A facile, solid-state reaction assisted synthesis of a berry-like NaNbO3 perovskite structure for binder-free, highly selective sensing of dopamine in blood samples

Article abstract of DOI:10.1039/c9nj02282a

Herein, we report a facile synthesis of sodium niobiate (NaNbO₃) perovskite nanomaterial using a solid-state reaction (SSR) via solvothermally grown niobium pentoxide (Nb₂O₅) nanopowder for the excellent sensitive detection of dopamine (DA) in simulated blood serum with high selectivity. The X-ray Diffraction (XRD) pattern and Raman spectrum revealed an orthorhombic phase formation for NaNbO₃ and the presence of a NbO₆ octahedra site, while the scanning electron microscopy (SEM) images confirmed berry-like cluster formations of NaNbO₃. The NaNbO₃ modified glassy carbon electrode (GCE) sensor showed an excellent selectivity against interfering species like Na⁺, Cl^-, Ca²⁺, glucose, ascorbic acid (AA), anduric acid, a sensitivity of 99 nA nM^-1 cm^-2 in the wide dynamic range of 10 nM to 500 μM, and a limit of detection (LOD) of 6.8 nM towards DA sensing, making it suitable for detecting physiological levels of DA in human blood. The sensing mechanism for DA was ascribed to the presence of NbO₆ octahedral sites in the NaNbO₃ perovskite structure that interacted with the oxidase of DA (dopamine-o-quinone) through Nb⁵⁺/Nb⁴⁺ niobium states, resulting in an increase in the oxidation peak current. This sensor did not exhibit any kind of surface fouling effects due to the reduction mechanism and the Na⁺ ions stabilizing the perovskite structure. The as-fabricated NaNbO₃/GCE sensor was further assessed for the detection of DA in simulated blood serum, which showed an excellent recovery percentage. This novel, binder-free, NaNbO₃ perovskite-based modified electrode offers a promising platform for developing high performance, non-enzymatic electrochemical sensors for numerous bioanalytical applications.

Full text of DOI:10.1039/c9nj02282a

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A facile, solid-state reaction assisted synthesis of berry-like Na
D
N
OI
b
O
perovskite structure for binder-free, highly selective sensing of dopamine in
blood samples
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Lignesh Durai , and Sushmee Badhulika
1Department of Electrical Engineering, Indian Institute of Technology, Hyderabad,
5
02285, India.
Corresponding author: E-mail: sbadh@iith.ac.in; Telephone: +040-23018443
Graphical Abstract
*
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A facile, solid-state reaction assisted synthesis of berry-like Na
D
N
OI
b
O
perovskite structure for binder-free, highly selective sensing of dopamine in
blood samples
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Lignesh Durai , and Sushmee Badhulika
1Department of Electrical Engineering, Indian Institute of Technology, Hyderabad,
02285, India.
Corresponding author: E-mail: sbadh@iith.ac.in; Telephone: +040-23018443
5
*
Abstract
Herein, we report, a facile synthesis of sodium niobiate (NaNbO
using solid-state reaction (SSR) via solvothermal grown niobium pentoxide (Nb
3
) perovskite nanomaterial
2
O )
5
nanopowder for excellent sensitive detection of dopamine (DA) in simulated blood serum with
high selectivity. The X-Ray Diffraction (XRD) pattern and the Raman spectrum revealed the
orthorhombic phase formation of the NaNbO
respectively while scanning electron microscope (SEM) images confirmed berry like cluster
formations of NaNbO3. The NaNbO modified glassy carbon electrode (GCE) sensor showed
3
and the presence of NbO
6
octahedra site,
3
+
-
2+
an excellent selectivity against interfering species like Na , Cl , Ca , glucose, ascorbic acid
2
(
AA) and uric acid, sensitivity of 99 nA/nM cm in the wide dynamic range of 10nM to 500M
and limit of detection (LOD) of 6.8nM towards DA sensing making it suitable for detecting
physiological levels of DA in the human blood. The sensing mechanism towards DA can be
ascribed to the presence of NbO
6
octahedra sites of the NaNbO
3
perovskite structure that
5
+
4+
interacts with oxidase of DA (dopamine-o-quinone) through Nb /Nb states of niobium,
which in turn increases the oxidation peak current. This sensor does not exhibit any kind of
+
surface fouling effect due to the reduction mechanism and the property of the Na ions that
stabilizes the perovskite structure. The as-fabricated NaNbO /GCE sensor was further assessed
3
2

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for the detection of DA in the simulated blood serum which showed an excellent recovery
percentage. This novel, binder-free, NaNbO perovskite based modified electrode offers a
3
promising platform for developing high performance, non-enzymatic electrochemical sensors
for numerous bioanalytical applications.
