Characterization of electrodes modified with nanocomposites of cobalt tetraaminophenoxyphthalocyanine, reduced graphene and multi-walled carbon nanotubes

Article abstract of DOI:10.1080/00958972.2019.1621299

Glassy carbon electrodes or plates were modified with nanocomposites consisting of cobalt tetraaminophenoxyphthalocyanine (CoTAPhPc), reduced graphene oxide nanosheets (rGONs) and multi-walled carbon nanotubes (MWCNTs). The modified electrodes were charac

Full text of DOI:10.1080/00958972.2019.1621299

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Characterization of electrodes modified
with nanocomposites of cobalt
tetraaminophenoxyphthalocyanine, reduced
graphene and multi-walled carbon nanotubes
Munyaradzi Shumba, Stephen Nyoni, Jonathan Britton & Tebello Nyokong
To cite this article: Munyaradzi Shumba, Stephen Nyoni, Jonathan Britton & Tebello
Nyokong (2019): Characterization of electrodes modified with nanocomposites of cobalt
tetraaminophenoxyphthalocyanine, reduced graphene and multi-walled carbon nanotubes, Journal
of Coordination Chemistry, DOI: 10.1080/00958972.2019.1621299
To link to this article: https://doi.org/10.1080/00958972.2019.1621299
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1621299
Characterization of electrodes modified with nanocomposites
of cobalt tetraaminophenoxyphthalocyanine, reduced
graphene and multi-walled carbon nanotubes
Munyaradzi Shumba, Stephen Nyoni, Jonathan Britton and Tebello Nyokong
Center for Nanotechnology Innovation, Chemistry Department, Rhodes University, Grahamstown,
South Africa
ABSTRACT
ARTICLE HISTORY
Glassy carbon electrodes or plates were modified with nanocom-
posites consisting of cobalt tetraaminophenoxyphthalocyanine
Received 22 February 2019
Accepted 8 May 2019
(
CoTAPhPc), reduced graphene oxide nanosheets (rGONs) and
KEYWORDS
Reduced graphene oxide
nanosheets; cobalt
tetraaminophenoxyphthalo-
cyanine; multi-walled
carbon nanotubes;
multi-walled carbon nanotubes (MWCNTs). The modified electro-
des were characterized using cyclic voltammetry, scanning elec-
trochemical microscopy (SECM) and time-of-flight-secondary ion
mass spectrometer (TOF-SIMS). The electrocatalytic activity of the
modified electrode was tested for detection of L-cysteine. The
presence of CoTAPhPc on sequential layers of MWCNT and rGONs
resulted in improved detection currents compared to CoTAPhPc
alone or when MWCNT/rGONs are mixed in CoTAPhPc–MWCNT/
rGONs (mix)–glassy carbon electrode (GCE). CoTAPhPc–MWCNT–GCE
L-cysteine; electrocatalysis
(
without rGONS) showed higher sensitivity toward L-cysteine as
compared to the probes incorporating rGONs with a catalytic rate
4
ꢁ1 ꢁ1
constant of 4.62ꢀ 10 M
s
and a detection limit of 30 nM. The
presence of rGONs improved the stability of the electrode.
CONTACT Stephen Nyoni
nyonistephen77@gmail.com; snyoni@cut.ac.zw
Center for Nanotechnology
Innovation, Chemistry Department, Rhodes University, Grahamstown, South Africa
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
M. SHUMBA ET AL.
1. Introduction
Carbon-based nanomaterials such as single and multi-walled carbon nanotubes and
different graphene moieties have applications in electrocatalytic systems providing a
platform for the detection of a wide range of environmental pollutants and physio-
logical molecules [1–7]. This is mainly due to their superior electrical properties and
the abundance of carbon sources in nature. In this work, we combine reduced gra-
phene oxide nanosheets (rGONs) and multi-walled carbon nanotubes (MWCNTs) in the
presence of an extended metallophthalocyanine (MPc) for electrocatalysis. Graphene-
based materials have been reported to exhibit excellent electrochemical properties [8].
Recently, graphene-based composite materials have been used in catalysis of the oxy-
gen evolution reaction and reported results demonstrated high activity, low overpo-
tential and high current density [9–11]. These results attest to graphene as a good
catalytic platform for a wide range of applications. However, graphene sheets can
undergo p-stacking which compromises their electrochemical properties. The out-
standing properties of graphene nanosheets can be harnessed by incorporating mate-
rials such as MWCNTs into the graphene sheets, which inhibit the agglomeration and
improve the electrode conductivity. The aim of this study is to determine the best
way to modify electrodes with nanocomposites consisting of MWCNTs and rGONs. The
order in which materials are added to the electrode has a profound influence on their
electrocatalytic (or electron transfer) properties. The work compares drop drying a pre-
mixed rGONs and MWCNTs against the sequential drop dry of each separately fol-
lowed by addition of phthalocyanines as an electrocatalyst.
The combination of rGONs and MWCNTs has been reported to exhibit excellent
electrocatalytic activity [7, 12], hence this combination is employed in this work
together with phthalocyanines (Pcs) for electrocatalysis. Pcs are well known for their
electrocatalytic activity [13]. Thus, combining the known electrocatalysts (MPcs) with
rGONs and MWCNTs, which improve electron transfer at the electrode, is expected to
result in improved electrocatalysis. Pcs have been combined with carbon nanomateri-
als for electrocatalytic purposes [12, 14–16]. However, there is only one study combin-
ing MPcs, rGONs and MWCNTs for electrode modification [12]. We continue our work
on these nanomaterials using a cobalt tetraaminophenoxyphthalocyanine (CoTAPhPc,
Scheme 1) which contains bulky substituents and hence will result in less aggregation
compared to cobalt tetraaminophthalocyanine used before [12, 15]. Cobalt is
employed as a central metal due to the well-known electrocatalytic activity of CoPc
derivatives [13] and L-cysteine is used as a test analyte since its electrocatalysis using
CoPc derivatives is also well known [13].
2. Experimental
2.1. Materials
Buffer tablets (pH 4) and quinoline were purchased from Merck; alumina (10 mm), L-
cysteine, sodium borohydride, multi-walled carbon nanotubes (MWCNT, 110–170 nm
length), graphene oxide nanosheets (GONS), K Fe(CN)6 and K Fe(CN)6 were from
3
4
Sigma-Aldrich. Cobalt chloride was purchased from Saarchem (Pty) Ltd. 1,8-

