Carbon supported olivine type phosphate framework: A promising electrocatalyst for sensitive detection of dopamine

Article abstract of DOI:10.1039/c8ra05034a

In this study, a layered olivine-type LiMnPO₄/functionalized-multiwall carbon nanotube (f-MWCNTs) composite is used as an electrochemically active material for the real-time detection of dopamine. A wet-chemical ultrasonication process is used to combine LiMnPO₄ with f-MWCNTs at room temperature. The composite was subjected to various structural, morphological and electrochemical studies. The blending of olivine-type LiMnPO₄ into the f-MWCNTs is revealed by TEM analysis. The electrochemical activities of the LiMnPO₄/f-MWCNTs composite are systematically investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) for the real-time detection of dopamine. Furthermore, the applicability of the as prepared LiMnPO₄/f-MWCNTs composite was extended for the detection of human serum (E48) and rat brain-serum (C7) samples with satisfactory recoveries for the real-time applications. All these studies revealed that the layered olivine-type LiMnPO₄/f-MWCNTs composite is a potential candidate in the field of electrochemical sensing.

Full text of DOI:10.1039/c8ra05034a

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Carbon supported olivine type phosphate
framework: a promising electrocatalyst for
Cite this: RSC Adv., 2018, 8, 27775
sensitive detection of dopamine†
*
Raja Nehru and Shen-Ming Chen
In this study, a layered olivine-type LiMnPO4/functionalized-multiwall carbon nanotube (f-MWCNTs)
composite is used as an electrochemically active material for the real-time detection of dopamine.
A wet-chemical ultrasonication process is used to combine LiMnPO4 with f-MWCNTs at room
temperature. The composite was subjected to various structural, morphological and
electrochemical studies. The blending of olivine-type LiMnPO4 into the f-MWCNTs is revealed by
TEM analysis. The electrochemical activities of the LiMnPO4/f-MWCNTs composite are
systematically investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV)
for the real-time detection of dopamine. Furthermore, the applicability of the as prepared LiMnPO4/
f-MWCNTs composite was extended for the detection of human serum (E48) and rat brain-serum
(C7) samples with satisfactory recoveries for the real-time applications. All these studies revealed
that the layered olivine-type LiMnPO4/f-MWCNTs composite is a potential candidate in the field of
electrochemical sensing.
Received 12th June 2018
Accepted 13th July 2018
DOI: 10.1039/c8ra05034a
rsc.li/rsc-advances
techniques have some limitations, particularly high-cost, being
time-consuming and the usage of environmentally hazardous
1. Introduction
solvents. The electrochemical sensing technique is considered as
the potential alternative for the real-time detection of DA because it
is facile, cost-effective, easy to operate, eco-friendly, rapidly
responsive, highly sensitive, and selective. In the electrochemical
sensing technique, the detection of DA is limited by the interfering
biomolecules having the similar oxidation potential such as
ascorbic acid (AC), uric acid (UA) and glucose, which results in
superimpose responses making their differences difficult.8,9 To
overcome this problem, modied electrodes are designed for the
selective and sensitive detection of DA. Intriguingly, the oxidation
peak potential of DA appeared to be more positive over carbon-
based modied electrodes. Nevertheless, the low catalytic
behavior limits the electrochemical performance of the carbon-
based electrodes as an active sensor for the real-time detection
of DA. In order to enhance the electrochemical properties of the
carbon-based electrodes, it has been suggested to form the
composite with highly electrocatalytic materials. Mainly, the
formation of a composite with CNTs using high active electro-
chemical material is believed to be more effective for the sensor
applications. Also, the formation of nano-composite is sufficient to
reduce the fouling effect that is commonly occurred in pristine
MWCNTs.
Human brain function is systematically regulated by neuro-
transmitters, which are endogenous molecules made by
neurons used for the communication and transmission of
signals between neurons and the central nervous system.
