Electronic Supplementary Material on the Microchimica Acta publication entitled

Amperometric sensing of hydroquinone using a glassy carbon electrode modified with a composite consisting of graphene and molybdenum disulfide

Huayu Huang1,2[1], Jiangyi Zhang1,2, Meimei Cheng1,2, Kunping Liu3*, Xingyu Wang1,2

1 College of Urban and Environmental Science, NorthwestUniversity, Xi’an 710127, China

2Shaanxi Key laboratory of Earth surface System and Environmental Carrying Capacity, NorthwestUniversity, Xi’an 710127, China

3 Key Laboratory of Medicinal and Edible Plants Resources Development of Sichuan Education Department, Sichuan Industrial Institute of Antibiotics, Chengdu University, Chengdu 610052, China

Preparation of graphene

GO was prepared from natural graphite powder according to the modified Hummer’s method. The graphite powder was first oxidized by concentrated sulfuric acid and potassium permanganate at 80℃for 5 h. The resultant product was oxidized using 0.1 mol L−1 of sulfuric acid and hydrogen peroxide in an ice bath for 2 h. Finally, the mixture was sonicated for 15 min, centrifuged and washed with a 10% HCl solution to remove ions.

Graphene oxide (GO, 200 mg) was dispersed in water via ultrasonication. The pH value of the dispersed solution was adjusted to 10 with a NaOH aqueous solution (0.5 mol L−1), and the mixture was sonicated for 1 h until it became yellow-brown. Then, NaBH4 (400 mg) was added to the mixture with vigorous stirring for 6 h at 35℃. Finally, a black graphene powder was obtained by filtration and dried under vacuum.

Optimization of the electrochemical detection

(a)Working pH value

To obtain a higher sensor sensitivity, three experimental parameters were optimized, pH of the testing buffer, amount of graphene/MoS2 composite and accumulation time. Fig. S1A shows the effects of the pH from 4.0 to 10.0 on the hydroquinone oxidation peak current. As shown, the hydroquinone measurement reached the highest sensitivity when the pH value of PBS was 7.5. pH 7.5 PBS was used as the detection solution. The oxidation potential (Epa) of hydroquinone varied linearly in the range of pH values from 4.0 to 10.0 (Fig. S1B). The regression equation is Epa (V) = −0.06791pH + 0.5768 (R = 0.9921). This result demonstrates that a proton participated in the electrochemical reaction of hydroquinone. Moreover, the electron transfer in the electrochemical process was accompanied by an equal number of protons because the slope value of the equation was close to the theoretical value of −59 mV/pH.

Fig. S1 (A) DPV curves of the graphene/MoS2 GCE in 0.1 mol L−1phosphate buffer containing hydroquinone at different pH values (4.0, 5.0, 6.0, 7.0, 7.5, 8.0, 9.0, and 10.0); (B) The relationship between the oxidation potential and the pH value (n = 3).

(b)Amount of graphene/MoS2

Fig. S2 shows the variations in the peak current with the increasing amount of the graphene/MoS2 suspension. The electrochemical response of hydroquinone increased as the amount of the graphene/MoS2 suspension increased from 2 to 8 μL. The graphene/MoS2 film greatly promoted the active area on the GCE surface and reached a higher accumulation efficiency. As indicated from Fig. S2, the modification with 10−12 μL of graphene/MoS2 suspension exhibited a saturated state of graphene/MoS2 on the GCE surface. Therefore, 8 μL of the graphene/MoS2 suspension was selected to fabricate the electrochemical sensor.

Fig. S2 Effects of the graphene/MoS2suspension volumes on the oxidation peak current for 1.010−4 mol L−1 hydroquinone (n = 3).

(c)Accumulation time

The accumulation time for the oxidation peak current of hydroquinone was also studied on the graphene/MoS2 GCE. As seen from Fig. S3, the oxidation peak current initially increased the accumulation time increased and reached an equilibrium after 60 s. A longer time did not cause a significant response in the electrochemical sensor because the adsorption amount of hydroquinone on the surface of the graphene/MoS2 film tended to be a limiting value. To reduce the measurement time, 60 s was chosen for the electrochemical detection.

Fig. S3Effects of the accumulation time on the oxidation peak current for 1.010−4 mol L−1 hydroquinone (n = 3).

The following experimental conditions were found to give the best results: (a) A sample pH value of 7.5, (b) 8 μL of the graphene/MoS2 suspension (1 mg L−1), and (c) an accumulation time of 60 s.

Interferences

The catechol (Epa, 0.20 V) and resorcinol (Epa, 0.54 V) oxidation potentials were different from that of hydroquinone (Fig. S4). Three well-distinguished anodic peaks were observed. This result indicated that the oxidation of the dihydroxybenzene isomers in the mixed solution occurred independently on the graphene/MoS2 electrode.

Fig. S4 DPV curve for a mixture of hydroquinone (1.0×10−4 mol L−1), catechol (1.0×10−4 mol L−1) and resorcinol (4.0×10−4 mol L−1) on a graphene/MoS2-modified GCE.

*Corresponding authors.

Tel: 0086-29- 88308427. Email addresses: (H. Huang), (K. Liu).