ELECTROCATALYTIC DETECTION OF DRUGS 

OF ABUSE USING ONION-LIKE CARBON 

BASED ELECTROCATALYSTS 

  

 

Submitted by 

Tobechukwu Justice Ehirim (1569304) 

 

Supervisor:  Prof. Kenneth I. Ozoemena 

“A thesis submitted to the Faculty of Science, University of Witwatersrand 

Johannesburg in fulfilment for the degree of 

Doctor of Philosophy in Chemistry” 

Johannesburg 



November 2022 



   i 
 

DECLARATION 

I solemnly declared that this thesis is self-reliant and independently studied. It is 

being submitted to the School of Chemistry, Faculty of Science, University of the 

Witwatersrand, Johannesburg for the award of the degree of Doctor of Philosophy. It 

has not been submitted previously to any other institution of higher learning for 

consideration of any degree. 

 

 

 

Tobechukwu Justice Ehirim                                          24 = 02 - 2023 
 

                Name                                                                            Date 

  

                                                                                                                                                                       

              Signature 

 

 

 

 

 

 



   ii 
 

DEDICATION 

This Ph.D. work is dedicated to my late father, Chief. Livinus Uzoma Nlewedimanya 

Ehirim (Uzoudo 1 of Amazano) who taught me never to get tired of developing 

myself through life-long acquisition of knowledge, and my mother, Lolo. Dorathy 

Adaeze Ehirim (Ugo-si-Mba) who offered herself as a reminder and launching pad 

unto this Academic work long after the passing of my father, and specially dedicated 

to Jehovah Chineke Bu Eze, God Almighty who grants me the grace to so do. 

 

 

 

 

 

 

 

 

 

 

 

 



   iii 
 

ACKNOWLEDGEMENTS 

My deepest gratitude and indebtedness go to my supervisor, Professor Kenneth 

Ikechukwu Ozoemena, a world-renowned chemist and researcher, for the 

opportunity given me, his dedicated guidance, encouragement and support that 

made this accomplishment possible. A special and heartfelt appreciation to my 

beautifully brilliant and wonderfully made wife, Lolo. Vivienne Ogechi T.J. - Ehirim, 

my academically supportive children, Ubasinachi J.TJ-Ehirim, Chimzikora Mikayla 

TJ-Ehirim, T. Jehovah Bu Eze TJ-Ehirim, Chiamaka Rhianda TJ-Ehirim and Chidera 

E. TJ-Ehirim. Not forgetting my siblings and relatives who has been serious sources 

of encouragement and motivation, Chika Blaise Ehirim, Igwebuike Bede Ehirim, 

Uche Barbara Ehirim-Umunnakwe, Chidera Pat, Ehirim-Uka and Chinedu Ashinobi, 

Uchenna Chukwumezie, Ifeanyi Orji, Patrick Obi, Dr. Rita Ozoemena, Uche 

Olebuike, amongst others.  

I wish to specially acknowledge and thank my co-researcher, and co-author, 

Okoroike Celestine Ozoemena who put in so much dedication to enhance the 

achievement of this feat through his academic collaborations. My gratitude also goes 

to my colleagues and co-researchers at the Ozoemena research group, at The 

University of the Witswatersrand, Dr. Aderemi Haruna, Dr.Thapelo Mofokeng, Dr. 

Patrick Mwonga, Dr. Adewale Ipadeola, Siwaphiwe Pheteni, Kelechi Lebechi, 

Refilwe Modise, Lesego Goalahle, Ms. Itumeleng Mokhosi, Thokozane Tsoari, 

Sebenzile Shabalala, Seremo Podile, Dr. Tobile Khawula, Bokome Shaku, Dr. Victor 

Mashindi, Coolani Fakude, Tebogo Tsekedi, Funanani Tshivase, also not forgetting 

Prof. Careen Billing of the Wits School of Chemistry for her support through the use 

of her laboratory, and a host of others.     

Finally, I want to acknowledge my colleagues at work and my Pharmacy Profession 

who have in no little way supported me through this academic journey amongst 

whom are S.Choma, D,Bayerver, R.Lotter, N.P. Mthetwa, C,Hadebe, S. Dikgang, L. 

Maswabi, L.V. Khambule, R. M. Mashile, A. Paruk, T. Tshabalala, N. H. Ndlovu, F. 

Asmal, T. Shabangu, H. Pabhoo,  D.E. Dlamini, V. Adoons, T. Ncube, amongst 

others. 

 



   iv 
 

 

RESEARCH OUTPUTS 

 LIST OF PUBLICATIONS 

1) Tobechukwu J. Ehirim, Okoroike C. Ozoemena, Patrick V. Mwonga, Aderemi B. 
Haruna, Thapelo P. Mofokeng, Karolien De Wael, and Kenneth I. Ozoemena, 
Onion-Like Carbons Provide a Favorable Electrocatalytic Platform for Sensitive 
Detection of Tramadol Drug, ACS Omega 7, 51 (2022) 47892-47905. 
doi.org/10.1021/acsomega.2c05722 
 

2) Okoroike C. Ozoemena, Nsovo S. Mathebula, Tobechukwu J. Ehirim, Tobile 
Maphumulo, Goodness M. Valikpe, Jerry L. Shai, Kenneth I. Ozoemena. Onion-
like carbon re-inforced electrospun polyacrylonitrile fibres for ultrasensitive 
electrochemical immunosensing of Vibrio cholerae toxin. Electrochimica Acta 356 
(2020) 136816. doi.org/10.1016/j.electacta.2020.136816 
 

3) Okoroike C. Ozoemena· Tobechukwu J. Ehirim· Tobile Khawula· Katlego 
Makgopa· Leshweni J. Shai· Kenneth I. Ozoemena. Bovine Serum   

Albumin‑Dependent Charge‑Transfer Kinetics Controls the Electrochemical 
Immunosensitive Detection: Vibrio cholerae as a Model Bioanalyte, 
Electrocatalysis 12 (2021) 595-604. doi.org/10.1007/s12678-021-00673-8. 

 
4) Tobechukwu J. Ehirim, Okoroike C. Ozoemena, Adewale K, Ipadeola, Patrick 

V. Mwonga, Aderemi B. Haruna, Thapelo P. Mofokeng, Karolien De Wael, and 
Kenneth I. Ozoemena, Utilizing the unique electronic interactions between Pd 
and CeO2@onion carbon for enhanced electrocatalytic oxidation of ethanol and 
sensing, Electrochimica Acta (under review). 

 

PODIUM PRESENTATIONS AT INTERNATIONAL CONFERENCES 

5) Tobechukwu J. Ehirim, Siwaphiwe Pheteni, Adewale Ipadeola, Kenneth I. 
Ozoemena. Electrochemical detection of Tramadol at onion-like carbon modified 
electrode. 70th Annual Meeting of the International Society of Electrochemistry. 
Held August 4 – 9, 2019 at the International Conference Centre, Durban, South 
Africa. 
 

6) Tobechukwu J. Ehirim, Okoroike C. Ozoemena, Adewale Ipadeola, Kenneth I. 
Ozoemena. Electrochemical detection of drugs of abuse. 4th African 
Nanotechnology Conference/ Workshop. Held 19th – 23rd of July at University of 
Nigeria, Nsukka (UNN), Enugu State, Nigeria. 

  

 

https://doi.org/10.1021/acsomega.2c05722
https://doi.org/10.1016/j.electacta.2020.136816


   v 
 

 

MEMBERSHIP OF PROFESSIONAL BODIES AND ASSOCIATIONS 

1. The South African Pharmacy Council (SAPC). 



   vi 
 

ABSTRACT 

Substance abuse is a serious problem worldwide. Among abused substances, 

tramadol and alcohol are one of many. There is an urgent need to use 

electrochemical method for their detection since electrochemistry methods are 

simple, low-cost, high sensitivity and can easily be miniaturised. This PhD work 

reports the first investigation on the application of nanodiamond-derived onion-like 

carbons (OLC) and conductive carbon black (CB) as (i) electrocatalysts for the 

detection of tramadol, and (ii) as support materials for nano-sized palladium 

electrocatalysts (Pd/OLC, Pd/CB, Pd-CeO2/OLC) for the detection of ethanol. The 

catalysts were characterised with X-ray diffraction (XRD), Raman, Brunauer–

Emmett–Teller (BET), Thermal gravimetric analysis (TGA), X-ray photoelectron 

spectroscopy (XPS), Scanning electron microscopy (SEM) and Transmission 

electron microscopy (TEM).  

For the detection of tramadol, OLC gave the best sensing performance compared to 

CB. Theoretical calculations (DFT simulations) predict that OLC is better because it 

allows for weaker interaction energy with tramadol (Ead = -26.656 eV) than CB (Ead = 

-40.174 eV). OLC-modified glassy carbon electrode (GCE-OLC) shows a wide linear 

calibration curve (55 – 392 M), high sensitivity (0.0315 A /M), low limit of 

detection (LoD) and quantification (LoQ) of 3.8 and 12.7 M, respectively. OLC-

modified screen-printed electrode (SPE-OLC) successfully detected tramadol in real 

tramadol drug and human serum. The OLC-based electrochemical sensor promises 

to be useful for sensitive and accurate detection of tramadol in clinics, quality control 

and routine quantification of tramadol in pharmaceutical formulations. 



   vii 
 

For the oxidation and detection of ethanol, Pd/CB, Pd/OLC and Pd-CeO2/OLC, were 

studied as catalysts. In comparison, adding ceria (CeO2) to Pd/OLC, the performance 

was enhanced significantly than in carbon-only support for palladium. GCE/Pd-

CeO2/OLC shows the best electrocatalytic performance (i.e., high current density, 

fast electron transport, etc). DFT calculation, supported by XPS and HRTEM data, 

predict that this high activity may be related to CeO2 modulating the electronic 

properties of the catalyst. GCE/Pd-CeO2/OLC gave wide linear range for ethanol 

sensing (38.5 – 286 mM), excellent sensitivity (0.00024 mA mM-1) and LoD of ~ 8.7 

mM. The GCE/Pd-CeO2/OLC shows excellent potential for application in real 

samples of commercial alcoholic beverages and human serum, with satisfactory 

recoveries (89 – 108 %).  