According to Otto Loewi study, based on the chemical and
molecular characteristics, the neurotransmitters have been
categorized into different classes, such as biogenic amine,
amino acids, peptides and gaseous transmitters.1 Among them,
dopamine (DA; IUPAC: 4-(2-aminoethyl) benzene-1,2-diol) plays
an essential role in several vital body functions, such as circa-
dian rhythm, memory management, mood regulation, motor
functions, and prolactin secretion. The 3D-chemical structure
of DA is depicted in the ESI (Fig. S4†). Typically, the low-level
secretion of DA cause Alzheimer's, schizophrenia, Hunting-
ton's diseases and HIV infections.2 On the other hand, the high-
level secretion of DA leads to loneliness as well as abnormal
activity or mental disorder in a central nervous system.3
Therefore, accurate and real-time detection of DA is highly
crucial in clinical diagnostics.4 There are several types of analytical
techniques available for the detection of DA including high-
performance liquid chromatography (HPLC),5 spectrometry,6
calorimetry, and electrophoresis.7 However, the above-mentioned
Phospho-olivines with general formula LiMPO4 (M ¼ Fe, Mn,
Co, or Ni) is emerging as the potential electrode materials for
electrochemical energy storage devices, electrocatalysts and
water splitting applications.10 Recently, the enhanced electro-
catalytic activity was demonstrated using multi-component
Department of Chemical Engineering and Biotechnology, National Taipei University of
Technology, Taipei 10608, Taiwan. E-mail: Smchen1957@gmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra05034a
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Scheme 1 Schematic illustration of GCE/LiMnPO4/f-MWCNTs composite and its real-time applications of DA.
olivine-type transition metal (M ¼ Co, Ni) phosphates for by using NaOH/H2SO4. The de-ionized (DI) water was used to
glucose oxidation.11 These olivine-type phosphates are eco- prepare all electrolyte solutions for electrocatalytic studies. The
friendly and economically viable.12–14 As mentioned earlier, f- samples such as Human serum (E48), and rat brain serum (C7)
MWCNTs are well-known sensor material because of their were acquired from Chang Gung University, Taiwan. The
large electrode surface area and conductivity. However, their experimental protocols were carried out on approval of the
low catalytic behavior limits the performance as a practical Institutional Animal Ethics Committee.
sensor. By combining olivine-type phosphate and f-MWCNTs is
believed to be useful to improve the electrochemical perfor-
mance of the sensing applications.
2.2. Functionalization of MWCNTs
Prior to composite formation, the MWCNTs are subjected to
surface functionalization. Typically, surface functionalized
multi-walled carbon nanotube (f-MWCNTs) possess large
surface area for the more active reaction center. Therefore,
hydrophilic functional groups, such as –COOH and –OH were
functionalized on the surface of the MWCNTs according to the
previously reported procedure.15 Briey, the pristine–MWCNTs
(0.5 g) were dispersed into 50 mL (1 : 1 v/v) ratio of HNO3 (67%)
In this study, electrochemically active olivine-type LiMnPO4
combined with f-MWCNTs to investigate the electrochemical
properties for the real-time detection of dopamine (DA). The
microstructural properties and morphology of the as-prepared
nanocomposite were examined. Besides, the LiMnPO4/f-
MWCNTs was used to determine the real samples, such as
human serum (E48) and rat brain-serum samples (C7) with
functional recovery. All these studies revealed that the LiMnPO4/
f-MWCNTs composite is efficient for real-time electrochemical
sensing application. Furthermore, superior electrochemical
performances prove that the olivine-type LiMnPO4/f-MWCNTs
frameworks an efficient and promising sensing material for
real-time detection of dopamine with high selectivity.
ꢀ
and H2SO4 (97%) and kept at 60 C for 30 min under constant
magnetic stirring. Then, the solution was subjected to ultra-
sonication for 6 h and nally cooled down to room temperature.
