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Nafion composite film
Journal of Electroanalytical Chemistry 736 (2015) 55–60
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Solid-state tris(2,20 -bipyridyl)ruthenium(II) electrogenerated chemiluminescence sensor based on ionic liquid/sol–gel titania/Nafion composite film Junho Jang, Won-Yong Lee ⇑ Department of Chemistry, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 July 2014 Received in revised form 9 October 2014 Accepted 24 October 2014 Available online 29 October 2014 Keywords: Chemiluminescence ECL Ionic liquid Sol–gel titania Composite film
a b s t r a c t The feasibility of being able to control the selectivity of solid-state tris(2,20 -bipyridyl)ruthenium(II) (Ru(bpy)2+ 3 ) electrogenerated chemiluminescence (ECL) sensor is proposed by selecting ionic liquids with appropriate hydrophobicity in the sol–gel titania/Nafion composite film. Four different kinds of ionic liquids with different hydrophobicity such as 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6), 1-ethyl-3-methylimidazolium hexafluorophosphate (EMImPF6), 1-hexyl-3-methylimidazolium hexafluorophosphate (HMImPF6), 1-benzyl-3-methylimidazolium hexafluorophosphate (BnMImPF6) were incorporated in the sol–gel titania/Nafion composite films. The sequence of the observed ECL intensities for hydrophobic analytes is closely related to the sequence of the hydrophobicity of the ionic liquids used in the composite-based ECL sensor. In contrast, the observed ECL sequence for hydrophilic analytes such as ascorbic acid and oxalate is the opposite to the sequence of the hydrophobicity of the ionic liquids used in the composite film. Due to the decreased electron transfer resistance in the ionic liquid-based sol– gel titania/Nafion composite films, the present ECL sensor based on the BMImPF6/titania/Nafion composite film gave a remarkable detection limit (S/N = 3) of 0.48 nM for TPA. Ó 2014 Published by Elsevier B.V.
1. Introduction Since the first research of tris(2,20 -bipyridyl)ruthenium(II) 2+ (Ru(bpy)2+ 3 ) as an electrochemiluminophore [1], the Ru(bpy)3 electrochemiluminescence (ECL) has attracted great attention as a powerful detection method due to its inherent sensitivity and wide linear dynamic range for a great variety of compounds [2–5] such as alkylamines [6], oxalate [7], NADH [8,9], amino acids [10,11], DNA [12], and a number of pharmaceutical compounds [13–15]. In the ECL process, the Ru(bpy)2+ 3 is a regenerable luminophore and thus can be everlastingly reused in theory when it is immobilized on the electrode surface. Accordingly, a lot of efforts have been directed toward the development of stable immobilization methods of Ru(bpy)2+ 3 on a solid electrode surface in order that ECL-based chemical sensors, biosensors and detectors in flowing streams for real analytical and pharmaceutical use are developed [16,17]. Despite the excellent advantages of the Ru(bpy)2+ 3 ECL as a detection method, it still has difficulty in controlling the selectivity
⇑ Corresponding author. Tel.: +82 2 2123 2649; fax: +82 2 364 7050. E-mail address: [email protected] (W.-Y. Lee). http://dx.doi.org/10.1016/j.jelechem.2014.10.027 1572-6657/Ó 2014 Published by Elsevier B.V.
of the ECL detection system for particular analytes. For example, almost all amine compounds can produce ECL signals although their responses are strongly dependent upon the chemical structure of the amines in the order of tertiary > secondary > primary amine. Therefore, in order to improve the selectivity of the Ru(bpy)2+ 3 ECL detection system in real analytical problems, the detection system is often coupled with enzymatic reactions [18–23] or some types of separation techniques such as highperformance liquid chromatography (HPLC) [24–32] and capillary electrophoresis (CE) [33,34]. On the other hand, the ECL detection can be carried out on the modified electrode surface [15,35]. For example, a lauric acid modified electrode has been used in the Ru(bpy)2+ 3 ECL detection for the selective preconcentration and selective detection of analyte [35]. DNA-modified electrode has been used for the highly selective and sensitive the Ru(bpy)2+ 3 ECL determination of phenothiazine drugs in urine sample [15]. In the present work, we propose the feasibility of being able to control the selectivity of solid-state Ru(bpy)2+ 3 ECL sensor by the introduction of ionic liquids with appropriate hydrophobicity in the sol–gel titania/Nafion composite film. Ionic liquids (IL) are a group of materials containing an organic cation (e.g. N-alkylpyridinium, N,N0 -dialkylimidazolium, etc.) and an organic or inorganic anion (e.g. Cl, Br, I, AlCl 4 , PF6 , BF4 , CF3SO3 , CF3COO , etc.),
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which are molten salts at low temperatures. Ionic liquids have many advantages in the fields of chemistry, such as high chemical and thermal stability, good solvating properties, negligible volatility, low toxicity, high ionic conductivity and wide electrochemical windows [36–41]. For that reason, ionic liquids have received considerable attention in the fields of electroanalysis such as modified materials as well as supporting electrolyte, solvent and capillary electrophoresis selectors [42–51,34,52]. For example, Li et al. reported Ru(bpy)2+ 3 ECL in two kinds of ionic liquids of tetraalkylammonium as well as imidazolium, and interpreted the differences of the ECL response in terms of the polarity of the ionic liquids [34]. Xu et al. demonstrated the enhanced solution-phase Ru(bpy)2+ 3 ECL on glassy carbon electrode by the introduction of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) as additives to the electrolytes, where the adsorption of ionic liquid rendered the electrode surface more hydrophobic, facilitating the selective concentration of ECL coreactants based on the hydrophobicity [52]. In this study, we report a highly selective solid-state ECL sensor based on Ru(bpy)2+ 3 immobilized on ionic liquid/sol–gel titania/ Nafion composite film. The selectivity of solid-state ECL sensor can be tuned by the introduction of various ionic liquids with appropriate hydrophobicity in the sol–gel titania/Nafion composite film. Since the introduction of ionic liquids in Nafion is known to improve the conductivity of the composite [38,53], the incorporation of ionic liquid within the sol–gel titania/Nafion composite films not only decreases the electron transfer resistance in the composite film but also increases the oxidation rate of an ECL co-reactant, tripropylamine (TPA) on the composite-modified glassy carbon electrode, which enhances the sensitivity of the ECL sensor.
