Electrochemical Methods

There are more than 30 methods can be employed in electrochemistry. In this chapter, we will discuss the fundamental methods of the Cyclic Voltammetry (CV), Linear Sweep Voltammetry (LSV), Square wave Voltammetry (SWV), Differential Pulse Voltammetry (DPV), Amperometry (IT), and Open Circuit Potential-Time (OCP).

1.2.2.1 Linear Sweep Voltammetry and Cyclic Voltammetry


Voltammetery is one of most widely employed electrochemical method in the world. Basically, it is used to get electrochemical information of analyte by measuring the current of analyte as the function of potential and time. Its waveform goes with continuously or steps like a ladder. In continuous waveform, it divided into linear sweep a segment and cyclic scanning which named LSV and CV. (see Figure 1(a) and Figure 3(a)) For example, 30 mM of ferricyanide was oxidized at a screen printed carbon electrode with the method of LSV as shown in Figure 1 and 2, the potential started at Eintial and ended at Efinal with a continuous waveform (see Figure 1 (a)), and the resulted current versus time is illustrated as shown in Figure 1(b). It is clearly that the oxidized current with applied potential was started obtaining at the time of 6th second and getting steady state after 12th second.



Moreover, cyclic voltammetry, applied potential was scanned forward to the final potential and then reverse scanned to the initial potential and thus forms a triangular potential waveform cycle as shown in Figure 3(a). Figure 3(b) represents the obtained current as a function time with applied cyclic potential sweep, the ferricyanide started to be oxidized at around 2.2 second and got maximum current at around 2.4 second, and in the opposite scanning, the reduced reaction was started at around 4 second and got maximum at around 4.2 second within a complete cycle. With these information, the electron transfer rate can be calculated and analyzed for qualification and quantification of the analyte.

Figure 4 represents the typical cyclic voltammogram for redox process of ferricyanide. In the forward scan, it goes with the anodic reaction and get the anodic current (Ipa) and anodic potential (Epa), in the opposite way, reverse scan, it belongs to the cathodic reaction and obtains the cathodic current (Ipc) and cathodic potential (Epc) of the analyte. Anodic reaction is an oxidizing reaction which the analyte lose one electron or more to the electrode for analysis. On the other hand, the cathodic reaction means the analyte gets one electron or more from the electrode. However, forward scan can be anodic or cathodic, it depends on the direction of scan, and so does the reverse scan.



• 1.2.2.2 Differential Pulse Voltammetry

  
  Sir Geoffrey Barker, the inventor of Pulse Voltammetry, gives us the technique to measure trace level of analyte by decreasing the charging current and thinner diffusion layer. The differential pulse voltammetry named from the DIFFERENCE of two sampling current which are the first sample point before the pulse application and another point around 40ms late of the pulse. Figure 5 (a, b) represents the detailed description of waveform of differential pulse voltammetry that gives the fixed amplitude of pulses with an increasing linear potential sweeping. In Figure 5 (c), the current is sampled twice, which are it1 and it2. Therefore, it1 is subtracted from it2 to being the current difference and then to be plotted as a function of potential as shown in Figure 6. It is clearly that the signal of DPV is larger than the signal of LSV in the presence of 30 mM ferricyanide and can be used to quantify the analyte with low detection limit.
  

• 1.2.3 Square Wave Voltammetry

  
  

As illustrated in Figure 7 (a, b), the symmetrical square wave potential is applied to the electrode as a function of time with a twice larger amplitude. The measured current is also sampled twice as well as DPV, one is for forward pulse and the other is for reverse pulse. (see Figure 7 (c)) The difference between forward and reverse gives a current plot and resulting its sensitivity is higher than the differential pulse voltammetry in a reversible redox system, in which the signal of SWV is larger than the signal of DPV in the presence of 30 mM ferricyanide as shown in Figure 8. Moreover, the advantage of SWV is its fast scan rate which is equal frequency multiplied increasing potential.

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  1.2.2.4 Amperometry

  
  Amperometry is one of the most employed method in electroanalysis It is applied a fixed potential ( and measure the current as a function of time as illustrated in Figure 9 It is widely used in the quantification of biomolecule in point of care device such as glucose sensor device
  
  



• 1.2.2.5 1.2.5 Open circuit potential

  
  Open circuit potential is used to measure the potential difference between two terminals of a device which the circuit is not closed. (i.e., no current flow through.)

Reference

Please click the link below

• 1.Allen J. Bard, Larry R. Faulkner, Electrochemical Methods and Applications 2nd, 2001
• 2.Joseph Wang, Analytical Electrochemistry 2nd ed., 2000