Electrochemical preparation and
characterization of gold nanoparticles
graphite electrode: Application to
antioxidant analysis
Guan H. Tan, Ng Khan Loon, Khor Sook Mei and Lee See Mun
Contents
 Introduction
 Literature reviews
 Objectives
 Experimental
 Results and discussion
 Conclusions
 References
Introduction
Graphite
- Good electrical conductance
- Renewable surface
- Chemical inertness
- Abundantly available (such as used batteries)
Graphite possess a honeycomb laminar structure
Alpha
Carbon
Beta
Carbon
Source from www.pixshark.com
The beta carbon of the upper layer is positioned above the cavity, therefore it possesses a free
valence electron, which can form Van der Waals interaction with metals (Appy et al., Prog. Surf. Sci., 2014 vol. 89,
pp. 219)
Introduction
Limitations with graphite
- High activation overpotential (Wring et al., Analyst, 1992, 117, pp 1215), which limits its application in electroanalysis.
-
To overcome this, the surface of the graphite could be modified with nobel metal nanoparticles such as gold and
platinum.
Anti-oxidant analysis required high oxidation potential window, instead of reduction potential
Metal
Max Oxidation
potential (at acidic
range, pH 2-5)
Silver
0.4 V
Platinum
1.0 V
Gold
1.4 V
Palladium
0.8 V
Ruthenium
0.8 V
Gold
Silver
Source : F. Campbell, R. Compton., The use of nanoparticles in electroanalysis: an updated review. Analytical and bioanalytical
chemistry, 2010, 396, Pg 241
Introduction
Simple
preparation
- 1-step preparation: Auric acid solution and
Cyclic voltammetry
- High reproducibility with simple control
parameter (deposition cycles, and Auric
acid concentration)
Rapid
Electrodeposition
- Does not require any
incubation time.
- Surface can be easily
cleaned using tape and
re-used.
Low cost
- Less chemical usage
Introduction
Anti-oxidant
 Anti-oxidant compounds are substances that can inhibit oxidation caused by
free radicals, peroxide, and oxygen.
Electro-active compound
that could be analyzed
using electrochemical
method
Electron
donating
peroxide
Anti-Oxidants
Natural
Myricetin
Quercetin
Rutin
Tocopherol
Synthetic
Propyl gallate
Butylated hydroxyanisole (BHA)
Butylated hydroxytoluene (BHT)
Tert-butylhydroquinone (TBHQ)
Introduction
Myricetin
- Abundantly available in fruits and vegetables such as grape,
tomato, cabbage and carrot ( Huang et al., Toxicology in vitro. 2010, pp
21)
- Possess anti-oxidant properties, which is significant to health:
a) anti-cancer (Shiomi et al., Food Chem., 2013, 139, pp 910)
b) therapeutic potential for diabetes mellitus( Li et al., Food science
and human wellness, 2012, pp 19)
Analytical procedure used in the analysis of myricetin in food
a) Liquid chromatography (Flores et al., Food Composition and Analysis, 2015, 39 , pp 55)
b) Gas chromatography ( Kumar et al., Analytica Chimica Acta, 2009, 631, pp 177)
Accurate and precise method but tedious sample preparation and not
possible for field analysis
Alternatively – electrochemical method could provide a rapid, and
possible for on-field analysis (screen printed electrode)
Introduction
BHA, BHT and TBHQ
 Anti-oxidants used as the additives in food
 International food safety standards, limit the usage at 200 ppm in edible
oil products, biscuits, chewing gum, cream-based products,etc.
CH3
OH
OH H3C
H3C
CH3
H3C
CH3
O
HO
H3C
Butylated hydroxytoluene (BHT)
HO
Tert - Butylhydroquinone (TBHQ)
Butylated hydroxyanisole (BHA)
Safety assessment study – high concentration level in food above 3000 ppm could promote
cancer (G. M. Williams, M. J. Iatropoulos, and J. Whysner, Food Chem. Toxicol., 1999, 37,
pp 1027)
Literature reviews
No
Analyte
Sensor type
References
1
Ascorbic acid
AOx/Au-NPs/Graphite
Dodevska et al., 2013
2
Dopamine and
uric acid
Pencil graphite
Alipour et al., 2013
3
NADH
Quercetin modified
Pencil graphite
Dilgin et al., 2013
4
Glucose
GOx/Au-NPs/Graphite
German et al., 2014
5
Lorazepam
Polypyrrole/AuNPs/pencil graphite
Rezaei et al., 2014
6
DNA interaction
Topotecan immobilized
pencil graphtie
Congur et al., 2015
AOx: ascorbic oxidase, GOx: glucose oxidase
Significance of the modified graphite electrode studies
- Renewable surface ( Ref. 2)
- Disposable sensor (Ref.3, 5 and 6)
- Improved performance – increase surface area, electrocatalytic, and
overpotential (Ref. 1,4, and 5)
Literature reviews
No
Analyte
Method
Sample
References
1
Myricetin
Abrasive stripping voltammetry
not performed
Komorsky, S.Novak, Ivana.
