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Functionalized graphene sensors for real time monitoring fermentation processesChinnathambi, Selvaraj

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98

Chapter 4

99

Fabrication of hydrothermally reduced graphene oxide electrodes for

potentiometric and chemiresistive pH measurements

Abstract

In this chapter, we report the synthesis of hydrothermally reduced graphene oxide (HRGO) as

a reagent-less sensing probe for the construction of a potentiometric and chemiresistive pH

sensor. We used cyclic voltammetry (CV) to study the chemical and electrochemical nature of

the functional groups present on the HRGO. We found that HRGO contains quinone-like

functional groups. The CV of HRGO showed reversible quinone/hydroquinone-like redox

couples in different buffers with a pH range from 2-8. In the presence of dissolved oxygen,

the HRGO electrode showed an oxygen reduction peak, which is absent when the electrode is

placed in an N2 saturated buffer. The absence of the O2 reduction peak indicates that the

quinone-like groups on HRGO have catalytic activity towards oxygen reduction. The HRGO

modified electrode, used as a potentiometric sensing probe, showed a sensitivity of 66 mV /

pH for a freshly prepared electrode and after a few exposures to different buffers, a stable

sensitivity of 55 mV / pH was obtained in a pH range from 2 – 12. In the chemiresistive

sensing mode, the HRGO electrode showed a sensitivity of 1000 Ω / pH in a pH range from

4-7. Reduced graphene oxide also was prepared by chemical and electrochemical reduction

methods to compare the influence of the reduction process on the pH sensor performance.

Although (electro)-chemically reduced graphene oxide electrodes contain surface functional

groups similar to HRGO, the electrodes showed a poor response in buffers with a pH higher

than 7.

100

4.1. Introduction

The pH is an important analytical parameter in many chemical and biological processes.

There are several techniques available to detect pH, which includes potentiometry,

amperometry, and chemiresistive methods. The glass-electrode is the most successfully used

potentiometric pH sensor and is widely accepted in the laboratory and industrial applications.

The ongoing miniaturization of laboratory equipment also requires the availability of small

sensors. This led to the exploration of new pH sensing techniques for use in applications that

require small sensors, e.g., microtiter plates, and in vivo tissue measurements. ISFET and

optical pH sensors are alternative sensors and can be constructed in tiny housings. However,

the drift and limited pH range is a significant drawback of ISFET and optical pH sensors [1-

2].

Oxygen-rich carbon materials are attractive for the construction of a pH sensing probe [3].

The carbon surface of the material contains a variety of functional groups, including

carboxylic acid, phenol, quinones, and carbonyl groups [4,5]. These functional groups are

sensitive towards pH and they undergo protonation and de-protonation reactions depending

on the pH. The presence of these functional groups on the surface turns the material into a

potential candidate for the construction of a reagent-less pH sensing probe. The pH-sensitive

molecules can be immobilized onto carbon surfaces in several ways. Chemical and

electrochemical activation, physical adsorption of pH-sensitive molecules, and composite

formation with carbon are popular immobilization methods [6-15]. The pH-sensitive

molecules are attached to the carbon surface through covalent and non-covalent bonding [15-

17]. Electrochemical oxidation is one of the easiest and efficient ways to covalently attach

pH-sensitive molecules to the surface. The carbon electrode is oxidized by applying a

potential higher than 1.0 V to electrochemically produce oxo functional groups like COOH,

101

C=O, and C-OH. The electrochemical reduction of aryl diazonium compounds at the carbon

surface also results in covalent attachment of pH-sensitive molecules [17-20].

Hydrothermally reduced graphene oxide (HRGO) is a typical material with abundant oxo

functional groups. It is produced through the bulk oxidation of graphite into graphene oxide

and then reduced under hydrothermal conditions. The hydrothermal reduction is one of the

greenest ways for the synthesis of reduced graphene oxide [21, 22]. At hydrothermal

conditions, super-heated water molecules catalyze the reduction of the oxo functional groups.

