Electrochemiluminescence sensor for dopamine with a dual molecular recognition strategy based on graphite-like carbon nitride nanosheets/3,4,9,10-perylenetetracarboxylic acid hybrids

Article abstract of DOI:10.1039/c5ra03154k

Graphite-like carbon nitride nanosheets/3,4,9,10-perylenetetracarboxylic acid hybrids (g-C₃N₄ NSs-PTCA) were synthesized via π-π conjugate action. Based on novel dual molecular specific recognition of oxyethyl groups to diol and carboxyl to amine groups, a signal-on electrochemiluminescence (ECL) sensor for dopamine (DA) was constructed utilizing g-C₃N₄ NSs-PTCA as asignal probe and K₂S₂O₈ as a coreactant. Under the optimal conditions, the prepared ECL sensor presented a response to DA with a linear range from 6.0 pM to 30.0 nM. The detection limit (LOD) and limit of quantification (LOQ) of the sensor were 2.4 pM and 7.9 pM, respectively. The g-C₃N₄ NSs-PTCA signal probe coupling with a novel dual molecular recognition strategy would provide a promising ECL sensing platform for detecting DA sensitively and selectively.

Full text of DOI:10.1039/c5ra03154k

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Electrochemiluminescence sensor for dopamine
with a dual molecular recognition strategy based
on graphite-like carbon nitride nanosheets/
3,4,9,10-perylenetetracarboxylic acid hybrids†
Cite this: RSC Adv., 2015, 5, 42698
a
a
b
a
a
a
*
Xiaomin Fu, Jiahui Feng, Xingrong Tan, Qiyi Lu, Ruo Yuan and Shihong Chen
Graphite-like carbon nitride nanosheets/3,4,9,10-perylenetetracarboxylic acid hybrids (g-C3N4 NSs–PTCA)
were synthesized via p–p conjugate action. Based on novel dual molecular specific recognition of oxyethyl
groups to diol and carboxyl to amine groups, a signal-on electrochemiluminescence (ECL) sensor for
dopamine (DA) was constructed utilizing g-C3N4 NSs–PTCA as asignal probe and K2S2O8 as
a
coreactant. Under the optimal conditions, the prepared ECL sensor presented a response to DA with a
linear range from 6.0 pM to 30.0 nM. The detection limit (LOD) and limit of quantification (LOQ) of the
sensor were 2.4 pM and 7.9 pM, respectively. The g-C3N4 NSs–PTCA signal probe coupling with a novel
dual molecular recognition strategy would provide a promising ECL sensing platform for detecting DA
sensitively and selectively.
Received 19th February 2015
Accepted 6th May 2015
DOI: 10.1039/c5ra03154k
www.rsc.org/advances
Furthermore, compared with bulk g-C3N4, g-C3N4 NSs show
improved photocatalytic property and water-dispersibility, and
higher surface-area-to-volume ratio.17,18 The g-C3N4 NSs possess
1. Introduction
Dopamine (DA) as an important catecholamine neurotrans-
mitter plays a signicant role in the renal, hormonal and
cardiovascular systems.1 Abnormal levels of DA are relevant to
several serious diseases, including Parkinson's disease and
schizophrenia.2 Recently, various techniques have been reported
for the determination of DA, such as UV-vis spectrophotometry,3
high performance liquid chromatography,4 uorometry and
electrochemical techniques.5,6 Peculiarly, when an electro-
chemiluminescence (ECL) technique was used in analytical
detection, this analytical method is of interest because it exhibits
a low background signal, easy controllability, high sensitivity,
and good selectivity.1,7,8 In the eld of ECL sensors, luminol,9,10
quantum dots,11,12 Ru complexes13,14 and gold nanocluster15,16
have been applied as ECL luminophores. However, these ECL
luminophores show some insufficiencies, such as high-cost,
poisoning or a complicated synthesis process.
