**3. Elucidation of the mechanism of the potentiometric signal generation of calix[4]pyrrole, calix[4]phyrin and corrole –ISEs upon stimulation by undissociated phenol derivatives**

The results we have obtained for calix[4]pyrrole [ 16, 17, 19, 20 ], calix[4]phyrin or corrole [14, 15] modified membranes and the results reported for macrocyclic polyamines [5, 6, 38-40] suggest that the intermolecular recognition processes between the ligands investigated and undissociated nitrophenol isomers occurring at the organic/aqueous interface, leading to the potentiometric signal generation, is a general phenomenon.

of a supramolecular complex with phenolic guests is not possible. To confirm that such complex could block the cavity of the calix[4]pyrrole, in our previous investigation we have tested the membrane modified with a calix[4]pyrrole substituted with bromine atoms at the β - carbons. The lack of any response towards nitrophenol isomers of a membrane incorpo‐ rating bromine derivatives of calix[4]pyrrole, even in strong acid solution supports this assumption [14, 17]. The electron withdrawing bromine atoms increase the affinity of cal‐

in this case the cavity of calix[4]pyrrole is blocked and the supramolecular ligand – phenol

The lack of any response of the membrane modified with calix[4]phyrin or corrole towards dinitrophenol isomers and very low response towards nitrophenol isomers in alkaline medium

negatively charged surface of polymeric membranes. Dinitrophenols in such circumstances also exist in the anionic forms. Thus, the host-guest electrostatic repulsion force is probably

The magnitude sequence of the potentiometric signals observed for calix[4]pyrrole, cal‐ ix[4]phyrin and corrole membranes suggests that the acidity of the target molecules is one of the important factors affecting the process of potentiometric signal generation. A similar relation was observed in the case of macrocyclic polyamine-ISEs [5, 6]. In Table 2 the values

On the other hand, the recognition of phenol derivatives by calix[4]pyrrole calix[4]phyrin and corrole is such that guests respectively bury themselves into the host cavity or are perpendic‐ ular to the macrocycle, with (in both cases) the phenolic OH pointed toward the pyrrole end, where the hydrogen bonds with NH units are formed [14, 17, 42 ]. Because of this, the sensitivity sequences observed for nitrophenol as well dinitrophenol isomers reflect also the magnitude of the steric obstacle, which is the lowest in the case of *para-*nitrophenol and 2,4-dinitrophenol. Therefore, for these two guests, the strongest potentiometric responses of ISEs studied were observed. A comparison of the results we have obtained for calix[4]pyrrole and [16, 17] calix[4]phyrin and corrole [14, 21, 42] and the results reported for macrocyclic amines [5, 6, 38-40] showed that the potential generation by the membranes modified with nitrogencontaining macrocyclic compounds after stimulation with phenol derivatives is a general phenomenon. The most important parameters governing this phenomenon are the differences between the acidity of the OH group of the target molecules and the ability of the NH group of the macrocyclic ligands to create the hydrogen bonds with OH group of the phenol

The signal magnitude sequence based on the host molecules incorporated into the membranes

This sequence reflects the sequence of availability of hydrogen atoms coming from NH or NH <sup>+</sup> group of ligand for OH group of phenol derivatives. This availability is crucial for host –guest

complex based on hydrogen bonds can not be created [14, 17].

the main reason preventing interaction between them.

(Table 1) could be explained as follow. The consequence of host-OH -

of pKa and logPoct of target molecules investigated were collected.

polyamines > calix 4 pyrroles > calix 4 phyrins >corroles

. Such a complex could be created even in acidic medium. Therefore,

Potentiometry for Study of Supramolecular Recognition Processes Between Uncharged Molecules

complex creation is the

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

501

ix[4]pyrrole towards OH-

derivatives.

is as follows:

complex creation.

The results show a higher potentiometric response in all of discussed membranes for the more acidic guest. This fact confirms the influence of the acidity and the lipophilicity of the neutral guest on the signal generation process by membranes incorporating calix[4]phyrin, corrole or calix[4]pyrrole derivatives.

