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A novel solid-state electrochemiluminescence quenching sensor for detection of aniline based on luminescent composite nanofibers
Journal of Luminescence 156 (2014) 229–234
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A novel solid-state electrochemiluminescence quenching sensor for detection of aniline based on luminescent composite nanofibers Xiaoying Wang n, Yu Yang, Huiwen Gao Key Laboratory of Environmental Medicine and Engineering, Ministry of Education, School of Public Health, Southeast University, Nanjing 210009, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 June 2014 Received in revised form 13 July 2014 Accepted 6 August 2014 Available online 19 August 2014
A novel solid-state electrochemiluminescence (ECL) quenching sensor based on the luminescent composite nanofibers for detection of aniline has been developed. The gold nanoparticles (AuNPs) and Ruthenium (II) tris-(bipyridine) (RuðbpyÞ23 þ ) doped nylon 6 (PA6) luminescent composite nanofibers (Ru–AuNPs–PA6) were successfully deposited to the bare glassy carbon (GC) electrode by a one-step electrospinning technique. The Ru–AuNPs–PA6 nanofibers maintained the photoelectric properties of the RuðbpyÞ23 þ ions completely and exhibited excellent ECL behaviors. A high quenching effect on the ECL signal of the Ru–AuNPs–PA6/C2 O24 system was obtained with the presence of low concentration aniline compounds. The potential of analytical application was explored by use of the inhibited ECL. The quenching efficiencies of the five kinds of aniline compounds were compared by monitoring the anilinedependent ECL intensity change. The magnitude of quenching depended linearly upon the concentration of aniline in the investigated concentration range of 10–10 mM. The detection limit for aniline is 5.0 nM, which is comparable or better than that in the reported assays. The solid-state ECL quenching sensor exhibited high sensitivity and good stability. This study may provide new insight into the design of advanced electrospun nanofibers-based ECL sensors for detection and analysis of a variety of active molecules. & 2014 Elsevier B.V. All rights reserved.
Keywords: Solid-state electrochemiluminescence RuðbpyÞ23 þ Luminescent composite nanofibers Aniline compounds Quenching
1. Introduction Aniline and some of its derivatives often attract great concerns due to their wide utilization associated with arising environmental problems [1,2]. They are used on a large scale as raw materials for various industrial applications, including the production of urethanes, the manufacturing of intermediates for herbicides and other pesticides, the manufacturing of dyes and pigments, and the production of accelerators and antioxidants for the rubber industry. However, aniline creates severe health problems such as anoxia, erythrocyte damage, spleen effects and is also considered as a suspected carcinogen [3,4]. Considering high risk to human health through aniline exposure the development of a reliable and sensitive method for its determination is of great importance. Thus far, a variety of methods for measuring aniline have been reported, such as spectrophotometry [4,5], near infrared (NIR) spectrometry [6], spectrofluorometry [7], solid-phase microextraction (SPME) [8], electrochemical techniques [9–11], gas chromatography (GC) [12], capillary zone electrophoresis (CZE) [13] and high-performance liquid chromatography (HPLC) [14]. However, these approaches are time consuming, highly skilled labor, less
n
Corresponding author. Tel.: þ 86 25 83272563; fax: þ86 25 83272561. E-mail address: [email protected] (X. Wang).
http://dx.doi.org/10.1016/j.jlumin.2014.08.021 0022-2313/& 2014 Elsevier B.V. All rights reserved.
sensitive and expensive equipment. It has been reported that aniline compounds exhibit a strong quenching effect on the electrochemiluminescence (ECL) signals of the Ruthenium (II) tris-(bipyridine) (RuðbpyÞ23 þ ) due to the energy transfer from the excited state RuðbpyÞ23 þ n to the electro-oxidation species of aniline compounds [15]. A flow injection procedure with ECL quenching detection has been developed for determination of aniline [15,16]. The method exhibits good reproducibility and stability with a low detection limit for aniline. But the above solution-phase RuðbpyÞ23 þ ECL system can create large consumption of expensive ECL reagent RuðbpyÞ23 þ , environmental pollution and the high cost. Solid-state ECL, viz. immobilizing the ECL substrate on the electrode surface, can reduce the consumption of expensive reagent, enhance the ECL signal, simplify experimental design and create regenerable sensors [17–19]. The potential of analytical application was challenged by use of the inhibited solid-state ECL for determination of aniline. RuðbpyÞ23 þ is the most used ECL substrate. Until now, a growing number of different methods have been employed to fabricate solid-state ECL sensors by immobilizing RuðbpyÞ23 þ on electrode surfaces, such as Langmuir–Blodgett [20], self-assembled monolayers [21], layer-by-layer assembly [22], cation exchange polymers [23] and RuðbpyÞ23 þ -doped nanomaterials [24,25]. Although much progress has been made to improve sensitivity, robustness, and regenerability of the solid-state ECL sensors, many new
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methods and materials are still needed to further improve sensor performance in practical applications [26]. An ideal ECL electrode should possess long-term stability, well-defined electron transfer property of the immobilized tags, low cost, high surface enrichment efficiency, good adhesion, and good permeations of analyte molecules through the matrices to enhance the ECL signal readout. Electrospinning has been proved to be a simple, versatile and cost-effective approach to fabricate polymeric nanofibers from natural and synthetic materials with controllable diameter in nanometer scale [27]. Compared with conventional planar materials, the ultrafine nanofibers with unique 3D nanostructure have several advantages of uniformity, porosity, high surface area to volume ratio and mechanical strength [28,29]. Thus, electronspun nanofibers can be used as the attractive host matrix for the available loading of guest molecules (RuðbpyÞ23 þ ) in the electrode modification [30,31]. Moreover, the target molecules can penetrate freely into the matrix and react with the immobilized guest molecules and result in the enhanced response. So, the fibrous membranes show a great potential for sensors of high sensitivity. Several reports have investigated the development of nanofiberRuðbpyÞ23 þ based solid-state ECL sensors for molecular detection, such as phenol [30], hydroquinone [32], catechol [33] and atropine [34]. But the solid-state ECL sensor based on electrospun luminescent composite nanofibers has never been applied to detect the aniline compounds. Herein, a novel solid-state ECL quenching sensor based on the luminescent composite nanofibers for detection of aniline has been developed. The gold nanoparticles (AuNPs) and RuðbpyÞ23 þ doped nylon 6 (PA6) luminescent composite nanofibers (Ru–AuNPs–PA6) were successfully fabricated by a one-step electrospinning technique, as shown in Fig. 1. The Ru–AuNPs–PA6 nanofibers exhibit some advantages over the common RuðbpyÞ23 þ ECL system in terms of ease of fabrication, strong ECL intensity and sensitive response to aniline. The solid-state ECL quenching sensing strategy based on the Ru–AuNPs–PA6 for detection of aniline has been designed. The characteristics of the sensing system for the detection of aniline and the analytical performance were evaluated.
