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Graphene quantum dots enhanced electrochemiluminescence of cadmium sulfide nanocrystals for ultrasensitive determination of pentachlorophenol Qian Liu,ab Kun Wang,*b Juan Huan,b Gangbing Zhu,c Jing Qian,b Hanping Maob and Jianrong Cai*a An ultrasensitive electrochemiluminescence (ECL) sensor based on the ECL amplifying behavior of the graphene quantum dots–CdS nanocrystals (GQDs–CdS NCs) was constructed for the detection of pentachlorophenol (PCP). Because of the presence of doped GQDs, the resulting GQDs–CdS NCs exhibited 5-fold enhanced ECL intensity than pure CdS NCs with the ECL onset potential positively shifted by 80 mV. Furthermore, based on the effective inhibition of the ECL response of GQDs–CdS NCs
Received 12th February 2014 Accepted 21st March 2014
film by PCP, a simple method for ultrasensitive determination of PCP was devised, which showed a wide linear range of 0.01–500 ng mL1 and a low detection limit of 3 pg mL1 (S/N ¼ 3) with good stability, reproducibility and applicability for PCP detection in real water samples. Thus, it can be expected that
DOI: 10.1039/c4an00307a
GQDs-based composites with excellent performance may play a more important role in pesticide
www.rsc.org/analyst
determination.
Introduction Signal amplication strategy has attracted considerable interest because of the need for ultrasensitive electrochemical sensors, and employing nanoscaled materials with different sizes, shapes and compositions for signal amplication has become an important topic in recent years.1,2 In particular, because of their outstanding electric conductivity, carbon-based nanomaterials, such as one-dimensional carbon nanotubes (CNTs) and two-dimensional graphene sheets, can accelerate electron transfer on the electrode surface to amplify electrochemical signals.3 Graphene quantum dots (GQDs), which are emerging zero-dimensional carbon-based nanomaterials, have been widely used for designing various optical applications such as bioimaging4 and photoluminescent5,6 and electroluminescent sensing7 because of their low toxicity, better surface graing, high uorescent activity, and excellent water solubility.8–10 In addition to these properties, GQDs exhibit outstanding electric conductivity compared with one-dimensional CNTs or twodimensional graphene sheets;11 thus, they are ideal materials in the eld of signal amplication for the fabrication of a
School of Food and Biological Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China. E-mail: [email protected]
b
Key Laboratory of Modern Agriculture Equipment and Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China. E-mail: [email protected]; Fax: +86 511 88791708; Tel: +86 511 88791800
c School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China
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ultrasensitive electrochemical sensors. However, little attention has been paid to this eld, probably because the existence of oxygen-rich functional groups at the surfaces and edges of GQDs makes them water soluble12 and thus restricts their practical applications for signal amplication. As a result, it is imminently desired to achieve GQDs-based signal amplication for the construction of ultrasensitive solid-state electrochemical sensors, which has not been reported yet to our knowledge. Pentachlorophenol (PCP), classied as a priority persistent organic pollutant by the United States Environmental Protection Agency, has been widely used as a pesticide, bactericide, disinfectant and wood preservative.13–15 Owing to its high toxicity, long persistence, and recalcitrance to degradation, large-scale use of PCP has led to severe contamination of terrestrial and aquatic ecosystems.16,17 Thus, it is necessary and urgent to develop a highly sensitive method for rapid and accurate determination of PCP. Although traditional methods, such as gas chromatography, gas chromatography coupled with mass spectrometry or electron capture detectors18–20 and highperformance liquid chromatography,21 enable accurate determination of PCP, they require complicated pretreatment steps, considerable labor resources, and expensive equipment; thus, they cannot be used for on-site determination of PCP. Recently, alternative approaches for PCP determination such as total internal reection ellipsometry phosphorescence22 and spectrouorimetric detection methods23 have been put forward for simplifying the detection procedure and reducing expenses; however, these approaches generally require long testing times.
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Therefore, development of a convenient and effective approach for rapid and accurate detection of PCP with high sensitivity and rapid response remains a challenge. Electrochemiluminescence (ECL), the production of light from electrochemically generated reagents, has received considerable attention recently due to its low cost, high sensitivity, good selectivity, low background noise, and ease of operation.24,25 Many kinds of semiconductor nanocrystals, such as ZnO,26,27 CdO,28 CdS,29 and CdSe,30 have been accepted as ECL indicators because of their effective and valuable ECL behavior. Recently, with the development of nanoscience and ECL technique, efforts have been made toward ECL detection of PCP based on various nanomaterials because of their promising ECL properties.16,31,32 However, to our knowledge, ECL detection of PCP based on GQD-based signal amplication has not been reported yet. This work is aimed to provide a potential sensitive and quantitative application platform to monitor PCP contamination based on signal amplication of a GQD-based system. Herein, a novel composite of GQDs–CdS nanocrystals (GQDs– CdS NCs) was successfully fabricated via a facile one-pot reaction, which exhibited enhanced ECL intensity compared to pure CdS NCs. Based on the inhibitory effect of PCP on the ECL intensity of the GQD-based system, an effective method for ECL detection of PCP was proposed. This result offers a new perspective for applying signal amplication of a GQDs-based system in pesticide determination.