the photomultiplier tube (PMT) window and enclosed in a light– tight black box. 2.3. Preparation of the ECL sensor Glassy carbon (GC) electrodes were polished with 1.0 lm and 0.05 lm alumina powder and cleaned by ultrasonication with ethanol and deionized–distilled water for 30 min. The titania (TiO2) sol was prepared by the hydrolysis and condensation by mixing 1 mL of 0.15 M Ti(OC4H9)4 dissolved in 2-propanol and 5 mL of deionized water according to the previous report [54]. The same volume of Nafion was added to the titania sol (1:1, v/v) forming the sol–gel titania/Nafion composite solution. A given amount of ionic liquid was solubilized in the sol–gel titania/Nafion composite solution and then the solution was stirred for more than 1 h at room temperature. 0.10 M Ru(bpy)2+ 3 solution was added to the ionic liquid/sol–gel titania/Nafion mixture (1:20, v/v). An aliquot of 3 lL composite solution was hand-cast on the surface of a GC electrode. The composite films were dried for 1 h at room temperature. The Ru(bpy)2+ 3 ECL sensor was placed in 0.05 M phosphate buffer solution at pH 7.0 for 30 min for swelling and removing nonspecifically bound Ru(bpy)2+ 3 and then it was ready for use. 2.4. CV and ECL measurements The CV and ECL behavior was monitored by running consecutive cyclic potential scans between 0 and 1.3 V vs. Ag/AgCl (3 M NaCl) at a scan rate of 100 mV/s. The voltage of PMT is 900 mV and the reported ECL values were the average of at least three scans. All experiments were conducted in 0.05 M phosphate buffer solution at pH 7.0 and ambient conditions at 25 °C.
2. Materials and methods 2.1. Chemicals 1-Butyl-3-methylimidazolium hexafluorophosphate (BMImPF6), 1-ethyl-3-methylimidazolium hexafluorophosphate (EMImPF6), 1-benzyl-3-methylimidazolium hexafluorophosphate (BnMImPF6), titanium(IV) isopropoxide (99.999% trace metals basis), NafionÒ perfluorinated resin solution (5 wt. % in lower aliphatic alcohol and water, contains 15–20% water), tripropylamine (TPA, 99%), sodium oxalate (99.5%), L-ascorbic acid (99%), (-)-erythromycin hydrate (96%), b-nicotinamide adenine dinucleotide, reduced disodium salt hydrate (NADH, 97%), promazine hydrochloride, tris(2,20 -bipyridyl)dichlororuthenium(II) (Ru(bpy)2+ 3 , 98%), and multiwalled carbon nanotube (99%) were purchased from Sigma– Aldrich Chemical Co. (Milwaukee, WI, USA). 1-Hexyl-3-methylimidazolium hexafluorophosphate (HMImPF6) and 2-propanol were obtained from Fluka. Hydrochloric acid was purchased from Burdick & Jackson. Water for all solutions was purified using a Milli-Q water purification system (Millipore, Bedford, MA). 2.2. Instrumentation Cyclic voltammetric experiments were performed with an EG&G 263A potentiostat (Oak Ridge, TN). All experiments were carried out using a conventional three-electrode system in a 10 mL electrochemical quartz cell. The working electrode was glassy carbon (0.07065 cm2) coated with the composite films, and the counter electrode was a platinum wire. All of the potentials quoted here were relative to an Ag/AgCl (3 M saturated NaCl) reference electrode. The photon-counting system used was a Hamamatsu Photonics HC 135-02 photon-counting module (Hamamatsu City, Japan) in conjunction with a computer for recording the output. The ECL cell was placed directly on top of
3. Results and discussion 3.1. Electrochemical behavior In the present work, 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6) as a representative of hydrophobic ionic liquid was introduced in the sol–gel titania/Nafion composite solution to form BMImPF6/sol–gel titania/Nafion composite, which was effectively adhered on the glassy carbon electrode as previously reported with BMImPF6/Nafion composite [52]. A hydrophobic ionic liquid, BMImPF6 (ca. 1%, v/v), can be well solubilized in the sol–gel titania/Nafion composite solution to form the BMImPF6/ sol–gel titania/Nafion composite. Electrochemical behavior of Ru(bpy)2+ immobilized in the BMImPF6/sol–gel titania/Nafion 3 composite films on a GC electrode were studied using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Fig. 1 shows typical cyclic voltammograms of Ru(bpy)2+ 3 immobilized at the sol–gel titania/Nafion composite-modified electrode and at the BMImPF6/sol–gel titania/Nafion modified electrode in 0.05 M phosphate buffer solution (pH 7.0). The voltammograms for the composite-modified electrodes were recorded after those electrodes had reached a steady-state current of Ru(bpy)2+ 3 in the 0.05 M phosphate solution. As shown in Fig. 1, the increase in both oxidation and reduction currents was observed at the BMImPF6/ sol–gel titania/Nafion composite-modified GC electrode compared to those observed at the sol–gel titania/Nafion composite-modified GC electrode. In addition, the surface coverage, C, was determined for the BMImPF6/sol–gel titania/Nafion composite-modified GC electrode by the integration of background-corrected cyclic voltammogram. The calculated C was 4.4 109 mol/cm2, which is slightly higher than that for the titania/Nafion composite-modified GC electrode (3.3 109 mol/cm2). The incorporation of hydrophobic ionic liquid, BMImPF6, within the sol–gel titania/Nafion
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Fig. 1. Cyclic voltammograms of Ru(bpy)2+ 3 immobilized at sol–gel titania/Nafion composite (dash line) and at BMImF6/sol–gel titania/Nafion composite (solid line)modified GC electrode in 0.05 M phosphate buffer solution at pH 7 at a scan rate of 100 mV/s.