2. Experiment section 2.1. Reagents and apparatus Ru(bpy)3Cl2 6H2O (99.95%), HAuCl4, aniline, m-phenylenediamine, o-phenylenediamine, p-phenylenediamine, phenylenediamine and benzidine were purchased from Sigma (USA). Nylon 6 (PA6) material was purchased from Debiochem (China). Other reagents were of analytical reagent grade. All the solutions were prepared with ultrapure water from a Millipore Milli-Q system. The field-emission scanning electron microscope (FESEM) images were recorded using S-4800 FE-SEM (JEOL, Japan). The fluorescence photographs were recorded using an IX71-F22FL/PH
Fig. 1. Schematic representation of the one-step process for fabricating the Ru–AuNPs–PA6 luminescent composite nanofibers.
inverted fluorescence microscope (Olympus, Japan). ECL was measured with a MPI-E electrochemiluminescence analyzer (Remax Electronic Science Tech. Co. Ltd., China), cyclic voltammogram (CV) was recorded with a CHI 660D electrochemical analyzer (CHI instruments Inc., Chenhua Corp., China). 2.2. Preparation of the Ru–AuNPs–PA6 nanofibers electrode Au nano-particles (AuNPs) with a diameter of 16 nm were prepared by citrate reduction of HAuCl4 in aqueous solution according to the literature [35]. In brief, 100 mL of solution containing 0.01 g of HAuCl4 was brought to reflux, and then 2.5 mL of 1% sodium citrate solution was introduced with stirring. The solution was kept boiling for 30 min and cooled to room temperature. The product was stored in dark glass bottle at 4 1C for further use. The electrospinning apparatus was similar to those described before [31]. A series of electrospinning solutions were prepared by dissolving PA6, AuNPs and RuðbpyÞ23 þ in mixture of cresol, formic acid and trifluoroethanol (4:4:2, V/V) at desired weight ratios. In order to obtain a homogeneous solution, the mixed solution (containing PA6 (30.6 wt%), AuNPs (1.2 wt%) and RuðbpyÞ23 þ (0.2 wt.%)) was under vigorous stirring for 6 h at room temperature. The surface of glassy carbon (GC) electrode (3 mm in diameter) was carefully polished with 0.3 μm Al2O3 powders successively, rinsed with water and ethanol in an ultrasonic bath briefly and then allowed to dry at room temperature. Electrospinning was carried out with a 20 mL syringe (1.2 mm diameter spinneret) at electrical potential of 15 kV over a 15 cm gap between the spinneret and the clean GC electrode with a rate of 1.0 mL/h. The ambient temperature and relative humidity for electrospinning were kept at 25 1C and 40%, respectively. It took 3 min to deposit the nanofibers on the surface of the clean GC electrode by electrospinning the mixed solution. The as-prepared electrode was denoted as the Ru–AuNPs–PA6 nanofibers electrode. 2.3. Measurement The ECL determinations were performed at room temperature in a 10 mL homemade quartz cell. A three-electrode system was used with the Ru–AuNPs–PA6 nanofibers electrode as the working electrode, an Ag/AgCl (sat.) as the reference electrode and a platinum wire as the counter electrode. Cyclic voltammetry mode with continuous potential scanning from 0.0 to 1.2 V and scanning rate of 100 mV s 1 was applied to achieve ECL signal in 50 mM phosphate buffer solution (PBS) containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5). A high voltage of 800 V was supplied to the photomultiplier for luminescence intensity determination. The ECL and CV curves were recorded simultaneously.
3. Results and discussion 3.1. The micromorphologies of the Ru–AuNPs–PA6 luminescent composite nanofibers The micromorphologies of the Ru–AuNPs–PA6 luminescent composite nanofibers were analyzed using FESEM and an inverted fluorescence microscope, as shown in Fig. 2. The Ru–AuNPs–PA6 composite nanofibers with smooth surface (Fig. 2A), evenly distributed on the substrate with their random orientations, form a porous 3D structure nanofibrous membrane (Fig. 2B). The diameters of the Ru–AuNPs–PA6 nanofibers range from 100 to 500 nm. The fibers diameters can be adjusted by varying the conditions of the electrospinning process. The fluorescence photographs (Fig. 2C) show that the Ru–AuNPs–PA6 luminescent
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Fig. 2. (A, B) FESEM images of the Ru–AuNPs–PA6 luminescent composite nanofibers. PA6 (30.6 wt%), AuNPs (1.2 wt%), RuðbpyÞ23 þ (0.2 wt%), 7 spinneret, 15 cm gap, 0.8 mL h 1 rate, 15.1 kV electrical potential, the diameter of the nanofibers was 250 nm. (C) Fluorescence photograph of the Ru–AuNPs–PA6 luminescent composite nanofibers magnified at 400 times.