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Keywords: NaNbO , Solid State Reaction, Neurotransmitter, Dopamine, Electrochemical
3
sensor.
1
. Introduction
Dopamine (DA) is one among the utmost researched neuro-transmitters due to its vital
functions in the human body related to metabolism, cardiovascular, central neural system in
line with renal and hormonal systems [1]. The inadequate level of dopamine in the blood leads
to the neurological diseases or disorders like attention deficit hyperactivity disorder,
schizophrenia, depression and parkinson’s disease [2, 3]. There are wide range of methods for
the detection of DA like conventional chromatography or capillary electrophoresis which have
a major drawback due to the mandate sophisticated pre-treatment processes with long operation
times [3]. Apart from these in-vitro conventional techniques, there are colorimetric or
fluorescent optical detection techniques which demonstrate enhanced sensitivity towards DA,
but are mainly restricted to lab-based demonstrations with major restrictions for samples with
living cells or intact biological samples [4]. On the other hand, electrochemical techniques
overcome the above-mentioned limits and have offered numerous merits such as low cost,
applicability of in vivo analysis, enhanced non-enzymatic analysis with faster responses, and
real time analysis [5].
There are many reports on electrochemical sensing of DA using various nanomaterials such as
transitional metal oxide, graphene oxide, metal chalcogenides and polymer-based composites.
[6-8] These materials involve complex synthesis procedures, have low material stability, high
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limit of detection and moderate dynamic range of detection towards DA. The major challenges
involved in sensing DA in blood samples is its low concentration which makes it difficult for
accurate sensing and the role of interfering compounds [2], such as AA and UA that co-exist
with DA. So, there is a need for developing a simple, effective DA sensor which can detect DA
with high specificity along with a low limit of detection below the physiological levels of its
concentration in blood.
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In this work, we report a novel NaNbO
DA with excellent selectivity. The perovskite-based materials for bioanalytical measurements
have gained major interest due to their low toxicity and high stability in line with their ABO
structure Where, A is the cation material with the cuboctahedra coordination and B is the
3
perovskite nanomaterial for the sensitive detection of
3
.
transition metal ion with octahedral coordination. Herein the A site cation helps in the structural
retention and contributes to the stability of the crystals whereas B site cation favours various
functionalities of the material through the octahedral coordination [9]. Among perovskite
materials, NaNbO
transition from rhombohedral phase to cubic along with tetragonal and orthorhombic
phase [10]. NaNbO is one of the major lead-free perovskite materials which is environment
friendly [11], shows enhanced electrocatalytic properties at nanoscale [12], and high chemical
stability and exhibits a high bandgap of 2.4eV [13]. As a result, NaNbO and its composites
3
perovskite is one of the most complex materials which has a series of phase
3
3
have also been reported for humidity sensors, photocatalysis applications and piezoelectric
nanogenerator.
There are several reports on the synthesis of NaNbO through hydrothermal techniques using
3
niobium oxides with sodium hydroxide precursors [14], wet chemical methods [15] and solid-
state reactions (SSR) [16]. Among these techniques, the SSR technique offers major
advantages in terms of control over the phase transition of the material with respect to the
synthesis temperature and high productivity. Hence, we demonstrate SSR synthesized NaNbO
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modified GCE electrode for electrochemical sensing of DA. The as-fabricated NaNbO
3
/GCE
electrode showed an excellent dynamic range of detection, selectivity and sensitivity. The
enhanced electrocatalytic properties of the NaNbO towards DA were attributed to the 3d
orbital of Nb and the active NbO octahedra sites with the Nb /Nb oxidation states in the
NaNbO perovskite structure which favours the delocalization of charge carriers during
catalytic reaction [12, 18]. To the best our knowledge, this is the first report on NaNbO for
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3
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DA sensing which showed a very low LOD, a wide dynamic range of detection with excellent
selectivity towards the DA thus proving to be an excellent material for bioanalytical
applications.
2
2
. Experimental
.1 Materials
Reagents of Analytical grade like niobium chloride (NbCl
5
), sodium carbonate (Na
2
CO ),
3
dopamine hydrochloride ((HO) CH CH NH ·HCl) and ethanol were procured from
2
C
6
H
3
2
2
2
sigma aldrich and were used without any additional purification process. Deionized (DI) water
was collected from millipore system with a resistivity of 18.2MΩ .cm at room temperature and
used for preparing aqueous solutions. An electrolytic solution of 1M KCl containing 5mM of
-3/-4
Fe(CN)
solution was used as electrolyte for the electrochemical studies.