intriguing advantages including non-toxicity, high stability, good
biocompatibility, easy functionalization and rich p electron,19 thus
exhibiting great potential applications over the conventional ECL
luminophores. For instance, an ECL immunosensor based on Au
nanoparticles-functionalized g-C3N4 NSs was prepared to detect
carcinoembryonic antigen with a low detection limit (6.8 pg mLꢀ1),
excellent specicity, reproducibility and stability.19 Recently, our
group established an ECL sensor based on gold nano-
ower@graphitic carbon nitride polymer nanosheet–polyaniline
hybrids for dopamine with a detection limit of 1.7 ꢁ 10ꢀ9 M.20
In recent years, the derivatives of perylene, such as
3,4,9,10-perylenetetracarboxylic acid (PTCA) and ammonolysis
product
of
3,4,9,10-perylenetetracarboxylic
dianhydride
(PTC–NH2), have been widely investigated owing to their attrac-
tive structure and properties. For example, PTCA with aromatic
pyrenyl group and carboxylic-functionalized interface has been
used to functionalize the material via p–p stacking.21,22 In
addition, Zhuo et al. and Zhao et al. have reported that PTCA
could effectively enhance the ECL emission of S2O82ꢀ–O2 system
and Ru(II) system, thus was used in sensing elds.23,24 In our
previous work,25 we investigated the cathodic ECL behavior of
the PTC–NH2 in aqueous solutions with potassium perox-
ydisulfate (K2S2O8) as the coreactant. Based on the fact that DA
could quench the ECL signals of the PTC–NH2, the detection of
DA was achieved at the PTC–NH2 modied electrode.
Graphite-like carbon nitride (g-C3N4) is a new-style of carbon-
based materials with strong ECL activity. Recently, g-C3N4 nano-
sheets (g-C3N4 NSs) have been obtained by a simply and environ-
mentally exfoliative process from bulk g-C3N4 in water.
aKey Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest
University), Ministry of Education, College of Chemistry and Chemical Engineering,
Southwest University, Chongqing 400715, China. E-mail: cshong@swu.edu.cn; Fax:
+86-23-68253172; Tel: +86-23-68253172
bDepartment of Endocrinology, 9 th People's Hospital of Chongqing, Chongqing
400700, China
The dual molecular recognition strategy, which is
a
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra03154k
sequential recognition process at different recognition sites
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between a molecule and other molecules to improve the speci- 3-Aminopropyltriethoxysilane (APTES) and 3,4,9,10-perylenete-
city, usually was applied in the chiral sensing, the preparation tracarboxylic dianhydride (C24H8O6, PTCDA) were received from
of supramolecular structures, and sandwich immunoassays.26,27 Lian Gang Dyestuff Chemical Industry Co. Ltd. (Liaoning, China).
Recently, it was also used to detect small biomolecules. For Melamine (2,4,6-triamino-1,3,5-trazine, 99%) was obtained from
example, Zhang et al. developed an ECL sensor for dopamine Aladdin Ltd. (Shanghai, China). N-(3-Dimethylaminopropyl)-N0-
using a dual molecular recognition strategy based on the ethylcarbodiimidehydrochloride (EDC), mercaptan and N-hydroxy-
specic recognition of N-hydroxysuccinimide ester of func- succinimide (NHS) were got from Sigma-Aldrich Chem. Co.
tionalized CdTe quantum dots (QDs) to amine group of DA and (St. Louis, MO, USA). Potassium persulfate (K2S2O8) was purchased
boronic acid to diol of DA.26 This sensor showed a wide linear from Shanghai Chemical Reagent Co. (Shanghai, China). Phos-
response range from 50 pM to 10 nM, low detection limit down phate buffer solutions (PBS) with various pH values were prepared
to 26 pM, and high selectivity. However, the shortcomings of by mixing the stock solution of 0.10 M KH2PO4 and Na2HPO4.
cadmium-containing QDs including complex synthesis and 0.10 M KCl was used as a supporting electrolyte. Doubly distilled
environmental poisonousness restricted their further applica- water was used throughout the experiments.
tions. Therefore, developing a better-performing dual molecular
recognition strategy to achieve an eco-friendly sensor is of great
interesting. It was reported that oxyethyl group of 3-amino-
propyltriethoxysilane (APTES) could react with the hydroxy of
graphene oxide to form stable cyclic esters,28 which is similar to
the reaction between boronic acids and cis-diol-containing
molecules. Since boronic acids and cis-diol-containing mole-
cules based reaction has been applied in the construction of DA
sensor,29,30 furthermore, also has been involved in the dual
molecular recognition strategy based DA sensor,26 the reaction
based on APTES and cis-diol-containing molecules may be well
applied to prepare a DA sensor with a novel construction
strategy.
