Voltammetric determination of catechol based on a glassy carbon electrode modified with a composite consisting of graphene oxide and polymelamine

The authors describe an voltammetric catechol (CC) assay based on the use of a glassy carbon electrode (GCE) modified with a composite consisting of graphene oxide and polymelamine (GO/PM). The modified GCE was characterized by field emission scanning electron microscopy, elemental analysis, Raman spectroscopy and FTIR. Cyclic voltammetry reveals a well-defined response to CC, with an oxidation peak current that is distinctly enhanced compared to electrodes modified with GO or PM only. The combined synergetic activity of GO and PM in the composite also results in a lower oxidation potential. Differential pulse voltammetry (DPV) shows a response that is linear in the 0.03 to 138 μM CC concentration range. The detection limit is 8 nM, and the sensitivity is 0.537 μA⋅μM−1⋅cm−2. The sensor is selective for CC even in the presence of potentially interfering compounds including hydroquinone, resorcinol and dopamine. The modified GCE is highly reproducible, stable, sensitive, and shows an excellent practicability for detection of CC in water samples. Graphical abstract Voltammetric sensing of catechol at polymelamine entrapped graphene oxide composite. Voltammetric sensing of catechol at polymelamine entrapped graphene oxide composite.


Introduction
Catechol (CC) is an ortho-isomer of benzenediols, has been common used as a building block in organic synthesis [1]. It has been widely used in variety of applications including production of pesticides and precursor to perfumes and pharmaceuticals [2,3]. In addition, CC has classified as a periodic environmental pollutant and Group 2B human carcinogen due to its toxicity and low degradability in the ecological environment [4,5]. Continues exposure of CC can result in the prolonged rise of blood pressure and depression of central nervous system in animals [6].
Therefore, considerable attention has been paid for the fabrication of sensitive devices for low level detection of CC in environmental samples. Various analytical methods have been used for the sensitive detection of CC such as high performance liquid chromatography, gas chromatography, mass spectrometry, spectrophotometry, chemiluminescence, flow injection analysis and electrochemical methods [7][8][9][10][11][12]. The electrochemical methods are found simple and cost effective than other available aforementioned analytical methods [12]. The modified electrodes have played an important role in the fabrication of sensitive CC sensors due to their high selectivity, low oxidation potential and high sensitivity. On the other hand, the unmodified electrodes displayed a poor selectivity and sensitivity along with fouling of signals towards detection of CC [12,13].
Graphene oxide (GO) has attached much attention in the scientific community owing to its extreme hydrophilicity and presence of abundant functional groups [14,15]. In addition, GO has been widely used for surface modification with different molecules due to the presence of different functional groups on the surface [15]. Instead, polymelamine (PM) is known as a class conducting polymer, has been widely used for electrode modification owing to its high stability, strong adherence to electrode surface and presence of abundant nitrogen and amine groups [16,17]. 4 Furthermore, the PM modified electrodes have been used for electrochemical sensing of different analytes such as gallic acid, dihydroxybenzene isomers and neurotransmitters [16][17][18][19][20].
Considering the aforementioned properties of PM and GO, one may have assumed that an electrode modified with PM combined with GO can provide good electrocatalytic activity.
However, only limited reports are available for PM based electrochemical sensors [16][17][18][19][20][21]. To the best of our knowledge, the GO/PM composite modified electrode has not been used yet for any electrochemical applications.
In this paper, we report the fabrication of a simple and sensitive CC electrochemical sensor based on GO/PM composite modified glassy carbon electrode (GCE). The GO/PM composite modified electrode shows an enhanced catalytic activity for oxidation of CC than PM and GO modified electrodes. The sensitivity, selectivity, stability and practicality of the sensor was evaluated and discussed in detail.

Materials and method
Natural graphite was purchased from Sigma Aldrich and used as received

Fabrication of GO/PM composite modified electrode
Scheme 1 Schematic representation for the fabrication of GO/PM modified electrode and its electro-oxidation towards CC. 6 The GO and GO dispersion (5 mg mL -1 in double distilled water) were prepared by our previously reported methods [22,23]. To prepare GO@PM composite modified electrode, about 8 µL (optimum) of GO dispersion was coated on pre-cleaned GCE, and dried in an air oven. The resulting GO modified electrode was immersed in the electrochemical cell containing 1 mM melamine and 0.01 M HCl, and performed 10 constitutive cycles in the potential ranging from 0 to 1.5 V at a scan rate of 50 mV s -1 [18].

