Electrochemical Sensing Techniques

In electrochemical sensor, there are a number of sensing techniques can be used to determine the interested analytes at low concentration level. In this work, we have used two different voltammetric techniques (linear sweep voltammetry (LSV), and cyclic voltammetry (CV)), three different pulse-voltammetric techniques (differential pulse voltammetry (DPV) and square wave voltammetry (SWV) and amperometric technique (chronoamperometry (CA)). It was chosen based on which sensing technique shows

3.5.1 Linear Sweep Voltammetry (LSV)

Linear sweep voltammetry (LSV) technique is one of the voltammetric methods that used to identify the existence of analytes in solution. The basic principle of LSV involves in monitoring the electroanalytical current signals generated upon applying a voltage to the WE that was immersed in an electrolyte solution containing target analytes. The electrode potential was varied linearly between two limits and the scan rate was kept constant throughout the process. The result obtained from LSV was presented as a plot of oxidation or reduction current response against the applied voltage that is the potential for working electrode. The current is measured between the WE and the counter electrode. The oxidation or reduction of analyte was represented by a peak or trough in the current signal at the potential at which the species begins to be oxidized or reduced.

3.5.2 Cyclic Voltammetry (CV)

Cyclic voltammetry (CV) is one of the sensing techniques that are frequently used to detect the presence of bioanalyte in the solution due to its simple procedure. In CV, the applied potential was ramped in the opposite direction to return to the initial potential after it reached the set potential and these cycles of ramps in potential can be repeated. Basically, CV is the extension of LSV where two linear sweeps run back to back, however, CV has a few advantages over LSV. For example, CV can give more information about the chemical reactions existing at the electrode surface by observing the peaks that appeared in CV. The reversibility of a redox couple can be evaluated by determines the potential difference between the anodic and cathodic peak potential of corresponding redox couple. Moreover, CV allowing the conversion of a species back to its original form and in the same time prevents the accumulation of unwanted species due to the reverse scanning in CV.

3.5.3 Differential Pulse Voltammetry (DPV)

Differential pulse voltammetry (DPV) is one of the pulse voltammetry techniques that is highly sensitive to trace the levels of the analytes. Theoretically, the potential wave form in DPV composed of small pulses with constant amplitude normally in the range of 10 to 100 mV. These small pulses were superimposed on a slowly changing base potential. The current was sampled twice in each pulse period that is at the beginning and ending of the same pulse to permit the decay of the nonfaradaic (charging) current. The final result was displayed in a plot of the current difference between these two points for each single pulse versus base potential. The height of DPV is directly proportional to the concentration of analyte. In DPV, the species of target analytes can be identified by observing the peak potential, therefore it give advantage especially for simultaneous detection of analytes. As compared to CV, the background current in DPV is smaller because the charging current contribution is negligible. In order to increase its sensitivity and achieve lowest limit of detection, several parameters need to be optimized including the modulation amplitude, step potential, and step width.

3.5.4 Square Wave Voltammetry (SWV)

The most advanced technique in the group of pulse voltammtery technique is the square wave voltammetry (SWV). The potential wave form in SWV was made up from a superimposed of symmetrical square wave pulses with constant amplitude on a staircase wave form of step height. In this case, the forward pulse of the square wave coincides with the staircase step. The current in this technique was determined at two points, intially at the end of the forward potential pulse and then at the end of the reverse potential pulse. The current measured at these two points are known as oxidation and reduction current and the difference between these two currents was calculated to obtain the net current in SWV. This net current is directly proportional to

the concentration of the analyte which often give a lowest detection limit in the range of nanomolar, thus make it more sensitive compared to DPV. The advantage of SWV over other techniques is the speed as it allows the experiments to be performed repeatedly and increase the signal to noise ratio. Moreover, the fast technique offered by SWV enable us to study the kinetics of fast electron transfer reactions and the kinetics of rapid chemical reactions coupled to the target analytes.

3.5.5 Amperometry

The amperometry is one of the sensing techniques belongs to the family of controlled potential technique. The relatively simple technique owned by amperometry making it the most frequently used for direct determination of the analyte concentration. In amperometry, a steady state current was measured as a function of time upon applying a constant square-wave potential to the working electrode. The constant potential value was selected based on the existing well-established essential point of reference provided by CV. In this work, the point of reference will be the oxidation or reduction potential for our target analytes. The mass transfer throughout this process is solely governed by diffusion. The final data collected from amperometry will be translated in the plot of current-time (I-t) dependence. The value of peak current measured over a linear potential range is directly proportional to the bulk concentration of the analyte.

3.6 Optical Characterization Technique