**3. BPA sensor based on usual transducer functionalized by Fe(III)TMPP**

In order to check the attachment of the Fe(III)TMPP on the bare Au electrode, electrochemical impedance spectroscopy (EIS) has been selected as a method of characterization thanks to the interfacial charge transfer properties that it can provide for the electrode surface during the modification process [30, 31].

Usually for EIS studies, the first step is to optimize the polarization potential by reducing the warburg diffusion component at low frequencies, which facilitates the interpretation of physico-chemical phenomena [32]. For our study, different negative potentials (E = 0.6 V, 0.7 V, 0.8 V and 0.9 V) have been applied in a large frequency range (100 kHz to 30 mHz). We observe in **Figure 2** an obvious decrease of the half circle diameter correlated with the decrease of the applied

*Novel Sensor Based on Nanocarbon Transducer Functionalized by Iron (III) Porphyrin… DOI: http://dx.doi.org/10.5772/intechopen.98560*

**Figure 2.**

*Optimization of the polarization potential of Au/Fe(III)TMPP electrode within a frequency range from 100 kHz to 30 mHz and an amplitude of 10 mV sinusoidal modulation in 0.1 M PBS (pH = 7).*

potential. This decrease can be explained by the reduction of the charge transfer resistance as a function of the *dc* potential [33], which makes possible the observation of the kinetics of cations at the membrane/solution interface. As a result, we have chosen a continuous polarization of 900 mV throughout our next measurements.

**Figure 3** shows the recorded Nyquist plots for bare Au before and after its modification by Fe(III)TMPP membrane in a PBS solution without any additional external redox probe. The semicircle diameter is related to the charge transfer resistance (Rct) of the electronic transfer from the porphyrin to the electrode [34]. Upon the modification of the Au transducer using Fe(III)TMPP membrane, the semicircle diameter increased, which means the increase of Rct. This is indicative of a better Au electrode surface coverage with Fe(III)TMPP accompanied by the decrease in the electronic charge transfer.

The Nyquist diagram of Au/Fe(III)TMPP electrode can be modeled by an equivalent electrical circuit formed by electrical components. This modeling is done thanks to a software "FRA2" which makes it possible to draw the proposed equivalent electrical circuit optimized by iteration. The choice of the latter is made according to the best fit which corresponds to the lowest value of the total error χ<sup>2</sup> as well as the error on each parameter. The values of the electrical components given for each Nyquist plot illustrate the electronic properties between the transducer/membrane/electrolyte interfaces which facilitate the interpretation of the electronic phenomena during the modification process.

In this study, the choice of the equivalent electric circuit was chosen based on the shape of the Nyquist diagram of the Au/Fe(III)TMPP electrode as well as on its corresponding bode diagram (**Figure 4**). As can be seen in **Figure 4A**, the total impedance plot of the Au/Fe(III)TMPP electrode has an enlarged shape. Consequently the Nyquist diagram of the modified electrode can be considered as an

**Figure 3.** *Nyquist plots of bare Au and Au/Fe(III)TMPP in PBS (0.1 M PBS, pH = 7).*

**Figure 4.** *(A) Nyquist and (B) Bode plots of the Au/Fe(III)TMPP electrode with the fit result.*

overlap of two closely interacting semicircles [35] which can indicate the presence of more than one dipole in the equivalent electrical circuit model [36]. **Figure 4B** shows the bode diagram fit of Au/Fe(III)TMPP electrode. This figure reveals that phase plot presents one phase pic maxima. Consequently, the equivalent electric circuit is analyzed as one dipole [36]. Taking into account the interpretation of Nyquist and bode diagrams of Au/Fe(III)TMPP, the best circuit that has been chosen is shown in **Figure 5**. This circuit has shown the best fit with low total error χ<sup>2</sup> . This circuit consists of an electrolyte resistance denoted Rs placed in series with one electrical dipole which divides itself into two dipoles. The first dipole is formed by (Rm, CPE1) corresponding to the first high frequency loop and describes the

*Novel Sensor Based on Nanocarbon Transducer Functionalized by Iron (III) Porphyrin… DOI: http://dx.doi.org/10.5772/intechopen.98560*

### **Figure 5.**

*Equivalent circuit used to fit the impedance spectra of Au/Fe(III)TMPP electrode.*

electrochemical phenomena occurring at the electrolyte/membrane interface where Rm is the resistance of the membrane and CPE1 is called the constant phase element. The second dipole is made up by (Rct, CPE2) describing the second loop at low frequencies and describes the electrochemical phenomena occurring at the membrane/electrode interface where Rct represents the charge transfer resistance at the membrane/electrode interface.

In our previous optical study, Fe(III)TMPP was able to detect BPA molecules through the strong coordination ability of Fe(III) cation to oxygen atoms [26]. Thus, in this study, the Au/Fe(III)TMPP electrode has been used to test its ability towards the impedimetric detection of BPA and 2 other interferent molecules. These interfering molecules are 2,2-'biphenol and catechol, which have a structure similar to BPA. The choice of EIS technique to detect these phenolic compounds was made because of its several advantages, such as simplicity, label-free, high sensitivity, and serving as a way to interface recognition events and signal transduction.

