Disposable Carbon Dots Modified Screen Printed Carbon Electrode Electrochemical Sensor Strip for Selective Detection of Ferric Ions

Abstract

A disposable electrochemical sensor strip based on carbon nanodots (C-Dots) modified screen printed carbon electrode (SPCE) was fabricated for selective detection of ferric ions (Fe ³⁺ ) in aqueous solution. C-Dots of mean diameters within the range of 1–7 nm were synthesized electrochemically from spent battery carbon rods. The analytical performance of this electrochemical sensor strip was characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The deposition of C-Dots had enhanced the electron-transfer kinetics and current intensity of the SPCE remarkably by 734% as compared to that of unmodified SPCE. Under optimized conditions, the electrochemical sensor strip exhibited a linear detection range of 0.5 to 25.0 ppm Fe ³⁺ with a limit of detection (LOD) of 0.44±0.04 ppm (at S/N ratio = 3). Validation of results by the electrochemical sensor strip was done by comparing analysis results obtained using an Atomic Absorption Spectrometer (AAS).

Full text

Research Article
Disposable Carbon Dots Modified Screen Printed
Carbon Electrode Electrochemical Sensor Strip for
Selective Detection of Ferric Ions
Shao Chien Tan, Suk Fun Chin, and Suh Cem Pang
Department of Chemistry, Faculty of Resource Science and Technology, Universiti Malaysia Sarawak,
94300 Kota Samarahan, Sarawak, Malaysia
Correspondence should be addressed to Suk Fun Chin; sukfunchin@gmail.com
Received  November ; Accepted  January ; Published  February 
Academic Editor: Biplab Paul
Copyright ©  Shao Chien Tan et al. is is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
A disposable electrochemical sensor strip based on carbon nanodots (C-Dots) modied screen printed carbon electrode (SPCE)
was fabricated for selective detection of ferric ions (Fe3+) in aqueous solution. C-Dots of mean diameters within the range of – nm
were synthesized electrochemically from spent battery carbon rods. e analytical performance of this electrochemical sensor strip
was characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). e depositionof C-Dots had
enhanced the electron-transfer kinetics and current intensity of the SPCE remarkably by % as compared to that of unmodied
SPCE. Under optimized conditions, the electrochemical sensor strip exhibited a linear detection range of . to .ppm Fe3+ with
a limit of detection (LOD) of 0.44 ± 0.04ppm (at 𝑆/𝑁 ratio = ). Validation of results by the electrochemical sensor strip was done
by comparing analysis results obtained using an Atomic Absorption Spectrometer (AAS).
1. Introduction
Ferricions(Fe
3+) are transition metal ions which play essen-
tial roles in biological activities, such as oxygen carriers in
haemoglobin [] and growth nutrients for phytoplankton [,
]. Deciency in iron can result in anemia [], yet high level
of iron in human body may result in serious health problems,
for instance, Alzheimer and Parkinson diseases [, ]. Iron
may speed up the formation of reactive oxygen species in
redox-active forms [, ]; hence overdose of iron may result
in diseases. erefore it is important to monitor the level of
iron in human body or in tap water supplies.
Conventionally, Atomic Absorption Spectroscopy (AAS)
andInductivelyCoupledPlasmaMassSpectroscopy(ICP-
MS)arebeingusedforheavymetalsanalysisduetotheirwide
range of detection and high sensitivity []. However, these
instruments are costly, time-consuming, and bulky as well as
not portable for on-site testing. Furthermore, samples have
to be transported from sites to laboratories, and preservation
of samples is normally required [, ]. A portable, highly
sensitive, and selective sensing system is highly desirable for
rapid and accurate detection of heavy metals ions especially
for in situ environmental monitoring.
Several ion-selective electrodes for detection of Fe3+
have been reported. An ion-selective electrode based on
𝜇-bis(tridentate)ligandwasshowntobehighlyselective
towards Fe3+ with a limit of detection (LOD) of . ppm
(evaluated as 5.0 × 10−6 M) []. Some researchers had
proposed the use of poly(vinyl chloride) (PVC) membrane
electrode incorporating ,󸀠-dimethoxybenzil bisthiosemi-
carbazone (DBTS) and porphyrins as receptors [, ]. Fong
et al. [] had reported a uorescence chemosensor based on
carbon nanoparticles (CNP) synthesized from sodium algi-
nate using nanoprecipitation and thermal acid carbonization
method. is sensor worked by determining the uorescence
quenching of CNP in the presence of Fe3+ and a LOD of
. 𝜇M was reported. However, these sensors for Fe3+ ions
still posed challenges of requiring the use of hazardous or
expensive chemicals and complicated fabrication process.
erefore, a low-cost, portable, ecofriendly, and highly sen-
sitive electrochemical sensor is highly desirable for on-site
rapid detection of Fe3+ ions.
Hindawi
Journal of Sensors
Volume 2017, Article ID 7576345, 7 pages
https://doi.org/10.1155/2017/7576345

