Ultrasensitive self-enhanced electrochemiluminescence sensor based on novel PAN?Ru?PEI?Nafion nanofiber mat

Article abstract of DOI:10.1039/c9tb02287b

In the present work, a novel self-enhanced electrochemiluminescence (ECL) composite was for the first time prepared by simply electrospinning a mixture of polyacrylonitrile (PAN), Ru(bpy)₃²⁺, poly(ethylenimine) (PEI) and Nafion, and defined as PAN?Ru?PEI?Nafion nanofiber mat (PAN?Ru?PEI?Nafion_nfm). Herein, Ru(bpy)₃²⁺was applied as an ECL reagent, and PEI was selected as a co-reactant due to the abundant tertiary amine groups in its backbone. In order to further improve the ECL efficiency of the Ru(bpy)₃²⁺-PEI system, we innovatively loaded Ru(bpy)₃²⁺and PEI into PAN nanofibers simultaneously by a one-step electrospinning technique. As a result, the electronic transmission distance between them was sharply shortened. Meanwhile, with the help of electrostatic interaction between positively charged Ru(bpy)₃²⁺or PEI and negatively charged Nafion, exfoliation of Ru(bpy)₃²⁺and PEI from the PAN nanofiber mat was effectively avoided. As expected, the luminous nanofibers exhibited excellent ECL performance including high efficiency and good stability. On this basis, we fabricated a new self-enhanced ECL sensor with excellent analysis performance. Using a-naphthol as an analytical probe, the sensor showed a wide linear range from 1.0 × 10^-12to 1.0 × 10^-7M with a low detection limit of 1.0 × 10^-12M. When it was applied to detect a-naphthol from carbaryl hydrolysis, the results were well consistent with those obtained from the high efficiency liquid chromatography-diode array detector (HPLC-DAD) method, indicating the potential application of the ECL sensor in real sample assays.

Full text of DOI:10.1039/c9tb02287b

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Materials Chemistry B
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Ultrasensitive self-enhanced electrochemi-
luminescence sensor based on novel
Cite this:DOI: 10.1039/c9tb02287b
PAN@Ru@PEI@Nafion nanofiber mat†
a
b
a
a
Libo Li, Limin Zhou, Xiaohong Liu and Tianyan You
*
In the present work, a novel self-enhanced electrochemiluminescence (ECL) composite was for the first time
2+
prepared by simply electrospinning a mixture of polyacrylonitrile (PAN), Ru(bpy)
Nafion, and defined as PAN@Ru@PEI@Nafion nanofiber mat (PAN@Ru@PEI@Nafionnfm). Herein, Ru(bpy)
applied as an ECL reagent, and PEI was selected as a co-reactant due to the abundant tertiary amine groups
3
, poly(ethylenimine) (PEI) and
2+
3
was
2+
3
in its backbone. In order to further improve the ECL efficiency of the Ru(bpy)
–PEI system, we innovatively
and PEI into PAN nanofibers simultaneously by a one-step electrospinning technique. As a
result, the electronic transmission distance between them was sharply shortened. Meanwhile, with the help of
2
+
loaded Ru(bpy)
3
2+
3
electrostatic interaction between positively charged Ru(bpy)
or PEI and negatively charged Nafion,
2+
3
exfoliation of Ru(bpy)
and PEI from the PAN nanofiber mat was effectively avoided. As expected, the
luminous nanofibers exhibited excellent ECL performance including high efficiency and good stability. On this
Received 16th October 2019,
Accepted 4th December 2019
basis, we fabricated a new self-enhanced ECL sensor with excellent analysis performance. Using a-naphthol as
ꢁ12
ꢁ7
an analytical probe, the sensor showed a wide linear range from 1.0 ꢀ 10
to 1.0 ꢀ 10 M with a low
ꢁ12
detection limit of 1.0 ꢀ 10
M. When it was applied to detect a-naphthol from carbaryl hydrolysis, the results
DOI: 10.1039/c9tb02287b
were well consistent with those obtained from the high efficiency liquid chromatography-diode array detector
(HPLC-DAD) method, indicating the potential application of the ECL sensor in real sample assays.
rsc.li/materials-b
2+
1,4
Ru(bpy)
3
–TPA system in the field of ECL sensing. Nevertheless,
Introduction
in a typical ECL system, TPA is usually added into the working
Electrochemiluminescence (ECL), integrating the distinct advan- buffer solution or immobilized on the sensing interface, which
5,6
tages of electrochemistry and spectroscopy, has always been one vastly sophisticates the operation. Moreover, the electron transfer
of the most widely used technologies in the field of analysis and often occurs between the luminescent reagent and the co-reactant
testing due to its excellent sensitivity, simple operation process and via intermolecular interaction, accompanied by a loss of energy,
1
0
2+
6
fast response speed. Ruthenium(II) tris(2,2 -bipyridyl) (Ru(bpy)
3
)
and further poor ECL efficiency and stability.
In this case, many self-enhanced ECL research studies have
and its derivatives are the most employed ECL luminophores
because of their high luminescence efficiency, excellent chemical been carried out by scientists.