The f-MWCNTs residue is washed with DI water to remove acids
by centrifugation. The washing and centrifugation process
repeated several times until the supernatant liquid to reach the
pH value 7, indicating that the acids have been completely
removed from the solution. The residual black-powder of f-
2. Experimental
2.1. Materials
All chemicals, such as Li3PO4 (Chempur 99.99%), MnCO3
(Aldrich 99.99%), NH4H2PO4 (Chempur 99.99%), MnCl2
(99.999% Aldrich) and pure-MWCNTs were used as received
without any further purication. The supporting electrolyte
solution, 0.05 M phosphate buffer (PB) was prepared by using
sodium phosphate (dibasic and monobasic, anhydrous; Sigma-
Scheme
2 A schematic illustration of electrochemical oxidation
Aldrich). The pH of PB solution was adjusted to neutral (pH-7) mechanism of DA by using LiMnPO4/f-MWCNT composite.
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Fig. 1 (a) Three-dimensional crystal structure view of LiMnPO4 (b) X-ray powder diffraction patterns of LiMnPO4 recorded at room temperature
along with Rietveld analysis.
MWCNTs is dried at room temperature. The well-dried f-
MWCNTs is used to combine with olivine LiMnPO4 by sono-
chemical synthesis method.
2.3. Synthesis of LiMnPO4/f-MWCNTs composite
Fine powder of LiMnPO4 obtained by grinding the high-quality
crystals was used for the preparation of LiMnPO4/f-MWCNTs
Fig. 2 Structural and phase purity characterization of the LiMnPO4 nanoparticles. (a) Representative TEM image of the as-synthesized LiMnPO4
particles; (b) f-MWCNTs (inset; SAED pattern of f-MWCNTs) (c) LiMnPO4/f-MWCNTs composite (inset: higher magnification image) and (d)
HRTEM image and associated ED pattern of the LiMnPO4/f-MWCNTs composite.
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Fig. 3 (a) The CV curve response of 200 mM of DA on the GCE/LiMnPO4/f-MWCNTs (curve a0), GCE/f-MWCNT (curve b0), GCE/LiMnPO4 (curve
c0) and bare GCE (curve d0) in 0.05 M PB solution (scan rate: 50 mV sꢁ1), (b) CV response of DA at GCE/LiMnPO4/f-MWCNTs in different pH in the
ranging from 3.0 to 11.0, (c) plot of peak current vs. different pH and (d) corresponding calibration plot for pH vs. formal potential (E00).
composites. Initially, f-MWCNTs (5 mg mLꢁ1) was dispersed into a working electrode (surface area: 0.071 cm2), Ag/AgCl as
100 mL of DI water and then ultrasonicated for 45 min. The ne a reference electrode and Pt mesh as a counter electrode. The
powder of LiMnPO4 (10 mg) was added to the suspension of f- detection of DA was accomplished by using two different tech-
MWCNTs solutions. The resulting mixture was ultrasonicated for niques, such as cyclic voltammetry (CV) and differential pulse
an hour aerward; the suspension was settled down at the bottom voltammetry (DPV) from CHI 1205C and CHI 900 (CH Instru-
like a black residue. Finally, the water was decanted entirely from ments, Electrochemical Analyzer, USA), respectively.
the residue. The resulting LiMnPO4/f-MWCNTs composite was
dried at room temperature. The well-dried LiMnPO4/f-MWCNTs
composite was used for the electrochemical analysis. The sche-
matic illustration with real time sensing application of LiMnPO4/f-
2.5. Fabrication of modied electrode (LiMnPO4/f-
MWCNTs)
MWCNTs composite is shown in (Scheme 1).
Before modication, the GCE was well polished (mirror-like) with
0.05 mM alumina slurry and followed by the removal of alumina
particles on the surface with DI water. The well-polished GCE was
sonicated for few seconds with water and ethanol (1 : 1 ratio).
