Electronic Supporting Material

(ESM)

Simultaneous voltammetric determination of hydroquinone and catechol by using a glassy carbon electrode modified with carboxy-functionalized carbon nanotubes in a chitosan matrix and decorated with gold nanoparticles

Yu Shen1, 2, Dejiang Rao1, Qinglin Sheng1* and Jianbin Zheng1*

1 Institute of Analytical Science, Shaanxi Provincial Key Laboratory of Electroanalytical Chemistry, Northwest University, Xi’an, Shaanxi 710069, China.

2 Xi’ an Northwest Geological Institute for Nonferrous Metals Co. Ltd, Xi’ an, Shaanxi 710054, China

*E-mail address: [email protected] (QL Sheng); [email protected] (JB Zheng)

Experimental

The specified experimental parameters used for conducting EIS, DPV and CV experiments Cyclic voltammetry (CV) studies were done in the potential range from -0.12 to +0.3 V, at a scan rate of 100 mV s-1 unless otherwise stated. Differential pulse voltammograms (DPV) were recorded between -0.12 and +0.3 V for different concentrations of HQ and CC solutions prepared in 0.1 mol L -1 PBS (pH 7.0), under optimized instrumental parameters (pulse amplitude 50 mV, sampling width 17 ms, pulse width 200 ms, pulse period 500 ms). The electrochemical impedance spectroscopy (EIS)

-1 4-/3- measurements were performed in 0.1 mol L PBS (pH 7.0) containing 5.0 mM [Fe(CN)6] at open- circuit potential conditions with the AC amplitude of 5.0 mV. They were recorded with a frequency range from 105 to 10-2 Hz.

Synthesis of c-MWCNTs On the basis of previous study, the c-MWCNTs were prepared by

16 ultrasound in a mixture of concentrated H2SO4 and HNO3 (3:1, v/v ratio). Briefly, 50 mg of pristine c-MWCNTs were sonicated in the above mixed acid (100 mL) for 4 h. After that, the black c- MWCNTs were collected by centrifugation and washed with deionized water to neutral pH. At last, the resulting products were dried in air.

Fig. S1 SEM images of c-MWCNTs (A), c-MWCNTs/CTS composites (B) and c-MWCNTs/CTS/Au nanocomposites (C, D).

Fig. S1 shows the SEM images of c-MWCNTs (A), c-MWCNTs/CTS composites (B) and c- MWCNTs/CTS/Au nanocomposites (C, D). As shown in Fig. S1A and B, the various-length c- MWCNTs with different orientation and the c-MWCNTs/CTS composites with compact network structure were clearly observed. It can be explicitly observed that many bright AuNPs were deposited on the surface of the c-MWCNTs/CTS composites (Fig. S1C). By further observation, in Fig. S1D it can be seen that the prepared AuNPs were uniformly anchored on the surface of compact network structure and without obvious aggregation.

60 y t i s n

e 40 t

n I 20

10 100 Diameter/nm

Fig. S2 The particle size distribution measured by DLS.

Fig. S2 is the Nanoparticles size deduced from DLS. The synthetized nanoparticles are near- spherical with a diameter of about 12 ± 5 nm which is in agreement with the size measured by TEM.

Fig. S3 TEM images of c-MWCNTs/CTS/Au nanocomposites .

It can be observed from Fig. S3 that a distribution of the synthesized AuNPs on the c- MWCNTs/CTS composites, suggesting c-MWCNTs/CTS/Au nanocomposites were successfully synthesized by in-situ reduction.

75 (A) Epa(HQ) (B) I (HQ) 0.3 pa Epa(CT) I (CT) 60 pa 0.2 A V /  a / p a 45 p

E 0.1 I

30 0.0

15 5 6 7 8 9 5 6 7 8 9 pH pH

A 45  / a

p I 30

0.2 0.3 0.4 0.5 0.6 V /mL HAuCl 4

−1 Fig. S4 Effect of pH value of 0.1 mol L PBS (pH: 5.0, 6.0, 7.0, 8.0 and 9.0) on the Epa and Ipa of c- MWCNTs/CTS/Au/GCE in the presence of 0.2 mM HQ and 0.2 mM CT (A, B); the highest current response (Ipa) values of the c-MWCNTs/CTS/Au/GCE with different volumes of HAuCl4 solution

(VHAuCl4: 0.2, 0.3, 0.4, 0.5 and 0.6 mL) in the presence of 0.2 mM HQ and 0.2 mM CT (C).

