Arabian Journal of Chemistry (2018) xxx, xxx–xxxKing Saud University
Arabian Journal of Chemistry
www.ksu.edu.sa
www.sciencedirect.comORIGINAL ARTICLEPolydopamine-functionalized graphene nanoplatelet
smart conducting electrode for bio-sensing
applications* Corresponding author at: Department of Chemistry, Physical &
Theoretical Chemistry Laboratory, University of Oxford, South Parks
Road, Oxford OX1 3QZ, UK.
E-mail addresses: prosper.kanyong@chem.ox.ac.uk, p.kanyong@-
waccbip.org (P. Kanyong).
Peer review under responsibility of King Saud University.
Production and hosting by Elsevier
https://doi.org/10.1016/j.arabjc.2018.01.001
1878-5352 
 2018 Production and hosting by Elsevier B.V. on behalf of King Saud University.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Kanyong, P. et al., Polydopamine-functionalized graphene nanoplatelet smart conducting electrode for bio-sensing appli
Arabian Journal of Chemistry (2018), https://doi.org/10.1016/j.arabjc.2018.01.001Prosper Kanyong a,b,*, Francis D. Krampa a,c, Yaw Aniweh a,
Gordon A. Awandare a,caWest African Centre for Cell Biology of Infectious Pathogens (WACCBIP), University of Ghana, Legon, Accra, Ghana
bDepartment of Chemistry, Physical & Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1
3QZ, UK
cDepartment of Biochemistry, Cell & Molecular Biology, University of Ghana, Legon, Accra, GhanaReceived 17 November 2017; accepted 1 January 2018KEYWORDS
Polyethylene glycol;
Screen-printed electrodes;
Bio-sensing;
Mediated sensors;
Thin films;
Dopamine;
OrganocatalystsAbstract The development of a novel polydopamine (PDA)-functionalized graphene nanoplatelets
(GNPs)-based disposable sensor is described. The sensor was fabricated by drop-coating
PDA@GNPs in polyethylene glycol (PEG) and poly(3,4-ethylenedioxythiophene (PEDOT):poly(
styrenesulfonate) (PSS) aqueous suspension onto the working area of a screen-printed electrode
(SPE). The final sensor, designated as PDA@GNPs/PPP/SPE, was characterized by scanning elec-
tron microscopy (SEM), Raman spectroscopy, Faradaic electrochemical impedance spectroscopy
(FEIS) and cyclic voltammetry (CV). Mediated detection of hydrogen peroxide (H2O2) via the
redox properties of PDA was achieved. It showed excellent selectivity and sensitivity towards
H2O2 with a limit of detection and sensitivity of 0.55 mM (S/N = 3) and 3.0 mA mM

1 cm
2,
respectively. Thereafter, glucose oxidase (GOx) was immobilized onto the electrode to develop
GOx/PDA@GNPs/PPP/SPE sensor. The glucose biosensor exhibited a limit of detection of 0.25
lM (S/N = 3) and a sensitivity of 0.51 lA lM
1 cm
2; thus, proving its potential suitability for
bio-sensing applications.

 2018 Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access
article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).1. Introduction
Graphene and its derivatives are widely used as surface modi-
fiers for electrodes including glassy carbon and screen-printed
electrodes during the preparation of electrochemical bio-
(chemo) sensors (Kanyong et al., 2016a; Ratinac et al., 2011).
Electrodeposition, drop-casting and physical adsorption are
the most commonly employed methods for the modificationcations.
2 P. Kanyong et al.of these electrode surfaces (Ratinac et al., 2011; Cinti et al.,
2015). Several graphene-based electroanalytical methods have
been developed for the individual and simultaneous detection
of molecules such as ascorbic acid, uric acid, acetaminophen
and dopamine. The use of graphene-based electrodes for simul-
taneous analysis has been useful in solving some of the main
drawbacks associated with the use of traditional electrodes,
namely, low reproducibility due to electrode fouling and poor
selectivity arising from overlapping voltammetric peaks
(Kanyong et al., 2016b, 2016c, 2015, 2013; Chen et al., 2013;
Huang et al., 2011). Another possible route for resolving over-
lapping voltammetric peaks is the use of redox mediators. Typ-
ically, the use of mediators such cobalt phthalocyanine and
ferrocene has enabled the detection of hydrogen peroxide
(H2O2) at low potentials (Kanyong et al., 2012, 2016d; Rabti
et al., 2016) and, the subsequent development of sensitive and
selective sensors and biosensors (Kanyong et al., 2013, 2015).
Polydopamine (PDA) films can be formed on various mate-
rials such as glass, metals and silica. The deposition of PDA
films particularly from aqueous solution constitutes a new
and versatile way of functionalizing surfaces (Liu et al.,
2014) and the films are generally considered to be robust,
non-poisonous, inert and biocompatible. Although, both a
fundamental understanding of the mechanism of formation
of PDA and its structure is still under investigation, there is
a consensus that it consists of quinone and hydroquinone sub-
units as well as its semi-oxidized and semi-reduced forms
(Mrowczynki et al., 2014; Luo et al., 2013); thus, these sub-
units can undergo electron-transfer reactions. The reductive
capabilities of these quinone moieties have been exploited for
the covalent immobilization of enzymes and other biomole-
cules (Luo et al., 2013). More recently, PDA was employed
to selectively analyze uric acid and guanine in real samples
(Huang et al., 2014; Kanyong et al., 2016e).