Fig. 2. Electrochemical impedance spectra obtained at sol–gel titania/Nafion composite (a), BMImF6/sol–gel titania/Nafion composite-modified GC electrodes (b), and bare GC (c) electrode in the presence of 5.0 mM K3Fe(CN)6/K4Fe(CN)6 (1:1, v/v) solution prepared in 0.05 M phosphate buffer (pH 7.0).
composite films might render the composite-modified GC electrode surface more hydrophobic, which could lead to the increased amount of Ru(bpy)2+ containing hydrophobic bipyridyl ligands 3 immobilized at the composite film via both an ion-exchange process and hydrophobic interactions between Ru(bpy)2+ 3 complex and the BMImPF6/sol–gel titania/Nafion composite. These experimental results indicate that the BMImPF6/sol–gel titania/Nafion composite is an effective medium for the immobilization of Ru(bpy)2+ 3 luminophore. The apparent diffusion coefficient, Dapp, for Ru(bpy)2+ 3 at the BMImPF6/sol–gel titania/Nafion composite-modified GC electrode was calculated from the slope (from linear regression analysis) of the anodic peak current vs square root of the scan rate plot by the Randles–Sevick equation. The apparent diffusion coefficient, Dapp, was found to be 9.562 108 cm2/s, which is slightly larger than the previously reported diffusion coefficient for the Ru(bpy)2+ 3 at the titania/Nafion composite-modified GC electrode (4 109 cm2/s) [54]. The experimental results suggest that the incorporation of ionic liquid within the sol–gel titania/Nafion composite films can increase the mass transport rate within the composite film on GC electrode surface. The electrochemical impedance spectroscopy (EIS) measurements have been carried out in the presence of 5.0 mM K3Fe(CN)6/ K4Fe(CN)6 (1:1, v/v) prepared in 0.05 M phosphate buffer solution (pH 7.0) to get information on the interfacial property at the BMImPF6/sol–gel titania/Nafion composite-modified electrode surface. The impedance spectra were plotted in the form of Nyquist plot as shown in Fig 2. From the EIS experiment, the electron transfer resistance (Ret) of the redox probe at the sol–gel titania/Nafion composite-modified GC electrode (a), the BMImPF6/sol–gel titania/ Nafion composite-modified GC electrode (b), and bare GC electrode (c) were found to be 2.5 MX, 41 kX, and 3.6 kX, respectively. The Ret value measured at the titania/Nafion composite-modified GC electrode was very high because of the repulsive interactions (electrostatic and steric) between the negatively charged redox probe ions, [Fe(CN)6]3/4 and the negatively charged Nafion in the modified GC electrode surface as we previously reported [55]. However, the presence of ionic liquid BMImPF6 in the composite film dramatically reduces the electron transfer resistance as shown in Fig. 2. This is consistent with a previous report observed at the BMImPF6/Nafion composite [43]. The experimental results suggest that the incorporation of ionic liquid within the sol–gel titania/Nafion composite films can increase the electron transfer within the composite film on GC electrode surface due to the high conductivity of ionic liquid.
3.2. ECL behavior The ECL behavior of the present ECL sensor based on Ru(bpy)2+ 3 incorporated into the ionic liquid (BMImPF6)/sol–gel titania/Nafion composite-modified GC electrode was studied using tripropylamine (TPA) as a co-reactant. Fig. 3 shows cyclic voltammograms (A) and corresponding ECL–potential profile (B) simultaneously
Fig. 3. (A) Cyclic voltammograms of Ru(bpy)2+ 3 immobilized at BMImPF6/sol–gel titania/Nafion composite-modified electrode in the presence (solid line) and the absence (dash line) of 500 lM TPA in 0.05 M phosphate buffer at pH 7.0 at a scan rate of 100 mV/s. (B) ECL-potential curves recorded during the CV scan. Inset shows the relative ECL intensity obtained at the ECL sensor based on pure Nafion, sol–gel titania/Nafion composite, and BMImPF6/sol–gel titania/Nafion composite in the same experimental conditions.
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obtained at the present ECL sensor in the absence (dashed line) and the presence (solid line) of 500 lM TPA in 0.05 M phosphate buffer at pH 7.0 at a scan rate of 100 mV/s. The presence of TPA causes the anodic peak current to increase and the cathodic peak current to decrease slightly, which corresponds to the typical electrocatalytic reaction mechanism on the ECL emission of Ru(bpy)2+ 3 –TPA system [46–51]. The ECL emission of Ru(bpy)2+ 3 –TPA system occurs when the deprotonated TPA radical (TPA) reacts with Ru(bpy)3+ 3 to form * [Ru(bpy)2+ 3 ] , which then decays to produce orange emission (Eqs. 1, 2a, 2b, 3, 4). TPA radical was formed via oxidation by Ru(bpy)3+ 3 (Eq. (2a)) and direct electrode oxidation (Eq. (2b)) [1,46–51]. 2þ
3þ
RuðbpyÞ3 ! RuðbpyÞ3 þ e 3þ
ð1Þ
2þ
RuðbpyÞ3 þ TPA ! RuðbpyÞ3 þ TPA
ð2aÞ
TPA ! TPA þ e
ð2bÞ 2þ
3þ
RuðbpyÞ3 þ TPA ! ½RuðbpyÞ3 2þ
2þ
½RuðbpyÞ3 ! RuðbpyÞ3 þ hm ð610 nmÞ
ð3Þ
Fig. 4. Cyclic voltammograms of 10 mM TPA (solid line) and 0.05 M phosphate buffer (dash dot line) at BMImPF6/sol–gel titania/Nafion composite-modified electrode at a scan rate of 100 mV/s. Inset shows the cyclic voltammograms of 10 mM TPA (dot line) and 0.05 M phosphate buffer (dash line) at sol–gel titania/ Nafion composite at a scan rate of 100 mV/s.