A comparison of the results for investigated host molecules shows that calix[4]pyrrole modified membranes are the considerably more sensitive towards phenolic guests that the calix[4]phyrin or corrole [15, 17, 19]. While calix[4]pyrrole, calix[4]phyrin and corrole modified membranes do not respond towards the dissociated form of phenol derivatives, the polyamine modified membranes do respond [38-40].

Investigated membranes displayed the signal magnitude sequence as follow:

calix 4 pyrrole >calix 4 phyrin > corrole

The generation of membrane potential changes after stimulation with undissociated isomers of phenols derivatives could be explained as follows. In the first step, a supramolecular complex between the host molecules located at the surface of liquid membrane phase, and the neutral phenol guest placed at the surface of the aqueous phase is formed. This interaction relays on the hydrogen bond creation between the OH group of nitrophenol and pyrrole NH groups from the macrocyclic hosts. This was proved by NMR measurements [14].

The formation of such a supramolecular complex, according to mesomeric and inductive effects, causes an increase of acidity of the phenol OH function. This may decrease the pKa of the phenol derivatives at the surface of the polymeric phase. This leads to the dissociation of OH group and finally to proton ejection from the interface to the aqueous layer adjacent to the organic phase. The energy gained from the proton solvation process is probably the driving force allowing for the dissociation of phenol derivatives at the aqueous/organic membrane interface. This event is responsible for the generation of an anionic response of calix[4]pyrrole, calix[4]phyrin and corrole incorporating membranes after their stimulation with undissociated phenols.

ArOH(aq) ↔ ArOH(mem) extraction ( i)

Host(membrane) + ArOH(mem) ↔ Host---ArOH(membrane) complexation (ii)

Host---ArOH(membrane) <sup>↔</sup> Host---ArO- + H<sup>+</sup> dissociation and proton ejection to water surface (iii)

The increase of the proton concentration in the very thin aqueous layer containing 1.0 x 10-2 M *para-*nitrophenol in 1.0 x 10-2 M of KCl (pH 4.0), adjacent to the calix[4]pyrrole or corrole membrane surface, supported this assumption [14, 17]. According to the mechanism proposed, the lack of the potential changes of calix[4]pyrrole, calix[4]phyrin and corrole ISEs observed in the presence of phenolic guests at alkaline pH could be explained as follows.

At this pH the concentration of OH in the water phase is high enough to block the cavity of all investigated host molecules by creation of host –OH complex [14, 17]. Thus, the formation of a supramolecular complex with phenolic guests is not possible. To confirm that such complex could block the cavity of the calix[4]pyrrole, in our previous investigation we have tested the membrane modified with a calix[4]pyrrole substituted with bromine atoms at the β - carbons. The lack of any response towards nitrophenol isomers of a membrane incorpo‐ rating bromine derivatives of calix[4]pyrrole, even in strong acid solution supports this assumption [14, 17]. The electron withdrawing bromine atoms increase the affinity of cal‐ ix[4]pyrrole towards OH- . Such a complex could be created even in acidic medium. Therefore, in this case the cavity of calix[4]pyrrole is blocked and the supramolecular ligand – phenol complex based on hydrogen bonds can not be created [14, 17].

undissociated nitrophenol isomers occurring at the organic/aqueous interface, leading to the

An Integrated View of the Molecular Recognition and Toxinology - From Analytical Procedures to Biomedical

The results show a higher potentiometric response in all of discussed membranes for the more acidic guest. This fact confirms the influence of the acidity and the lipophilicity of the neutral guest on the signal generation process by membranes incorporating calix[4]phyrin, corrole or

A comparison of the results for investigated host molecules shows that calix[4]pyrrole modified membranes are the considerably more sensitive towards phenolic guests that the calix[4]phyrin or corrole [15, 17, 19]. While calix[4]pyrrole, calix[4]phyrin and corrole modified membranes do not respond towards the dissociated form of phenol derivatives, the polyamine

The generation of membrane potential changes after stimulation with undissociated isomers of phenols derivatives could be explained as follows. In the first step, a supramolecular complex between the host molecules located at the surface of liquid membrane phase, and the neutral phenol guest placed at the surface of the aqueous phase is formed. This interaction relays on the hydrogen bond creation between the OH group of nitrophenol and pyrrole NH