Reduced graphene oxide, obtained through hydrothermal reduction, contains more oxygen-

containing functional groups compared to other reduction processes. Under hydrothermal

conditions, the reduction occurs due to the acid-catalyzed dehydration through the protonation

of oxide functional groups in a reversible manner. As a result of this reversibility, some of the

functional groups like epoxides and alcohols are still present after the reduction process has

been completed [21]. C-13 NMR was used to identify the functional groups present on the

graphene oxide after hydrothermal reduction. The abundant oxygen functionalities on the

HRGO was explored for supercapacitor applications [23]. The contribution of pseudo-

capacitance, due to redox reactions of the functional groups, contributed to the high

capacitance of the material. However, the chemical/electrochemical nature of these functional

groups is not explored further. In this chapter, we investigated the nature of the functional

groups using cyclic voltammetry and studied the properties of HRGO as a potential reagent-

less pH sensing probe.

102

4.2. Experimental methods and physiochemical characterization

4.2.1. Material preparation

Graphite oxide (GO) was prepared according to Hummers’ method [24]. In a typical

procedure, 3 g of graphite flakes (Sigma-Aldrich) was dispersed in 69 ml of H2SO4 (Merck),

and 1.5 g of sodium nitrite (Sigma-Aldrich) was added to the suspension while stirring. Then

the suspension was placed in an ice bath and continuously stirred, followed by the slow

addition of 9 g KMnO4 (Sigma-Aldrich). Subsequently, 400 ml of distilled water was

cautiously added to the mixture. The temperature quickly rose to 90° C, and this temperature

was maintained for 15 minutes. Afterward, 7.5 ml of 30% H2O2 (Sigma-Aldrich) was added,

and the color of the suspension changed from brown to yellow. Finally, the GO suspension

was washed several times with 5% HCl (Merck) and Milli Q water.

For HRGO preparation, 50 mg GO was dispersed in 50 ml ultra-pure water sonicated for 12

hrs. Afterward, the dispersion was autoclaved at 130 ºC for 6 hours. After hydrothermal

treatment, the black dispersion was separated by centrifugation. Then the suspension was

repeatedly washed with water and re-dispersed in isopropanol. For comparison, reduced

graphene oxide also prepared by chemical and electrochemical methods. Two types of

chemically reduced graphene oxide (CRGO) electrodes were prepared using sodium

dithionite (sodium hydrosulfite) or sodium borohydride as reducing agents [25].

4.2.2. Electrochemical pH sensing

The responses of the HRGO modified electrodes were measured by immersing the electrode

in solutions with different pH values (pH 2.0 – pH 12). The Britton and Robinson (B-R)

universal buffer solution (0.04 M H3PO4, 0.04 M CH3COOH, and 0.04 M H3BO3) was titrated

with 0.2 N NaOH to adjust the pH to the desired value [26]. For the potentiometric sensor, a 2

103

mm gold disc, platinum wire, and Ag / AgCl were used as working, counter, and reference

electrode, respectively (CH Instruments, Austin, Texas, USA). The electrochemical

measurements were performed with a CH-Instruments potentiostat (CH600 and CH760). A

three-compartment electrochemical cell was used for the analyses. Potentiometric responses

were obtained by measuring the open circuit potential (OCP) against an Ag / AgCl reference

electrode, and changes in the potential values were used as the sensor signal. For cyclic

voltammetry, an R-B buffer was used for the pH range from 2-6, and 0.2 M phosphate buffer

was used for the pH range from 7-8. For all pH measurements, 0.1 M KCl was used as a

supporting electrolyte.

For chemiresistive sensing, 2 µl of HRGO dispersed in isopropanol was drop-casted on the

interdigitated gold-electrode. Two leads of the electrode were connected to the potentiostat

for data acquisition. A potential of 100 mV was applied between the source and the drain, and

the output current was measured over time. The resistance value of the HRGO-deposited

electrode was obtained through Ohms law.

