Novel composite electrodes:Preparation and application to the electroanalytical study of two...

by TCHIENO MELATAGUIA Francis Merlin

POSTGRADUATE SCHOOL

DEPARTMENT OF CHEMISTRY

UNIVERSITY OF DSCHANG

DSCHANG SCHOOL OF SCIENCES AND TECHNOLOGY

LABORATORY OF NOXIOUS CHEMISTRY AND ENVIRONMENTAL ENGINEERING

Doctorat/PhD Defence in Chemistry

The PresidentEmmanuel NGAMENI Professor Univ. of Dschang The RapporteurIgnas TONLE KENFACK Ass. Professor Univ. of DschangThe ExaminersAchille NASSI Ass. Professor Univ. of DoualaEmmanuel DJOUFAC W. Ass. Professor Univ. of Yaoundé I ANAGHO Solomon G. Ass. Professor Univ. of Dschang

Novel composite electrodes: Preparation and application to the electroanalytical study of two pharmaceutically active study of two pharmaceutically active

molecules, viz. mangiferin and quercetin

[email protected]

Presentation outline

1. Introduction

Motivations and objectives

Xanthones and mangiferin

2. Brief literature review

1

3. Experimental procedures

4. Results and discussion

Conclusion and perspectives

Flavonoids and quercetin

Composite electrodes

1. Introduction : motivations

African drug market fast growing;

Rapid population growth;

People in dire need of medicationsPeople in dire need of medicationsespecially from their immediateenvironment;

Much criticism has been laid on indigenous drugs;

2


Lack of knowledge on their therapeutic and toxic effects;

Quantification of bioactive molecules can Quantification of bioactive molecules can improve pharmacotherapy;

Necessity to investigate the ingredientsof indigenous drugs.

3


For such investigations, a number oftechniques are available

Electrochemical methods and sensorsare the most attractive and suitable forelectroactive species:electroactive species:

Simplicity, Low cost, Rapid response time, High sensitivity, Selectivity.

4

1. Introduction : objectives

Our work therefore had as objectives to:

elaborate and characterise new compositematerials using electrochemical and physico-chemical techniques;

use these materials as electrode modifiers tobuild amperometric sensors;

5

apply the obtained electrodes to theelectroanalytical study of some pharmaceuticallyactive molecules.


MG and its therapeutic activitiesO

O

O

OH

HO

HO

OH

OH

OH

HO

1

2

34 5

6

7

81'

2'

3'

4' 5'6'

Xanthones and mangiferin (MG): description

Scheme 1: Molecular structure of MG

HO3'

antioxidant anti-inflammatory anti-diabetic

activities have been reported for MG(Wauthoz et al. Inter. J. Biomed. Pharm. Sci. 1(2) (2007) 112-119) 6

anti-allergic, anti-tumor, antimicrobial

Flavonoids and quercetin (QCT): description

QCT and its therapeutic activities

O

OH

OH

HO

OH

A

B

C

1'

2'

3'

4'

5'

6'

1

2

345

6

7

8


OOH

45

Scheme 2: The molecular structure of QCT

anti-inflammatory,anti-bacterial,anti-gastric ulcer,

7

anti-cancer, anti-diabetic, anti-tumor.

(Havsteen (1983) Biochemical Pharmacology, 32(7), 1141-1148.

Izzo (1994) Phytotherapy Research, 8, 179-185)

Composite electrodes: types

Type 1: Carbon paste electrodes (CPEs)Type 1: Carbon paste electrodes (CPEs)

(Carbonaceous material + Binder + Modifier)

Advantages:


Advantages:

Ease of renewal Chemical inertness Low cost Wide potential window Flexible (electrodes of desired composition)

8

Drawbacks:


Type 1: Carbon paste electrodes (CPEs)Type 1: Carbon paste electrodes (CPEs)

(Carbonaceous material + Binder + Modifier)


Drawbacks:

Experience of the user determines success,

Regular removal of the electrode’s surface layer,

Poor diffusion of analyte (organic binder).

9

Deposited or fixed chemical species with specific properties on solid electrodes (Pt, GC, SnO2).

Type 2: Film modified electrodes (FMEs)Type 2: Film modified electrodes (FMEs)



• Drop-coating,

• Spin-coating,

• Electrodeposition,

10• Self-assemble layers.