In this study, we demonstrate the possibilities of using the
redox species of PDA in conjunction with nanomaterials,
namely, graphene nanoplatelets (GNPs) to develop a sensitive
and selective platform for bio-sensing applications. A one-step
facile procedure was employed for the fabrication of
PDA@GNPs. Prior to this, a smart conducting composite
consisting of polyethylene glycol (PEG) and poly(3,4-ethylene
dioxythiophene (PEDOT):poly(styrenesulfonate) (PSS) in
H2O was formulated and the as-prepared PDA@GNPs dis-
persed within it to form a nanocomposite. The nanocomposite
was then used to modify a screen-printed electrode (SPE).
Details of the sensor fabrication, characterization and
exploitation of the redox properties of PDA for bio-sensing
applications are described and discussed. This study is of rele-
vance in mediated bio-sensing and fields where PDA films are
presumed to be inert coatings.
2. Experimental
2.1. Apparatus and reagents
Electrochemical experiments were conducted using
PGSTAT204 Autolab Potentiostat/Galvanostat/EIS
FRA32M Module (Metrohm-Autolab, The Netherlands) with
Nova 2.1 Software for data acquisition and experimental con-
trol. Electrochemical impedance spectroscopy in 5.0 mM
potassium hexacyanoferrate ([Fe(CN) ]3/4
6 ) was carried outPlease cite this article in press as: Kanyong, P. et al., Polydopamine-functionalized
Arabian Journal of Chemistry (2018), https://doi.org/10.1016/j.arabjc.2018.01.001at open circuit within the frequency range of 100 kHz–0.1 Hz
at an applied potential of 0.25 V. The disposable screen-
printed carbon electrodes (Ref DS 410) used for fabricating
the sensor have a carbon working electrode, carbon counter
electrode and silver reference electrode (Scheme 1) and were
purchased from DropSens, Spain. Scanning electron micro-
scopy (SEM) was performed by JEOL JSM-610PLUS/LA
SEM (JEOL Ltd, Japan). Sessile contact angle measurements
were performed using CAM200 Optical Contact Angle Meter
(KSV Instruments Ltd., Finland). Centrifugation was per-
formed using Eppendorf Centrifuge 5804 Manual (Ham-
burg, Germany). Raman spectra were acquired with a
LabRAM 300 system (HORIBA Scientific, UK) using He-
Ne (632.8 nm) laser.
Hydrogen peroxide (H2O2), glucose oxidase (GOx), poly(3,
4-ethylenedioxythiophene) (PEDOT): poly(4-styrenesulfonate)
(PSS), potassium hexacyanoferrate ([Fe(CN) ]3/4
6 ), phosphate
buffered saline (PBS) tablets and dopamine hydrochloride were
purchased from Sigma-Aldrich, USA. Graphene nanoplatelets
(thickness of 2–10 nm) were purchased from the American
Society Material, LLC (Product No.: GNNP0051), USA.
Ultra-Pure Tris (Lot No.: 2937C302) was purchased from
VWR Life Sciences, USA. Polyethylene glycol 6000 (Product
No.: 29577) was purchased from BDH Chemicals Ltd (Poole,
England). All other chemicals were of analytical grade and used
without further purification.
2.2. Procedures
2.2.1. Preparation of PDA@GNPs hybrid nanocomposite
The PDA@GNPs nanocomposite was prepared by using a
modified version of a protocol previously reported elsewhere
(Feng et al., 2016). Briefly, graphene nanoplatelets (GNPs,
50.0 mg) were dispersed in 5.0 mM dopamine hydrochloride
aqueous solution (dopamine hydrochloride was dissolved in
10.0 mL of 5.0 mM Tris-buffer, pH 7.5). The suspension was
then stirred on a VWR Rocking Platform Shaker for 48 h
at room temperature. The resulting black product was cen-
trifuged at 5000 rpm for 30 min to separate the sediment and
supernatant. The sediment was washed several times with
deionized H2O, dried by freeze-drying and named as
PDA@GNPs.