 

 

 

 

 

 

 

 

 

 



   viii 
 

 



ix 
 

TABLE OF CONTENTS 

 

DECLARATION ....................................................................................................... i 

DEDICATION ........................................................................................................... ii 

ACKNOWLEDGEMENTS ....................................................................................... iii 

RESEARCH OUTPUTS .......................................................................................... iv 

ABSTRACT ............................................................................................................. vi 

TABLE OF CONTENTS .......................................................................................... ix 

LIST OF FIGURES ................................................................................................ xv 

LIST OF TABLES ................................................................................................... xx 

LIST OF SYMBOLS .............................................................................................. xxi 

LIST OF ABBREVIATIONS.................................................................................. xxii 

CHAPTER 1 ............................................................................................................ 1 

INTRODUCTION .................................................................................................... 1 

1.1 Research Problems .......................................................................................... 1 

1.2 Rationale or Justifications ................................................................................. 2 

1.3 Research Aim and Objectives ........................................................................... 3 

1.3.1 Aim of the study ...................................................................................... 3 

1.3.2 Objectives of Study ................................................................................. 4 

1.4 Outline of the thesis .......................................................................................... 4 

1.5 References ........................................................................................................ 6 



x 
 

CHAPTER 2 ............................................................................................................ 8 

LITERATURE REVIEW........................................................................................... 8 

2.1 Rationale for the detection of tramadol and alcohol .......................................... 8 

2.2 Brief Overview of Current Detection Methods of Ethanol: Pros and Cons ...... 13 

2.2.1 UV-Visible spectroscopy ....................................................................... 13 

2.2.3 Mass Spectroscopy .............................................................................. 14 

2.2.4 High Performance liquid chromatography (HPLC) ................................ 14 

2.2.5 Electrochemical detection (Advantages & Disadvantages). .............. 15 

2.2.6 Electrode materials ............................................................................ 16 

2.2.6.1 Onion-Like Carbon (OLC)  ........................................ ……… .16 

2.2.6.2 Palladium Carbon on Carbon and Carbon-Ceria Supports as 

Catalysts ............................................................................... ……… 18 

2.2.7 Tramadol and Ethanol .......................................................................... 19 

2.2.7.1 Tramadol .............................................................................  19 

2.2.7.2 Ethanol ................................................................................  19 

2.2.8 Ferri-Ferrocyanide Solution. ................................................................. 20 

2.2.9 Amstel Beer (4.0 % Alc)........................................................................ 20 

2.2.10 Nederberg Wine (13.0 % Alc) ............................................................. 20 

2.2.11 Interferents ......................................................................................... 20 

2.3 References ................................................................................................. 22 

CHAPTER 3 .......................................................................................................... 27 

MATERIALS AND METHODS .............................................................................. 27 



xi 
 

3.1 Material Characterization ................................................................................ 28 

3.1.1 Powder X-ray Diffraction (PXRD) ......................................................... 28 

3.1.2 Raman Spectroscopy ........................................................................... 28 

3.1.3 Scanning electron microscopy (SEM) ................................................... 29 

3.1.4 Energy-dispersive X-ray spectroscopy (EDX) ....................................... 29 

3.1.5 Transmission electron microscopy (TEM)  ............................................ 29 

3.1.6 Thermogravimetric analysis (TGA) ....................................................... 30 

3.1.7 Brunauer-Emmett-Teller (BET) surface area analysis .......................... 30 

3.1.8 X-ray Photoelectron Spectroscopy (XPS) ............................................. 30 

3.2 Electrochemical Characterization Techniques ................................................ 31 

3.2.1 Cyclic voltammetry ............................................................................ 31 

3.2.2 Square wave voltammetry ................................................................. 31 

3.2.3 Differential pulse voltammetry (DPV) ................................................ 32 

3.2.4 Electrochemical impedance spectroscopy ........................................ 32 

3.2.5 Chronoamperometry ......................................................................... 32 

3.3 Evaluation of Electrochemical Sensor‟s Figures of Merit. ............................... 33 

3.3.1 Sensitivity ............................................................................................. 33 

3.3.2 Limit of detection .................................................................................. 33 

3.3.3 Limit of quantification ............................................................................ 34 

3.3.4 Specificity / Selectivity .......................................................................... 34 

3.3.5 Repeatability / Stability ......................................................................... 34 



xii 
 

3.4 References ...................................................................................................... 35 

CHAPTER 4 .......................................................................................................... 37 

4.1 Introduction ..................................................................................................... 38 

4.2 Experimental section ....................................................................................... 40 

4.2.1 Materials, reagents and methods ......................................................... 40 

4.2.2 Physical property characterization ........................................................ 41 

4.2.3 Electrochemical detection procedure .................................................... 41 

4.2.4 Density functional theory (DFT) calculations ........................................ 43 

4.3 Results and discussion ................................................................................... 44 

4.3.1 Materials characterization ..................................................................... 44 

4.3.2 Cyclic voltammetry: mass transport and charge-transfer kinetics ......... 47 

4.3.3 Electrochemical Impedance Spectroscopy: Charge-Transfer Kinetics . 52 

4.3.4 Electrocatalytic detection of tramadol ................................................... 55 

4.3.5 Predicting the High Electrocatalytic Activity of OLC Over CB Towards 

the Detection of Tramadol: DFT Calculations… ............................................ 58 

4.3.6 Analytical application of OLC-based electrochemical sensors .............. 61 

4.3.6.1 Determination of raw tramadol in PBS (pH 7.40……………....61 

4.3.6.2 Determination of tramadol in pharmaceutical sample and 

human serum………. ........................………………………………62 

4.4 Conclusion ...................................................................................................... 66 

4.5 References ...................................................................................................... 67 

CHAPTER 5 .......................................................................................................... 77 



xiii 
 

5.1 Introduction ..................................................................................................... 78 

5.2 Experimental ................................................................................................... 80 

5.2.1 Materials and reagents ......................................................................... 80 

5.2.2 Synthesis procedures ........................................................................... 81 

5.2.2.1 Synthesis of Pd/carbon. .................................................. …. 81 

5.2.2.2 Synthesis of OLC-CeO2 support .........................................  81 

5.2.2.3 Synthesis of Pd-CeO2/OLC .................................................  82 

5.2.3 Physical property characterization ........................................................ 82 

5.2.4 Density functional theory (DFT) calculations ........................................ 83 

5.2.5 Electrochemical and physical characterization ..................................... 84 

5.3 Results and Discussion ................................................................................... 85 

5.3.1 Materials characterization ..................................................................... 85 

5.3.2 Cyclic voltammetry and EIS: Charge-transfer kinetics .......................... 92 

5.3.3 Electrocatalytic Oxidation and Detection of Ethanol in Alkaline Medium

 ...................................................................................................................... 96 

5.3.4 Reasons for the high performance of the Pd-CeO2/OLC electrocatalyst 

towards ethanol oxidation reaction .............................................................. 100 

5.3.5 Analytical application of Pd-CeO2/OLC-based electrochemical sensor

 .................................................................................................................... 106 

5.3.5.1 Alcohol detection in alkaline medium………………………….106 

5.3.5.2 Determination of ethanol in real alcoholic samples…………107 

5.4 Conclusion .................................................................................................... 108 



xiv 
 

5.5 References .................................................................................................... 109 

CHAPTER 6 ........................................................................................................ 120 

6.1 Electrocatalytic Sensing of Tramadol ............................................................ 120 

6.2 Electrocatalytic Sensing of Alcohol ............................................................... 121 

APPENDIX .......................................................................................................... 123 

 



xv 
 

LIST OF FIGURES 

 
Figure 2.1 Isomeric structures of 

tramadol……………………………………………………………………………………..12 

Figure 2.2 Chemical Structure of 

Ethanol………………………………………………………………………………………13 

Figure 2.3 Pictures depicting faces of alcohol and drug abuse in the society 

(Courtesy: Bing images, assessed 15 March 

2022)……..…………………………………………………………………………………13 

Figure 2.4 (A) HRTEM images of onion-like carbon (OLC) and (B) its structural 

representation. (Ugarte,1995)…………………………………………………………….18 

Figure 4.1 Isomeric structures of commercial tramadol 

hydrochloride………………………………………………………………………………41 

Figure 4.2 Electrode modification for tramadol electrocatalytic oxidation and sensing. 

The images are modified from the photographs obtained from electrode suppliers: 

(A) courtesy of BASi, copyright 2022; and (B) courtesy of Metrohm Dropsens, 

copyright 

2022).………………………………………………….…………………………………….45 

Figure 4.3 SEM images of (A) CB and (B) OLC, and (C) HRTEM image of OLC….46 

Figure 4.4 Comparing powder XRD (A), Raman spectra (B), BET adsorption 

desorption curve, and (inset) pore size distribution curves (C), and TGA and their 

corresponding derivative TGA (D) of OLC and 

CB……………………………………………………………………………………………47 



xvi 
 

Figure 4.5 Cyclic voltammograms of the electrode at (A) 10 mV/s ansd (B) 150 

mV/s; CV at different scan rates (10 – 150 mV/s) for the (C) OLC and (E) CB, and 

their corresponding plots of peak currents versus square root of scan rate (D) and 

(F), respectively. All data were collected in a redox probe ([Fe(CN)6]
4−/[Fe(CN)6]

3− in 

0.1 M 

KCl)………………………………………………………………………………………….50 

Figure 4.6 (A) Nyquist plots and (B-D) Bode plots of the GCE, GCE-OLC and GCE-

CB. Data points are experimental, while lines are fitted data using the electrical 

equivalent circuit (inset in Figure 6A). All data were collected in redox probe 

([Fe(CN)6]4−/[Fe(CN)6]3−) in 0.1 M 

KCl…………………………………………………………………………………………..55 

Figure 4.7 (A) CV evolutions of the GCE, GCE-CB and GCE-OLC in PBS (pH 7.4) 

containing 0.5 mM tramadol at 20 mV/s, (B) Comparing the CVs of GCE-OLC in PBS 

alone and PBS containing 0.5 mM tramadol at 20mV/s, (C) CV evolution showing the 

effect of changing scan rates on the current responses, (D) plot of peak potential 

versus log scan rate. All data acquired in PBS containing 0.5 mM tramadol………..58 

Figure 4.8 Adsorption sites of tramadol at the surfaces of CB and OLC. The dark 

blue dots (shades) show the possible site tramadol molecules are adsorbed. Inset is 

used to determine the isosurface potentials of tramadol @+/- 0.05181 isovalue(a.u.), 

for tramadol 

molecule…………………………………………………………………………………….61 

Figure 4.9 Band structures and PDOS for tramadol adsorbed on (A,B) CB and  

(C,D) OLC 

electrodes…………………………………………………………………………………..63 



xvii 
 

Figure 4.10 (A) Typical square wave voltammograms obtained in PBS (pH 7.4) 

containing different concentrations of tramadol (0 – 392 M), and (B) plot of peak 

current response versus concentration of 

tramadol…………………………………………………………………………………….64 

Figure 4.11 Typical SWV curves (A, C) obtained in real sample analysis using 

screen-printed carbon electrode (SPCE) modified with OLC (SPCE-OLC) for 

tramadol in drug capsule (A) and in human serum solution (C); and the plot of 

background-subtracted peak current responses versus the concentrations of 

tramadol (B,D)……………………………………………………………………………..66 

Figure 5.1 TGA of CB and OLC and their derivative 

TGA………………………………………………………………………………………….87 

Figure 5.2 Raman spectra (A-C) and XRD patterns (D-F) of the 

electrocatalysts…………………………………………………………………………….88 

Figure 5.3 Typical low-resolution HRTEM images of (A) Pd/C, (B) Pd/OLC and (C) 

