Subscriber access provided by UNIV OF ALABAMA HUNTSVILLE
Article
Supersensitive Detection of Chlorinated Phenols by Multiple Amplification Electrochemiluminescence Sensing Based on Carbon Quantum Dots/Graphene Shanli Yang, Jiesheng Liang, Sheng-Lian Luo, Chengbin Liu, and Yanhong Tang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac400874h • Publication Date (Web): 24 Jul 2013 Downloaded from http://pubs.acs.org on July 29, 2013
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
Supersensitive Detection of Chlorinated Phenols by Multiple
2
Amplification Electrochemiluminescence Sensing Based on Carbon
3
Quantum Dots/Graphene
4 5
Shanli Yang a, Jiesheng Liang a, Shenglian Luo a,*, Chengbin Liu a,*, Yanhong Tang b
6 7
a
8
Changsha 410082, P. R. China
9
b
10
State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University,
College of Materials Science and Engineering, Hunan University, Changsha 410082,
P. R. China
11 12 13 14 15 16 17 18 19 20
————————————
21
*
22
E-mail:
[email protected].(S.L. Luo);
[email protected] (C.B. Liu)
Corresponding authors. Tel.: +86 731 88823805; Fax: +86 731 88823805
23 1
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 25
1
ABSTRACT
2
A novel electrochemiluminescence (ECL) sensor based on carbon quantum dots
3
(CQDs) immobilized on graphene (GR) has been firstly developed for the
4
determination of chlorinated phenols (CPs) in water. The detection is based on the
5
ECL signals from the interaction between the analytes and the excited CQDs (C*+)
6
using S2O82− as coreactant. GR facilitates both C•− and SO4•− production, resulting in a
7
high yield of C*+, and the multistage amplification effect leads to a nearly 48-fold
8
ECL amplification. Pentachlorophenol (PCP) is often monitored as an important
9
indicator for CPs in real environmental samples, but its ultra-trace and real-time
10
analysis is an intractable issue in environmental monitoring. The resulting ECL sensor
11
enables the real-time detection of PCP with unprecedented sensitivity reaching 1.0 ×
12
10−12 M concentration in a wide linear range from 1.0 × 10−12 to 1.0 × 10−8 M. The
13
ECL sensor showed high selectivity to CPs, especially to PCP. The practicability of
14
the sensing platform in real water sample showed ideal recovery rates. It is envisaged
15
that the eco-friendly and recyclable sensor could be employed in the identification of
16
key CPs in the environment.
17 18
Keywords:
PCP
ultra-trace
detection;
Carbon
19
Electrochemiluminescence; Multistage amplification
quantum
20
2
ACS Paragon Plus Environment
dots;
Graphene;
Page 3 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
INTRODUCTION
2
Chlorinated phenols (CPs) are a typical group of persistent organic pollutants in
3
the environment, and several of them have been listed as priority pollutants by the US
4
Environmental Protection Agency and classified as a group 2B environmental
5
carcinogen by International Agency for Research on Cancer.1−3 One of the most
6
commonly used and strongest carcinogens in the CPs family is pentachlorophenol
7
(PCP), which not only poses a health threat, but is also of great environmental concern
8
because of soil and water contamination. The content of PCP correlates well with the
9
total CPs content in contaminated environmental samples, and thus PCP is often
10
monitored as a model compound for CPs in real environmental samples. Moreover,
11
due to the strict restrictions on the maximum admissible concentration of CPs in
12
drinking water by European Union and the US Environmental Protection Agency (0.5
13
ppb and 1 ppb respectively), extensive work has been carried out on the trace and
14
ultra-trace detection of CPs in the last decade.4−8 However, those methods are usually
15
time-consuming, and require bulky equipment, tedious sample preparation and an
16
expert operator. In addition, considering critical importance for practical use, those
17
systems lack the ability to perform a real-time analysis since they cannot be
18
miniaturized.