2.2.2. Fabrication of PDA@GNPs-based sensor
The smart conducting sensor was prepared using a simple
drop-coating method. PEDOT:PSS aqueous solution was
ultrasonicated and used to formulate different percentages of
polyethylene glycol (PEG). This formulation is termed PEG/
PEDOT:PSS. The as-prepared PDA@GNPs was then dis-
persed in the PEG/PEDOT:PSS aqueous solution to form
PDA@GNPs/PEG/PEDOT:PSS nanocomposite. Thereafter,
2.0 lL of the resultant solution was dropped on the working
area (0.1257 cm2) of the screen-printed electrode (SPE),
allowed to dry in hot air oven for 1 h at 40 C to form
PDA@GNPs/PEG/PEDOT:PSS/SPE. This sensor is desig-
nated as PDA@GNPs/PPP/SPE. A schematic representation
of the SPE and sensor fabrication process is illustrated in
Scheme 1. The same procedure was used to fabricate
PEDOT:PSS/SPE, PEG/PEDOT:PSS/SPE and GNPs/PEG/
PEDOT:PSS/SPE, designated as PP/SPE, PPP/SPE and
GNPs/PPP/SPE, respectively. The surfaces of all the modifiedgraphene nanoplatelet smart conducting electrode for bio-sensing applications.
Polydopamine-functionalized graphene nanoplatelet bio-sensor 3
Scheme 1 Schematic representation of the SPE (left) and fabrication of the PDA@GNPs/PPP/SPE sensor.SPEs were thoroughly rinsed in PBS to remove any unbound
materials. Once prepared, the sensors were stored under room
temperature conditions.Fig. 1 (A) Cyclic voltammograms recorded at bare SPE, PP/
SPE and PPP/SPEs containing different percentages of PEG at
100 mV s
1 scan rate; (B) Anodic (Ipa) and cathodic (Ipc) peak
currents for [Fe(CN) ]3/4
6 vs. amount of PEG (%). CVs were
recorded in 5.0 mM [Fe(CN) ]3/4
6 in PBS (pH 7.4) containing 0.1
M KCl. P; polyethylene glycol, PP; PEDOT:PSS; 0*; bare SPE,
0%; PP/SPE.
Please cite this article in press as: Kanyong, P. et al., Polydopamine-functionalized
Arabian Journal of Chemistry (2018), https://doi.org/10.1016/j.arabjc.2018.01.001Glucose oxidase (GOx) was subsequently immobilized onto
the surface of the PDA@GNPs/PPP/SPE and the resulting
biosensor, designated as GOx/PDA@GNPs/PPP/SPE, used
to analyze glucose.2.2.3. Sessile contact angle measurement
Contact angle measurements were carried out by the sessile
drop technique; a water droplet was placed onto a flat surface
of the bare SPE and PDA@GNPs/PPP/SPE, and the contact
angle of the droplet with the surface measured. Reported val-
ues are the average contact angle (right and left) of 10 droplets.
During the measurement time (50 s), no change in contact
angle was observed. A variation of 5 is generally considered
to be sufficient to differentiate materials (Kanyong et al.,
2016e).Scheme 2 A schematic representation of the mechanism of
electrocatalytic enhancement of the SPE by PEDOT:PSS doped
with PEG.
graphene nanoplatelet smart conducting electrode for bio-sensing applications.
4 P. Kanyong et al.3. Results and discussion
3.1. Electrocatalytic study
The optimum amount of PEG in PEDOT:PSS with the highest
electrocatalytic activity towards [Fe(CN) ]3/4
6 redox couple
was optimized by coating the SPE with nanocomposites con-
taining different percentages (1, 3, 5, 10 and 15%) of PEG in
PEDOT:PSS and studying their voltammetric responses.
Fig. 1A illustrates cyclic voltammograms obtained at the bareFig. 2 (A) Cyclic voltammograms of SPE, PP/SPE, PPP/SPE,
GNPs/PPP/SPE and PDA@GNPs/PPP/SPE in PBS (pH 7.4) at
50 mV s 
1 scan rate; (B) PDA@GNPs/PPP/SPE in PBS (pH 7.4)
at scan rates of 10, 25, 50, 75, 100, 150, 175 and 200 mV s
1; (C)
Plot of Ip vs v
1/2.
Please cite this article in press as: Kanyong, P. et al., Polydopamine-functionalized
Arabian Journal of Chemistry (2018), https://doi.org/10.1016/j.arabjc.2018.01.001SPE and representatives of the modified electrodes while
Fig. 1B shows a plot of the peak current vs. the percentages
of PEG in the PEDOT:PSS aqueous suspension.
The PEDOT:PSS coated SPE shows higher electrocatalytic
activity towards [Fe(CN) ]3/4
6 in comparison with the bare
SPE and on doping the PEDOT:PSS with PEG, the electrocat-
alytic activity is markedly increased from 1% up to 10%
Fig. 1A,B).
Scheme 2 represents the proposed mechanism for the
enhanced electrocatalytic behavior of PEDOT:PSS doped
with PEG. The electrode comprises of PEDOT:PSS poly-
mer chains wherein the PEDOT molecules present in the
core of the insulating PSS are highly conductive in nature.
On being doped with PEG, the core-shell structure becomes
partially linear due to the ejection of the PSS molecules.