Pd-CeO2/OLC, and typical HRTEM images of (D) Pd/CB; (E) Pd/OLC and (F) Pd-

CeO2/OLC. The particle size distribution of (G) PdCB, (H) Pd/OLC and (I) Pd-

CeO2/OLC…………………………………………………………………………………..90 

Figure 5.4 XPS data showing the wide spectra of (A) Pd/OLC and (B) Pd/CeO2-

OLC; C 1s scans of (C) Pd/OLC and (D) Pd-CeO2/OLC; O 1s scans of (E) Pd/OLC 

and (F) Pd-CeO2/OLC; Pd 3d scans of (G) Pd/OLC and (H) Pd-CeO2/OLC; and Ce 

3d scan of (I) of Pd-

CeO2/OLC………………………………………………………………………...91 



xviii 
 

Figure 5.5 XPS data for the core Pd 3d scans of (A) Pd/CB, (B) Pd/OLC, (H) Pd-

CeO2/OLC; and Ce 3d scan of (D) Pd-

CeO2/OLC………………………………………………………………………………….93 

Figure 5.6 Typical comparative (A) cyclic voltammograms at 100 mVs-1, and (B) 

Nyquist plots of the electrocatalysts immobilized onto GCE surface obtained at 0.22 

V (vs Ag|AgCl, 3 M KCl). All data were collected in a redox probe 

([Fe(CN)6]
4−/[Fe(CN)6]

3− in 0.1 M 

KCl)…………………………………………………………………………………………94 

Figure 5.7 Cyclic voltammograms of the electrode at (A) 10 mV/s ansd (B) 150 

mV/s; CV at different scan rates (10 – 150 mV/s) for the (C) OLC and (E) CB, and 

their corresponding plots of peak currents versus square root of scan rate (D) and 

(F), respectively. All data were collected in a redox probe ([Fe(CN)6]
4−/[Fe(CN)6]

3− in 

0.1 M 

KCl)………………………………………………………………………………………….96 

Figure 5.8 Cyclic voltammograms (A) 1.0 M KOH alone and (B) mixture of 1.0 M 

KOH and 1.0 M ethanol of the Pd-based 

electrocatalysts……………………………………………………………………………97 

Figure 5.9 (A-C) Scan rate studies and (D) plots of the current against the square 

root of scan rates of the electrocatalysts toward ethanol oxidation 

reaction…………………………………………………………………………………….99 

Figure 5.10 Acetyl group adsorption sites at the surfaces of (A) Pd/CB, (B) Pd/OLC 

and (C) Pd-CeO2/OLC. The dark blue dots (shades) show the possible site ethanol 

molecules are 

adsorbed…………………………………………………………………………………103 



xix 
 

Figure 5.11 HRTEM showing the interfacial interaction between Pd and CeO2 in Pd-

CeO2/OLC, and the valence band spectra of the (B) Pd/CB (C) Pd-CeO2/CB and (D) 

Pd-CeO2/OLC catalysts………………………………………………………………….106 

Figure 5.121 (A) Typical chronoamperometric curves obtained in 1 M KOH 

containing different concentrations of ethanol (0 – 666.7 mM), and (B) plot of peak 

current response versus concentration of 

ethanol……………………………………………........................................................108 

 

 

 

 

 

 

 

 

 

 

 

 

 



xx 
 

LIST OF TABLES 

Table 4.1 Surface parameters CB and OLC materials………………………………. 48 

Table 4.1 CV parameters of the OLC- and CB-based electrodes obtained in a 

solution of redox probe..…………………………………………………………  51 

Table 4.2 EIS parameters for the GCE, GCE-CB and GCE-

OLCs……………………………………………………………………………………….56 

Table 3.4 Predicted energy values from the DFT calculations accompanying the 

interaction of tramadol with the surfaces of OLC and 

CB…………………………………………………………………………………………..61 

Table 4.4 Summary of results obtained in this work for electrochemical sensing of 

tramadol, using the DPV method…………………………………………………………68 

Table 5.1 BET surface areas and porosity of the 

electrocatalysts……………………………………………………………………………..90 

Table 5.2 CV parameters of the OLC- and CB-based electrodes obtained in a 

solution of redox 

probe………………………………………………………………………………………...94 

Table 5.3 Predicted energy values from the DFT calculations accompanying the 

interaction of tramadol with the surfaces of Pd/CB, Pd/OLC and Pd-

CeO2/OLC…………………………………………………………………………………105 

Table 5.4 Summary of results obtained in this work for electrochemical sensing of 

tramadol, using the DPV 

method……………………………………………………………………………………..110 

 

 

 

  



xxi 
 

LIST OF SYMBOLS 

 

 

 

 

 

 

Α Electron transfer coefficient   

Αa Anodic electron transfer coefficients  

Αc Cathodic electron transfer coefficients 

ʋ  Scan rates  

Δ Standard deviation 

Λ Wavelength 

∆E Peak potential separation 

C Analytes bulk concentration (mol/L) 

℃ 
Degree Celsius 

D Diffusion coefficient of analytes (cm2/s) 

E1/2                              Half – wave potential 

Ω                          Ohms  

µm                            Micrometre 

Z'  Real impedance                                             

Z"  Imaginary impedance 

Ma mill Ampere  

G Negative change in the Gibb‟s free energy 



xxii 
 

LIST OF ABBREVIATIONS 

A Electrodes electroactive area (cm2)    

AA Ascorbic acid 

API   Application Programming Interface 

BET Brunauer-Emmett-Teller 

CA Chronoamperometry  

Cad Cathode   

 CB Carbon black 

CE Counter electrode 

DC Direct current  

µ-DEFC Direct ethanol micro-fuel cell 

Cdi  Double layer capacitance 

CE Counter electrode 

CNF    Carbon Nano Fibers 

CNT Carbon nanotube 

COOH Carboxylic acid 

CV Cyclic Voltammetry 

D Diffusion coefficient 

DI Deionized 

 DMF N, N-Dimethylformamid 

DPV Differential Pulse Voltammetry 

E Potential  

 ECD Electrochemical deposition 



xxiii 
 

ECSA Electrochemical active surface area 

EDC 1-Ethyl-3-(3-dimethylaminopropyl carbodiimide hydrochloride 

EEC Electrical equivalent circuits  

EG Ethylene glycol  

Epa Peak anodic potential 

Epc Peak cathodic potential 

EIS Electrochemical Impedance Spectroscopy 

EMS Energy management system  

EtOH Ethanol  

 EOR Ethanol oxidation reaction   

FC Fuel cell  

 FCBrAS Fuel cell-based breath alcohol sensor 

FCEV Fuel cell electric vehicles 

FTIR Fourier transform infrared spectroscopy  

Ferri/Ferro Potassium Ferrocyanide/Potassium Ferricyanide 

FRA  Frequency response analyser (analysis) 

GCE Glassy carbon electrode  

GHG Greenhouse gas  

GPES General purpose electrochemical system 

HCl Hydrochloric acid  

HER Hydrogen evolution reaction  

HOR Hydrogen oxidation reaction 

Ip Peak current 

I Current 

 



xxiv 
 

Ipa  Peak anodic current 

Ipc  Peak cathodic current 

KCl Potassium chloride 

K-L Koutecky-Levich 

KHz kilohertz  

 kV Kilovolt 

 LoD Limit of Detection 

LoQ   Limit of Quantification 

M Molar 

 mHz                              Milli hertz 

 Min Minutes  

 Mg Milligram 

 ml                                 Millilitre  

 mM                              Millimolar concentration 

mW Milliwatt   

 N2 Nitrogen 

 O2 Oxygen 

 OLC Onion-like carbon 

ORR Oxygen reduction reaction 

Pd Palladium 

 Pt Platinum 

 RC Resistant-current 

Rct Charge transfer resistance 

Ret Electron transfer resistance 



xxv 
 

Rs Ohmic resistance of the electrolyte solution 

SEM Scanning Electron Microscope 

SWV Square Wave Voltammetry  

TEM Transmission Electron Microscopy 

UA Uric acid 

 V Volt 

 Vs Versus 

 W Watt 

 WE Working electrode 

WHO World Health Organisation 

WITS University of the Witwatersrand 

XAS X-ray Absorption Spectroscopy 

XPS X-ray Photoelectron 

XRD X-ray Diffraction 

Z Impedance 

Zw Warburg impedance 

 

  

 

 

 

 

 



1 
 

CHAPTER 1 

INTRODUCTION 

The research problems, research aims and objectives, significance of the study 

and outline of the thesis are summarised in this chapter. 

1.1 Research Problems  

Drugs of abuse (DOA) are normally described as psychoactive substances 

used by individual or people for numerous reasons including (i) curiosity and 

peer influence amongst youth, (ii) prescribed drugs, that are initially deliberated 

for pain relief, are rather used for recreational activities and become addictive, 

(iii) religious belief (people use them for rituals), and (iv) an avenue to get 

creative inspiration. However, the DOA poses severe negative consequences 

on humans‟ health and the society at large. The consequences of the DOA on 

human body makes it to be classified into three categories: 

 Depressants: These substances may cause depression. 

 Stimulants: These substances stimulate the brain for alertness and 

accelerate activities like rapid heat rate, dilated pupils, increased 

blood pressure, nausea or vomiting and behavioral change (i.e., 

agitation, and impaired judgment). 

 Hallucinogens: These substances results in hallucinations and an 

“out of the world” feeling of dissociation from one‟s comportment, 

causing distorted sensory perception, delusion, paranoia and even 

depression.     

 



2 
 

Some of the DOA are tramadol, alcohol, cocaine, tobacco, marijuana, heroin, 

opioids etc. Tramadol and alcohol are amongst the most abused substances in 

the world that cause significant global risk factors, resulting in disability and 

premature loss of life [1]. The health implication of the tramadol and alcohol 

abuse leads to major economic cost such as health care and law enforcement 

expenditure, low productivity, and other unforeseen cost (i.e., harm to others) 

[2]. A study has shown that 1-15% of drivers on the highway drive in the 

influence of one or more DOA, thereby impairing their driving ability culpable of 

an accident [3]. Abusing of drugs and substances, particularly tramadol and 

alcohol is becoming a major challenge amongst our teeming youths in the 

society. Hence, there is need for law enforcement agents to be provided with 

devices that can detect the abused substances in individual or people on site, 

so that they could be easily prosecuted and perhaps may serve as deterrent to 

other youths and the elderly to desist from abusing these drugs. 

  

1.2 Rationale or Justifications 

The need for these drugs of abuse to be detected and controlled more 

effectively using less cumbersome, much simpler, faster and cost-effective 

techniques than the conventional techniques has become imperative. Different 

conventional methods such as Uv-visible spectroscopy, Raman spectroscopy, 

high-performance liquid chromatography, gas chromatography, 

spectrofluorometry and electrochemical technique have been proffered for 

detecting these abused substances, but there is limitation to the use of all the 

techniques for on-site detection of DOA, except the electrochemical techniques, 



3 
 

since the method consistently produce exceptional performance. Furthermore, 

the method is easy to carry out and can provide rapid detection and selectivity 

and sensitivity [4]. This finding informed our interest to design and fabricate the 

electrochemical-based devices for the detection of tramadol and alcohol, 

leading to the main crux of this study. This research holds a number of 

promises to the body of knowledge in that amongst other things it can 

potentially: 

 Provide us with faster and more sensitive techniques to determine drugs and 

substances of abuse like tramadol or alcohol in humans 

 Provide us with a more selective technique in the determination of tramadol 

and Alcohol. 