BPA is an electron-rich system favorable for the strong interaction with porphyrin ring. Hence, we explore the use of the electrostatic interaction between BPA and cell porphyrin membrane without using any external redox indicator. Thus the detection is directly proportional to the change in the electrical properties of the electroactive Fe(III)TMPP membrane. **Figure 6** shows the Nyquist plot of Au/ Fe(III)TMPP for different BPA, 2,2-'biphenol and catechol concentrations (from 10�<sup>12</sup> to 10�<sup>7</sup> M). Au/Fe(III)TMPP electrode shows a large increase in diameter of the semicircle, after BPA, 2,2'-biphenol and catechol attachment, correlated with their increasing concentrations, indicating much higher Rct values. This increase can be associated with the multilayer adsorption of BPA, 2,2<sup>0</sup> -biphenol and catechol molecules on the surface of Fe(III)TMPP membrane, which leads to the modification of electrochemical properties at the interface.

To confirm this mechanism of detection, the electrochemical proprieties of the sensor have been quantified by fitting the Nyquist plots using the equivalent circuit shown in **Figure 5**. The different fits are done with a total error value (χ<sup>2</sup> ) less than 10�<sup>3</sup> . **Figure 7** shows the variation of the membrane resistance Rm as a function of the BPA, 2,2<sup>0</sup> -biphenol and catechol concentrations cologarithm p[X] = �log[CX], where X = BPA, 2,2<sup>0</sup> -biphenol and catechol. We can observe that the value of Rm increases when increasing the concentration of the three studied analytes.

**Figure 6.**

*Nyquist diagrams of Au/Fe(III)TMPP electrode for different (A) BPA, (B) 2,2*<sup>0</sup> *-biphenol and (C) catechol concentrations in PBS (0.1 M, pH = 7). Frequency range [100 kHz–30mHz]. Potential polarization E =* �*900 mV. The points presenting the experimental data and the line is the fit obtained with the equivalent circuit shown below.*

This result indicates an increase in the thickness of the Fe(III)TMPP membrane, which is proportional to Rm according to the following Eq. (1).

$$\mathbf{R\_m} = \frac{d}{\delta \mathbf{S}}\tag{1}$$

Where d is the membrane thickness, *δ* is the conductivity of the membrane and S the active area.

Consequently, the increase of the thickness of the Fe(III)TMPP membrane correlated with the increase of BPA, 2,2<sup>0</sup> -biphenol and catechol concentrations proves the adsorption of these molecules on the surface of the Fe(III)TMPP membrane through their π electron system [37].

To fully understand the charge transfer kinetics at the interfaces, the Rct values after BPA, 2,2<sup>0</sup> -biphenol and catechol attachment were obtained from the equivalent circuit model and used to generate **Figure 8**. This figure describes the evolution of the charge transfer resistance Rct characteristic of the Au/Fe(III)TMPP interface, as a function of the logarithm of the concentration of BPA, 2,2<sup>0</sup> -biphenol and catechol. We report, from the curve of Rct = f (p [X]), that the charge transfer resistance increased with the increase of BPA, 2,2<sup>0</sup> -biphenol and catechol concentrations leading to down the electron transfer to Au/Fe(III)TMPP electrode. This is due to the steric hindrance favored by the multi layers adsorption of the target molecules, which seems logical since it has been shown from Eq. (1) that the sensitive membrane thickness has already increased upon the increasing of the analyte concentrations.

*Novel Sensor Based on Nanocarbon Transducer Functionalized by Iron (III) Porphyrin… DOI: http://dx.doi.org/10.5772/intechopen.98560*

**Figure 7.** *Variation of the Fe(III)TMPP membrane resistance (Rm) plots versus p[X].*

### **Figure 8.**

*Variation of the Fe(III)TMPP charge transfer resistance (Rct) plots versus p[X].*

The Rct slope values of the Au transducer coated by Fe(III)TMPP membrane for the impedimetric detection of BPA, 2,2<sup>0</sup> -biphenol and catechol are illustrated in **Table 1**. As can be seen, the slope associated with the catechol molecule has a lower


**Table 1.**

*Comparison of the Rct slope values of Au/Fe(III)TMPP electrode for different phenolic analytes.*

### **Figure 9.**

*Chemical structure of BPA, 2,2*<sup>0</sup> *-biphenol and catechol showing the presence of two aromatic rings in bisphenol A and 2,2*<sup>0</sup> *-biphenol and the presence of one aromatic ring in catechol.*

value, whereas the slope associated with BPA and 2,2<sup>0</sup> -biphenol molecules has a higher value. These results show a better sensitivity of Fe(III)TMPP towards BPA molecules. This can be explained by the higher number of aromatic groups in the structures of BPA and 2,2<sup>0</sup> -biphenol than in the structure of catechol (**Figure 9**). According to these results, we have chosen BPA as the target molecule for Fe(III)TMPP membrane throughout our next measurements.