This exclusion of PSS from the surface is perhaps responsi-
ble for conformational changes in the polymer film that in
turn results in increased electrocatalytic activity. The addi-
tion of ethylene glycol to aqueous suspension of PEDOT:
PSS is known to decrease the columbic interactions between
PEDOT and PSS molecules which in turn contribute to the
reorientation of the polymer chains, resulting in improved
charge carrier mobility (Wei et al., 2013). This behavior
has been confirmed by Fourier transform infrared spec-
troscopy and X-ray photoelectron spectroscopic studies
(Kumar et al., 2015).
The SPE coatedwith PEDOT:PSS dopedwith 10%PEGhas
been termed a smart conducting electrode. In other studies, the
electrocatalytic behavior of PEDOT:PSS dipped in methanol,
H2SO4 and ionic liquid was markedly enhanced (Alemu et al.,
2012; Krampa et al., 2017). In this study, the effect of PEG on
the electrocatalytic behavior of PEDOT:PSS has been investi-
gated. We observed that the addition of different percentages
of PEG to PEDOT:PSS increased the electrocatalytic behavior
up to 10%; thereafter, any further increase in the percentage of
PEG does not result in an increase in the electrocatalytic behav-
ior. Consequently, 10% PEG was chosen as the optimum
amount of PEG required to be present in the PEDOT:PSS aque-
ous solution to give the highest electrocatalytic response.Fig. 3 Nyquist plots observed for electrochemical impedance
spectroscopy at bare SPE (curve a) and PDA@GNPS/PPP/SPE
(curve b) in PBS (pH 7.4) containing 5.0 mM [Fe(CN) ]3/4
6 and
0.1 M KCl.
graphene nanoplatelet smart conducting electrode for bio-sensing applications.
Polydopamine-functionalized graphene nanoplatelet bio-sensor 5
Fig. 4 (A) Scanning electron micrograph of the surface of (A) bare SPE and (B) PDA@GNPs/PPP/SPE; (C) cross-section of
PDA@GNPs/PPP/SPE; (D) Raman spectrum of PDA@GNPs/PPP/SPE.Thereafter, we incorporated PDA@GNPs into the 10%
PEG/PEDOT:PSS matrix for further electrochemical studies
since GNPs exhibits excellent electrochemical properties
(Kavan et al., 2011). The voltammetric behavior of the bare
SPE, PP/SPE, PPP/SPE, GNPs/PPP/SPE and
PDA@GNPs/PPP/SPE in PBS (pH 7.4) is shown in
Fig. 2A. No redox peaks were observed when the voltammo-
grams of the bare SPE, PP/SPE, PPP/SPE and GNPs/PPP/
SPE in buffer were recorded. However, a pair of well-
defined reversible anodic and cathodic peak potentials located
at 271.1 mV and 107.6 mV, respectively, were observed on the
PDA@GNPs/PPP/SPE; this confirms the presence of redox
species. The peaks can be attributed to the oxidation and
reduction of quinone units present in PDA (Kanyong et al.,
2016e; Mrowczynski et al., 2014). The effect of scan rate on
the voltammetric behavior of the PDA in the sensor was also
examined by cyclic voltammetry (Fig. 2B). At the scan rates
investigated, the anodic and cathodic peak potentials
remained unchanged while the anodic (Ipa) and cathodic
(Ipc) peak currents increased linearly with scan rate
(Fig. 2C). This suggests an electrochemical process that is a
mixture of diffusion and adsorption-controlled. It also indi-
cates a behavior consistent with surface confined voltamme-
try and corresponding ‘thin layer’ type voltammetry (Lee
et al., 2014). Moreover, the Ipc-to-Ipa (Ipc/Ipa) was found to
be close to unity, which is a criterion for a quasi-reversible
electrode reaction and indicates stable redox products at the
electrode surface (Krampa et al., 2017).Please cite this article in press as: Kanyong, P. et al., Polydopamine-functionalized
Arabian Journal of Chemistry (2018), https://doi.org/10.1016/j.arabjc.2018.01.0013.2. Electrochemical impedance analysis of PDA@GNPs/PPP/
SPE
The PDA@GNPs/P/PP/SPE interface was examined by
Faradaic electrochemical impedance spectroscopy (FEIS) in
the presence of 5.0 mM [Fe(CN) ]3/4
6 ]. In a Nyquist plot impe-
dance spectrum, the diameter of the semicircle at the high fre-
quency region represents charge-transfer resistance (RCT) at
the electrode surface (Kanyong et al., 2016e). Fig. 3 shows
the impedance spectra of the bare SPE and PDA@GNPs/
PPP/SPE. There was a significant decrease in the RCT value
after coating the SPE with PDA@GNPs nanocomposite, indi-
cating enhanced electron transfer occurring at the
PDA@GNPs/PPP/SPE surface.3.3. Morphological and Raman spectroscopic characterization of
PDA@GNPs/PPP/SPE
The surface morphological features of the SPE and
PDA@GNPs/PPP/SPE were examined by SEM and Raman
spectroscopy. Fig. 4A–C shows electron micrographs of the
carbon SPE and PDA@GNPs modified smart conducting
SPE, respectively. The morphology of the SPE is typical for
graphite materials with grains that are stacked in flakes
(Fig. 4A). The heterogeneous distribution of graphene nano-
platelets sheets of submicron dimension is clearly visible
throughout the substrate with wrinkles on the surfacegraphene nanoplatelet smart conducting electrode for bio-sensing applications.