Experimental Reagents Pentachlorophenol (PCP) and Naon (5%) were purchased from Sigma-Aldrich (USA). Carbon bers were directly purchased from Zhenjiang Hengshen Co. Ltd. H2O2 (30% w/v, solution) and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. CdS NCs were synthesized and characterized as per our previous work.29 PBS (0.1 M) was prepared by mixing standard stock solutions of NaH2PO4 and Na2HPO4, and adjusting the pH value with 0.1 M H3PO4 or NaOH. All solutions were prepared with twice-distilled water. Apparatus Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with a JEOL 2100 transmission electron microscope (Japan) operated at 200 kV. X-ray photoelectron spectrometry (XPS) analyses were carried out on an ESCALAB MKII X-ray photoelectron spectrometer. Fourier transform infrared (FT-IR) and Raman spectra were obtained from a Nicolet Nexus 470 FTIR spectrometer and an RM 2000 microscopic confocal Raman spectrometer, respectively. Electrochemiluminescence (ECL) curves were recorded by a Model MPI-A ECL analyzer system (Xi'an Remax Electronic Science and Technology Co. Ltd., Xi'an, China) in 0.1 M PBS (pH 9.0) with 10 mM H2O2. The photomultiplier tube was biased at 800 V in the ECL process. All cyclic voltammetric (CV) and ECL curves were recorded with a
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Analyst
conventional three-electrode system, in which a glassy carbon electrode (GCE, 3 mm in diameter) was used as a working electrode, a Ag/AgCl (saturated KCl solution) was used as a reference electrode and a platinum wire was used as a counter electrode. Electrochemical impedance spectroscopy (EIS) was performed in a 0.1 M KCl solution containing 5 mM Ru(NH3)6Cl3 at 200 mV with a frequency range from 0.1 Hz to 10 kHz, and the amplitude of the applied sine wave potential in each case was 5 mV, which was obtained with a ZENNIUM electrochemical workstation (Zahner Instruments, Germany). Preparation of graphene quantum dots–CdS NCs composite The GQDs were synthesized from carbon bers according to the reported method.33 To obtain GQDs–CdS NCs, 40 mL GQDs solution (1 mg mL1) was added to 50 mL 0.035 M Cd(NO3)2 solution under stirring. Then, H2S was bubbled through this dispersion for 1 h to form a yellow GQDs–CdS NCs precipitation. By using H2S as a sulde source and reducing agent, the in situ formation of CdS NCs and the partial reduction of GQDs were performed simultaneously during the reaction. The resulting solids were centrifuged and washed three times with distilled water and acetone, and then dried in vacuum at 45 C for 24 h. Fabrication of the modied electrode Prior to modication, the GCE was rst polished with sandpaper, followed by 1.0, 0.3, and 0.05 mm alumina slurry, and was then ultrasonicated in a water bath to remove any residues. GQDs–CdS NCs (2 mg mL1) suspension was obtained via dispersing GQDs–CdS NCs in 0.5% Naon/ethanol solution under ultrasonication. Then, 6 mL of the suspension was cast on the pretreated GCE surface and dried in air at room temperature to form GQDs–CdS NCs modied GCE (GQDs–CdS NCs/GCE). For comparison, CdS NCs/GCE and GQDs/GCE were fabricated by a similar procedure.
Results and discussion Characterization of GQDs–CdS NCs TEM was employed to characterize the morphology of the resulting GQDs (Fig. 1A) and GQDs–CdS NCs (Fig. 1B). It is obvious that the as-prepared GQDs are highly dispersed with their diameters mainly distributed in the range of 3–6 nm. The HRTEM image (Fig. 1A inset) indicated the high crystallinity of the GQDs and a lattice parameter of 0.21 nm, which was consistent with the (102) diffraction planes of sp2 graphitic carbon.34 Furthermore, the typical TEM image of GQDs–CdS NCs displayed in Fig. 1B indicates that the products mainly consist of uniform NCs with sizes ranging from 10 to 15 nm. To conrm the formation of GQDs–CdS NCs, an HRTEM image was recorded (Fig. 1B inset). The interplanar spacing of 0.21, 0.34 and 0.31 nm were clearly observed, which corresponded to the (102) facet of graphite and (002) and (101) facets of CdS and NCs, respectively. XPS measurements were carried out to further prove the formation of GQDs–CdS NCs (Fig. 1C). A dominant graphitic C
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TEM images of (A) GQDs and (B) GQDs–CdS NCs (inset: corresponding HRTEM image); (C) XPS survey spectra of GQDs and GQDs–CdS NCs; (D) high-resolution XPS S 2p and Cd 3d of GQDs– CdS NCs. Fig. 1
1s peak at 285 eV and O 1s peak at 532 eV were observed for original GQDs and GQDs–CdS NCs.33 In addition, the XPS spectrum of GQDs–CdS NCs conrms the existence of cadmium and sulfur species on the surface of NCs from the appearance of a characteristic Cd 3d5/2 peak at 404.6 eV, a Cd 3d3/2 peak at 411.5 eV and an S 2p peak at 161.2 eV (Fig. 1D).35 In addition, the characteristic Cd peaks had a binding energy shi of 1.2 eV compared to that of CdS NCs (gure not shown), which indicated that the local chemical state was slightly inuenced by the incorporation of GQDs into the CdS lattice.36 Raman scattering has been proven to be an essential tool for characterizing graphene materials, particularly for distinguishing ordered and disordered crystal structures of carbon.37 Fig. 2A shows the Raman spectra of GQDs and GQDs– CdS NCs. The frequencies of the G and D bands in the GQDs– CdS NCs are similar to those observed in the GQDs, although the GQDs–CdS NCs have an increased D/G intensity ratio relative to the GQDs. This change suggests a decrease in the size of the in-plane sp2 domains, which demonstrates that GQDs in the GQDs–CdS NCs have been reduced during the chemical process.38 The FT-IR results of the as-prepared GQDs and GQDs–CdS NCs are displayed in Fig. 2B. The spectrum of GQDs illustrates the presence of C–OH (nC–OH at 1421 cm1) and C]O (nC]O at 1640 cm1 in carboxylic acid and carbonyl moieties).39 Compared with GQDs, the adsorption bands of the oxygen functional groups in the GQDs–CdS NCs weakened, further conrming that the GQDs have been deoxygenated. The deconvoluted C 1s XPS core levels of the GQDs and GQDs–CdS NCs are presented in Fig. 2C and D. The deconvoluted peak located at the binding energy of 285 eV is attributed to the C–C and C]C bonds, and the deconvoluted peaks centered at the binding energies of 286.2 and 287.5 eV are attributed to the C–OH and C]O oxygen-containing carbon
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(A) Raman and (B) FT-IR spectra of the as-prepared GQDs and GQDs–CdS NCs; peak deconvolution of C 1s XPS core level of (C) GQDs and (D) GQDs–CdS NCs.