Enlightened by above observations, in this work, the g-C3N4
NSs–PTCA hybrids were synthesized via p–p stacking between
g-C3N4 NSs and PTCA. With g-C3N4 NSs–PTCA as signal probe, a
signal-on ECL sensor was constructed for DA by coupling with a
novel dual molecular recognition strategy based on a specic
recognition of the oxyethyl group of APTES to the diol of DA and
the amine group of DA to the carboxyl group of g-C3N4
2.2 Apparatus
The ECL emission was inspected by a model MPI-A electro-
chemiluminescence analyzer (Xi'an Remax Electronic Science &
Technology Co. Ltd., Xi'an, China), equipped with a photo-
multiplier tube (PMT). Electrochemistry measurements were
performed with a CHI 600B electrochemical work-station
(Shanghai CH Instruments, China). Scanning electron micros-
copy (SEM) was performed using a hitachi scanning electron
microscope (SEM, Hitachi, S-4800, Japan). The UV-vis absorp-
tion spectrum was conducted with an UV-2450 UV-vis spectro-
photometer (Shimadzu, Japan). The FT-IR spectra were
measured using a Nexus 670 FT-IR spectrophotometer (Nicolet
Instruments). The chemical composition of the samples were
characterized by X-ray diffraction (XRD) (XRD, Purkinje General
InstrumentXD-3) with Cu-Ka radiation (l ¼ 0.15406 nm). All
experiments were carried out at room temperature.
2.3 Synthesis of g-C3N4 NSs
NSs–PTCA. First, gold nanoparticles (AuNPs) were electro- Bulk g-C3N4 was prepared through the pyrolysis of melamine in
deposited on a glassy carbon electrode (GCE) for assembling semiclosed system.31 Briey, melamine (20.0 g) was put in an
APTES via Au–N interaction. Subsequently, DA was modied on alumina crucible with a cover and heated at 600 ꢂC for 2 h with
the electrode via the specic recognition of the diol of DA to the heating speed of 5 ꢂC minꢀ1, generating the yellow bulk g-C3N4
oxyethyl group of APTES to form stable cyclic esters. Then, powder. Then, 250 mg bulk g-C3N4 was sonicated with 250 mL
g-C3N4 NSs–PTCA hybrids as signal probes were further modi- doubly distilled water for 10 h. Aerwards, the obtained
ed onto the electrode through the special reaction between the suspension was centrifuged at 5000 rpm for 10 min to remove
carboxyl group of g-C3N4 NSs–PTCA and the amine group of DA. the unexfoliated g-C3N4. Ultimately, the resulting g-C3N4 NSs
In this strategy, with the increase of DA incubated onto the dispersion was dried to get g-C3N4 NSs powder.
electrode through APTES, ECL signal would enhance due to the
increase in the amount of g-C3N4 NSs–PTCA captured onto the
modied electrode. Such a signal-on sensor integrated the
excellent ECL performance of g-C3N4 NSs–PTCA hybrids as
signal probes and the novel dual molecular recognition strategy
based on the specic recognition of the oxyethyl group of APTES
to the diol of DA and the amine group of DA to the carboxyl
group of g-C3N4 NSs–PTCA, thus would open a new avenue to
construct ECL sensing.
2.4 Synthesis of PTCA composite
According to the literature,32 PTCA was prepared by hydrolyzing
PTCDA. In this work, 20.0 mg PTCDA was dissolved in NaOH
(4.0 mL, 0.050 M), and then 1.0 M HCl was gradually added to
the mixture solution to neutralize the excess NaOH and the pH
of the solution was maintained at slightly acidic. Then, the
obtained PTCA precipitate was centrifugated and washed with
double-distilled water. Conclusively, PTCA was dried to get the
power at room temperature.