6 P. Kanyong et al.(Fig. 4B) and large porous open surfaces (Fig. 1C). The
Raman spectrum (Fig. 4D) shows characteristics D, G and
2D peaks, which are typical features of thick graphene stacks
(Cancado et al., 2008). The G band is associated with the
stretching of the bonds within the chains and rings while the
D band can be attributed to the breathing mode of sp2 atoms.
It is also a characteristic of defects and substituted sites on a
material (Ferrari, 2007). The 2D band is attributable to the
stacking order and has been used to estimate the number of
graphene layers. It can also be seen that 2D band has lower
peak intensity than the G band, which is indicative of a mate-
rial composed of many layers (Ferrari, 2007); a characteristic
feature of graphene nanoplatelets.
3.4. Sessile contact angle analysis
The measurement of water contact angle on the surface of the
bare SPE and PDA@GNPs/PPP/SPE was performed. The
water contact angle for the bare SPE was found to be
73.9. However, on the PDA@GNPs/PPP/SPE the contact
angle significantly decreased to 44.3. This increase in
hydrophilicity of the modified SPE means that properties of
the PDA coated GNPs can be manipulated particularly in buf-
fer; thus, making the sensor a suitable tool for biofunctional-
ization, which is of great importance in a variety of
applications including bio-sensing and for studying biointer-Fig. 5 (A) Cyclic voltammograms recorded using PDA@GNPs/PPP
H2O2; (B) Chronoamperograms obtained at PDA@GNPs/PPP/SPE i
H2O2 in buffer (pH 7.4). Insert; icat/ibuffer vs. t
1/2 plot derived from ch
Linear segments of plot i vs. t
1/2 for (a) 0.5; (b) 1.25; (c) 2.5 and (d) 5.
of H2O2.
Please cite this article in press as: Kanyong, P. et al., Polydopamine-functionalized
Arabian Journal of Chemistry (2018), https://doi.org/10.1016/j.arabjc.2018.01.001faces (Kanyong et al., 2016e). The stability of the
PDA@GNPs modified SPE is crucial for bio-sensing applica-
tions. In order to investigate the stability and durability of the
electrocatalytic activity of the sensor, 25 repetitive cyclic
voltammograms were recorded in buffer. In general, unstable
electrodes have unstable voltammograms, however, the anodic
and cathodic peak currents (Ipa, Ipc) for the redox species on
the PDA (shown in Fig. 2B) exhibited a standard deviation
values of 1.72% and 1.39%, respectively. These standard devi-
ation values indicate that the redox activity of the PDA is
highly stable.
3.5. Application of PDA@GNPs/PPP/SPE to H2O2 analysis
The mediation of H2O2 oxidation and reduction was analyzed
as a proof of concept for evaluating the electrocatalytic prop-
erties of PDA@GNPs/PPP/SPE for non-enzymatic detection
of H2O2. Cyclic voltammograms of PDA@GNPs/PPP/SPE
with and without H2O2 is shown in Fig. 5A. Both the anodic
and cathodic peak currents increased when the concentration
of H2O2 was increased. These results suggest that the PDA
redox species catalyzed the oxidation of H2O2 to O2 or reduc-
tion to H2O and the GNPs-based nanocomposite efficiently
promotes the direct electron transfer activity of the PDA.
Therefore, the proposed mechanism for these reactions is that
both the oxidation of H2O2 to O2 and the reduction of H2O2 to/SPE at 50.0 mV s
1 scan rate in (a) buffer; (b) 2.5 and (c) 5.0 mM
n the presence of (a) 0; (b) 0.5; (c) 1.25; (d) 2.5 and (e) 50.0 mM
ronoamperometric data for buffer (a) and 0.5 mM H2O2 (b); (C)
0 mM H2O2; (D) plot of the slopes from graph C vs. concentration
graphene nanoplatelet smart conducting electrode for bio-sensing applications.
Polydopamine-functionalized graphene nanoplatelet bio-sensor 7H2O can be catalyzed by the quinone/hydroquinone moieties
in PDA, which is thought to be a 2e
, 2H+ system
(Kanyong et al., 2016d, 2016e).