Attapulgite clayAttapulgite clay

Natural crystalline hydrated magnesium aluminium silicate

Zeolite-like channels with cross-sectional dimensions 3.7 Å x 6.0 Å(Frost & Ding, 2003, Thermochimica Acta, 397, 119-128)

Scheme 3: Ideal structure of attapulgite (Bradley, 1940, American Mineralogist, 25, 405-410).

11

low cost,

high chemical stability,


Attapulgite clayAttapulgite clay

high adsorption and penetrability due to formation ofwell-ordered coatings with large surface area on electrodesurfaces (Chen et al., 2011, Talanta, 86, 266-270)

ion-exchange properties,

12

3. Experimental setup and procedures

Preparation of organoattapulgite clay

Purification of the attapulgite sample

Fine attapulgite fractions obtained by sedimentation based on Stokes’ law

13

≤ 10 µm attapulgite particles were siphoned at x = 20 cm after 33 min

2p fgd - x

V18 t

Preparation of organoattapulgite clay

Grafting procedure

2 g Attapulgite

15 mLToluene

+4 mL

[3-(2-aminoethylamino)propyl]trimethoxysilane

+


trimethoxysilane

N2 atm 3 h reflux

Washing with Toluene and Isopropanol

Drying at 100 °C for 14 h

AttaNHAttaNH22 14

Characterisation of the grafted clay

Scanning electron microscopy (SEM)

Elemental analysis (EA)


X-ray diffraction (XRD) analysis

Fourier transform infrared (FTIR) spectroscopy15

Preparation of working electrodes

Organoattapulgite film modified GCE

10 mg AttaNH2 1 mL H2O+

Ultrasonication


10 mg AttaNH2 1 mL HCl (pH1)+

Ultrasonication

AttaNH2 suspension

5 min drying (110 °C)

GCE/AttaNH2

Deposition on GCE

6 µL

16

5 min drying (110 °C)

GCE/ AttaNH3+

Deposition on GCE

6 µL

AttaNH3+ suspension


Activated chitosan CPE

Silicone oil (pasting liquid)

Graphitepowder

Chitosanpowder

+ +


Activated CPE-CHI

Final paste filled into Teflon tube

Homogenisation

Activation 5 cyclic voltametricscans in 0.5 M HCl

17


Why chitosan?


Film forming ability,

Biocompatibility,

Good adhesion,

Susceptibility to chemical modification.

Biocompatibility,

(Martinez-Huitle et al., 2010, Portugaliae Electrochimica Acta, 28(1), 39-49) 18


1-ethylpyridinium bromide/CPE

Silicone oil (pasting liquid)

Graphitepowder

1-Ethylpyridiniumbromide

+ +


Final paste filled into Teflon tube

Homogenisation

CPE-EPB 19


Why an ionic liquid as modifier?


Creation of a variety of interactions(hydrogen bonds, dipole-dipole, electrostatic),

20

(hydrogen bonds, dipole-dipole, electrostatic),

Presence increase rate of electron transferby decreasing the overpotential

(Maleki et al., 2007, Analytical Biochemistry, 369(2), 149-153.Safavi et al., 2008, Electrochemistry Communications, 10, 420-423)

Electroanalytical techniques used

Electrochemical impedance spectroscopy (EIS)

Cyclic voltammetry (CV)


Cyclic voltammetry (CV)

Differential pulse voltammetry (DPV)

Chronocoulometry

21

4. Results and discussion: findings

CV analysis

1

(a)

2 µA

Cu

rren

t

(b)

2 A

Cu

rren

t

Electrochemical study of MG at the activated CPE-CHI

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

43

2

1

Cu

rren

t

Potential (V) vs Ag/AgCl

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

(i)

(ii)

Cu

rren

t Potential (V) vs Ag/AgCl

Figure 1: (a) CV response of 20.4 µM MG in PB (pH 5) at CPE-CHI (3%): (1) scan 1, (2) scan 2, (3) scan 3 and (4) blank. (b) Comparison of CV responses

at (i) bare CPE and (ii) activated CPE-CHI (3%). v = 75 mV/s. 22

Optimisation of experimental parameters by DPV



Effect of accumulation time and electrolysis potential

4.0

4.5 (a)

6

7(b)

Pea

k c

urr

ent

(µA

)

23

0 50 100 150 200 250 3001.0

1.5

2.0

2.5

3.0

3.5

4.0

Pea

k c

urr

ent

(µA

)

Accumulation time (s)

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

3

4

5

6

Pea

k c

urr

ent

(µA

)

Electrolysis potential (V)

Figure 2: Dependence of current response at CPE-CHI (3%) for 10.3 µM MG in 0.1 M PB (pH 5) on (a) accumulation time and (b) electrolysis potential.