19
Quantum dots (QDs) based electrochemiluminescence (ECL) sensor is a
20
powerful potential detection method, owing to its miniaturization, inexpensive
21
instrumentation, excellent detection sensitivity and high selectivity.9 Its applications
22
for the detection of chemical and biological species have been widely reported.10−12
3
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
However, reports on concerning the environmental detection utilizing QDs based ECL
2
sensor are relatively scarce though the ECL sensor has so many advantages. The
3
reasons are mainly attributed to the following factors: 1) inherently high toxicity of
4
Cd-based QDs used in ECL;13−15 2) high cost and complex experimental design of
5
ECL involved in Cd-based QDs and non-recycle of these solution-phase QDs;10−15
6
and 3) most importantly, poor reversibility and low ECL intensity due to the poor
7
stability of Cd-based QDs at high excited electrochemical potentials. Therefore,
8
nontoxicity, low cost, and reproducibility are urgently needed for the development of
9
QDs based ECL sensors in environmental monitoring.
10
The newly emerging carbon quantum dots (CQDs) may be a promising
11
alternative because of their low toxicity, low cost, high stability and good ECL
12
reversibility.16 To the best of our knowledge, CQDs based ECL sensors have rarely
13
been reported, which is hampered by the low ECL intensity of CQDs and the
14
non-recycle of CQDs in aqueous system. Although extrinsic coreactant S2O82− has
15
been employed to enhance CQDs ECL intensity,16 it is hard to satisfy the trace and
16
ultra-trace detection of CPs in the environment. Thus, the key point lies in how to
17
further amplify CQDs ECL signal and solve the non-recyclable problem
18
simultaneously.
19
Graphene (GR), a two-dimensional sp2-hybridized carbon sheet, has recently
20
attracted great attention due to its unique electrical, optical, and mechanical properties
21
as well as potential use in various fields such as electronics, sensors and composite
22
materials.17−19 Benefiting from the large surface-to-volume ratio, good chemical
4
ACS Paragon Plus Environment
Page 4 of 25
Page 5 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
stability and flexibility, GR is a suitable immobilization platform for many
2
nanostructural materials;20,21 moreover, due to its good electrical conductivity and
3
excellent electrocatalytic activity, GR could efficiently amplify the ECL signal of
4
CdSe QDs.22
5
Herein, for the first time, a novel one-step electrochemical reduction technology is
6
proposed for the preparation of carbon quantum dots/graphene (CQDs/GR) hybrid
7
(See Supporting Information, Scheme S1), and an advanced ECL sensor based on the
8
CQDs/GR hybrid was fabricated for the supersensitive, rapid, and real-time detection
9
of CPs through multistage amplification of the CQDs ECL signal by GR and S2O82−.
10
GR serves as both the ECL amplification reagent and the immobilization platform for
11
CQDs, which can not only enhance the detection sensitivity, but also achieve the
12
recyclability of CQDs.
13 14
METERIALS AND METHODS
15
Materials. Titanium foil (99.8%, 0.127 mm thickness) was purchased from
16
Aldrich (Milwaukee, WI). Graphite powder in size of about 50 µm was purchased
17
from Shanghai Carbon Co., Ltd. Graphite oxide was synthesized from graphite
18
powder by the Hummers’ method,23 and then the prepared graphite oxide was
19
dissolved in 0.067 M PBS (pH 7) to form 1 mg/mL GO solution. Wood based
20
activated carbon powder (200 mesh) and 1−30 kDa molecular weight cut off (MWCO)
21
membranes (Amicon Ultra-4, Millipore) were purchased from the local suppliers, and
22
the oxidized carbon quantum dots (OCQDs) was synthesized from activated carbon
5
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
powder by the Dong’s method.24 Other chemicals of analytical reagent grade were
2
obtained from commercial sources and used without further purification. Ultrapure
3
water was used throughout the experiments.
4
Fabrication of CQDs/GR hybrid. 1 mL of OCQDs (5 mg) and 9 mL of GO
5
(9mg) solution were mixed under stirring for 1 h to form OCQDs/GO solution. A
6
titanium ribbon (1.0 cm×3.5 cm ) was ultrasonically cleaned in a dilute HF solution
7
and then in water for 5 min each. The top part of the titanium ribbon was used as the
8
electrical contact position, and an electrically insulated Pt wire was attached on the
9
top by jointing. The as-prepared titanium ribbon was immersed into the above
10
OCQDs/GO solution under stirring and then subjected to scanning from −1.4 to +1.0
11
V at 50 mV/s with 10 electrodeposition cycles to obtain CQDs/GR hybrid. Based on
12
the measurement of electrochemical properties, the above conditions were optimized.