Journal of Sensors
Herein, we report a disposable, sensitive, and selective
detection of Fe3+ ions using C-Dots modied SPCE based
electrochemical sensor strip. C-Dots were derived electro-
chemically from spent battery carbon rods, using a cost
eective and ecofriendly process which involved the use of
only ultrapure water as the electrolyte. Spent battery carbon
rodsservedasthecarbonsourceinthepreparationofC-Dots,
which were, in turn, used for the fabrication of electrochemi-
cal sensor. As-prepared C-Dots were observed to be selective
towards Fe3+ ions without the need of surface modication as
demonstrated by this work.
2. Experimental
2.1. Reagents and Materials. All chemicals were purchased
from Sigma-Aldrich Company, Merck Company, and Ham-
burg Company. Ultrapure water (∼. MΩ⋅cm, ∘C) was
prepared using the Water Purifying System (ELGA Model
Ultra Genetic). Hydrochloric acid (HCl) was obtained from
R&MChemicals.CarbonrodsofspentEVEREADYSuper
HeavyDutyAAsizeprimarybatterywereusedfortheprepa-
ration of C-Dots. Screen printed carbon electrodes (SPCE)
consisting of carbon-based working and counterelectrodes
and a silver/silver chloride (Ag/AgCl) reference electrode
were purchased from a local vendor, Rapid Labs Sdn Bhd.
Mineral water was purchased from Blue Ice Natural Mineral
Water.
2.2. Electrochemical Preparation of C-Dots. C-dots were pre-
pared by adopting the previously reported method []. A
direct current power supply (GPR-D) was used as the
power source. Two carbon rods (diameter = . mm) were
used as both anode and cathode which were set parallel to
eachotherandseparatedatadistanceofcminmLof
ultrapure water. A constant voltage ( V) was applied to the
electrochemical cell and the electrolyte was constantly stirred
forhours.Attheendoftheprocess,theelectrolyteturned
black indicating the formation of C-Dots. e electrolyte was
ltered using the quantitative lter disc Sartorius Grade .
e ltrate dispersion was centrifuged at , RPM for
 min to remove coarse graphite particles, and the super-
natant was oven-dried at ∘C to obtain C-Dots.
2.3. Characterization of C-Dots. e size and morphology
of C-Dots were characterized using a transmission electron
microscope (TEM) (JEOL JEM ). UV-Vis absorption
spectra of the C-Dots were measured using a UV/Vis spec-
trophotometer (Jasco V-). Fourier Transform Infrared
(FTIR) spectra of C-Dots were obtained from KBr/sample
pellets within the range of – cm−1 using FTIR spec-
trometer (ermo Scientic, Nicole iS).
2.4. Fabrication and Characterization of C-Dots Modied
SPCE Sensor Strip. Commercial SPCE strips were modied
with C-Dots using a simple drop-coating method. Simply,
 𝜇L of C-Dots dispersion was added onto the surface of
working electrode of SPCE. e SPCE was then dried in an
oven at ∘C for  min. e C-Dots modied SPCE sen-
sor strips were characterized by Cyclic voltammetry (CV)
and electrochemical impedance spectroscopy (EIS) using a
Potentiostat (Princeton Applied Research, PARSTAT ) in
. M of HCl with a scan rate of  mVs−1 within a potential
scan range of −. to . V. e response of C-Dots modied
SPCE strip was optimized by varying both the quantity of
C-Dots deposited and the concentration of HCl electrolyte.
 𝜇L C-Dots dispersion of dierent concentrations (.–
. 𝜇g) was drop-coated onto the working electrode of SPCE
and oven-dried. ese C-Dots modied SPCE strips were
characterized by CV and EIS. HCl solutions of dierent
concentrations (.–. M) were purged with nitrogen gas
for  min before use. Scheme  depicts the process on the
fabrication of C-Dots modied SPCE working electrode.
2.5. Metal Ions Selective Study. Stock solutions of various
metal ions at concentration of . ppm were prepared.
Among metal ions used for the selectivity studies included
Ba2+,Ca
2+,Cd
2+,Co
2+,Fe
3+,Hg
2+,Na
+,K
+,Sn
2+,andZn
2+
ions. ese metal ions were more commonly found in water
and associated with heavy metal pollution.  𝜇Lofthese
metal ions stock solutions was separately added dropwise
onto the surface of C-Dots modied working electrode of
SPCE,dried,andcharacterizedbyCVandEIS.Atomic
Absorption Spectrophotometer (AAS) (ermo Fisher Sci-
entic TCE ) was used for determining the Fe3+ ions
concentration in real water samples.
3. Results and Discussion
3.1. Preparation and Characterization of C-Dots. C-Dots of
uniform size were successfully prepared by electrochemical
oxidation of carbon rods of spent batteries. e preparation
of carbon nanoparticles by electrochemical oxidation of gra-
phite electrodes had been previously reported [, ]. TEM
micrograph of the as-prepared C-Dots is shown in Fig-
ure (a). e C-Dots were observed to be spherical in shape
with a size range of –nm and a mean diameter of . nm
(inset Figure (a)). Figure (b) shows the UV-Vis absorption
spectrum with the characteristic absorption peak of C-Dots at
nmwhichwasattributedtothe𝜋-𝜋transition of aromatic
carbon [, ].
Figure (a) shows FTIR spectrum of spent battery carbon
rod. Carbon rod showed  distinctive peaks at  cm−1 (O–
H),  cm−1 (C=C), and  cm−1 (C–O) which were in
consonance with functional groups of commercial graphite
rodasreportedinanotherwork[].Figure(b)showsFTIR
spectrum of as-synthesized C-Dots. Absorption peaks at
 cm−1 and  cm−1 were attributed to the COO−group
[]. Absorption peaks at  cm−1 and  cm−1 were
assigned to O–H and C=O [], whereas peaks at  cm−1
and  cm−1 were attributed to C–O stretching vibration
[].AsshownintheFTIRspectra,COO
−groups were
formed during electrochemical oxidation on the as-prepared
C-Dots which therefore required no surface modication for
their use in the detection of Fe3+ ions.