.2 Synthesis of NaNbO
6
was used to observe the electrochemical characteristics of NaNbO . 0.1M PBS
3
2
3
The synthesis of NaNbO
solvothermal technique was used for the synthesis of Nb
the synthesis of NaNbO . In brief, 0.1M of NbCl was dissolved in 80mL of ethanol under
3
was carried out using solvothermal technique followed by SSR. The
2
O
5
that acts as starting material for
3
5
constant stirring at room temperature for 30 minutes. The resultant solution was fed into 100mL
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teflon lined stainless steel autoclave at 180 C for 15h. And then reaction was allowed to cool
down to room temperature without any external processing. The reaction can be explained
using the equation (1) in which the formation of chloromethane as is shown.
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2NbCl
5
+ 5C
2
H
6
O
Nb
2
O
5
+ 5C
2
H
6
Cl ----------------------- (1)
2
In order to remove chloromethane, the product was washed using ethanol by centrifugation.
o
Finally, the collected product was dried at 100 C under vacuum for 12 hours to obtain Nb
2
O
5
nanopowder.
Later, Nb
2
O
5
nanopowder and Na
2
CO under stoichiometric calculations were well grinded
3
o
together and the resultant mixture was reacted in muffle furnace at 450 C with a rate of
o
30 C/min for 6h. The reaction was allowed to cool down to room temperature without any
external cooling process. The as obtained product was grinded well to get NaNbO
nanopowder. The reaction mechanism is shown as follows.
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Nb
2
O
5 (s)+ Na
2
CO3(s)
NaNbO3 (s)+ CO2 (g)----------------------- (2)
2
.3 Material Characterization
The as-synthesised Nb
2
O
5
and NaNbO were characterized using zeiss EVO 18 scanning
3
electron microscope (Germany)., X-Ray Diffraction (XRD) using Cu-Kα with λ=1.54 Å,
lineon X'pert PRO X-ray diffractometer at the 2θ range of 15-60°, Raman studies were
explored with senterra, bruker spectrometer with the spectral range from 200 to 1800 cm-1
using a laser source of  = 532nm and FTIR (Fourier Transform Infra-Red) spectrum analysis
-1
-1
using IR affinity at the spectrum ranging from 500cm to 3500 cm .
.4 Sensor fabrication and Electrochemical analysis
All the electrochemical measurements of NaNbO were done using the three-electrode cell
2
3
setup with the NaNbO
3
/GCE, as the working electrode, 1M KCl Ag|AgCl as the reference
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electrode and platinum wire electrode was used as the counter electrode connected to CHI 660E
electrochemical work station. For fabrication of sensor, NaNbO nanopowder was dispersed in
3
dimethylformamide (DMF) using ultra sonicator for 30 minutes to achieve uniform dispersion
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and drop casted on to GCE with a quantity of 6 L and dried using a hot air oven at 60 C for
1
0 minutes. This NaNbO
3
modified GCE (NaNbO /GCE) was used as working electrode
3
towards electrochemical sensing of DA. The entire electrochemical sensing of DA was
performed using 0.1M phosphate buffer solution (PBS; pH - 7.2) as supporting electrolyte at
the room temperature in a potential window from -0.2 to 0.8V. The concentration of NaNbO
for the sensor fabrication was also optimized and studied for the effective sensing of DA. The
electrochemical performance of NaNbO was analysed using CV and DPV techniques.
3
3
3
3
. Results and Discussion
.1 Physiochemical Characterization of NaNbO
3
The morphological analysis of the SSR synthesized NaNbO
SEM as shown in the fig. 1. Fig. 1(a) displays the formation of berry-like NaNbO
with an average particle size of ~96nM to ~296nM. The Fig. 1(b) depicts the magnified image
of NaNbO nanopowder. Here, the size of the particles remained to be in nanometres while the
cluster growing up to micrometres due to the agglomeration of the individual nanoparticles
17]. The Energy dispersive X-ray analysis (EDX) spectrum depicted in Fig. 1(c) confirms the
existence of Na and Nb with oxygen in the structure and also proves no impurities in the as-
synthesised NaNbO nanopowder.
3
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3
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[
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Figure 1 (a) SEM image of NaNbO
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nanopowder (b) High magnification SEM image (c)
EDX spectrum of NaNbO
spectrum.
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nanopowder; inset: image of the sample area scanned for the
The crystal phase of Nb
XRD and Raman spectra analysis along with the CV response of Nb
in the supporting information (SI) as Fig. S1. In brief, the Nb was observed as amorphous
2
O
5
and NaNbO
3
were studied using the XRD spectrum. The SEM,
2
O
5
are discussed in-details
2
O
5
nanomaterial with the size distribution form ~10nM to 20nM agglomerated to form
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The crystalline phase formation of the NaNbO perovskite was studied using XRD pattern
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shown in the Fig. 2(a) observed with sharp and intense peaks at

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2 =                           
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0

                           


















o
        and 77.8 indexed to (110), (112), (104), (113), (200), (202), (121),
(
115), (122), (106), (024), (116), (124), (025), (220), (026), (108), (302), (224), (311), (133),
313), (028), (208), (230), (135), (232), (136), (228), (043), (236), (331), (334) and (415) planes
(
in the NaNbO
the JCPDS data card 073-0803 which confirmed the formation of orthorhombic- NaNbO
the Pbcm space group. The peak at 2 = 46.3 confirms the formation of orthorhombic phase in
the SSR synthesized NaNbO [18], with lattice constants a = 5.577, b = 5.524 and c = 15.579.