Electrochimica acta, 2013.
2
BHA
Linear sweep voltammetry using
gold nanoparticles-PVPgraphene-GCE
Soybean oil,
flour
Wang et al., Talanta, 2015.
3
BHA and
BHT
Linear sweep voltammetry gold
disc electrode
Mineral oil
M.Tomaskova et al., Fuel,
2014
4
TBHQ
Differential pulse voltammetry
using glassy carbon,
Mayonnaise
Goulart et al,. Fuel, 2014
5
BHA and
BHT
Carbon composite/Cu3PO4
/Polyester resin
Soybean biodiesel
K.Freitas, O. Fatibello.,
Talanta, 2010
Objectives
The objectives of this research study :
- Fabrication of a graphite electrode from a used battery and
surface modification with gold nanoparticles.
- Electrochemical and morphology study of the fabricated working
electrode to assess the electrode performance.
- Application of the surface modified working electrode in antioxidant analysis and determination in food samples.
Experimental design
Experimental
Fabrication of Au-NPs/graphite
Used battery
Graphite rod,
diameter 3mm
PTFE insulation
Surface polish with
emery paper and
alumina silicate
powder
Dry in oven
at 130 °C
1.0 mM HAuCl4
Cyclic voltammetry
(electrodeposition)
4, 8, 12, 16, 20 and 24
cycles
Cleaning –
Sonication in
UPW and ethanol
Surface Activation
(0.5 M H2SO4)
Experimental – Electrode characterization
FE-SEM
CV
- Gold nanoparticles size
- Distribution
- Element analysis by EDXrF
Au-NPs/graphite
Characterization
- Cyclic voltammetry - scan
rates study
- Ferri/ferro cyanide redox
EIS
- Nyquist Plot
- Randless-circuit
Experimental
Application of Au-NPs/graphite electrode
Method development flow
Buffer and electrolyte
optimization. Example
Britton-Robinson buffer,
Phosphate buffer,
Method validation
- LOD, LOQ, Linearity,
sample analysis
pH optimization
Electrochemical technique
- Linear sweep voltammetry
- Square wave voltammetry
Results and discussion
Results and discussions
Results and discussion
 Gold nanoparticles deposition
Constant peak current –
thermodynamic favorable
nucleation growth of gold.
Anodic scan - oxidation
Graphite
0.7109 V
8 cycles
Reduction
0.5497 V
16 cycles
Peak potential shifted toward more positive potential suggesting
a favorable deposition of the Au on the metal rather than carbon
substrate.
Results and discussion
 Activation of Au-NPs/graphite in 0.5 M H2SO4
suppression in Au oxide formation
after 20 CV scan
1st CV scan
activated
Reduction
of Au Peak
inactivated
20th CV scan
Au-NPs without activation impedes the
performance of the Au-NPs/graphite
bare
The CV of the ferri/ferro redox of
the activated Au-NPs/graphite
showed an improvement in the
overpotential (51mV).
Peak potential difference (Epa - Epc) / V, activated = 78 mV, Bare and inactivated = 183 mV
Results and discussion
Morphology evaluation with FE-SEM
A
Bare
graphite
24th Deposition cycles
D
180 nm
B
8th Deposition
cycles
75 nm
C
16th Deposition
cycles
110 nm
Au peak
Results and discussion
 Electrochemical characterization
Effective surface area (A) (Randless-Sevcik)
Heterogeneous electron transfer (HET)rate
(Laviron equation)
Electron transfer resistance
Randless Circuit fitting
(c)
Results and discussion
 Electrochemical characterization (cont.)
0.1M Ferricyanide solution
Activated AuNPs/graphite
inactivated AuNPs/graphite
Bare graphite
Nyquist Plot –
at16th deposition
cycle
Anodic potential at 0.3210 V (solid
line) shifted to 0.2698 V (dotted
line)
The overpotential of the activated Au-NPs/graphite was improved and the measured
current was much higher than the bare graphite. The peak separation between the
anodic and cathodic is much closer to the theoretical 59.16 mV (Nernst equation).
