**4. BPA sensor based on carbon nanotubes/Fe(III)TMPP nanocomposite**

In order to enhance the sensitivity of the Fe(III)TMPP towards the detection of BPA, carbon nanotubes (CNTs) has been used to dope iron (III) porphyrin as it has attracted considerable interest due to its excellent electrochemical properties and its high ability to amplify the detection signal [38, 39]. Hence, Fe(III)TMPP was doped

### **Figure 10.**

*Nyquist diagrams of (A) Au/2%CNTs/Fe(III)TMPP electrode and (B) Au/4%CNTs/Fe(III)TMPP electrode for different BPA concentrations.*

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

### **Figure 11.**

*Schematic presentation of the non-interaction of most aromatic rings of Fe(III)TMPP with CNT because of their unsuitable structures.*

by 2% CNTs and 4% CNTs, then the obtained 2% CNTs/Fe(III)TMPP and 4%CNTs/ Fe(III)TMPP nanocomposites have been used for the detection of different concentrations of BPA (**Figure 10**). As can be seen, the Nyquist plots of 2%CNTs/ Fe(III)TMPP and 4%CNTs/Fe(III)TMPP modified Au electrodes show a very small variation with increasing BPA concentrations. This result shows that the doping of porphyrin by CNTs did not improve its sensitivity towards the detection of BPA molecules. This can be explained by the planar structure of Fe(III)TMPP making difficult the *π*- *π* interaction with the rolled up tubular structure of CNT (**Figure 11**). These results led us to think about improving the sensitivity of the Au/Fe(III)TMPP sensor using another carbon material having a 2D planar structure which can make easier the *π*- *π* interaction with Fe(III)TMPP and preserves its structure. Because of its fascinating electronic properties and extremely high specific surface area, this chosen carbon material is known as reduced graphene oxide (RGO), a graphene derivate that has attracted a lot of attention in improving the sensing ability of BPA sensors [40].

## **5. BPA sensor based on nanocarbon transducer functionalized by Fe(III)TMPP**

After proving the good affinity of Fe(III)TMPP towards BPA molecules, we aim to improve the sensitivity of Au/Fe(III)TMPP sensor by involving the use of nanosized electrodes based on nanocarbon transducers. Hence, owing to its strong electrocatalytic activity and minimal surface fouling, RGO has been chosen as a nanocarbon material to functionalize it on Au electrode to form a new Au/RGO nanocarbon transducer with enhanced charge transfer ability than the Au transducer. Then, the Au/RGO nanocarbon transducer has been prepared and functionalized with Fe(III)TMPP membrane to form Au/RGO/Fe(III)TMPP platform.

With the objective of highlighting the interaction between the Fe(III)TMPP and the Au/RGO nanocarbon transducer, UV/vis has been used. Hence, the prepared ITO/RGO, ITO/Fe(III)TMPP and ITO/RGO/Fe(III)TMPP electrodes were characterized by UV–vis (**Figure 12**). As shown in **Figure 12**, the UV/vis absorption spectrum of ITO/Fe(III)TMPP exhibits a strong peak at 438 nm and a weak peak at 527 nm ascribed to the Soret band and the Q-band of porphyrin respectively. After we have deposited the Fe(III)TMPP on ITO/RGO, we have observed a decrease in the Soret and Q band intensities with a red shift from 438 nm to 431 nm for the Soret band and the Q-band shift from 527 nm to 536 nm. These shifts prove the strong π-π interactions between the aromatic rings of RGO and the Fe(III)TMPP macrocycles [41, 42]. These results confirm the formation of the graphene–porphyrin complex, being in agreement with the proposed hypothesis of the easier interaction of the flattened structure of porphyrin with the 2D surface of graphene (**Figure 13**).

Bare Au, Au/Fe(III)TMPP, Au/RGO and Au/RGO/Fe(III)TMPP electrodes have been characterized by EIS in PBS (0.1 M, pH = 7) with an optimized potential of 0.9 V (**Figure 14**).

As shown in **Figure 14**, the total impedance plot of Au/RGO shows a semi cercle with a diameter smaller than obtained with bare Au. Consequently, the Rct value of Au/RGO decreased compared to that of bare Au. This result proves that the fabricated Au/RGO nanocarbon transducer has improved charge transfer ability than the usual transducer (Bare Au) thanks to the electron catalyst role of RGO. As illustrated in **Figure 14**, the impedance plot of Au/RGO/Fe(III)TMPP structure presents a very smaller semi cercle diameter compared to Au/Fe(III)TMPP electrode which is explained by smaller Rct value for Au/RGO/Fe(III)TMPP, thus better charge transfer ability at the interface electrode/electrolyte. This result demonstrates that the Au/ RGO nanocarbon transducer ensured the good attachment of the porphyrin on its surface via π-π interaction as we have proved by UV–visible which leads to enhanced kinetic charge transfer of the Au/RGO/Fe(III)TMPP structure.

