Electrode modification using iron metallophthalocyanine through click chemistry and axial ligation with pyridine

Abstract

Electrochemical grafting of 4-azidobenzenediazonium salt and click chemistry with ethynylpyridine was used to modify a glassy carbon electrode surface, and iron phthalocyanine was subsequently attached through axial ligation to the surface pyridine groups. The strong axial bond formed by the interaction between the central metal and the lone pair of the nitrogen in the pyridine group resulted in stable modified electrodes. The electrocatalytic ability of this sensor was shown using hydrazine as a test analyte, with a linear range from 1.0 × 10⁻⁵ to 3.4 × 10⁻⁴ M and a limit of detection of 10.0 ± 1.3 μM.

Introduction

Modification of electrodes is commonly undertaken as a means to improve their sensitivity, stability and selectivity for a target analyte. In the formation of stable monolayers on electrodes, grafting of diazonium complexes through electrochemical reduction has been used on a wide range of conductive and semi-conductive surfaces, making this one of the most versatile methods currently being used in surface chemistry [1], [2], [3], [4], [5], [6], [7], [8], [9]. This technique has proved especially useful when used with the click chemistry reaction proposed by Demko and Sharpless [10], [11], [12] as a means to functionalise electrode surfaces with a variety of groups, and shows great potential for further applications [1], [13], [14]. This click chemistry reaction uses Huisgen 1,3-dipolar cycloaddition between an alkyne and an azide in the presence of a Cu(I) catalyst to form a triazole link between a wide range of substituted azides and alkynes [1], [10], [11], [12], [13], [14], [15]. This reaction has been used a great deal in the organic synthesis of various complex compounds [15], [16], [17], [18], [19], but has only recently been studied for its potential in surface modification. The versatility, high yields, mild conditions and stability of the resulting 1,2,3-triazoles makes them particularly attractive for surface modification, and thus sensor development [1], [13], [14], [15], [20], [21], [22], [23], [24].

For this purpose, metallophthalocyanines (MPcs) have proved useful in the electrocatalysis of analytes such as 2-mercaptoethanol, hydrazine, and amitrole [25], [26], [27], when electroactive central metals such as cobalt, iron and nickel, or manganese are used [28]. MPc complexes have been used to modify electrodes by several methods such as self-assembled monolayer, electropolymerisation and dip or drop dry [28]. Some of the methods require the synthesis of complex phthalocyanines, and in other cases the electrode surface is not reproducible. This works aims at using a simple Pc for the electrode modification using click chemistry.

The combination of grafting and click chemistry offers a useful approach for covalent attachment of MPcs to an electrode surface. MPcs bearing the required azide and alkyne groups are required for click chemistry. The synthesis of such MPc complexes is often complex and commonly requires the use of protecting groups or extensive purification [29], [30], [31], [32], [33], [34], [35], [36]. To eliminate the need for costly and complex substituents, an alternative approach utilising the strong bond between iron phthalocyanine (FePc) and pyridine has been investigated. It is well known that MPcs can coordinate to bases containing nitrogen, and FePc in particular can form strong axial bonds to pyridine [37], [38]. Axial ligation to pyridine adducts has been used to attach MPcs to gold surfaces through the use of a self-assembled monolayer of 4-mercaptopyridine [39]. In this work, we investigate the use of commercially-available 4-ethynylpyridine in the click chemistry approach, where the pyridine group can be linked to a grafted glassy carbon surface and then bonded through axial ligation to transition metal bearing phthalocyanines such as iron or cobalt phthalocyanine (Scheme 1). This represents a new simple way of electrode modification in electroanalytical chemistry. The modified electrode is used for the detection of hydrazine as a test molecule since its electrochemical behaviour on phthalocyanine modified electrodes is well established [28], [40], [41], [42].