4.3. Results and discussion

4.3.1. Material characterization

The formation of HRGO was characterized by FT-IR spectroscopy. The FT-IR spectrum of

GO (Fig. 4.1) showed strong peaks at 1700 cm-1, and 1010 cm-1, which are due to -C=O and -

C-O stretching of the COOH and epoxide functional groups, respectively. The intensity of

these peaks is reduced drastically after the hydrothermal reduction. This indicates the removal

of oxo functional groups and partial restoration of the sp2 hybridized carbon conductive

network [25, 27].

104

Figure 4.1: The FT-IR spectrum of GO and HRGO

The Raman spectra of GO and HRGO show two broad peaks around 1350 cm-1 and 1500 cm-

1, corresponding to the D and G mode of vibration (Fig 4.2). The G peak relates to E2g in the

plane vibration mode of graphite lattices, and the D peak corresponds to the K2 phonons of

the A2g symmetrical vibration mode [25, 27]. The D peak is considered to be indicative for the

number of defective sites in the graphene sheets. The higher the D peak, the higher are the

number of the defects.

Figure 4.2: Raman spectra of GO and HRGO

500 1000 1500 2000 2500 3000 3500500 1000 1500 2000 2500 3000 3500500 1000 1500 2000 2500 3000 3500

Inte

nsit

y / a

.u.

ID/I

G - 0.99

wavenumber / cm-1

HRGOID/I

G - 1.03

D+G2D

GO film

105

Transmission electron microscopic (TEM) images of parent GO and after the hydrothermal

reduction (HRGO) were taken and shown in Fig. 4.3. The TEM images showed a thin

graphene film with wrinkle formation and crumpled graphene sheets.

Figure 4.3: TEM images of GO (a and b), and HRGO (c and d).

4.3.2. Potentiometric pH sensing

The potentiometric response of the HRGO modified electrode was obtained for the pH range

from 2 to 12. The measurements are carried out by recording the potential differences against

Ag / AgCl. The potential differences occur because of the reversible

protonation/deprotonation of the oxo functional groups present on HRGO and follow the

Nernst equation shown in Eqn.1.

E = E° + [2.303RT / F] log [H+] (1)

(a))

(b)

(c) (d)

106

The HRGO electrode gave a near Nernstian response of 66 mV / pH for the first measurement

(Fig. 4.4 (a,b). After repeated experiments, the electrode showed a stable response of 55 mV /

pH (Fig. 4.4 (c,d)). The change of sensitivity in consecutive experiments is due to irreversible

de-protonation of the oxo functional groups at higher pH values. The response time and

stability of HRGO were obtained by continuously monitoring the open circuit potential over

some time while the pH of the solution was changed by the addition of 0.2 N NaOH (Fig. 4.4

(a,c)). From the graph, it can be seen that the HRGO electrode has good stability with a

response time of a few seconds.

The high sensitivity of the HRGO modified electrode indicates that it contains

electrochemically active pH-sensitive oxo functional groups that dominate the sensing

response. The hydrothermal reduction of GO is based upon acid-catalyzed dehydration of oxo

functional groups. Some of the oxo functional groups were not reduced during the

hydrothermal treatment because of the reversible nature of the dehydration process [21].

These remaining oxo functional groups are responsible for the pH-dependent potentiometric

response of the HRGO electrode.

107

Figure 4.4: Potentiometric measurements. The Continuous potential measurement of the

HRGO electrode in a buffer with increasing pH from 3 to 12 with respect to time (A),

corresponding potential versus pH plot for the first measurements (B), the potential

measurements of the electrode in a buffer with the pH from 3 to 12 and 12 to 2 with respect to

time (C), and the corresponding potential versus pH plot (D).

The pH response of chemically reduced GO (CRGO) has also been tested. Two reducing

agents, sodium dithionite and sodium borohydride, were used for the chemical reduction of

GO. Fig. 4.5 shows the potentiometric response of CRGO obtained using sodium dithionite

(CRGO-SS) when exposed to buffers with different pH. The CRGO-SS electrode showed a

linear response in the pH range from 2 to 9 with a sensitivity of 52 mV / pH (Fig. 5a,b) but a

poor response at a pH higher than 7 (Fig. 4.5 (c,d)).

