*3.3. Square Wave Voltammetry*

SWV is a pulsed method in which the waveform is defined by step potential, amplitude, and period. The excitation signal used consists of a symmetrical square wave pulse superimposed on the stepped waveform, where the forward pulse of the waveform corresponds to the step to form a rectangular wave [89]. The current intensity is obtained at the end of each applied pulse during the potential sweep cycle. The difference between positive and negative pulse currents is recorded at the pulse time, and the difference between the forward and reverse currents over the same period is called the net current. The advantages of SWV are improved speed, background discrimination and sensitivity, and good discrimination of non-Faraday or charging currents.

In recent years, screen-printed electrodes (SPE) have been widely used in the design of biosensors. Their diverse functions, low cost, and easy use have attracted great interest. Moreover, the SPE can be modified with nanomaterials to enhance its electrochemical performance. Fakude et al. [90] first proposed an E-apt sensor for Cd2+ detection based on flexible polyester SPE. A Cd2+ join makes the fit body configuration change; an iron/ferrocyanide REDOX probe can then go more easily through the electrode surface, and the carbon nanofibers (CNF) can promote a simple electron transfer reaction. CNF after acid treatment on the polyester SPE is increased by the electrical activity of the electrode surface area and catalytic REDOX process of the iron/ferrocyanide, which enhances electron flow. Fakude [91] also modified the SPE with carbon black nanoparticles and AuNPs. The strategy is to deposit AuNPs by CV after modifying the electrode with carbon nanoparticles, so the Faraday current can increase up to 80% with a detection limit of 0.14 ppb. This strategy has also been used to detect As3+ in water [92], but the electrode used in this sensor is a glassy carbon electrode (GCE). The interface properties of the electrode are characterized by charge transfer resistance and double layer capacitance. The presence of nanoparticles on the detection limit GCE shows a significant decrease in the Rct value. The detection limit was 0.092 ppb and the specificity was well.

Si et al. [93] proposed an electrochemical biosensor based on aptamer-terminal deoxynucleotidyl transferase (TdT), which catalyzed the continuous polymerization of adenine bases, resulting in the formation of long polyA which enabled Si-DNA to be anchored on the electrode surface and enhanced electrical signals. The introduction of Hg2+ leads to the formation of the T-Hg2+-T complex, which prevents TdT from forming polyA, resulting in the absence of Si-DNA on the electrode surface and the decrease in the electrochemical signal.
