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

IntroductionGraphite-Good electrical conductance -Renewable surface -Chemical inertness -Abundantly available (such as used batteries)

Source from www.pixshark.com

Alpha Carbon

Beta Carbon

Graphite possess a honeycomb laminar structure

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.

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

Source : F. Campbell, R. Compton., The use of nanoparticles in electroanalysis: an updated review. Analytical and bioanalytical chemistry, 2010, 396, Pg 241

Anti-oxidant analysis required high oxidation potential window, instead of reduction potential

GoldSilver

Introduction

- 1-step preparation: Auric acid solution and Cyclic voltammetry- High reproducibility with simple control parameter (deposition cycles, and Auric acid concentration)

- Less chemical usage

- Does not require any incubation time.

- Surface can be easily cleaned using tape and re-used.

Anti-oxidant Anti-oxidant compounds are substances that can inhibit oxidation

caused by free radicals, peroxide, and oxygen.

Electron donating

Electro-active compound that could be analyzed using electrochemical method

Anti-Oxidants peroxide

Synthetic Natural

MyricetinQuercetinRutinTocopherol

Propyl gallateButylated 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

CH3

CH3

CH3

CH3

CH3

CH3

OH

Butylated hydroxytoluene (BHT) OH

O

Butylated hydroxyanisole (BHA)

OH

OH

Tert - Butylhydroquinone (TBHQ)

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)

Introduction

Literature reviewsNo 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/Au-NPs/pencil graphite

Rezaei et al., 2014

6 DNA interaction Topotecan immobilized pencil graphtie

Congur et al., 2015

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)

AOx: ascorbic oxidase, GOx: glucose oxidase

Literature reviewsNo 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-PVP-graphene-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 resinSoybean bio-diesel

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 anti-oxidant analysis and determination in food samples.

Experimental design

ExperimentalFabrication of Au-NPs/graphite

Cleaning – Sonication in UPW and ethanol

Dry in oven at 130 °C

Cyclic voltammetry (electrodeposition)

Surface Activation (0.5 M H2SO4)

Graphite rod, diameter 3mm

PTFE insulation

Surface polish with emery paper and alumina silicate powder

Used battery

1.0 mM HAuCl4

4, 8, 12, 16, 20 and 24 cycles

Experimental – Electrode characterization

- Gold nanoparticles size - Distribution - Element analysis by EDXrF

- Cyclic voltammetry - scan rates study

- Ferri/ferro cyanide redox - Nyquist Plot- Randless-circuit

Buffer and electrolyte optimization. Example Britton-Robinson buffer, Phosphate buffer,

pH optimization

Electrochemical technique -Linear sweep voltammetry-Square wave voltammetry

Method validation - LOD, LOQ, Linearity,

sample analysis

ExperimentalApplication of Au-NPs/graphite electrode

Method development flow

Results and discussionResults and discussions

Results and discussion Gold nanoparticles deposition

0.5497 V

Anodic scan - oxidation

Reduction

Constant peak current – thermodynamic favorable nucleation growth of gold.

Peak potential shifted toward more positive potential suggesting a favorable deposition of the Au on the metal rather than carbon substrate.

0.7109 V

Graphite

8 cycles

16 cycles

Activation of Au-NPs/graphite in 0.5 M H2SO4

suppression in Au oxide formation after 20 CV scan

1st CV scan

20th CV scanReduction of Au Peak

Au-NPs without activation impedes the performance of the Au-NPs/graphite

The CV of the ferri/ferro redox of the activated Au-NPs/graphite showed an improvement in the overpotential (51mV).

activatedinactivated

bare

Peak potential difference (Epa - Epc) / V, activated = 78 mV, Bare and inactivated = 183 mV

Results and discussion

Morphology evaluation with FE-SEM

Bare graphite

8th Deposition cycles

16th Deposition cycles

24th Deposition cyclesA

B

C

D

75 nm

110 nm

180 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.)

Activated Au-NPs/graphite

Bare graphite

inactivated Au-NPs/graphite

Nyquist Plot – at16th deposition cycleThe 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).

Anodic potential at 0.3210 V (solid line) shifted to 0.2698 V (dotted line)

0.1M Ferricyanide solution

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

Results and discussion

From Nernst equation, it suggests an equal ratio of electron to proton transfer, i.e. n=1.

3 oxidation potential

BA

Oxidation mechanism at peak , 0.4 V

C. Goncalo et. al. J. Mol. Chem., 2010, 16, 863S. Komorsky-Lovrić et. al.,Electrochim. Acta, 2013, 98, 53.

The first oxidation occurs at the 2nd hydroxyl group of pyrogallol group also reported by Goncalo et. al and Komorsky et al.

Application to anti-oxidant analysis – myricetin

Au-NPs /graphite

Bare graphite

Au-NPs/graphite - Improvement in the sensitivity of myricetin analysis.

Au-NPs /graphite

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

Results and discussion

Sample Myricetin /mg Kg-1 RSD / %

Green tea 16.9 4.33

Analysis in green tea samples (n =2)

Other Anti-Oxidants in Food

TBHQ (Tertiary Butyl Hydroquinone) BHA (Butylated Hydroxy Anisole) BHT (Butylated Hydroxy Toluene)

Results and Discussion

TBHQ

BHABHT

Au-NPs/Graphite

Bare graphite

Results and Discussion

BHA

TBHQ

BHT

(b)

64 µg mL-1

4 µg mL-1

(a) BHA

TBHQ

BHT

Blank

Linear sweep voltammetry (LSV) analysis of TBHQ, BHA and BHT standards

Linear correlation of peak current against concentration

SampleTBHQ , mgKg-1 BHA, mg Kg-1 BHT, mg Kg-1

Result Std error Result Std error Result Std errorMargarine - - 55.2 2.3 - -Ghee - - 140.8 3.2 - -Mayonaise - - - - - -

Peanut Butter 33.6 1.2 - - - -Sunflower Oil 190.3 8.3 - - - -Biscuit - - - - - -Corn Oil - - - - - -

Salad Dressing - - 105.0 3.0 - -

Results and Discussion

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 Au-NPs/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/PG177-2014B),

- 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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