Fig. 2
bands, respectively.40,41 For comparison, the peak of oxygenated groups in the spectrum of GQDs–CdS NCs is slightly weaker than that in the spectrum of GQDs, indicating a partial removal of oxygenated groups during the chemical process.34 This result is consistent with observations from Raman and FT-IR results in Fig. 2A and B. Amplied ECL response of the GQDs–CdS NCs modied electrode CV curves were recorded in 0.1 M PBS (pH 9.0) for CdS NCs/GCE and GQDs–CdS NCs/GCE between 0.1 V and 1.5 V, as displayed in Fig. 3A inset. It is obvious that a pair of redox peaks of CdS NCs appeared at ca. 0.88 V and 1.23 V at both electrodes. In addition, the redox peak current of CdS NCs for the GQDs– CdS NCs modied electrode was enhanced compared to that for the CdS NCs electrode. Further, the DEp of CdS NCs on the GQDs–CdS NCs modied electrode was much smaller than that
(A) ECL-potential curves of GQDs/GCE (a), CdS NCs/GCE (b) and GQDs–CdS NCs/GCE (c) in 0.1 M PBS (pH 9.0) containing 10 mM H2O2 (inset: CV curves of CdS NCs/GCE (red) and GQDs–CdS NCs/ GCE (green) in 0.1 M PBS (pH 9.0)). (B) EIS of CdS NCs/GCE (a) and GQDs–CdS NCs/GCE (b) in 0.1 M KCl containing 5 mM Ru(NH3)6Cl3 at 200 mV vs. SCE (inset: equivalent circuit used for fitting the impedance data). Fig. 3
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of the CdS NC lm, which indicates that the GQDs can facilitate the electrochemical redox process of CdS NCs.42,43 The ECL behaviors of CdS NCs/GCE, GQDs/GCE and GQDs– CdS NCs/GCE were investigated in 0.1 M PBS (pH 9.0) with 10 mM H2O2. As shown in Fig. 3A, there was no obvious light emission at the GQDs modied electrode (curve a), and obvious ECL signals were observed at CdS NCs (curve b) and GQDs–CdS NCs modied electrodes (curve c), suggesting that the ECL emission can be attributed to the introduction of CdS NCs. Furthermore, the ECL intensity from the GQDs–CdS NCs lm was about ve times higher than that observed from the CdS NCs lm, which indicates that the incorporation of GQDs in the GQDs–CdS NCs can enhance the ECL intensity of CdS NCs effectively. Moreover, the ECL onset potential of the GQDs–CdS NCs lm was 80 mV more positive than that of CdS NCs, indicating that the GQDs can clearly decrease the potential barriers of the ECL reaction, which is accordance with the CV response of the modied electrodes shown in the Fig. 3A inset. EIS is widely used to obtain information on electron transfers between electrolytes and electrode surfaces. The EIS normally includes a semicircular part and a linear part, and the semicircle diameter at higher frequencies corresponds to the electron-transfer resistance.44 Fig. 3B shows the impedance
Fig. 4 (A) ECL-potential curves of GQDs–CdS NCs/GCE in 0.1 M PBS (pH 9.0) containing 10 mM H2O2 in the absence (a) and presence (b) of 1 ng mL1 PCP. (B) Time-dependent ECL signals collected at 1.26 V of GQDs–CdS NCs/GCE in 0.1 M PBS (pH 9.0) with 10 mM H2O2.
Scheme 1
spectra at different modied electrodes using [Ru(NH3)6]3+ as a redox probe. It is obvious that the GQDs–CdS NCs modied electrode (curve b) exhibited fast electron transfer as evidenced by a minimal semicircular portion in the Nyquist diagram, which proves that the GQDs doped in the CdS NCs can further enhance the electron transfer.45
ECL quenching of GQDs–CdS NCs/GCE by PCP The addition of PCP into this ECL system resulted in a signicant decrease in ECL intensity, as shown in Fig. 4A. The possible ECL detection mechanism is intuitively shown in Scheme 1 and is described as follows: CdS NCs are reduced to generate nanocrystal species (CdS). With H2O2 as the coreactant, the electrochemical reduction of H2O2 can produce hydroxyl radical (OHc), which can easily inject a hole in the HOMO of CdS efficiently to form oxidized CdS+. Then, the recombination of the CdS and CdS+ results in the formation of excited states (CdS*) and emission of light in an aqueous solution.46,47 When PCP is added, it is oxidized into TCQ, which has an energy level between the HOMO and LUMO of CdS. Then, the quenching effect takes place due to the energy-transfer process, which reduces the production of excited CdS* and results in a decrease in ECL.48 Furthermore, the ECL stability of GQDs–CdS NCs modied electrode was also investigated in 0.1 M PBS (pH 9.0) with 10 mM H2O2, as shown in Fig. 4B. The ECL response of GQDs–CdS NCs/GCE was repeated under continuous cyclic scans of 0.1 to 1.5 V for 14 cycles. Note that the ECL intensity was ca. 6800 a.u. and did not show obvious change. This result indicates that the ECL intensity of the modied electrode was very stable and benecial for construction of the ECL sensor.