 Provide us with a less cumbersome, cost-effective and miniature facility to 

detect drugs and substances of abuse, e.g., tramadol, alcohol. 

This study can potentially add to the efforts in enhancing control of drug abuse 

(e.g., tramadol abuse,) and substances abuse (e.g., Alcohol abuse, and 

dangerous habits like driving under the influence of substances of abuse in 

society. The results obtained here can be extended to other drugs and 

substances of abuse. 

1.3 Research Aim and Objectives  

1.3.1 Aim of the study 

The aim of this study was to investigate the performance of nanocarbons- (i.e., 

onion-like carbon (OLC), carbon black Nanotubes and palladium-based (i.e., 

palladium on carbon (Pd/CB), palladium on carbon-ceria composite (Pd-CeO2/CB), 

palladium on OLC (Pd/OLC), palladium on OLC-ceria composite (Pd-CeO2/OLC) 



4 
 

electrode materials in the electrocatalytic detection of tramadol and alcohol, 

respectively. 

1.3.2 Objectives of Study 

The aim was achieved with the following:  

(i) To synthesize the nanocarbon-based materials (OLC, CB) and palladium-

based materials (Pd/C, Pd/C-CeO2, Pd/OLC and Pd/OLC-CeO2) using 

microwave-assisted methods. 

(ii) To characterize the nanocarbon-based materials (OLC, CB) and palladium-

based materials (Pd/C, Pd/C-CeO2, Pd/OLC and Pd/OLC-CeO2) using  

scanning electron microscopy (SEM), transmission electron microscopy 

(TEM), energy-dispersive X-ray spectroscopy (EDX), ultraviolet-visible (UV-

vis) spectroscopy,  Fourier transform infra-red (FTIR), X-ray diffraction (XRD), 

Raman spectroscopy, X-ray photoelectron spectroscopy (XPS)  and 

Brunauer-Emmett Teller (BET), thermogravimetric analysis (TGA). 

(iii) Use of electrochemical technique such as cyclic voltammetry (CV), 

chronoamperometry (CA), square wave voltammetry (SWV), and 

electrochemical Impedance spectroscopy (EIS) to determine or measure the 

performances of the nanocarbon-based and palladium-based materials for the 

detection of tramadol and alcohol, respectively.  

1.4 Outline of the thesis  

Chapter 1 (Introduction): This is the introductory chapter of the thesis that explains 

the rationale, and objectives of this study, as well as illustrates the research 

problems, the research aims and objectives of the study.  



5 
 

Chapter 2 (Literature review): This chapter reviews recent literature that explains 

developmental trends for the detection of drugs and substances of abuse, 

particularly tramadol and alcohol, their mode of operations and application, electrode 

materials and their synthetic approaches, and description of electrochemical 

techniques employed in this study.  

Chapter 3 (Materials Characteristics Methods): This chapter explains, illustrates, 

and demonstrates all methodologies for the physical and electrochemical 

characterizations employed in this study.  

Chapter 4 (Electro-catalytic detection of tramadol): This chapter explains and 

discusses in detail the electro-catalytic detection of tramadol using the identified 

Nano-Carbons. 

Chapter 5 (Electro-catalytic detection of Ethanol):  This chapter explains and 

discusses in detail the electro-catalytic detection of ethanol using the identified 

Nano-Carbons. 

Chapter 6 (Conclusions and Recommendations): This chapter deals with the 

conclusions drawn from the study results and recommendations therefrom.  



6 
 

1.5 References 

1) S. S. Lim, T. Vos, A. D. Flaxman, G. Danaei, K. Shibuya, H. Adair-Rohani, et  

al. Lancet 2012, 380, 2224–2260. 

2) J. Rehm, C. Mathers, S. Popova, M. Thavorncharoensap, Y. 

Teerawattananon, J. Patra.  Lancet 2009, 373, 2223–2233. 

3) R. Penning, J. Veldstra, A. P. Daamen, B. Oliver, J. C. Joris, Current Drug 

Abuse Reviews 2010, 3, 23-32. 

4) K. I. Ozoemena, S. Musa, R. Modise, A. K. Ipadeola, L. Gaolatlhe, S. Peteni, 

G. Kabongo, Current Opinion in Electrochemistry 2018, 10, 82-87.  

5) A. V. Tripkovi, K. D. Popovi, B. N. Grgur, B. Blizanac, P. N. Ross, and N. M. 

Markovi, “Methanol electrooxidation on supported Pt and PtRu catalysts in 

acid and alkaline solutions,” Electrochim. Acta, 2002, 47, 3707-3714. 

6) J. Prabhuram and R. Manoharan, “Investigation of methanol oxidation on 

unsupported platinum electrodes in strong alkali and strong acid,” J. Power 

Sources, 1998, 74(1), 54-61.  

7) A. S. Arico, V. Baglio, and V. Antonucci, Direct methanol fuel cells, 2010. New 

York: Nova Science Publishers. https://doi.org/10.1201/9781439833148. ch5  

8) J. R. Varcoe, R. C. T. Slade, and E. Lam How Yee, “An alkaline polymer 

electrochemical interface: A breakthrough in application of alkaline 

anionexchange membranes in fuel cells,” Chem. Commun., 2006, 1428-1429. 

9) J. R. Varcoe and R. C. T. Slade, “An electron-beam-grafted ETFE alkaline 

anion-exchange membrane in metal-cation-free solid-state alkaline fuel cells,” 

Electrochem. commun., 2006, 8, 839-843. 

10) J. R. Varcoe, R. C. T. Slade, E. L. H. Yee, S. D. Poynton, and D. J. Driscoll, 

“Investigations into the ex situ methanol, ethanol and ethylene glycol 

https://doi.org/10.1201/9781439833148


7 
 

permeabilities of alkaline polymer electrolyte membranes,” J. Power Sources, 

2007, 173, 194-199.  



8 
 

CHAPTER 2 

LITERATURE REVIEW 

2.1  Rationale for the detection of tramadol and alcohol  

For some years detection of drugs and substances of abuse (notably, tramadol and 

alcohol which are the main focus of this PhD project) had been undertaken by health 

workers, providers, Doctors, Pharmacists, Nurses, researchers, and government 

departments to determine causes of some unpalatable symptoms experienced by 

some patients, and to ascertain the actual drugs or substances of abuse being used 

by such patients to effectively diagnose and prescribe accurately for the treatment of 

the symptoms.  

Sometimes public health units and departments undertake drug detections in areas 

where there are indications of environmental contamination sometimes from known 

drugs and substances of abuse (in their sources of water, in their soil, in the air etc) 

which by extension affects the health of the public to find interventions and 

mitigations in such occurrences. In recent times, societies have learnt to adopt 

preventive measures over and above curative interventions so, reasons or rationale 

for detecting drugs of abuse had extended beyond keeping the streams, ponds and 

sources of domestic water supply devoid of contaminants capable of causing danger 

to public health, the rationale for detecting drugs and substances of abuse has 

extended to over and above health professionals, police and other security agencies 

proactively and preventatively testing drivers, sportsmen and women, and citizens in 

general to identify illicit use of such drugs and substances on individuals which has 

critical undesirable effects not only on individuals but on society at large especially 



9 
 

as some of such drugs and their opioid and euphoric effects enable commission of 

social misdemeanours and violent crimes in the society.  

Drugs of abuse such as tramadol (Figure 2.1) and alcohol (Figure 2.2) are prevalent 

in many countries including Africa and Asian countries. Tramadol exhibits an effect in 

patients similar to that of opioid agonists, and tramadol abuse just like opioids as 

indicated seem to be a problem on the rise for a number of countries. Over and 

above other violent crimes associated with the abuse of tramadol, the relationship 

between tramadol and sexual function appears to be controversially linked to its 

increasing abuse. It is recorded that men with premature ejaculation may benefit 

from taking tramadol off label; however, these patients live “on a knife's edge” and 

are more likely to develop other sexual dysfunctions and dependencies. According to 

the work done by Hamid et al. [1] on tramadol abuse and sexual function, the 

problem of tramadol abuse is controversially driven by the pursuit of the supposed 

sexual benefits as against the potential risks of tramadol on different sexual functions 

including ejaculation, orgasm, erection, desire, and testosterone levels. 

Originally tramadol was never thought of being among abused substances and 

dependence potentials worldwide [1], however, recently in some African, Middle 

East, and West Asia countries it has become a serious issue. The 11 clinical trials 

conducted to evaluate tramadol benefit of premature ejaculation and were evaluated 

by 6 systematic reviews and 3 of which pooled data in a meta-analysis. Regarding 

abuse of tramadol for other sexual challenges such as low libido, erectile 

dysfunction, hypogonadism, and anorgasmia the evidence is inadequate. 

Tramadol may be effective substance in treating premature ejaculation. Early studies 

had challenges namely, selection, allocation, or assessment bias. Over the years, 

https://www.sciencedirect.com/topics/medicine-and-dentistry/opiate-agonist
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fields to research drug abuse are emerging. Clinical research is one of the fields and 

studies are carried out on the negative effects, dose, and safety of drugs. The off-

label use of tramadol [2] which are in the increase costing the various countries 

heavily. 

On the part of ethanol, alcohol is the favorite beverage all over the world. Society 

consumes it to have fun or stress reliever. However, regular consumption may put 

health condition susceptible to chronic problems such as cardiovascular diseases, 

cancer, and digestive tract conditions [3-5]. Moreover, over consumption of alcohol 

at a short space of time may lead to violence and accidents [6]. Road accidents due 

to alcohol is a trend that occurs each an every year, especially in South Africa during 

festive season. Africa follow the global pattern. In 2004, young African males 

between the age of fifteen to twenty-four years experienced a disability-adjusted life 

years due to alcohol. The nexus between alcohol and infectious diseases are 

general not recognized and/or poorly addressed in Africa. Rehm et al.,[7] estimated 

that there was an increase in infectious diseases attributed to alcohol by 50% in 

Africa. Despite the increasing evidence, most cities and nations neglect importance 

of risk factor of alcohol on the epidemics burden of infectious diseases such as HIV 

and Tuberculosis. Studies conducted in 2004 and 2010 by Rehm et al., [7, 8] 

assessed alcohol-related diseases and injuries around the world and identified 

conditions of contributing factor. 

