Electronic Supplementary Material
Sensing of picric acid with a glassy carbon electrode modified with CuS nanoparticles deposited on nitrogen-doped reduced graphene oxide
Krishnan Giribabu1,2†, Seo Yeong Oh2†, Ranganathan Suresh3, Sivakumar Praveen Kumar1, Ramadoss Manigandan1, Settu Munusamy1, Govindhan Gnanamoorthy1, Jun Yeong Kim2, Yun Suk Huh2*, and Vengidusamy Narayanan1*
1Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai-600025, India
2Department of Biological Engineering, Biohybrid Systems Research Center (BSRC), Inha University, Incheon 402-751, Republic of Korea
3Department of Chemistry, SRM University, Ramapuram Campus, Chennai – 600 116, India
†These authors contributed equally to this work
*Corresponding authors: Email: (Y.S. Huh), Email: (V. Narayanan)
Synthesis of copper-bis(hexadecyldithiocarbamate)
In a typical synthesis, hexadecylamine (2.41 g, 0.01 mol) was added to ethanol containing potassium hydroxide (0.561g, 0.01 mol) with stirring. The mixture was cooled in an ice bath for 1 h, to this mixture carbon disulphide (0.8 mL, 0.010 mol) was added dropwise with continuous stirring for 30 min in an ice bath. The formed hexadecyldithiocarbamate was washed with chloroform, several times and dried under vacuum.
Copper complex of hexadecyldithiocarbamate was prepared as follows: Methanolic solution of the copper sulphate was added dropwise with stirring to a stoichiometric amount of potassium salt of hexadecyldithiocarbamate taken in 25 mL of methanol. Copper –bis(hexadecyldithiocarbamate) complex forms rapidly and precipitates out, which is filtered and washed with water, methanol thrice and dried in vacuum.
Graphene oxide (GO) was prepared by using Hummers and Offeman method by reacting commercially obtained graphite powder with a mixture of H2SO4, NaNO3, and KMnO4. In a typical reaction, briefly, 2 g of graphite powder was mixed with 3 g of NaNO3 and introduced to 300 mL of concentrated H2SO4, and the mixture was cooled down to 0 ºC in an ice bath. Then, 20 g KMnO4 was added slowly while stirring and maintaining the temperature below 5 ºC. The cooling bath was then removed and the suspension was brought up to room temperature. After that, 100 mL of distilled water was added and the temperature was increased to 90 ºC. The mixture was further diluted with 500 mL of double distilled water, stirred for 30 min and then treated with 50 mL of 5% H2O2, filtered and washed with deionized water until the pH was 7. The GO powder was dried under vacuum.
Synthesis of N-rGO/CuS nanohybrids
N-rGO/CuS nanohybrids were prepared by in situ microwave irradiation. To synthesize N-rGO/CuS(25/75), 2.269 g of hexadecylamine, 0.50 g of KOH, 3 mL of carbon disulphide and 1.173 g of CuSO4.5H2O were placed in 20 mL of methanol and stirred well for 2 h. The resulting product was collected by centrifugation and purified by washing with methanol several times. To this product, 0.250 g of graphene oxide suspended in 20 mL of DMF were added and heated in a conventional microwave oven at 640 W for 1 min. The synthesized N-rGO/CuS(25/75) nanohybrids were washed several times with methanol and acetone, then dried under vacuum. For N-rGO/CuS(50/50), 1.446 g of hexadecylamine, 0.40 g of KOH, 2 mL of carbon disulphide and 0.747 g of CuSO4.5H2O were placed in 20 mL methanol and stirred well for 2 h. The resulting sample was then purified by washing with methanol several times. To this mixture, 0.5 g of g GO suspended in 20 mL of DMF was added and heated in a conventional microwave oven at 640 W for 1 minute. For N-rGO/CuS(75/25), 0.723 g of hexadecylamine, 0.30 g of KOH, 2 mL of carbon disulphide and 0.373 g of CuSO4.5H2O were placed in 20 mL methanol and stirred well for 2 h. The resultant mixture was purified by washing with methanol several times. To this mixture, 0.750 g of graphene oxide suspended in 20 mL of DMF was added and heated in a conventional microwave oven at 640 W for 1 min.
Fig. S1. Raman spectrum of pure CuS nanoparticles.
Fig. S2. N 1s core level spectrum of pure CuS nanoparticles.
Fig. S3. N 1s core level spectrum of NrGO/CuS(50/50) nanohybrids.
Fig. S4. FTIR spectra of (a) CuS, (b) N-rGO/CuS(25/75), (c) N-rGO/CuS(50/50), and (d) N-rGO/CuS(75/25) nanohybrids.
Fig. S4 depicts the FTIR spectra of N-rGO/CuS nanohybrids along with pure CuS nanoparticles. Fig. S6 shows bands at 3349, 3120, 2923, 2848, 1402, 1226, 1033, 959, 682, 640, 570, 487 and 464 cm-1. The general range of 3300-3100 cm-1 is related to antisymmetrical and symmetrical N-H stretching vibrations which may be assigned for amine groups. The bands at 2920 and 2848 cm−1 are due to the stretching of C-H of hexadecyl chain present on the surface of CuS. The band at 1402 cm−1 is assigned to C-S stretching vibration. The bands located at 1033 and 1226 cm−1 are characteristic vibrations of the S2C-N and wagging mode of C-H group. The peak at 959 cm-1 corresponds to the asymmetric stretching of CS2 (C-S) groups. The band obtained at 640 cm−1 has been attributed to the stretching mode of CH2-S present in the sample. The peak at 456 cm-1 is attributed to the presence of Cu-S bond. In case of N-rGO/CuS nanohybrids, additional peaks were observed at 1714, 1617, 1576, 1469, 1358 and 1173 cm-1 in all the samples and the peaks in the region of 2920-2830 cm-1 becomes more dominant as the wt % of N-rGO increases. The bands at 1714 and 1617 cm-1 are due to the stretching of C=O group and the asymmetric vibrations of the carboxylate group of N-rGO. The peak at 1576 cm-1 is attributed to the asymmetric stretching of CH2 of (sp2) C=C bonds. The bands at 1469, 1358 and 1173 cm-1 are attributed to the presence of keto groups (i.e., quinone like structure), C-OH groups and edge phenol groups on the surface of N-rGO nanohybrids. The FTIR spectra clearly demonstrates the formation of CuS on the surface of N-rGO, and the presence C-O, C=O, C=C, C-OH groups are also detected. The Cu-S bands were observed at 453, 450, 443 cm-1 in case of N-rGO/CuS (25/75, 50/50, 75/25) nanohybrids. The slight shift towards the lower wavenumber in the region of 453 - 443 cm-1 clearly demonstrates the interactions between the CuS and N-rGO.
Fig. S5. Cyclic voltammograms of CuS/GCE in the pH of (a) 3, (b) 4, (c) 5, (d) 6, (e) 7, (f) 8, (g) 9, (h) 10 at the scan rate of 50 mVs-1.
Fig. S6. Cyclic voltammograms of CuS/GCE in presence of 1×10-4 M PA in various pH (a to h: 3 to 10) at a scan rate of 50 mVs-1. Inset figure shows the plot of pH vs Epc.
Fig. S7. Double log plot of PA at CuS/GCE (A), N-rGO/CuS (25/75) (B), N-rGO/CuS(50/50) (C) and N-rGO/CuS(75/25) (D).
Fig. S8. Plot of ln(υ) vs Epc of PA at CuS/GCE(A), N-rGO/CuS (25/75) (B), N-rGO/CuS(50/50) (C), N-rGO/CuS(75/25) (D).
Fig. S9. Plot of sensitivity towards detection of PA vs % CuS in the NrGO/CuS nanohybrid.
Table S1. Oxidation and reduction peaks of CuS at various pH
pH of Medium / Oxidation peaks(V vs SCE) / Reduction peaks
(V vs SCE)
3 / 0.24, 0.37, 0.51 / -0.30, -0.60
4 / 0.26, 0.40, 0.68 / -0.19, -0.60
5 / 0.15, 0.33, 0.70 / -0.32, -0.57
6 / 0.31, 0.64 / -0.27, -0.53
7 / 0.05 / -0.22, -0.45, -0.64
8 / 0.01 / -0.28, -0.51, -0.67
9 / 0.38, 0.65 / -0.32
10 / 0.36,0.52 / -0.31