240 s - 0.1 V

Effect of pH of supporting electrolyte

(a)

Cu

rren

t

5 A

1.0

0.6

0.7 (b)

Pea

k p

ote

nti

al

(V)

Optimisation of experimental parameters by DPV



0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Potential(V) vs Ag/AgCl

Cu

rren

t

6.0

3.1 2.0

4.2

5.1

1 2 3 4 5 60.3

0.4

0.5

Pea

k p

ote

nti

al

(V)

pH of supporting electrolyte

Figure 3: (a) Effect of detection medium pH on the anodic peak current of MG, at CPE-CHI (3%). (b) Variation of the peak

Potential as a function of pH of the detection medium. 24

Calibration Curve and limit of detection

(a)20 µA

Cu

rren

t

30

40

50

(b)

Pea

k c

urr

ent

(µA

)



0.4 0.5 0.6 0.7 0.8 0.9

p

a

Cu

rren

t


0 10 20 30 40 50 60 700

10

20

Pea

k c

urr

ent

(µA

)[Mangiferin] (µM)

Figure 4: (a) DPV curves obtained under optimised conditions in HCl/KCl buffer (pH 1) at CPE-CHI (3%) for various concentrations of MG (a-p): 0 - 67.4 µM MG.

(b) Corresponding calibration graph of MG. 25

LOD: 1.84 µM

Calibration Curve and limit of detection in human urine

(a)

6 µA

Cu

rren

t

20

25 (b)

Pea

k c

urr

ent

(µA

)



0.48 0.54 0.60 0.66 0.72 0.78

h

a

Cu

rren

t

Potential (V) vs Ag/AgCl0 10 20 30 40 50 60

5

10

15

Pea

k c

urr

ent

(µA

)

[Mangiferin] (µM)

Figure 5: (a) DPV curves obtained under optimised conditions in HCl/KCl buffer (pH 1)/human urine mixture at CPE-CHI (3%) for various concentrations of MG

(a-h): 0 - 59.5 μM MG. (b) Corresponding calibration graph of MG. 26

LOD: 2.98 µM

Interference of some organic moleculesTable 1: Influence of some organic compounds on the determination of

10.1 µM MG

Interfering organic

species

Concentration

(µM)

Increase in peak

current of MG



species (µM) current of MG

Ascorbic acid 51.1 ≈ 19 %

Uric acid 51.1 ≈ 30 %

Dopamine 51.1 ≈ 63 %

L-Aspartic acid 51.1 ≈ -2 %

Citric acid 51.1 ≈ 1 %

D-(+)-Glucose 51.1 ≈ 18 % 27

SEM micrographs of attapulgite and AttaNH2


Physico-chemical characterisation of amine-grafted attapulgite

Figure 6: SEM micrographs of Attapulgite (a and b) and AttaNH2 (c and d). 28

Elemental analysis

Elemental analysis: 13.40% C, 3.60% H and 5.55% N

Theoretical content: C (13.71%), H (4.40%) and N (4.57%)



OH

OH

Si NH

NH2

OCH3

OCH3

H3CO+ 2

Att

apu

lgit

e

Reflux

Toluene

Si NH

NH2

OCH3

OCH3

O

Att

apu

lgit

e

Si NH

NH2

OCH3

OCH3

O

+ 2 CH3OH

Scheme 4: Aminoattapulgite grafting process.

Si8O20Mg5(OH2)4•4H2O[OSi(OCH3)2(CH2)3NH(CH2)2NH2]2

29

X-ray diffractograms of attapulgite and AttaNH2

AttaNH2

(b)

Inte

nsi

ty (

a. u

.)



4 8 12 16 20 24

(a)

Inte

nsi

ty (

a. u

.)