ð4Þ
The ECL signal increased considerably in the presence of TPA. The onset of ECL occurred at around 0.9–1.0 V. As can be expected from the Ru(bpy)2+ 3 –TPA ECL mechanism, the onset of ECL seen at around 0.9–1.0 V was coincident with the onset potential where Ru(bpy)2+ immobilized in the BMImPF6/sol–gel titania/Nafion 3 composite films is electrochemically oxidized. The ECL intensity obtained at the BMImPF6/sol–gel titania/Nafion compositemodified GC electrode was 21-fold and 3-fold higher than that obtained at pure Nafion and sol–gel titania/Nafion compositemodified GC electrodes, respectively, as shown in the inset of Fig. 3B. The increased ECL response at the BMImPF6/sol–gel titania/Nafion composite-modified GC electrode is probably attributed to many possible reasons. First, the ECL response of the Ru(bpy)2+ 3 – TPA system in a solid-state sensor format is known to be proportional to the amount of Ru(bpy)2+ 3 immobilized on the electrode surface [19]. Thus, as discussed in the CV results, the increased amount of Ru(bpy)2+ 3 immobilized at the BMImPF6/sol–gel titania/Nafion composite-modified GC electrode could lead to an increase in the ECL response. Second, it is well known that the surface hydrophobicity of an electrode surface strongly affect the ECL response for TPA [50]. The addition of hydrophobic ionic liquid BMImPF6 in the sol–gel titania/Nafion composite films might cause the composite-modified GC electrode surface to become more hydrophobic, which may lead to the increased oxidation rate of hydrophobic co-reactant TPA as shown in Fig. 4. Finally, as discussed in the CV results, the increased charge and mass transport within the BMImPF6/sol–gel titania/Nafion composite films could also lead to the increased ECL response. Since the amount of ionic liquid incorporated into sol–gel titania/Nafion composite film significantly affects the ECL behavior, the effect of the quantity of ionic liquid in the composite film on the ECL response of 500 lM TPA was studied. As shown in Fig. 5, the ECL intensity steadily increased with the amount of BMImPF6 in the sol–gel titania/Nafion composite up to 50 mM (ca. 1%, v/v). However, as the amount of ionic liquid in the sol–gel titania/Nafion composite increased further, the ECL response decreased. In that case, too much ionic liquid led to the destabilization of the sol– gel titania/Nafion, thus leading to poor composite film condition. Therefore, an optimum amount of ionic liquid at 50 mM was used in all the subsequent experiments. Calibration curves for TPA have been constructed using the BMImPF6/sol–gel titania/Nafion composite-modified electrode as shown in Fig. 6. Each point of calibration curves is a mean of five or more ECL signals obtained by consecutive cyclic potential scans
Fig. 5. Dependence of ECL intensity for 500 lM TPA in 0.05 M phosphate buffer (pH 7.0) on the amount of ionic liquid (BMImPF6) incorporated into the sol–gel titania/ Nafion composite at a scan rate of 100 mV/s. Error bars show standard deviation for at least three different ECL measurements.
Fig. 6. Calibration curve for TPA obtained at the ECL sensor based on BMImPF6/sol– gel titania/Nafion composite film.
(100 mV/s) at a given concentration. The Ru(bpy)2+ 3 ECL sensor based on the ionic liquid/sol–gel titania/Nafion composite gave a linear response (R2 = 0.99924) for TPA ranging from 1.0 nM to 1.0 mM with a detection limit (S/N = 3) of 0.48 nM. The detection
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J. Jang, W.-Y. Lee / Journal of Electroanalytical Chemistry 736 (2015) 55–60 Table 1 Performance comparison of the ECL sensors based on different immobilization composite. Immobilization composite Nafion Silica/Nafion TiO2/Nafion V2O5/Nafion ZrO/Nafion CNT/Nafion CNT/TiO2/Nafion CNT/ZrO/Nafion Pt–CNT/Zirconia/Nafion Graphene/Nafion Ionic liquid/TiO2/Nafion
Linear dynamic range (M) 6
3
1.0 10 –1.0 10 1.0 107–1.0 103 1.0 107–1.0 103 5.0 108–1.0 103 5.0 108–1.0 103 1.0 107 – 1.0 103 5.0 108–1.0 103 1.0 108–1.0 103 3.0 109–1.0 103 1.0 107–1.0 104 1.0 109–1.0 103
Detection limit (M) 6
1.0 10 1.0 107 1.0 106 1.0 108 5.0 108 5.0 108 1.0 108 5.0 109 1.0 109 5.0 108 4.8 1010
Long-term stability
References
Less than 1 day Less than 2 weeks 60% in 3 weeks 60% in 3 weeks 83% in 3 weeks 90% in 2 months Not changed in 4 months 92% in 4 weeks 94% in 5 weeks 85% in 1 months Not changed in 2 months
[9,19] [18] [19] [20] [23] [21,22] [22] [23] [23] [25] This study
response for TPA [50]. These selectivity test results strongly indicate that the hydrophobicity of the ionic liquid has significant impact on the resulting ECL intensity. As the hydrophobicity of the ionic liquid increases, the ECL intensity increases for hydrophobic analyte (NADH) but decreases for hydrophilic analytes (oxalate and ascorbic acid) as shown in Fig. 7. In addition to the effect of hydrophobicity, the enhanced ECL intensity for NADH could be attributed to the strong p–p interaction between aromatic rings of NADH and ionic liquids [53]. The present selectivity test suggests that the selective detection for a variety of analytes could be realized through the proper tuning of the hydrophobicity of ionic liquid added in the sol–gel titania/Nafion composite film. 4. Conclusions
limit is lower than that obtained from the pure Nafion, sol–gel/Nafion, CNT/sol–gel ceramics/Nafion [18,19,22,23,34] and graphene/ Nafion [25] composite-modified ECL sensors. The present Ru(bpy)2+ 3 ECL sensor based on the ionic liquid/sol–gel titania/Nafion composite films showed good reproducibility and long-term storage stability in the determination of TPA. The relative standard deviation of the ECL intensity for 20 CV scans in 0.05 M phosphate buffer at pH 7.0 containing 500 lM TPA was 4.34%. The stability of the present Ru(bpy)2+ 3 ECL sensor was superior than that obtained from the pure Nafion, sol–gel/Nafion, and graphene/Nafion composite-modified ECL sensors, and comparable to the ECL sensor based on CNT/sol–gel titania/Nafion composite. Performance comparison of the ECL sensors based on different immobilization matrices is summarized in Table 1.