No other peaks of the reactants, or by-products were found in the XRD pattern, which confirms
the high purity orthorhombic phase formation in the NaNbO perovskite. The chemical
composition of the NaNbO perovskite was studied using the Raman spectra shown in the Fig.
(b) where, the band situated below 100cm corresponds to the translational movements of
3
perovskite, respectively. The diffraction peaks of the pattern can be indexed to
3
with
3
3
3
-1
2
+
Na cations against the NbO
list of six different vibrational modes such as v
(F2u), modes [20]. Where, A1g, E , F2g are the Raman active modes. The F1u, F2u are infrared
active and inactive modes, respectively [21]. The presence of NbO octahedra can be
determined with vibrational modes identified in the band region between 90 to 170 cm . The
6
octahedron [19]. Generally, an isolated NbO
6
octahedron has a
1
(A1g), v
2
(E ), and v
g
3
(F1u), v
4
(F1u), v (F2g),
5
v
6
g
6
-1
-1
internal vibrational modes of NbO
6
were observed between the region of 150-1000cm , where
-1
the band region between 150 to 300 cm were due to the triply degenerated v
6
(F2u) and v
5
(F2g)
-1
modes [22]. The band at 400cm can be attributed to v
4
(F1u) mode and band around 450–750
cm-1 can be assigned to v
(A1g), v
(E
), and v (F1u) modes, respectively. The band around
1
2
g
3
-1
the region from 800–950 cm was due to the centro-symmetric space group and it can be
9

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assigned with v
1
+ v combination mode having F2g as irreducible representation [23]. The
5
existence of the functional groups in the surface of the NaNbO
3
were analysed through FTIR
spectra. The Fig. 2(c) shows the FTIR spectrum of the NaNbO
3
in which the transmittance peak
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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8
9
0
1
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
observed at the region from 500 - 850cm can be ascribed to Nb-O while the peak observed at
-1
-1
-1
~
1100cm , ~1500cm and ~1700cm were ascribed to C-OH, N=O and C=O, respectively.
-
-1
The Antisymmetric COO stretch can be observed at the IR band of ~1650cm (22) and a low
-1
intensity transmittance peak at band ~2300cm was due to the C≡N.
Figure 2 – Structural and compositional analysis of NaNbO Perovskite using (a) XRD
3
pattern, (b) Raman spectrum and (c) FTIR spectrum.
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3
.2 Electrochemical Measurements
The optimization in the weight percentage (wt.%) of electrode material plays a critical role in
sensing performance and electrocatalytic activity of any modified electrodes. The optimum
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
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7
8
9
0
1
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0
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1
2
3
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5
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7
8
9
0
wt.% of NaNbO
using 0.1, 0.3, 0.5, 0.7 and 1.0 wt.% of NaNbO
PBS electrolytic solution. The maximum peak current value was observed for 0.5wt% of
NaNbO /GCE whilst the lowest peak current was observed for 0.1wt% of NaNbO /GCE. This
behaviour can be ascribed to the rapid charge separation at the electrode-electrolyte interface
resulting in effective oxidation of DA at the active catalytic sites of the NaNbO . To achieve
better sensitivity in DA sensing the 0.5wt% of NaNbO /GCE was used as the working electrode
3
nanopowder dispersed in DMF was selected through an CV study performed
3
/GCE in the presence of 10M DA at 0.1M
3
3
3
3
for all electrochemical measurements performed in this work. The detailed optimization study
can be found in fig S2 in SI.
The electrochemical active surface area and the surface coverage of the optimized
NaNbO
3
/GCE were studied using CV and chronocoulometric techniques. The measurements
were done in the presence of 1M KCL electrolytic solution containing 5mM of Fe (CN) -3/-4
as
6
supporting electrolyte. The peak potential separation and active surface area of the
NaNbO /GCE were examined using CV technique. Fig. 3(a) shows the CV curves at different
3
scan rates from 10mV/s to 250mV/s. It was witnessed that, when the scan rate of the process
was increased, the redox peak current had a linear increase. The linear relationship between the
redox current and the log of scan rate was studied in order to determine the type of process.