2 (°)

Attapulgite

Figure 7: X-ray powder diffraction patterns of (a) pristineattapulgite and (b) amine-grafted attapulgite. 30

FTIR spectra of attapulgite and AttaNH2

AssignmentWavenumber (cm-1)

Attapulgite AttaNH2

ν (Mg)3-OH 3613 3615

νas physisorbed

H2O

3545 3535

νas C-H - 2929

νs C-H or ν O-CH3 - 2851

(b)425

469

974

1361

14722851 2929

Ab

sorb

ance

(a

.u)



Figure 8: FTIR spectra of (a) pristine attapulgite and (b) amine-grafted attapulgite.

δ physisorbed H2O 1650 1651

δas C-H - 1472

δs C-H - 1361

ν Si-O-Si

1193

1030

974

1190

1125

978

δ (Mg)3-OH 911 905

δ Si-O 507

469

503

438

ν Si-O-Mg 425 423

31

500 1000 1500 2000 2500 3000 3500 4000

Wavenumber (cm-1)

(a)

507

911

1030

1193 1650 3545 3613

Ab

sorb

ance

(a

.u)

Electrochemical characterisation by EIS

8

10

12

14

16

(a))



0 5 10 15 20 25 30 35 40

0

2

4

6

8

(c)(b)

-Z''

(

)

Z' ()

Figure 9: Nyquist plot of the EIS data obtained at (a) bare GCE, (b) GCE/AttaNH2

and (c) GCE/AttaNH3+ in 0.2 M KCl containing 0.1 mM [Fe(CN)6]

3-/4-. 32

EIS data fitting

Z' (Ω)-Z

" (

Ω)

(a)

bare GCE


(b)

Z' (Ω)

-Z"

(Ω

)

Z' (Ω)

Z' (Ω)

-Z"

(Ω

)

(c)

GCE/AttaNH2 GCE/AttaNH3+

33Figure 10: Experimental ( ) and fitted ( ) EIS spectra for (a) bare GCE, (b) GCE/AttaNH2 and (c) GCE/AttaNH3

+.

EIS data fitting

Rct W

RΩ

Cdl


EIS characterisation of amine-grafted attapulgite

Electrode RΩ (Ω) Rct (Ω) Cdl (µF)

Bare GCE 537 14063 0.27

GCE/AttaNH2 183 4892 2.06

GCE/AttaNH3+ 272 3785 5.32

Table 2: Circuit parameter values from the EIS experimental datarecorded at bare GCE, GCE/AttaNH2 and GCE/AttaNH3

+.

Scheme 5: Randles equivalent electrical circuit of an electrochemical cell for a simple electrode process.

34

Electrochemical characterisation by permeation studies

(a)

1 µA

Cu

rren

t

(b)

30

1

2 µA

Cu

rren

t


Electrochemical characterisation of amine-grafted attapulgite

Figure 11: Multisweep cyclic voltammograms recorded in 0.2 M KCl containing0.1 mM [Fe(CN)6]

3- on GCE coated with (a) AttaNH2. Inset shows response at pristine attapulgite. (b) AttaNH3

+. 35

0.0 0.2 0.4 0.6

Cu

rren

t


-0.2 0.0 0.2 0.4 0.6

1 µA

Cu

rren

t


0.0 0.1 0.2 0.3 0.4 0.5 0.6

1

Cu

rren

tPotential (V) vs Ag/AgCl

CV response

(iii)

4 µA

Cu

rre

nt

Electrochemical quantification of MG at the grafted attapulgite film modified GCE

aα n FlogI = constant + E

2.3RT

Tafel equation

Tafel region


-0.4 0.0 0.4 0.8 1.2

(ii)

(iii)

(i)

Cu

rre

nt


Figure 12: (a) Cyclic voltammograms recorded at(i) blank , (ii) bare GCE and (iii) GCE/AttaNH3

+ with78.1 µM MG added. Potential scan rate was 75 mV/s. 36

2.3RT

GCE/AttaNH3+ αanα = 0.56

bare GCE αanα = 0.37

αa = 0.56

Tafel region

CV response: successive scans

5 AC

urr

ent



-0.4 0.0 0.4 0.8 1.2

3

2

1

Cu

rren

t


Figure 13: CV response of 40 µM MG in HCl/KCl (pH 1) at GC/AttaNH3+:

(1) scan 1, (2) scan 2 and (3) scan 3. Potential scan rate 75 mV/s. 37

m

a

15 µAC

urr

ent


CV response: variation of scan rate


0.0 0.3 0.6 0.9 1.2 1.5

Cu

rren

t


Figure 14: Cyclic voltammograms of 200 µMMG at different scan rates (a-m):15 - 300 mV/s. 38



Laviron equation


0.84

0.88

Ag/A

gC

l

Figure 15: The plot of Epa against Inѵ.

