Self-Assembled Ionic Liquid-Phosphomolybdic Acid/Reduced Graphene Oxide Composite Modiﬁed Electrode for Sensitive Determination of Dopamine

A novel reduced graphene oxide (RGO) coated glassy carbon electrode (IL-PMo 12 /RGO/GCE) modiﬁed with ionic liquid (IL, [BMIM][BF 4 ]) and phosphomolybdic acid (PMo 12, H 3 PMo 12 O 48 ) was fabricated via electrostatic self-assembling. The phospho- molybdic acid anionic monolayer could electrostatically adsorbed ionic liquid cations to form an organic and inorganic hybrid ﬁlm on graphene sheets. The modiﬁed electrode was used for the determination of dopamine (DA) in the presence of uric acid (UA). This electrode anticipated the good electron mobility and large surface area of graphene, high ionic conductivity of ionic liquid besides the electron transfer property of phosphomolybdic acid. Optimization of the sensor’s performance is presented and resulted in a better current signal. The linear range of the modiﬁed electrode was from 0.1–100 μ M for determination of DA with an R-Square 0.9924 and with a detection limit of 3.3 × 10 − 8 M (S/N = 3). The IL-PMo 12 /RGO/GCE also presented good stability and reproducibility. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of

In the last ten years graphene, a two-dimensional all-sp 2hybridized carbon, has received growing research interest as an electrode material due to its large surface area, good chemical stability, high electrical and thermal conductivity and broad electrochemical window. [1][2][3][4][5] Nevertheless, graphene usually suffer from serious aggregation caused by van der Waals interactions between the carbon sheets, resulting in reduced active specific surface area for electrochemical performance. 6 One of the most common routes to resolve this problem is functionalization of graphene, including reactions of graphene (and its derivatives) with organic and inorganic molecules, chemical modification of the large graphene surface, and the general description of various covalent and noncovalent interactions with graphene. [7][8][9] So far, the most economical way to produce graphene is reduction of graphene oxide (GO). This kind of graphene is also called chemically reduced graphene oxide (RGO), usually has abundant structure defects and functional groups 10,11 which are advantageous for its electrochemical applications. 12 Recently, RGO or RGO composite-based modified electrodes have attracted much attention in the field of electrochemical sensors and biosensors, 13 electroanalysis 14 and electrochemical determination of metal ions, 15 et al.
Polyoxometalates are Keggin-type heteropolyanions of molybdenum and tungsten, which are particularly attractive because of their ability to adsorb irreversibly on carbon and metal surfaces by selfassembling to form structured films. 16 Such polyoxometallates are rigid inorganic metal-oxygen compounds which undergo reversible stepwise multi-electron transfer reactions of importance to such technologies as electrocatalysis, molecular electronics and sensing. [17][18][19] It has been reported that phosphomolybdic acid (H 3 PMo 12 O 48 , PMo 12 ) can form an anionic monolayer on the carbon materials, 20 PMo 12functionalization could exfoliate the stacked graphene sheets into individual ones through their electrostatic repulsion. The negatively charged PMo 12 on graphene sheets (GSs) may facilitate a uniform deposition of positively charged molecules such as conductive polymers and ionic liquids, which reducing the interfacial resistance. 21 Ionic liquid (IL) is a kind of solvent, shows not only a good solubility to organic substances but also to inorganic substances,and itself has a = These authors contributed to this work equally. z E-mail: wangzonghua@qdu.edu.cn good temperature stability and chemical stability. [22][23][24] In this work, a small amount of PMo 12 was self-assembled on graphene sheets to form a functionally anionic monolayer. The anionic monolayer could electrostatically adsorbed ionic liquid cations to form a self-assembled organic and inorganic hybrid film, which was used as modified material on glassy carbon electrode. The electrochemical behaviors of the as-prepared modified electrode were investigated for electrochemical determination of dopamine (DA) in the presence of uric acid (UA).
Instrumentation.-SEM images were taken by JEOL JSM-6700F. All electrochemical experiments were performed using a CHI 660C electrochemical workstation (Shanghai, China) with a conventional three-electrode system. A glassy carbon electrode (GCE, 3 mm in diameter) was used as the working electrode, while a platinum wire and a saturated calomel electrode (SCE) were used as counter and reference electrode, respectively. All the potentials reported in this work were with respect to the SCE reference.
Preparation of modified electrodes.-Before modification, glassy carbon electrode substrates were activated by polishing with successively finer grade aqueous alumina slurries (0.05 μm after 0.3 μm) on a polishing cloth. Preparation of the ionic liquid-PMo 12 /reduced graphene oxide hybrid film modified glassy carbon electrode (IL-PMo 12 /RGO/GCE) was achieved via the following immersion scheme. Graphene oxide (GO) was prepared based on the modified Hummers method. 25 A 250 mL three-necked flask was charged with a stirring bar, graphite (0.6 g), NaNO 3 (1.0 g) and 35 mL of concentrated H 2 SO 4 . The flask was placed into an ice bath, allowed remain here to reactor for 60 min. 3.0 g KMnO 4 was added into the flask with vigorous stirring, keeping the temperature lower than 20 • C. Then, 150 mL of water was slowly poured into the flask with vigorous agitation for 45 min. After the temperature of the suspension was raised to 98 • C by heating, it was poured into a beaker containing 60 • C water. Finally, 10 mL of 30% H 2 O 2 was added to the suspension. The product was washed by 5% HCl and water for several times. Graphene oxide powder was obtained by vacuum drying the filter mass for 12 h. Chemical reduction of graphene oxide was carried out according to the method mentioned in our previous work. 26 Graphene oxide powder (50 mg) was dispersed in a flask containing 20 mL water by ultrasonication for 60 min to obtain a uniform dispersion. After 0.5 mL of hydrazine monohydrate was added, the flask was heated with constant vigorous magnetic stirring to 90 • C and reacted for 24 h. The product was filtration washed by water for several times and vacuum dried for 12 h to obtain the reduced graphene oxide (RGO). As shown in Scheme 1, the first step was the preparation of graphene modified glassy carbon electrode (RGO/GCE). 1 mg of RGO was ultrasonically dispersed in 1 mL of ethanol to generate a homogeneous black suspension. 5 μL of the suspension was dropped onto a GCE and dried at room temperature to form RGO/GCE. Then RGO/GCE surface was modified with a self-assembled PMo 12