The formation of such a supramolecular complex, according to mesomeric and inductive effects, causes an increase of acidity of the phenol OH function. This may decrease the pKa of the phenol derivatives at the surface of the polymeric phase. This leads to the dissociation of OH group and finally to proton ejection from the interface to the aqueous layer adjacent to the organic phase. The energy gained from the proton solvation process is probably the driving force allowing for the dissociation of phenol derivatives at the aqueous/organic membrane interface. This event is responsible for the generation of an anionic response of calix[4]pyrrole, calix[4]phyrin and corrole incorporating membranes after their stimulation with undissociated

The increase of the proton concentration in the very thin aqueous layer containing 1.0 x 10-2 M *para-*nitrophenol in 1.0 x 10-2 M of KCl (pH 4.0), adjacent to the calix[4]pyrrole or corrole membrane surface, supported this assumption [14, 17]. According to the mechanism proposed, the lack of the potential changes of calix[4]pyrrole, calix[4]phyrin and corrole ISEs observed

dissociation and proton ejection to water surface (iii)

in the water phase is high enough to block the cavity of

complex [14, 17]. Thus, the formation

Investigated membranes displayed the signal magnitude sequence as follow:

groups from the macrocyclic hosts. This was proved by NMR measurements [14].

Host(membrane) + ArOH(mem) ↔ Host---ArOH(membrane) complexation (ii)

in the presence of phenolic guests at alkaline pH could be explained as follows.

potentiometric signal generation, is a general phenomenon.

calix[4]pyrrole derivatives.

Applications

500

phenols.

modified membranes do respond [38-40].

calix 4 pyrrole >calix 4 phyrin > corrole

ArOH(aq) ↔ ArOH(mem) extraction ( i)

Host---ArOH(membrane) <sup>↔</sup> Host---ArO- + H<sup>+</sup>

At this pH the concentration of OH-

all investigated host molecules by creation of host –OH-

The lack of any response of the membrane modified with calix[4]phyrin or corrole towards dinitrophenol isomers and very low response towards nitrophenol isomers in alkaline medium (Table 1) could be explained as follow. The consequence of host-OH complex creation is the negatively charged surface of polymeric membranes. Dinitrophenols in such circumstances also exist in the anionic forms. Thus, the host-guest electrostatic repulsion force is probably the main reason preventing interaction between them.

The magnitude sequence of the potentiometric signals observed for calix[4]pyrrole, cal‐ ix[4]phyrin and corrole membranes suggests that the acidity of the target molecules is one of the important factors affecting the process of potentiometric signal generation. A similar relation was observed in the case of macrocyclic polyamine-ISEs [5, 6]. In Table 2 the values of pKa and logPoct of target molecules investigated were collected.

On the other hand, the recognition of phenol derivatives by calix[4]pyrrole calix[4]phyrin and corrole is such that guests respectively bury themselves into the host cavity or are perpendic‐ ular to the macrocycle, with (in both cases) the phenolic OH pointed toward the pyrrole end, where the hydrogen bonds with NH units are formed [14, 17, 42 ]. Because of this, the sensitivity sequences observed for nitrophenol as well dinitrophenol isomers reflect also the magnitude of the steric obstacle, which is the lowest in the case of *para-*nitrophenol and 2,4-dinitrophenol. Therefore, for these two guests, the strongest potentiometric responses of ISEs studied were observed. A comparison of the results we have obtained for calix[4]pyrrole and [16, 17] calix[4]phyrin and corrole [14, 21, 42] and the results reported for macrocyclic amines [5, 6, 38-40] showed that the potential generation by the membranes modified with nitrogencontaining macrocyclic compounds after stimulation with phenol derivatives is a general phenomenon. The most important parameters governing this phenomenon are the differences between the acidity of the OH group of the target molecules and the ability of the NH group of the macrocyclic ligands to create the hydrogen bonds with OH group of the phenol derivatives.