Inset of Fig. 3(a) displays the linear calibration plot of the redox peak current with respect to
log of scan rate. The linear regression equation obtained with respect to anodic peak current
2
was Ipa(A) = -51.799(log (scan rate)) + 40.05309 (R =0.97781) and for cathodic peak current
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was Ipc(A) = 44.9561(log (scan rate)) - 41.33105 (R =0.97781) this confirms that the overall
electrochemical redox reaction was a diffusion controlled process under the ferricyanide
solution[25]. Fig. 3(b) shows the comparative CV analysis between the NaNbO
3
/GCE and
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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8
9
0
1
2
3
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0
1
2
3
4
5
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9
0
GCE with 1M KCL electrolytic solution containing 5mM of Fe (CN) -3/-4
6
.. The half-cell
potential (E1/2) and peak to peak separation (E
V vs Ag|AgCl and 0.104 V respectively. The GCE was found with 0.223 V vs Ag|AgCl as E1/2
and the E as 0.119 V. This decrement in the E shows the rapid electron transfer behaviour
after the oxidation by potential cycling, at the surface of the NaNbO /GCE electrode (24). The
p
) of the NaNbO3/GCE were observed as 0.235
p
p
3
effective surface area was examined using the standard Randle-Sevik equation shown below,
equation (3):
5
1/2
3/2
1/2
i
pa or ipc= 2.69x10 xA xD xn xv xc ----------------------- (3)
e
-6
2
where, D is diffusion coefficient (i.e. 7.6x10 cm /sec), c is the redox couple concentration, n
is the number of electrons involved in the redox reaction (n=1), v is the scan rate used for CV
measurement and A
e
is the effective surface area which will be calculated. The active surface
2
area with respect to anodic peak current for NaNbO
3
/GCE was found to be 0.079 cm which
was 77.19% greater than that of the GCE. The OH and oxygen functional groups ascribed to
the enhancement of electrochemical active sites of the modified GCE electrode. Fig. 3 (c)
shows the chrono-coulometry study as a comparison between NaNbO
study was performed in order to estimate the change in active surface coverage of the NaNbO
The Active surface coverage area () was calculated using the following equation, equation 4:
3
/GCE and GCE. This
3
.
Q = nFA ----------------------- (4)
where, Q is the difference between the forward and reverse intercepts; A is the area of electrode
2
(
0.0707cm ); n is the number of electrons transferred, F is the faraday constant and  is the
active surface coverage area. The  value, for NaNbO
/GCE was found to be 0.545 n mol cm-
3
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which was 87.59% greater than that of GCE. This increase in the value validates the formation
of oxy functional groups which is in agreement with the electrochemical treatment.
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2
2
2
2
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2
2
2
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2
3
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1
2
3
4
5
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8
9
0
Figure 3 – Typical CV analysis of NaNbO /GCE with the supporting electrolyte of 1M
3
-3/-4
KCl containing 5mM of Fe(CN)
6
(a) CV performed at different scan rates from
1
0mV/s to 250mV/s inset: corresponding calibration plot of I vs log(scan rate (mV/s))
displaying the linear regression line for both Ipa and Ipc (b) CV for comparison between
GCE and the NaNbO /GCE . (c) Chronocoulometric response of GCE and
NaNbO /GCE.
3
3
3
.3 Electrochemical Sensing of Dopamine
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To examine the electrochemical properties of NaNbO /GCE, CV and DPV experiments were
3
performed in the potential range of -0.2 to 0.8V vs. Ag|AgCl in 0.1M PBS (pH = 7.2),
supporting electrolytic solution in presence of 300M DA. The study on impact of electrolyte
pH on electrooxidation of DA and to determine the optimal pH value of 0.1M PBS electrolyte
0
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for sensing, the CV of NaNbO
shown in the fig. S3. Fig. 4(a) shows the electrocatalytic activity of GCE and NaNbO
the presence of the 300M DA. In presence of the 300M DA the NaNbO /GCE exhibited a
distinct oxidation peak at potential of 0.28V, indicating the excellent electrocatalytic property
of the NaNbO /GCE to the electro-oxidation of DA. The presence of reduction peak at ~0.12V
shows that the electro-oxidation of DA at the surface of NaNbO /GCE is a reversible reaction.
