Electrocatalytic actvity of modified gold electrodes based on self-assembled monolayers of 4-mercaptopyridine and 4-aminothiophenol on Au(111) surfaces chemically functionalized with substituted and unsubstituted iron phthalocyanines
Introduction
Self-assembled monolayers (SAMs) of thiols on gold surfaces have become the focus of intensive research [1]. These systems offer the possibility of designing bio inert devices with controllable chemical functionality. When using a given architecture it is possible to build an appropriate electrochemical interface for a given reaction. Transition metal phthalocyanines (MPc) are versatile molecules for generating interface with a chemical reactivity that can be controlled by using a given central metal or by using substituents on the macrocyclic ligand with different electron-withdrawing properties [2]. These compounds are known for their electrocatalytic activity for the oxidation of l-cysteine [2], [3], [4], [5], [6], [7], [8], an important aminoacid in living systems. Different strategies for incorporating these catalysts on electrode substrates have been studied including the use of self-assembled monolayers, which can anchor the phthalocyanine molecules on the gold surface (Au/SAMs/MPc). One of the interesting advantages of using self-assembled organic monolayer systems functionalized with MPc is that they provide electrochemically active systems with stable and highly ordered structures. Early strategies for producing Au/SAMs/MPc systems involved the incorporation of anchoring SH groups on the periphery of the macrocyclic ligand. Porter et al. [9] and Murray et al. [10], [11] have reported the immobilization of a Co porphyrin in different orientations with respect to the electrode, by using porphyrins with pendant thiol groups located on the ligand (octopus-like porphyrins). These assemblies presented electrocatalytic activity for the reduction of molecular oxygen. Ozoemena et al. have reported that Co and Fe octa(hydroxyethylthio)-phthalocyanine posses electrocatalytic activity for the oxidation of l-cysteine, homocysteine, penicillamine and thiocyanate [12]. Other authors have published the synthesis of macrocycles functionalized with the anchoring groups like SH and SS, and the successful modification of gold surfaces, in this way achieving self-assembled molecular films (SAMs) of the macrocycles [13], [14], [15], [16], [17], [18]. However, these strategies present two difficult problems: the first is that the metal macrocyclics need to be synthesized with alkanethiols or with a thiol group linked to the ligand, and the syntheses can be complex and difficult [19]. The second is the formation of stable multilayers in view of the good interaction between the sulfur of the thiol and the metal center of the macrocycle [20], hindering the control of the deposition of the catalyst and in some cases blocking the central metal catalytic center.
In recent years an interesting and simple strategy has been reported for modifying gold electrodes with SAMs/MPc interface. It consists of first adsorbing a 4-mercaptopyridine over a gold electrode forming a SAMs–Au, a thiol that would act as a molecular anchor, linking the sulfur atom with the gold surface and leaving a pyridine group exposed to the outermost portion of the SAMs [21]. This pyridine group then can act as an axial ligand for a metal macrocylic complex like Fe phthalocyanine. (Fig. 1) [22], [23], [24], [25]. This approach has been successfully used for Co and Fe tetraphenylporphyrin, and a Co phthalocyanine [22], [23]. The SAMs/MPc assemblies of 4-mercaptopyridine on gold, chemically functionalized the Fe phthalocyanine exhibit catalytic activity for the oxidation of hydrazine, thiocyanate and cysteine, suggesting their use as possible sensors for the detection of these substances [24], [25], [26], with FePc having the best electrocatalytic response for the l-cysteine oxidation reaction, compared to CoPc (cobalt phthalocyanine) and MnPc (manganese phthalocyanine) anchored in the same fashion. This route has been reported as a simple strategy for making electrochemically active molecular assemblies that can be used as very selective electrochemical sensors. However, these studies do not evaluate the effect of the axial ligand that can modulate the electrocatalytic activity of a SAMs/MPc system, so we studied the reactivity of a series of iron phthalocyanines with different substituents in the periphery of the macrocycle. In this work, we study the dependence of the redox potential of the catalysts on Hammet's parameters of the substituents as report in other studies [2]. We have also modified the SAMs by giving them different N-donor character, by using 4-aminothiophenol, which was previously used in the modification of gold surfaces anchoring CoPc, Co and Fe tetraphenylporphyrin [22], [27]. Finally, we have focused our attention on studying the effect of the thiol-end-FePc spacer on the electron transfer rate from l-cysteine to the gold-SAM-FePc assembly, and also on the effect of substituents on the phthalocyanine ligand on the ET kinetics.
Section snippets
Chemicals
Iron phthalocyanine (FePc) and iron hexadecachlorophthalocyanine (16(Cl)FePc) complexes were purchased from Aldrich and used as received. Iron carboxyphthalocyanine (4β(COOH)FePc) was donated by Dr. A.A. Tanaka of the Universidad Federal do Maranhao, Brazil [28]. Iron tetraaminophthalocyanine (4β(NH2)FePc) and iron tetranitrophthalocyanine (4β(NO3)FePc) were obtained from Mid Century Chemical, U.S.A. 4-mercaptopyridine (4MPy, 95%) and 4-aminothiophenol (4ATP, 97%) were obtained from Aldrich,
Results and discussion
Fig. 2 shows cyclic voltammetry curves obtained with Au(111) substrates modified with self-assembled monolayers of 4-mercaptopyridine and 4-aminothiophenol functionalized with the Fe phthalocyanine. The voltammograms were obtained in a pH 4.0 buffer electrolyte at v = 0.05 V/s, over a potential range from −0.5 to 0.5 V. Fig. 2 also shows the voltammetric response of the unmodified Au(111) substrate and the response of the organic monolayer of 4-mercaptopyridine and 4-aminothiophenol (blank)
Conclusions
It can be concluded that in all cases, the aromatic thiols acting as axial ligands increase the catalytic activity of the different phthalocyanines for the oxidation of l-cysteine compared to phthalocyanines adsorbed directly on gold. This enhancement in the catalytic activity has also been observed for the reduction of O2 [21], [29] using FePc linked to gold via SAMs and for FePc linked to carbon nanotubes via a pyridine unit covalently bound to a carbon atom [30] so the presence of the axial
Acknowledgments
J.F. Silva acknowledges with thanks the support of Programa Bicentenario en Ciencia Tecnología PDA-23 of Conicyt (Chile), and Fondecyt Project 11100450. Support of Fondecyt is also gratefully acknowledged by J. Pavez, Project 1131062, J.H. Zagal Project 1100773 and C.P. Silva is thankful to Conicyt for a Doctoral fellowship. JZ and JP thank support of Núcleo Milenio de Ingeniería Molecular P07-006-F.
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