13
The morphologies of CQDs/GR hybrid were characterized on a scanning electron
14
microscope (SEM, JSM-6700F, JEOL, Japan) and a transmission electron microscope
15
(TEM, JEM-3010, JEOL, Japan). Photoluminescence (PL) spectra were acquired with
16
F-2500 fluorescence spectrometer (Hitachi, Japan). Electrochemical measurements
17
were carried out on CHI-660D electrochemistry station (Chenhua Instrument Inc.,
18
China) with a three-electrode system where the modified titanium ribbon (1.0 cm×
19
3.5 cm) was used as the working electrode with a Pt foil as the counter electrode and a
20
saturated calomel electrode (SCE) as the reference electrode.
21
ECL Measurements. The ECL of CQDs/GR in PBS solution containing 100 mM
22
K2S2O8 was measured with a MPI-E multifunctional chemiluminescent analyzer
6
ACS Paragon Plus Environment
Page 6 of 25
Page 7 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
(Xi’an Rimax Electronics Co. Ltd., China) with a photomultiplier tube voltage set at
2
600 V, using CQDs/GR modified titanium ribbon, a Pt foil,and a SCE as the working
3
electrode, the counter electrode and the reference electrode, respectively. During the
4
measurements, the potential supplied by the MPI-E electrochemical analyzer was
5
applied to the CQDs/GR working electrode by a cyclic voltammetric technique with a
6
potential range from −1.6 to +0.2 V at 400 mV/s. At the same time, the ECL emission
7
was recorded by the MPI-E multifunctional chemiluminescence analyzer.
8 9
RESULTS AND DISCUSSIONS
10
Characterization and Evaluation of CQDs/GR Hybrid. Recently, Dong et al.
11
reported a new kind of oxidized carbon quantum dots (OCQDs) with an abundant of
12
-COOH groups on their surfaces, showing a good dispersity in aqueous solution.24
13
Inspired by this, utilizing the difference in solubility of OCQDs (good dispersity in
14
aqueous solution) and CQDs (insoluble in aqueous solution), we can speculate that
15
when the OCQDs in direct contact with an electrode accept electrons to suffer from
16
electroreduction to CQDs, the resulted insoluble CQDs will directly attach onto the
17
electrode surface. We would like to point out that, to the best of our knowledge, it is
18
the first report about CQDs immobilization by directly utilizing the solubility
19
difference between OCQDs and CQDs and also about CQDs/GR hybrid construction
20
by one-step electrochemical reduction technique.
21
The cyclic voltammograms (CVs) for OCQDs electrolysis on titanium ribbon
22
electrode and on the more sensitive glassy carbon electrode (GCE) are shown in
7
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
Figure S1A and S1B (See Supporting Information), respectively. Both of the
2
reduction currents regularly decreased with successive sweeps, highlighting the
3
persistent deposition of poor conductive CQDs onto the electrodes. The GCE after
4
OCQDs electrolysis was then scanned in PBS buffer solution (Figure S1C) where
5
there is no irreversible electroreduction peak of OCQDs to CQDs at −1.3 V (peak III
6
in Figure S1B), showing that part of oxygen-containing groups of OCQD had been
7
irreversibly reduced during OCQDs electrolysis. Figure S1D shows the Fourier
8
transform infrared reflectance (FTIR) spectra of OCQDs and CQDs where the
9
absorption intensities of C=O, C-O, and O-H in OCQDs were much stronger than
10
those in CQDs, confirming the electroreduction of OCQDs to CQDs. The CQDs were
11
in an average size of 7.5 nm on the electrode surface (Figure S1E). The cyclic
12
voltammogram (CV) for GO electrolysis on titanium ribbon is shown in Figure S1F
13
where the cathodic peak at −1.0 V was attributed to the irreversible electrochemical
14
reduction of GO to GR;25 moreover, in the Raman spectra of GO and GR (Figure
15
S1G), there appeared a decreased intensity ratio (ID/IG) of D band and G band, which
16
further confirmed the electroreduction of GO to GR. Figure S1H shows the CV for
17
OCQDs/GO co-electrolysis on titanium ribbon retaining the reduction peak of GO.