The GCE showed an electrocatalytic response towards the electro oxidation of DA in presence
of 300M DA which was 4.94 times lesser than the response of NaNbO /GCE. This substantial
3
/GCE was investigated at different pH from 7.0 -7.4 pH as
3
/GCE in
3
3
3
3
enhancement in the electrocatalytic property and the observed increase in the oxidation peak
current of the modified sensor in presence of DA can be attributed towards the presence of Nb
4
+
5+
in the NaNbO
3
perovskite. The electro oxidation of Nb produces Nb complex on the surface
of NaNbO /GCE electrode catalysing the oxidation of DA to dopamine-o-quinone (DQ) which
3
4
+
later generates Nb complex during the cathodic scan of the cycle. The overall structure of the
+
NaNbO
3
nanopowder was not affected due to the presence of Na ion in the perovskite
structure. The peak separation between the anodic and cathodic peak was found to be 161mV
which indicates the fast electron transfer kinetics. The multivalent states of the Nb and Na in
the complex oxide structure can easily generate the oxygen vacancies and delocalized electrons
as defects which also act as active centres for effective sensing [25]. The electron transfer
kinetics of the DA oxidation on the surface of NaNbO /GCE at 0.1M PBS (7.2pH) was studied
3
in presence of 300M DA under different scan rates. Fig. 4(b) depicts the CV at different scan
rates from 50mV/s to 200mV/s where the peak current increased linearly as the scan rate was
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increased. This behaviour is due to the existence of the quasi reversible electrochemical
reaction mechanism. The corresponding calibration plot is shown in fig. 4(c) and the linear
regression equation for both anodic and cathodic peaks were given in equation (5) and (6),
respectively.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
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0
1
2
3
4
5
6
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9
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1
2
3
4
5
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9
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1
2
3
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0
-1
2
I
pa(A) = 3.26038 (log (scan rate(mVs )) – 1.53838 (R = 0.968) ----------------------- (5)
-1
2
I
pc(A) = -4.15802 (log (scan rate(mVs )) – 4.77522 (R = 0.977) ----------------------- (6)
The slope value observed here is above 0.5, which indicates that the DA oxidation at the surface
of NaNbO /GCE is a purely an adsorption-controlled mechanism. The surface coverage of the
3
redox species with the number of electrons engaged in the mechanism were calculated using
the Laviron’s equation; shown in equation (7)
2
2
푛 퐹
푛퐹
푖푝 = 4 퐴푣휏 =
푄푣
----------------------- (7)
푅푇
4푅푇
-1
-1
Where, F = 96,500 C/mol is the Faradays constant, R = 8.314 J K mol is the gas constant, n
is the number of electrons and T = 298K the temperature at which the reaction was explored
(
here room temperature). Q is the charge corresponding to the anodic oxidation peak of DA for
-6
300M at 50mV/s; 4.482x10 C and the v = slope of the anodic peak calibration curve. The
number of electrons was calculated as ~2
The relationship between the peak potential (E
which depicts the increase in the E as the scan rate increases. The linear regression equation
for anodic and cathodic peaks are shown in the equation (8) and (9) respectively.
p
) and log of scan rate is shown in the fig. 4(d)
p
-1
2
E
pa(V) = 0.06907 (log (scan rate(mVs )) + 0.16256 (R = 0.988) ----------------------- (8)
-1
2
E
pc(V) = -0.05355 (log (scan rate(mVs )) + 0.21587 (R = 0.991) ----------------------- (9)
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The kinetic parameters such as  − the charge transfer coefficient and k – the apparent charge
s
transfer rate constant can also be calculated using laviron’s model (25, 26),
푅푇
훼ꢀ
푙ꢁ 푚
표
퐸푝푎 = 퐸 +
----------------------- (10)
훼ꢀ푛퐹
0
1
2
3
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6
7
8
9
0
1
2
3
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9
0
푅푇
훼ꢂ
푙ꢁ 푚
표
퐸 = 퐸 +
----------------------- (11)
푝푐
훼ꢂ푛퐹
Where, m = 0.025(k
s
/n), k the apparent charge transfer rate constant, n is the number of
s
o
electrons transferred, E is the reduction potential and  is the charge transfer coefficient.
푅푇
푛퐹∆ꢇꢈ
lo g 푘 = ꢃ푙ꢄ푔(1 − ꢃ) + (1 − ꢃ)lo g ꢃ − 푙ꢄ푔 ꢅ ꢆ − ꢃ (1 − ꢃ) ꢉ.3푅푇
---(12)
푠
푛퐹휗
The calibration plot gives two straight linear regression line with slopes equivalent to
.3RT/ nF and -2.3RT/ nF for the anodic and cathodic peaks, respectively (27). The values
of  and  was calculated as 0.427 and 0.550 respectively. On substituting, charge transfer
coefficient  in equation (14), the average value of k was determined to be as 0.712 cm/s.
2
a
c
a
c
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Figure 4 (a) CV curves of NaNbO
of 300M DA (b) CV curves for different scan rates ranging from 50mV/s to 200mV/s in
the presence of 300M DA (c) Corresponding calibration curve for the peak current i vs
log (scan rate(mV/s)) for both anodic and cathodic peaks (d) Calibration curve for peak
potential E vs log(scan rate(mV/s)) for both anodic and cathodic peaks.