1
,4,7–12
In the self-enhanced ECL
stability, reversible electrochemical behavior and wide dynamic complexes, the ECL-active luminophore and the corresponding
2–4
range. However, a luminophore itself can only produce a very co-reactant are suitably designed and covalently bound in the
weak ECL signal. Previous research has indicated that introducing same molecule or composite. Thus, the ECL signal is generated
2+
a co-reactant into the Ru(bpy)3 -related ECL system can effectively through intramolecular interaction. Compared to the intermo-
enhance the signal intensity and further improve the analytical lecular ECL pattern, self-enhanced ECL exhibits a relatively
sensitivity. The most representative co-reactants are alkyl amines, high and stable luminous signal because of the shortened
1
and it has been confirmed that tripropylamine (TPA) provides the electronic transmission distance and reduced energy loss. In
optimum ECL signal and has led to wide application of the addition, since the self-enhanced ECL complexes themselves can
produce efficient ECL signals, introduction of extra co-reactants is
a
Key Laboratory of Modern Agricultural Equipment and Technology, Ministry of
Education, School of Agricultural Equipment Engineering, Institute of Agricultural
Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China.
avoided. Up to now, many Ru(II)-based self-enhanced ECL com-
2+
plexes have been prepared by binding Ru(bpy)3 or its derivatives
5,8,10,13
with novel co-reactants, such as poly(ethylenimine) (PEI),
E-mail: youty@ujs.edu.cn; Fax: +86-511-88982992; Tel: +86-511-88982992
Anyang Institute of Technology, Anyang, Henan 455000, China
14,15
16
b
polyamidoamine (PAMAM),
tris(3-aminopropyl)amine (TAPA),
6,17
9
12,18
L-lysine,
CdTe quantum dots, carbon-based nanodots
and
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9tb02287b
so on. During the synthesis procedure of most self-enhanced
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ECL emitters, auxiliary nanomaterials including metal–organic (PEI), Nafion (5 wt%) and a-naphthol were purchased from
6
13
layers (MOLs) or metal–organic frameworks (MOFs), SiO
2
Sigma-Aldrich. Dimethylformamide (DMF, Z99.8%) was supplied
by Beijing Chemical Reagent Factory (Beijing, China). Double-
8
–10,12
14
16,17
19,20
nanospheres,
nanowires, nanorods,
nanoparticles
21
and nanocages are widely used as carriers to increase the loading sided conductive copper foil was bought from Junye adhesive
amount of Ru(II) complexes and further improve the ECL efficiency. products Technology Co. Ltd (Guangzhou, China). Na HPO and
2
4
2 4
However, the synthesis approaches are always relatively compli- NaH PO were used to prepare phosphate buffer solutions (PBSs).
cated, involving the preparation of nanomaterials, binding of All aqueous solutions in the experiments were prepared with
luminophores with co-reactants, loading of self-enhanced ECL doubly distilled water.
reagents into the support and so on. Therefore, there is a great
Apparatus
urgency to develop a one-step and simple method to prepare novel
self-enhanced ECL complexes with excellent ECL properties.
The scanning electron microscopy (SEM) characterization was
Recently, electrospun nanofibers have attracted great attention operated on a PHILIPS XL-30 ESEM with the accelerating voltage
and have been widely utilized in filtration, drug delivery, tissue of 20 kV. High-angle annular dark field (HAADF)-scanning transmis-
engineering, sensing and light sources due to their large surface sion electron microscopy (STEM) was performed on a JEM-2010 (HR)
22–29
area, high porosity and abundant reaction sites.
In particular, microscope operated at 200 kV. A Thermo ESCALAB250 instrument
the porous 3D nanofibrous membrane and large surface area with Al Ka radiation (VG Scientific, UK) was used to record the X-ray
endow electrospun nanofibers with ideal nanocarrier potential for photoelectron spectroscopy (XPS) spectra. A Nicolet iS10 Fourier
the decoration of ECL luminophores, and many electrospun transformed infrared (FT-IR) spectrometer was used to characterize
3
0–34
nanofiber-based ECL sensors have been generally fabricated.
the functional groups on the nanofiber mat. Ultraviolet-visible
2+
It has been indicated that incorporating Ru(bpy)3 into electro- adsorption (UV-vis) spectra and fluorescence (FL) spectra were
spun nanofibers can effectively enhance the ECL intensity. For recorded on a Hitachi U-4100 UV-Vis spectrophotometer and a
instance, in Shan’s work, a glassy carbon electrode modified Hitachi F-4500 fluorescence spectrophotometer, respectively. For
3
0
with electrospun poly(acrylonitrile-co-acrylic acid) nanofiber mat the characterization of optical properties, the self-enhanced ECL
2
3
+
(PAN-co-PAAnfm/GCE) was immersed into a Ru(bpy)
solution nanofiber mat was exfoliated from double-sided conductive
to immobilize the luminophore. The introduction of PAN-co- copper foil before it was cut into small pieces (0.5 cm ꢀ 2 cm).
PAAnfm resulted in 100 times enhanced ECL intensity than that Then, the nanofiber mat was placed into a special cuvette for
3
3
using a normal PAN-co-PAA solution. Zhou et al. added film or solid powder to record the UV-Vis and FL spectra.