Optimization of the ECL response at GQDs–CdS NCs/GCE In order to achieve the optimal ECL response for PCP determination, the effects of solution pH and H2O2 concentration on
Illustrative ECL detection mechanism for PCP based on GQDs–CdS NCs/GCE.
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Fig. 5
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Effects of (A) solution pH and (B) H2O2 concentration on the ECL intensity of the GQDs–CdS NCs/GCE.
then tended to stabilize. Thus, 10 mM was chosen as the optimal concentration of H2O2 in this experiment. ECL detection of PCP
Fig. 6 (A) Effect of PCP concentration on ECL behavior of GQDs–CdS NCs/GCE in 0.1 M PBS (pH 9.0) with 10 mM H2O2. (B) Calibration curve for PCP detection. Error bars: S.D., n ¼ 3.
the ECL behaviors of GQDs–CdS NCs/GCE were considered carefully, as shown in Fig. 5. The effect of solution pH on the ECL behaviors of the GQDs– CdS NCs modied electrode was determined in 0.1 M PBS with a pH range of 6.0–10.0. As shown in Fig. 5A, the ECL intensity increased with an increase in pH from 6.0 to 9.0, and then decreased when the pH increased from 9.0 to 10.0; thus, a pH of 9.0 was selected in subsequent experiments. Furthermore, the effect of H2O2 concentration on the ECL responses of the GQDs– CdS NCs lm was considered carefully in our work. As shown in Fig. 5B, the ECL intensity increased gradually with an increase in H2O2 concentration; however, when the concentration of H2O2 reached 10 mM, the ECL intensity increased slowly and
Based on the inhibition of PCP on the ECL intensity of GQDs– CdS NCs/GCE, an effective method for ECL detection of PCP was proposed, as shown in Fig. 6. It is obvious that the ECL intensity of GQDs–CdS NCs/GCE decreased gradually with an increase in PCP concentration (Fig. 6A). Moreover, the ECL intensity was linearly dependent on the logarithm of PCP concentration in the range from 0.01 to 500 ng mL1 with a detection limit of 3 pg mL1 (S/N ¼ 3) (Fig. 6B), indicating that the proposed ECL sensor is remarkably reliable for the ultrasensitive determination of PCP. Reproducibility, stability, specicity and real sample analysis The reproducibility was investigated for ve identically made electrodes with a relative standard deviation (RSD) of 7.3%, while 5 measurements for a single electrode were made upon the addition of 0.5 ng mL1 PCP in 0.1 M PBS (pH 9.0) with a RSD of 4.9%, demonstrating excellent reproducibility. In order to evaluate the stability of the sensor, the ECL responses to 0.5 ng mL1 PCP were measured in 0.1 M PBS (pH 9.0) at two-day intervals. As shown in Fig. 7A, the proposed sensor retained about 87% of its initial ECL response aer three weeks, suggesting effective stability.
Fig. 7 (A) Stability of the ECL sensor over 23 days in 0.1 M PBS (pH 9.0) with 0.5 ng mL1 PCP. (B) Quenching of the ECL responses of the sensor in 0.1 M PBS (pH 9.0) by 0.5 ng mL1 DCP, TCP, DNP and PCP.
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Paper Table 1
Analyst Recovery studies of PCP in tap and river water samples (n ¼ 3)
Sample
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Tap water
River water
Taken (pg mL1) 0.00 20.0 50.0 100.0 0.00 20.0 50.0 100.0
Found (pg mL1)
Recovery (%)
RSD (%)
Not detected 21.7 48.3 97.5 15.4 34.6 66.9 119.6
—
—
108.5 96.6 97.5 — 97.7 102.3 103.6
5.6 5.2 4.4 5.1 4.2 5.7 6.5
The selectivity of the ECL sensor to PCP was also investigated by measuring the ECL responses to analogs of PCP, such as 2,3,5-trichlorophenol (TCP), 2,6-dichlorophenol (DCP) and 2,4dinitrophenol (DNP), as shown in Fig. 7B. The result indicated that the addition of 0.5 ng mL1 DCP, TCP and DNP resulted in only 4.6%, 2.7% and 3.1% decreases in ECL intensity, respectively, whereas the addition of 0.5 ng mL1 PCP resulted in a 28.6% decrease in ECL intensity. Thus, a relatively high selectivity was achieved. The reason may be that these selected interferences are more stable than PCP, and thus could not result in consumption of the excited CdS NCs*. These results indicate that the ECL detection of PCP is not affected by the presence of the analogs of PCP. To demonstrate the practicality of the proposed ECL sensor, a recovery test was carried out by adding different amounts of PCP into tap and river water samples. The results of this test are presented in Table 1, and they are in good agreement with the given concentration with average recoveries ranging from 96.6% to 108.5% (n ¼ 3). These results indicate that this method can be used for analysis of real samples.
Conclusions A novel ultrasensitive ECL sensor for PCP detection was fabricated based on the ECL amplifying behavior of GQDs–CdS NCs. The GQDs doped in the GQDs–CdS NCs not only enhanced the ECL intensity of CdS NCs effectively, but also decreased their ECL onset potential. Furthermore, based on the effective inhibition of PCP on the ECL response of GQDs–CdS NCs, a simple ECL sensor for the determination of PCP was constructed. The resulting ECL sensor showed excellent performance with the merits of wide linear range, low detection limit, good stability, reproducibility and ne applicability for the detection of PCP in real samples. This result enables a new application of GQDs as an efficient signal amplication material for the construction of high-performance ECL devices for pesticide determination.
Acknowledgements The present work was supported by the National Natural Science Foundation of China (no. 21175061 and 21375050), Key Laboratory of Modern Agriculture Equipment and Technology
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(no. NZ201109), the Natural Science Foundation of Jiangsu province (no. BK20130481), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Innovation Project of Science and Technology for College Graduates of Jiangsu Province (no. CXZZ12_0704).