3
/GCE and GCE in 0.1M PBS, in presence and absence
p
p
To overcome the effect of non-faradic current and to obtain high sensitivity with improved
response time for DA sensing, DPV technique was preferred. In brief, first the DPV curve was
recorded for GCE and NaNbO
oxidation peak throughout the potential sweep from -0.2 to 0.8V. Fig.5 (a) depicts the DPV
curve of NaNbO /GCE in the presence of 10nM to 1mM DA. The oxidation peak current
3
in absence of DA (data not shown). This study confirmed no
3
amplified with increase in the DA concentrations at the potential ~0.21V. This increase in
oxidation peak current can be ascribed to the electron release under the oxidation mechanism.
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Fig. 5(b) shows the calibration plot for the DPV with peak current and the respective DA
concentration for n=4 devices for the range 10-50nM. Linear regression equations were
2
calculated as Ip (A) =0.0070154c (nM) + 0.9765 (R =0.984). Likewise, for 100-500nM of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
DA the calibration curve is shown in fig. 5(c) Ip (A) = 0.0054673c (nM) + 1.0098 (R =0.982)
and finally for 1-500M, the calibration curve is shown in fig. 5(d) with a liner regression
2
equation Ip (A) = 0.0053122c (M) + 1.0941 (R =0.994). The limit of detection (LOD) was
calculated to be as 6.8nM based on the equation, LOD = 3s/m. where, s is the standard deviation
and m is the slope of the calibration curve. The LOD was found to be below the biological
-8
-6
levels of DA observed in the human body (i.e. 10 M to 10 M). This proves that this sensor
can be used to detect DA in the real samples such as human blood, urine samples. Here the
overall dynamic range of detection was 10nM to 500M of DA. The sensitivity of
NaNbO
3
/GCE towards DA in the linear range of 10nM to 50nM DA detection was found to be
2
9
7
9nA/nM cm . Similarly, the sensitivity found for the linear range from 100nM to 500nM was
2
7nA/nM cm and for the linear range from 1M upto 500M the sensor exhibited a sensitivity
2
of 75nA/M. cm . The table 1 displays the performance comparison of the NaNbO
3
/GCE
sensor with previously reported perovskite-based DA sensors. Notably, the proposed sensor
exhibited a wide dynamic range for DA detection, highest sensitivity and the lowest LOD
among the previously reported perovskite-based DA sensors. Further, the surface fouling
effect that often plagues electrodes was avoided with the help of rapid redox mechanism
followed by the presence of Na in the structure ensuring the stability of the perovskite.
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3
4
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0
1
2
3
4
5
6
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8
9
0
Figure 5 (a) DPV analysis of GCE and NaNbO
0nM to 1mM (b) corresponding calibration curve for 10-50nM DA with n=4 devices (c)
calibration curve for 100-500nM range of DA with n=4 devices (d) calibration curve for
-500M range of DA with n=4 devices
The overall sensing mechanisms are shown below in equation 13 and 14.
3
/GCE at range of DA concentration from
1
1
4
+
-
-
5+
-
NaNbO
3
-Nb + OH → NaNbO
3
(OH )-Nb + e ----------------------- (13)
-
5+
4+
NaNbO
3
(OH )-Nb + DA → NaNbO -Nb + DQ----------------------- (14)
3
This mechanism was entirely supported by the 3d orbital of the niobium and the formation of
NbO octahedra sites in NaNbO perovskite structure which favours the delocalization of
6
3
charge carriers under catalytic reaction [12, 18]. The graphical representation of the DA sensing
mechanism is illustrated in fig. 6.
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Figure 6 The sensing mechanism of DA using NaNbO /GCE electrode.
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Table 1 Comparison on the analytical performance of NaNbO /GCE electrode with other
3
perovskite based modified electrodes reported for the electrochemical detection DA.
Dynamic
Range
Real
Recovery
range
pH
Electrode
Material
Synthesis
of
Sensitivity
Sample
Analysis
Refer
ence
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
LOR
Technique
Detection
(%)
(M)
Human
Blood
-
98-102
-
8.0
7.2
7.0
7.2
LaFeO
3
/GCE
10 nM
62 nM
10-180
0.082-16
1-100
57.7 nA/M
[25]
[26]
[27]
SSR
LaFeO
s
3
Dendrite
Hydrothermal
-
-
LaCoO
Nanostructures
LaFeO
Microspheres
GCE
3
Hydrothermal
3
.53M
-
-
-
-
3
Wet Chemical
process
59 nM
0.020-1.6
-
[28]
/
2
9
9 nA/nM cm (for range
Simulated
Blood
98.35-
102.42
7.2
1
0-50nM)
2
77 nA/nM cm (for range
Serum
This
3
NaNbO /GCE
SSR
6.8 nM
0.010-500
100nM - 500nM)
work
2
7
5 nA/mM cm
(for range 1M - 500M)
Reproducibility of the developed NaNbO
four different electrodes with identical fabrication procedures, in 0.1M PBS in presence of
0nM DA (data not shown). The NaNbO /GCE sensor displayed a relative standard deviation
3
sensor towards DA sensing was determined using
1
3
(RSD) of ∼ 4.59%, indicating a good reproducibility of the sensor.