2
+
Ru(bpy)3 into the electrospinning precursor, and prepared a
Cyclic voltammetry (CV) measurements were carried out on
2+
novel Ru(bpy)3 -doped Nafion (ERDN) nanofiber. Compared to the a CHI 832 electrochemical workstation (Shanghai Chenhua
precursor solution, the ECL signal of ERDN nanofibers was about Apparatus Inc., China) with a conventional three-electrode
5-fold enhanced. However, the co-reactant of Ru(bpy) , mainly system, where a modified indium tin oxide (ITO) electrode was
3
2
+
1
TPA, is needed and added into the detection solution in the used as a working electrode, and platinum wire and Ag/AgCl
previous reports. Loading both the luminophore and its (saturated KCl) electrodes were used as a counter electrode and a
co-reactant into the electrospun nanofibers at the same time to reference electrode, respectively. ECL responses were recorded
prepare a self-enhanced ECL nanofiber mat has not been reported. on an MPI-E ECL analyzer (Xi’An Remax Electronic Science &
In the present work, a novel self-enhanced ECL nanofiber Technology Co. Ltd, China) with the voltage of 800 V supplied to
mat was first prepared by one-step electrospinning a mixture of the photomultiplier tube (PMT).
2
+
polyacrylonitrile (PAN), Ru(bpy)
3
, PEI and Nafion, and named
2
+
Preparation of PAN@Ru@PEI@Nafionnfm
PAN@Ru@PEI@Nafionnfm. In the composite, Ru(bpy)
3
was
selected as a model ECL species and PEI was selected as a The self-enhanced ECL composite PAN@Ru@PEI@Nafionnfm
co-reactant, and an electrospun PAN nanofiber mat and Nafion was simply prepared by one-step electrospinning a mixture
2
+
ꢁ1
2+
ꢁ1
were employed to immobilize Ru(bpy)
3
3
and PEI. In this way, solution of 8 wt% PAN, 2 mg mL Ru(bpy) , 20 mg mL
2+
large amounts of Ru(bpy)3 and PEI were loaded simultaneously, PEI and 0.25 wt% Nafion in DMF, as shown in Scheme 1. The
and the electronic transmission distance between them was electrospinning process was carried out at an electric field of
shortened. Therefore, PAN@Ru@PEI@Nafionnfm achieved a 20 kV over a 15 cm gap between the spinneret and the collector,
significant enhancement in ECL efficiency and stability. A novel which was covered with double-sided conductive copper foil.
ꢁ
1
self-enhanced ECL sensor was fabricated based on the luminous The feed rate of the electrospinning was controlled at 1.0 mL h
.
nanofibers, and excellent analysis properties were obtained All preparation experiments were performed at the ambient
using a-naphthol as a probe, indicating that a potential ECL temperature of 25 1C and the relative humidity of 30%. For
sensing platform was proposed.
comprehensive investigation, nanofiber mats including PANnfm,
PAN@Runfm, PAN@Ru@PEInfm and PAN@Ru@Nafionnfm were
prepared under the same conditions.
Experimental
Materials and reagents
Fabrication of the working electrode
0
Polyacrylonitrile (PAN, M
ruthenium(II) hexahydrate (Ru(bpy)
w
= 150 000), tris(2,2 -bipyridyl) dichloro- An ITO electrode (4.0 cm ꢀ 0.8 cm) was ultrasonically cleaned
Cl O), poly(ethylenimine) in acetone, ethanol and distilled water thoroughly and dried at
ꢂ6H
3
2
2
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filtrating with a 0.22 mm filter membrane. The production was
then adjusted to neutral with H SO and diluted to 250 mL. The
2 4
content of a-naphthol in the sample solution was measured by
using the developed ECL method after diluting several times.
Results and discussion
Characterization of the self-enhanced ECL nanofiber mat
Fig. 1(A–D) shows the micro-morphology of different nanofiber
mats including PANnfm (A), PAN@Runfm (B), PAN@Ru@PEInfm
(
C) and PAN@Ru@PEI@Nafionnfm (D) characterized by using
the SEM technique. As displayed, all the prepared luminescent
Scheme 1 Schematic illustration of the one-step electrospinning process nanomaterials exhibit exactly the same fibrous structure and a
for the preparation of self-enhanced ECL PAN@Ru@PEI@Nafionnfm
.
fairly smooth surface without any beads. These nanofibers
randomly cross together to form a stable 3D porous structure
and evenly distribute on the substrate, which is beneficial to mass
room temperature. To fabricate the modified working electrode,
double-sided conductive copper foil covered with the self-enhanced
ECL nanofiber mat was cut into small pieces (1.0 cm ꢀ 0.8 cm) and
directly pasted on the ITO surface, as shown in Scheme 2A–C. And
the as-prepared working electrode was denoted as PAN@Ru@PEI@
Nafionnfm/ITO (Scheme 2D). For comparison, PANnfm/ITO, PAN@
Runfm/ITO, PAN@Ru@PEInfm/ITO and PAN@Ru@Nafionnfm/ITO
were fabricated via the same simple method. In addition, the
PAN@Ru@PEI@Nafion mixture solution modified electrode
13,16
transfer and further results in an excellent ECL response.