Journal of Sensors 
SPCE
SPCE
SPCE
Working electrode of
unmodied SPCE
Characterization by
CV and EIS
C-Dots drop-coated and
oven-dried C-Dos modied SPCE
Fe3+ions drop-coated and
oven-dried
Fe3+-C-Dots modied SPCE
sensor strip
sensor strip
S : Schematic diagram of the fabrication of C-Dots modied SPCE sensor strip and its use for the detection of Fe3+ ions.
2345671
Particle size (nm)
0
10
20
30
40
50
Frequency
80 kV ×300000
(a)
0
0.1
0.2
0.3
0.4
0.5
0.6
Absorbance (A)
220 240 260 280 300200
Wavelength (nm)
(b)
F : (a) TEM micrograph of C-Dots electrochemically synthesized from carbon rods of spent batteries. (b) UV-Vis absorption spectrum
of C-Dots. Inset in (a) shows the particle size distribution of C-Dots.
5001000150020002500300035004000
3450
1635
% T
1422
(a)
(b)
3215 1697
1585
14151228 1089
Wavenumbers (cm−1)
F : FTIR spectra of (a) carbon rod of spent batteries and (b)
C-Dots.
3.2. Performance and Optimization of C-Dot Modied SPCE
Sensor Strip. e analytical performances of unmodied
SPCE and C-Dots-modied SPCE sensor strip were inves-
tigated by cyclic voltammetry and EIS. Several detection
parameters were modulated in order to optimize the detec-
tion parameters of C-Dots modied SPCE strip. Dilute HCl
solution was chosen as it was commonly being used as the
electrolyte for electrochemical detection of heavy metal ions
[], and chloride ions (Cl−) were able to stabilize the SPCE
potential[,].eeectsofHClconcentrationandmass
of C-Dots on the current intensity were investigated in order
to optimize the analytical performance of C-Dots modied
SPCEsensorstrip.AsshowninFigure,thehighestcurrent
intensity was achieved by using . M HCl as electrolyte
and the deposition of  𝜇g of C-Dots onto SPCE. However,
deposition of  𝜇g of C-Dots did not lead to further increase
in the current intensity but resulted in a thicker C-Dots layer

