Talanta 76 (2008) 1097–1104 Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Electrocatalysis of reduced l-glutathione oxidation by iron(III) tetra-(N-methyl-4-pyridyl)-porphyrin (FeT4MPyP) adsorbed on multi-walled carbon nanotubes Rita C.S. Luz a,∗ , Flavio S. Damos b , Auro A. Tanaka c , Lauro T. Kubota a , Yoshitaka Gushikem a a Institute of Chemistry, UNICAMP, P.O. Box 6154, 13084-971 Campinas, SP, Brazil b Department of Chemistry, UFVJM, Rua da Gl´ oria, 187, 39100-000 Diamantina, MG, Brazil c Department of Chemistry, UFMA, Avenida dos Portugueses s/n, 65085-580 S˜ ao Lu´ ıs, MA, Brazil article info Article history: Received 16 February 2008 Received in revised form 6 May 2008 Accepted 9 May 2008 Available online 17 May 2008 Keywords: Reduced l-glutathione Iron(III) tetra-(N-methyl-4-pyridyl)-porphyrin Carbon nanotubes Erythrocyte abstract The development of a highly sensitive voltammetric sensor for reduced l-glutathione (GSH) using a basal plane pyrolytic graphite (BPPG) electrode modified with iron(III) tetra-(N-methyl-4-pyridyl)-porphyrin (FeT4MPyP) adsorbed on multi-walled carbon nanotubes (MWCNT) is described. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to verify the morphologies and composition of the MWCNT after modification with the FeT4MPyP complex. The modified electrode showed very efficient electrocatalytic activity for l-glutathione oxidation, substantially decreasing the oxi- dation peak to -0.025 V vs Ag/AgCl. A linear response range from 5 mol L -1 to 5 mmol L -1 was obtained with a sensitivity of 703.41 A L mmol -1 . The detection limit for GSH determination was 0.5 mol L -1 and the relative standard deviation (R.S.D.) for 10 determinations of 250 mol L -1 GSH was 1.4%. The modified electrode was applied for GSH determination in erythrocyte samples and the results were in agreement to those obtained by a comparative method described in the literature. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Carbon nanotubes (CNT) have attracted enormous interest in recent years due to their exceptional electrical, chemical and mechanical properties, which make them attractive candidates for many applications [1,2], particularly in the area of chemical sensors with electrochemical detections [3,4], once CNTs can be functionalized with organic compounds without destroying their electrical and chemical properties [5–10]. In this context, studies of catalytic activity of modified multi-walled carbon nanotubes (MWCNT) electrodes with several electrocatalysts have been per- formed. The choice of a metalloporphyrin molecule was based on the fact that porphyrins play an important role in several biochemi- cal processes [11] and the observed electrochemical reversibility of iron-porphyrin derivatives on several electrode materials, which make them perfectly suitable electron transfer mediators in prepar- ing chemically modified electrodes. Iron-porphyrins are also well known for their excellent electrocatalytic properties toward the detection of many important analytes, such as nitric oxide [12], ∗ Corresponding author. Tel.: +55 19 3521 3053; fax: +55 19 3521 3023. E-mail address: rcsluz@iqm.unicam.br (R.C.S. Luz). neurotransmitters [13], oxygen [14], hydrogen peroxide [15], nitrite [16], superoxide [17], sulfur oxyanions [18] and tryptophan and its derivatives [19]. Unfortunately, most of the modified electrodes based on iron-porphyrin derivatives have shown poor stability and selectivity, and some of the methods that have been employed for surface modifications are laborious and expensive. In addition, given the widespread involvement of thiols and the corresponding disulfides in many essential biological functions, much effort has been made to develop sensitive and selective meth- ods for the detection of such compounds. Specifically, l-glutathione (Fig. 1a) can exist in oxidized (GSSG) or reduced (GSH) forms and it is an essential compound in many biological processes such as catabolism and transportation [20]. The reduced form of l-glutathione (GSH) is required to maintain the iron(II) form of hemoglobin in its reduced state and for maintaining the structure of red blood cells [21,22]. Although many studies on GSH electro- chemical oxidation using various modified electrodes have been reported, to our best knowledge, there is no report about the use of a basal plane pyrolytic graphite (BPPG) electrode with FeT4MPyP (Fig. 1b) complex and MWCNT. In this sense, a simple one-step procedure was used for modification of a BPPG electrode with Fe(III)T4MPyP and MWCNT by non-covalent (–( stacking, aiming at designing a new special nanostructured interface to improve the electron transfer. 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.05.009