composite nanofibers emit bright orange light at 610 72 nm and the luminosities are uniform when the photographs magnified at 400 times. The results are in agreement with the characteristics of free RuðbpyÞ23 þ ion, which convinces that the Ru–AuNPs–PA6 luminescent composite nanofibers with a porous 3D structure properly maintain the luminescent property of RuðbpyÞ23 þ ions. Furthermore, the existence of the polymer hybrid matrixes can improve the thermal and optical stability of the RuðbpyÞ23 þ complex. Consequently, the Ru–AuNPs–PA6 luminescent composite nanofibers may find applications in nanodevices. 3.2. The characterization of the Ru–AuNPs–PA6 luminescent composite nanofibers The electrochemical characterizations of the Ru–AuNPs–PA6 luminescent composite nanofibers were analyzed using CV and ECL as shown in Fig. 3. The Ru–AuNPs–PA6 luminescent composite nanofibers had a pair of obvious redox peaks of RuðbpyÞ23 þ and the oxidation current signal at þ1.13 V versus Ag/AgCl (sat.) potential in the CV curve (Fig. 3A). The ECL intensity–time curve of the Ru–AuNPs–PA6 nanofibers electrode in 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) is shown in Fig. 3B. The ECL signal of the Ru–AuNPs–PA6 nanofibers electrode in the presence of 3.0 mM C2 O24 (Fig. 3B, curve c) was stronger than that obtained without C2 O24 (Fig. 3B, curve a), which is almost the
same as that of free RuðbpyÞ23 þ under the same experimental conditions. These results are in agreement with the characteristics of free RuðbpyÞ23 þ ion [36,37], which convinces that the Ru–AuNPs–PA6 luminescent composite nanofibers properly maintain the electrical properties of the RuðbpyÞ23 þ ions. In the paper, we employed 3 min to deposit the Ru–AuNPs–PA6 nanofibers on the surface of the GC electrode by electrospinning 10 mL mixed solution containing PA6 (30.6 wt%), AuNPs (1.2 wt%) and RuðbpyÞ23 þ (0.2 wt%) with a rate of 1.0 mL/h. After conversion, the quality of RuðbpyÞ23 þ in the Ru–AuNPs–PA6 nanofibers was 100 μg, and the quality of the AuNPs was 6 times of the RuðbpyÞ23 þ . In order to demonstrate the advantage of the Ru–AuNPs–PA6 luminescent composite nanofibers in ECL amplification, we compared the different ECL behaviors between the bare GC electrode in the 50 mM PBS containing 100 μg RuðbpyÞ23 þ , 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) (solution-phase RuðbpyÞ23 þ ECL system), and Ru–AuNPs–PA6 nanofibers electrode in the 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) (solid-state RuðbpyÞ23 þ ECL system). As shown in Fig. 3B, the ECL intensity of the bare GC electrode in the 50 mM PBS containing 100 μg RuðbpyÞ23 þ , 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) is 506 a.u. during continuous CV scans for 3 cycles (curve b). For the Ru–AuNPs–PA6 nanofibers electrode, the ECL intensity was 912 a.u (curve c). Since the quality of RuðbpyÞ23 þ in the solution and in the Ru–AuNPs–PA6 nanofibers are the same, the ECL intensity of the Ru–MWNTs–PA6 nanofibers
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Fig. 3. (A) The cyclic voltammogram of the Ru–AuNPs–PA6 nanofibers electrode in 50 mM PBS (pH 6.5). Scan rate: 100 mV s 1, scan range: 0.0–1.25 V (versus Ag/AgCl). (B) The ECL intensity of the Ru–AuNPs–PA6 nanofibers electrode in absence (a) and presence (c) of 3.0 mM C2 O24 50 mM PBS (0.4 M KNO3, pH 6.5). The ECL intensity of the bare GC electrode (b) in the 50 mM PBS containing 100 μg RuðbpyÞ23 þ , 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5).
is about 1.8 times higher than that of the solution during continuous CV scans as shown in Fig. 3B. Thus, electronspun nanofibers can be used as the attractive host matrix for the available loading of guest molecules (RuðbpyÞ23 þ ) in the electrode modification. It might be attributed to the high surface area to volume ratio, and the uniform, stable and efficient immobilization of RuðbpyÞ23 þ U C2 O24 , a small molecule, acted as an ECL coreactant, can penetrate freely into the Ru–AuNPs–PA6 nanofibers matrix and react with the immobilized guest molecules (RuðbpyÞ23 þ ). As a result, an enhanced response presented due to the porous structure of the electrospun nanofibers. Moreover, a trace amount of AuNPs might greatly promote the electron-transfer reactions of the Ru–AuNPs–PA6 nanofibers electrode surface. Finally, the Ru– AuNPs–PA6 nanofibers exhibited excellent ECL behaviors on GC electrode. The ECL intensity of the Ru–AuNPs–PA6 nanofibers electrode almost unchanged during continuous CV scans for 30 cycles. It indicates a good stability of the ECL signal of the electrospun nanofibers. The Ru–AuNPs–PA6 nanofibers electrode was immersed in 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) for 12 h, and no ECL signal was detected for the above PBS. It is attributed to the facts that negatively charged AuNPs immobilized stably a large number of positively charged RuðbpyÞ23 þ in the form of composites via electrostatic interaction in the porous 3D structure nanofibrous membrane, and the amide groups in the main chain segment unit of PA6 were well known to bind strongly with AuNPs [38,39]. As a result, the combination of the above facts made the Ru–AuNPs–PA6 nanofibers more durable under operating conditions.
only the outstanding charge-transport characteristics but also the good stability. In order to examine the effect of doped concentration of RuðbpyÞ23 þ on the luminescent intensity, four different amounts of RuðbpyÞ23 þ (0.05, 0.1, 0.15, 0.2, 0.3 wt%) were doped to the nanofibers. It is obvious that the luminescent intensity increases with the increment of doping concentration, and reaches the maximum at the doping concentration of 0.2 wt%. The luminescent intensity decreases at 0.3 wt%. It indicates that the concentration quenching may dominate when the concentration is higher than 0.2 wt%. In other words, when the concentration is higher than 0.2 wt%, the strong self-quenching and triplet–triplet annihilation between ruthenium(II) complex in the composite fibers may decrease the emission efficiency. The determination buffer solution has great impact on ECL intensity. The concentration effects of C2 O24 and the pH of the PBS on the ECL intensity of the Ru–AuNPs–PA6 nanofibers electrode were investigated. The ECL intensity increased with the increasing of the C2 O24 concentration from 0.5 to 2.5 mM with an increment of 0.5 mM, and reached plateau regions at C2 O24 concentration of 3.0 mM. Thus, the optimal C2 O24 concentration was chosen at 3.0 mM. The ECL intensity increased greatly with the pH value increasing until it reached a plateau in the pH 6.0–6.5 and declined sharply at the pH 47.0. So, 6.5 was chosen as the pH value of the PBS at the following experiment. The amount of the KNO3 concentration of the detecting PBS solution was selected according to the literature [16]. So, 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) was selected as the ECL detecting solution.
3.3. Optimization of experimental conditions
3.4. The analytical performance of the solid-state ECL quenching sensor
In order to maximize the detection sensitivity of the proposed solid-state ECL quenching sensor, various conditions were optimized by single factor experiments. The depositing time of the Ru–AuNPs–PA6 nanofibers on the surface of the GC electrode was investigated. In 3 min, the Ru–AuNPs–PA6 nanofibers were deposit on the surface of the GC electrode, and were directly visible. So, we employed 3 min to deposit the Ru–AuNPs–PA6 nanofibers on the surface of the GC electrode by electrospinning the solution (containing PA6 (30.6 wt%), AuNPs (1.2 wt%) and RuðbpyÞ23 þ (0.2 wt%)). In such conditions, Ru–AuNPs–PA6 nanofibers electrode has not
In this work, the effect of aniline on the ECL intensity of the Ru–AuNPs–PA6 nanofibers was systematically studied, as shown in Fig. 4. A strong ECL signal appeared when the Ru–AuNPs–PA6 nanofibers electrode was in the 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) (curve a), while a sharp decrease of the ECL signal was observed when the quenching molecule of aniline was added in the above ECL detecting solution (curve b). The quenching efficiency is defined as ðI 0ECL I ECL Þ=I 0ECL , where IECL and I 0ECL represent the ECL intensities with and without the quencher, respectively. So, the quenching efficiency of the aniline
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Fig. 4. The ECL intensity–potential curves of the Ru–AuNPsvPA6 nanofibers electrode in 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) with (b) and without (a) 100 μM aniline.