JOURNAL OF COORDINATION CHEMISTRY
3
Scheme 1. Synthesis of cobalt tetraaminophenoxyphthalocyanine (CoTAPhPc).
Diazabicyclo [5.4.0] undec-7-ene (DBU) was from Fluka and dimethyl formamide (DMF)
was from B & M Scientific. Aqueous solutions of L-cysteine in pH 4 buffer were pre-
pared using millipore water from Milli-Q Water Systems (Millipore Corp. Bedford, MA,
USA). We previously reported the synthesis of tetraamminophenoxyphthalocya-
nine [17].
2.2. Equipment
Scanning electrochemical microscopy (SECM) experiments were carried out using
Uniscan Model 370 equipment and a 25 lm Pt microelectrode (Uniscan) as the tip.
SECM approach curves were done using the Pt microelectrode with a Pt counter elec-
trode and Ag|AgCl wire as the pseudo-reference electrode. Images were obtained by
maintaining the tip at a constant Z position and scanning in the X–Y plane over the
desired area (constant-height mode of SECM) and monitoring changes in the steady-
state current of K [Fe(CN) ]/K [Fe(CN) ] following oxidation at 0.1 V versus Ag|AgCl.
3
6
4
6
Glassy carbon plates (Goodfellow, UK) of 1 ꢀ 1 cm and 2 mm thick were used as sub-
strates for SECM. Transmission Electron Microscopy (TEM) images were obtained from
a Zeiss Libra TEM 120 model operated at 90 kV.
Time-of-flight-secondary ion mass spectrometer (TOF-SIMS) data were recorded
with an ION TOF GmbH TOF SIMS 5–100 run in micro-raster mode. The raster area
was 3000 lm ꢀ 3000 lm, and the sample was run in both positive and negative ion
modes. The analyzer was set to a standard operating mode with a cycle time of 100
ls; the primary beam was a Bi ion cluster gun with a current of 0.4 pA and an energy
3