2. Experimental
2.1 Reagents and chemical
2.5 Synthesis of g-C3N4 NSs–PTCA hybrids
Gold chloride tetrahydrate (HAuCl4$4H2O) and dopamine (DA) The g-C3N4 NSs–PTCA hybrids were synthesized via a p–p
were obtained from Sigma Chemical Co. (St. Louis, MO, USA). stacking approach. Briey, 3.0 mg g-C3N4 NSs and 3.0 mg PTCA
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were mixed completely in 3.0 mL double-distilled water by
successive ultrasonication for 8 h. The dispersion was centri-
fuged to obtain the sediment. Aer that, 1.0 mg mLꢀ1 EDC
solution and 1.0 mg mLꢀ1 NHS solution (volume ratio ¼ 4 : 1)
were added into the sediment to activate the carboxyl of PTCA
with stirring for overnight. Finally, the g-C3N4 NSs–PTCA
hybrids were puried by centrifugation at 12 000 rpm for
15 min and stored in the refrigerator at 4 ꢂC.
3. Results and discussion
3.1 Characterization of g-C3N4 NSs–PTCA
SEM was employed to characterize the morphologies of g-C3N4
NSs, PTCA, and g-C3N4 NSs–PTCA. As seen in Fig. 1A, a lamellar
structure of g-C3N4 NSs was observed clearly, which was
consistent with the reported literature.33 Fig. 1B presented the
irregularly quadrateshaped structure of PTCA. For g-C3N4
NSs–PTCA (Fig. 1C), both the overlapping layered structure of
g-C3N4 NSs and irregularly quadrateshaped structure of PTCA
were clearly observed, indicating that g-C3N4 NSs–PTCA hybrids
were successfully synthesized.
The UV-vis absorption spectra of PTCA, g-C3N4 NSs and
g-C3N4 NSs–PTCA were measured to testify the interaction
between PTCA and g-C3N4 NSs. As presented in Fig. 2A, the
UV-vis absorption spectrum of g-C3N4 NSs (curve a) showed a
strong peak at 320 nm, which was in agreement with the liter-
ature.34 The absorption spectrum of PTCA (curve b) displayed a
characteristic absorption peak at 535 nm ascribed to p–p*
transition in the perylene core of PTCA and a weak absorption
peak at 241 nm, respectively. For g-C3N4 NSs–PTCA hybrids
(curve c), the characteristic absorption peak of g-C3N4 NSs at
320 nm shied to 325 nm, which was attributed to the p–p
conjugate action between g-C3N4 NSs and PTCA.
The FT-IR spectra of g-C3N4 NSs (curve a), PTCA (curve b),
and g-C3N4 NSs–PTCA (curve c) are shown in Fig. 2B. At curve
a, the vibration of triazine ring of g-C3N4 NSs at 810 cmꢀ1
could be observed. Meanwhile, the peaks at 1200–1650 cmꢀ1
were assigned to the stretching vibration of connected units of
C–N(–C)–C (full condensation) or C–NH–C (partial conden-
sation), and the peaks at 3000–3500 cmꢀ1 were corresponded
to the N–H stretching and hydrogen-bonding interactions of
g-C3N4 NSs. This result was accordant with the report.31 In the
curve b, three characteristic peaks of PTCA could be observed
at 1595 cmꢀ1, 1773 cmꢀ1 and 3446 cmꢀ1, which were assigned
to the stretching vibration of C]C, C]O and O–H, respec-
tively.35 As seen in curve c, the FT-IR spectra of the g-C3N4
NSs–PTCA presented the characteristic peaks of both g-C3N4
NSs and PTCA, indicating the successful synthesis of g-C3N4
NSs–PTCA hybrids.