In closing, the negative impact of abuse of tramadol and ethanol (Figure 2.3) as 

indicated and its attributable injuries and disease burden in developing countries 

especially African nations is on the rise, hence confirming the relevance and 

importance of this study to develop more efficient, cost-effective methods of 

detection of these drugs. Various researchers such as [8,9], present the 



11 
 

epidemiological situation regarding alcohol as it relates to disease burden in Africa 

specifically giving details on alcohol exposure, and its impact on deaths and 

disability, using estimates from the World Health Organization (WHO) Global Health 

Estimates for outcome data and the WHO Global Status Report on Alcohol and 

Health (GSRAH) for risk relations. In addition, researchers present impact of ethanol 

abuse under different scenarios which include the impact of alcohol on HIV/AIDS 

incidence, economic costs of excessive alcohol consumption, alcohol abuse and 

dependence, excessive use of ethanol and impaired productivity, alcohol abuse and 

criminal justice system costs etc. [10] 

 

 

 

Figure 2.1: Isomeric structures of tramadol 



12 
 

Figure 2.2 Chemical Structure of Ethanol  

 

Figure 2.3 Pictures depicting faces of alcohol and drug abuse in the society 
(Courtesy: Bing images, assessed 15 March 2022) 

 



13 
 

2.2  Brief Overview of Current Detection Methods of Ethanol: Pros and Cons 

There are various detection methods already on record, and in literature to have 

been used to detect tramadol such as UV-visible spectroscopy, Spectrophotometry, 

High-performance Liquid Chromatography (HPLC), mass spectroscopy, 

Electrochemistry. etc.  

2.2.1 UV-Visible spectroscopy 

UV-visible spectroscopy “is a “form of Absorption spectroscopy. Absorption 

spectroscopy in the UV-Visible region is to be one of the oldest and most frequently 

employed technique in pharmaceutical analysis for qualitative, quantitative, and 

structural analysis of a substance in solution. UV-Vis spectroscopy is a widely used 

technique in many areas of science ranging from bacterial culturing, drug 

identification and nucleic acid purity checks and quantitation, to quality control in the 

beverage industry and chemical research [11]. This article will describe how UV-Vis 

spectroscopy works, how to analyze the output data, the technique's strengths and 

limitations and some of its applications.” 

Light has a certain amount of energy which is inversely proportional to its 

wavelength. Thus, “shorter wavelengths of light carry more energy and longer 

wavelengths carry less energy. A specific amount of energy is needed to promote 

electrons in a substance to a higher energy state which we can detect as absorption. 

Electrons in different bonding environments in a substance require a different 

specific amount of energy to promote the electrons to a higher energy state. This is 

why the absorption of light occurs for different wavelengths in different substances 

[12]. Humans are able to see a spectrum of visible light, from approximately 380 nm, 

which we see as violet, to 780 nm, which we see as red.1 UV light has wavelengths 

shorter than that of visible light to approximately 100 nm. Therefore, light can be 



14 
 

described by its wavelength, which can be useful in UV-Vis spectroscopy to analyze 

or identify different substances by locating the specific wavelengths corresponding to 

maximum absorbance (see the Applications of UV-Vis spectroscopy section) [12-

14].” 

 2.2.2.   Spectrophotometry    

Spectrophotometry “deals with measurement of the radiant energy transmitted or 

reflected by a body as a function of the wavelength. Here, the intensity of the energy 

transmitted is compared to that transmitted by some other system that serves as a 

standard [15,16]. It is a procedure often used for determining how much light is 

reflected by a chemical material by measuring the strength of light as a light beam 

travels through the sample solution.” 

 

2.2.3 Mass Spectroscopy 

Mass spectroscopy, also referred to as mass spectrometry, “is an analytic technique 

by which chemical substances are identified by the sorting of gaseous ions in electric 

and magnetic fields according to their mass-to-charge ratios. Simply put, mass 

spectroscopy separates the components of a sample by their mass and electrical 

charge [17,18]. The instruments used in such studies are called mass spectrometers 

and mass spectrographs.” 

 

2.2.4 High Performance liquid chromatography (HPLC) 

High Performance liquid Chromatography “was formerly referred to as High Pressure 

Liquid Chromatography. It is a technique in analytical chemistry used to separate, 

identify, and quantify each component in a mixture [19]. These existing techniques 



15 
 

employed for the detection of tramadol, alcohol and other drugs and substances of 

abuse have their advantages and disadvantages [20]. These techniques such as 

HPLC for instance used in the detection of tramadol exhibit the following:” 

  

(i) Sensitivity such that they can detect a very low concentration of tramadol.  

(ii) Selectivity, in that they can selectively detect tramadol in the presence of 

impurities and interferents.  

(iii) Re-usability such that they are stable enough to give the same results under 

similar conditions over time. 

However, they are fraught with some critical drawbacks in that some are 

(i) Cumbersome and not simple to use. 

(ii) Requires a well-skilled personnel and technical know-how to operate. 

(iii) Requires complicated set-up and does not allow on-site operation. 

Yet, electrochemical techniques have been reported to be more sensitive, more 

selective and more cost-effective.   

 

2.2.5 Electrochemical detection (Advantages & Disadvantages). 

Electrochemical detection is a powerful analytical method that can detect electric 

currents generated from oxidative or reductive reactions in test compounds. Its 

variants had been utilized in the detection of tramadol recorded in literature [22]. Its 

design can be simple and compact allowing construction of portable devices [23]. 

This electro-analytical technique can be divided into different types of 

conductometric, potentiometric, voltametric and amperometric methods etc. 

Electrochemical detection method has its advantages and disadvantages. The 



16 
 

advantages of electrochemical detection method relative to other detection methods 

include: 

 It has a low power requirement  

 Linear output and good resolution 

 High sensitivity and selectivity 

 Excellent repeatability 

 Wireless networking 

 Simple, compact and can be constructed into a portable device. 

The drawbacks of electrochemical detection methods include: 

 Short lifespan 

 Temperature fluctuation 

 Possible cross-sensibility 

 

2.2.6 Electrode materials   

2.2.6.1 Onion-Like Carbon (OLC)   

The Onion-Like Carbons (OLC) also referred to as Carbon Nano Onion (CNOs) or 

carbon onions are usually quasi-spherical nanoparticles made up of multi-layer 

concentric defective graphitic shells like multi-shell fullerene [33]. They are the latest 

discoveries of the carbon nanostructures which are currently very popular research 

in electrocatalytic analyses. They have properties different from other carbon 

nanostructures such as graphite [26, 34, 37, 41].  

The defective interconnects between the shells allow for the intercalation of small 

ions and molecules. They have a large surface area, small particle sizes and are 

porous with hectohedral and hexahedral structures which are synthesized by very 



17 
 

high temperatures (annealing) into small fine structures of about 5 -10µm [1, 23, 26]. 

This OLC is thermodynamically more stable than graphene because it is 

characterized by a curved structure which creates strong strains that causes a 

hybridization with partials sp3 character. 

The myriad properties enable the OLC to be utilized in areas such as: 

 Pharmaceutical drug delivery in the body. 

 Bioimaging. 

 Electrochemical sensing.  

 Electromagnetic shielding. 

 Electrical energy storage.  

 catalyst (and catalyst support),   

 cell imaging and selective sensing  

The Figure 2.4 below shows the  images of Onion Like Carbon. 

 

  

Figure 2.4 (A) HRTEM images of onion-like carbon (OLC) and (B) its structural 
representation. (Ugarte,1995).  



18 
 

2.2.6.2 Palladium Carbon on Carbon and Carbon-Ceria Supports as Catalysts  

Palladium based materials have recently been explored as excellent electrocatalysts 

for anion membrane fuel cells (AMFCs) [23,24]. The configurative architectural 

design of the AMFCs is advantageously mimic for the detection of ethanol in this 

study.  Platinum-based electrocatalysts have been exhaustively used and studied as 

catalysts for low temperature fuel cells, especially the direct ethanol fuel cell and in 

the preparation of sensors for detection of ethanol. However, the commercially 

available FC devices are too expensive and that could be traced to the use of bulk 

platinum (Pt) electrocatalysts to drive the reactions in acidic conditions.  

The reaction at the cathode is of major concern in fuel cell and ethanol detection 

devices, as it is extremely sluggish and necessitated the use of high loadings of Pt 

electrocatalysts [38] as opposed to the anodic reaction, which is about five times 

faster than the reaction at the cathode, with very little loading of Pt [39]. Moreover, 

the Pt electrocatalyst is rarely readily available due to scarcity. Nanotechnology is 

one strategy that has been employed to minimize the cost of the Pt electrocatalyst 

and MEA for the fabrication of fuel cell or ethanol detection devices through 

electrocatalysis [29–31]. Nanotechnology has brought about research ideas to 

reduce the loading of Pt electrocatalysts (ca. 20 %). The utilization of various carbon 

supports to achieve this end has been variously reported [32]. Another strategy that 

has been employed in these electrocatalytic technologies is a replacement of the 

acidic media of operation with alkaline media [26] as electrocatalysts have shown 

great performance in alkaline media due to the impressive characteristics of alkaline 

media. Consequently, it became the interest of this research work to explore Pd-

based carbon materials in alkaline conditions for the electrocatalytic detection of 

ethanol. 



19 
 

2.2.7 Tramadol and Ethanol    

2.2.7.1 Tramadol  

Tramadol is an opiate pain medication used in the treatment of moderate to 

moderately severe acute or chronic pain.  It is metabolized in the human body by 

Cytochrome P2D6 (CYP2D6) enzymes to produce its therapeutic effects through its 

active metabolites, O-desmethyltramadol [35,36]. Tramadol is formulated in different 

dosage forms including tablets, powders, injections etc. Tramadol acts in the central 

nervous system (CNS) by binding to the µ-opioid receptor neuron and is also a 

serotonin-norepinephrine reuptake inhibitor. It is these actions on the CNS that make 

tramadol cause dependency effect leading to addiction which poses a potential 

danger to consumers and the society at large. This gives rise to the need to detect 

tramadol in its traces and control the same amongst individuals and in society 

[35,36]. 

2.2.7.2 Ethanol 

Ethanol is an organic chemical compound. It is a simple alcohol with the chemical 

formula C₂H₆O. Its formula can be also written as CH ₃−CH ₂−OH or C ₂H ₅OH, and 

is often abbreviated as EtOH. Ethanol is a volatile, flammable, colorless liquid with a 

slight characteristic odour [37]. When ingested in large quantity causes drowsiness, 

blurred vision, impairs concentration and may lead to addiction in humans. The 

abuse of alcohol leads to health hazards and is often linked to criminal activities and 

public insecurity including traffic offences.  The Ethanol used in this work was 

absolute ethanol (ACS reagent, 99 % purity, and procured from MK Chemical). 



20 
 

2.2.8 Ferri-Ferrocyanide Solution.  

Ferrocyanide/Ferricyanide often referred to as Ferri/Ferro is a redox reaction couple 

commonly used in electrochemistry as a mediator for shuttling electrons between 

electro -active species dissolved in a solution and a working electrode [39,40]. The 

working electrodes (WE) used for this particular work are glass carbon electrode 

(GCE) and screen plated electrode (SPE) modified with carbon nano-materials with 

a Platinum wire (PI) counter electrode (CE) and the varying current or voltage is 

measured with respect to reference electrode, Ag/AgCl. 

2.2.9 Amstel Beer (4.0 % Alc) 

This is an alcoholic beverage whose brand originated from Amsterdam but currently 

manufactured and marketed locally in South Africa. This beer contains 4.0% alcohol 

and we utilized this to run a comparative ethanol sensing in this Project. 