In
addition, it can be clearly seen that the dimension distribution is
uniform and the average dimensions are 274 nm, 215 nm, 253 nm
and 289 nm, respectively. The difference in dimension size should
be attributed to the properties of electrospinning solution, such as
the viscosity, surface tension, conductivity and so on. HAADF-STEM
results (Fig. 1E) of PAN@Ru@PEI@Nafionnfm suggest that C, N, O,
Ru, F and S are uniformly distributed in the nanofibers, indicating
2+
3
the successful introduction of Ru(bpy)
and Nafion into PAN nano-
(
PAN@Ru@PEI@Nafion/ITO) was also prepared by spreading
.4 mL of the precursor solution on the surface of the copper foil
1.0 cm ꢀ 0.8 cm) modified ITO electrode.
fibers and the feasibility of the electrospun nanofibers as carriers.
XPS was further conducted to characterize the elemental
composition of the electrospun nanofiber mats. For PANnfm, a
6
(
Treatment of the practical sample carbaryl
Carbaryl bought from a local market was treated according to a
3
5
previous report. Briefly, 0.05 g of carbaryl was mixed with
0 mL of 10% NaOH and stirred at 80 1C for 10 h, followed by
2
Scheme 2 Fabrication process of the ITO electrode modified with the
Fig. 1 SEM images of (A) PANnfm, (B) PAN@Runfm, (C) PAN@Ru@PEInfm, and
self-enhanced ECL nanofiber mat.
(D) PAN@Ru@PEI@Nafionnfm and (E) STEM-HAADF of PAN@Ru@PEI@Nafionnfm.
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wide-scan XPS spectrum presents the peaks of C1s, N1s and O1s
at 283.0, 398.0 and 530.0 eV, respectively (not shown). Upon the
2
+
addition of Ru(bpy)3 , PAN@Runfm, PAN@Ru@PEInfm and
PAN@Ru@PEI@Nafionnfm exhibit the Ru3d signal at around
1
3,36
280.5 eV (Fig. S1A, ESI†).
Fig. S1B (ESI†) shows the F1s
spectra for different nanofiber mats. It can clearly be seen that
for PANnfm, PAN@Runfm and PAN@Ru@PEInfm, no peaks are
obtained in the range of 676–695 eV. But, as expected, a peak at
6
88.7 eV corresponding to F1s is observed in PAN@Ru@PEI@ Fig. 3 (A) UV-Vis spectra and (B) FL spectra of (a) PANnfm, (b) PAN@Runfm,
3
7
2+
Nafionnfm
,
further confirming the successful loading of (c) PAN@Ru@PEInfm, (d) PAN@Ru@PEI@Nafionnfm and (e) Ru(bpy)
3
solution.
Inset of (B): photographs of electrospinning precursor solution (left) and
PAN@Ru@PEI@Nafionnfm (right) under 365 nm UV light illumination.
Nafion into the self-enhanced ECL nanofiber mat.
The functional groups on the self-enhanced ECL nanofiber mat
and original materials were compared via FT-IR characterization.
As shown in Fig. 2, the characteristic absorption peaks of C–N
The FL spectrum of PANnfm (Fig. 3B, curve a) indicates that
ꢁ1
38
stretching (2243.80 cm ) in PAN, structural dipyridyl groups no FL emission is obtained in the range of 500–850 nm (excited
ꢁ
1
2+ 39
(777.10 and 731.08 cm ) in Ru(bpy)
3
,
N–H bending vibra- at 310 nm) due to the absence of any ECL reagent. While all the
ꢁ
1
40
tion (1648.10 cm ) in PEI and C–F symmetric stretching other nanofiber mats present an obvious FL emission band at
ꢁ1
ꢁ1 41
(1108.86 cm ) and SQO symmetric stretching (1044.23 cm ) in about 605 nm (curve b–d), corresponding to the transition of
2
+
Nafion are clearly observed for the self-enhanced ECL nanofiber the triplet MLCT to the ground state of Ru(bpy)3 (curve e). And
mat. The characterization results further verify the ideal application it can be seen that the maximum emission wavelengths of the
of electrospun PAN as a nanocarrier and successful immobiliza- prepared luminescent nanofibers are blue-shifted by B20 nm
2
3
+
2+
tion of Ru(bpy)
, PEI and Nafion into the nanofiber mat.
in comparison with Ru(bpy)
3
solution, which should be
2
8,44
attributed to the interaction between the precursors.
The
photographs (inset of Fig. 3B) display that the electrospinning
solution (left) and PAN@Ru@PEI@Nafionnfm (right) emit bright
Optical properties of the self-enhanced ECL nanofiber mat
Optical properties of the electrospun nanofiber mats were orange light under 365 nm UV light due to the addition of
2
+ 32,34
investigated by using UV-Vis and FL spectrophotometry. Ru(bpy)3
,
and the luminosity is very uniform. The UV-Vis
2+
Fig. 3A illustrates that PANnfm shows a characteristic absorption and FL results indicate that Ru(bpy)3 is successfully encapsulated
4
2,43
peak of cyanide at 281 nm (curve a).
After incorporating into PAN nanofibers and preserves its optical properties intact.
2
+
Ru(bpy)3 , an extra absorption band at 458 nm is observed for
PAN@Runfm (curve b), matching well with that of free Ru(bpy)3
in aqueous solution (curve e). The peak should be assigned to
2
+
ECL investigation of the prepared self-enhanced ECL nanofiber
mat
2
+
the metal-to-ligand charge transfer (MLCT) band of Ru(bpy)
3
.