**Figure 12.** *UV/vis spectra of ITO/Fe(III)TMPP and ITO/RGO/Fe(III)TMPP structures.*

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

### **Figure 13.**

*Schematic presentation of the π-π interaction between aromatic rings of Fe(III)TMPP and RGO thanks to their planar structures.*

**Figure 14.** *Nyquist plot of Bare Au, Au/Fe(III)TMPP, Au/RGO and Au/RGO/Fe(III)TMPP electrodes in PBS (0.1 M, pH = 7).*

The Nyquist diagram of Au/RGO/Fe(III)TMPP electrode, as shown in **Figure 15**, reveals the presence of a small semicircle at high frequency, corresponding to a small phase pic maxima in the phase diagram, and a large second semicircle at low

**Figure 15.** *Nyquist and Bode plots of the Au/RGO/Fe(III)TMPP electrode with the fit result.*

frequency, corresponding to an intense phase pic maxima in the phase diagram. Hence, the proposed equivalent electrical circuit should be composed by more than one dipole [28]. The best fit was done using an equivalent electrical circuit formed by a parallel association of two dipoles placed in series with the electrolyte resistance (Rs) (**Figure 16**). The first dipole, which is attributed to the high frequency loop and the electrochemical phenomena occurring at the electrolyte**/**membrane interface, was formed by a membrane resistance (Rm) and a membrane capacitance (Cm). The second dipole, is attributed to the second loop at low frequencies and describes the electrochemical phenomena taking place at the membrane/electrode interface, was composed of charge transfer resistance (Rct) and constant phase element (CPE).

The sensing properties of Au/RGO/Fe(III)TMPP electrode towards BPA concentrations in PBS electrolyte have been studied and presented in **Figure 17**. The semicircle diameter related to the charge transfer resistance (Rct) from the RGO/ Fe(III)TMPP to the electrode is observed to increase upon the addition of BPA concentration. This considerable dependance affirms the high affinity of Au/RGO/ Fe(III)TMPP sensor towards BPA molecules.

**Figure 16.** *Equivalent circuit used to fit the impedance spectra of Au/RGO/Fe(III)TMPP electrode.*

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

**Figure 17.** *EIS responses of the Au/RGO/Fe(III)TMPP electrode in the absence and the presence of different BPA concentrations.*

The electrical parameters values of Au/RGO/Fe(III)TMPP electrode after BPA attachment were obtained from the equivalent circuit model presented in **Figure 16.** Then a comparative study of Rm and Rct variations versus p[BPA] for Au/Fe(III)TMPP and Au/RGO/Fe(III)TMPP electrodes has been studied (**Figure 18**).

As can be observed in **Figure 18**, Rm and Rct variations increase with the successive addition of BPA concentration for the Au/Fe(III)TMPP and Au/RGO/Fe(III)TMPP electrodes. This result confirms the adsorption mechanism of BPA on the surface of the two modified electrodes, resulting in an increase of their

### **Figure 18.**

*Variation of the relative changes of (A) the membrane resistance and (B) the charge transfer resistance of Au/ Fe(III)TMPP and Au/RGO/Fe(III)TMPP electrodes versus BPA concentration cologarithm (p[BPA]). ΔRct = Rct Rct0 where Rct is the charge transfer resistance of the sensing membrane for each BPA concentration and Rct0 is the charge transfer resistance without any addition of concentration.*


**Table 2.**

*Comparison of the Rm and Rct variations slopes values of Au/Fe(III)TMPP and Au/RGO/Fe(III)TMPP electrodes for BPA detection.*

thickness membranes correlated with a decrease of the electron transfer ability at the membrane/electrode interface. The slope of Rm and Rct variations curves versus p[BPA] for Au/Fe(III)TMPP and Au/RGO/Fe(III)TMPP electrodes are summarized in **Table 2**. This table shows that the slopes of Rm and Rct variations curves obtained with Au/RGO/Fe(III)TMPP electrode are 2 times higher than those obtained with Au/Fe(III)TMPP electrode. This result proves the doubled sensitivity with the Au/ RGO/Fe(III)TMPP electrode towards BPA. This improved sensitivity comes from the catalyst role of RGO [43] which has created a nanocarbon transducer with enhanced electrical property and signal transfer. Based on the calibration curve (**Figure 18B**), the sensitivity (slope of the calibration curve) of our proposed sensor Au/RGO/Fe(III)TMPP was found to be 0.4218 per decade with an intercept of 5.6509. The correlation regression coefficient was obtained at 0.9987 in the concentration range from 10<sup>12</sup> M to 10<sup>8</sup> M. The detection limit of the sensor was estimated to be 2.1 10<sup>13</sup> M (signal-to-noise ratio of 3 independent measurements).