18
Notably, the peak currents persistently decreased with the successive co-deposition of
19
GR and CQDs but it is remarkably larger than that of individual GO or OCQDs
20
deposition, indicating the higher conductivity of CQDs/GR hybrid than that of GR or
21
CQDs. It is thought that the insertion of CQDs between GR sheets prevented GR
22
sheets from agglomeration and increased the surface area of GR, thus resulting in the
8
ACS Paragon Plus Environment
Page 8 of 25
Page 9 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
improved electrochemical activity. This inference is in agreement with the CV results
2
of GR, CQDs, and CQDs/GR on titanium ribbon in PBS buffer solution (Figure S2A)
3
and the redox probe Fe(CN)63−/4− solution (Figure S2B), respectively.
4
Figure 1A and B show the SEM and TEM images of CQDs/GR on titanium
5
ribbon, respectively. The CQDs particles in a size of about 7.5 nm were
6
homogeneously and densely distributed on the surface of GR sheets. Figure 1C shows
7
the PL spectra of GR (curve a), CQDs (curve b) and CQDs/GR hybrid (curve c). No
8
obvious excitation and emission peaks were observed for only GR, while being
9
excited at 365 nm, both CQDs and CQDs/GR demonstrate dramatic emission peaks at
10
about 420 nm, which were attributed to PL emission of CQDs. The PL peaks of both
11
CQDs/GR and CQDs were sharp, reflecting the homogeneous size distribution of
12
CQDs on the GR surface, consistent with the SEM and TEM results. No luminescence
13
from the supernatant after centrifugation of the CQDs/GR hybrid (curve d) suggested
14
the robust adhesion of CQDs to GR.
15
Multistage Amplified ECL of Carbon QDs. As shown in Figure 2A, in the
16
presence of coreactant S2O82−, there was no ECL on bare titanium ribbon (curve a)
17
and GR/titanium ribbon (curve b), while there was one ECL emission peak at −1.5 V
18
on both CQDs/titanium ribbon (curve c) and CQDs/GR/titanium ribbon (curve d),
19
resulting from the reaction between CQDs and S2O82−.16,24 The ECL intensity of
20
CQDs/GR is 7 times higher than that of CQDs, indicating that GR is a good ECL
21
signal amplifier. Notedly, the ECL intensity of CQDs/GR with S2O82− is 48 times
22
higher than that of CQDs in the absence of GR and S2O82− (Figure S3), indicating that
9
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
the proposed multistage amplification strategy is efficient in greatly enhancing the
2
ECL of CQDs. It is also worth noting that the ECL curve of the CQDs/GR shows an
3
onset potential at about −0.75 V that is more positive than that of CQDs (−0.9 V).
4
This indicates that the presence of GR decreased the potential barrier of the ECL
5
reduction due to the accelerated electron transport between the electrode and CQDs
6
by GR.22 The CVs of the CQDs and CQDs/GR electrodes were recorded
7
simultaneously (Figure 2A, inset), and both of them showed only one cathodic peak
8
attributed to the reduction of S2O82−.10,26 The more positive reduction potential and
9
higher reduction current of CQDs/GR electrode again suggest that GR could
10
accelerate electron transfer between the electrode and S2O82−. Figure 2B shows the
11
ECL emission of CQDs/GR with S2O82− under continuous 22 cyclic voltammetric
12
sweeps. The ECL emission is highly repeatable, meaning a good stability of the
13
proposed ECL multistage amplification system for CQDs.
14
Condition Optimization. The ECL emission intensity is closely related with the
15
performance of ECL detection. Accordingly, before the ECL detection of PCP, it is
16
necessary to investigate the factors that influence the ECL emission intensity.