2.6 Construction of ECL sensor
The glassy carbon electrode (GCE) was polished with 0.3 and
0.05 mm alumina powder on a polishing cloth, followed by
rinsing with distilled water. Then, the electrode was sonicated
in doubly distilled water and ethanol, successively. Aer that,
gold nanoparticles (AuNPs) were deposited on the pre-
processed GCE surface by electrodeposition at the potential of
ꢀ0.2 V for 30 s. Subsequently, 20.0 mL of APTES solution
(5.56 mM) was dropped onto the electrode for 30 min. In order
to eliminate non-specic binding effect, 20.0 mL of mercaptan
solution (1.0 mM) was coated on the APTES/AuNPs layer for
50 min to block possible remaining active sites of AuNPs. Next,
the electrode was incubated with 20.0 mL of DA with various
concentrations for 2 h. Finally, the electrode was coated with
20.0 mL of g-C3N4 NSs–PTCA dispersion (1.0 mg mLꢀ1) at room
temperature for 6 h. Physical adsorption was removed by
washing the electrode with doubly distilled water before next
modication step. The diagrammatic sketch of the sensor
preparation is shown in Scheme 1.
2.7 ECL detection
The ECL detection was performed in 3.0 mL 0.10 M PBS con-
taining 0.10 M K2S2O8 (pH 7.0). A scan rate of 100 mV sꢀ1 was
applied to obtain the ECL signal. The voltage of the PMT was set
at 600 V and the potential scan was from ꢀ1.3 to 0 V for the
detection. A classical three-electrode system was used with a
Ag/AgCl as the reference electrode, a platinum wire as the
counter electrode and a modied glass carbon electrode (GCE,
F ¼ 4.0 mm) as the working electrode. The measurement was
based on the change of ECL signal (DI ¼ It ꢀ I0). Here, It and I0
are the ECL intensity with and without g-C3N4 NSs–PTCA,
respectively.
Scheme 1 Schematic diagrams of the preparation of ECL sensor and Fig. 1 SEM images of (A) g-C3N4 NSs, (B) PTCA and (C) g-C3N4 NSs–
g-C3N4 NSs–PTCA hybrids.
PTCA.
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Fig. 2 UV-vis spectra (A), FT-IR spectra (B) and XRD spectra (C) of (a)
g-C3N4 NSs, (b) PTCA, and (c) g-C3N4 NSs–PTCA.
Fig. 3 ECL responses (A) in 0.10 M PBS (pH 7.0) containing 0.10 M
K2S2O8 and CV responses (B) in 5.0 mM K3[Fe(CN)6]/K4 [Fe(CN)6] of (a)
bare GCE, (b) AuNPs/GCE, (c) APTES/AuNPs/GCE, (d) mercaptan/
APTES/AuNPs/GCE, (e) DA/mercaptan/APTES/AuNPs/GCE, and (f)
g-C3N4 NSs–PTCA/DA/mercaptan/APTES/AuNPs/GCE. The scan
The X-ray diffraction (XRD) patterns of a series of samples
are presented in Fig. 2C. For g-C3N4 NSs (curve a), a diffraction
peak at 27.4ꢂ was observed, which was consistent with previous
report.31 PTCA exhibited diffraction peaks at 9.14ꢂ, 11.96ꢂ, 27.2ꢂ,
and 24.36ꢂ (curve b), respectively, which were similar to those of
PTCDA reported in the literature,36 since PTCA was prepared
from PTCDA. As seen form curve c, the characteristic diffraction
peaks of g-C3N4 NSs and PTCA were simultaneously observed at
g-C3N4 NSs–PTCA. Besides, the diffraction peak of g-C3N4 NSs
and PTCA at approximately 27ꢂ have been overlapped.
rates were 100 mV sꢀ1
.