2.2.10 Nederberg Wine (13.0 % Alc)  

Nederberg wines is one of South Africa's largest and most prominent lines of wines 

produced in the country. It is located beneath the Drakenstein Mountains in Paarl, 

Western Cape and produces a wide range of wines from across the Western Cape, 

from classic South African varieties. For this Project the wine utilized contains 13.0% 

alcohol. It is employed as one of proof of concepts of sensing ethanol in commercial 

products, discussed in the later section of this work.   

2.2.11 Interferents 

These are chemical substances that are found in real samples such as serum, blood, 

urine and utilized to ascertain the selectivity of sensor(s) in distinctly detecting the 

various sample materials in the presence of such materials. The examples of 



21 
 

interferents are uric acid, citric acid, ascorbic acid, Sodium Chloride Potassium 

Chloride, Urea, Ammonium Sulphate, Calcium etc, 



22 
 

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4) N. K. Morojeje, E. W. Dumbili, I.S. Obot, C.D.H. Parry. “Alcoholic 

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10) E. Bouchery; H. Hardwood; J. Sacks; Et al. Economic costs of excessive 

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Medicine 2011, 41: 516-524.   

11) T. Jiustin; UV-Vis Spectroscopy:  Principles, Strengths and Limitations and 

Applications; by Technology Networks, Analysis and separations, 

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principle-strengths-and-limitations-and-applications-349865, June 2021. 

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using UV-Vis spectra”. Nanoscale 2015, 7: 5134-5139. 

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366-380.  

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production by advanced proton exchange membrane (PEM) water 

electrolysers-Reduced energy consumption by improved electrocatalysis,” 

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catalysts for PEMFCs,” Applied Catalysis B: Environmental. 2005, 56: 9- 35. 

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fuel cells,” J. Power Sources, 2003, 113: 37-43.  

31) Leandro L. carvalho, Auro Tanaka, Flavio Colmati; “Palladium-platinum 

electrocatalysts for the ethanol oxidation reaction: comparison of 

electrochemical activities in acid and alkaline media”. Journal of Solid State 

Electrochemistry 2018, 22: 1471–1481. 

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G. Kabongo. “Fuel cell-based breath-alcohol sensors: Innovation-hungry old 

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Metabolite in Human Urine by the Gas Chromatography-Mass Spectrometry 

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G. Kabongo, Current Opinion in Electrochemistry, 2018, 10: 82-87. 

 



27 
 

CHAPTER 3  

MATERIALS AND METHODS 

 

INTRODUCTION 

The experimental and characterization techniques employed in this research are 

briefly summarised under this chapter. However, more information on the techniques 

and appropriate references are provided in chapters 4 and 5 that discuss in detail the 

results and discussion.  

3.1 Chemical Reagents  

The chemicals used in this work are listed in Table 3.1.  

 

Table 3.5 Chemical reagents used in this work 

Chemical Name Source 

Tramadol hydrochloride Sigma Aldrich / Merck 

Absolute Ethanol MK Chemical 

Potassium Hydroxide pellets Association of Chemical Entrepreneur 

(ACE) 

Detonated diamond particles NaBond Technologies / Gelon, China 

Carbon Black (Timical Super C45) Gelon (China) 

Human serum (Product #: H6914)  Sigma-Aldrich / Merck 

Austell Tramadol capsule 50 mg, 

tramadol HCl 50 mg, AUSTELL 

South Africa; Batch No.: MG 20567; 

Exp: 08/2023 

Local pharmacy 

KH2PO4 and K2HPO4 Sigma Aldrich / Merck 

K4Fe(CN)6 and K3Fe(CN)6 Sigma Aldrich / Merck 

Dimethyl Formamide (DMF) Sigma Aldrich / Merck 



28 
 

Nafion Sigma Aldrich / Merck 

Amstel Beer SAB MILLER 

Nederberg Wine Nederberg Wine 

Pure Nitrogen (N2) Afrox 

Alumina Powder Sigma Aldrich 

 

3.1 Material Characterization 

3.1.1 Powder X-ray Diffraction (PXRD) 

The powder XRD is a rapid non-destructive analytical technique with the operation of 

constructive interference of monochromatic X-ray, employed basically to determine 

phases and structures of crystalline samples, giving crystallographic information of 

the crystalline materials [1-3].  

3.1.2. Raman Spectroscopy 

Raman spectroscopy is a non-contacting, non-destructive technique employed to 

obtain both qualitative and quantitative molecular information from a sample. It is 

used to characterize a material by its unique vibrational and crystallographic 

information, which is derived from the material‟s electronic states and photon energy 

dispersion. A Raman spectrum is obtained by exciting a sample with a laser. The 

inelastic scattering of photons from the vibrations within the molecules is then 

measured at low wave number resolution. It is performed at room temperature and 

ambient pressure [4-10]. Raman spectroscopy is very useful in the characterization 

of carbon nanomaterials such as OLC, CB e.t.c. (their detailed structure and 

configurations). 



29 
 

3.1.3 Scanning electron microscopy (SEM)  

SEM was used to collect images of the samples. These images reveal information 

about the morphology and surface properties of each catalyst material [11]. It is a 

technique commonly used in the field of nanotechnology to visualize and examine 

nanoparticles and nanocomposites more closely. The procedure involves fixing a 

carbon tape to an SEM sample holder, and the sample (powdered) is sprinkled onto 

the carbon surface. A thin layer of platinum, gold, or carbon coating is then sputtered 

(≈100 Å) on the sample to ensure that the surface is conductive and not charging. 

This is a requirement for charging samples. The sample is then examined in a SEM 

chamber. Here, a fine beam of electrons is focused into a small probe which is used 

to scan over a sample. The interaction between the beam and the sample creates an 

emission of electrons and photons. The signals (back-scattered electrons, secondary 

electrons, transmitted electrons) emitted by the sample are observed on the 

detectors. The final product of the interaction of beams is the sample‟s image. A 

scanning electron microscope employs two image monitors. One monitor is used to 

observe the specimen and the other one is connected to a photo camera and is used 

to take images of the specimen.  

3.1.4 Energy-dispersive X-ray spectroscopy (EDX)  

EDX is employed for the identification of the elements in a given material sample. In 

other words, it is used for elemental analysis of the sample. The sample preparation 

for EDX is same as that of the SEM. It plays a complimentary role to the SEM 

analysis. (Page 29 highlighted) 

3.1.5 Transmission electron microscopy (TEM) analysis  

TEM is usually employed in the examination of the morphology of nanomaterials 

[12]. It helps to examine the microstructure of nanoparticles and nanocomposites. 



30 
 

TEM employs a nanometer-size electron probe with which it uniquely identifies and 

quantifies the structures of individual nanocrystals in a sample. TEM and SEM are 

similar, except that that the beam passes through the sample during TEM, while 

SEM studies the extrinsic structural morphology of the prepared Pd-based catalysts. 

Also, TEM can provide high-resolution images (up to 0.5 nm) of the samples from a 

high-powered beam of electrons accelerated at 120 kV to examine their intrinsic 

structural properties. In this research, a FEI Tecnai T12 TEM was used. 

3.1.6 Thermogravimetric analysis (TGA)  

The TGA is an analytical technique utilized with monitoring the weight changes of a 

material while the temperature is changed over time under a controlled temperature 

atmosphere [13]. This technique is used to analyze the thermal stability of a given 

sample. In this work, a Perkin Elmer Thermogravimetric analyzer (TGA)/Differential 

Thermogravimetric analysis (DTGA) 6000 was used for the TGA of the samples.  

3.1.7 Brunauer-Emmett-Teller (BET) surface area analysis  

Brunauer-Emmett-Teller (BET) analysis is employed to get this information about the 

specific surface area and porosity of materials. In this work, a Micromeritics TriStar II 

3000 was used. 

3.1.8 X-ray Photoelectron Spectroscopy (XPS)  

XPS is a surface-sensitive analytical technique that provides information on 

quantitative atomic composition and chemistry. It identifies elements in a sample, as 

well as their chemical states [14]. In this work, the XPS facility used was the Thermo 

brand of ESCAlab 250Xi (monochromatic Al kα (1486.7 eV, x-ray power of 300 W, 

and spot size of 900 µm, pass energy (survey) of 100 eV, pass energy  (hi-

Resolution) 20 eV, and pressure of <10-8 mBar).  



31 
 

3.2 Electrochemical Characterization Techniques 

3.2.1 Cyclic voltammetry 

This is a direct current (DC) electrochemical technique, which records the response 

in a current while a potential scan is applied to the working electrode at a constant 

scan rate in the forward and reversed directions, once or several times [15-17]. 

Cyclic Voltammetry (CV) is a standard technique used for determining the redox 

properties of a materials. It is the most popularly used electrochemical technique. In 

this technique, current signal is measured at a fixed potential range or window. It is 

used to acquire information about the properties and the characteristics of 

electrochemical process like the reversibility of a reaction, adsorption process, 

electron transfer kinetics, to determine the presence of intermediate in a redox 

reaction etc. The electrochemical equipment (potentiostat) is used to measure the 

current generated by the redox reaction, which is then plotted as current against 

applied potential, known as Voltammogram. In this process, the positive scan is the 

anodic (oxidation) process, while the backward scan is the cathodic (reduction) 

process. In general, the CV provides unique information about the redox properties 

of the material of interest such as reversibility or irreversibility, equilibrium potential, 

etc. This technique was used extensively in this PhD thesis (see Chapters 4 and 5 

for detail). 

3.2.2 Square wave voltammetry   

This technique is a form of linear potential sweep voltammetry that uses a combined 

square wave and staircase potential applied to a stationary electrode. It has found 

numerous applications in various fields, including medicinal and various sensing 

communities, hence useful in the characterization of materials used in the detection 

of tramadol.  



32 
 

3.2.3 Differential pulse voltammetry (DPV)  

Differential pulse voltammetry (DPV) (also differential pulse polarography, DPP) is 

a voltammetry method used to make electrochemical measurements and a 

derivative of linear sweep voltammetry or staircase voltammetry, with a series of 

regular voltage pulses superimposed on the potential linear sweep or stairsteps [15-

17].   

3.2.4 Electrochemical impedance spectroscopy 

Electrochemical impedance spectroscopy (EIS) is a powerful analysis technique that 

helps to understand the electrode-electrolyte interfacial properties [16-18]. EIS helps 

to measure the electrical resistance during electrochemical reaction; measures the 

relationship between the imaginary part of the impedance (Z” or Zim) and the real 

part of the impedance (Z‟ or Zre). The plot of “Z” “versus Z‟ is called the Nyquist plot. 

The EIS parameters are usually obtained by fitting the experimental date with the 

electrical equivalent circuit, such as the Randles circuit shown below (Figure 3.3b). 

The Nquist plot (figure 3.3a) is a semi-circle at high frequency and a 45-degree rise 

at the low frequency region. From The Randles circuit (figure 3.3b), the Nyquist plot 

comprises resistance due to the electrolyte (Rs), resistance due to charge- or 

electron-transfer (Rct or Ret), double layer capacitance (Cdl) and the Warbug 

impedance (ZW) due to the diffusion of ions from the electrolyte to the electrode 

interface.” 