Table 1 The quenching efficiencies of the aniline compounds in 50 mM PBS containing 3.0 mM C2 O24 , 0.4 M KNO3 and 100 μM aniline compounds (pH 6.5). Compounda
(I0 I)/I0 (%)
Aniline m-Phenylenediamine o-Phenylenediamine p-Phenylenediamine Benzidine Aniline/phenylenediamineb Aniline/benzidineb
52.2 63.8 59.3 57.9 70.2 56.1 67.9
a The concentration (C) of the compound is 100 μM, the fluctuation range of the quenching efficiency is 7 5%. Scan rate: 100 mV s 1, scan range: 0.0–1.2 V b Aniline and phenylenediamine, aniline and benzidine (1:1, C:C).
was 52.2%. The results verify that the ECL of the Ru–AuNPs–PA6 nanofibers can be efficiently quenched by the aniline. The mechanism for quenching ECL has been explained as bimolecular energy or electron transfer between RuðbpyÞ23 þ n and benzoquinone or their derivative, the oxidized species of aniline along with suppression of radical reactions [15,16]. The different quenching efficiencies between aniline, m-phenylenediamine, o-phenylenediamine, p-phenylenediamine and benzidine were observed. It was found that all the tested compounds showed an ECL inhibiting signal. In comparison of the tested compounds, the quenching efficiencies of the aniline derivatives were obviously stronger than that of the aniline. As shown in Table 1, benzidine exhibits highest quenching efficiencies. With a concentration of 100 μM, it displayed quenching efficiencies of 70.2%. The three isomers of phenylenediamine, m-phenylenediamine, o-phenylenediamine and p-phenylenediamine exhibited completely different quenching efficiencies. The magnitude of ECL inhibition was related to the position of the substituting group in the benzene ring and decreased in the following order: meta4ortho4para. Furthermore, the quenching efficiencies of the mixtures of aniline and phenylenediamine (1:1, C:C), aniline and benzidine (1:1, C:C) had also been studied. It was found that the mixtures presented medium quenching efficiencies compared with the two kinds of mixed substrates (Table 1). It resulted from the facts that the amine derivatives interfere with aniline detection. The result suggests that the Ru–AuNPs–PA6 nanofibers-based solidstate ECL sensor is ideal for simultaneous determination of the
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aniline compounds by use of inhibited ECL coupled with a separation technique. The relative standard deviation (RSD) of the ECL intensity for the 8 repeated detections of aniline (100 μM) was 4.71%, indicating the good reproducibility of this method. The long-term stability of the Ru–AuNPsvPA6 nanofibers electrode was found to be satisfactory. Rather small changes ( o4%) in the ECL quenching response were observed for a given electrode over a period of at least 10 days storage at room temperature. So, the solid-state ECL aniline sensor displayed good performance including long shelf life and excellent reproducibility. Moreover, after the aniline test, the Ru–AuNPs–PA6 nanofibers electrode can be regenerated by immersing it into the ultrapure water, and rinsing it for several times to remove the adsorbed aniline and C2 O24 thoroughly. After the treatments, the Ru–AuNPs–PA6 nanofibers electrode is ready to incubate with the aniline again and for next aniline detection. Under identical conditions, the Ru–AuNPs–PA6 nanofibers electrode can be continuously reused for 8 times, while still keeping the ECL response to 91.8% of its original state. As 90% of the original signal response is generally acceptable, our results indicate that this platform can be reused at least for 8 times repeatedly. 3.5. The calibration curve of aniline detection The sensitivity of the solid-state ECL quenching sensor was investigated. ECL responses of the Ru–AuNPs–PA6 nanofibers electrodes in 50 mM PBS containing various concentrations of aniline are shown in Fig. 5. The ECL intensity difference [ΔIECL ¼I 0ECL (the ECL intensities without aniline) IECL(the ECL intensities with aniline)] of the Ru–AuNPs–PA6 nanofibers electrodes grew when the aniline concentration increased, and the ΔIECL was found to be linear with the logarithm of aniline concentration in the range from 10 nM to 10 mM (as shown in the inset). The equation for the resulting calibration plot was y¼43lg x þ42 (x was the concentration of aniline divided by 10 nM, y was ΔIECL), the correlation coefficient was 0.9992, and a detection limit of 5.0 nM was estimated by using 3σ (where σ is the relative standard deviation of a blank solution, n ¼11). Consistent data is obtained as shown in the error bar in the inset of Fig. 5 when the experiment is repeated thrice.
Fig. 5. The ECL responses of the Ru–AuNPsvPA6 nanofibers electrodes in 50 mM PBS containing various concentrations of aniline. The concentrations of aniline are 0 M (a), 10 nM (b), 100 nM (c), 1 mM (d), 10 mM (e), 100 mM (f) and 1 mM (g), respectively. Inset: the calibration curves of aniline detection. The concentrations of aniline are 10 nM, 100 nM, 1 mM, 10 mM, 100 mM and 1 mM, respectively. ECL signals are achieved in 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5). Scan rate: 100 mV s 1, scan range: 0.0–1.2 V.