Choice of materials
The fabrication of simple and robust sensors for the determination of CC is of interest to the analytical chemists. The CC is highly electro-active on carbon modified electrodes, and they are often shows poor selectivity, sensitivity and reproducibility. Hence, the modified electrodes have been widely used for the sensitive determination of CC. In the present work, we have chosen the GO and PM composite for sensitive and selective determination of CC due to the unique properties of GO and PM. The abundant oxygen functional groups of GO are more favourable to form the stable composite with PM. In addition, the abundant nitrogen and amine groups of PM can possibly interact with more number of CC molecules and result into the enhanced sensitivity, lower oxidation potential and low detection limit of CC on GO/PM composite. In addition, the 7 sensor is more simple, inexpensive and has appropriate analytical features towards CC than previously reported carbon nanomaterial based CC sensors ( Table 1).  also noted that the surface morphology of PM on GO is similar to previous reports [20,21]. The FTIR (Fig. S1) and Raman spectra (Fig. S2) findings also confirmed the formation of GO/PM composite and the detailed discussions can be found in electronic supplementary material (ESM).

Electrochemical behavior of CC on different modified electrodes
The cyclic voltammetry was used to investigate the electrochemical behavior of CC at different modified electrodes. electrode. In addition, the Ep of CC was 84 mV lower than unmodified electrode, and the anodic peak current was lower than unmodified electrode. It addition, the GO widely known as an insulating material and has poor electrocatalytic activity. The PM modified electrode shows an enhanced oxidation peak current to CC than bare and GO modified electrodes, which indicates the high adsorption ability of PM towards CC. The observed Ep of CC was 153 and 69 mV lower than those observed at bare and GO modified electrodes. However, the GO/PM composite modified electrode shows a well-defined redox couple 10 with 3-fold higher oxidation peak current response for CC than PM modified electrode. The oxidation peak potential of CC was appeared at 0.223 V. As shown in Fig. 3

Determination of CC
DPV was used to determine the CC using GO/PM composite modified electrode due to its high sensitivity than other voltammetric methods [22].  To evaluate the novelty and superiority of the sensor, the analytical features of the sensor were compared with previously reported carbon nanomaterials and polymers based CC sensors.

Selectivity
The selectivity of sensor is more crucial for practical applications, hence we have investigated the selectivity of sensor in the presence of range of potentially active compounds such as hydroquinone (HQ), resorcinol (RC), dopamine (DA), ascorbic acid (AA) and uric acid (UA). 13 These compounds can possibly interfere with CC due to an oxidation potential close to CC [22].

Determination of CC in water samples
To verify the practical applicability of the sensor, we have determined the CC in different water samples by GO/PM modified electrode. DPV method was used to detect the CC and the experimental conditions are similar as of in Fig. 3A. The recovery of CC in water samples were calculated using the standard addition method [22]. The modified electrode does not show appropriate signal for the ground and tap water samples, which shows that CC was absent in the water samples. Then, a known concentration of CC containing ground and tap water samples were spiked into the pH 7 and the recoveries were calculated. The recovery values for CC in the ground and tap water samples are summarized in Table ST1. The average recoveries about 95.5 and 94.2 14 % of CC were found in ground and tap water samples using GO/PM composite modified electrode.
In addition, the practical ability of the composite modified electrode is comparable with results obtained from spectrophotometric method. The results revealed that the fabricated sensor can be used for the determination of CC in water samples.
The repeatability and reproducibility of the sensor (not shown) were investigated using cyclic voltammetry, and the experimental conditions are similar to Hence, the result proves that the fabricated sensor is highly stable for long time use.

Conclusions
In summary, a sensitive and selective CC sensor has been developed using an electrochemically derived GO/PM composite modified electrode. The sensor exhibited a low LOD (8 nM) with an appropriate analytical features (sensitivity and linear response range) than previously reported nanomaterials based CC sensors. As a proof of concept, the sensor was also successfully applied for the detection of CC in different water samples, and the recoveries of CC were highly satisfactory with the standard method. The sensor showed a high selectivity towards 15 CC in the presence of potentially active interfering compounds. However, the sensor also has some limitations, such as selectivity in the presence of high concentrations of HQ. As a future perspective, the GO/PM composite can be used for accurate detection of CC in environmental samples.