4
M. SHUMBA ET AL.
Table 1. Electrochemical parameters for the modified electrodes.
Surface roughness
E/V for L-cysteine
oxidation in
pH 4 buffer
a
3
ꢁ/4ꢁ
3ꢁ/4ꢁ
DE
p
for Fe(CN)
6
Fe(CN)
6
Surface coverage
2
ꢁ
GCE modifier
in 0.1 M KCl
in 0.1 M KCl
(mol cm
)
a
Bare
0.083
0.12
0.078
0.098
0.070
0.088
0.19
0.30
0.11
0.14
0.14
1.60
1.49
1.67
1.45
1.97
1.58
1.34
0.73
1.57
1.08
1.03
11
11
11
11
11
11
CoTAPhPc-MWCNT/rGONs (mix)
CoTAPhPc–MWCNT–rGONs
CoTAPhPc–rGONs–MWCNT
CoTAPhPc–MWCNT
CoTAPhPc–rGONs
CoTAPhPc
MWCNT–rGONs
rGONs–MWCNT
MWCNT
2.08 ꢀ 10ꢁ
0.38
0.46
0.41
0.42
0.43
0.a41
ꢁ
7.46 ꢀ 10
ꢁ
3.63 ꢀ 10
ꢁ
3.15 ꢀ 10
ꢁ
1.58 ꢀ 10
ꢁ
1.32 ꢀ 10
a
a
a
a
a
a
a
rGONs
aNo L-cysteine peak.
of 3000 eV (also termed as spectrometry mode). The Bi cluster and electron flood gun
3
were used to get a better ion signal from the sample. Charge compensation was used
to account for the electron flood gun. The raw data were processed using the Surface
Lab 6.5 software provided by ION TOF. Glassy carbon plates (Goodfellow, UK) of
1
ꢀ 1 cm and 2 mm thick were used as substrates for TOF-SIMS.
Infrared spectra were recorded on a Perkin Elmer 100 ART FT-IR spectrometer.
Elemental analyses were carried out on a Vario EL III MicroCube CHNS Analyzer. Mass
spectral data were collected with a BrukerAutoFLEX III Smartbeam TOF Mass spectrom-
eter. The spectra were acquired using a-cyano-4-hydroxycinnamic acid as the
MALDI matrix.
All electrochemical experiments (cyclic and chronoamperometric) were performed
using an Autolab Potentiostat PGSTAT 302 (Eco Chemie, Utrecht, The Netherlands)
driven by the general purpose Electrochemical System data processing software (GPES,
software version 4.9).
2.3. Preparation of reduced graphene oxide nanosheets
Following literature methods [18] graphene oxide sheets were reduced as follows: 1 g
of graphene oxide sheets were dispersed in 100 mL Millipore water through ultrasoni-
ꢂ
cation. NaBH (0.57 g, 15 mmol) was added and the mixture stirred for 3 h at 70 C to
4
reduce the graphene oxide sheets. The black solid product was obtained by filtration
over a fritted glass funnel, washed with copious amounts of water (5 ꢀ 50 mL) and
ꢂ
dried in an oven at 70 C. The resulting sample was further treated with 98% H SO
2
4
and is designated as rGONs [18].
2
.4. Electrode modification
Glassy carbon electrode (GCE) was used as the working electrode, silver/silver chloride
saturated silver chloride) as the reference electrode and platinum wire as the counter
(
electrode in all electrochemistry experiments. The GCE was polished on a Buehler-felt
pad using alumina (<10 mm). Between each polishing step, Millipore water was used
to remove any impurity by sonicating for 30 s. The electrode was then rinsed with