In order to demonstrate the advantages of the self-enhanced
Upon further addition of PEI (curve c) and Nafion (curve d), the ECL composite, ECL behaviors of ITO electrodes modified
2
+
ꢁ1
characteristic peak locations of PAN at 281 nm and Ru(bpy)
3
with different electrospun nanofiber mats in 0.1 mol L PBS
at 455 nm exhibit a maximum 27 nm red-shift and 34 nm blue- (pH 7.5) were investigated. As shown in Fig. 4A, there is no ECL
shift with the absorbance intensity decreasing successively. The response signal on PANnfm/ITO (curve a), because neither
shift may be attributed to the interaction between the electro- luminescent molecule nor its co-reactant exist in PANnfm. As
2
6
spun precursors.
expected, PAN@Runfm/ITO shows an ECL signal (160 a.u.)
beginning at 0.9 V (Fig. S2, ESI†) and peaking at 1.35 V
(
Fig. 4A, curve b). The ECL intensity should be ascribed to the
2
+
successful introduction of Ru(bpy)
and good retention of its ECL property.
3
into the nanofiber mat
3
3,45
The luminescence
phenomenon confirms the ideal feasibility of electrospun
2
+
nanofibers as carriers to immobilize Ru(bpy)3 . Upon the
further addition of PEI, the ECL intensity improves significantly
by 15-times (Fig. 4A, curve c). The enhanced ECL intensity can
2
+
be attributed in part to the catalytic effect of PEI on Ru(bpy)
3
2
1
due to abundant tertiary amines in the backbone of PEI. On
2
+
the other hand, the simultaneous presence of both Ru(bpy)
3
and PEI in the same electrospun nanofibers produces a novel
self-enhanced ECL composite. As a result, the electron transfer
path is shortened and the energy loss is decreased. Therefore,
5
the ECL efficiency is significantly enhanced. In comparison, the
2+
ECL intensity of the PAN@Runfm/ITO electrode in PBS containing
1 mg mL PEI is only one third that of PAN@Ru@PEInfm/ITO in
Fig. 2 FT-IR characterization of (a) PAN, (b) Ru(bpy)
and (e) self-enhanced ECL nanofiber mat.
3
, (c) PEI, (d) Nafion
ꢁ
1
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PAN@Ru(II)@PEI@Nafionnfm–2e -
ꢃ
+
PAN@Ru(III)@PEI @Nafionnfm
(1)
ꢃ
+
PAN@Ru(III)@PEI @Nafionnfm
-
ꢃ
+
PAN@Ru(III)@PEI @Nafionnfm + H
(2)
(3)
PAN@Ru(II)*@PEI@Nafionnfm
-
Fig. 4 (A) ECL–potential curves of (a) PANnfm/ITO, (b) PAN@Runfm/ITO,
(
c) PAN@Ru@PEInfm/ITO, (d) PAN@Ru@PEI@Nafionnfm/ITO, and (e) PAN@
PAN@Ru(II)@PEI@Nafionnfm + hn
(4)
ꢁ1
Ru@PEI@Nafion/ITO in 0.1 mol L pH 7.5 PBS. (B) ECL emission spectrum
ꢁ1
of PAN@Ru@PEI@Nafionnfm/ITO in 0.1 mol L pH 7.5 PBS and the FL
spectrum of Ru(bpy)
First, the PAN@Ru(II)@PEI@Nafionnfm luminophore is electro-
chemically oxidized and loses two electrons to form PAN@
2+
3
solution.
ꢃ
+
Ru(III)@PEI @Nafionnfm (Formula 1), which then generates
ꢃ
PBS (Fig. S3A, ESI†), further confirming the superiority of the self- PAN@Ru(III)@PEI @Nafion
by losing a proton (Formula 2).
nfm
enhanced ECL nanofiber mat.
However, because both Ru(bpy)
charged, the repulsive force may cause exfoliation of Ru(bpy)
Next, the light-emitting species, PAN@Ru(II)*@PEI@Nafionnfm,
2
+
3
and PEI are positively is produced and finally emits orange light at 620 nm by the
2
+
ꢂ
3
electron transfer from PEI to Ru(III) (Formula 3 and 4).
and PEI during electrospinning and scanning procedures,
leading to a weak and unstable ECL signal. Therefore, we
Optimization of experimental conditions
skillfully introduce Nafion into the electrospun nanofiber To obtain the highest detection sensitivity of the self-enhanced
2
+
mat, as it is a widely used carrier to immobilize Ru(bpy)3
ECL nanofiber mat, concentration of electrospinning solution,
due to its large ion-exchange selectivity coefficient (5.7 ꢀ 10 ) collection time and pH value of the buffer solution were
6
2
3
+ 46
for Ru(bpy)
the ECL intensity of PAN@Ru@Nafionnfm/ITO (B500 a.u.)
increases by about three fold in comparison with PAN@Runfm
.
It can clearly be seen from Fig. S3B (ESI†) that optimized in detail (Fig. S4, ESI†).