The incorporation of hydrophobic ionic liquid of 1-butyl-3methylimidazolium hexafluorophosphate (BMImPF6) within the sol–gel titania/Nafion composite films renders the compositemodified GC electrode surface more hydrophobic, which leads to not only the increased amount of Ru(bpy)2+ 3 immobilized on the ECL sensor but also the increased oxidation rate of a hydrophobic ECL co-reactant, tripropylamine (TPA). Due to the high conductivity of ionic liquid, the electron transfer resistance has been significantly decreased in the composite-modified GC electrode. Therefore, the present ECL sensor based on the BMImPF6/sol–gel titania/Nafion composite film showed improved ECL sensitivity, linear dynamic range, long-term stability and limit of detection for TPA compared to those obtained at the ECL sensors based on the other sol–gel ceramics/Nafion composite films and even carbon nanomaterials (e.g. CNT, graphene)/Nafion composite films. In addition, the present study implies that the selectivity of the ECL sensor can be tuned by proper tuning of the hydrophobicity of ionic liquid added in the sol–gel titania/Nafion composite film.
3.3. Selectivity
Conflict of interest
Fig. 7. Selectivity studies of the ECL sensor based on ionic liquids/sol–gel titania/ Nafion composite film for various analytes.
Ionic liquid is an important factor that determines the character of the composite films. Selectivity tests of the ECL sensor based on ionic liquid/sol–gel titania/Nafion composite were carried out with a number of different ionic liquids for various analytes. When hydrophilic analytes such as ascorbic acid and oxalate are used as co-reactants, the sequence of the resulting ECL intensities was EMIm+ > HMIm+ > BnMIm+ as shown in Fig. 7(a) and (b). The observed ECL sequence was the opposite to the sequence of the hydrophobicity of the ionic liquids used in the fabrication of the composite-based ECL sensor [56,57]. In contrast, the sequence of the observed ECL intensities for a hydrophobic analyte such as NADH was BnMIm+ > HMIm+ > EMIm+ as shown in Fig. 7(c), which accords closely with the sequence of the hydrophobicity of the ionic liquids used in the composite film. It is well known that the surface hydrophobicity of an electrode surface strongly affect the ECL
There is no conflict of interest. Acknowledgements Financial support for this work has been provided by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-R1A1A2006994). References [1] [2] [3] [4] [5]
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Solid-state tris(2,20 -bipyridyl)ruthenium(II) electrogenerated chemiluminescence sensor based on ionic liquid/sol–gel titania/Nafion composite film Junho Jang, Won-Yong Lee ⇑ Department of Chemistry, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 July 2014 Received in revised form 9 October 2014 Accepted 24 October 2014 Available online 29 October 2014 Keywords: Chemiluminescence ECL Ionic liquid Sol–gel titania Composite film
a b s t r a c t The feasibility of being able to control the selectivity of solid-state tris(2,20 -bipyridyl)ruthenium(II) (Ru(bpy)2+ 3 ) electrogenerated chemiluminescence (ECL) sensor is proposed by selecting ionic liquids with appropriate hydrophobicity in the sol–gel titania/Nafion composite film. Four different kinds of ionic liquids with different hydrophobicity such as 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6), 1-ethyl-3-methylimidazolium hexafluorophosphate (EMImPF6), 1-hexyl-3-methylimidazolium hexafluorophosphate (HMImPF6), 1-benzyl-3-methylimidazolium hexafluorophosphate (BnMImPF6) were incorporated in the sol–gel titania/Nafion composite films. The sequence of the observed ECL intensities for hydrophobic analytes is closely related to the sequence of the hydrophobicity of the ionic liquids used in the composite-based ECL sensor. In contrast, the observed ECL sequence for hydrophilic analytes such as ascorbic acid and oxalate is the opposite to the sequence of the hydrophobicity of the ionic liquids used in the composite film. Due to the decreased electron transfer resistance in the ionic liquid-based sol– gel titania/Nafion composite films, the present ECL sensor based on the BMImPF6/titania/Nafion composite film gave a remarkable detection limit (S/N = 3) of 0.48 nM for TPA. Ó 2014 Published by Elsevier B.V.
1. Introduction Since the first research of tris(2,20 -bipyridyl)ruthenium(II) 2+ (Ru(bpy)2+ 3 ) as an electrochemiluminophore [1], the Ru(bpy)3 electrochemiluminescence (ECL) has attracted great attention as a powerful detection method due to its inherent sensitivity and wide linear dynamic range for a great variety of compounds [2–5] such as alkylamines [6], oxalate [7], NADH [8,9], amino acids [10,11], DNA [12], and a number of pharmaceutical compounds [13–15]. In the ECL process, the Ru(bpy)2+ 3 is a regenerable luminophore and thus can be everlastingly reused in theory when it is immobilized on the electrode surface. Accordingly, a lot of efforts have been directed toward the development of stable immobilization methods of Ru(bpy)2+ 3 on a solid electrode surface in order that ECL-based chemical sensors, biosensors and detectors in flowing streams for real analytical and pharmaceutical use are developed [16,17]. Despite the excellent advantages of the Ru(bpy)2+ 3 ECL as a detection method, it still has difficulty in controlling the selectivity
⇑ Corresponding author. Tel.: +82 2 2123 2649; fax: +82 2 364 7050. E-mail address: [email protected] (W.-Y. Lee). http://dx.doi.org/10.1016/j.jelechem.2014.10.027 1572-6657/Ó 2014 Published by Elsevier B.V.