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4. Conclusions A novel and simple solid-state ECL quenching sensor based on the Ru–AuNPs–PA6 luminescent composite nanofibers for detection of aniline has been described. The Ru–AuNPs–PA6 nanofibers with unique 3D nanostructure, large specific surface area and long-term stability were successfully fabricated by a one-step electrospinning technique, and exhibited excellent ECL behaviors on GC electrodes. A high quenching effect on the ECL signal of the Ru–AuNPs–PA6/C2 O24 system was obtained with the presence of low concentration aniline and its derivatives. In comparison of the tested aniline compounds, the quenching efficiencies of the aniline derivatives were obviously stronger than that of the aniline. The magnitude of ECL inhibition was related to the position of the substituting group in the benzene ring and decreased in the following order: meta 4 ortho4 para. The assay allows detection at levels as low as 5.0 nM of the aniline. The solid-state ECL sensor displayed wide linear range, high sensitivity and good stability. The results demonstrate that the strategy could be extended to develop various solid-state ECL quenching sensors for the detection and quantification of aniline derivatives. Acknowledgment Project supported by the National Natural Science Foundation of China (No. 81302472), the Doctoral Program of Higher Education of China (No. 20110092120055) and the Students' Innovation Experimental Project of China (No. S2014118). References [1] E.H. Seymour, N.S. Lawrence, E.L. Beckett, J. Davis, R.G. Compton, Talanta 57 (2002) 233. [2] C.T. Yan, T.S. Shih, J.F. Jen, Talanta 64 (2004) 650. [3] T. Spǎtaru, N. Spǎtaru, A. Fujishima, Talanta 73 (2007) 404. [4] L.M. Zhang, B. Li, Spectrochim. Acta A 74 (2009) 1060.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A novel solid-state electrochemiluminescence quenching sensor for detection of aniline based on luminescent composite nanofibers Xiaoying Wang n, Yu Yang, Huiwen Gao Key Laboratory of Environmental Medicine and Engineering, Ministry of Education, School of Public Health, Southeast University, Nanjing 210009, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 June 2014 Received in revised form 13 July 2014 Accepted 6 August 2014 Available online 19 August 2014
A novel solid-state electrochemiluminescence (ECL) quenching sensor based on the luminescent composite nanofibers for detection of aniline has been developed. The gold nanoparticles (AuNPs) and Ruthenium (II) tris-(bipyridine) (RuðbpyÞ23 þ ) doped nylon 6 (PA6) luminescent composite nanofibers (Ru–AuNPs–PA6) were successfully deposited to the bare glassy carbon (GC) electrode by a one-step electrospinning technique. The Ru–AuNPs–PA6 nanofibers maintained the photoelectric properties of the RuðbpyÞ23 þ ions completely and exhibited excellent ECL behaviors. A high quenching effect on the ECL signal of the Ru–AuNPs–PA6/C2 O24 system was obtained with the presence of low concentration aniline compounds. The potential of analytical application was explored by use of the inhibited ECL. The quenching efficiencies of the five kinds of aniline compounds were compared by monitoring the anilinedependent ECL intensity change. The magnitude of quenching depended linearly upon the concentration of aniline in the investigated concentration range of 10–10 mM. The detection limit for aniline is 5.0 nM, which is comparable or better than that in the reported assays. The solid-state ECL quenching sensor exhibited high sensitivity and good stability. This study may provide new insight into the design of advanced electrospun nanofibers-based ECL sensors for detection and analysis of a variety of active molecules. & 2014 Elsevier B.V. All rights reserved.
Keywords: Solid-state electrochemiluminescence RuðbpyÞ23 þ Luminescent composite nanofibers Aniline compounds Quenching
1. Introduction Aniline and some of its derivatives often attract great concerns due to their wide utilization associated with arising environmental problems [1,2]. They are used on a large scale as raw materials for various industrial applications, including the production of urethanes, the manufacturing of intermediates for herbicides and other pesticides, the manufacturing of dyes and pigments, and the production of accelerators and antioxidants for the rubber industry. However, aniline creates severe health problems such as anoxia, erythrocyte damage, spleen effects and is also considered as a suspected carcinogen [3,4]. Considering high risk to human health through aniline exposure the development of a reliable and sensitive method for its determination is of great importance. Thus far, a variety of methods for measuring aniline have been reported, such as spectrophotometry [4,5], near infrared (NIR) spectrometry [6], spectrofluorometry [7], solid-phase microextraction (SPME) [8], electrochemical techniques [9–11], gas chromatography (GC) [12], capillary zone electrophoresis (CZE) [13] and high-performance liquid chromatography (HPLC) [14]. However, these approaches are time consuming, highly skilled labor, less
n
Corresponding author. Tel.: þ 86 25 83272563; fax: þ86 25 83272561. E-mail address: [email protected] (X. Wang).
http://dx.doi.org/10.1016/j.jlumin.2014.08.021 0022-2313/& 2014 Elsevier B.V. All rights reserved.
sensitive and expensive equipment. It has been reported that aniline compounds exhibit a strong quenching effect on the electrochemiluminescence (ECL) signals of the Ruthenium (II) tris-(bipyridine) (RuðbpyÞ23 þ ) due to the energy transfer from the excited state RuðbpyÞ23 þ n to the electro-oxidation species of aniline compounds [15]. A flow injection procedure with ECL quenching detection has been developed for determination of aniline [15,16]. The method exhibits good reproducibility and stability with a low detection limit for aniline. But the above solution-phase RuðbpyÞ23 þ ECL system can create large consumption of expensive ECL reagent RuðbpyÞ23 þ , environmental pollution and the high cost. Solid-state ECL, viz. immobilizing the ECL substrate on the electrode surface, can reduce the consumption of expensive reagent, enhance the ECL signal, simplify experimental design and create regenerable sensors [17–19]. The potential of analytical application was challenged by use of the inhibited solid-state ECL for determination of aniline. RuðbpyÞ23 þ is the most used ECL substrate. Until now, a growing number of different methods have been employed to fabricate solid-state ECL sensors by immobilizing RuðbpyÞ23 þ on electrode surfaces, such as Langmuir–Blodgett [20], self-assembled monolayers [21], layer-by-layer assembly [22], cation exchange polymers [23] and RuðbpyÞ23 þ -doped nanomaterials [24,25]. Although much progress has been made to improve sensitivity, robustness, and regenerability of the solid-state ECL sensors, many new
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methods and materials are still needed to further improve sensor performance in practical applications [26]. An ideal ECL electrode should possess long-term stability, well-defined electron transfer property of the immobilized tags, low cost, high surface enrichment efficiency, good adhesion, and good permeations of analyte molecules through the matrices to enhance the ECL signal readout. Electrospinning has been proved to be a simple, versatile and cost-effective approach to fabricate polymeric nanofibers from natural and synthetic materials with controllable diameter in nanometer scale [27]. Compared with conventional planar materials, the ultrafine nanofibers with unique 3D nanostructure have several advantages of uniformity, porosity, high surface area to volume ratio and mechanical strength [28,29]. Thus, electronspun nanofibers can be used as the attractive host matrix for the available loading of guest molecules (RuðbpyÞ23 þ ) in the electrode modification [30,31]. Moreover, the target molecules can penetrate freely into the matrix and react with the immobilized guest molecules and result in the enhanced response. So, the fibrous membranes show a great potential for sensors of high sensitivity. Several reports have investigated the development of nanofiberRuðbpyÞ23 þ based solid-state ECL sensors for molecular detection, such as phenol [30], hydroquinone [32], catechol [33] and atropine [34]. But the solid-state ECL sensor based on electrospun luminescent composite nanofibers has never been applied to detect the aniline compounds. Herein, a novel solid-state ECL quenching sensor based on the luminescent composite nanofibers for detection of aniline has been developed. The gold nanoparticles (AuNPs) and RuðbpyÞ23 þ doped nylon 6 (PA6) luminescent composite nanofibers (Ru–AuNPs–PA6) were successfully fabricated by a one-step electrospinning technique, as shown in Fig. 1. The Ru–AuNPs–PA6 nanofibers exhibit some advantages over the common RuðbpyÞ23 þ ECL system in terms of ease of fabrication, strong ECL intensity and sensitive response to aniline. The solid-state ECL quenching sensing strategy based on the Ru–AuNPs–PA6 for detection of aniline has been designed. The characteristics of the sensing system for the detection of aniline and the analytical performance were evaluated.