3.2.5 Chronoamperometry 

This “is an electrochemical technique in which the potential of the working electrode 

is stepped and the resulting current from faradaic processes occurring at the 

electrode (caused by the potential step) is monitored as a function of time. In this 



33 
 

work, chronoamperometric technique was used to measure current–time 

dependence of the concentrations of ethanol in electrolyte” solution (see Chapter 5). 

3.3 Evaluation of Electrochemical Sensor’s Figures of Merit. 

In this work, the electrochemical sensors of the glass carbon electrode (GCE), and 

screen-printed electrode (SPE) were modified with different nanomaterials and 

nanocomposites which were designed for the detection of tramadol and ethanol. The 

different modified electrodes were evaluated using the following criteria viz: 

sensitivity, limit of detection, limit of quantification, specificity studies, selectivity 

studies, and stability studies for possible application for use in real samples. These 

sensors showed good precision and accuracy as was recorded in the results section 

of this work. The different criteria used for the evaluation are discussed below: 

3.3.1 Sensitivity  

With sensitivity, “the extent to which the true positive will not be missed is tested 

using the modified electrodes to test different substrates in a solution and making 

sure that the true positive is not overlooked. The sensitivity in other words is the 

measure of the degree to which the sensor can dictate the concentration of the test 

materials. The sensitivity of the modified electrodes used was tested for the 

detection of tramadol and ethanol respectively and each result was recorded 

according to their performance in the results section. This parameter is determined 

as the gradient or slope of the plots of current response to concentration of the 

analytes (tramadol or alcohol).” 

3.3.2 Limit of detection  

Limit of detection (LoD) is an analytical expression used to determine the sensitivity 

of a given detection technique. LoD is the lowest analyte concentration likely to be 



34 
 

reliably detected by the sensor(s). It can also be defined as the lowest concentration 

of analyte that can be obtained which is statistically different from the blank sample 

at a given level of confidence. The (LoD) can be computed from the equation below:   

     
    

 
                                                                                                                                

where δ, m and LoD are the standard deviation of the plot, the slope of the fitted plot 

of current from voltammogram against the analyte concentrations and limit of 

detection, respectively. 

3.3.3 Limit of quantification  

The term means the analyte is detected at its lowest concentration with precise and 

accurate quantification. LoQ is given in equation below.  

     
   

 
          (3.2) 

where δ, m and LoQ are the standard deviation of the plot, the slope of the fitted plot 

of current from voltammogram against the analyte concentrations and limit of 

quantification, respectively. 

3.3.4 Specificity / Selectivity  

Specificity simply means the extent to which the electrochemical sensor can detect 

specific analyte irrespective of other interfering species.  

 

3.3.5 Repeatability / Stability   

The repeatability or stability of a sensor is its ability to be used repeatedly (over and 

over again) without a significant loss of activity or sensitivity.  

 



35 
 

3.4 References 

1) R. Heimann and R. Heimann, “X-Ray Powder Diffraction (XRPD),” in The 

Oxford Handbook of Archaeological Ceramic Analysis, 2016. 

(https://dokumen.pub/the-oxford-handbook-of-archaeological-ceramic-

analysis-oxford-handbooks-1nbsped-0199681538-9780199681532.html), May 

2022 

2) J. Lodge and J. P. Lodge, “X-Ray Powder Diffraction,” in Methods of Air 

Sampling and Analysis, 2018. https://www.amazon.in/Methods-Air-Sampling-

Analysis-3rd/dp/0367457571, May 2022. 

3) O. D. Neikov and N. A. Yefimov, “Powder Characterization and Testing,” in 

Handbook of Non-Ferrous Metal Powders, 2019, 3-62. 

https://www.sciencedirect.com/book/9780081005439/handbook-of-non-

ferrous-metal-powders, May 2022. 

4) D. D. Le Pevelen, “NIR FT-raman,” in Encyclopedia of Spectroscopy and 

Spectrometry, 2016, 98-109. https://www.elsevier.com/books/encyclopedia-of-

spectroscopy-and-spectrometry/lindon/978-0-12-803224-4, May 2022. 

5) P. S. Goh, A. F. Ismail, and B. C. Ng, “Raman Spectroscopy,” in Membrane 

Characterization, 2017, 31-46. DOI: 10.1016/b978-0-444-63776-5.00002-4 

6) P. K. Chu and L. Li, “Characterization of amorphous and nanocrystalline 

carbon films,” Mater. Chem. Phys., 2006, 96, 253-277.  

7) TUINSTRA F and KOENIG JL, “RAMAN SPECTRUM OF GRAPHITE,” J. 

Chem. Phys., 1970 53, 1126.  

8) A. Ferrari and J. Robertson, “Interpretation of Raman spectra of disordered 

and amorphous carbon,” Phys. Rev. B - Condens. Matter Mater. Phys., 2000, 

61. 

https://dokumen.pub/the-oxford-handbook-of-archaeological-ceramic-analysis-oxford-handbooks-1nbsped-0199681538-9780199681532.html
https://dokumen.pub/the-oxford-handbook-of-archaeological-ceramic-analysis-oxford-handbooks-1nbsped-0199681538-9780199681532.html
https://www.amazon.in/Methods-Air-Sampling-Analysis-3rd/dp/0367457571
https://www.amazon.in/Methods-Air-Sampling-Analysis-3rd/dp/0367457571
https://www.sciencedirect.com/book/9780081005439/handbook-of-non-ferrous-metal-powders
https://www.sciencedirect.com/book/9780081005439/handbook-of-non-ferrous-metal-powders
https://www.elsevier.com/books/encyclopedia-of-spectroscopy-and-spectrometry/lindon/978-0-12-803224-4
https://www.elsevier.com/books/encyclopedia-of-spectroscopy-and-spectrometry/lindon/978-0-12-803224-4
https://www.sciencegate.app/app/redirect#aHR0cHM6Ly9keC5kb2kub3JnLzEwLjEwMTYvYjk3OC0wLTQ0NC02Mzc3Ni01LjAwMDAyLTQ=


36 
 

9) A. C. Ferrari and J. Robertson, “Resonant Raman spectroscopy of disordered, 

amorphous, and diamondlike carbon,” Phys. Rev. B - Condens. Matter Mater. 

Phys., 2001, 64.  

10) M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus, L. G. Cançado, A. Jorio, 

and R. Saito, “Studying disorder in graphite-based systems by Raman 

spectroscopy,” Physical Chemistry Chemical Physics. 2007,9, 1276-1290.  

11) A. Ul-Hamid, A Beginners‟ Guide to Scanning Electron Microscopy. 2018. 

12) S. J. Pennycook et al., “Seeing inside materials by aberration-corrected 

electron microscopy,” Int. J. Nanotechnol., 2011, 8, 935 - 947.  

13) “W. M. Groenewoud, “Chapter 2 - Thermogravimetry,” in Characterisation of 

Polymers by Thermal Analysis, 2001, 61-76. 

https://www.sciencedirect.com/book/9780444506047/characterisation-of-

polymers-by-thermal-analysis. Doi.org/10.1016/B978-0-444-50604-7.X5000-6. 

14) H. Konno, “X-ray Photoelectron Spectroscopy (XPS),” Zairyo-to-Kankyo, 

1993, 42, 27-36. 

15) R.G. Compton, C.E. Banks, Understanding voltammetry, World Scientific 

2018. 

16) P.M. Monk, Fundamentals of electroanalytical chemistry, John Wiley & Sons 

2008. 

17) A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and 

Applications, 2nd ed., Wiley 2001. 

18) M.E. Orazem, B. Tribollet, Electrochemical impedance spectroscopy, 2nd 

ed.2017. 

 

 

https://www.sciencedirect.com/book/9780444506047/characterisation-of-polymers-by-thermal-analysis
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https://doi.org/10.1016/B978-0-444-50604-7.X5000-6


37 
 

CHAPTER 4 

Comparative Electrocatalytic Oxidation and Detection of Tramadol 

on Carbon-Based Materials 

Synopsis 

This Chapter discusses the first interrogation of the possible application of 

nanodiamond-derived onion-like carbons (OLC), in comparison with conductive 

carbon black (CB), as carbon electrocatalysts for the detection of tramadol (an 

important drug of abuse). It is clearly proven that OLC exhibits excellent 

electrocatalysis towards tramadol compared to the CB counterpart. The physico-

chemical properties of OLC and CB were determined using XRD, Raman, SEM, BET 

and TGA. The OLC exhibits, amongst others, higher surface area, more surface 

defects, and higher thermal stability than CB. Theoretical calculations (DFT 

simulations) predicts that the underlying science for the high performance of OLC is 

related to its weaker surface binding of tramadol (Ead = -26.656 eV) compared to the 

CB (Ead = -40.174 eV).  

 

 

 

 

 

 



38 
 

4.1 Introduction 

The abuse of addiction drugs is one of the major global health challenges with 

tramadol (TR) regarded as one of the most abused drugs [1, 2]. TR belongs to the 

phenanthrene opium alkaloids and, albeit being a weak opioid, it is a strong pain-

reliever and regularly used in the treatment of moderate to severe pain, especially in 

post-operative care [1, 3, 4]. TR hydrochloride is marketed as a racemic mixture of 

R- and S-stereoisomers, i.e., “(1RS,2RS)-2-[(dimethylamino)methyl]-1-(3-

methoxyphenyl)-cyclohexan-1-ol hydrochloride” (Figure 4.1) [5, 6] because the two 

isomers strongly complement the analgesic properties of each other. Unlike 

conventional opioid-based analgesics, its mechanism of easing pain is somewhat 

different: it inhibits the re-uptake of norepinephrine and serotonin thereby increasing 

their release [7]. TR is an addictive drug that has become a menace in most 

countries including African countries such as Nigeria, Ghana, etc leading to wide ban 

in African countries [8, 9]. TR is regarded as relatively safe when used at the 

prescribed dosage. However, tramadol overdose or intoxication leads to high levels 

of tramadol in the blood and serious consequences such as cardiac complications 

and death [10, 11]. In addition, TR is being implicated as an environmental risk [12-

14], especially in urban water pollution. High concentration of TR is required to 

achieve the therapeutic effect in patients. However, only 65 – 70% of the TR dosage 

is metabolized and absorbed by the body, while the remaining unmetabolized drugs 

(ca. 35%) is excreted in the urine which can find its way into the surface and 

underground water reserves [12-14]. 