2.4. Characterization
The formation of LiMnPO4/f-MWCNTs composite is conrmed The prepared LiMnPO4/f-MWCNTs composite was re-dispersed
by X-ray diffraction (XRD) and TEM analysis. Blending of the in DI water and was sonicated for 15 min to obtain the suspen-
LiMnPO4 into f-MWCNTs is examined by transmission electron sion of LiMnPO4/f-MWCNTs composite. About 6 mL of obtained
microscopy (TEM – TECNAI G2). XPS measurements were suspension was drop-casted on the surface of well-polished GCE.
carried out with a Kratos Axis Ultra spectrometer, using The drop-casted GCE was allowed to dry at room temperature.
a focused monochromatized AlKa X-ray source (1486.6 eV). The modied electrode was further used for electrochemical
Electrochemical studies were performed using the conventional investigation. The redox reaction involved in the DA is given the
three-electrode system, such as glassy carbon electrode (GCE) as following equation (Scheme 2).
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Fig. 4 (a) The CV response obtained at GCE/LiMnPO4/f-MWCNTs for different concentrations (0 to 200 mM) of DA in 0.05 M PB solution at scan
rate of 50 mV sꢁ1, (b) corresponding calibration plot for E/V vs. (Ag/AgCl) vs. peak current response, (c) the CV response of GCE/LiMnPO4/f-
MWCNTs modified electrode in the presence 0.05 M PB solution containing 200 mM of DA at different scan rates (a0 ¼ 10 to t0 ¼ 200) and (d)
corresponding calibration plot for scan rate (mV sꢁ1) vs. peak current response.
is evident that the LiMnPO4 crystallites elegantly intercalated by
the tangle of f-MWCNTs. In order to conrm the intercalation of
LiMnPO4, the composite is characterized by high-resolution TEM
3. Results and discussion
3.1. Structure and morphology analysis
(HRTEM) analysis. From shows the HRTEM image (Fig. 2(d)), the
intercalated single crystalline LiMnPO4 diffraction pattern clearly
observed. The lattice spacing of 1.03 and 0.47 nm can be assigned
to {100} and {001} crystal planes of olivine-type LiMnPO4
respectively. From the HRTEM image (Fig. 2(d)), the blended
single crystalline LiMnPO4 diffraction pattern visibly observed.
The lattice spacing of 1.03 and 0.47 nm can be assigned to {100}
and {001} crystal planes of layered olivine-type LiMnPO4,
respectively.
The structure and purity of the olivine type LiMnPO4, before the
composite formation with f-MWCNTs, was conrmed by powder
X-ray diffraction (XRD) study. The XRD pattern of LiMnPO4 is
shown in Fig. 1(b). The observed XRD pattern is well correlated to
the orthorhombic crystal structure with Pnma space group. The
estimated lattice parameters from the observed XRD pattern,
˚
˚
˚
such as a ¼ 10.4561(4) A, b ¼ 6.1081(2) A, c ¼ 4.7478(2) A and a ¼
b ¼ g ¼ 90ꢀ are in good agreement with the earlier report.16 There
are no additional diffraction peaks related to the secondary or
impurity phases in the XRD pattern, which species the high
purity of the LiMnPO4 sample. The formation of LiMnPO4/f-
MWCNTs composite and its morphology is examined by the 3.2. Electrochemical properties
TEM analysis. The single-crystalline nature of the LiMnPO4
The electrochemical activity of LiMnPO4/f-MWCNTs composite
nanoparticle is evinced from the SAED pattern in the inset of
Fig. 2(a). The bundles of interlinked f-MWCNTs with the inset of
amorphous SAED are clearly shown in Fig. 2(b). From Fig. 2(c), it
was studied by two different techniques, such as cyclic vol-
tammetry (CV) and differential pulse voltammetry (DPV) for the
detection of dopamine.