17
Generally, the ECL emission intensity is dependent on the pH value of solution and
18
the concentration of coreactant S2O82−. Figure S4A shows that the strongest ECL
19
intensity was observed at pH 7. In acidic solution, the reduction of proton to hydrogen
20
would occur at the applied negative potential, which might inhibit the reduction of
21
S2O82−, while in basic solution, the intermediate SO4•− from S2O82− reduction would be
22
scavenged by OH•−, resulting in a decrease in the ECL intensity.27 Figure S4B shows
10
ACS Paragon Plus Environment
Page 10 of 25
Page 11 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
that the ECL intensity increased with the concentration of S2O82− in the range from 25
2
mM to the saturated concentration of 200 mM. As the ECL intensity obtained in 200
3
mM S2O82− reached the upper detection limit of the instrument, 100 mM S2O82− was
4
chosen to ensure an adequate sensitivity. Figure S4C shows that the ECL intensity
5
obviously increased with the increasing electrodeposition cycle from 1 to 10, but
6
when the electrodeposition cycle was more than 10, the ECL intensity kept basically
7
stable. So the optimum electrodeposition cycle was chosen to be 10.
8
ECL Detection of PCP. Figure 4 shows that the ECL intensity decreased with
9
increasing PCP concentration under the optimum condition. Taking the relative
10
intensity change of I−I0 / I0 as the function of the concentration of PCP, where I0 was
11
the initial intensity and I−I0 was the PCP-induced decrease in ECL intensity, the I−I0 /
12
I0 was linear dependent on the logarithm of PCP concentration in the range of 1.0 ×
13
10−12 -1.0 × 10−8 M with a correlation coefficient of 0.995 (Figure 4 inset) and the
14
limit of detection (LOD) was 1.0×10−12 M (S/N=3). Besides, the achieved LOD has
15
been much lower than the reported values: 8.6×10−8 M from a molecularly imprinted
16
polymer-based room-temperature phosphorescence optosensor,28 1.9×10−9 M from a
17
spectrophoto-metricassay,29 and 1.9×10−7 M from a gas chromatography–mass
18
spectrometry.30
19
The selectivity of the ECL sensor was also investigated by measuring the ECL
20
responses to the addition of some common cations and anions, 2,4,6-trinitrophenol
21
(2,4,6-TNP), p-dioxybenzene (p-DOB), p-chlorophenol (p-CP), 2,6-dichlorophenol
22
(2,6-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and PCP, respectively. As shown in
11
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
Table 1, several points are noteworthy: 1) comparing the ECL inhibition ratio of the
2
common ions (cations and anions) and the non-chlorinated phenols (2,4,6-TNP and
3
p-DOB) with those of the chlorinated phenols (CPs) (p-CP, 2,6-DCP, 2,4,6-TCP and
4
PCP), the ECL sensor showed high selectivity to CPs, especially to PCP; 2) the
5
compound with higher oxidation potential (Figure S5) made less decrease in the ECL
6
intensity, which suggested a oxidization detection mechanism; and 3) as the analogs
7
of PCP, p-CP, 2,6-DCP and 2,4,6-TCP showed less ECL inhibition ratios than PCP
8
due to their more stable chemical properties,31 making them harder to be oxidized.
9
The low LOD and high selectivity of the proposed sensor will guarantee the quick
10
scanning of PCP. PCP in the concentrations of 0 M, 5×10−11 M, 10×10−11 M, and
11
20×10−11 M was respectively spiked in real soil samples, including the sludge from
12
Taozi Lake and the soil from the experimental field nearby Xiangjiang River. All the
13
naturally dried soil samples (5.0 g) were extracted, respectively, with a mixture of
14
methanol/ethylene glycol (4:1, v/v), and then the extraction solutions were tested
15
using the proposed method and GC-MS, respectively. The testing results of PCP in
16
the sludge and the farmland soil by the proposed method were good agreement with
17
those achieved by GC-MS (Table 2), indicating that the ECL sensing system could be
18
employed in the identification of CPs in the environment.