The role of interfering species that co-exists with the analyte of interest determines the
efficiency of a sensor used for non-enzymatic sensing. Hence, the impact of the interfering
+
-
2+
species presents in the blood such as AA, UA, Na , Cl , Ca and glucose were studied in
presence of DA using DPV technique. Fig. 7(a) shows the DPV response of NaNbO /GCE for
DA sensing in the presence of interfering species with 2-fold high concentration than DA.
Insignificant changes in peak current (I in %) were observed, displaying the excellent
selectivity of the sensor towards DA sensing as shown in Table S1.
3
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3
.4 Real Sample Analysis
In order to examine the practical performance of the proposed sensor in detecting DA, the
sensor (NaNbO /GCE) was used to detect the unknown concentration of DA in the simulated
DOI: 10.1039/C9NJ02282A
3
0
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blood serum. The simulated blood Serum was prepared by dissolving 1.5mM of calcium
chloride, 4.8mM of potassium chloride and 0.1M of sodium chloride in 50 mL of DI water
under RT. The experiment was carried out by standard addition method using DPV technique.
Initially, the unknown concentration of DA in simulated blood serum was spiked and recorded,
then the known DA was spiked into the electrolyte. The change in peak current was observed
with respect to the added DA concentrations as shown in fig. 7(b). The corresponding
calibration curve obtained was linear as shown in the Fig. 7(b) inset. The sensor exhibited a
good recovery percentage between ~98% to ~102% as shown in table 2 and also RSD values
were under acceptable range that shows, thus confirming its efficacy towards detecting DA in
real blood samples.
Figure 7 (a) DPV response of NaNbO
3
/GCE towards sensing of DA in presence of 2-fold
+
-
2+
higher concentration of Na , Cl , Ca , AA, UA and glucose. (b) DPV response of
simulated blood sample analysis for DA sensing under standard addition technique;
Inset: the calibration curve for the oxidation peak current with respect to the
concentration of DA added.
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Table 2 Determination of DA in real samples using as-fabricated NaNbO
standard addition method. (Real sample - Simulated Blood Serum)
3
/GCE under
DOI: 10.1039/C9NJ02282A
S.No. Sample
Dopamine
Dopamine Dopamine Relative
Recovery
%)
Concentration Spiked
recovered Standard
(
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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4
4
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4
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
Deviation
( M)
Simulated 1
Blood
Sample
(M)
(M)
(%)
1.
1
2
3
0.983
2.049
3.0327
1.65
2.42
1.06
98.35
102.42
101.06
While some reported DA sensors use binders such as nafion to screen the effect of AA, it also
blocks the active sites of the material which leads to reduction in overall catalytic activity of
the sensor [29]. This work presents a simple, binder free technique for fabrication of DA sensor
that offers numerous advantages in terms of very low LOD of 6.8nM, excellent reproducibility,
selectivity and sensitivity towards DA, when compared to previous reports. Also, the sensor
was more favourable towards real-time samples analysis as it demonstrated a successful
sensing of DA in simulated blood serum with excellent recovery percentage.
4. Conclusion
In this work, we demonstrated solid state reaction for synthesis of NaNbO
sensing of neurotransmitter such as Dopamine (DA). The enhanced electrocatalytic property
of the NaNbO /GCE sensor was attributed towards the presence of 3d orbital in the NaNbO
perovskite structure and NbO octahedra sites supported by the oxidation states of Niobium
Nb /Nb ). The sensor exhibited an excellent dynamic range of detection from 10nM to
00M of DA concentrations. The limit of detection was estimated as 6.8nM which is below
3
perovskite for
3
3
6
5+
4+
(
5
the range of physiological levels of DA in the human blood. The sensor showed a sensitivity
2
of 99 nA/nM cm towards sensing of DA. The sensor also showed an excellent reproducibility
and selectivity across various possible interfering species. The sensor was employed for the
2
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real time analysis of simulated blood serum and observed to have recovery percentage, i.e.
above 98%, this is a remarkable recovery percentage value for the real time samples analysis
when compared to the previous reports on DA sensing using various perovskite materials. Also,
0
1
2
3
4
5
6
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9
0
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as the basic credentials the NaNbO
3
was a lead-free perovskite with excellent electrocatalytic
properties, this NaNbO
3
sensor acts as an effective material for numerous bioanalytical
applications.
Acknowledgements
S.B. acknowledges the financial assistance from the Department of Science and Technology,
Government of India, under INSPIRE Faculty Fellowship Grant
DST/INSPIRE/04/2014/015132.
#
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