JOURNAL OF COORDINATION CHEMISTRY
5
Millipore water and dried before use. The GCE were modified via the drop dry
method. MWCNT, rGONs or CoTAPhPc (1 mg each) were added separately to 1 mL
DMF (corresponding to 1 lmol/mL for CoTAPhPc). An aliquot (0.5 mL) of each was
ꢂ
placed on the polished GCE and dried in an oven at 70 C before use. The electrodes
are designated as MWCNT–GCE, rGONs–GCE or CoTAPhPc–GCE, respectively. For modi-
fication of MWCNT–GCE or rGONs–GCE with CoTAPhPc, 0.5 mL of 1 mg/mL of
CoTAPhPc was added on top of MWCNT or rGONs electrodes followed by drying. The
electrodes are represented as CoTAPhPc–MWCNT–GCE or CoTAPhPc–rGONs–GCE,
respectively. For electrode modification with all three modifiers (CoTAPhPc, MWCNT
and rGONs), MWCNT and rGONs were premixed at a ratio 1:10 in DMF, followed by
sonication for 5 h, then 0.5 mL of the mixture was placed on the electrode and allowed
to dry. Then 0.5 mL of CoTAPhPc was added onto the electrode followed by drying;
the resulting electrode is represented as CoTAPhPc–MWCNT/rGONs (mix)–GCE.
Experiments were also performed where 0.5 lL of 1 mg/mL DMF solutions of MWCNT
and r-GONS were not mixed, but added in sequence onto the electrode before the
addition of CoTAPhPc. Before addition of CoTAPhPc, the electrodes are represented as
MWCNT–rGONs–GCE (when rGONs are placed on the electrode first) and
rGONs–MWCNT–GCE (when MWCNTs are placed on the electrode first). Following add-
ition of CoTAPhPc, the electrodes are represented as CoTAPhPc–MWCNT–rGONs–GCE
(
(
where rGONs were added to the electrode first) or CoTAPhPc–rGONs–MWCNT–GCE
where MWCNTs were added to the electrode first). Table 1 lists all the electro-
des employed.
3. Results and discussion
3.1. Synthesis and characterization of CoTAPhPc and carbon materials
CoTAPhPc was synthesized by template cyclization of 4-aminophenoxyphthalonitrile in
the presence of CoCl in quinoline. CoTAPhPc was characterized by elemental analysis
2
and several spectroscopic techniques. Elemental analysis results (reported before [17])
are in agreement with the reports that Pcs are often isolated as solvates [19, 20]. The
UV–Vis spectrum of CoTAPhPc is shown in Figure 1. The Q band for CoTAPhPc is at
678 nm, vibronic band at 618 nm and the Soret band at 349 nm. The broadness of the
Q band is typical for amino substituted phthalocyanines [21]. NMR spectra were not
recorded due to the paramagnetic nature of Co.
The TEM images show multi-walled carbon nanotubes (Figure 2A) and rGONs are
characterized by thick flat sheets (Figure 2B) that are less transparent than the ones
mixed with MWCNT (Figure 2C). This may be attributed to the tendency of the sheets
stacking together. The introduction of multi-walled carbon nanotubes disturbs the
stacking leaving single sheets, hence more transparent. This is expected to increase
the conductivity of the electrode surface.
3.2. Characterization of modified electrodes
The carbon-based materials (MWCNT and/or rGONs) were deposited on the electrode first
followed by CoTAPhPc. The latter is the electrocatalyst and placed on top. When CoTAPhPc

6
M. SHUMBA ET AL.
Figure 1. UV–Vis spectra of cobalt tetraaminophenoxyphthalocyanine in DMF.
was placed on the electrode before the carbon nanomaterials, the electrode did not show
much catalytic activity for L-cysteine. It has also been reported that thiols are catalytically
oxidized on the surface of cobalt phthalocyanines [22], hence it should be placed on top.
Carbon materials are expected to enhance electron transfer. They also reduce p–p stacking
in rGONs, providing an increased catalytic platform/surface area for oxidation of the target
analyte. Thus, in the absence of MWCNTs, there is less catalytic activity.
3.2.1. Time of flight secondary ionization mass spectrometry (TOF-SIMS)
Carbon plates were modified as described above before TOF-SIMS analysis. Figure 3
shows the TOF-SIMS images for the CoTAPhPc-MWCNT-GCE. The images mainly show
the presence of carbocations. The presence of cobalt in the images coupled with the
presence of nitrogens confirms the presence of the phthalocyanine. The cobalt image
also shows small dots of concentrated ion signal. The other main relevant carbocations
þ
þ
þ
þ
þ
present are C3H7 , C4H7 , C7H7 and C H N , with C H N showing the most ion
3
9
2
3
9
2
concentration overall and cobalt the least, which makes sense since cobalt would be
in the smallest abundance in a Pc. Similar molecular ions were also reported for highly
ordered pyrolytic graphite [23] and pyridinic N doped graphene [24].
3.2.2. Cyclic voltammetric analysis
Figure 4 shows the cyclic voltammograms for a selection of modified electrodes in
3
ꢁ/4ꢁ
1
mM Fe(CN)6
in 0.1 M KCl. A low peak potential separation (DE) reflects good
electron transfer ability (Table 1). Electron transfer ability improved in the order
MWCNT–rGONs–GCE (DE ¼ 0.30 V) < CoTAPhPc–GCE (DE ¼ 0.19 V) < MWCNT–GCE
(
DE ¼ 0.14 V) ¼ rGONs (DE ¼ 0.14 V) < CoTAPhPc–MWCNT/rGONs (mix–GCE (DE ¼ 0.12
V) ꢃ rGONs–MWCNT–GCE (DE¼ 0.11 V) < CoTAPhPc–rGONs–MWCNT–GCE (DE¼ 0.098V) <
CoTAPhPc–rGONs–GCE V) < Bare GCE V) <
CoTAPhPc–MWCNT–rGONs–GCE (DE¼ 0.078 V) < CoTAPhPc–MWCNT–GCE (DE¼ 0.070 V).
It is clear from the DE values that mixing MWCNT and rGONs before adsorbing them
onto the electrode (in CoTAPhPc–MWCNT/rGONs (mix)–GCE) is less favored compared to
(DE ¼ 0.088
(DE ¼ 0.083
sequentially
adding
the
carbon
materials
to
the
electrode
(in

