(Invited) Tuning the Electrocatalytic Activity of Fe Phthalocyanine Via Axial Ligation to Multiwalled Carbon Nanotubes and Via Ligand Substitution: Electrochemistry of l-Cysteine and l-Cystine

Abstract

Multiwalled carbon nanotubes (MWCNTs) are excellent platforms for the fabrication of active electrodes for electrocatalysis and electrochemical sensors [1], especially by chemical modification with molecules of known activity. We have studied the electrocatalytic activity of hybrid electrodes containing Fe-phthalocyanine (FePc) axially linked to carbon nanotubes and tested these electrodes for l-cysteine oxidation and l-cystine reduction. It is important to remark that the l-cysteine/l-cystine is a biological relevant redox couple that plays a very important role in many living systems. We have compared the electrocatalytic activity of pristine multi-walled carbon nanotubes and functionalized with –COOH, –NH2groups (MWCNT-p, MWCNT-c and MWCNT-a). These MWCNTs were chemically modified with iron phthalocyanine (FePc). These hybrid systems were characterized using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and AFM. The Fe(II)/(I) and Fe(III)/(II) redox processes of the surface confined FePc are slightly affected by the presence of MWCNTs. The activity for both reactions increases substantially by the presence of modified MWCNTs essentially by an area effect. However, when the currents are normalized by the amount of active sites estimated from the surface coverages of FePc, the activities plotted versus the redox potentials give volcano-shape curves. The data for l-cysteine oxidation fits well on a volcano correlation published previously [1] using several metal phthalocyanines and metal porphyrins (see Figure down below). This shows once more that a Mz+/M(z-1)+redox potential value does exist for which a maximum catalytic activity is observed and is an excellent reactivity descriptor for these complexes, as observed for other reactions. The reactivity trends do not change after the incorporation of FePc to MWCNTs with different chemical functionalities. Acknowledgements The authors are grateful to Fondecyt Projects 1140199, 1181037, Nucleo Milenio RC 120001. Fondecyt Postdoctoral Project 3150271 and Conicyt Scholarships M.P.O. 21130168.Fondequit, EQN160036. References E. Banks, R. G. Compton, Analyst,131(2006) 15-21.A. Gutierrez, J.F. Silva, F.J. Recio, S. Griveau, F. Bedioui, C.A. Caro, J.H. Zagal, Electrocatalysis, 5 (2014) 426–437. Figure 1