αa = 0.58

a a a

0 spa

RT RTk RT= + ln + lnE E

α n F α n F α n F

39

Average αa = 0.573.0 3.6 4.2 4.8 5.4 6.0

0.72

0.76

0.80

Ep

a (

V) vs

Ag/A

gC

l

In (v/mVs-1)




8

10

12

Randles-Sevcik equation

0.1 0.2 0.3 0.4 0.5 0.6

2

4

6

I p (

µA

)

1/2

((V/s)1/2

)

Figure 16: The plot of Ip against ѵ1/2. 40

n = 2.11 ≈ 2

)paI1/25 1/2α n= )n( ACD(2.99x10 a 1/2

Electrochemical oxidation mechanism of MG

O OHHO O OHO



O

Glu OH

OH O

Glu O

OH

- 2e-

- 2H+

Scheme 6: Proposed electrochemical oxidation mechanism of MG.

41

DPV response

0.3 A

Cu

rren

t

(a)

0.8

1.0

1.2

1.4

Pea

k c

urr

ent

(µA

)

(b)



0.5 0.6 0.7 0.8 0.9

a

h

Cu

rren

t


0 2 4 6 8 10 12

0.2

0.4

0.6

0.8

Pea

k c

urr

ent

(µA

)

[Mangiferin] (µM)

Figure 17: (a) DPV curves obtained in HCl/KCl buffer (pH 1) using GCE/AttaNH3+

(a-h): 0 - 10.57 µM MG. (b) The corresponding calibration curve obtained .

LOD: 0.275 µM MG

42

Chronocoulometric studies

Effective surface areas of GCE and GCE/AttaNH3+

bare GCE: 0.125 cm2

GCE/AttaNH + : 0.223 cm2

1/2 1/2

c ads1/2

2nFAD CtQ = + +Q Q

π



GCE/AttaNH3+ : 0.223 cm2

Anson equation

π

43

D = 2.18 x 10-5 cm2/s Diffusion coefficient of MG

Monolayer adsorption (Inzelt (2010) Chronocoulometry. In: Scholz F. (Ed.), Electroanalytical methods: Guide to experiments and applications, 2nd

edition, Springer-Verlag, Berlin, Germany)

Γ = 1.81 x 10-11 mol/cm2

Γ = 1.09 x 1013 MG molecules/cm2

adsQΓ =

nFA

Amount of adsorbed MG

Interference study

0.3

0.4

0.5

0.6

Sel

ecti

vit

y c

oef

fici

ent

(Kam

p)



Citr

ic a

cid

Uri

c aci

d

Dop

amin

e

Asc

orbic

aci

d

L-Asp

artic

aci

d

D-(+

)-Glu

cose

0.0

0.1

0.2

0.3

Sel

ecti

vit

y c

oef

fici

ent

(K

Figure 18: Selectivity coefficients for some potential interferents in the presence of MG. 44

Application to a real sample

Table 3: Determination of MG in human urine samples.

Sample Urine dilution MG added (µM) MG found (µM) Mean recovery (%)



Sample Urine dilution MG added (µM) MG found (µM) Mean recovery (%)

1 x 100 5.70 6.72 117.9

2 x 200 5.70 5.98 104.9

3 x 300 5.70 5.84 102.5

4 x 300 8.49 8.55 100.7

45

EIS characterisation

1.2

1.5

1.8 (a)

0.5

0.6

0.7(b)

Electrocatalytic sensing of QCT at the 1-ethylpyridinium bromide CPE


0.0 0.5 1.0 1.5 2.0 2.5

0.3

0.6

0.9

1.2

-Z''(k

)

Z'(k)

0.9 1.0 1.1 1.2 1.3 1.4 1.5

0.1

0.2

0.3

0.4

-Z''(k

)

Z'(k)

Figure 19: Nyquist plot of the EIS data obtained at (a) bare CPE and (b) CPE-EPB (10%) in 0.2 M KCl containing 0.1 mM [Fe(CN)6]

3-/4-. 46

EIS data fitting

(a) (b)



Figure 20: Experimental ( ) and fitted ( ) EIS spectra for (a) bare CPE and (b) CPE-EPB (10%).