Journal of Sensors
0.01 0.05 0.1 0.5 1
Concentration of HCl (M)
0
1
2
3
Current intensity (𝜇A)
(a)
0
1
2
3
4
5
6
Current intensity (𝜇A)
10 20 30 40 500
Mass of C-Dots (𝜇g)
(b)
F : Eect of (a) concentration of HCl and (b) concentration of C-Dots on the current intensity of C-Dots modied SPCE sensor strip.
(Error bars were calculated from the mean value, 𝑠/𝑛 =.)
(I)
(II) (III)
(IV)
−25
−20
−15
−10
−5
0
5
10
15
20
Current intensity (𝜇A)
0.30.20.10 0.4 0.5
−0.2−0.3−0.4 −0.1−0.5
Potential (V) versus Ag/AgCl
(a)
(I)
(II)
(III)
(IV)
0
50
100
150
200
250
300
350
−Z󳰀󳰀 (ohm)
100 200 300 400 500 600 700 8000
Z󳰀(ohm)
(b)
F : (a) Cyclic voltammogram and (b) Nyquist plot of (I) Fe3+
immobilized C-Dots modied SPCE, (II) C-Dots modied SPCE,
(III) Fe3+ immobilized unmodied SPCE, and (IV) unmodied
SPCE.BothCVandEISwereconductedin.MHClatascanrate
of  mVs−1 and within the frequency range of . Hz to .kHz,
respectively.
which could be detached easily from the SPCE. As such, the
optimum  𝜇g of C-Dots was deposited to modify SPCE for
studies on metal ions detection.
3.3. Ferric Ions (Fe3+) Detection by C-Dot Modied SPCE
Sensor Strip. As shown in Figure (a) C-Dots-modied
SPCE (curve II) exhibited a remarkable % higher anodic
peak current intensity as compared with that of unmodied
SPCE (curve IV), indicating that the deposition of C-Dots
had substantially enhanced the overall electrical conductivity
of SPCE. Upon addition of  𝜇Lof.ppmFe
3+ ions onto
C-Dots-modied SPCE, the current intensity was observed
to have increased substantially (curve I). However, addition of
the same quantity of Fe3+ ions onto unmodied SPCE did not
result in any notable change of current intensity (curve III).
As shown in Figure  (b), both Nyquist plots of unmodied
SPCE and Fe3+-unmodied SPCE were nearly identical with
semicircular features of similar large diameters, indicating
high electron-transfer resistance (𝑅et). In contrast, both C-
Dots-modied SPCE and Fe3+ ions immobilized C-Dots-
modied SPCE showed semicircular features of substantially
smaller diameter, indicating much lower 𝑅et. ese observa-
tions were consistent with results of CV.
Figure (a) shows the cyclic voltammograms of C-Dots
modied SPCE in the presence of various concentrations of
Fe3+ ions. e current intensity of C-Dots-modied SPCE
wasobservedtoincreaselinearlywithincreasingconcen-
trations of Fe3+ ions, within the range of . to . ppm
(Figure (b)). Such increase in current intensity could be
attributed to more Fe3+ ions being bound to –COO−groups
on the surfaces of C-Dots, which led to higher electrical
conductivity of the C-Dots-modied SPCE. Under optimized
conditions, the LOD for the detection of Fe3+ ions by the
C-Dots modied SPCE sensor strip was determined to be
0.44 ± 0.04 ppm.
3.4. Selectivity Analysis of C-Dots Modied SPCE Sensor Strip.
e selectivity of C-Dots-modied SPCE sensor strip was
investigated with a wide range of metal ions including Ba2+,
Ca2+,Cd
2+,Co
2+,Hg
2+,Na
+,K
+,Sn
2+,andZn
2+ ions which
were commonly associated with heavy metal pollution in
water. Selectivity tests were conducted under experimental
conditions optimized in this study. As shown in Figure ,
the current intensity of C-Dots-modied SPCE sensor strip
was the highest in the presence of Fe3+ ions among all metal
ions evaluated. Addition of  𝜇Lof.ppmofFe
3+ ions had
led to increase in the current intensity of C-Dots-modied

