**4. Potentiometric responses of Mn(III)-porphyrin and dipyrromethene Cu(II) containing sensors toward paracetamol**

The potentiometric responses of these sensors toward paracetamol were measured in 0.01 M phosphate buffer at pH = 5.5 [22]. Under these conditions, paracetamol (pKa= 9.5) exists as the undissociated compound in solution. The polymeric liquid membrane and carbon paste based sensor were tested toward paracetamol. Both of the sensors contain Mn(III)-porphyrin as the host molecule.

The generation of membrane potential changes after stimulation with undissociated parace‐ tamol molecules could be explained as follows. In the first step, chloride ligated Mn(III) porphyrin creates an aqua-complex via simple binding of water as a sixth ligand. The creation of such a complex was described by Meyerhoff *et al.* [47]:

$$\text{Mn(TPP)Cl} + \text{H}\_2\text{O} \quad \leftrightarrow \quad \text{Mn(TPP)Cl}\_2\text{O} \tag{1}$$

$$\begin{aligned} \text{Mn(TPP)ClH}\_2\text{O} + \text{parameter} &\leftrightarrow \text{Mn(TPP)ClH}\_2\text{O} - \text{parameter} \text{mol} \\ \text{Mn(TPP)Cl} &\dots \text{(partamento)} \text{ complexation} + \text{H}\_2\text{O} \end{aligned} \tag{2}$$

$$\begin{array}{rcl} \text{Mn(TPP)Cl} \text{---} \text{parameter} \text{mol} & \leftrightarrow & \text{Mn(TPP)Cl} \text{---} \text{parameter} \text{mol} "+ \text{H}^\* \\ \text{dissociation of parentanol and ejection of proton to water phases} \end{array} \tag{3}$$

In the next step, a second-sphere supramolecular complex of paracetamol molecules with Mn(TPP)ClH2O is created. The existence of such a complex at the surface of a polymeric liquid membrane modified with metalloporphyrins was postulated by Kibbey *et al.* [48]. This secondsphere interaction with paracetamol molecules occurs probably at a low sample concentration. When the concentration of paracetamol increases, an exchange of second-sphere coordinated paracetamol for inner-sphere water ligands occurs, and, as a consequence, a complex between the Mn(III) centers and paracetamol, *via* the oxygen atom from the amide group is created.

In the measuring condition (pH = 5.5), paracetamol molecules exists in undissociated form (pKa =9.5). The formation of the above mentioned complex, according to a combination of mesomeric and inductive effects, causes an increase of the acidity of the phenolic OH function from paracetamol molecule. As a consequence, this leads to a more facile dissociation of the OH group and finally to H+ ejection from the interface to the aqueous layer adjacent to the organic phase.

such as: aminoaniline, chloroaniline, hydroxyaniline, methylaniline, methoxyaniline and

Ligand 1 Ligand 2

**NH**5**C**6

**OH R R**

**N**

**OH OH**

**R R N N**

**NH**4**C**6**NO**2

**OH**

Potentiometry for Study of Supramolecular Recognition Processes Between Uncharged Molecules

**OH**

**OH**

**OH**

**OH**

**OH**

**NH**5**C**6

**OH OH**

**OH**

**R R**

**OH OH**

**OH**

**OH R R**

**OH**

**OH**

**OH OH**

**R R**

**OH OH**

**OH R R**

**OH**

http://dx.doi.org/10.5772/52803

505

Ligand 3

Ligand 4 Ligand 5

**C**6**H**5**N**

**OH**

**Br**

**N OH OH**

nitroaniline obtained with using potentiometry.

**OH**

**O**2**NC**6**H**4**N**

**OH**

**N**

**OH OH**

**R R N N**

> **N OH OH**

> > **C**6**H**5**N**

**OH OH**

**Br**

**R R**

**OH OH**

**Br**

**OH R R**

**OH**

**OH**

**Br**

**Figure 2.** The structure of host molecules.

**O**2**NC**6**H**4**N**

**OH R R**

**OH**

**OH**

**NH**4**C**6**NO**2

**OH**

This event is responsible for the generation of an anionic response of the polymeric liquid membranes modified with metalloporphyrins after their stimulation with undissociated paracetamol.

The reaction of the metalloporphyrin complex with paracetamol was confirmed by spectro‐ scopic measurements at the border between water and the polymeric membrane. The UV-Vis absorption spectrum of a thin membrane film containing Mn(III)-porphyrin deposited onto glass slides conditioned in 0.01 M phosphate buffer solution (pH = 5.5) exhibited one main band at 470 nm and two weaker bands at 376 and 400 nm. After conditioning in phosphate buffer with an increasing concentration of paracetamol, the absorbance maximum decreased and shifted to shorter wavelength. This blue shift was expected due to the increase in electron density around the Mn(III) centers by the coordinated amide. These data confirm the creation of a complex between the Mn(III)-porphyrin and paracetamol [22].

The electrochemically active Cu(II) dipyrromethene complex immobilized on the surface of gold electrodes previously modified with a dodecanethiol monolayer was successfully applied for voltammetric determination of paracetamol [23]. The interaction of paracetamol with Cu(II) redox centers was a base of analytical signal generation. The presence of human plasma in the measuring solution influence very little on the sensor performance. Its linear dynamic range ( 0.2-3.2 mM) was sufficient for controlling the toxic level of paracetamol in human plasma [23].