Z' (Ω)

-Z"

(Ω)

Z' (Ω)

-Z"

(Ω)

47

EIS data fitting

Rct W

RΩ

Cdl



Scheme 7: Randles equivalent electrical circuit of an electrochemical cell for a simple electrode process.

Table 4: Circuit parameter values from the EIS experimental data recorded at bare CPE and CPE-EPB (10%).

Electrode RΩ (Ω) Rct (Ω) Cdl (µF)

Bare CPE 120 1729 0.16

CPE-EPB (10%) 90 885 1.01

48

Electrochemical oxidation of QCT

30 µA

(b)

2

1


OH

(a)

15 µA


-0.2 0.0 0.2 0.4 0.6 0.8 1.0

(i)

(ii)3

Cu

rren

t


Figure 21: (a) Cyclic voltammetric response in HCl/KCl (pH 1) on bare CPE. (b) DPV at (i) bare CPE and (ii) CPE-EPB (10%) in 40.6 μM QCT. 49

O

O

OH

OH

HO

OH

A

B

C

1'

2'

3'

4'

5'

6'

1

2

345

6

7

8

0.0 0.2 0.4 0.6 0.8 1.0 1.2

(ii)

(i) 3

1 2

Cu

rren

t


Variation of scan rate

(a) 2

1100 µA

Cu

rren

tElectrocatalytic sensing of QCT at the 1-ethylpyridinium bromide CPE

0.70

0.77

0.84(b) 2

(V)

vs A

g/A

gC

l


-0.2 0.0 0.2 0.4 0.6 0.8 1.0

Cu

rren

t


Figure 22: Cyclic voltammograms of 100 µM QCT at CPE-EPB (10%) in PB pH 3 at different scan rates (a-g): 50 - 500 mV/s. (b) Corresponding Ep against lnv plots.

n1 = 2.6 ≈ 3 and n2 = 1.3 ≈ 1

50

3.5 4.0 4.5 5.0 5.5 6.0 6.5

0.49

0.56

0.63

0.70

1Ep

a(V)

vs A

g/A

gC

lln(v/mVs

-1)

Optimisation of experimental conditions by DPV



Optimisation of the modified CPE composition

30

35P

eak

cu

rren

t (µ

A)

10% EPB

0 5 10 15 205

10

15

20

25

Pea

k c

urr

ent

(µA

)

Mass of EPB (mg)

Figure 23: Variation of peak currents of 20.7 μM QCT with mass of EPB in 100 mg paste. The DPV curves were recorded in 0.1 M PB (pH 3). 51




Effect of pH

50 µA 2.11

1.08(a)

50

60

0.4

0.5

0.6

Pea

k c

urr

ent

(µA

)

(c)

Peak

poten

tial (V

)

pH 3.08

-0.30 -0.15 0.00 0.15 0.30 0.45 0.60

8.00

6.986.00

5.03

3.08

4.05

9.04

Cu

rren

t


0 2 4 6 8 1010

20

30

40

0.0

0.1

0.2

0.3

0.4

Pea

k c

urr

ent

(µA

)

pH of supporting electrolyte

(b)

Peak

poten

tial (V

)

Figure 24: (a) Effect of detection medium pH (1.08 - 9.04) on the anodic peak position of QCT, at CPE-EPB (10%) in 20.7 μM QCT. (b) Variation of the peak current as a function of pH of the detection medium. (c) Peak potential as a function of the pH of the detection medium. 52




Effect of preconcentration time and electrolysis potential

65

70

75(a)

Pea

k c

urr

ent

(µA

) 50

60 (b)

Pea

k c

urr

ent

(µA

)

0 30 60 90 120 150 180

45

50

55

60

65

Pea

k c

urr

ent

(µA

)

Preconcentration time (s)

-0.6 -0.5 -0.4 -0.3 -0.2

20

30

40

Pea

k c

urr

ent

(µA

)