Results and Discussion
Morphology and characterization.-The morphologies of RGO and IL-PMo 12 /RGO composite were characterized by scanning electron microscopy (SEM). As shown in Fig. 1A, RGO has a typically curved, layer-like structure. Fig. 1B shows that IL-PMo 12 film uniformly coat on the surface of RGO, which attributes to the electrostatically self-assembling between PMo 12 anions and ionic liquid cations.   Fig. 2 and Fig. 3, compare with the bare GCE, RGO/GCE and IL-PMo 12 /GCE which act as the blank under exactly the same conditions.  (Figs. 2D and 2C). The RGO/GCE shows an increase for the oxidation of DA and UA (Fig. 2B), which is contributes to the outstanding electron transport capabilities of graphene. The obvious enhancement of the peak current and good separation for DA and UA oxidation are obtained at the IL-PMo 12 /RGO/GCE (Fig. 2A). These improved performances are mainly due to the self-assembled organic and inorganic hybrid structure and synergistic effects of RGO with well-defined layered structure, biocompatible PMO 12 and conductive ILs.  ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.83 Downloaded on 2018-07-21 to IP 0.30 V, corresponding to a peak current of 11.4 μA and 14.9 μA. The potential difference between the two peaks is 0.15 V, which is enough to well distinguish DA from UA.

Effect of pH on the oxidation of DA and UA.-In order to fabricate
an efficient sensor, different factors including PH value of the solution were investigated. The effect of pH value on the electrochemical response of the IL-PMo 12 /RGO/GCE toward the oxidation of DA was studied. As shown in Fig. 4, the anodic peak current of DA increased with an increase in the solution pH until it reaches 7.0 and then the anodic peak current decreased with the increase of pH value. 27 In this work, a 0.2 M PBS solution with pH 7.0 was chosen as the optimal pH value.
Effect of scan rate.-The CV curves of IL-PMo 12 /RGO/GCE in the 0.2 M PBS solution at pH 7.0 in the presence of DA recorded at various scans from 50 mV/s to 300 mV/s was shown in Fig. 5a, which shows an increasing peak current along with the scan rate. As we can see in Fig. 5b, there is a liner correlation between the cathodic current and scanning rates (v), suggesting that the kinetics of the oxidation process of DA surface-controlled process. 28 This result is well indirectly indicated that IL-PMo 12 /RGO/GCE exhibited a positive charge in aqueous solution due to BMIM cations existed on the surface of the self-assembled film. It is noteworthy that the anodic oxidation peak potential shifts slightly to more positive potentials with increasing of the scan rate. These results   Table I shows a comparison of several modified electrodes reported in literature for the determination of dopamine. The prepared modified electrode in this work exhibits the lowest detection limit and relatively wide linear range compared to other reported electrodes.

Reproducibility, stability of the IL-PMo 12 /RGO/GCE.-Under
the optimum conditions, eight electrodes were independently fabricated through the same procedures as mentioned above. The fabrication reproducibility was investigated by comparing the oxidation peak currents of DA and UA in a mixed solution. From the results, the RSDs were 2.76% and 2.58% for DA and UA, respectively. The repeatability of one electrode was also examined by successive measurements. The RSD of 1.40% showed a good reproducibility of the fabricated IL-PMo 12 /RGO/GCE. Additionally, the stability of the modified electrode was investigated by storing it at 4 • C and measuring once a day over a period of 3 weeks. Only a small decrease of the oxidation peak current was observed (the signal changes were 4.90% for DA and 4.31% for UA), which could be attributed to the excellent long-term stability of IL-PMo 12 /RGO/GCE.
Interference of coexisting substances.-The possible interferences of some common inorganic ions and other organic compounds on the determination of DA and UA were also investigated. The results in Table II shown that 1000-fold concentration of Na + , K + , NH 4 + , Ca 2+ , Mg 2+ , Cu 2+ , Zn 2+ , NO 3 − and 100-fold concentration of Fe 3+ , phenol or resorcinol had no effects on the detection signals of DA and UA (signal change ≤5%). The above results reveal the excellent anti-interference ability of our method.  analytical results were shown in Table III. The recoveries were between 99.46% and 100.50%, which revealed that the proposed modified electrode could be effectively applied to the detection of DA in real samples.

Conclusions
It has been demonstrated that the ionic liquid-phosphomolybdic acid, organic-inorganic hybrid film was fabricated on the surface of a reduced graphene oxide modified glassy carbon electrode based on electrostatically self-assemble process. The as-prepared modified electrode (IL-PMo 12 /RGO/GCE) presented good stability, high sensitivity and wide linear range for determination of DA in the existence of UA. The self-assembling on the surface of graphene sheets changed the surface property and improved uniformity of the hybrid film, resulted in good electro catalytic performance toward the oxidation of DA and UA. The application in serum samples revealed that the proposed modified electrode could be effectively applied to the detection of DA in real samples.