The effect of Nafion in the electrospinning solution on the
ECL signal was first investigated. Smooth nanofibers can be
/
ITO (B160 a.u.), indicating that the introduction of Nafion can obtained with a low content of Nafion. The ECL intensity
2
+
indeed avoid a fall off of Ru(bpy)
3
from the obtained nanofibers improves sharply with an increase in the concentration of
47,48
by electrostatic interaction.
PAN@Ru@PEI@Nafionnfm/ITO reaches up to 4500 a.u. (Fig. 4A, that Nafion can load more Ru(bpy)3 and PEI. However, when
curve d), which is about double the intensity of PAN@Ru@PEInfm
the content is higher than 0.25%, a stable electrospinning
ITO. Moreover, compared with the electrode modified directly with procedure and a favorable nanofiber mat cannot be obtained
Interestingly, the ECL intensity of Nafion (Fig. S4A, ESI†). This should be ascribed to the merit
2
+
/
3
3
an electrospinning precursor solution (1400 a.u., Fig. 4A, curve e), due to the occasional blocking of the polymer jet. Therefore,
the ECL signal of the electrode modified by the nanofiber mat was 0.25% Nafion is chosen.
about 3 times higher. This is because of the fact that after the
Since the concentration of PAN is the decisive factor that
precursor solution is electrospun to form nanofibers, the inherent affects the entanglement of a polymer molecular chain in the
porous structure of the nanofibers can provide a large surface area solution, an appropriate PAN content in the solution is very
2+
and huge active sites to load Ru(bpy)3 and PEI and promote the important to achieve a strong ECL intensity. It is obvious that
mass diffusion, thereby resulting in the enhancement of ECL the luminescence intensity reaches the maximum at the PAN
3
3
signal.
concentration of 8% (Fig. S4B, ESI†). A lower concentration
probably leads to a poor uniform structure of nanofibers with
lots of beads, which may hinder the immobilization of
Possible luminous mechanism of the self-enhanced ECL
nanofiber mat
2+
Ru(bpy)3 and PEI, as well as the electron transfer rate.
In order to explore the luminous mechanism of the self-enhanced However, the dimension of electrospun nanofibers enlarges
ECL nanofiber mat, we recorded the ECL emission spectrum of with an increase in PAN concentration, thereby resulting in a
ꢁ1
PAN@Ru@PEI@Nafionnfm/ITO in 0.1 mol L PBS and the FL smaller surface area and an unsatisfactory ECL response.
2
+
2+
spectrum of free Ru(bpy)
3
aqueous solution (Fig. 4B). It can
The concentration of Ru(bpy)
3
was measured in the range
ꢁ1
2+
obviously be observed that the maximal ECL emission wavelength of 1.0–2.5 mg mL . More Ru(bpy)
occurs at 625 nm, which is identical to the Ru(bpy)
system. Moreover, the ECL spectrum is well consistent with the FL exceeds 2.0 mg mL , ECL emission efficiency begins to
spectrum of Ru(bpy)3 . This indicates that Ru(bpy)3 * is still the decrease because of the strong self-quenching and triplet–
emitter in the electrospun PAN@Ru@PEI@Nafionnfm, and PEI acts triplet annihilation in the ruthenium(II) complex. Thus, the
According to the ECL behaviors and the concentration of Ru(bpy)
the possible ECL mechanism of PAN@Ru@
PEI@Nafionnfm can be expressed by the following equations:
3
molecules produce higher
solution ECL signals (Fig. S4C, ESI†). Whereas, when the concentration
2
+
3
ꢁ
1
2+
2+
3
4
3
2,33
2+
ꢁ1
as a co-reactant.
previous reports,
3
is selected as 2.0 mg mL
Meanwhile, the effect of PEI concentration was also investi-
gated in the range of 5.0–22 mg mL (Fig. S4D, ESI†). The ECL
.
13,21
ꢁ1
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intensity enhances with the increase of PEI concentration, and detecting phenolic compounds. However, separation or biosensing
ꢁ
1
ꢁ1
it reaches a plateau at 20 mg mL , showing that 20 mg mL is technology should be introduced when the ECL sensor is applied to
the optimum amount of PEI to enhance the ECL. Therefore, real samples due to the mutual interference between the phenolic
ꢁ
1
20 mg mL PEI was used.
compounds.
Next, a-naphthol was selected as a model molecule to
The effect of collection time on ECL intensity is derived from
the immobile amount of Ru(bpy)
electrospinning proceeds, more Ru(bpy)
2
+
3
(Fig. S4E, ESI†). As the determine the sensitivity of the PAN@Ru@PEI@Nafionnfm
-
2
+
3
molecules are based ECL sensor. The inset of Fig. 5B shows that the ECL
assembled into the nanofibers to produce higher ECL signals. intensity of PAN@Ru@PEI@Nafionnfm/ITO decreases with the
However, declined ECL efficiency is obtained after 2 h. There- increase of a-naphthol concentration. The decrease in the value
fore, we chose 2 h as the optimal time.
of ECL intensity (DI = I
0
ꢁ I) exhibits a linear response to the
The effect of the pH value of PBS on the ECL intensity was logarithm of a-naphthol concentration in the range of 1.0 ꢀ
ꢁ12
ꢁ7
ꢁ1
investigated in the range from 6.0–8.5. As shown in Fig. S4F 10 –1.0 ꢀ 10 mol L with a determined detection limit of
ꢁ
12
ꢁ1
(
ESI†), as the pH value increases from 6.0 to 7.5, ECL intensity 1.0 ꢀ 10
improves accordingly, but it decreases when the pH value is DI = 642.05 ꢀ log C + 7978.69 with a relative coefficient of 0.998.