0 5000 10000 15000 20000 25000

-0.1

0.0

0.1

0.2

0.3

0.4

0.5pH 2

pH 11Po

ten

tial (V

) V

s A

g / A

gc

l

Time / s

pH 3

pH 4

pH 5

pH 6

pH 7

pH 8

pH 9

pH 10

pH 12

pH 3

pH 4

pH 5

pH 6

pH 7

pH 8

pH 9

pH 10

pH 11

(c)

2 4 6 8 10 12

-0.1

0.0

0.1

0.2

0.3

0.4

Po

ten

tia

l (V

) v

s A

g /

Ag

cl

pH

pH 3-12

E = 0.548 - 0.052 pH

pH 12-2

E = 0.550 - 0.52 pH

(d)

2 4 6 8 10 12

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

pH

Po

ten

tia

l (V

) v

s A

g /

Ag

cl

(b)

E = 0.748 - 0.066 pH

0 1000 2000 3000 4000 5000 6000

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

pH 12

pH 9

pH 8

pH 7

pH 6

pH 5

pH 4

pH 10

Po

ten

tia

l (V

) (A

g /

Ag

Cl)

Time / s

pH 3 (a)

108

Figure 4.5: Potentiometric response (a) and corresponding calibration curve (b) of the CRGO-

SS electrode immersed in different buffers with an increasing pH (pH 2-9) and decreasing pH

(pH 9-2). The potentiometric response with the pH range pH 2- 12 and pH 12-2 (c), and

corresponding calibration curve for pH 2-12-3 (d).

Similarly, CRGO obtained by reduction with sodium borohydride (CRGO-SB) also showed a

linear response between pH 2 and 9 but had a poor response in buffers with a pH higher than

7 (Fig. 4.6 (a-c)). The CRGO-SB-electrode showed a sensitivity of 50 mV / pH for pH 2 to 9

and 47 mV / pH for pH 2-12.

1 2 3 4 5 6 7 8 9 100.0

0.1

0.2

0.3

0.4

Po

ten

tia

l (V

) v

s A

g /

Ag

Cl

pH

pH 2-9

pH 9-2

E = 0.507-0.52pH

(b)

0 5000 10000 15000 200000.0

0.1

0.2

0.3

0.4

Po

ten

tia

l (V

) v

s A

g /

Ag

Cl

Time / s

(a)

pH 3

pH 5

pH 6

pH 7

pH 8

pH 9

pH 2

pH 9

pH 4

pH 9

pH 2

pH 3

pH 4

pH 5

pH 6

pH 7

pH 8

0 5000 10000 15000 20000

-0.1

0.0

0.1

0.2

0.3

0.4

pH 7

pH 9

pH10

pH 12

Po

ten

tial (V

) vs

Ag

/ A

gC

l

Time / s

pH 2

pH 3

pH 4

pH 5

pH 6

pH 7

pH 8

pH 9

pH 11

pH 10

pH 6

pH 5

pH 4

pH 3

pH 8

(c)

0 2 4 6 8 10 12-0.2

-0.1

0.0

0.1

0.2

0.3

0.4P

ote

nti

al

(V)

vs

Ag

/ A

gC

l

pH

(d)

109

Figure 4.6: Potentiometric response (a) of the CRGO-SB electrode immersed in different

buffers with an increasing pH (pH 2-9) and the corresponding calibration curve for pH 2-9 (b)

and pH 2-12 (c).

From the three RGO’s (HRGO, CRGO-SS, CRGO-SB), only the electrode constructed with

HRGO showed good sensitivity and linear response in the pH range 2-12. The CRGO-SS and

CRGO-SB electrodes were sufficiently sensitive but showed a longer response time in buffers

with a pH above 7. This indicates that HRGO contains functional groups that are active at a

higher pH. To further understand the nature of the functional groups present in these

materials, cyclic voltammetry was used to characterize the materials.