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modied electrode CV responses were carried out at different
acidic and alkaline mediums in the range of pH from 3 to 11 at
the same concentration of DA, which is shown Fig. 3(b). The DA
sensing is observed with higher oxidation peak current at pH-7,
this is because of the electroactive species that are highly active
in the neutral medium. The electrons and protons participate in
the redox behavior of biomolecules (DA). The redox peak
current and potential are pH dependent,17 hence the oxidation
peak shi toward the negative potential. As shown in Fig. 4(b),
the LiMnPO4/f-MWCNTs composite generated the highest
sensing activity at pH-7 and sensing activity gradually decreased
when the pH was increased to neutral and alkaline values.
Moreover, the pH dependence current shi of the LiMnPO4/f-
MWCNTs composites at gives robust electrocatalytic activity
towards the detection of dopamine (DA). For the comparison of
oxidation peak current versus potential at different pH value is
shown in Fig. 3(c). From Fig. 3(c), the oxidation peak current is
much higher at pH-7, which indicates that the GCE/LiMnPO4/f-
MWCNTs have shown good electrochemical activity at neutral
PH value for the detection of DA. Furthermore, the peak
3.2.1. Electrochemical determination of DA by CV. The
cyclic voltammetry is one of the promising techniques to detect
the DA concentrations with modied electrodes. The CV
responses of bare GCE, GCE/LiMnPO4, GCE/f-MWCNTs, GCE/
LiMnPO4/f-MWCNTs in 0.05 M PB solution (pH 7) with 200 mM
of DA at the scan rate of 50 mV sꢁ1 are shown in Fig. 3(a). The
redox reaction dopamine to dopamine-o-quinone (DA–DAQ,
anodic peak) and dopamine-o-quinone to dopamine (DAQ–DA,
cathodic peak) is presented in Scheme 2. The GCE modied
with LiMnPO4/f-MWCNTs composite exhibits better response
as compared to the bare GCE and GCE/LiMnPO4, GCE/f-
MWCNTs for the detection of DA. From CV, well-dened
redox peaks appeared at 0.2160 V (Epa) and 0.1845 V (Epc) for
the GCE/LiMnPO4/f-MWCNTs. From Fig. 3(a), the peak-to-peak
potential difference (DEp) is lower for the GCE/LiMnPO4/f-
MWCNTs as compared to other modied electrodes, which is
attributed by the kinetics and electron transfer behavior of
LiMnPO4/f-MWCNTs composite. Furthermore, it is important
to study the electrochemical activity at different pH value for the
real-time detection of DA. Therefore, the LiMnPO4/f-MWCNTs
Fig. 5 (a) DPV response of GCE/LiMnPO4/f-MWCNTs modified electrode in the presence of 0.05 M PB solution containing different addition of
DA, the DA oxidation ranging from (0.1 mM to 49 mM), (b) the corresponding calibration plot response for peak current response vs. concentration,
(c) the DPV response at GCE/LiMnPO4/f-MWCNTs with successive addition of DA (a0) in the presence of 10 mM was investigated with 10-folds
excess of interfering biological compounds such as (b0) ascorbic acid, (c0) uric acid, (d0) hydroquinone, (e0) glucose, (f0) catechol and (d) the
corresponding response for the effect of 10-fold interfering biological compounds vs. DA current response.
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Table 1 Comparison of GCE/LiMnPO4/f-MWCNTs sensor with other existed electrochemical DA sensors
Electrode
GEF/CFE
Morphology
Method
LOD (mM)
Linear range (mM)
Ref.
21
DPV
0.5
0.7–45.21
Pd-NC/rGO/GCE
Graphene/Pt/GCE
Pt/MWCNT/GCE
MWCNT/GCE
Amperometry
Amperometry
DPV
7.02
0.05
0.027
0.13
20–220
22
23
24
24
0.03–8.13
0.04–620
0.015–7.8
DPV
Pt/rGO
DPV
DPV
DPV
0.25
0.01
3.33
10–170
0.1–30
10–320
25
26
27
Pt–CNT–GR/GCE
rGO/PAMAM/MWCNT/AuNP/GCE
Pd3Pt1/PDDA-rGO
DPV
0.04
4–200
28
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Table 1 (Contd.)