+
2ꢀ
g-C3N4 + SO4cꢀ / g-C3N4 + SO4
(4)
(5)
g-C3N4cꢀ + g-C3N4 / g-C3N4* + g-C3N4
+
nally,
3.2 ECL and CV behaviors of the modied electrodes
g-C3N4* / g-C3N4 + hn
(6)
The ECL behaviors of the differently modied electrodes were
studied and the results are shown in Fig. 3A. As seen from curve
a to e (Fig. 3A, insert), in the absence of g-C3N4 NSs–PTCA, a
weak ECL signal was observed at the bare electrode. Further-
more, aer AuNPs, APTES, mercaptan and DA were stepwise
modied onto the electrode, the obtained corresponding elec-
trodes also presented a weak ECL signal. Such a weak ECL signal
was attributed to the light emission of S2O82ꢀ–O2 system.37
However, the ECL signal dramatically enhanced aer g-C3N4
NSs–PTCA was modied onto electrode (curve f). Obviously, this
enhanced ECL signal resulted from the light emission of g-C3N4
The stepwise fabrication of the electrode was also character-
ized by cyclic voltammetry (CV) and Fig. 3B presents corre-
sponding cyclic voltammograms (CVs) of different modied
electrodes in 0.10 M PBS (pH 7.0) containing 5.0 mM
K3[Fe(CN)6]/K4 [Fe(CN)6]. Compared with the bare GCE (curve a),
the peak currents apparently increased aer AuNPs were elec-
trodeposited on the bare GCE (curve b), indicating that the
AuNPs as conducting wire could promote the electron transfer
between the redox probe and the electrode. When APTES was
immobilized on the electrode surface (curve c), the redox peaks
currents decreased due to the non-conductive properties of
APTES. The redox peak currents further decreased while
mercaptan was coated on the electrode (curve d), which may be
caused by the strong electron transfer hindering of mercaptan.
In comparison with curve d, the peak currents increased aer DA
coating (curved e) due to the fact that DA could promote the
electron transfer. Aer the g-C3N4 NSs–PTCA was captured on
the electrode, the peak currents apparently increased (curve f),
suggesting that g-C3N4 NSs can facilitate the electron transfer.
2ꢀ
NSs–S2O8 system. Compared with the ECL signal of g-C3N4
NSs–S2O82ꢀ system, the ECL signal of S2O82ꢀ–O2 system could be
neglected. The possible ECL emission mechanisms of g-C3N4
2ꢀ
with S2O8 as coreactant are presented as follows:38
g-C3N4 + eꢀ / g-C3N4cꢀ
(1)
(2)
(3)
S2O8 + eꢀ / SO4 + SO4cꢀ
2ꢀ
2ꢀ
2ꢀ
g-C3N4cꢀ + SO4cꢀ / g-C3N4* + SO4
and/or
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sensor.38 Thus, pH 7.0 was chosen as the optimal pH in the
further study.
3.3 Electrochemical behavior DA at the modied electrodes
Electrochemical behaviors of DA at the modied electrodes
were investigated and the results are shown in Fig. S1.† For both
g-C3N4 NSs–PTCA/mercaptan/APTES/AuNPs/GCE (without DA
modied on the electrode) and g-C3N4 NSs–PTCA/DA/
mercaptan/APTES/AuNPs/GCE (with DA modied on elec-
trode), no oxidation reduction peaks were observed at the CVs
in the absence of DA in detecting cell containing 0.10 M PBS
(pH 7.0). However, an oxidation peak of DA was observed at the
CVs for two kinds of modied electrodes when DA was added in
the detecting cell. The reason was as following. DA was modied
on the electrode via the specic recognition of the diol of DA to
the oxyethyl group of APTES to form stable cyclic esters, leading
to the inactivation of the electroactive group. In this case, DA
would not exhibit electrochemical activity. However, DA added
in detecting cell could show electrochemical activity, thus
leading an oxidation peak observed at the modied electrode.
Actually, such a fact that DA modied on the electrode did not
exhibit electrochemical activity just indicated the successful
immobilization of DA on the electrode via the specic recog-
nition of the diol of DA to the oxyethyl group of APTES and the
carboxyl group of g-C3N4 NSs–PTCA to the amine group of DA.