0 4000 8000 12000 16000

-0.1

0.0

0.1

0.2

0.3

0.4

pH 10

pH 2

Po

ten

tia

l (V

) v

s A

g /

Ag

Cl

Time / s

pH 3

pH 4

pH 5

pH 6

pH 7

pH 8

pH 9

pH 11

pH 12

(a)

2 4 6 8 10 12

-0.1

0.0

0.1

0.2

0.3

0.4

Po

ten

tial (V

) V

s A

g / A

gC

l

pH

E = 0.470-0.047 pH

(c)

1 2 3 4 5 6 7 8 9 100.0

0.1

0.2

0.3

0.4

E = 0.483-0.050 pH

Po

ten

tial (V

) V

s A

g / A

gC

l

pH

(b)

110

4.3.3. Cyclic Voltammetry of pH-dependent HRGO

Cyclic voltammetry (CV) measurements were carried out to understand the electrochemical

nature and redox properties of the oxo functional groups that are responsible for pH sensing.

Initially, voltammograms were recorded in 1M H2SO4 electrolyte, and a potential range of -

0.3V to 0.8 V was applied (Fig. 4.7). During the positive scan, the oxidation potential

appeared at 0.380 V (vs Ag/AgCl), and during the backward scan, the reduction potential

appeared at 0.320 V (vs Ag/AgCl). These redox potentials were related to the

quinone/hydroquinone redox-couple [26-27]. The anodic potential corresponds to

hydroquinone oxidation, and the cathodic potential refers to quinone reduction. The

difference between anodic and cathodic potential is 60 mV, which is indicative of a two-

electron and two-proton reduction process related to quinone/hydroquinone-like redox-

couples. Voltammograms measured at a scan rate of 50 and 100 mV/s showed that the current

increases with increasing scan rate (Fig 4.7). At a higher scan rate, the effect of background

capacitance is higher; therefore, a low scan rate of 1 mV/s was selected for further pH

dependent CV measurements (Fig. 4.8-4.12). Subsequently, cyclic voltammograms were

measured for the HRGO electrode immersed in buffers with a different pH (Fig. 4.8). The

voltage limit was adjusted as a function of the pH.

111

Figure 4.7: CV measurements of the HRGO electrode immersed in 1 M H2SO4 from -0.3 V to

0.8 V at a scan rate of 50 and 100 mV / s.

When the pH buffer changed from 2 to 8, the redox potential became more negative and

shifted towards the cathodic direction. When the pH increases, well-defined oxidation peaks

were observed up to pH 8, but cathodic peaks are not clearly seen for pH 7 and 8. The anodic

potential values obtained with the HRGO electrode were plotted against the pH of the buffer.

The slope of the regression line was 57 mV / pH with an R2 of 0.9988, indicating Nernstian

behavior (Fig. 4.8(b)).

Figure 4.8: CV measurement of the HRGO electrode immersed in different R-B buffers with a pH

from 2 to 8 at a scan rate of 1 mV / s (A); Plot of the anodic potential of the HRGO electrode with

respect to the pH (B).

-0.2 0.0 0.2 0.4 0.6 0.8-3

-2

-1

0

1

-0.2 0.0 0.2 0.4 0.6 0.8

(a)

Cu

rre

nt

/ A

Potential (V) Vs Ag / AgCl

pH 2

pH 3

pH 4

pH 5

pH 6

pH 7 pH 8

1 2 3 4 5 6 7 8 9

0.00

0.07

0.14

0.21

0.28

0.35

pH

Po

ten

tia

l V

s A

g /

Ag

Cl

Slope - 57 mV / pH

R2 - 0.9988

E = 0.435-0.057 pH

(b)

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-0.30

-0.15

0.00

0.15

0.30

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-0.30

-0.15

0.00

0.15

0.30

Potential (V) Vs Ag / Agcl

Cu

rre

nt

/ m

A

112

The results suggested that the pH response of HRGO was due to the presence of oxo

functional groups on the graphene sheets. There are three different oxo functional groups

present on GO (carboxylic acid, alcohol, and epoxides). The redox peaks on the

voltammograms suggests that the -OH group presents on the adjacent carbon atom or at the

para position at the basal plane.