19
Detection Mechanism. In ECL system, there are two proposed mechanisms: one
20
is cationic and anionic radical annihilation ECL and the other is coreactant ECL.16
21
Figure 3 shows the ECL transients of CQDs by applying 1 Hz potential steps between
22
−1.8 and +1.5V without S2O82−. At the first positive step, only cationic CQD radical
12
ACS Paragon Plus Environment
Page 12 of 25
Page 13 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
(C•+) was produced without ECL signals; however, both anodic and cathodic ECL
2
signals were evidently observed at the subsequent potential steps, indicating that both
3
cationic CQD radical (C•+) and anionic CQD radical (C•−) are stable enough to
4
produce ECL via the annihilation between C•+ and C•−. However, it was noteworthy
5
that the ECL signals was much lower than that of the CQDs with S2O82− (Figure S3),
6
indicating that S2O82− played an important role in the ECL process where the
7
coreactant ECL dominantly contributed to the whole ECL. Furthermore, due to the
8
extraordinary electron transport ability of GR (Figure 2A), electrons were transferred
9
from electrode to CQDs and S2O82− through GR in the cathodic ECL process,
10 11 12
promoting the generation of C•− and SO4•−. By referencing the proposed model,32 the possible ECL detection mechanism is elucidated as below:
13
C + e−
C•−
14
S2O82− + e-
15
2SO4•− + C•−→ 2SO42− + C*+
(3)
16
C*+ + e− → C + hν
(4)
17
C*+ + PCP → C + TCQ
(5)
18
Scheme 1 intuitively shows the mechanism. Primarily, electrons were transferred
19
from electrode to the CQDs and S2O82− through GR, resulting the generation of C•−
20
and SO4•− (reactions 1 and 2). The cathodic ECL herein was originated from the
21
formation of excited-state CQDs (C*+) via electron transfer annihilation of C•− and
22
SO4•− radicals (reactions 3 and 4). While PCP was added, the PCP would be absorbed
(1) SO42− + SO4•−
(2)
13
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
onto the CQDs surface, and be oxidized by C*+ resulting in a decrease in ECL
2
(reaction 5). The chloranil (TCQ) produced during the oxidization of PCP was
3
determined by mass spectroscopy analysis (not shown here), confirming the
4
oxidization detection mechanism.
5 6
CONCLUSION
7
In conclusion, a novel ECL sensing platform based on CQDs/GR hybrid was
8
fabricated by one-step electrochemical reduction technology. The new ECL sensor
9
exhibited the supersensitive and selective detection of PCP through a multistage
10
CQDs ECL signal amplification mechanism by combining GR with coreactant S2O82−.
11
Moreover, the sensing system is eco-friendly and recyclable. This work may present
12
an important strategy to design QDs ECL sensors and expanded their applications of
13
ultra-trace environmental sensing.
14 15
ACKNOWLEDGMENTS
16
This work was supported by the National Natural Science Foundation of China
17
(51178173, 51202065, 51078129), Innovation Research Team in University
18
(IRT1238), Program for New Century Excellent Talents in University (11-0126), the
19
Key Program of National Natural Science Foundation of China (51238002), and
20
Hunan Province graduate student scientific research innovation plan (No.
21
521298769).
22
14
ACS Paragon Plus Environment
Page 14 of 25
Page 15 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
REFERENCES
2
(1) Shiu, W. Y.; Ma, K. C.; Varhanickova, D.; Mackay, D. Chemosphere 1994, 29,
3
1155–1224.
4
(2) Keith, L. H.; Telliard, W. A. 1979, 13, 416–423.
5
(3) Toxic Substance Control Ac; U.S. Environmental Protection Agency:
6
Washington, DC, 1979.
7
(4) Leblance, Y. G.; Gilbert, R.; Hubert, J. Anal. Chem. 1999, 71, 78–85.
8
(5) Mardones, C.; Palma, J.; Sepulveda, C.; Berg, A.; Von Baer, D. J. Sep. Sci. 2003,
9
26, 923–926.
10
(6) Tayal, A.; Das, L.; Kaur, I. Biomed. Chromatogr. 1999, 13, 220–224.
11
(7) Fischer, W.; Bund, O.; Hauck, H. E. J. Anal. Chem. 1996, 354, 889–891.