Results and discussion
 Application to anti-oxidant analysis – myricetin
Method: Square wave voltammetry (SWV)
Electrolyte: Britton-Robinson Buffer 0.1M
Highest peak current at pH 2
At 0.0591, n =1
Nernst slope
Results and discussion
From Nernst equation, it suggests an equal ratio of electron to proton
transfer, i.e. n=1.
A
B
3 oxidation
potential
Oxidation mechanism at peak , 0.4 V
The first oxidation occurs at the 2nd hydroxyl group of pyrogallol group also reported by Goncalo et.
al and Komorsky et al.
C. Goncalo et. al. J. Mol. Chem., 2010, 16, 863
S. Komorsky-Lovrić et. al.,Electrochim. Acta, 2013, 98, 53.
Results and discussion
Application to anti-oxidant analysis – myricetin
Au-NPs /graphite
Bare graphite
Au-NPs /graphite
Au-NPs/graphite - Improvement in
the sensitivity of myricetin
analysis.
Bare graphite
Results and discussion
SWV of myricetin with concentration increment
corresponding to 0.2, 0.4, 0.6, 0.8 and 1.0 µg mL-1 .
The detection limit (LOD)of myricetin = 0.4 µg mL-1
The limit of quantitation (LOQ) was calculated based
on 10 times the standard deviation of LOD (n=5).
The LOQ of myricetin = 0.8 µg mL-1
Results and discussion
 To test the method accuracy and precision in sample analysis, solutions of myricetin
in ethanol were prepared at 0.8 and 1.0 µg mL-1 ( n=5)
Concentration /
µg mL-1
SWV Analysis /
µg mL-1
Standard error /
µg mL-1, (p=0.05)
Recovery (%)
0.80
0.81
0.03
98.31
1.00
1.00
0.03
99.32
Analysis in green tea samples (n =2)
Sample
Green tea
Myricetin /mg Kg-1
16.9
RSD / %
4.33
Other Anti-Oxidants in Food
TBHQ (Tertiary Butyl Hydroquinone)
BHA (Butylated Hydroxy Anisole)
BHT (Butylated Hydroxy Toluene)
Results and Discussion
BHA
Au-NPs/Graphite
Bare graphite
TBHQ
BHT
Results and Discussion
Linear sweep voltammetry (LSV) analysis of TBHQ,
BHA and BHT standards
(a)
Linear correlation of peak current
against concentration
(b)
BHA
BHA
BHT
64 µg mL-1
TBHQ
4 µg mL-1
TBHQ
Blank
BHT
Results and Discussion
TBHQ , mgKg-1
BHA, mg Kg-1
BHT, mg Kg-1
Sample
Result
-
Std error
-
Result
55.2
140.8
-
Std error
2.3
3.2
-
Result
-
Std error
-
Sunflower Oil
Biscuit
Corn Oil
33.6
190.3
-
1.2
8.3
-
-
-
-
-
Salad Dressing
-
-
105.0
3.0
-
-
Margarine
Ghee
Mayonaise
Peanut Butter
Analysis of TBHQ, BHA and BHT in food samples using linear sweep
voltammetry and Au-NPs/graphite working electrode. (n =5)
All within the allowed limits of 200 mg/kg
Conclusions
 In this study a gold nanoparticles graphite electrode was successfully
fabricated from a used battery graphite.
 The electrochemical and morphology characterization showed
improvement in the effective surface area, overpotential and
heterogeneous electron transfer rate of the Au-NPs/graphite electrode.
 It can be inferred that at the 16th deposition cycle the Au-NPs/graphite
electrode reached an optimum performance.
 The Au-NPs/graphite was successfully applied in the myricetin analysis
using square wave voltammetry. The electrode sensitivity was improved
by 2.5 fold when compared to the bare graphite. The LOD and LOQ
were determined at 1.26 x 10-6 mol L-1 and 2.51 x 10-6 mol L-1
 The 3 anti-oxidants can be simultaneously analyzed using the AuNPs/graphite electrode. It was successfully applied in the determination
of TBHQ, BHA, and BHT in some food samples.
Acknowledgment
 This work was financially supported by :
- The University of Malaya Research Grant (UMRG-Programme RP012C-14SUS/PG1772014B),
- Fundamental Research Grant Scheme (FRGS) from the Ministry of Higher Education of
Malaysia (MOHE) FP014-2013A and FP058-2014A.
Publication
This work has been accepted by the journal of Analytical Sciences:
Khan Loon Ng, See Mun Lee, Sook Mei Khor, Guan Huat Tan. 2015. Electrochemical
preparation and characterization of gold nanoparticles graphite electrode: Application to
myricetin antioxidant analysis. Analytical Sciences. Accepted for publication. (ISI-Cited
Publication)
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