Electrochemical interfacial adsorption mechanism of polyphenolic molecules onto Hanging Mercury Drop Electrode surface (HMDE) Evangelos Giannakopoulos a,b,⇑ , Yiannis Deligiannakis b , Georgios Salahas c a Laboratory of Physical Processes and Signals, Department of Automation, Technological Educational Institute of Mesologhi, Nea Ktiria, 30200 Mesologhi, Greece b Laboratory of Physical Chemistry of Materials & Environment, Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, 30100 Agrinio, Greece c Laboratory of Plant Physiology and Biochemistry, Department of Greenhouse Crops and Floriculture, Technological Educational Institute of Mesologhi, Nea Ktiria, 30200 Mesologhi, Greece article info Article history: Received 5 August 2011 Received in revised form 29 October 2011 Accepted 4 November 2011 Available online 11 November 2011 Keywords: Electrode reaction Electro-adsorption Polyhydroxybenzoic acid Deconvolution Stripping voltammetry abstract The electrochemical interfacial adsorption of a series of polyphenolic molecules (i.e. polyhydroxyben- zoic acids) at the Hanging Mercury Drop Electrode (HMDE)–electrolyte interfaces were investigated using Square Wave-Adsorption Cathodic Stripping Voltammetry (SW-AdCSV) at pH 7.5. Polyhydroxy- benzoic acids bearing one 4-hydroxybenzoic acid, two 3,4-dihydroxybenzoic acid (protocatechuic) or three 3,4,5-trihydroxybenzoic acid (gallic) OH-groups at positions C3, C4 and C5 on the benzene ring were studied. The complex interfacial electrochemical behaviour of these molecules has been decon- voluted to (i) adsorption evens and (ii) redox formations at the HMDE interface. The approach pre- sented is based on the comparative analysis of the SW-AdCSV signals in conjunction with the molecular structure of the molecules. Accordingly, theoretical calculations, involving nonlinear-fit, resulted in the identification of the interfacial reaction mechanism of adsorbed gallic acid on the HMDE. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Phenolic compounds are secondary plant metabolites found in all fruits and vegetables [1]. In addition, phenolic compounds are fundamental structural units i.e. soil humic substances [2–5]. It has been shown that the antioxidant activity [6] and radical-scav- enging activity [4] of plant-phenols and humic substances depend critically on (a) the number and (b) geometrical arrangement of phenolic hydroxy groups. A study of the antioxidant capacity of eight phenolic compounds [7] has shown that the antioxidant activity of the compound increases proportionally with the num- ber of OH-groups linked to the aromatic ring [7]. In addition, the pyrogallol-type triphenols carrying three adja- cent hydroxy groups on a benzene ring scavenge radicals more effectively than catechol-type o-diphenols [8,9]. Electrochemical measurements allow determination of physicochemical parame- ters for antioxidants, such as redox potential, the number of elec- trons transferred, the electrode-reaction rate constant, which are relevant not only for evaluating the antioxidative abilities of phe- nols [10], but also for understanding their reaction mechanisms. The half-wave electrochemical potential, E 1/2 , has been sug- gested to be correlated with the radical scavenging activity of hydroxyphenols [11]. Thus, a phenol with a low E 1/2 value is pre- dicted to be good radical scavenger [12]. This has been rationalised based on the idea that free-radical formation involves the electro- chemical oxidation and deprotonation of one phenolic OH, produc- ing the phenoxyl radical and H + , in one electron and one H + transfer reaction [11]. The oxidation mechanism of phenols is of interest e.g. particularly in connection with the biosynthetic oxidative cou- pling reactions. Oxidation can occur either in a single one-electron or in two one-electron steps [11,13]. Among natural polyphenols gallic acid (3,4,5-trihydroxybenzoic acid [GA]), exhibits the most considerable antioxidant capacity in plants. Moreover, recent works by our group [4,5], based on the Electron Paramagnetic Resonance spectroscopy, provided evidence that GA is a good model for the radicals of Humic Acids. GA is a natural product of tannins’ hydrolysis and is present in food of plant origin [14]. It has been documented that GA possesses a higher radical scavenging activity than alkylgallate derivatives [15] as well as protocatechuic acid and 4-hydroxybenzoic acid [9,16]. Recent theoretical calculations for GA [3,4], and other phenolic compounds provided a theoretical foundation for their 1572-6657/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.11.008 ⇑ Corresponding author at: Laboratory of Physical Processes and Signals, Depart- ment of Automation, Technological Educational Institute of Mesologhi, Nea Ktiria, 30200 Mesologhi, Greece. E-mail address: egiann@teimes.gr (E. Giannakopoulos). Journal of Electroanalytical Chemistry 664 (2012) 117–125 Contents lists available at SciVerse ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem