Electronic Supporting Information (ESI):

The influence of lateral flake size in Graphene/Graphite

paste electrodes: an electroanalytical investigation

Alejandro García-Miranda Ferraria, Hadil M. Elbardisya,b, Valentine Silvaa, Tarek S. Belalc,

Wael Talaatb, Hoda G. Daabeesd, Craig E. Banksa and Dale A. C. Brownsona*

a: Faculty of Science and Engineering, Manchester Metropolitan University, Chester Street,

Manchester M1 5GD, UK.

b:Pharmaceutical Analysis Department, Faculty of Pharmacy, Damanhour University,

Damanhour, 22511, Egypt.

c: Department of Pharmaceutical Analytical Chemistry, Faculty of Pharmacy, Alexandria

University, Alexandria, 21521, Egypt.

d: Pharmaceutical Chemistry Department, Faculty of Pharmacy, Damanhour University,

Damanhour, 22511, Egypt.

*To whom correspondence should be addressed.

Email: d.brownson@mmu.ac.uk; Tel: ++(0)1612476561

Electronic Supplementary Material (ESI) for Analytical Methods.This journal is © The Royal Society of Chemistry 2020

Figure S1. (A) A schematic showing that the graphite/graphene pastes are mixed with Nujol

(60-40 % ratio respectively) before then being inserted into a polymeric-composite electrode

shell with an inner diameter of 4.5 mm. The electrode material is in contact with copper foil

as electrode connector. After polishing, the working electrode (WE) is ready to be used in

conjunction with reference (RE) and counter (CE) electrodes in a three-electrode cell

configuration. (B) Depicts the different working electrodes used in this manuscript.

(C) Shows the different graphene (AO1, AO3, AO4, AO2 and C1) and graphite (HCNG and G250)

powders used as electrode materials in this manuscript.

Figure S2. Voltammetric profiles for 72.36 μg mL-1 of COC (A) and MDMA (B) in B-R pH 9 and

PBS pH 7.4 respectively, using the range of graphene (AO1, AO4, AO2 and C1) and graphite

(HCNG and G250) paste electrodes. Scan rate: 50 mV s-1 (vs. Ag/AgCl). Solid lines represent

the largest (HCNG) and the smallest (G250) flakes used; dotted lines represent the

intermediate flake sizes (AO1, AO3, AO4, AO2 and C1). Data recorded using the AO3 electrode

has not been included due to a very high capacitive background current that does not allow

one to distinguish a redox peak.

Figure S3. Calibration plots of COC (A) and MDMA (B) in B-R pH 9 and PBS pH 7.4 respectively,

using the graphene (AO1, AO4, AO2 and C1) and graphite (HCNG and G250) paste electrodes.

Analytical sensitivities of these calibration plots are shown in Table S3. Scan rate: 50 mV s-1

(vs. Ag/AgCl). Solid lines represent the largest (HCNG) and the smallest (G250) flakes used;

dotted lines represent the intermediate flake sizes (AO1, AO3, AO4, AO2 and C1).

Data recorded using the AO3 electrode has not been included due to a very high capacitive

background current that does not allow one to distinguish a redox peak.

Table S1. Percentage Relative Standard Deviation (%RSD) values obtained at the various

electrode materials towards the detection of DA, UA, AA, NADH and MA. Values relative to

the calibration plots depicted in Figure 3. Scan rate: 50 mV s-1 (vs. Ag/AgCl) (N = 3).

Table S2. Comparison of the peak position (Ep, in V) obtained at the various electrode

materials towards the detection of 100 μg mL-1 COC (in B-R pH 9) and MDMA (in PBS pH 7.4).

Scan rate: 50 mV s-1 (vs. Ag/AgCl) (N = 3).

RSD DA

/ % RSD UA

/ % RSD AA

/ % RSD NADH

/ % RSD MA

/ %

GRAPHITE HCNG 8.0 13.0 5.3 9.1 10.2

GRAPHENE

AO1 5.5 5.5 10.2 7.3 7.5 AO3 5.3 2.5 9.9 5.5 4.3 AO4 9.2 3.7 7.3 6.8 17.3 AO2 5.7 10.8 9.0 7.5 8.9 C1 3.5 7.2 8.2 5.9 11.1

GRAPHITE G250 5.1 9.5 4.4 7.0 13.5

COC / V MDMA / V

GRAPHITE HCNG 0.925 1.096

GRAPHENE

AO1 0.922 1.099 AO3 0.960 1.096 AO4 0.926 1.101 AO2 0.908 1.153 C1 0.939 1.085

GRAPHITE G250 0.955 1.070

Table S3. Comparison of the analytical sensitivities (in A μg-1 mL) obtained at the various

electrode materials towards the detection of COC (in B-R pH 9) and MDMA (in PBS pH 7.4).

Scan rate: 50 mV s-1 (vs. Ag/AgCl) (calculated from gradient of calibration plots depicted in

Figure S3 between 11.86–72.36 μg mL-1 for COC and 5.96–72.36 μg mL-1 for MDMA) (N = 3).

Table S4. Comparison of the electrochemical limit of detection (LOD; LOD = 3*σ / slope)

obtained at the various electrode materials towards the detection of COC (in B-R pH 9) and

MDMA (in PBS pH 7.4). Scan rate: 50 mV s-1 (vs. Ag/AgCl) (calculated from gradient of

calibration plots depicted in Figure S3 between 11.86–72.36 μg mL-1 for COC and 5.96–72.36

μg mL-1 for MDMA) (N = 3).

COC / A μg-1 mL MDMA / A μg-1 mL

GRAPHITE HCNG 0.107 0.108

GRAPHENE

AO1 0.052 0.082 AO3 0.031 0.156 AO4 0.043 0.099 AO2 0.024 0.103 C1 0.046 0.109

GRAPHITE G250 0.160 0.124

COC / μg mL-1 MDMA / μg mL-1

GRAPHITE HCNG 1.04 3.12

GRAPHENE

AO1 1.45 0.28 AO3 13.02 18.65 AO4 0.25 0.84 AO2 1.63 0.27 C1 0.87 0.46

GRAPHITE G250 0.51 1.27

Figure S4. Scanning Electron Microscopy (SEM) images of the various powders utilised to

fabricate the electrodes studied herein. HCNG (A), AO1 (B), AO3 (C), AO4 (D), AO2 (E), C1 (F)

and G250 (G). Note that these are representative images obtained from ‘batch

characterisation’ of the samples that have been reported previously in Ref. [54].

Figure S5. Raman spectroscopy characterisation of the various graphitic powders: HCNG (A);

AO1 (B); AO3 (C); AO4 (D); AO2 (E); C1 (F); and G250 (G). Note that these are representative

spectra obtained from ‘batch characterisation’ of the samples and have been reported

previously in Ref. [54].