Evaluation of various strategies to formation of pH responsive hydroquinone-terminated films on carbon electrodes
Introduction
Covalently modified surfaces have long been used for many different applications, in particular in electroanalytical and electrocatalytical chemistry [1]. The most commonly used electrochemical procedures for derivatizing carbon surfaces consist of the electrochemical oxidation of amines [2], [3] or the electrochemical reduction of aryldiazonium salts [4]. Recently, we reported on a new method based on the reduction of iodonium salts which allows the introduction of alkynyl groups in addition to aryl groups [5]. In these procedures, reactive radical species are generated directly at the electrode surface and attack the surface to form covalent CC or CO bonds. These bonds ensure a high stability of the modified electrode and applications have been described in several fields [6], [7].
Numerous studies have already been reported on hydroquinone (H2Q) terminated films on various electrode materials. In particular, the use of self-assembly of thiols on gold has led to many interesting studies of, e.g. pH sensing [8], adsorption kinetics [9], electron transfer properties [10], and electrochemical integrity [11].
Carbon provides a cheap, multi configurational, and easily accessible electrode material for e.g. analytical sensors. However, results from self-assembled films of H2Q on other materials cannot be directly inferred to carbon, in part because films grafted on carbon are unable to self-organize [12]. On the other hand, the possible formation of CC and CO bonds on carbon surfaces, potentially lead to much more stable films than obtainable with e.g. Au–S bonds. Not only is the accessible potential window much larger for the carbon based electrodes, but the chemical and thermal stability is also much higher [1].
In this paper a number of strategies for the covalent attachment of alkyl-spaced H2Q centers on a carbon surface are provided. In general, well-documented procedures for the initial functionalization of the carbon are employed in conjunction with conventional organic synthesis to ensure a strong attachment of these moieties. In particular, we wish to present a comparative study of three different methods for the initial derivatization, i.e. electrooxidation of the surface in aqueous solvent to afford carboxylic acid functionalities, immobilization of 4-aminobenzoic acid by oxidation in ethanol, and finally plasma deposition using a gaseous form of maleic anhydride. The analysis of the modified electrodes was accomplished with a combination of electrochemical and surface microscopic techniques, such as atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).
Section snippets
Chemicals and instrumentation
Dichloromethane (DCM) was purchased from Lab-Scan and used as received. Water was triple distilled. Bromine end-group alkyl-functionalized dimethoxybenzenes were synthesized from dimethoxybenzene using the procedure described by Hong and Park [10] The subsequent transformation into the corresponding n-(2,5-dimethoxyphenyl) alkan-1-amine (n-DMPAA) where n is the number of methylene units in the alkane was accomplished via another literature synthesis [13]. All other compounds were commercial and
Results and discussion
The electrochemical oxidative treatment of the GC electrodes in aqueous acidic solution produces a substantial number of superficial carboxylic acid groups in addition to other oxygenated functionalities such as phenols [1]. The carboxylic groups may be used as anchoring points for the immobilization of, e.g. amino-tethered molecules. In our procedure, the immobilization of DMPAA allows us to tether a molecular precursor to a stable pH responsive H2Q/Q redox system.
The surface atomic
Conclusions
The hydroquinone/quinone (H2Q/Q) redox system was tethered via aminoalkyl chains to an electrochemically pre-oxidized glassy carbon surface containing carboxylic acid groups. The resultant surfaces (Γ ∼ 3 × 10−10 mol cm−2) exhibit the expected chemical reversibility in aqueous solution with a pH-sensitive position of the formal potential (∼55 mV/pH unit). Interestingly, the peak potential separation increases relatively slowly from 0.02 V for 1 methylene unit to 0.21 V for 12 units. The electrode
Acknowledgments
Statens Naturvidenskabelige Forskningsråd is gratefully acknowledged for funding. We thank Mingdong Dong of the iNANO center at the University of Aarhus for performing the AFM analysis. AHH acknowledges the Villum Kann Rasmussen Foundation for a postdoctoral grant. We dedicate the paper to the memory of Torben Bo Christensen.
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