2. Experiment section 2.1. Reagents and apparatus Ru(bpy)3Cl2 6H2O (99.95%), HAuCl4, aniline, m-phenylenediamine, o-phenylenediamine, p-phenylenediamine, phenylenediamine and benzidine were purchased from Sigma (USA). Nylon 6 (PA6) material was purchased from Debiochem (China). Other reagents were of analytical reagent grade. All the solutions were prepared with ultrapure water from a Millipore Milli-Q system. The field-emission scanning electron microscope (FESEM) images were recorded using S-4800 FE-SEM (JEOL, Japan). The fluorescence photographs were recorded using an IX71-F22FL/PH
Fig. 1. Schematic representation of the one-step process for fabricating the Ru–AuNPs–PA6 luminescent composite nanofibers.
inverted fluorescence microscope (Olympus, Japan). ECL was measured with a MPI-E electrochemiluminescence analyzer (Remax Electronic Science Tech. Co. Ltd., China), cyclic voltammogram (CV) was recorded with a CHI 660D electrochemical analyzer (CHI instruments Inc., Chenhua Corp., China). 2.2. Preparation of the Ru–AuNPs–PA6 nanofibers electrode Au nano-particles (AuNPs) with a diameter of 16 nm were prepared by citrate reduction of HAuCl4 in aqueous solution according to the literature [35]. In brief, 100 mL of solution containing 0.01 g of HAuCl4 was brought to reflux, and then 2.5 mL of 1% sodium citrate solution was introduced with stirring. The solution was kept boiling for 30 min and cooled to room temperature. The product was stored in dark glass bottle at 4 1C for further use. The electrospinning apparatus was similar to those described before [31]. A series of electrospinning solutions were prepared by dissolving PA6, AuNPs and RuðbpyÞ23 þ in mixture of cresol, formic acid and trifluoroethanol (4:4:2, V/V) at desired weight ratios. In order to obtain a homogeneous solution, the mixed solution (containing PA6 (30.6 wt%), AuNPs (1.2 wt%) and RuðbpyÞ23 þ (0.2 wt.%)) was under vigorous stirring for 6 h at room temperature. The surface of glassy carbon (GC) electrode (3 mm in diameter) was carefully polished with 0.3 μm Al2O3 powders successively, rinsed with water and ethanol in an ultrasonic bath briefly and then allowed to dry at room temperature. Electrospinning was carried out with a 20 mL syringe (1.2 mm diameter spinneret) at electrical potential of 15 kV over a 15 cm gap between the spinneret and the clean GC electrode with a rate of 1.0 mL/h. The ambient temperature and relative humidity for electrospinning were kept at 25 1C and 40%, respectively. It took 3 min to deposit the nanofibers on the surface of the clean GC electrode by electrospinning the mixed solution. The as-prepared electrode was denoted as the Ru–AuNPs–PA6 nanofibers electrode. 2.3. Measurement The ECL determinations were performed at room temperature in a 10 mL homemade quartz cell. A three-electrode system was used with the Ru–AuNPs–PA6 nanofibers electrode as the working electrode, an Ag/AgCl (sat.) as the reference electrode and a platinum wire as the counter electrode. Cyclic voltammetry mode with continuous potential scanning from 0.0 to 1.2 V and scanning rate of 100 mV s 1 was applied to achieve ECL signal in 50 mM phosphate buffer solution (PBS) containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5). A high voltage of 800 V was supplied to the photomultiplier for luminescence intensity determination. The ECL and CV curves were recorded simultaneously.
3. Results and discussion 3.1. The micromorphologies of the Ru–AuNPs–PA6 luminescent composite nanofibers The micromorphologies of the Ru–AuNPs–PA6 luminescent composite nanofibers were analyzed using FESEM and an inverted fluorescence microscope, as shown in Fig. 2. The Ru–AuNPs–PA6 composite nanofibers with smooth surface (Fig. 2A), evenly distributed on the substrate with their random orientations, form a porous 3D structure nanofibrous membrane (Fig. 2B). The diameters of the Ru–AuNPs–PA6 nanofibers range from 100 to 500 nm. The fibers diameters can be adjusted by varying the conditions of the electrospinning process. The fluorescence photographs (Fig. 2C) show that the Ru–AuNPs–PA6 luminescent
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Fig. 2. (A, B) FESEM images of the Ru–AuNPs–PA6 luminescent composite nanofibers. PA6 (30.6 wt%), AuNPs (1.2 wt%), RuðbpyÞ23 þ (0.2 wt%), 7 spinneret, 15 cm gap, 0.8 mL h 1 rate, 15.1 kV electrical potential, the diameter of the nanofibers was 250 nm. (C) Fluorescence photograph of the Ru–AuNPs–PA6 luminescent composite nanofibers magnified at 400 times.
composite nanofibers emit bright orange light at 610 72 nm and the luminosities are uniform when the photographs magnified at 400 times. The results are in agreement with the characteristics of free RuðbpyÞ23 þ ion, which convinces that the Ru–AuNPs–PA6 luminescent composite nanofibers with a porous 3D structure properly maintain the luminescent property of RuðbpyÞ23 þ ions. Furthermore, the existence of the polymer hybrid matrixes can improve the thermal and optical stability of the RuðbpyÞ23 þ complex. Consequently, the Ru–AuNPs–PA6 luminescent composite nanofibers may find applications in nanodevices. 3.2. The characterization of the Ru–AuNPs–PA6 luminescent composite nanofibers The electrochemical characterizations of the Ru–AuNPs–PA6 luminescent composite nanofibers were analyzed using CV and ECL as shown in Fig. 3. The Ru–AuNPs–PA6 luminescent composite nanofibers had a pair of obvious redox peaks of RuðbpyÞ23 þ and the oxidation current signal at þ1.13 V versus Ag/AgCl (sat.) potential in the CV curve (Fig. 3A). The ECL intensity–time curve of the Ru–AuNPs–PA6 nanofibers electrode in 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) is shown in Fig. 3B. The ECL signal of the Ru–AuNPs–PA6 nanofibers electrode in the presence of 3.0 mM C2 O24 (Fig. 3B, curve c) was stronger than that obtained without C2 O24 (Fig. 3B, curve a), which is almost the
same as that of free RuðbpyÞ23 þ under the same experimental conditions. These results are in agreement with the characteristics of free RuðbpyÞ23 þ ion [36,37], which convinces that the Ru–AuNPs–PA6 luminescent composite nanofibers properly maintain the electrical properties of the RuðbpyÞ23 þ ions. In the paper, we employed 3 min to deposit the Ru–AuNPs–PA6 nanofibers on the surface of the GC electrode by electrospinning 10 mL mixed solution containing PA6 (30.6 wt%), AuNPs (1.2 wt%) and RuðbpyÞ23 þ (0.2 wt%) with a rate of 1.0 mL/h. After conversion, the quality of RuðbpyÞ23 þ in the Ru–AuNPs–PA6 nanofibers was 100 μg, and the quality of the AuNPs was 6 times of the RuðbpyÞ23 þ . In order to demonstrate the advantage of the Ru–AuNPs–PA6 luminescent composite nanofibers in ECL amplification, we compared the different ECL behaviors between the bare GC electrode in the 50 mM PBS containing 100 μg RuðbpyÞ23 þ , 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) (solution-phase RuðbpyÞ23 þ ECL system), and Ru–AuNPs–PA6 nanofibers electrode in the 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) (solid-state RuðbpyÞ23 þ ECL system). As shown in Fig. 3B, the ECL intensity of the bare GC electrode in the 50 mM PBS containing 100 μg RuðbpyÞ23 þ , 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) is 506 a.u. during continuous CV scans for 3 cycles (curve b). For the Ru–AuNPs–PA6 nanofibers electrode, the ECL intensity was 912 a.u (curve c). Since the quality of RuðbpyÞ23 þ in the solution and in the Ru–AuNPs–PA6 nanofibers are the same, the ECL intensity of the Ru–MWNTs–PA6 nanofibers
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Fig. 3. (A) The cyclic voltammogram of the Ru–AuNPs–PA6 nanofibers electrode in 50 mM PBS (pH 6.5). Scan rate: 100 mV s 1, scan range: 0.0–1.25 V (versus Ag/AgCl). (B) The ECL intensity of the Ru–AuNPs–PA6 nanofibers electrode in absence (a) and presence (c) of 3.0 mM C2 O24 50 mM PBS (0.4 M KNO3, pH 6.5). The ECL intensity of the bare GC electrode (b) in the 50 mM PBS containing 100 μg RuðbpyÞ23 þ , 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5).