At pH > 6, the anodic peaks are not visible because they are concealed by the oxygen

reduction peak in the presence of dissolved oxygen. The CV measurements were carried out

in O2, and N2 saturated buffers at pH 3, 5, and 7 to understand the oxygen interference in

more detail (Fig. 4.9). It can be seen that in the absence of oxygen, explicit oxidation and

reduction peaks corresponding to the quinone-hydroquinone redox couples are present.

Although oxygen interfered with the measurement, oxygen did not affect the position of

oxidation potential.

Figure 4.9: CV measurements of the HRGO electrode immersed in pH buffers 3, 5, and 7,

saturated with oxygen (a) and saturated with N2 (b). The scan rate was 1 mV / s.

The CV of the HRGO electrode was compared with the CV of the ERGO electrode to look

for indications that hydroquinone redox peaks are present in ERGO as well (Fig. 4.10). The

CV of the ERGO electrode also showed a redox peak corresponding to the

-0.8 -0.4 0.0 0.4 0.8

-4.5

-3.0

-1.5

0.0

1.5(a)

Cu

rre

nt

/ A

Potential (V) Vs Ag / AgCl

pH 3

pH 5

pH 7

-0.8 -0.4 0.0 0.4 0.8

-1.6

-0.8

0.0

0.8

1.6

Cu

rre

nt

/ A

Potential (V) Vs Ag / AgCl

pH 3

pH 5

pH 7

(b)

113

quinone/hydroquinone redox couple but with a reduced current intensity compared to the

redox peak of the HRGO electrode. The anodic peak position also shifted to a more positive

potential in the case of the ERGO electrode. The main differences occurred in the reversibility

of the redox couples. The redox peak in ERGO is quasi-reversible with a difference of 190

mV while HRGO contains a highly reversible redox peak with a difference of 78 mV.

Figure 4.10: Comparison of cyclic voltammetry of an HRGO and ERGO electrode immersed

in an N2 saturated R-B buffer with pH 3 (A) and pH 7 (B). The scan rate was 1 mV / s.

Similarly, the CRGO-SS and CRGO-SB electrodes were electrochemically characterized

with CV. Fig.4.11 shows the cyclic voltammetry measurements of the CRGO-SS electrode.

The CV shows highly reversible quinone/hydroquinone redox-couples as it was also the case

with the HRGO and ERGO electrodes. The redox peak shifted cathodically with increasing

pH (Fig. 4.11 (a)). For every increasing pH unit, there is a potential shift of 51 mV in the

negative direction (Fig. 4.11(b)).

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-0.16

-0.08

0.00

0.08

0.16

0.24

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

ERGO-pH 3

Potential (V) Vs Ag / AgCl

Cu

rre

nt

/ A

HRGO-pH 3

(a)

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-0.1

0.0

0.1

0.2

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5(b)

Potential (V) Vs Ag / AgCl

ERGO-pH 7

Cu

rre

nt

/ A

HRGO-pH 7

114

Figure 4.11: Cyclic voltammetry of the CRGO-SS electrode in pH buffers from 3 to 8 (a);

Plot of the potential shift of the CRGO-SS electrode versus the pH of the buffer (b). The scan

rate was 1 mV / s.

The CV of the CRGO-SB electrode showed a similar behavior as the CRGO-SS electrode

(Fig. 4.12). The pH-dependent redox peak corresponding to the quinone/hydroquinone redox

couples were clearly visible in the pH range 2 to 8.

Figure 4.12: CV measurements of the CRGO-SB electrode immersed in different buffers with

a pH from 2 to 8 (a). Plot of the potential shift of the CRGO-SB electrode versus the pH of

the buffer (b). The scan rate was 1 mV / s.