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 2. TEM images of MWCNTs (A), rGONs (B) and MWCNT/rGONs (mixed) (C).
Figure 3. 2D TOF-SIMS images for CoTAPhPc–MWCNT–GCE in positive mode.
CoTAPhPc–rGONs–MWCNT–GCE and CoTAPhPc–MWCNT–rGONs–GCE). When rGONs are
placed on the electrode alone for CoTAPhPc–rGONs–GCE (DE¼ 0.088 V) or before MWCNT
and CoTAPhPc-MWCNT–rGONs–GCE (DE¼ 0.078 V) there is better electron transfer proper-
ties than when MWCNTs are placed before rGONs (for CoTAPhPc–rGONs–MWCNT
(DE¼ 0.098 V)). Placing rGONs on the electrode first might assist in reducing the stacking of
rGONs. CoTAPhPc–MWCNT (DE¼ 0.070 V) alone shows good electron transfer properties.
MWCNTs improve the properties of rGONs electrode if the latter is placed first on the elec-
trode in CoTAPhPc–MWCNT–rGONs–GCE (DE¼ 0.078 V). Direct mixing of rGONs and
MWCNT does not harness the electron transfer properties of either material. Thus, it is sug-
gested that in order to use these materials effectively they must be used to modify electro-
des in a sequential manner.
The surface roughness factor for the modified electrodes were determined using
3ꢁ/4ꢁ
[Fe(CN)6]
redox system and applying Randles–Sevcik Equation (1) for reversible sys-
tems [25]:
Ip ¼ 2:69 ꢀ 10 n3=2ACD1=2v1=2
5
(1)
where I , n, A, C, D and v are the peak current, the number of electrons involved, the
p
3ꢁ/4ꢁ
3ꢁ/4ꢁ
surface area, the concentration of [Fe(CN)6]
, the diffusion coefficient of [Fe(CN)6]

8
M. SHUMBA ET AL.
3ꢁ/4ꢁ
Figure 4. Cyclic voltammograms (A) in 1 mM Fe(CN)6
solution. Scan rate ¼ 100 mV/s.
in 0.1 M KCl and (B) in pH 4 buffer
ꢁ
6
2
ꢁ1
and the scan rate, respectively. From the D value for K [Fe(CN) ]¼ 7.6ꢀ 10 cm s [26]
3
6
and n¼ 1, the surface roughness factors (ratio of I experimental/I theoretical) were
pa
pa
used to calculate real electrode areas f(roughness factor ꢀ theoretical surface area
2
(
¼0.071 cm )g. Surface coverages were calculated using Equation (2) [27] (Table 1), and
the cyclic voltammograms for modified electrodes (containing CoTAPhPc, where there is a
III
II
peak due to Co /Co ) in pH 4 buffer are shown in Figure 4B,
Q
u ¼
(2)
nFA
where u is the film surface coverage, Q is the charge passed, n is the number of trans-
ferred electrons, F is the Faraday constant, and A is the effective area of the electrode
ꢁ
10
ꢁ2
area. u-values in Table 1 for all the catalysts are less than 1 ꢀ 10
mol.cm which is
expected for a monolayer of a Pc molecule lying flat on the electrode surface [28].
The values suggest that the molecules are not lying flat on the electrode. The largest
surface coverage is observed for CoTAPhPc–MWCNT–rGONs–GCE and the least for
CoTAPhPc–GCE. Thus, when rGONs are deposited first on the electrode, followed by
MWCNTs, the surface coverage is larger. The redox couple near 0.1 V in Figure 4B is
III/II
attributed to Co
in comparison with literature [29], and will be involved in electro-
catalytic oxidation of L-cysteine.
3.2.3. Scanning electrochemical microscopy
Scanning electrochemical microscopy was used to characterize the probes. Area scans
were done to show the conductivity of the modified electrodes. Figure 5(A–C) shows
that bare GCE electrode has an almost uniformly conducting surface and the least
recorded currents. Modified electrodes (using CoTAPhPc–MWCNT–rGONs–GCE and
CoTAPhPc–MWCNT–GCE as examples) show improved currents compared to the bare
GCE which is indicative of possible catalytic nature. CoTAPhPc–MWCNT (Figure 5B) has
larger currents and also had the lower DE value in Table 1 compared to
other electrodes.