Electrode
Morphology
Method
DPV
LOD (mM)
Linear range (mM)
Ref.
29
MWCNT/CCE
0.33
5–50
CHI/VSG/PPy scaffold
DPV
DPV
0.0194
0.024
0.1–200
30
31
g-WO3/GCE
0.1–50, 50–600
LiMnPO4/f-MWCNT/GCE
DPV
0.019
0.1–49
Present study
potential (EPa and EPc) shis towards positive and negative redox peak current is found to increases linearly with an
direction at the lower and higher pH value, respectively in the increase of the DA concentration from 0 to 200 mM.
!
presence of 0.05 M containing PB solution. The Fig. 3(d) shows
ꢀ
ꢁ
A
V sꢁ1
Ipa ðmAÞ ¼ 2:5590 ꢂ 10ꢁ5
ꢁ 0:995 R2 ¼ 0:9894
(1)
the corresponding calibration plot for pH vs. formal potential.
0
0
The formal potential (E ) of DA between the range of pH from 3
to 11 shows the proper linear range with a slope value of
50.3 mV pHꢁ1. The obtained slope value is very near to the
theoretical value (59.0 mV pHꢁ1 z 50.3 mV pHꢁ1) that was re-
ported foꢁr the redox reaction involving equal number of elec-
trons (2e ) and protons (2H+).18 In overall consideration, the
electrochemical behavior of DA purely depends on pH of the
electrolyte solution. Furthermore, the electrochemical redox
reaction of DA involves with an equal number of electrons (2eꢁ)
and protons (2H+). Typically, the peak current in biosensors is
directly proportional to the concentration of the analyte.
Fig. 4(a) and (b) shows the modied electrode GCE/LiMnPO4/f-
MWCNTs accomplished by the effect of different concentration
!
ꢀ
ꢁ
A
Ipc ðmAÞ ¼ ꢁ1:062 ꢂ 10ꢁ5
ꢁ 4:72 V R2 ¼ 0:9838 (2)
V sꢁ1
The electrochemical kinetic behavior of DA was evaluated
by performing the CV at different scan rates. The measure-
ment was carried out at 200 mM of DA in 0.05 M PB solution by
varying the scan rate from 10 to 200 mV sꢁ1 as in Fig. 4(c).
From Fig. 4(c), the redox peak current is steadily increasing
with the increase of scan rate from 10 to 200 mV sꢁ1. The
oxidation and reduction peak current at different scan rate was
estimated using linear regression eqn (3) and (4), respectively.
Thus, the obtained redox peak currents of DA are plotted
against the various scan rates ranging from 10 to 200 mV sꢁ1 as
shown in Fig. 4(d), in which the linear increase of anodic and
cathodic currents with an increase of scan rate has been
observed.
of DA in neutral 0.05 M PB solution at a scan rate 50 mV sꢁ1
.
The oxidation and reduction peak current at different
concentration of DA was estimated using linear regression eqn
(1) and (2), respectively. From Fig. 4(a), it is shown that the
redox peak current increases linearly with an increase of the DA
concentration from 0 to 200 mM. Typically, the peak current in
biosensors is directly proportional to the concentration of the
analyte.19 Fig. 4(a) and (b) shows the modied electrode GCE/
LiMnPO4/f-MWCNTs accomplished by the effect of different
concentration of DA was estimated using linear regression eqn
(1) and (2), respectively. From Fig. 4(a), it is shown that the
!
ꢀ
ꢁ
A
V sꢁ1
Ipa ðmAÞ ¼ 5:21 ꢂ 10ꢁ5
þ 8:4464 R2 ¼ 0:9998
(3)
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Fig. 6 DPV response of GCE/LiMnPO4/f-MWCNTs modified electrode in the presence of 0.05 M PB solution containing real samples at different
concentration of DA: (a) human serum sample at different concentration of DA (a0) 20, (b0) 30, (c0) 40, (d0) 50, (b) corresponding calibration plot for
peak current response vs. concentration [DA]/mM, (c) rat brain-serum sample at different concentration of DA (a0) 20, (b0) 30, (c0) 40, (e0) 50 and (d)
corresponding calibration plot for peak current response vs. concentration [DA]/mM.