Journal of Sensors 
Concentration of Fe(III) ion
0.5 ppm
25.0 ppm
0.1 0.3 0.5
−0.3 −0.1−0.5
Potential (V) versus Ag/AgCl
−25
−20
−15
−10
−5
0
5
10
15
20
Current intensity (𝜇A)
(a)
y = 0.277x + 4.790
R2= 0.999
5 10152025300
Concentration of Fe(III) ions (ppm)
4
5
6
7
8
9
10
11
12
13
Current (𝜇A)
(b)
F : (a) Cyclic voltammogram of C-Dots modied SPCE with
various concentrations of Fe3+ ions and (b) relationship between
current intensity and Fe3+ ions concentration within the range of .
to . ppm. (Error bars were calc ulated from the mean value, 𝑠/𝑛 =
.)
Wat e r
Ba(II)
Ca(II)
Cd(II)
Co(II)
Fe(III)
Hg(II)
Na(I)
K(I)
Sn(II)
Zn(II)
Metal ions
0
2
4
6
8
10
12
14
Current intensity (𝜇A)
F : Current response of C-Dots modied SPCE for various
metalions(.ppm)(waterwasusedasthecontrol).
SPCE sensor strip by % as compared to the blank signal. In
contrast, all other metal ions were observed to have negligible
eects on the current intensity of C-Dots-modied SPCE
sensor strip. Hence, the C-Dots-modied SPCE sensor strip
was observed to exhibit high selectivity towards Fe3+ ions.
In addition, the interference study was conducted in order
to evaluate the selectivity of C-Dots-modied SPCE sensor
strip for the detection of Fe3+ ions in the presence of other
T : Relative error in the determination of Fe3+ ions concentra-
tion in the presence of other metal ions at  ppm.
Metal ions Relative error of Fe3+ ions concentration (%)
Ba(II) .
Ca(II) .
Cd(II) .
Co(II) .
Hg(II) .
K(I) .
Na(I) .
Sn(II) .
Zn(II) .
metal ions. e relative errors of Fe3+ ions concentration
determined by the sensor strip due to interferences of other
metalionswerecalculatedasshowninTable.Interferences
from Sn2+ and Na+were observed to cause comparatively
higher errors of .% and .% in the determination of
Fe3+ ions concentration. e presence of Sn2+ ions on C-Dots
modied SPCE sensor strip which exhibited higher current
intensity than that of C-Dots modied SPCE sensor strip
alone. Hence, the addition of Fe3+ ions onto the working
electrode of the sensor strip would result in higher current
intensity which in turn gave rise to higher relative error in
the Fe3+ ions concentration determination. e selectivity
of C-Dots-modied SPCE sensor strip was attributed to the
presence of the carboxylate groups (–COO−)onthesurfaces
of C-Dots with strong coordination anity towards Fe3+ ions
[, ]. e formation of Fe3+–COO−complexes could have
givenrisetohigherelectricalconductivityofthesensorstrip.
3.5. Real Sample Analysis. e potential application of C-
Dots modied SPCE sensor strip for the detection and quan-
tication of Fe3+ ions in real samples was evaluated by using
samples of tap water, reverse osmosis (RO) water, and com-
mercial mineral drinking water. Water samples were ltered
withlterpaperandthenspikedtopreparewatersamples
containing . ppm of Fe3+ ions. As shown in Table , the
%recoveryofFe
3+ ions from these samples as determined by
using the C-Dots modied SPCE sensor strip ranged between
.% and .% with relative standard deviation (RSD) of
.% to .%. ese analysis results were further validated
against those obtained by AAS. Analysis results for all real
samples obtained by both AAS and C-Dots modied SPCE
sensor strips were consistent and comparable. e C-Dots
modied SPCE sensor strip was shown to be sensitive and
selective for the detection of Fe3+ ions in aqueous samples.
4. Conclusion
A disposable C-Dots modied SPCE sensor strip for sensitive
and selective detection of Fe3+ ions in aqueous samples had
been fabricated. C-Dots were prepared from carbon rods of
spent batteries using a green electrochemical method. Under
optimized conditions, the LOD for Fe3+ ions by C-Dots

Journal of Sensors
T : Percentage recovery of spiked Fe3+ ions for various water samples using C-Dots modied SPCE sensor strips and AAS.
Real sample Sample
Fe(III) (ppm)
Spiked Fe(III)
(ppm)
Measured
Fe(III) (ppm) Recovery (%) RSD (%) Measured
Fe(III) (ppm) Recovery (%) RSD (%)
AAS C-Dots modied SPCE sensor strip
Tap w a t er <. []  . . . . . .
Reverse osmosis
water ND  . . . . . .
Mineral water ND  . . . . . .
ND = not detected.
modied SPCE sensor strip was determined to be 0.44 ±
0.04ppm. High percentage recovery of Fe3+ ions from various
water samples with low % RSD and low % relative error in
the presence of other metal ions showed that the C-Dots
modied SPCE sensor strip could potentially be used for
sensitive and selective detection of Fe3+ ions in real samples.
e detection limit can further be improved in the future by
doping the C-Dots with other elements such as nitrogen (N)
and sulfur (S) for more sensitive detection and application in
water quality studies.
Competing Interests
e authors declare that there are no competing interests
regarding the publication of this paper. e authors of this
paperhavenodirectnancialrelationwiththecommercial
entities mentioned in this paper.
Acknowledgments
is work was funded by Ministry of Higher Education,
Malaysia, via the award of a Exploratory Research Grant
[Grant no. ERGS/STG()//()]. MyBrain and
Zamalah Graduate Scholarship (ESSU) were gratefully
acknowledged.
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