is about 1.8 times higher than that of the solution during continuous CV scans as shown in Fig. 3B. Thus, electronspun nanofibers can be used as the attractive host matrix for the available loading of guest molecules (RuðbpyÞ23 þ ) in the electrode modification. It might be attributed to the high surface area to volume ratio, and the uniform, stable and efficient immobilization of RuðbpyÞ23 þ U C2 O24 , a small molecule, acted as an ECL coreactant, can penetrate freely into the Ru–AuNPs–PA6 nanofibers matrix and react with the immobilized guest molecules (RuðbpyÞ23 þ ). As a result, an enhanced response presented due to the porous structure of the electrospun nanofibers. Moreover, a trace amount of AuNPs might greatly promote the electron-transfer reactions of the Ru–AuNPs–PA6 nanofibers electrode surface. Finally, the Ru– AuNPs–PA6 nanofibers exhibited excellent ECL behaviors on GC electrode. The ECL intensity of the Ru–AuNPs–PA6 nanofibers electrode almost unchanged during continuous CV scans for 30 cycles. It indicates a good stability of the ECL signal of the electrospun nanofibers. The Ru–AuNPs–PA6 nanofibers electrode was immersed in 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) for 12 h, and no ECL signal was detected for the above PBS. It is attributed to the facts that negatively charged AuNPs immobilized stably a large number of positively charged RuðbpyÞ23 þ in the form of composites via electrostatic interaction in the porous 3D structure nanofibrous membrane, and the amide groups in the main chain segment unit of PA6 were well known to bind strongly with AuNPs [38,39]. As a result, the combination of the above facts made the Ru–AuNPs–PA6 nanofibers more durable under operating conditions.
only the outstanding charge-transport characteristics but also the good stability. In order to examine the effect of doped concentration of RuðbpyÞ23 þ on the luminescent intensity, four different amounts of RuðbpyÞ23 þ (0.05, 0.1, 0.15, 0.2, 0.3 wt%) were doped to the nanofibers. It is obvious that the luminescent intensity increases with the increment of doping concentration, and reaches the maximum at the doping concentration of 0.2 wt%. The luminescent intensity decreases at 0.3 wt%. It indicates that the concentration quenching may dominate when the concentration is higher than 0.2 wt%. In other words, when the concentration is higher than 0.2 wt%, the strong self-quenching and triplet–triplet annihilation between ruthenium(II) complex in the composite fibers may decrease the emission efficiency. The determination buffer solution has great impact on ECL intensity. The concentration effects of C2 O24 and the pH of the PBS on the ECL intensity of the Ru–AuNPs–PA6 nanofibers electrode were investigated. The ECL intensity increased with the increasing of the C2 O24 concentration from 0.5 to 2.5 mM with an increment of 0.5 mM, and reached plateau regions at C2 O24 concentration of 3.0 mM. Thus, the optimal C2 O24 concentration was chosen at 3.0 mM. The ECL intensity increased greatly with the pH value increasing until it reached a plateau in the pH 6.0–6.5 and declined sharply at the pH 47.0. So, 6.5 was chosen as the pH value of the PBS at the following experiment. The amount of the KNO3 concentration of the detecting PBS solution was selected according to the literature [16]. So, 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) was selected as the ECL detecting solution.
3.3. Optimization of experimental conditions
3.4. The analytical performance of the solid-state ECL quenching sensor
In order to maximize the detection sensitivity of the proposed solid-state ECL quenching sensor, various conditions were optimized by single factor experiments. The depositing time of the Ru–AuNPs–PA6 nanofibers on the surface of the GC electrode was investigated. In 3 min, the Ru–AuNPs–PA6 nanofibers were deposit on the surface of the GC electrode, and were directly visible. So, we employed 3 min to deposit the Ru–AuNPs–PA6 nanofibers on the surface of the GC electrode by electrospinning the solution (containing PA6 (30.6 wt%), AuNPs (1.2 wt%) and RuðbpyÞ23 þ (0.2 wt%)). In such conditions, Ru–AuNPs–PA6 nanofibers electrode has not
In this work, the effect of aniline on the ECL intensity of the Ru–AuNPs–PA6 nanofibers was systematically studied, as shown in Fig. 4. A strong ECL signal appeared when the Ru–AuNPs–PA6 nanofibers electrode was in the 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) (curve a), while a sharp decrease of the ECL signal was observed when the quenching molecule of aniline was added in the above ECL detecting solution (curve b). The quenching efficiency is defined as ðI 0ECL I ECL Þ=I 0ECL , where IECL and I 0ECL represent the ECL intensities with and without the quencher, respectively. So, the quenching efficiency of the aniline
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Fig. 4. The ECL intensity–potential curves of the Ru–AuNPsvPA6 nanofibers electrode in 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5) with (b) and without (a) 100 μM aniline.