12
(8) Kang, Q.; Yang, L. X.; Chen, Y. F.; Luo, S. L.; Wen, L. F.; Cai, Q. Y.; Yao, S. Z.
13
Anal. Chem. 2010, 82, 9749–9754.
14
(9) Lei, J. P.; Ju, H. X. Tren. in Anal. Chem. 2011, 30, 1351–1359.
15
(10) Jie, G. F.; Zhang, J. J.; Wang, D. C.; Cheng, C.; Chen, H. Y.; Zhu, J. J. Anal.
16 17 18 19 20
Chem. 2008, 80, 4033–4039. (11) Wang, Y.; Lu, J.; Tang, L. H.; Chang, H. X.; Li, J. H. Anal. Chem. 2009, 81, 9710 –9715. (12) Liu, X.; Zhang, Y.; Lei, J.; Xue, Y.; Cheng, L.; Ju, H. Anal. Chem. 2010, 82, 7351–7356.
21
(13) Bae, Y.; Myung, N.; Bard, A. J. Nano Lett. 2004, 4, 1153–1161.
22
(14) Ding, S. N.; Xu, J. J.; Chen, H. Y. Chem. Commun. 2006 , 34, 3631–3633.
15
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2 3 4 5 6
(15) Dirk, M.; Guldi, G. M.; Aminur, R.; Vito, S.; Nicholas, A. K.; Davide, B.; Maurizio, P. J. Am. Chem. Soc. 2006, 128, 2315–2323. (16) Zheng, L.; Chi, Y.; Dong, Y.; Lin, J.; Wang, B. J. Am. Chem. Soc. 2009, 131, 4564–4565. (17) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong,; B. H. Nature 2009, 457, 706–710.
7
(18) Geim, A. K. Science 2009, 32, 1530–1534.
8
(19) Katsnelson, M. I. Mater. Today 2007, 10, 20–27.
9
(20) Liu, C. B.; Wang, K.; Luo, S. L.; Tang, Y. H.; Chen, L.Y. Small 2011, 7,
10 11 12 13 14 15 16 17 18 19 20 21 22
1203-1206. (21) Yoo, E. J.; Okata, T.; Akita, T.; Kohyama, M.; Nakamura, J.; Honma, I. Nano. Lett. 2009, 9, 2255–2259. (22) Ling, L. L.; Liu, K. P.; Yang, G. H.; Wang, C. M.; Zhang, J. R.; Zhu, J. J. Adv. Funct. Mater. 2011, 21, 869–878. (23) William, S.; Hummers, J. R.; Richard, E. O. J Am. Chem. Soc. 1958, 80, 1339–1339. (24) Dong, Y. Q.; Zhou, N. N.; Lin, X. M.; Lin, J. P.; Chi, Y. W.; Chen, G. N. Chem. Mater. 2010, 22, 5895–5899. (25) Chen, L.Y.; Tang, Y. H.; Wang, K.; Liu, C. B.; Luo, S. L. Electrochem. Communica. 2011, 13, 133–137. (26) Jie, G. F.; Li, L. L.; Chen, C.; Xuan, J.; Zhu, J. J. Biosens. Bioelectron. 2009, 24, 3352–3358.
16
ACS Paragon Plus Environment
Page 16 of 25
Page 17 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2
Analytical Chemistry
(27) Yamashita, K.; Yamazaki-Nishida, S.; Harima, Y.; Segawa, A. Anal. Chem. 1991, 63, 872–876.
3
(28) Wang, H. F.; He, Y.; Ji, T. R.; Yan, X. P. Anal. Chem. 2009, 81, 1615–1621.
4
(29) Mufeed Awawdeh, A.; Harmon, H. J. Biosens. Bioelectron. 2005, 20,
5
1595–1601.
6
(30) Gremaud, E.; Turesky, R. J. J. Agric. Food Chem. 1997, 45, 1229–1233.