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 5. Scanning electrochemical microscopy images: Bare GCE (A), CoTAPhPc–MWCNT (B),
3ꢁ/4ꢁ
CoTAPhPc–MWCNT–rGONs (C). (D) Approach curves using 2.5 mm UME in 5 mM [FeCN6]
in 0.1
M KCl.
The approach curves in Figure 5D show changes in tip current as the ultramicro
electrode (UME) tip approaches the substrate in a feedback mode experiment in the
2
þ
3þ
presence of Fe /Fe redox mediator. When a sufficiently positive potential is applied
2
þ
3þ
to the tip, the oxidation of Fe to Fe occurs. When the tip is closer to a conductive
substrate surface, the Fe diffuses to the substrate where it is reduced back to Fe
This results in additional flux of Fe to UME tip, resulting in increased currents. For
insulating surface, the current decreases. Figure 5D shows approach curves for con-
ducting surfaces of bare-GCE, CoTAPhPc-based electrodes and non-conducting
Teflon surface.
3þ
2þ
.
2
þ
The difference in conductivity is also reflected in the catalytic behavior of the
probes as shown in the next section. The largest currents were observed for
CoTAPhPc–MWCNT–rGONs–GCE and CoTAPhPc–MWCNT–GCE, which also had the low-
est DE values in Table 1.
3.3. L-Cysteine detection
3
.3.1. Cyclic voltammetric analysis
Acidic medium (pH 4) was selected so that the oxidation product of L-cysteine does
not precipitate out of solution. Figure 6 shows the L-cysteine oxidation peak on the
different electrodes. L-cysteine peaks were only observed in the presence of CoTAPhPc

1
0
M. SHUMBA ET AL.
Figure 6. Cyclic voltammograms of modified electrodes in 2 mM L-cysteine. Insert shows the vol-
tammogram for CoTAPhPc–MWCNT–rGONs–GCE and CoTAPhPc–rGONs electrodes.
and are listed in Table 1. The dip in the current on the return scan for the oxidation
of L-cysteine on modified surfaces (Figure 6) is typical of electrocatalysis for some ana-
lytes such as hydrazine and L-cysteine on CoPc-modified electrodes [30–32].
The oxidation potentials for L-cysteine on CoTAPhPc–MWCNT–GCE,
CoTAPhPc–rGONs–MWCNT–GCE, CoTAPhPc–rGONs–GCE, CoTAPhPc–GCE and
CoTAPhPc–MWCNT/rGONs
(mix)–GCE
were
similar
at
ꢄ0.40
V.
CoTAPhPc–MWCNT–rGONs–GCE (where rGONs are deposited first on the electrode) gave
a slightly more positive potential at ꢄ-0.46 V. This electrode also had a larger surface
coverage. These potentials are lower than reported for L-cysteine oxidation on cobalt tet-
raaminophthalocyanine (CoTAPc) – MWCNT at 0.63 V [15], showing the importance of the
Pc with the spacer between the amino group and the Pc ring. CoTAPhPc–MWCNT–GCE
was the best probe in terms of currents for cysteine electrooxidation (Figure 6).
To test for reusability of the modified surface, the electrode was continuously
cycled in the presence of 2 mM L-cysteine (figures not shown). The presence of both
rGONs and MWCNT in CoTAPhPc–MWCNT/rGONs(mix)–GCE resulted in less passivation
of the electrode. Generally, the inclusion of rGONs in the probes resulted in resistance
to passivation for all electrodes.
CoTAPhPc–MWCNT–GCE (as an example) was employed for the determination of
Tafel slopes due to its high electrocatalytic activity (in terms of current) for L-cysteine
detection compared to the other electrodes. Figure 7A and B show linear plots of cur-
2
rent versus root of scan rate (R ¼ 0.98) and the plot of peak potential versus log scan
2
2
rate (R ¼ 0.99) for L-cysteine oxidation, respectively. The R value for the plot in
Figure 7A confirms that catalytic oxidation of L-cysteine is diffusion controlled. The
relationship between peak potential and scan rate for an irreversible diffusion-con-
trolled process is given by Equation (3) [27]:
2
:3 RT
Ep ¼
logv þ K
(3)
2
ð1ꢁaÞnaF