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-2

-1

0

1

2

Cu

rre

nt

/ A

Potential (V) vs Ag / AgCl

pH 3

pH 4

pH 5

pH 6

pH 7

pH 8

(a)

3 4 5 6 7 8

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Po

ten

tia

l (V

) V

s A

g /

Ag

Cl

pH

E = 0.417-0.051 pH

(b)

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-2

-1

0

1

2

Cu

rre

nt

/ A

Potential (V) Vs Ag / AgCl

pH 2

pH 3

pH 4

pH 5

pH 6

pH 7

pH 8

(a)

2 3 4 5 6 7 8

0.0

0.1

0.2

0.3

Po

ten

tial

(V)

vs

Ag

/ A

gC

l

pH

E = 0.386 - 0.051 pH

(b)

115

The CV investigations of the RGO’s indicate that the three RGO’s contain quinone and

hydroquinone-like functional groups that showed pH-dependent redox peaks.

4.3.4. Chemiresistive sensing of HRGO

For chemiresistive sensing, HRGO was dispersed in isopropanol and drop-casted on an

interdigitated gold electrode and dried at 100 0C for 12 hours. The I-V characteristics of the

HRGO electrode were measured between 0.1 to 1 V (Fig 13). A linear relationship was

obtained indicative of ohmic contact formation between the HRGO sheets and the gold

electrode surface. It can also be seen that after Nafion coating, the resistance of the HRGO

electrode was increased.

Figure 4.13: I-V curve of the HRGO electrode.

The chemiresistive response of the HRGO electrode was studied in different buffers with a

pH between 3 and 7 (Fig. 4.14). The HRGO electrode showed a response when exposed to pH

buffer from 3to 7. It showed a poor response for the pH below 3 and also for pH above 7.

0.0 0.2 0.4 0.6 0.8 1.00.0

-0.2

-0.4

-0.6

-0.8

-1.0

HRGO-1.083 K

HRGO-NA-1.534 K

Cu

rre

nt

/ m

A

Voltage / V

116

Figure 4.14: Chemiresistive response of the HRGO-NA electrode immersed in different

buffers with the pH between 3 and 7.

4.4. Conclusion

The role of oxygen functional groups present on the reduced graphene oxide was investigated

for pH sensing application. Three types of reduced graphene oxide was prepared by

hydrothermal (HRGO) and chemical reduction (CRGO) method. For the chemical reduction

method sodium thionite (CRGO-SS) and sodium borohydride (CRGO-SB) reducing agents

were used for the reduction process. All three reduced graphene oxide showed similar pH

response. HRGO showed linear response for the pH range 2-12 with the sensitivity of 52 mV

/ pH. The CRGO-SS and CRGO-SB also showed similar response. The notable differences in

the sensor response appeared for the pH higher than 7. HRGO showed linear response for pH

ranges from 2 – 12 while CRGO-SS and CRGO-SS showed poor response for the pH above

7.

The electrochemical nature of functional groups present on hydrothermally reduced graphene

oxide was investigated using cyclic voltammetry. The CV studies indicated that quinone-like

functional groups were present on the HRGO. The CVs showed reversible quinone and

0 500 1000 1500 2000 2500-84

-81

-78

-75

-72

pH 3

cu

rre

nt

/ A

Time / s

pH 4

pH 5

pH 6

pH 7

(a)

4000 6000 8000 10000

-92

-88

-84

-80

cu

rre

nt

/ A

Time / s

pH 3pH 4

pH 5

pH 6

pH 7

pH 4

pH 5

pH 6

pH 7

(b)

117

hydroquinone-like peaks in the pH range from 2-8. These quinone-like moieties promote the

reduction of dissolved oxygen. Hence, the removal of dissolved oxygen is necessary for the

accurate measurement of the reduction peak potential at a pH higher than 6. These redox

peaks were appeared with reduced current intensity for ERGO, which is indicative for a low

amount of quinone/hydroquinone species on the surface. The CRGO electrode also showed

quinone/hydroquinone redox peaks similar to the HRGO electrodes, but showed poor pH

response for a pH above 7.

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