The signal magnitude sequence based on the host molecules incorporated into the membranes is as follows:

polyamines > calix 4 pyrroles > calix 4 phyrins >corroles

This sequence reflects the sequence of availability of hydrogen atoms coming from NH or NH <sup>+</sup> group of ligand for OH group of phenol derivatives. This availability is crucial for host –guest complex creation.

Calix[4]pyrrole changes its conformation upon complexation of target molecules, and this macrocycle adopts a cone conformation with the four pyrrole NH groups forming hydrogen bonds with OH group of phenol derivatives. The similar reorganization of the calix[4]pyrrole cavity upon complexation with halide anions was reported [26, 27, 29]. In the case of corrole, such conformational change is not possible because of its rigid structure [41]. As a consequence a strong four-center hydrogen bond can not be created. This is probably the explanation of the weaker potential signal generated by the membrane modified with corrole stimulated with nitrophenols in comparison to membranes modified with calix[4]pyrrole. This statement is supported by results obtained for membranes modified with protonated macrocyclic polya‐ mines, which because of their highest flexibility and ionic character are able to create the strongest supramolecular complex based on hydrogen bonds [40]. This leads to the strongest polarization of O-H bond of phenol derivatives. Such membranes generated the strongest signal for nitrophenol derivatives and only these types of membranes are able to recognize dihydroxybenzene isomers, which are very weak acids [6, 40]. The relationship between acid –base properties of guest is in good agreement with the proposed mechanism. The dissociation of the O-H group at the liquid /liquid interface from the phenol – ligand complex is necessary for the generation of membrane potential changes. In the case of macrocyclic polyamines, the hydrogen bond between the N-H and OH phenolic group is the strongest, and as a conse‐ quence, this causes the highest increase of the acidity of proton from –OH. In the case of calix[4]pyrrole and corroles, the hydrogen bonds are weaker than macrocyclic polyamines. Therefore, the increase of the acidity of OH group from phenolic guest upon creation of supramolecular complexes with calix[4]pyrrole and corrole are lower in the comparison to this observed for polyamines.

A similar influence of both type of lipophilic salts were observed for calix[4]pyrrole liquid

Potentiometry for Study of Supramolecular Recognition Processes Between Uncharged Molecules

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

503

The addition of a lipophilic anion-exchanger into calix[4]pyrrole or corrole-incorporating

This supports the hypothesis, that in the mechanism of their potentiometric response towards pH or phenol derivatives, the reversible hydroxide transport form aqueous to organic phase

**4. Potentiometric responses of Mn(III)-porphyrin and dipyrromethene**

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

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

( ) ( ) <sup>2</sup> 2 Mn TPP Cl + H O Mn TPP ClH O « (1)

2

(2)

(3)

membranes induces the increase of their pH and phenol response.

**Cu(II) containing sensors toward paracetamol**

of such a complex was described by Meyerhoff *et al.* [47]:

( )

( ) ( )

2 2

Mn TPP ClH O + paracetamol Mn TPP ClH O---paracetamol

«

( ) ( ) Mn TPP Cl---paracetamol Mn TPP Cl---paracetamol + H - + dissociation of paracetamol and ejection of proton to water phases

«

Mn TPP Cl---paracetamol complexation +H O

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.

membrane electrodes [17].

is involved [17].

host molecule.

The response properties of ISEs based on ion carriers are strongly influenced by the membrane composition, in particular by the presence of ionic sites in the organic membrane [44-46]. The type of ionic sites depends on the charge of ionophore. In the case of ISEs based on neutral host molecules, ionic sites with the charged sign opposite to that of primary ions are necessary to obtain a Nernstian response, to decrease the membrane resistance, to reduce the ion interference, and to improve the detection limit and selectivity. On the other hand, in ISEs based on electrically charged host, ionic sites with the same charge sign as the primary ions are recommended. In case of potentiometric sensors destined for the detection of neutral compounds there is no general knowledge about the influence of ionic sites on response property.

The calix[4]pyrroles are neutral molecules. On the contrary, the corroles could exist in the three forms: cationic, neutral or anionic [32]. Therefore, two types of ionic sites (anionic and cationic) were used for additional modification of liquid membrane electrodes incorporating both hosts.