of the ECL detection system for particular analytes. For example, almost all amine compounds can produce ECL signals although their responses are strongly dependent upon the chemical structure of the amines in the order of tertiary > secondary > primary amine. Therefore, in order to improve the selectivity of the Ru(bpy)2+ 3 ECL detection system in real analytical problems, the detection system is often coupled with enzymatic reactions [18–23] or some types of separation techniques such as highperformance liquid chromatography (HPLC) [24–32] and capillary electrophoresis (CE) [33,34]. On the other hand, the ECL detection can be carried out on the modified electrode surface [15,35]. For example, a lauric acid modified electrode has been used in the Ru(bpy)2+ 3 ECL detection for the selective preconcentration and selective detection of analyte [35]. DNA-modified electrode has been used for the highly selective and sensitive the Ru(bpy)2+ 3 ECL determination of phenothiazine drugs in urine sample [15]. In the present work, we propose the feasibility of being able to control the selectivity of solid-state Ru(bpy)2+ 3 ECL sensor by the introduction of ionic liquids with appropriate hydrophobicity in the sol–gel titania/Nafion composite film. Ionic liquids (IL) are a group of materials containing an organic cation (e.g. N-alkylpyridinium, N,N0 -dialkylimidazolium, etc.) and an organic or inorganic anion (e.g. Cl, Br, I, AlCl 4 , PF6 , BF4 , CF3SO3 , CF3COO , etc.),
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which are molten salts at low temperatures. Ionic liquids have many advantages in the fields of chemistry, such as high chemical and thermal stability, good solvating properties, negligible volatility, low toxicity, high ionic conductivity and wide electrochemical windows [36–41]. For that reason, ionic liquids have received considerable attention in the fields of electroanalysis such as modified materials as well as supporting electrolyte, solvent and capillary electrophoresis selectors [42–51,34,52]. For example, Li et al. reported Ru(bpy)2+ 3 ECL in two kinds of ionic liquids of tetraalkylammonium as well as imidazolium, and interpreted the differences of the ECL response in terms of the polarity of the ionic liquids [34]. Xu et al. demonstrated the enhanced solution-phase Ru(bpy)2+ 3 ECL on glassy carbon electrode by the introduction of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) as additives to the electrolytes, where the adsorption of ionic liquid rendered the electrode surface more hydrophobic, facilitating the selective concentration of ECL coreactants based on the hydrophobicity [52]. In this study, we report a highly selective solid-state ECL sensor based on Ru(bpy)2+ 3 immobilized on ionic liquid/sol–gel titania/ Nafion composite film. The selectivity of solid-state ECL sensor can be tuned by the introduction of various ionic liquids with appropriate hydrophobicity in the sol–gel titania/Nafion composite film. Since the introduction of ionic liquids in Nafion is known to improve the conductivity of the composite [38,53], the incorporation of ionic liquid within the sol–gel titania/Nafion composite films not only decreases the electron transfer resistance in the composite film but also increases the oxidation rate of an ECL co-reactant, tripropylamine (TPA) on the composite-modified glassy carbon electrode, which enhances the sensitivity of the ECL sensor.
the photomultiplier tube (PMT) window and enclosed in a light– tight black box. 2.3. Preparation of the ECL sensor Glassy carbon (GC) electrodes were polished with 1.0 lm and 0.05 lm alumina powder and cleaned by ultrasonication with ethanol and deionized–distilled water for 30 min. The titania (TiO2) sol was prepared by the hydrolysis and condensation by mixing 1 mL of 0.15 M Ti(OC4H9)4 dissolved in 2-propanol and 5 mL of deionized water according to the previous report [54]. The same volume of Nafion was added to the titania sol (1:1, v/v) forming the sol–gel titania/Nafion composite solution. A given amount of ionic liquid was solubilized in the sol–gel titania/Nafion composite solution and then the solution was stirred for more than 1 h at room temperature. 0.10 M Ru(bpy)2+ 3 solution was added to the ionic liquid/sol–gel titania/Nafion mixture (1:20, v/v). An aliquot of 3 lL composite solution was hand-cast on the surface of a GC electrode. The composite films were dried for 1 h at room temperature. The Ru(bpy)2+ 3 ECL sensor was placed in 0.05 M phosphate buffer solution at pH 7.0 for 30 min for swelling and removing nonspecifically bound Ru(bpy)2+ 3 and then it was ready for use. 2.4. CV and ECL measurements The CV and ECL behavior was monitored by running consecutive cyclic potential scans between 0 and 1.3 V vs. Ag/AgCl (3 M NaCl) at a scan rate of 100 mV/s. The voltage of PMT is 900 mV and the reported ECL values were the average of at least three scans. All experiments were conducted in 0.05 M phosphate buffer solution at pH 7.0 and ambient conditions at 25 °C.