!
concentration. Fig. 5(a) shows DPV response attained at a low-
level concentration of DA that ranges from 0.1 mM to 49 mM in
the presence of N2 saturated PB solution. From Fig. 5(a), the
oxidation peak current was found to increase with increasing the
DA concentration. Also, for the lowest concentration of DA,
a well-dened anodic peak current at 0.1696 V caused due to the
oxidation of DA to DAQ is observed. The Fig. 5(b) shows the
oxidation peak current is plotted against the concentration of
[DA/mM]. It exhibits the good range from 0.1 mM to 49 mM with
the linear correlation coefficient of R2 ¼ 0.9872. The lower
detection limit (LOD) and sensitivity of DA are calculated from
the slope value of linear calibration plot. The lowest detection
limit is calculated to be 19 nM with a sensitivity of 37.18169 mA
mMꢁ1 cmꢁ2. The low detection limit at the nano-molar level of
biomolecules pronounces the better performances of the GCE/
LiMnPO4/f-MWCNTs composite. Comparison of the GCE/
LiMnPO4/f-MWCNTs sensor with other existed electrochemical
DA sensors are given in Table 1.21–31
ꢀ
ꢁ
A
V sꢁ1
Ipc ðmAÞ ¼ ꢁ3:808 ꢂ 10ꢁ5
ꢁ 0:0551 R2 ¼ 0:9999 (4)
The logarithmic plot of scan rate versus log(Ipa) is shown in
the ESI (Fig. S6).† The linear dependent of Ipa with scan rate has
been observed. From the ESI-Fig. S6,† the slope value estimated
to be 1.0289 with the regression coefficient (R2) is equal to
0.9994. According to the kinetic theory of electrode reaction, the
slope of log(Ipa) vs. log(scan rate) should be 0.5 V for pure
diffusion controlled process and 1 V for the pure adsorption
controlled process.20 Hence, the observed slope value of
1.0289 V indicates that the electron transfer reactions of DA on
GCE/LiMnPO4/f-MWCNTs modied electrodes are an
adsorption-controlled process.
3.2.2. Electrochemical sensitivity of DA by DPV. Differential
pulse voltammetry (DPV) plays a vital role in the electrochemical
study due to high sensitivity when compared to the other vol-
tammetry techniques. In this study, DPV was further used to
determine the sensitivity of DA in PB (0.05 M) solution at low
3.2.3. Selectivity and real sample analysis of DA sensor.
The DA, AA and UA generally coexist in biological samples, since
it plays vital roles in human body.32 Therefore, the selective
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determination of DA in the presence of some interfering bio- in Fig. S8 and S9,† respectively. From these study, the DA sensor
logical compounds (such as, AA, UA) is quit important. The shows good recovery from bio-samples (E48) and (C7) with
electrochemical detection of these compound has more respect to the GCE/LiMnPO4/f-MWCNTs. The bio-samples (E48
concern. Commonly, the basal DA concentration is very low, and C7) obtain a good recovery ranging from 96.1 to 121.3 and
while the concentration of AA is normally higher (0.1 mM). 116.3 to 141.3, respectively given in (Table 2). Hence, the
Herein, we demonstrate the selective detection of DA by fabricated sensor can be used for the precise detection of DA in
LiMnPO4/f-MWCNTs composite. Fig. 5(c) shows the DPV the real samples. This result endorses that the GCE/LiMnPO4/f-
response of DA (a0) (10 mM) is investigated in the presence of 10- MWCNTs composite sensor can be a good recovery for the
fold excess of interfering biomolecules, such as ascorbic acid determination of DA and it can be applied to the pharmaceu-
(b0), uric acid (c0), hydroquinone (d0), glucose (e0), and catechol tical industry.