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Graphene quantum dots enhanced electrochemiluminescence of cadmium sulfide nanocrystals for ultrasensitive determination of pentachlorophenol Qian Liu,ab Kun Wang,*b Juan Huan,b Gangbing Zhu,c Jing Qian,b Hanping Maob and Jianrong Cai*a An ultrasensitive electrochemiluminescence (ECL) sensor based on the ECL amplifying behavior of the graphene quantum dots–CdS nanocrystals (GQDs–CdS NCs) was constructed for the detection of pentachlorophenol (PCP). Because of the presence of doped GQDs, the resulting GQDs–CdS NCs exhibited 5-fold enhanced ECL intensity than pure CdS NCs with the ECL onset potential positively shifted by 80 mV. Furthermore, based on the effective inhibition of the ECL response of GQDs–CdS NCs
Received 12th February 2014 Accepted 21st March 2014
film by PCP, a simple method for ultrasensitive determination of PCP was devised, which showed a wide linear range of 0.01–500 ng mL1 and a low detection limit of 3 pg mL1 (S/N ¼ 3) with good stability, reproducibility and applicability for PCP detection in real water samples. Thus, it can be expected that
DOI: 10.1039/c4an00307a
GQDs-based composites with excellent performance may play a more important role in pesticide
www.rsc.org/analyst
determination.
Introduction Signal amplication strategy has attracted considerable interest because of the need for ultrasensitive electrochemical sensors, and employing nanoscaled materials with different sizes, shapes and compositions for signal amplication has become an important topic in recent years.1,2 In particular, because of their outstanding electric conductivity, carbon-based nanomaterials, such as one-dimensional carbon nanotubes (CNTs) and two-dimensional graphene sheets, can accelerate electron transfer on the electrode surface to amplify electrochemical signals.3 Graphene quantum dots (GQDs), which are emerging zero-dimensional carbon-based nanomaterials, have been widely used for designing various optical applications such as bioimaging4 and photoluminescent5,6 and electroluminescent sensing7 because of their low toxicity, better surface graing, high uorescent activity, and excellent water solubility.8–10 In addition to these properties, GQDs exhibit outstanding electric conductivity compared with one-dimensional CNTs or twodimensional graphene sheets;11 thus, they are ideal materials in the eld of signal amplication for the fabrication of a
School of Food and Biological Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China. E-mail: [email protected]
b
Key Laboratory of Modern Agriculture Equipment and Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China. E-mail: [email protected]; Fax: +86 511 88791708; Tel: +86 511 88791800
c School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China
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ultrasensitive electrochemical sensors. However, little attention has been paid to this eld, probably because the existence of oxygen-rich functional groups at the surfaces and edges of GQDs makes them water soluble12 and thus restricts their practical applications for signal amplication. As a result, it is imminently desired to achieve GQDs-based signal amplication for the construction of ultrasensitive solid-state electrochemical sensors, which has not been reported yet to our knowledge. Pentachlorophenol (PCP), classied as a priority persistent organic pollutant by the United States Environmental Protection Agency, has been widely used as a pesticide, bactericide, disinfectant and wood preservative.13–15 Owing to its high toxicity, long persistence, and recalcitrance to degradation, large-scale use of PCP has led to severe contamination of terrestrial and aquatic ecosystems.16,17 Thus, it is necessary and urgent to develop a highly sensitive method for rapid and accurate determination of PCP. Although traditional methods, such as gas chromatography, gas chromatography coupled with mass spectrometry or electron capture detectors18–20 and highperformance liquid chromatography,21 enable accurate determination of PCP, they require complicated pretreatment steps, considerable labor resources, and expensive equipment; thus, they cannot be used for on-site determination of PCP. Recently, alternative approaches for PCP determination such as total internal reection ellipsometry phosphorescence22 and spectrouorimetric detection methods23 have been put forward for simplifying the detection procedure and reducing expenses; however, these approaches generally require long testing times.
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Therefore, development of a convenient and effective approach for rapid and accurate detection of PCP with high sensitivity and rapid response remains a challenge. Electrochemiluminescence (ECL), the production of light from electrochemically generated reagents, has received considerable attention recently due to its low cost, high sensitivity, good selectivity, low background noise, and ease of operation.24,25 Many kinds of semiconductor nanocrystals, such as ZnO,26,27 CdO,28 CdS,29 and CdSe,30 have been accepted as ECL indicators because of their effective and valuable ECL behavior. Recently, with the development of nanoscience and ECL technique, efforts have been made toward ECL detection of PCP based on various nanomaterials because of their promising ECL properties.16,31,32 However, to our knowledge, ECL detection of PCP based on GQD-based signal amplication has not been reported yet. This work is aimed to provide a potential sensitive and quantitative application platform to monitor PCP contamination based on signal amplication of a GQD-based system. Herein, a novel composite of GQDs–CdS nanocrystals (GQDs– CdS NCs) was successfully fabricated via a facile one-pot reaction, which exhibited enhanced ECL intensity compared to pure CdS NCs. Based on the inhibitory effect of PCP on the ECL intensity of the GQD-based system, an effective method for ECL detection of PCP was proposed. This result offers a new perspective for applying signal amplication of a GQDs-based system in pesticide determination.