higher than 7.5. With the ECL mechanism described above, a The comparison of analytical performance of the proposed
mol L (Fig. 5B). The linear regression equation is
higher pH value is favorable for deprotonation, thus resulting ECL method with previously reported methods, mainly electro-
in a higher ECL signal. However, too high pH values may lead chemical, chromatography and spectroscopy methods, is listed
4
5
to the decomposition of some species. Hence, the pH value of in Table S1 (ESI†). It can be seen that the proposed ECL sensor
PBS is set as 7.5.
shows the widest linear range. More importantly, the detection
ꢁ
12
ꢁ1
limit obtained by the ECL method (1.0 ꢀ 10
mol L ) is two
orders of magnitude lower than that of the most sensitive method
Analytical performance of the luminous nanofiber mat-based
self-enhanced ECL sensor
ꢁ
10
ꢁ1
(1.0 ꢀ 10
mol L ). The excellent analytical performance of the
prepared ECL sensor is mainly because of the simultaneous
immobilization of the luminophore and its co-reactant into the
electrospun nanofiber mat, where the electron transfer distance is
effectively shortened. Besides, the introduction of Nafion greatly
It has been widely reported that phenolic compounds are a
serious threat to the environment and human health. And
2
+
many Ru(bpy)3 -based ECL sensors have been developed for
the detection of phenolic compounds due to the energy transfer
2+
avoids the exfoliation of Ru(bpy)3 and PEI from the nanofiber
2
+
from Ru(bpy)
compounds.
3
* to the electro-oxidation species of phenolic
Herein, the quenching effect of some phenol-
mat, thus excellent ECL stability can be received. And the relative
standard deviation (RSD) of the ECL intensity obtained by
continuous scanning for 10 cycles is calculated to be 1.96%
4
9,50
containing quenchers was studied to investigate the analytic per-
formance of the luminous nanofiber mat-based self-enhanced ECL
sensor. The quenching efficiency is defined as (I ꢁ I)/I , where I
(inset of Fig. 5B).
0
0
0
In order to define the reproducibility of the method, ECL
and I represent the ECL intensity in the absence and presence of
the quenchers, respectively. As shown in Fig. 5A, all the selected
phenolic compounds exhibit obvious quenching effects on the
fabricated ECL sensor, and a-naphthol and catechol display the
highest quenching efficiencies of 76.9% and 75.8%, respectively.
intensities of three PAN@Ru@PEI@Nafionnfm-based ECL sensors
fabricated in the same batch were measured. It is found that the
ECL intensities are in good agreement with the RSD of 3.4%.
Moreover, the RSD of the three sensors, which are prepared by
different batches, is calculated to be 5.1%. These data indicate that
the assay has reasonable reproducibility.
+
2+
2ꢁ
2ꢁ
While, several coexisting ions such as K , Ca , SO
4
, CO
3
and
ꢁ
NO
3
cause no significant ECL signal changes, indicating a great
potential of the PAN@Ru@PEI@Nafion based ECL sensor for Detection of a-naphthol in real tap water and hydrolyzed from
carbaryl
In order to verify the feasibility of the developed ECL method in
practical applications, detection of a-naphthol in real tap water
was performed via the standard addition method. As presented
in Table S2 (ESI†), no analyte was detected in the tap water.
ꢁ
9
ꢁ1
ꢁ9
ꢁ1
With the addition of 1.0 ꢀ 10 mol L and 5.0 ꢀ 10 mol L
a-naphthol, satisfactory recoveries in the range of 94.0–95.8%
with the RSD of 3.53–4.62% are obtained.
As exhibited in Fig. S5 (ESI†), carbaryl can easily hydrolyze to
3
5
produce a-naphthol under basic conditions. The content of
a-naphthol hydrolyzed from carbaryl was measured by the
developed ECL method and confirmed by the standard high
performance liquid chromatography-diode array detector
Fig. 5 (A) Quenching efficiencies of the phenolic compounds on the self-
enhanced ECL sensor and the interference of several ions upon a-naphthol
detection (n = 3). (B) Calibration curve of decreased ECL intensity (DI = I –I)
0
to logarithmic a-naphthol concentration (n = 3). Inset: ECL–time curves of
the sensor with different concentrations of a-naphthol and stability of the
developed sensor.
(HPLC-DAD) method to demonstrate the accuracy. The corres-
ponding results are summarized in Table S3 (ESI†). It can be
seen that the content of a-naphthol obtained by the prepared
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ECL sensor is well consistent with that of the HPLC method
with the RSD less than 10%. The results demonstrate that the
PAN@Ru@PEI@Nafionnfm-based ECL sensor offers a sensitive
and precise way for the analysis of a-naphthol in real samples.
8 L. M. Zhou, J. S. Huang, B. Yu and T. Y. You, Sci. Rep., 2016,
6, 22234.