The membranes containing corrole and lipophilic cationic salt, tridodecylmethyl -ammonium chloride (T-DDMACl), demonstrated a better sensitivity and a wider dynamic range of potentiometric response towards mono- and dinitrophenol isomers in comparison to mem‐ branes containing only corrole [14]. On the other hand, the corrole membranes additionally incorporating an anionic lipophilic salt, potassium tetrakis(p-chlorophenyl)borate (K-TpCPB), gave no response towards phenolic guests.

A similar influence of both type of lipophilic salts were observed for calix[4]pyrrole liquid membrane electrodes [17].

Calix[4]pyrrole changes its conformation upon complexation of target molecules, and this macrocycle adopts a cone conformation with the four pyrrole NH groups forming hydrogen bonds with OH group of phenol derivatives. The similar reorganization of the calix[4]pyrrole cavity upon complexation with halide anions was reported [26, 27, 29]. In the case of corrole, such conformational change is not possible because of its rigid structure [41]. As a consequence a strong four-center hydrogen bond can not be created. This is probably the explanation of the weaker potential signal generated by the membrane modified with corrole stimulated with nitrophenols in comparison to membranes modified with calix[4]pyrrole. This statement is supported by results obtained for membranes modified with protonated macrocyclic polya‐ mines, which because of their highest flexibility and ionic character are able to create the strongest supramolecular complex based on hydrogen bonds [40]. This leads to the strongest polarization of O-H bond of phenol derivatives. Such membranes generated the strongest signal for nitrophenol derivatives and only these types of membranes are able to recognize dihydroxybenzene isomers, which are very weak acids [6, 40]. The relationship between acid –base properties of guest is in good agreement with the proposed mechanism. The dissociation of the O-H group at the liquid /liquid interface from the phenol – ligand complex is necessary for the generation of membrane potential changes. In the case of macrocyclic polyamines, the hydrogen bond between the N-H and OH phenolic group is the strongest, and as a conse‐ quence, this causes the highest increase of the acidity of proton from –OH. In the case of calix[4]pyrrole and corroles, the hydrogen bonds are weaker than macrocyclic polyamines. Therefore, the increase of the acidity of OH group from phenolic guest upon creation of supramolecular complexes with calix[4]pyrrole and corrole are lower in the comparison to this

An Integrated View of the Molecular Recognition and Toxinology - From Analytical Procedures to Biomedical

The response properties of ISEs based on ion carriers are strongly influenced by the membrane composition, in particular by the presence of ionic sites in the organic membrane [44-46]. The type of ionic sites depends on the charge of ionophore. In the case of ISEs based on neutral host molecules, ionic sites with the charged sign opposite to that of primary ions are necessary to obtain a Nernstian response, to decrease the membrane resistance, to reduce the ion interference, and to improve the detection limit and selectivity. On the other hand, in ISEs based on electrically charged host, ionic sites with the same charge sign as the primary ions are recommended. In case of potentiometric sensors destined for the detection of neutral compounds there is no general knowledge about the influence of ionic sites on response

The calix[4]pyrroles are neutral molecules. On the contrary, the corroles could exist in the three forms: cationic, neutral or anionic [32]. Therefore, two types of ionic sites (anionic and cationic) were used for additional modification of liquid membrane electrodes incorporating both hosts.

The membranes containing corrole and lipophilic cationic salt, tridodecylmethyl -ammonium chloride (T-DDMACl), demonstrated a better sensitivity and a wider dynamic range of potentiometric response towards mono- and dinitrophenol isomers in comparison to mem‐ branes containing only corrole [14]. On the other hand, the corrole membranes additionally incorporating an anionic lipophilic salt, potassium tetrakis(p-chlorophenyl)borate (K-TpCPB),

observed for polyamines.

gave no response towards phenolic guests.

property.

Applications

502

The addition of a lipophilic anion-exchanger into calix[4]pyrrole or corrole-incorporating membranes induces the increase of their pH and phenol response.

This supports the hypothesis, that in the mechanism of their potentiometric response towards pH or phenol derivatives, the reversible hydroxide transport form aqueous to organic phase is involved [17].