2. Materials and methods 2.1. Chemicals 1-Butyl-3-methylimidazolium hexafluorophosphate (BMImPF6), 1-ethyl-3-methylimidazolium hexafluorophosphate (EMImPF6), 1-benzyl-3-methylimidazolium hexafluorophosphate (BnMImPF6), titanium(IV) isopropoxide (99.999% trace metals basis), NafionÒ perfluorinated resin solution (5 wt. % in lower aliphatic alcohol and water, contains 15–20% water), tripropylamine (TPA, 99%), sodium oxalate (99.5%), L-ascorbic acid (99%), (-)-erythromycin hydrate (96%), b-nicotinamide adenine dinucleotide, reduced disodium salt hydrate (NADH, 97%), promazine hydrochloride, tris(2,20 -bipyridyl)dichlororuthenium(II) (Ru(bpy)2+ 3 , 98%), and multiwalled carbon nanotube (99%) were purchased from Sigma– Aldrich Chemical Co. (Milwaukee, WI, USA). 1-Hexyl-3-methylimidazolium hexafluorophosphate (HMImPF6) and 2-propanol were obtained from Fluka. Hydrochloric acid was purchased from Burdick & Jackson. Water for all solutions was purified using a Milli-Q water purification system (Millipore, Bedford, MA). 2.2. Instrumentation Cyclic voltammetric experiments were performed with an EG&G 263A potentiostat (Oak Ridge, TN). All experiments were carried out using a conventional three-electrode system in a 10 mL electrochemical quartz cell. The working electrode was glassy carbon (0.07065 cm2) coated with the composite films, and the counter electrode was a platinum wire. All of the potentials quoted here were relative to an Ag/AgCl (3 M saturated NaCl) reference electrode. The photon-counting system used was a Hamamatsu Photonics HC 135-02 photon-counting module (Hamamatsu City, Japan) in conjunction with a computer for recording the output. The ECL cell was placed directly on top of
3. Results and discussion 3.1. Electrochemical behavior In the present work, 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6) as a representative of hydrophobic ionic liquid was introduced in the sol–gel titania/Nafion composite solution to form BMImPF6/sol–gel titania/Nafion composite, which was effectively adhered on the glassy carbon electrode as previously reported with BMImPF6/Nafion composite [52]. A hydrophobic ionic liquid, BMImPF6 (ca. 1%, v/v), can be well solubilized in the sol–gel titania/Nafion composite solution to form the BMImPF6/ sol–gel titania/Nafion composite. Electrochemical behavior of Ru(bpy)2+ immobilized in the BMImPF6/sol–gel titania/Nafion 3 composite films on a GC electrode were studied using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Fig. 1 shows typical cyclic voltammograms of Ru(bpy)2+ 3 immobilized at the sol–gel titania/Nafion composite-modified electrode and at the BMImPF6/sol–gel titania/Nafion modified electrode in 0.05 M phosphate buffer solution (pH 7.0). The voltammograms for the composite-modified electrodes were recorded after those electrodes had reached a steady-state current of Ru(bpy)2+ 3 in the 0.05 M phosphate solution. As shown in Fig. 1, the increase in both oxidation and reduction currents was observed at the BMImPF6/ sol–gel titania/Nafion composite-modified GC electrode compared to those observed at the sol–gel titania/Nafion composite-modified GC electrode. In addition, the surface coverage, C, was determined for the BMImPF6/sol–gel titania/Nafion composite-modified GC electrode by the integration of background-corrected cyclic voltammogram. The calculated C was 4.4 109 mol/cm2, which is slightly higher than that for the titania/Nafion composite-modified GC electrode (3.3 109 mol/cm2). The incorporation of hydrophobic ionic liquid, BMImPF6, within the sol–gel titania/Nafion
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Fig. 1. Cyclic voltammograms of Ru(bpy)2+ 3 immobilized at sol–gel titania/Nafion composite (dash line) and at BMImF6/sol–gel titania/Nafion composite (solid line)modified GC electrode in 0.05 M phosphate buffer solution at pH 7 at a scan rate of 100 mV/s.
Fig. 2. Electrochemical impedance spectra obtained at sol–gel titania/Nafion composite (a), BMImF6/sol–gel titania/Nafion composite-modified GC electrodes (b), and bare GC (c) electrode in the presence of 5.0 mM K3Fe(CN)6/K4Fe(CN)6 (1:1, v/v) solution prepared in 0.05 M phosphate buffer (pH 7.0).
composite films might render the composite-modified GC electrode surface more hydrophobic, which could lead to the increased amount of Ru(bpy)2+ containing hydrophobic bipyridyl ligands 3 immobilized at the composite film via both an ion-exchange process and hydrophobic interactions between Ru(bpy)2+ 3 complex and the BMImPF6/sol–gel titania/Nafion composite. These experimental results indicate that the BMImPF6/sol–gel titania/Nafion composite is an effective medium for the immobilization of Ru(bpy)2+ 3 luminophore. The apparent diffusion coefficient, Dapp, for Ru(bpy)2+ 3 at the BMImPF6/sol–gel titania/Nafion composite-modified GC electrode was calculated from the slope (from linear regression analysis) of the anodic peak current vs square root of the scan rate plot by the Randles–Sevick equation. The apparent diffusion coefficient, Dapp, was found to be 9.562 108 cm2/s, which is slightly larger than the previously reported diffusion coefficient for the Ru(bpy)2+ 3 at the titania/Nafion composite-modified GC electrode (4 109 cm2/s) [54]. The experimental results suggest that the incorporation of ionic liquid within the sol–gel titania/Nafion composite films can increase the mass transport rate within the composite film on GC electrode surface. The electrochemical impedance spectroscopy (EIS) measurements have been carried out in the presence of 5.0 mM K3Fe(CN)6/ K4Fe(CN)6 (1:1, v/v) prepared in 0.05 M phosphate buffer solution (pH 7.0) to get information on the interfacial property at the BMImPF6/sol–gel titania/Nafion composite-modified electrode surface. The impedance spectra were plotted in the form of Nyquist plot as shown in Fig 2. From the EIS experiment, the electron transfer resistance (Ret) of the redox probe at the sol–gel titania/Nafion composite-modified GC electrode (a), the BMImPF6/sol–gel titania/ Nafion composite-modified GC electrode (b), and bare GC electrode (c) were found to be 2.5 MX, 41 kX, and 3.6 kX, respectively. The Ret value measured at the titania/Nafion composite-modified GC electrode was very high because of the repulsive interactions (electrostatic and steric) between the negatively charged redox probe ions, [Fe(CN)6]3/4 and the negatively charged Nafion in the modified GC electrode surface as we previously reported [55]. However, the presence of ionic liquid BMImPF6 in the composite film dramatically reduces the electron transfer resistance as shown in Fig. 2. This is consistent with a previous report observed at the BMImPF6/Nafion composite [43]. The experimental results suggest that the incorporation of ionic liquid within the sol–gel titania/Nafion composite films can increase the electron transfer within the composite film on GC electrode surface due to the high conductivity of ionic liquid.