The catalytic rate constant (Kcat) and diffusion coefficient
(D) of H2O2 at the PDA@GNPs/PPP/SPE were estimated
by chronoamperometry. Chronoamperometric measurements
were carried out in buffer containing various concentrations
of H2O2 (0.5, 1.25, 2.5 and 5.0 mM) at an applied potential
of +0.2 V (Fig. 5B). The catalytic rate constant Kcat was cal-
c
ulatedusing the equation (Kanyong et al., 2016e) (Eq. (1)):
icat ¼ ð 1=2Kcat:C:pÞ :t1=2 ð1Þ
ibuffer
where icat/ibuffer are the currents obtained at the PDA@GNPs/
PPP/SPE for H2O2 and buffer, C is the concentration of H2O2
and t is time in seconds, respectively. Using the slope (here
0.6954 s
1/2) of icat/i
1/2
buffer vs. t plot (insert of Fig. 5B) for
0.5 mM H2O2, a Kcat value of 9.15  104 M
1 s
1 was
obtained, which reveals that the PDA film is suitable for devel-
oping biointerfaces for bio-sensing applications (Kanyong
et al., 2016e; Rotariu et al., 2014). The slope of the linear por-
tions of i vs. t
1/2 plots (Fig. 5C) for the varying concentrations
of H2O2 (0.5, 1.25, 2.5 and 5.0 mM) were selected and used for
the construction of the i.t1/2 vs. [H2O2] plot (Fig. 5D). The
slope of i.t1/2 vs. [H2O2] plot was used in conjunction with
the Cottrell expression (Rotariu et al., 2014) (Eq. (2)):

 
¼ nFAD
1=2C
i ð2Þ
p1=2t1=2
to estimate the diffusion coefficient (D) for H2O2; and a value
of 2.82  10
5 cm2 s
1 was found.Fig. 6 (A) Chronoamperometric responses recorded at PDA@GNPs
(B) Calibration plot for H2O2; (C) Differential pulse voltammogram (
using GOx/PDA@GNPs/PPP/SPE; and (D) Calibration plot for gluc
Please cite this article in press as: Kanyong, P. et al., Polydopamine-functionalized
Arabian Journal of Chemistry (2018), https://doi.org/10.1016/j.arabjc.2018.01.001A calibration plot was then performed for varying concen-
trations of H2O2 from 0.5 to 20.0 mM (Fig. 6A) using amper-
ometry in stirred solution at an applied voltage of +0.2 V. A
linear range (Fig. 6B) was recorded from 0.5 to 13.0 mM with
a sensitivity of 3.0 mA mM
1 cm
2 and a calculated limit of
detection of (S/N = 3) of 0.55 mM; these analytical perfor-
mance characteristics are superior to similar studies reported
elsewhere (Kanyong et al., 2016d; Fan et al., 2012; Ye et al.,
2012).
3.6. Application of PDA@GNPs/PPP/SPE to glucose bio-
sensing
This study also investigated the biofunctionalization of
PDA@GNPs/PPP/SPE via the immobilization of glucose
oxidase (GOx) as a model enzyme. Differential pulse voltam-
metric measurements show an increase in PDA oxidation cur-
rent upon successive additions of standard concentrations of
glucose (Fig. 6C). This behavior can be attributed to PDA
mediation of glucose oxidation and can be expressed by
Eq. (3):
a
ð3Þ/PPP/SPE +0.2 V in PBS (pH 7.4) for standard additions of H2O2;
DPV) recorded for standard additions of glucose in PBS (pH 7.4)
ose.
graphene nanoplatelet smart conducting electrode for bio-sensing applications.
8 P. Kanyong et al.The electrocatalytic current generated via the reaction in
Eq. (3) displayed a linear behavior in the range of 1.0 to
800.0 mM (Fig. 6D). The limit of detection was found to be
0.25 lM (based on S/N = 3) with a sensitivity of 0.51 lA
lM
1 cm
2; these analytical performance characteristics of
the biosensor were superior to similar published studies which
tended to have either lower limits of detection (Rabti et al.,
2016; Palanisamy et al., 2014a, 2014b) and/or poor sensitivity
(Palanisamy et al., 2014b; Razmi and Mohammad-Rezaei,
2013). Since the physiological concentrations of glucose in
human saliva ranges from 20.0 to 240.0 lM (Siu et al.,
2014), the glucose biosensor can be employed as a sensing tool
for noninvasive monitoring of salivary glucose.4. Conclusion
We have demonstrated that the functionalization of graphene
nanoplatelets (GNPs) with polydopamine (PDA) allowed for
the development of a smart conducting bio-(chemo)sensor.
The new PDA@GNPs-based sensor was well designed and
found to be suitable for the development of low-cost, dispos-
able bio-sensors; these are clearly required for point-of-need
biomedical diagnostics applications. The bio-sensor was
employed for non-enzymatic and enzymatic detection of
H2O2 and glucose, respectively. This work represents a prelim-
inary investigation of PDA functionalized-graphene nanopla-
telets preparation and, suggests that the bio-sensor is suitable
for practical routine sensing applications.Acknowledgements
This work was supported by funds from a World Bank African
Centers of Excellence grant (ACE02-WACCBIP: Awandare)
and a DELTAS Africa grant (DEL-15-007: Awandare).
Francis Krampa was supported by a WACCBIP-World Bank
ACE PhD fellowship and Yaw Aniweh was supported by a
DELTAS Africa postdoctoral fellowship. The DELTAS
Africa Initiative is an independent funding scheme of the
African Academy of Sciences (AAS)’s Alliance for Accelerat-
ing Excellence in Science in Africa (AESA) and supported by
the New Partnership for Africa’s Development Planning and
Coordinating Agency (NEPAD Agency) with funding from
the Wellcome Trust (107755/Z/15/Z: Awandare) and the UK
government. The views expressed in this publication are those
of the author(s) and not necessarily those of AAS, NEPAD
Agency, Wellcome Trust or the UK government.Author Contributions
P.K. conceived and designed the experiments; P.K. and F.D.