Based on the above health and environmental challenges of TR, there have 

been consistent efforts amongst sensor researchers on developing simpler methods 

for rapid, sensitive and selective detection of TR in its pharmaceutical formulation 



39 
 

and human biological samples. The detection of opioids in patients‟ samples involves 

the use of standard techniques, notably the high-performance liquid chromatography 

(HPLC) [15], including the HPLC integrated with diode array detectors (HPLC-DAD) 

[4], or with mass spectrometer (HPLC-MS) [3, 16], solid-state 

electrochemiluminescence [17], and solid-phase extraction with UV-vis 

spectrophotometry [18]. These HPLC-based techniques are bulky and expensive 

techniques that require high-level expertise to perform. The need for fast and 

accurate detection of TR to curb the abuse has necessitated the intense exploration 

of the electrochemical techniques as viable alternatives [13, 19-33]. Electrochemical 

methods are associated with several advantages including low-cost, rapidity of 

detection, simplicity of operation with basic training skills, and ease of miniaturization 

for point-of-care usage. 

 

 

Figure 4.1 Isomeric structures of commercial tramadol hydrochloride 

 

To date, carbon-based electrodes remain the most famous sensors for the 

detection of TR, which includes multi-walled carbon nanotubes (MWCNTs) [24], 

graphene modified with metal oxide [21], graphene oxide (GO) integrated with 



40 
 

MWCNTs [28], and carbon nanoparticles (CNP) [33]. These carbon materials have 

been reported to exhibit different sensitivity for electrocatalytic detection of TR. 

However, we are not aware of any report that provides a basic understanding on the 

possible reasons for the differences in the sensitivities.  

In this work, we investigate the application of onion-like carbon (OLC) as 

electrocatalyst for tramadol and compare its performance with a well-known 

conductive carbon black (CB) (i.e., Super C45). In 2002, Keller and colleagues [34] 

were the first to publish the use of OLC as a catalyst for the synthesis of styrene. 

There are few reports on the use of OLC in electrochemistry, especially in the field of 

energy storage,[35-39] but its use in electrocatalysis has rarely been explored.[40-

43] This work show OLC is a viable carbon catalyst for tramadol detection compared 

to the CB counterpart. Preliminary theoretical insights (DFT calculations) predict that 

the possible reason for the high electrocatalytic performance of the OLC is because 

the adsorption energy of tramadol onto the surface of OLC is weaker than on the 

surface of CB. 

4.2 Experimental section 

4.2.1 Materials, reagents and methods 

Conductive carbon black (TIMICAL SUPER C45, 45 m2/g) “was obtained from 

Gelon, China, while OLC was synthesized from high purity (98–99%) nanodiamond 

powder (NaBond Technologies) by annealing in a muffle furnace at 1300 oC for 3 h 

in an argon atmosphere. All other reagents were of analytical grade and purchased 

from Sigma-Aldrich without further purification. Human serum (product number: 

H6914) was obtained from Sigma-Aldrich / Merck, while tramadol pharmaceutical 

formulation (Austell tramadol capsule 50 mg, tramadol HCl 50 mg, AUSTELL South 



41 
 

Africa; Batch No.: MG 20567; Exp: 08/2023) was donated from a local pharmacy 

store.”  

 

4.2.2 Physical property characterization 

The powder X-ray diffraction “(PXRD) for OLC and CB were characterized using 

Bruker D2 Phaser (Cu-Kα X-rays at =1.5406 Å)” to investigate the extent of 

crystallinity. Morphology of the samples were established by “Scanning electron 

microscopy (SEM). The specific surface areas of the samples were obtained with the 

Brunauer-Emmett-Teller (BET) method using the Micromeritics TriStar II 3000 area 

and porosity analyzer instrument. Raman spectroscopy (Bruker Senterra laser 

Raman spectrometer) was used to verify the extent of graphitization of the carbon. 

bond vibration. Thermal properties were investigated with the Perkin Elmer 

Thermogravimetric analyzer (TGA) / Differential Thermogravimetric analysis (DTGA) 

6000. In a standard run, 10 mg of the samples were placed into a high-temperature 

alumina sample cup that was supported on an analytical balance located in the 

furnace chamber of the analyzer and the sample was heated in the air (5 °C min-1) 

from 35 to 900 °C. Initially, the instrument uses nitrogen gas for purging (20 mL min-

1) while holding at 35 °C for 5 min.” 

4.2.3 Electrochemical detection procedure 

Electrochemical measurements were conducted with the “SP300 Bio-Logic 

Potentiostat (running on EC-Lab software). A three-electrode configuration was 

used: glassy carbon electrode (GCE, diameter 3.0 mm, 0.071 cm2) modified with 

either OLC or CB ink as the working electrode, a platinum wire as the counter 

electrode, and a Ag|AgCl, 3 M KCl electrode as the reference electrode. Ultrapure 



42 
 

water of resistivity 18.2 MΩ cm was obtained from a Milli-Q Water System (Millipore 

Corp., Bedford, MA, USA) and used throughout for the preparation of solutions. 

Analytical grade KH2PO4 and K2HPO4 were used for the preparation of the 

phosphate buffer solutions (PBS, pH 7.4). The GCE was cleaned by proper polishing 

on a pad using alumina (Al2O3; nanopowder Aldrich) slurry followed by ultrasonic 

stirring in ethanol and acetone. The carbon ink was prepared by dispersing the 

powder (1 mg) in ethanol (1 mL) and adding 100 µL of Nafion (5 wt%) to increase the 

adhesion of the catalyst material on the GCE. The mixture was sonicated for 30 min 

to obtain a homogeneous mixture. The catalyst ink (10 µL) was then deposited on 

the GCE in a dropwise fashion and allowed to dry (Figure 4.2).  Electrochemical 

impedance spectroscopy (EIS) experiments were carried out in the frequency range 

of 100 kHz and 10 mHz at an amplitude of 10 mV. Redox probe (3 mM 

K4Fe(CN)6/K3Fe(CN)6 (1:1 mol ratio) dissolved in 0.1 M KCl) was used to determine 

the charge-transfer kinetics of the GCE-immobilized carbon catalysts. The EIS was 

performed at an equilibrium potential (E1/2) of the redox probe (0.15 V vs Ag|AgCl, 

3 M KCl) as observed from prior cyclic voltammetry experiments. For real sample 

analysis (i.e., commercial tramadol capsules and in human serum), only OLC-

modified screen-printed carbon electrodes (SPCE, DropSens) were used. The DPV 

parameters were 50 mV (pulse amplitude), 0.05 s (pulse width) and 0.2 s (pulse 

period).” 

 

 



43 
 

 

Figure 4.2 Electrode modification for tramadol electrocatalytic oxidation and sensing. 
The images are modified from the photographs obtained from electrode suppliers: 
(A) courtesy of BASi, copyright 2022; and (B) courtesy of Metrohm Dropsens, 
copyright 2022). 

“The modified electrodes (of GCE and SPE) were used to conduct the 

electrocatalysis and detection of TR drug samples (pure raw TR and pharmaceutical 

formulation). Prior to measurements of the TR, blank electrolyte scans were run 

several times until a stable background current was achieved. The DPV scans in 

PBS buffer solution were measured for several concentrations of the tramadol 

hydrochloride by successively injecting into the cell with a micropipette from a stock 

solution (0.5 nM TR). For the human serum measurements, the serum was diluted 

as 1:100 in PBS, and TR was injected from the stock solution.” 

 

4.2.4 Density functional theory (DFT) calculations 

DFT “simulations were performed at the super-computational facilities at the Centre 

for High-Performance Computing (CHPC, Cape Town, South Africa) using the 

BIOVIA Material Studio Suites and employing adsorption locator tool module. TR 



44 
 

was used as the adsorbate on both CB and OLC models. Supercells of 3 x 3 were 

modelled for carbon electrocatalysts, followed by geometric relaxation calculations 

with threshold energy set at 10-6 eV for convergence to be achieved. The modelled 

TR was cleaned using Material Studio cleaning tool, prior to its‟ adsorption. The 

minimum adsorption distance was set at 5 Å. DMoI3, another module of the BIOVIA 

Materials Studio, was used to calculate the electronic properties. The same 

threshold energy as that used for adsorption was set for the calculations of electronic 

and energy properties. Condensed-phase Optimization Molecular Potential for 

Atomistic Simulation Studies (COMPASS), forcefield was used since it guarantees 

reliable theoretical results.”   

4.3 Results and discussion 

4.3.1 Materials characterization 

Figure 4.3 shows the SEM images of (A) CB and (B) OLC, and the HRTEM image of 

the OLC. The SEM images show that CB comprises impure carbonaceous materials 

while the OLC is of high purity. The HRTEM images of the OLC clearly confirms its 

characteristic graphic concentric rings [39].  

 

 

Figure 4.3 SEM images of (A) CB and (B) OLC, and (C) HRTEM image of OLC. 



45 
 

 

Figure 4.4 compares the XRD patterns (Figure 4.4A) and Raman spectra 

(Figure 4.4B) of the two carbon materials. The XRD patterns (Figure 2A) show 

broad diffraction peak centred at 2 = 25o and a small diffraction peak at 2 = 3.2o, 

both of which are related to the (002) and (101) diffraction patterns of disordered 

carbon structure [44].  Structural information of CB and OLC was obtained with 

Raman spectroscopy (Figure 4.4B).  

 

Figure 4.4 Comparing powder XRD (A), Raman spectra (B), BET adsorption 
desorption curve, and (inset) pore size distribution curves (C), and TGA and their 
corresponding derivative TGA (D) of OLC and CB. 

The D band observed around the wavelength of 1350 cm−1 is due to the vibrations 

arising from the defect/disorder/amorphous carbon atoms (sp2-hybridized) while the 

G band observed around 1580 cm−1 is ascribed to the ordered/crystalline graphitic 



46 
 

carbon atoms (sp3-hybridized). The G‟ band observed at around wavelength 2700 

cm−1 is due to the process of two-photon elastic scattering. The intensity ratio of the 

D and G bands (ID/IG) is used to determine the extent of defects or graphitization 

presents in the carbon materials; the higher the ratio, the higher the defects (lesser 

the graphitization). In addition, the higher the ratio of the IG‟/IG, the higher the purity 

of the carbon [45-47]. The ID/IG values were estimated as 1.06 and 1.04 for the OLC 

and CB, respectively, meaning that OLC is slightly more defective than CB. The 

IG‟/IG ratio for OLC was 0.76 while that of CB was zero, suggesting that OLC is of 

high purity while CB is mostly of poor quality. The porosity of CB and OLC were 

studied using N2 adsorption-desorption measurements (BET). Specific surface area, 

pore volume and pore size are critical parameters for electrocatalysts. 

Figure 4.4C compares the nitrogen adsorption-desorption isotherms and pore size 

distribution curves of CB and OLC. Both carbon materials exhibit type IV isotherms 

with type H3 hysteresis loops, indicating that they are essentially mesoporous 

materials (i.e., 2 – 50 nm). The BET surface areas (Table 4.1) of CB and OLC were 

determined as 35.6986 and 375.3579 m2/g, respectively. 

 

Table 4.1 Surface parameters CB and OLC materials. 

 



47 
 

The specific surface area of OLC is more than 10 times higher than that of CB, 

confirming that OLC comprises much smaller nanoparticles than CB. The Barrett-

Joyner-Halenda (BJH) approach was used to assess the pore size distribution of CB 

and OLC. A continuous pore size distribution within the range of 5–100 nm is 

observed, which is nearly in direct proportion to