Experimental Reagents Pentachlorophenol (PCP) and Naon (5%) were purchased from Sigma-Aldrich (USA). Carbon bers were directly purchased from Zhenjiang Hengshen Co. Ltd. H2O2 (30% w/v, solution) and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. CdS NCs were synthesized and characterized as per our previous work.29 PBS (0.1 M) was prepared by mixing standard stock solutions of NaH2PO4 and Na2HPO4, and adjusting the pH value with 0.1 M H3PO4 or NaOH. All solutions were prepared with twice-distilled water. Apparatus Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with a JEOL 2100 transmission electron microscope (Japan) operated at 200 kV. X-ray photoelectron spectrometry (XPS) analyses were carried out on an ESCALAB MKII X-ray photoelectron spectrometer. Fourier transform infrared (FT-IR) and Raman spectra were obtained from a Nicolet Nexus 470 FTIR spectrometer and an RM 2000 microscopic confocal Raman spectrometer, respectively. Electrochemiluminescence (ECL) curves were recorded by a Model MPI-A ECL analyzer system (Xi'an Remax Electronic Science and Technology Co. Ltd., Xi'an, China) in 0.1 M PBS (pH 9.0) with 10 mM H2O2. The photomultiplier tube was biased at 800 V in the ECL process. All cyclic voltammetric (CV) and ECL curves were recorded with a
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Analyst
conventional three-electrode system, in which a glassy carbon electrode (GCE, 3 mm in diameter) was used as a working electrode, a Ag/AgCl (saturated KCl solution) was used as a reference electrode and a platinum wire was used as a counter electrode. Electrochemical impedance spectroscopy (EIS) was performed in a 0.1 M KCl solution containing 5 mM Ru(NH3)6Cl3 at 200 mV with a frequency range from 0.1 Hz to 10 kHz, and the amplitude of the applied sine wave potential in each case was 5 mV, which was obtained with a ZENNIUM electrochemical workstation (Zahner Instruments, Germany). Preparation of graphene quantum dots–CdS NCs composite The GQDs were synthesized from carbon bers according to the reported method.33 To obtain GQDs–CdS NCs, 40 mL GQDs solution (1 mg mL1) was added to 50 mL 0.035 M Cd(NO3)2 solution under stirring. Then, H2S was bubbled through this dispersion for 1 h to form a yellow GQDs–CdS NCs precipitation. By using H2S as a sulde source and reducing agent, the in situ formation of CdS NCs and the partial reduction of GQDs were performed simultaneously during the reaction. The resulting solids were centrifuged and washed three times with distilled water and acetone, and then dried in vacuum at 45 C for 24 h. Fabrication of the modied electrode Prior to modication, the GCE was rst polished with sandpaper, followed by 1.0, 0.3, and 0.05 mm alumina slurry, and was then ultrasonicated in a water bath to remove any residues. GQDs–CdS NCs (2 mg mL1) suspension was obtained via dispersing GQDs–CdS NCs in 0.5% Naon/ethanol solution under ultrasonication. Then, 6 mL of the suspension was cast on the pretreated GCE surface and dried in air at room temperature to form GQDs–CdS NCs modied GCE (GQDs–CdS NCs/GCE). For comparison, CdS NCs/GCE and GQDs/GCE were fabricated by a similar procedure.
Results and discussion Characterization of GQDs–CdS NCs TEM was employed to characterize the morphology of the resulting GQDs (Fig. 1A) and GQDs–CdS NCs (Fig. 1B). It is obvious that the as-prepared GQDs are highly dispersed with their diameters mainly distributed in the range of 3–6 nm. The HRTEM image (Fig. 1A inset) indicated the high crystallinity of the GQDs and a lattice parameter of 0.21 nm, which was consistent with the (102) diffraction planes of sp2 graphitic carbon.34 Furthermore, the typical TEM image of GQDs–CdS NCs displayed in Fig. 1B indicates that the products mainly consist of uniform NCs with sizes ranging from 10 to 15 nm. To conrm the formation of GQDs–CdS NCs, an HRTEM image was recorded (Fig. 1B inset). The interplanar spacing of 0.21, 0.34 and 0.31 nm were clearly observed, which corresponded to the (102) facet of graphite and (002) and (101) facets of CdS and NCs, respectively. XPS measurements were carried out to further prove the formation of GQDs–CdS NCs (Fig. 1C). A dominant graphitic C
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TEM images of (A) GQDs and (B) GQDs–CdS NCs (inset: corresponding HRTEM image); (C) XPS survey spectra of GQDs and GQDs–CdS NCs; (D) high-resolution XPS S 2p and Cd 3d of GQDs– CdS NCs. Fig. 1
1s peak at 285 eV and O 1s peak at 532 eV were observed for original GQDs and GQDs–CdS NCs.33 In addition, the XPS spectrum of GQDs–CdS NCs conrms the existence of cadmium and sulfur species on the surface of NCs from the appearance of a characteristic Cd 3d5/2 peak at 404.6 eV, a Cd 3d3/2 peak at 411.5 eV and an S 2p peak at 161.2 eV (Fig. 1D).35 In addition, the characteristic Cd peaks had a binding energy shi of 1.2 eV compared to that of CdS NCs (gure not shown), which indicated that the local chemical state was slightly inuenced by the incorporation of GQDs into the CdS lattice.36 Raman scattering has been proven to be an essential tool for characterizing graphene materials, particularly for distinguishing ordered and disordered crystal structures of carbon.37 Fig. 2A shows the Raman spectra of GQDs and GQDs– CdS NCs. The frequencies of the G and D bands in the GQDs– CdS NCs are similar to those observed in the GQDs, although the GQDs–CdS NCs have an increased D/G intensity ratio relative to the GQDs. This change suggests a decrease in the size of the in-plane sp2 domains, which demonstrates that GQDs in the GQDs–CdS NCs have been reduced during the chemical process.38 The FT-IR results of the as-prepared GQDs and GQDs–CdS NCs are displayed in Fig. 2B. The spectrum of GQDs illustrates the presence of C–OH (nC–OH at 1421 cm1) and C]O (nC]O at 1640 cm1 in carboxylic acid and carbonyl moieties).39 Compared with GQDs, the adsorption bands of the oxygen functional groups in the GQDs–CdS NCs weakened, further conrming that the GQDs have been deoxygenated. The deconvoluted C 1s XPS core levels of the GQDs and GQDs–CdS NCs are presented in Fig. 2C and D. The deconvoluted peak located at the binding energy of 285 eV is attributed to the C–C and C]C bonds, and the deconvoluted peaks centered at the binding energies of 286.2 and 287.5 eV are attributed to the C–OH and C]O oxygen-containing carbon
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(A) Raman and (B) FT-IR spectra of the as-prepared GQDs and GQDs–CdS NCs; peak deconvolution of C 1s XPS core level of (C) GQDs and (D) GQDs–CdS NCs.
