative-going (cathodic) part of the pulse and current for the reoxidation following the positive-going (anodic) edge of the pulse. One can plot the current sampled following either the cathodic-going or the anodic-going edge of the pulse, but the difference in the current samples at each voltage step produces a voltammogram that is as much as a factor of two more sensitive than either of current voltage curves alone. The usual SWV plot displays a difference current,Δi, on the vertical axis. The peak current is proportional to the bulk concentration of the analyte and inversely proportional to the square root of the pulse width in seconds [20].
The peak current is also a function of both the voltage pulse amplitude,Esw, (also called the square wave amplitude) and the voltage step size of the staircase waveform, ΔEs. The pulse amplitude has an optimum value of about 50/nmV (wherenis the num- ber of electrons transferred for reactant species) [20]. The pulse width is chosen to be equal to half the cycle time between pulses (or𝜏/2 in diagram at the bottom of Figure 5.17c).
If𝜏 is the time between the start of each pulse, then 1/𝜏 is the pulse frequency for the scan. Although the peak height increases with frequency, the optimum is about 200 Hz (𝜏=5 ms) for a reversible electrode process [20]. Beyond that rate, the current peak width increases making it more difficult to resolve peaks with similarEo′ values. The peak is centered around theEo′value for a reversible process. For electron transfer of decreasing reversibility, the peak potential will shift away from theEo′value (toward more negative voltages for negative-going scans). Also, the peak height decreases in sensitivity as the electron transfer rate decreases. Nevertheless, the SWV peak is still many times more sen- sitive for quantitative determinations than CV. Furthermore, the time needed to record a scan is very short. For example, a 1 V scan can be completed within 0.5 s using a step size (ΔEs) of 10 mV and a stepping frequency of 200 Hz. That allows one to take many scans during the elution of a solute from chromatographic column, for example, or to increase the precision of an analysis by recording and averaging many replicate scans quickly, such as during a flow-injection experiment.
Differential pulse voltammetry (illustrated in Figure 5.17b) is very similar to SWV in
k k current is plotted as the signal. This strategy also enhances the signal for reversible analytes
in quiet solutions because of recycling.