(f0), respectively. It can be seen that the interfering compounds
merged over the DA curve. It clearly reveals that there is no
electrocatalytic response observed for interfering biological
4. Conclusions
compounds. Since, it may due to the negatively charged nature
In summary, the layered olivine-type LiMnPO4/f-MWCNTs
composite is prepared by the wet sonochemical method to
investigate the electrochemical properties for the real-time
detection of dopamine. The LiMnPO4 nanoparticles are found
to be well blended between the wrinkles of the MWCNTs, which
is clearly evidenced by TEM analysis. Moreover, the retaining of
of AA {pKa ¼ 4.10} at pH 7,respectively. According to crystal eld
theory, the strong ligands donate their charges to metal ions
and therefore will acquire a negative charge.33 Herein, the DA is
positively charged {pKa ¼ 8.87} at pH (7.2 to 7.4). Therefore, the
negative charge of the metal ions easy to attract the positively
charged nature of DA. Moreover, the DA was adsorbed on the
single crystallinity of the LiMnPO4 nanoparticles in the
surface of LiMnPO4/f-MWCNTs composite, which implies that
the continues oxidation of DA was controlled adsorption
process. Fig. 5(d) shows the DPV current response of DA
exhibits enhanced electrochemical performance towards the
oxidation with various coexisting interfering compounds,
wherein the proposed sensor (LiMnPO4/f-MWCNTs composite)
obtained from various real samples, such as human serum and
would be selectively oxidized DA in the presence of other
interfering compounds. Hence, GCE/LiMnPO4/f-MWCNTs is
more suitable for the real-time detection of DA with better
selectivity.
The applicability of LiMnPO4/f-MWCNTs composite was
investigated ethically by performing conventional real sample
LiMnPO4/f-MWCNTs composite with real samples human
experimentation. The real samples such as human serum (E48)
LiMnPO4/f-MWCNTs composite is conrmed by the SAED
pattern and HRTEM study. The LiMnPO4/f-MWCNTs composite
oxidation of DA in the pure chemical form, as well as that is
rat brain-serum samples, respectively. The electrochemical
behavior of the GCE/LiMnPO4/f-MWCNTs composite via cyclic
voltammetry (CV) and differential pulse voltammetry (DPV)
revealed good sensitivity and selectivity towards the real-time
detection of dopamine. The validation of applicability of the
serum sample and rat brain-serum sample with satisfactory
and rat brain serum (C7) were acquired from Chang Gung
recoveries open up the broad scope in the further development
University, Taiwan. The bio-samples (E48) and (C7) were diluted
with 0.05 M PB solution (pH-7) and then directly used for
biological systems.
determination of at different concentrations (0.01 M [DA/mM])
of the novel electrochemical sensor for the detection of complex
as shown in Fig. 6(a) and (c). The Fig. 6(b) and (d) shows the
linear plot against the oxidation peak current and DA concen-
Conflicts of interest
tration [DA/mM]. From Fig. 6(b) and (d) the calculated linear
There are no conicts to declare.
correlation coefficient were R2 ¼ 0.9979 and R2 ¼ 0.9843,
respectively. The bio-samples (E07) and (N2) were diluted with
0.05 M PB solution (pH-7) and directly used for the determi-
nation of 0.001 M [DA/mM] at a different concentration as shown
The authors are grateful for the nancial support (MOST 106-
Acknowledgements
2113-M-027-003) from the Ministry of Science and Technology
Table 2 Determination of DA in a human serum sample (E48) and rat
(MOST), Taiwan.
brain-serum sample (C7) by using LiMnPO4/f-MWCNTs modified GC
electrode
Notes and references
Concentration
(mM)
Spiked
(mM)
Found
(mM)
Recovery
(%)
Sample
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