Table 1 The quenching efficiencies of the aniline compounds in 50 mM PBS containing 3.0 mM C2 O24 , 0.4 M KNO3 and 100 μM aniline compounds (pH 6.5). Compounda
(I0 I)/I0 (%)
Aniline m-Phenylenediamine o-Phenylenediamine p-Phenylenediamine Benzidine Aniline/phenylenediamineb Aniline/benzidineb
52.2 63.8 59.3 57.9 70.2 56.1 67.9
a The concentration (C) of the compound is 100 μM, the fluctuation range of the quenching efficiency is 7 5%. Scan rate: 100 mV s 1, scan range: 0.0–1.2 V b Aniline and phenylenediamine, aniline and benzidine (1:1, C:C).
was 52.2%. The results verify that the ECL of the Ru–AuNPs–PA6 nanofibers can be efficiently quenched by the aniline. The mechanism for quenching ECL has been explained as bimolecular energy or electron transfer between RuðbpyÞ23 þ n and benzoquinone or their derivative, the oxidized species of aniline along with suppression of radical reactions [15,16]. The different quenching efficiencies between aniline, m-phenylenediamine, o-phenylenediamine, p-phenylenediamine and benzidine were observed. It was found that all the tested compounds showed an ECL inhibiting signal. In comparison of the tested compounds, the quenching efficiencies of the aniline derivatives were obviously stronger than that of the aniline. As shown in Table 1, benzidine exhibits highest quenching efficiencies. With a concentration of 100 μM, it displayed quenching efficiencies of 70.2%. The three isomers of phenylenediamine, m-phenylenediamine, o-phenylenediamine and p-phenylenediamine exhibited completely different quenching efficiencies. The magnitude of ECL inhibition was related to the position of the substituting group in the benzene ring and decreased in the following order: meta4ortho4para. Furthermore, the quenching efficiencies of the mixtures of aniline and phenylenediamine (1:1, C:C), aniline and benzidine (1:1, C:C) had also been studied. It was found that the mixtures presented medium quenching efficiencies compared with the two kinds of mixed substrates (Table 1). It resulted from the facts that the amine derivatives interfere with aniline detection. The result suggests that the Ru–AuNPs–PA6 nanofibers-based solidstate ECL sensor is ideal for simultaneous determination of the
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aniline compounds by use of inhibited ECL coupled with a separation technique. The relative standard deviation (RSD) of the ECL intensity for the 8 repeated detections of aniline (100 μM) was 4.71%, indicating the good reproducibility of this method. The long-term stability of the Ru–AuNPsvPA6 nanofibers electrode was found to be satisfactory. Rather small changes ( o4%) in the ECL quenching response were observed for a given electrode over a period of at least 10 days storage at room temperature. So, the solid-state ECL aniline sensor displayed good performance including long shelf life and excellent reproducibility. Moreover, after the aniline test, the Ru–AuNPs–PA6 nanofibers electrode can be regenerated by immersing it into the ultrapure water, and rinsing it for several times to remove the adsorbed aniline and C2 O24 thoroughly. After the treatments, the Ru–AuNPs–PA6 nanofibers electrode is ready to incubate with the aniline again and for next aniline detection. Under identical conditions, the Ru–AuNPs–PA6 nanofibers electrode can be continuously reused for 8 times, while still keeping the ECL response to 91.8% of its original state. As 90% of the original signal response is generally acceptable, our results indicate that this platform can be reused at least for 8 times repeatedly. 3.5. The calibration curve of aniline detection The sensitivity of the solid-state ECL quenching sensor was investigated. ECL responses of the Ru–AuNPs–PA6 nanofibers electrodes in 50 mM PBS containing various concentrations of aniline are shown in Fig. 5. The ECL intensity difference [ΔIECL ¼I 0ECL (the ECL intensities without aniline) IECL(the ECL intensities with aniline)] of the Ru–AuNPs–PA6 nanofibers electrodes grew when the aniline concentration increased, and the ΔIECL was found to be linear with the logarithm of aniline concentration in the range from 10 nM to 10 mM (as shown in the inset). The equation for the resulting calibration plot was y¼43lg x þ42 (x was the concentration of aniline divided by 10 nM, y was ΔIECL), the correlation coefficient was 0.9992, and a detection limit of 5.0 nM was estimated by using 3σ (where σ is the relative standard deviation of a blank solution, n ¼11). Consistent data is obtained as shown in the error bar in the inset of Fig. 5 when the experiment is repeated thrice.
Fig. 5. The ECL responses of the Ru–AuNPsvPA6 nanofibers electrodes in 50 mM PBS containing various concentrations of aniline. The concentrations of aniline are 0 M (a), 10 nM (b), 100 nM (c), 1 mM (d), 10 mM (e), 100 mM (f) and 1 mM (g), respectively. Inset: the calibration curves of aniline detection. The concentrations of aniline are 10 nM, 100 nM, 1 mM, 10 mM, 100 mM and 1 mM, respectively. ECL signals are achieved in 50 mM PBS containing 3.0 mM C2 O24 and 0.4 M KNO3 (pH 6.5). Scan rate: 100 mV s 1, scan range: 0.0–1.2 V.
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4. Conclusions A novel and simple solid-state ECL quenching sensor based on the Ru–AuNPs–PA6 luminescent composite nanofibers for detection of aniline has been described. The Ru–AuNPs–PA6 nanofibers with unique 3D nanostructure, large specific surface area and long-term stability were successfully fabricated by a one-step electrospinning technique, and exhibited excellent ECL behaviors on GC electrodes. A high quenching effect on the ECL signal of the Ru–AuNPs–PA6/C2 O24 system was obtained with the presence of low concentration aniline and its derivatives. In comparison of the tested aniline compounds, the quenching efficiencies of the aniline derivatives were obviously stronger than that of the aniline. The magnitude of ECL inhibition was related to the position of the substituting group in the benzene ring and decreased in the following order: meta 4 ortho4 para. The assay allows detection at levels as low as 5.0 nM of the aniline. The solid-state ECL sensor displayed wide linear range, high sensitivity and good stability. The results demonstrate that the strategy could be extended to develop various solid-state ECL quenching sensors for the detection and quantification of aniline derivatives. Acknowledgment Project supported by the National Natural Science Foundation of China (No. 81302472), the Doctoral Program of Higher Education of China (No. 20110092120055) and the Students' Innovation Experimental Project of China (No. S2014118). References [1] E.H. Seymour, N.S. Lawrence, E.L. Beckett, J. Davis, R.G. Compton, Talanta 57 (2002) 233. [2] C.T. Yan, T.S. Shih, J.F. Jen, Talanta 64 (2004) 650. [3] T. Spǎtaru, N. Spǎtaru, A. Fujishima, Talanta 73 (2007) 404. [4] L.M. Zhang, B. Li, Spectrochim. Acta A 74 (2009) 1060.
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