K. performed the experiments; P.K. analyzed the data; G.A.
A. contributed reagents/materials/analysis tools; A.Y. pro-
vided day-to-day advice and guidance to F.D.K.; P.K. wrote
the paper. All the authors approved the final manuscript.Compliance with ethical standards
The authors declare that they have no conflict of interest.Please cite this article in press as: Kanyong, P. et al., Polydopamine-functionalized
Arabian Journal of Chemistry (2018), https://doi.org/10.1016/j.arabjc.2018.01.001References
Alemu, D., Wei, H.-Y., Ho, K.-C., Chu, C.-W., 2012. Highly
conductive PEDOT:PSS electrode by simple film treatment with
methanol for ITO-free polymer solar cells. Energy Environ. Sci. 5,
9662–9671.
Cancado, L.G., Takai, K., Enoki, K., Endo, M., Kim, Y.A., Mizusaki,
H., Speziali, N.L., Jorio, A., Pimenta, M.A., 2008. Measuring the
degree of stacking order in graphite by Raman spectroscopy.
Carbon 46, 272–275.
Chen, M., Zhang, C., Li, X., Zhang, L., Ma, Y., Zhang, L., Xu, X.,
Xia, F., Wang, W., Gao, J., 2013. A one-step method for reduction
and self-assembling of graphene oxide into reduced graphene oxide
aerogels. J. Mater. Chem. A 1, 2869–2877.
Cinti, S., Ardunini, F., Carbone, M., Sansone, L., Cacciotti, I.,
Moscone, D., Palleschi, G., 2015. Screen-printed electrodes mod-
ified with carbon nanomaterials: a comparison among carbon
black, carbon nanotubes and graphene. Electroanalysis 27, 2230–
2238.
Fan, L., Zhang, Q., Wang, K., Li, F., Niu, L., 2012. Ferrocene
functionalized graphene: preparation, characterization and efficient
electron transfer toward sensors of H2O2. Mater. Chem. 22, 616–
1170.
Feng, J., Fan, H., Zha, D., Wang, L., Jin, Z., 2016. Characterization of
the formation of polydopamine-coated halloysite nanotubes in
various pH environments. Langmuir 32, 10377–10386.
Ferrari, A.C., 2007. Raman spectroscopy of graphene and graphite:
disorder, electron-phonon coupling, doping and nonadiabatic
effects. Solid State Commun. 143, 47–57.
Huang, L., Huang, Y., Liang, J., Wan, X., Chen, Y., 2011. Graphene-
based conducting inks in electric circuits and chemical sensors.
Nano Res. 4, 675–684.
Huang, L., Jiao, S., Li, M., 2014. Determination of uric acid in human
urine by eliminating ascorbic acid interference on copper(II)-
polydopamine immobilized electrode surface. Electrochim. Acta
121, 233–239.
Kanyong, P., Pemberton, R.M., Jackson, S.K., Hart, J.P., 2012.
Development of a sandwich format, amperometric screen-printed
uric acid biosensor for urine analysis. Anal. Biochem. 428, 39–43.
Kanyong, P., Pemberton, R.M., Jackson, S.K., Hart, J.P., 2013.
Development of an amperometric screen-printed galactose biosen-
sor for serum analysis. Anal. Biochem. 435, 114–119.
Kanyong, P., Hughes, G., Pemberton, R.M., Jackson, S.K., Hart, J.P.,
2015. Amperometric screen-printed galactose biosensor for cell
toxicity applications. Anal. Letts. 49, 236–244.
Kanyong, P., Rawlinson, S., Davis, J., 2016a. A voltammetric sensor
based on chemically reduced graphene oxide-modified screen-
printed carbon electrode for the simultaneous analysis of uric acid,
ascorbic acid and dopamine. Chemosens 4, 25.
Kanyong, P., Rawlinson, S., Davis, J., 2016b. God nanoparticle
modified screen-printed carbon arrays for the simultaneous elec-
trochemical analysis of lead and copper in tap water. Microchim.
Acta 183, 2361–2368.
Kanyong, P., Pemberton, R.M., Jackson, S.K., Hart, J.P., 2016c.
Simultaneous electrochemical determination of dopamine and 5-
hydroxyindoleacetic acid in urine using a screen-printed graphite
electrode modified with gold nanoparticles. Anal. Bioanal. Chem.
https://doi.org/10.1007/s00216-016-9351-0.
Kanyong, P., Rawlinson, S., Davis, J., 2016d. A non-enzymatic sensor
based on the redox of ferrocene carboxylic acid on ionic liquid film-
modified screen-printed graphite electrode for the analysis of
hydrogen peroxide residues in milk. J. Electroanal. Chem. 766,
147–151.