Fig. 2
bands, respectively.40,41 For comparison, the peak of oxygenated groups in the spectrum of GQDs–CdS NCs is slightly weaker than that in the spectrum of GQDs, indicating a partial removal of oxygenated groups during the chemical process.34 This result is consistent with observations from Raman and FT-IR results in Fig. 2A and B. Amplied ECL response of the GQDs–CdS NCs modied electrode CV curves were recorded in 0.1 M PBS (pH 9.0) for CdS NCs/GCE and GQDs–CdS NCs/GCE between 0.1 V and 1.5 V, as displayed in Fig. 3A inset. It is obvious that a pair of redox peaks of CdS NCs appeared at ca. 0.88 V and 1.23 V at both electrodes. In addition, the redox peak current of CdS NCs for the GQDs– CdS NCs modied electrode was enhanced compared to that for the CdS NCs electrode. Further, the DEp of CdS NCs on the GQDs–CdS NCs modied electrode was much smaller than that
(A) ECL-potential curves of GQDs/GCE (a), CdS NCs/GCE (b) and GQDs–CdS NCs/GCE (c) in 0.1 M PBS (pH 9.0) containing 10 mM H2O2 (inset: CV curves of CdS NCs/GCE (red) and GQDs–CdS NCs/ GCE (green) in 0.1 M PBS (pH 9.0)). (B) EIS of CdS NCs/GCE (a) and GQDs–CdS NCs/GCE (b) in 0.1 M KCl containing 5 mM Ru(NH3)6Cl3 at 200 mV vs. SCE (inset: equivalent circuit used for fitting the impedance data). Fig. 3
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of the CdS NC lm, which indicates that the GQDs can facilitate the electrochemical redox process of CdS NCs.42,43 The ECL behaviors of CdS NCs/GCE, GQDs/GCE and GQDs– CdS NCs/GCE were investigated in 0.1 M PBS (pH 9.0) with 10 mM H2O2. As shown in Fig. 3A, there was no obvious light emission at the GQDs modied electrode (curve a), and obvious ECL signals were observed at CdS NCs (curve b) and GQDs–CdS NCs modied electrodes (curve c), suggesting that the ECL emission can be attributed to the introduction of CdS NCs. Furthermore, the ECL intensity from the GQDs–CdS NCs lm was about ve times higher than that observed from the CdS NCs lm, which indicates that the incorporation of GQDs in the GQDs–CdS NCs can enhance the ECL intensity of CdS NCs effectively. Moreover, the ECL onset potential of the GQDs–CdS NCs lm was 80 mV more positive than that of CdS NCs, indicating that the GQDs can clearly decrease the potential barriers of the ECL reaction, which is accordance with the CV response of the modied electrodes shown in the Fig. 3A inset. EIS is widely used to obtain information on electron transfers between electrolytes and electrode surfaces. The EIS normally includes a semicircular part and a linear part, and the semicircle diameter at higher frequencies corresponds to the electron-transfer resistance.44 Fig. 3B shows the impedance
Fig. 4 (A) ECL-potential curves of GQDs–CdS NCs/GCE in 0.1 M PBS (pH 9.0) containing 10 mM H2O2 in the absence (a) and presence (b) of 1 ng mL1 PCP. (B) Time-dependent ECL signals collected at 1.26 V of GQDs–CdS NCs/GCE in 0.1 M PBS (pH 9.0) with 10 mM H2O2.
Scheme 1
spectra at different modied electrodes using [Ru(NH3)6]3+ as a redox probe. It is obvious that the GQDs–CdS NCs modied electrode (curve b) exhibited fast electron transfer as evidenced by a minimal semicircular portion in the Nyquist diagram, which proves that the GQDs doped in the CdS NCs can further enhance the electron transfer.45
ECL quenching of GQDs–CdS NCs/GCE by PCP The addition of PCP into this ECL system resulted in a signicant decrease in ECL intensity, as shown in Fig. 4A. The possible ECL detection mechanism is intuitively shown in Scheme 1 and is described as follows: CdS NCs are reduced to generate nanocrystal species (CdS). With H2O2 as the coreactant, the electrochemical reduction of H2O2 can produce hydroxyl radical (OHc), which can easily inject a hole in the HOMO of CdS efficiently to form oxidized CdS+. Then, the recombination of the CdS and CdS+ results in the formation of excited states (CdS*) and emission of light in an aqueous solution.46,47 When PCP is added, it is oxidized into TCQ, which has an energy level between the HOMO and LUMO of CdS. Then, the quenching effect takes place due to the energy-transfer process, which reduces the production of excited CdS* and results in a decrease in ECL.48 Furthermore, the ECL stability of GQDs–CdS NCs modied electrode was also investigated in 0.1 M PBS (pH 9.0) with 10 mM H2O2, as shown in Fig. 4B. The ECL response of GQDs–CdS NCs/GCE was repeated under continuous cyclic scans of 0.1 to 1.5 V for 14 cycles. Note that the ECL intensity was ca. 6800 a.u. and did not show obvious change. This result indicates that the ECL intensity of the modied electrode was very stable and benecial for construction of the ECL sensor.