9 M. M. Chen, Y. Wang, S. B. Cheng, W. Wen, X. H. Zhang,
S. F. Wang and W. H. Huang, Anal. Chem., 2018, 90, 5075.
0 M. M. Chen, S. B. Cheng, K. L. Ji, J. W. Gao, Y. L. Liu,
W. Wen, X. H. Zhang, S. F. Wang and W. H. Huang, Chem.
Sci., 2019, 10, 6295.
1
Conclusion
2
+
11 J. J. Ling, M. Zhao, F. J. Chen, X. Y. Zhou, X. S. Li, S. J. Ding
Taking PAN, Ru(bpy)
3
, PEI and Nafion as a precursor solution,
and H. Tang, Biosens. Bioelectron., 2019, 127, 167.
a novel self-enhanced ECL nanofiber mat was prepared by using
a one-step electrospinning technique. In the ECL nanofiber mat,
PAN was used as a carrier to load more Ru(bpy)
further enhance the ECL efficiency. Negatively charged Nafion
was selected to fix Ru(bpy)
1
1
1
1
1
1
1
1
2
2
2 L. J. Luo, S. Ma, L. B. Li, X. H. Liu, J. Y. Zhang, X. Li, D. Liu
and T. Y. You, Food Chem., 2019, 292, 98.
3 C. Y. Xiong, W. B. Liang, Y. N. Zheng, Y. Zhuo, Y. Q. Chai
and R. Yuan, Anal. Chem., 2017, 89, 3222.
4 H. J. Wang, Y. L. Yuan, Y. Q. Chai and R. Yuan, Biosens.
Bioelectron., 2015, 68, 72.
5 L. Zhang, C. Y. Xiong, H. J. Wang, R. Yuan and Y. Q. Chai,
Sens. Actuators, B, 2017, 241, 765.
6 H. J. Wang, Y. L. Yuan, Y. Zhuo, Y. Q. Chai and R. Yuan,
Anal. Chem., 2016, 88, 2258.
7 L. Zhang, Y. He, H. J. Wang, Y. L. Yuan, R. Yuan and
Y. Q. Chai, Biosens. Bioelectron., 2015, 74, 924.
8 S. Carrara, F. Arcudi, M. Prato and L. D. Cola, Angew. Chem.,
Int. Ed., 2017, 56, 4757.
9 W. B. Liang, Y. Zhuo, C. Y. Xiong, Y. G. Zheng, Y. Q. Chai
and R. Yuan, Anal. Chem., 2015, 87, 12363.
2
3
+
and PEI, and
2
3
+
and PEI in the nanofiber mat and
ensured the stability of the ECL signal. As the luminophore and
its co-reactants were combined in one composite, the electronic
transmission distance between them was shortened, and the
energy loss was reduced. Therefore, the self-enhanced ECL
nanofiber mat exhibited better ECL performance than that of
its electrospinning solution. Based on the superiorities, the ECL
sensor fabricated by pasting the ECL nanofiber mat on the ITO
electrode surface showed a wide linear range and a low detection
limit using a-naphthol as an analytical probe. Finally, the sensor
was successfully applied to determine a-naphthol obtained from
carbaryl hydrolysis. Combining with some separation techniques
or bio-recognition elements, this work can provide a new
research strategy for the fabrication of sensitive and selective
self-enhanced ECL sensors.
0 A. Y. Chen, M. Zhao, Y. Zhuo, Y. Q. Chai and R. Yuan, Anal.
Chem., 2017, 89, 9232.
1 H. J. Wang, Y. He, Y. Q. Chai and R. Yuan, Nanoscale, 2014,
6, 10316.
Conflicts of interest
22 Kenry and C. T. Lim, Prog. Polym. Sci., 2017, 70, 1.
2
2
2
3 P. Wang, Y. P. Wang and L. M. Tong, Light: Sci. Appl., 2013,
There are no conflicts to declare.
2, e102.
4 K. C. Liu, Z. Y. Zhang, C. X. Shan, Z. Q. Feng, J. S. Li, C. L. Song,
Y. N. Bao, X. H. Qi and B. Dong, Light: Sci. Appl., 2016, 5, e16136.
5 Q. H. Guo, L. J. Liu, M. Zhang, H. Q. Hou, Y. H. Song,
H. D. Wang, B. Y. Zhong and L. Wang, Biosens. Bioelectron.,
Acknowledgements
This project was supported by the National Natural Science
Foundation of China (No. 21675065 and 61801195), the China
Postdoctoral Science Foundation (No. 2019T120399 and
2017, 92, 654.
2
2
2
2
6 Q. H. Guo, T. T. Wu, L. J. Liu, H. Q. Hou, S. L. Chen and
L. Wang, J. Mater. Chem. B, 2018, 6, 4610.
2018M632238), the Innovation/Entrepreneurship Program of
Jiangsu Province, and the Priority Academic Program Develop-
ment (PAPD) of Jiangsu Higher Education Institutions (No.
PAPD-2018-87).
7 M. F. Zhang, X. N. Zhao, G. H. Zhang, G. Wei and Z. Q. Su,
J. Mater. Chem. B, 2017, 5, 1699.
8 X. Y. Wanga, Y. J. Wanga, M. Jiang, Y. Q. Shan, X. Jin,
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9 T. J. Liu, Y. Q. Guo, Z. F. Zhang, Z. C. Miao, X. Y. Zhang and
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