Kanyong, P., Rawlinson, S., Davis, J., 2016e. Fabrication and
electrochemical characterization of polydopamine redox polymer
modified screen-printed carbon electrode for the detection of
guanine. Sens. Actuators B 233, 528–534.graphene nanoplatelet smart conducting electrode for bio-sensing applications.
Polydopamine-functionalized graphene nanoplatelet bio-sensor 9Kavan, L., Yum, J.-H., Gratzel, M., 2011. Graphene nanoplatelets
outperforming platinum as the electrocatalyst in co-bipyridine-
mediated dye-sensitized solar cells. Nano Lett. 11, 5501–5506.
Krampa, F.D., Aniweh, Y., Awandare, G.A., Kanyong, P., 2017. A
disposable amperometric sensor based on high-performance
PEDOT:PSS/ionic liquid nanocomposite thin film-modified
screen-printed electrode for the analysis of catechol in natural
water samples. Sensors 17, 1716.
Kumar, S., Kumar, S., Srivastava, S., Yadav, B.K., Lee, S.H., Sharma,
J.G., Doval, D.C., Malhotra, B.D., 2015. Reduced graphene oxide
modified smart conducting paper for cancer biosensor. Biosens.
Bioelectron. 73, 114–122.
Lee, P.T., Ward, K.R., Schulik, K.T., Chapman, G., Compton, R.G.,
2014. Electrochemical detection of glutathione using a poly(caffeic
acid) nanocarbon composite modified electrode. Electroanalysis 26,
366–373.
Liu, Y., Ai, K., Lu, L., 2014. Polydopamine and its derivative
materials: synthesis and promising applications in energy, environ-
mental and biological fields. Chem. Rev. 114, 5057–5115.
Luo, R., Tang, L., Wang, J., Zhao, Y., Tu, Q., Weng, Y., Ru, H.N.,
2013. Improved immobilization of biomolecules to quinone-rich
polydopamine for efficient surface immobilization. Colloids Surf. B
106, 66–73.
Mrowczynki, R., Bunge, A., Liebscher, J., 2014. Polydopamine- an
organocatalyst rather than an innocent polymer. Chem.- A Eur. J.
20, 8647–8653.
Mrowczynski, R., Bunge, A., Liebscher, J., 2014. Polydopamine- an
organocatalyst rather than an innocent polymer. Chem.-A Euro. J.
20 (28), 8647–8653.
Palanisamy, S., Karuppiah, C., Chen, S.M., 2014b. Direct electro-
chemistry and electrocatalysis of glucose oxidase immobilized onPlease cite this article in press as: Kanyong, P. et al., Polydopamine-functionalized
Arabian Journal of Chemistry (2018), https://doi.org/10.1016/j.arabjc.2018.01.001reduced graphene oxide and silver nanoparticles nanocomposite
modified electrode. Colloids Surf. B Biointerf. 114, 164–169.
Palanisamy, S., Cheemalapti, S., Chen, S.M., 2014a. Amperometric
glucose biosensor based on glucose oxidase dispersed in multi-
walled carbon nanotubes/graphene oxide hybrid biocomposite.
Mater. Sci. Eng. C 34, 207–213.
Rabti, A., Mayorga-Martinez, C.C., Baptista-Pires, L., Raouafi, N.,
Merkoci, A., 2016. Ferrocene-functionalized graphene electrode for
biosensing applications. Anal. Chim. Acta 926, 28–35.
Ratinac, K.R., Yang, W., Gooding, J.J., Thorndarson, P., Braet, F.,
2011. Graphene and related materials in electrochemical sensing.
Electroanalysis 23, 803–826.
Razmi, H., Mohammad-Rezaei, R., 2013. Graphene quantum dots as
a new substrate for immobilization and direct electrochemistry of
glucose oxidase: application to sensitive glucose determination.
Biosens. Bioelectron. 41, 498–504.
Rotariu, L., Istrate, O.-M., Bala, C., 2014. Poly(allylamine hydrochlo-
ride) modified screen-printed carbon electrode for sensitive and
selective detection of NADH. Sens. Actuators B 191, 491–497.
Siu, V.S., Feng, J., Flanigan, P.W., Palmore, G.T.R., Pacific, D., 2014.
A ‘‘plasmonic cuvette”: dye chemistry coupled to plasmonic
interferometry for glucose sensing. Nanophotonics 3, 125–140.
Wei, Q., Mukaida, M., Naitoh, Y., Ishida, T., 2013. Morphological
change and mobility enhancement in PEDOT:PSS by adding co-
solvents. Adv. Mater. 25, 2431–2836.
Ye, Y., Kong, T., Yu, X., Wu, Y., Zhang, K., Wang, X., 2012.
Enhanced nonenzymatic hydrogen peroxid sensing with reduced
graphene oxide/ferroferric oxide nanocomposite. Talanta 89, 417–
421.graphene nanoplatelet smart conducting electrode for bio-sensing applications.