Optimization of the ECL response at GQDs–CdS NCs/GCE In order to achieve the optimal ECL response for PCP determination, the effects of solution pH and H2O2 concentration on
Illustrative ECL detection mechanism for PCP based on GQDs–CdS NCs/GCE.
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Fig. 5
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Effects of (A) solution pH and (B) H2O2 concentration on the ECL intensity of the GQDs–CdS NCs/GCE.
then tended to stabilize. Thus, 10 mM was chosen as the optimal concentration of H2O2 in this experiment. ECL detection of PCP
Fig. 6 (A) Effect of PCP concentration on ECL behavior of GQDs–CdS NCs/GCE in 0.1 M PBS (pH 9.0) with 10 mM H2O2. (B) Calibration curve for PCP detection. Error bars: S.D., n ¼ 3.
the ECL behaviors of GQDs–CdS NCs/GCE were considered carefully, as shown in Fig. 5. The effect of solution pH on the ECL behaviors of the GQDs– CdS NCs modied electrode was determined in 0.1 M PBS with a pH range of 6.0–10.0. As shown in Fig. 5A, the ECL intensity increased with an increase in pH from 6.0 to 9.0, and then decreased when the pH increased from 9.0 to 10.0; thus, a pH of 9.0 was selected in subsequent experiments. Furthermore, the effect of H2O2 concentration on the ECL responses of the GQDs– CdS NCs lm was considered carefully in our work. As shown in Fig. 5B, the ECL intensity increased gradually with an increase in H2O2 concentration; however, when the concentration of H2O2 reached 10 mM, the ECL intensity increased slowly and
Based on the inhibition of PCP on the ECL intensity of GQDs– CdS NCs/GCE, an effective method for ECL detection of PCP was proposed, as shown in Fig. 6. It is obvious that the ECL intensity of GQDs–CdS NCs/GCE decreased gradually with an increase in PCP concentration (Fig. 6A). Moreover, the ECL intensity was linearly dependent on the logarithm of PCP concentration in the range from 0.01 to 500 ng mL1 with a detection limit of 3 pg mL1 (S/N ¼ 3) (Fig. 6B), indicating that the proposed ECL sensor is remarkably reliable for the ultrasensitive determination of PCP. Reproducibility, stability, specicity and real sample analysis The reproducibility was investigated for ve identically made electrodes with a relative standard deviation (RSD) of 7.3%, while 5 measurements for a single electrode were made upon the addition of 0.5 ng mL1 PCP in 0.1 M PBS (pH 9.0) with a RSD of 4.9%, demonstrating excellent reproducibility. In order to evaluate the stability of the sensor, the ECL responses to 0.5 ng mL1 PCP were measured in 0.1 M PBS (pH 9.0) at two-day intervals. As shown in Fig. 7A, the proposed sensor retained about 87% of its initial ECL response aer three weeks, suggesting effective stability.
Fig. 7 (A) Stability of the ECL sensor over 23 days in 0.1 M PBS (pH 9.0) with 0.5 ng mL1 PCP. (B) Quenching of the ECL responses of the sensor in 0.1 M PBS (pH 9.0) by 0.5 ng mL1 DCP, TCP, DNP and PCP.
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Paper Table 1
Analyst Recovery studies of PCP in tap and river water samples (n ¼ 3)
Sample
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Tap water
River water
Taken (pg mL1) 0.00 20.0 50.0 100.0 0.00 20.0 50.0 100.0
Found (pg mL1)
Recovery (%)
RSD (%)
Not detected 21.7 48.3 97.5 15.4 34.6 66.9 119.6
—
—
108.5 96.6 97.5 — 97.7 102.3 103.6
5.6 5.2 4.4 5.1 4.2 5.7 6.5
The selectivity of the ECL sensor to PCP was also investigated by measuring the ECL responses to analogs of PCP, such as 2,3,5-trichlorophenol (TCP), 2,6-dichlorophenol (DCP) and 2,4dinitrophenol (DNP), as shown in Fig. 7B. The result indicated that the addition of 0.5 ng mL1 DCP, TCP and DNP resulted in only 4.6%, 2.7% and 3.1% decreases in ECL intensity, respectively, whereas the addition of 0.5 ng mL1 PCP resulted in a 28.6% decrease in ECL intensity. Thus, a relatively high selectivity was achieved. The reason may be that these selected interferences are more stable than PCP, and thus could not result in consumption of the excited CdS NCs*. These results indicate that the ECL detection of PCP is not affected by the presence of the analogs of PCP. To demonstrate the practicality of the proposed ECL sensor, a recovery test was carried out by adding different amounts of PCP into tap and river water samples. The results of this test are presented in Table 1, and they are in good agreement with the given concentration with average recoveries ranging from 96.6% to 108.5% (n ¼ 3). These results indicate that this method can be used for analysis of real samples.
Conclusions A novel ultrasensitive ECL sensor for PCP detection was fabricated based on the ECL amplifying behavior of GQDs–CdS NCs. The GQDs doped in the GQDs–CdS NCs not only enhanced the ECL intensity of CdS NCs effectively, but also decreased their ECL onset potential. Furthermore, based on the effective inhibition of PCP on the ECL response of GQDs–CdS NCs, a simple ECL sensor for the determination of PCP was constructed. The resulting ECL sensor showed excellent performance with the merits of wide linear range, low detection limit, good stability, reproducibility and ne applicability for the detection of PCP in real samples. This result enables a new application of GQDs as an efficient signal amplication material for the construction of high-performance ECL devices for pesticide determination.
Acknowledgements The present work was supported by the National Natural Science Foundation of China (no. 21175061 and 21375050), Key Laboratory of Modern Agriculture Equipment and Technology
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(no. NZ201109), the Natural Science Foundation of Jiangsu province (no. BK20130481), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Innovation Project of Science and Technology for College Graduates of Jiangsu Province (no. CXZZ12_0704).
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