Electrolysis potential (V)

Figure 25: Dependence of current response at CPE-EPB (10%) in 0.1 M PB (pH 3) on (a) preconcentration time in 0.248 μM QCT and (b) electrolysis potential in 20.7 μM QCT. 53

- 0.3 V130 s

Variation of QCT Concentration and Calibration Graph

Cu

rren

t

(a)

25 A80

100

120

140(b)

Cu

rren

t (µ

A)



0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

h

a

Cu

rren

t


0 1 2 3 4 5 6 7 8

0

20

40

60

80

Cu

rren

t (µ

A)

[Quercetin] (µM)

Figure 26: (a) DPV curves obtained under optimised conditions in 0.1 M PB pH 3 at CPE-EPB (10%)for various concentrations of QCT (a-h): 0 - 7.434 µM QCT. (b) The corresponding calibration graph.

LOD: 0.0448 µM QCT

54


Chronocoulometry

bare CPE: 0.1984 cm2

CPE-EPB (10%): 3.962 cm2

Electrochemical effective surface area

1/2 1/2

c ads1/2

2nFAD CtQ = + +Q Q

π


CPE-EPB (10%): 3.962 cm2Anson equation

π

55

Diffusion coefficient of QCT D = 4.52 x 10-5 cm2/s

Molecular surface coverage

adsQΓ =

nFA

Γ = 6.95 x 10-11 mol/cm2

Γ = 4.18 x 1013 QCT molecules/cm2

Monolayer adsorption

Interferences

Table 5: Effect of some interfering organic species on the signal of 20.7 µM QCT.

Interfering organic

species

Concentration

(µM)

Increase in peak current

of QCT



Ascorbic acid 21.0 ≈ 6 %

Uric acid 21.0 ≈ 9 %

Dopamine 21.0 ≈ 13 %

L-Aspartic acid 21.0 ≈ –4 %

Citric acid 21.0 ≈ –4 %

D-(+)-Glucose 21.0 ≈ 6 %56

Real sample analyses

Table 6: Quantification of QCT in human urine samples.

Sample Urine dilution QCT added (µM) QCT found (µM) Mean recovery (%)



1 x 100 4.06 3.65 89.9

2 x 200 4.06 3.89 95.8

3 x 300 4.06 3.93 96.8

4 x 300 6.10 6.38 104.6

57


Linear range Detection limit

In HCl/KCl pH 1 2.06 µM to 67.4 µM 1.84 μM

In human urine 2.04 µM to 59.5 µM 2.98 µM

Activated chitosan modified CPE for MG determination

Amine-grafted Attapulgite modified GCE for MG determination


In HCl/KCl pH 1 0.61 µM to 10.57 µM 0.275 µM

Characterised by SEM, EA, FTIR, XRD and EIS


In PB pH 3 0.248 µM to 7.43 µM 0.0448 µM

1-Ethylpyridinium bromide modified CPE for QCT determination

58


Successful application to real sample analysis,

Successful determination of MG and QCT in the presence of interferents,

Envisage amperometric sensor with mediators forsimultaneous determination of molecules ofmedicinal importance,

Determination of such molecules in plants extracts.

59

Résumé

De nouvelles électrodes composites ont été mises au point,puis appliquées à l’électroanalyse de deux composés d’intérêtmédical de la famille des xanthones et des flavonoïdes:

une EPC modifiée par une poudre de chitosane activée pour l’électroanalyse de la mangiférine,

une EF d’attapulgite modifié par greffage covalente du [3-(2-aminoéthylamino)propyl]triméthoxysilane pour l’électroana-lyse de la mangiférine,

une EPC modifiée par un liquide ionique pour l’électronalysede la quercétine.

60

Acknowledgements

Great support from the following is acknowledged:

The University of Dschang

The International Foundation for Science (IFS)

The Academy of Sciences for the Developing World (TWAS)

Pr A. TAPONDJOU (DCH, FS, UDs),

Pr B. T. NGUELEFACK (DAB, FS, UDs),

Pr W. SCHUHMANN (Ruhr-University, Bochum, Germany),

Drs F. DOUNGMENE and G. KENNE-DEDZO,

The Members of Jury.61

Thank

you for your your

kind attention

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