The concentration of K2S2O8 is a key factor concerned to the
ECL signal. Fig. 4B depicts the effects of the concentration of
K2S2O8 on the DI of the sensor in 0.10 M PBS (pH ¼ 7) with 0.30
nM DA. As seen, the DI increased with the increase of K2S2O8
concentration in the range of 0.050–0.10 M and then decreased
from 0.10 M to 0.12 M. Therefore, 0.10 M was chosen as the
optimal concentration of K2S2O8. According to the ECL emis-
2ꢀ
sion mechanisms of g-C3N4 with S2O8 as coreactant, ECL
intensity increased when more g-C3N4* was generated from the
oxidation of g-C3N4cꢀ by SO4cꢀ. However, the formation of g-
2ꢀ
C3N4* was inhibited due to the fact that excess S2O8 would
react easily with g-C3N4cꢀ.39
Fig. 4C shows the effects of incubation time of g-C3N4
NSs–PTCA on the DI in 0.10 M PBS (pH ¼ 7) containing 0.10 M
K2S2O8 with 0.30 nM DA. As seen, the DI increased with the
incubation time of g-C3N4 NSs–PTCA on the electrode from 3 h
to 6 h and then showed a slight glide aer 6 h. Consequently, 6
h was chosen as the optimal incubation time of g-C3N4
NSs–PTCA.
Fig. 4D displays the effect of the scan rate on the ECL
intensity. The ECL intensity increased with the scan rates from
25 to 100 mV sꢀ1 and then dropped while the scan rates were
higher than 100 mV sꢀ1. According to the previous reports, at
3.4 Optimization of detection conditions
2ꢀ
high scan rates, the consumption of S2O8 on the electrode
K2S2O8 concentration, solution pH and incubation time of g-
C3N4 NSs–PTCA were optimized in this work. Fig. 4A shows the
effects of pH on the change in ECL intensity (DI) of the sensor in
0.10 M PBS containing 0.10 M K2S2O8 with 0.30 nM DA. The DI
increased when pH varied from 5.0 to 7.0, and then fell down as
pH ranged from 7.0 to 8.5. The reasonable explanation may be
as follows. At low pH, the electro-reduction of g-C3N4 NSs was
suppressed due to the fact that the proton could be readily
reduced at the negative potential. At too high pH, the strong
oxidant SO4cꢀ was consumed via reacting with OHꢀ. As a result,
too low or too pH led to the decrease in the ECL intensity of the
2ꢀ
interface would be much faster than the diffusion of the S2O8
from the solution to the electrode surface, resulting in a low
2ꢀ
transient concentration of S2O8 near the electrode surface
accompanying with a decrease of ECL signal.40 Thus, 100 mV
sꢀ1 was chosen as the optimal scan rate and used throughout
this work.
3.5 ECL response of the sensor to DA
With the increase of DA incubated onto the electrode through
APTES, ECL signal would enhance due to the increase in the
amount of g-C3N4 NSs–PTCA captured onto the modied elec-
trode. As shown in Fig. 5A, under optimal conditions, the ECL
intensity increased with the increase in the concentration of DA.
The calibration plot shows a good linear response between the
DI and the logarithmic value of DA concentration from 6.0 pM
to 30.0 nM (Fig. 5B) and the linear equation was DI ¼ 44 612.44
+ 3650.30 log c (R ¼ 0.995). Here, c is the concentration of DA.
The detection limit (LOD) and limit of quantication (LOQ) of
the sensor were 2.4 pM and 7.9 pM, respectively. The detailed
calculation method and concerning data are shown in ESI.†
Table S1† exhibits the comparison between our ECL sensor and
the reported sensor for the detection of DA. As can be seen, our
sensor in this work displayed a wider linear range and a lower
detection limit than those of other previous work. Because the
dual molecular specic recognition and “signal on” detection
strategy could decrease background signal, thus leading to a low
detection limit down to picomoles.25 Above facts indicated that
the prepared sensor exhibited preferable performance for the
detection of DA owing to the excellent performance of g-C3N4
NSs–PTCA and the dual molecular recognition strategy.
Fig. 4 The effects of pH (A), concentration of K2S2O8 (B), incubation
time of g-C3N4 NSs–PTCA (C) and scan rate (D) on the DI with 0.30 nM
DA. Error bars are obtained by the standard deviation from the mean of
three parallel experiments.
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injection sample using the standard addition method. As
shown in Table S2,† the recoveries are in the range of 94.2–
107%. Thus, the present method showed a promising applica-
tion for DA determination in real samples.
