Unusual pH - Dependent Electrochemical Activity of Nitrogen Doped Graphene Oxide Towards Selective and Simultaneous Determination of Hydroquinone and Catechol

Abstract

Hydroquinone (HQ) and Catechol (CC) are the two important dihydroxybenzene isomers that have found widespread usage in dyes, pesticides, cosmetics and pharmaceuticals manufacturing industries as synthetic intermediates [1]. However, due to their toxicity and low degradability, they have been declared as important environmental pollutants to be monitored [2]. Therefore, it is imperative to develop a rapid and facile analytical method for the simultaneous determination of HQ and CC. In this regard, electroanalytical techniques have been preferred over conventional analytical techniques because of their cost–efficiency and high–speed analysis [3]. Unfortunately, due to the presence of same electroactive groups, the redox peaks of both these isomers overlap together, thereby resolution of them at conventional working electrodes has been rendered impossible without chemical or electrochemical modifications. In the past few years, the doping of atoms such as Nitrogen, Boron and Sulfur into graphene or graphene oxide lattices have evoked lot of interest [4]. These doped graphene oxides have large amount of edge sites, high surface area and electrical conductivity which in turn leads to superior electrochemical and catalytic activities [5]. In the present work, a simple, facile and one – pot hydrothermal strategy is presented for the synthesis of Nitrogen–doped graphene oxide (NGO) by employing Amitrole (AMT) as the nitrogen source. The successful N–doping into the graphene oxide lattices have been confirmed through FT–IR, FT–Raman and XPS techniques while, other characteristic properties were investigated using powder XRD, SEM, TEM and TGA. As an application, an electrochemical sensor based on the NGO–AMT modified glassy carbon electrode (NGO–AMT/GCE) has been developed for the sensitive and simultaneous determination of HQ and CC. Interestingly, the NGO–AMT/GCE exhibited a pH–dependent electrochemical activity towards the simultaneous sensing of these two dihydroxybenzene isomers in 0.1 M PBS. Moreover, the characteristic twin oxidation peaks of HQ and CC were observed at pH = 7.08 with a peak separation of about 110 mV using DPV technique. On the other hand, at pH = 5.08 and pH = 9.08 the corresponding oxidation peaks of CC and HQ were alone observed. This unusual behavior has been rationalized through the protonation of pyridinic nitrogen at lower pH and de-protonation of carboxylic acid active sites at higher pH (Fig.1). The proposed sensor was thus examined at pH 7.08 and found to have a linear range from 1 to 500 μM. The limits of detection (LOD, S/N = 3) were 15 and 20 nM, respectively for HQ and CC. Furthermore, the interference analysis has been carried out with some typical interfering agents and the performance of NGO–AMT/GCE was found to be satisfactory. As a practical application, the developed sensor was utilized for analyzing these isomers in tap water and exhibited reliable and stable recovery results.