JOURNAL OF COORDINATION CHEMISTRY
11
1/2
Figure 7. Plots of (A) Ipc vs. v and (B) peak potential vs. log scan rate in L-cysteine oxidation on
CoTAPhPc–MWCNT–GCE.
where a is the transfer coefficient, n is the number of electrons involved in the rate
a
determining step, v is the scan rate, K is a constant, R is the universal gas constant and T
is the temperature (298 K). The plot of E versus log v (Figure 7B) gave a linear relation-
p
ꢁ
1
ship and the Tafel slope was 106 mV decade . The Tafel slope as observed in this work
suggests that transfer of a first one-electron is the rate determining step [33].
3.3.2. Chronoamperometric analysis
Chronoamperometry data were used to determine catalytic rate constant for oxida-
tion of L-cysteine. CoTAPhPc–MWCNT–GCE, CoTAPhPc–MWCNT–rGONs–GCE,
CoTAPhPc–rGONs–MWCNT–GCE and CoTAPhPc–MWCNT/rGONs (mixed) were
employed as examples. Figure 8A shows the chronoamperometric evolutions on
CoTAPhPc–MWCNT–GCE as an example. These chronoamperometric evolutions were
used to determine probe sensitivity.
Sensitivity of the probes showed the following trend: CoTAPhPc–MWCNT–GCE (71.7
2
1
21
mAM )> CoTAPhPc–MWCNT–rGONs–GCE (41.6 mAM )> CoTAPhPc–rGONs–MWCNT–
2
1
21
GCE (8.0 mAM )> CoTAPhPc–MWCNT/rGONs (mixed) (1.9 mAM ). Thus,
2
1
CoTAPhPc–MWCNT–GCE showed higher sensitivity (71.7 mAM ) and the lowest limit of
detection (LOD) of 0.03 mM using the 3d/slope ratio notation (where d is the standard
deviation of the blank), Table 2. The LOD value for CoTAPhPc–MWCNT–GCE is an
improvement compared to reported values in Table
2
[15, 34–36].
CoTAPhPc–MWCNT–GCE was followed by CoTAPhPc–MWCNT–rGONs–GCE in terms of sen-
sitivity but the latter was worse than CoTAPhPc–rGONs–MWCNT–GCE in terms of LOD.
Thus, even though the presence of MWCNT with CoTAPhPc gives good detection limits,
layer by layer modification of electrodes containing rGONs and MWCNT also gives reason-
able LOD values. The electron transfer properties of rGONs are improved when layered
with MWCNTs compared to when the two are mixed or for rGONs alone. The presence of
MWCNTs thus assist in enhancing the electron transfer properties when layered.
1/2
Figure 8B shows plots of (Icat/Ibuf) versus. t (Equation (5)) for L-cysteine oxidation.
The catalytic current (Icat) is dominated by the rate at which L-cysteine is oxidized on
1/2
CoTAPhPc–MWCNT–GCE. The slopes of the plots of (Icat/Ibuf) versus t were used to
calculate the rate constant according to Equation (4) [37]:
Icat
Ibuf
c1=2p1=2 ¼ p ðkcatCtÞ
1=2
1=2
¼
(4)