Fig. S4A and B illustrate the effect of pH value of 0.1 mol L−1 PBS (pH: 5.0, 6.0, 7.0, 8.0 and 9.0) on the Epa and Ipa of c-MWCNTs/CTS/Au/GCE in the presence of 0.2 mM HQ and 0.2 mM CT. As

−1 shown in Fig. S4A, the Epa shifted negatively and varied linearly with the pH value of 0.1 mol L PBS in the range from 5.0 to 9.0. The linear relation equation between Epa and pH value were evaluated to

2 2 be Epa (V) = 0.48 – 0.058 pH (r = 0.9991) for HQ and Epa (V) = 0.60 – 0.061 pH (r = 0.9995) for CT. According to the following Nernst equation: dEp/dpH = 2.303 mRT/nF (1)

Here, m is the number of proton, n is the number of electron, R is the gas constant, F is the Faraday constant and T is the temperature in Kelvin. In this study, m/n was calculated to be 1.00 for HQ and 1.01 for CT in their electrocatalytic oxidation reaction, and T is equal to 298.15 K. Therefore, the slopes of the two linear equations we obtained are approximately equal to the theoretical value of 0.059 V/pH, indicating two protons and electrons involved in the electrocatalytic process.Fig. S4B

−1 illustrate the effect of pH value of 0.1 mol L PBS (pH: 5.0, 6.0, 7.0, 8.0 and 9.0) on the Ipa of c- MWCNTs/CTS/Au/GCE in the presence of 0.2 mM HQ and 0.2 mM CT. From Fig. S4B we can know that the obvious responses were observed on Ipa of c-MWCNTs/CTS/Au/GCE in the presence of 0.2 mM HQ and 0.2 mM CT. However, it can be observed that the responses to the electrocatalytic oxidation of HQ and CT were the highest at the pH value of 7.0. Consequently, the 0.1 mol L−1 PBS (pH 7.0) was used for the next electrochemical study.

As the AuNPs can effectively improve the electron transfer rate in this electrocatalytic process, the effect of the volumes of HAuCl4 solution was also deeply studied. Fig. S4C shows the highest current response (Ipa) values of the c-MWCNTs/CTS/Au/GCE with different volumes of HAuCl4 solution (VHAuCl4: 0.2, 0.3, 0.4, 0.5 and 0.6 mL) in the presence of 0.2 mM HQ and 0.2 mM CT. The c- MWCNTs/CTS/Au nanocomposites with different amounts of AuNPs were synthesized, and then constructed the c-MWCNTs/CTS/Au/GCE to detect HQ and CT (each 0.2 mM) by CV, respectively. It is obvious that the current response to the electrocatalytic oxidation of HQ and CT were the highest at the VHAuCl4 of 0.4 mL. This may be attributed to the excessive AuNPs have been aggregated, and the electrocatalytic performance has been constrained significantly. Thus, the VHAuCl4 of 0.4 mL was used in this experiment. According to previous literatures, the redox process of HQ and CT at a surface modified electrode is a reversible process via the involvement of two electrons and protons. The probable electrocatalytic mechanism of HQ and CT at c-MWCNTs/CTS/Au/GCE is shown in Scheme S1. Scheme S1. Electrocatalytic mechanism of HQ and CT at c-MWCNTs/CTS/Au/GCE.

64 (A) 0.24 I (CT) 56 pa 0.16 48 A

E (CT) V  pa / / a p a 0.08 p

I 40 Epa(HQ) 32 0.00 Ipa(HQ) 24 -0.08 1 2 3 4 5 Number of electrodes

Fig. S5 The reproducibility of DPV responses for HQ and CT (each 0.2 mM) in 0.1 mol L−1 PBS (pH 7.0) at c-MWCNTs/CTS/Au/GCE (A); The stability of the c-MWCNTs/CTS/Au/GCE of DPV responses for HQ and CT (each 0.2 mM) in 0.1 mol L−1 PBS (pH 7.0) in a period of three weeks (B).

The reproducibility and stability of this c-MWCNTs/CTS/Au/GCE were carefully evaluated by DPV. Fig. S5 shows the reproducibility (A) and stability (B) of DPV responses for HQ and CT (each 0.2 mM) in 0.1 mol L−1 PBS (pH 7.0) at c-MWCNTs/CTS/Au/GCE. The reproducibility of five c-

MWCNTs/CTS/Au/GCEs was evaluated by comparing the Ipa of HQ and CT (each 0.2 mM). The experimental results are shown in Fig. S5A. As demonstrated in Fig. 10A, the Epa of HQ and CT were not shifted obviously. Moreover, the relative standard deviation (RSD) of this reproducibility experiment is 3.21% for HQ and 4.13% for CT, demonstrating a good reproducibility of the c- MWCNTs/CTS/Au/GCE. The experimental results of stability study are illustrated in Fig. S5B. We recorded the DPV responses (measure once a week) for HQ and CT (each 0.2 mM) in 0.1 mol L−1 PBS

(pH 7.0) at c-MWCNTs/CTS/Au/GCE in a period of three weeks. The Ipa remained 89.3% for HQ and 90.5% for CT of its initial DPV responses after storage in a fridge for three weeks, indicating a good stability of the c-MWCNTs/CTS/Au/GCE.