7
(31) Li, C. C.; Kang, Q.; Chen, Y. F.; Li, J. X.; Cai, Q. Y.; Yao, S. Z. Analyst 2010,
8 9 10
135, 2806–2810. (32) Li, J. X.; Yang, L. X.; Luo, S. L.; Chen, B. B.; Li, J.; Lin, H. L.; Cai, Q. Y.; Yao, S. Z. Anal. Chem. 2010, 82, 7357–7361.
11
17
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 18 of 25
Figures and Tables
1 2
(A) C
e-
C
eee-
(B)
ECL Signals
C·-
hv
*+
C
eeee-
h+
SO4·S2O82- SO42-
CQDs:
C C·-
h+
SO4·S2O82-
*+
PCP
hv TCQ
SO42-
ECL quenching
ECL emission RG:
ECL Signals
PCP:
TCQ:
3 4
Scheme 1 Illustrative ECL detection mechanism for PCP with CQDs/GR in S2O82−
5
solution.
6
18
ACS Paragon Plus Environment
Page 19 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1 2
Figure 1 (A) SEM and (B) TEM images of CQDs/GR. (C) Photoluminescence of (a)
3
GR, (b) CQDs, (c) CQDs/GR and (d) the supernatant after centrifugation of the
4
CQDs/GR hybrid.
5
19
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
2 3
Figure 2 (A) ECL–potential curves of (a) bare titanium ribbon, (b) GR, (c) CQDs,
4
and (d) CQDs/GR on titanium ribbon in 0.067 M pH 7 PBS containing 100 mM
5
S2O82−. Inset: CVs of (a) CQDs and (b) CQDs/GR on titanium ribbon. (B) ECL
6
emission from CQDs/GR with 100 mM S2O82− under continuous 22 cycles of cyclic
7
voltammetry.
8 20
ACS Paragon Plus Environment
Page 20 of 25
Page 21 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1 2
Figure 3 ECL transients of CQDs by applying 1 Hz potential steps between −1.8 and
3
+1.5 V without S2O82−.
4
21
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2
Figure 4 The ECL intensity response of CQDs/GR in 100 mM S2O82− solution (pH 7)
3
at different concentration of PCP (×10−12 M): (a) 1, (b) 10, (c) 100, (d) 1000, and (e)
4
10000; the insert plot is the calibration curve for PCP determination.
5
22
ACS Paragon Plus Environment
Page 22 of 25
Page 23 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
Analytical Chemistry
Table 1
The interferences on PCP detection.
Substance
Concentration /M
I−I0 / I
Na+
3.0×10−11
0.1%
K+
3.0×10−11
0.1%
Ca2+
3.0×10−11
0.2%
Mg2+
3.0×10−11
0.1%
Cl-
3.0×10−11
0.2%
NO3-
3.0×10−11
0.1%
HCO32-
3.0×10−11
0.3%
SO42-
3.0×10−11
0.2%
2,4,6-TNP
3.0×10−11
2.1%
p-DOB
3.0×10−11
3.2%
p-CP
3.0×10−11
6.2%
2,6-DCP
3.0×10−11
7.3%
2,4,6-TCP
3.0×10−11
9.1%
PCP
3.0×10−11
21.4%
CPs analogs
2
23
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
Table 2
Page 24 of 25
Recovery study for PCP with various real soil samples.
Sample
PCP
Proposed method
GC-MS
Added
Found
Found
(10−11 M)
(10−11 M)
(10−11 M)
0
3.2±0.2
2.8±0.1
5.0
7.9±0.1
7.8±0.2
10.0
13.1±0.3
12.5±0.4
20.0
22.9±0.5
22.4±0.3
0
2.1±0.1
1.7±0.2
5.0
7.2±0.3
6.9±0.1
10.0
11.9±0.2
11.4±0.3
20.0
19.9±0.6
19.7±0.2
sludge
farmland soil
2
24
ACS Paragon Plus Environment
Page 25 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
for TOC only
1 2 (A) eeee-
(B)
ECL Signals
C
C*+
C·-
hv
C eeee-
h+
SO4·S2O82- SO42-
3
ECL Signals
C·-
C*+ h+
SO4·S2O82-
PCP SO42-
ECL quenching
ECL emission
25
ACS Paragon Plus Environment
hv TCQ