4. Conclusion
Based on a novel dual molecular specic recognition of the
oxyethyl group of APTES to the diol of DA and the carboxyl of
g-C3N4 NSs–PTCA to the amine group of DA, a signal-on ECL
sensor for dopamine (DA) was constructed utilizing g-C3N4
NSs–PTCA as signal probe and K2S2O8 as coreactant. Such an
ECL sensing strategy shows a decreased background signal,
thus leading to a low detection limit down to picomoles.
Furthermore, compared with the current ECL materials, g-C3N4
NSs shows some charming merits such as low-cost, non-
poisonous, easier synthesis, and strong ECL activity. The inte-
gration of the novel dual molecular specic recognition strategy
and g-C3N4 NSs–PTCA as signal probe would provide a green
and promising platform for ECL sensing.
Fig. 5 (A) The ECL responses of the sensor towards DA with various
concentrations of (a) 6.0 pM, (b) 10 pM, (c) 30 pM, (d) 60 pM, (e) 0.3 nM,
(f) 10 nM, (g) 30 nM in 0.10 M PBS (pH 7.0) containing 0.10 M K2S2O8;
(B) calibration curve for DA detection (DI vs. logarithmic value of DA
concentration). Error bars are obtained by the standard deviation from
the mean of three parallel experiments.
3.6 Stability, selectivity and reproducibility of the ECL
sensor
Stability is one of the signicant focuses for practical applica-
tion of a sensor. Fig. S2† shows the ECL intensity of the
prepared sensor for the successive measurements (n ¼ 8) with
0.30 nM DA. The relative standard deviation (RSD) was 1.7%.
The stability was also investigated by monitoring the ECL
Acknowledgements
ꢂ
response of the sensor to 0.30 nM DA aer storage at 4 C for
This work was nancially supported by the NNSF of China
(21075100, 21275119, 21105081), Ministry of Education of
China (Project 708073), Research Fund for the Doctoral
Program of Higher Education (RFDP) (20110182120010),
Natural Science Foundation of Chongqing City (CSTC-
15 days. As shown in Fig. S3,† the ECL response declined 9.9%
compared with the original signal, showing an acceptable
stability for DA detection. The RSD (n ¼ 3) of ve different
sensors from the same batch and different batches at 0.30 nM
DA level were 5.4% and 5.6%, respectively, indicating an
acceptable precision and reproducibility.
2011BA7003,
CSTC-2014JCYJA20005,
CSTC-2010BB4121),
Science and Technology Commission of Beibei (2012-27),
Medical Scientic Research Projects of Health Bureau of
Chongqing (2012-2-286), and the Fundamental Research Funds
for the Central Universities (XDJK2013A008, XDJK2013A027,
XDJK2014A012), China.
Selectivity is another important parameter for a sensor.
Possible interferences with a concentration 10 time higher than
DA (0.30 nM), such as glucose, tyrosine, ascorbic acid, uric acid,
epinephrine, clenbuterol, K+, Na+, Ca2+, Clꢀ and SO42ꢀ, were
investigated and the results are displayed in Fig. S4.† As seen,
no obvious ECL response from above interferences was
observed at the present sensor. Additionally, in order to deter-
mine the preference species, the response of the sensor towards
clenbuterol and epinephrine was also tested since they have
reactive groups similar to those of DA (corresponding structure
shown in Fig. S5†). As shown in column g and h of Fig. S4,† the
sensor did not exhibit an obvious ECL response towards clen-
buterol or epinephrine. The reasonable explanation may be as
follows. Although the diol of epinephrine was similar to the diol
of DA, it was difficulty for secondary amine species to react with
carboxyl. Thus, the dual molecular recognition strategy may be
not achieved for epinephrine. In the case of clenbuterol, its
primary amine and hydroxyl were similar to those of DA, but
further reaction may be hindered by the steric hindrance. Thus,
the dual molecular recognition strategy also cannot be ach-
ieved. Perhaps, our ECL sensor could be used to distinguish DA
from clenbuterol and epinephrine. Above results demonstrated
a good selectivity of the prepared sensor.
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42704 | RSC Adv., 2015, 5, 42698–42704
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