1
2
M. SHUMBA ET AL.
Figure 8. (A) Chronoamperograms for CoTAPhPc–MWCNT–GCE in different concentrations of L-cyst-
1
/2
eine (A) plot of current vs. time. (B) Plots of (Icat/Ibuf) vs. t (0.27 mM, 0.40 mM, 0.44 mM, 0.50 mM,
.57 mM, 0.67 mM, 0.80 mM).
0
where Icat and Ibuf are currents in the presence and absence of L-cysteine, respectively,
kcat is the catalytic rate constant, C is the concentration of L-cysteine and t is the time
elapsed in seconds.
The plots of the square of slopes obtained from Figure 8B (using
CoTAPhPc–MWCNT–GCE as an example) versus concentration of L-cysteine were done
for CoTAPhPc–MWCNT–GCE, CoTAPhPc–MWCNT/rGONs (mix)–GCE, CoTAPhPc–MWCNT–
rGONs–GCE and CoTAPhPc–rGONs–MWCNT–GCE. Linear relationships were observed and
represented by Equation (5a) for CoTAPhPc–MWCNT–GCE, Equation (5b) for
CoTAPhPc–rGONs–MWCNT–GCE, Equation (5c) for CoTAPhPc–MWCNT–rGONs–GCE and
Equation (5d) for CoTAPhPc–MWCNT/rGONs(mix)–GCE:
ꢀ
ꢁ
ꢁ
1
s
1
2
y ¼ 145:21½lꢁcysteineꢅ
ꢁ 8:711 sꢁ ; R ¼ 0:96
(5a)
(5b)
(5c)
mM
ꢀ
ꢁ
ꢁ
1
s
y ¼ 2:76½Lꢁcysteineꢅ
ꢁ1:243sꢁ ; R ¼ 0:97
1
2
mM
ꢀ
ꢁ
ꢁ
1
s
þ 24:524sꢁ ; R ¼ 0:96
1
2
y ¼ 10:18½Lꢁcysteineꢅ
mM
ꢀ
ꢁ
ꢁ
1
s
2
y ¼ 0:93½Lꢁcysteineꢅ
ꢁ1:634; R ¼ 0:98
(5d)
mM
4
where the slopes of these plots are equal to pk and the value of k was 4.62 ꢀ 10
ꢁ
1 ꢁ1
3
ꢁ1 ꢁ1
M
s
for CoTAPhPc–MWCNT–GCE, 3.24 ꢀ 10 M
s
for CoTAPhPc–MWCNT–
2
ꢁ1 ꢁ1
2
rGONs–GCE, 8.78 ꢀ 10
M
s
for CoTAPhPc–rGONs–MWCNT–GCE and 2.94 ꢀ 10
ꢁ
1 ꢁ1
M
s
for CoTAPhPc–MWCNT/rGONs (mix)–GCE. The values for CoTAPhPc–
MWCNT–GCE and CoTAPhPc–MWCNT–rGONs–GCE are an improvement to that given
3
ꢁ1 ꢁ1
in literature (1.93 ꢀ 10 M
s ) for L-cysteine detection on Nile blue A modified elec-
trode [38]. This makes our probes suitable for the detection of L-cysteine. Thus, lower
detection limits and overpotential, higher sensitivity and catalytic rate constant, made
CoTAPhPc–MWCNT–GCE the best electrode; the presence of rGONs improved elec-
trode stability.

JOURNAL OF COORDINATION CHEMISTRY
13
Table 2. Comparative LODs for the electrocatalyzed oxidation of L-cysteine using different cata-
lytic systems.
Electrodes
pH
LOD (mM)
Reference
Mixed-CoTAPc–MWCN–GCE
MWCNT–FeTsPc (mixed)–GCE
CoPc–carbon paste
CoOHETPc–SAM–Au
FeOHETPc–SAM–Au
CoTAPhPc–MWCNT–GCE
CoTAPhPc–MWCNT–rGONs–GCE
CoTAPhPc–rGONs–MWCNT–GCE
CoTAPhPc–MWCNT/rGONs (mix)
4
7
2.4
4
4
4
4
4
4
0.28
1
[12]
[34]
[35]
[36]
[36]
This work
This work
This work
This work
0.52
0.52
0.03
2.24
0.11
6.39
OHETPc: octa(hydroxyethylthio)phthalocyanine; SAM: self-assembled monolayer.
4
. Conclusions
Glassy carbon electrode was modified CoTAPhPc in the presence of rGONs and
MWCNTs were used for the detection of L-cysteine as test analyte.
a
CoTAPhPc–MWCNT–GCE was a better sensor for L-cysteine than the other probes
incorporating rGONs in terms of sensitivity. Sequential layers of MWCNT and rGONs
improves conductivity compared to CoTAPhPc alone or when MWCNT/rGONs are
mixed. Inclusion of rGONs results in resistance to passivation. Generally the inclusion
of the carbon-based nanomaterials resulted in improvement of the properties of the
phthalocyanine toward electrical conductivity hence L-cysteine detection. The low
detection limit of 30 nM indicates CoTAPhPc–MWCNT–GCE can be used as a platform
for the detection and quantification of L-cysteine.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the African Laser Centre (ALC), the Department of Science and
Technology (DST), CSIR National Laser Centre, Rental Pool Programme, National Research
Foundation (NRF) of South Africa through DST/NRF South African Research Chairs Initiative for
Professor of Medicinal Chemistry and Nanotechnology (UID 62620), and Rhodes University.
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