3.2. ECL behavior The ECL behavior of the present ECL sensor based on Ru(bpy)2+ 3 incorporated into the ionic liquid (BMImPF6)/sol–gel titania/Nafion composite-modified GC electrode was studied using tripropylamine (TPA) as a co-reactant. Fig. 3 shows cyclic voltammograms (A) and corresponding ECL–potential profile (B) simultaneously
Fig. 3. (A) Cyclic voltammograms of Ru(bpy)2+ 3 immobilized at BMImPF6/sol–gel titania/Nafion composite-modified electrode in the presence (solid line) and the absence (dash line) of 500 lM TPA in 0.05 M phosphate buffer at pH 7.0 at a scan rate of 100 mV/s. (B) ECL-potential curves recorded during the CV scan. Inset shows the relative ECL intensity obtained at the ECL sensor based on pure Nafion, sol–gel titania/Nafion composite, and BMImPF6/sol–gel titania/Nafion composite in the same experimental conditions.
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obtained at the present ECL sensor in the absence (dashed line) and the presence (solid line) of 500 lM TPA in 0.05 M phosphate buffer at pH 7.0 at a scan rate of 100 mV/s. The presence of TPA causes the anodic peak current to increase and the cathodic peak current to decrease slightly, which corresponds to the typical electrocatalytic reaction mechanism on the ECL emission of Ru(bpy)2+ 3 –TPA system [46–51]. The ECL emission of Ru(bpy)2+ 3 –TPA system occurs when the deprotonated TPA radical (TPA) reacts with Ru(bpy)3+ 3 to form * [Ru(bpy)2+ 3 ] , which then decays to produce orange emission (Eqs. 1, 2a, 2b, 3, 4). TPA radical was formed via oxidation by Ru(bpy)3+ 3 (Eq. (2a)) and direct electrode oxidation (Eq. (2b)) [1,46–51]. 2þ
3þ
RuðbpyÞ3 ! RuðbpyÞ3 þ e 3þ
ð1Þ
2þ
RuðbpyÞ3 þ TPA ! RuðbpyÞ3 þ TPA
ð2aÞ
TPA ! TPA þ e
ð2bÞ 2þ
3þ
RuðbpyÞ3 þ TPA ! ½RuðbpyÞ3 2þ
2þ
½RuðbpyÞ3 ! RuðbpyÞ3 þ hm ð610 nmÞ
ð3Þ
Fig. 4. Cyclic voltammograms of 10 mM TPA (solid line) and 0.05 M phosphate buffer (dash dot line) at BMImPF6/sol–gel titania/Nafion composite-modified electrode at a scan rate of 100 mV/s. Inset shows the cyclic voltammograms of 10 mM TPA (dot line) and 0.05 M phosphate buffer (dash line) at sol–gel titania/ Nafion composite at a scan rate of 100 mV/s.
ð4Þ
The ECL signal increased considerably in the presence of TPA. The onset of ECL occurred at around 0.9–1.0 V. As can be expected from the Ru(bpy)2+ 3 –TPA ECL mechanism, the onset of ECL seen at around 0.9–1.0 V was coincident with the onset potential where Ru(bpy)2+ immobilized in the BMImPF6/sol–gel titania/Nafion 3 composite films is electrochemically oxidized. The ECL intensity obtained at the BMImPF6/sol–gel titania/Nafion compositemodified GC electrode was 21-fold and 3-fold higher than that obtained at pure Nafion and sol–gel titania/Nafion compositemodified GC electrodes, respectively, as shown in the inset of Fig. 3B. The increased ECL response at the BMImPF6/sol–gel titania/Nafion composite-modified GC electrode is probably attributed to many possible reasons. First, the ECL response of the Ru(bpy)2+ 3 – TPA system in a solid-state sensor format is known to be proportional to the amount of Ru(bpy)2+ 3 immobilized on the electrode surface [19]. Thus, as discussed in the CV results, the increased amount of Ru(bpy)2+ 3 immobilized at the BMImPF6/sol–gel titania/Nafion composite-modified GC electrode could lead to an increase in the ECL response. Second, it is well known that the surface hydrophobicity of an electrode surface strongly affect the ECL response for TPA [50]. The addition of hydrophobic ionic liquid BMImPF6 in the sol–gel titania/Nafion composite films might cause the composite-modified GC electrode surface to become more hydrophobic, which may lead to the increased oxidation rate of hydrophobic co-reactant TPA as shown in Fig. 4. Finally, as discussed in the CV results, the increased charge and mass transport within the BMImPF6/sol–gel titania/Nafion composite films could also lead to the increased ECL response. Since the amount of ionic liquid incorporated into sol–gel titania/Nafion composite film significantly affects the ECL behavior, the effect of the quantity of ionic liquid in the composite film on the ECL response of 500 lM TPA was studied. As shown in Fig. 5, the ECL intensity steadily increased with the amount of BMImPF6 in the sol–gel titania/Nafion composite up to 50 mM (ca. 1%, v/v). However, as the amount of ionic liquid in the sol–gel titania/Nafion composite increased further, the ECL response decreased. In that case, too much ionic liquid led to the destabilization of the sol– gel titania/Nafion, thus leading to poor composite film condition. Therefore, an optimum amount of ionic liquid at 50 mM was used in all the subsequent experiments. Calibration curves for TPA have been constructed using the BMImPF6/sol–gel titania/Nafion composite-modified electrode as shown in Fig. 6. Each point of calibration curves is a mean of five or more ECL signals obtained by consecutive cyclic potential scans
Fig. 5. Dependence of ECL intensity for 500 lM TPA in 0.05 M phosphate buffer (pH 7.0) on the amount of ionic liquid (BMImPF6) incorporated into the sol–gel titania/ Nafion composite at a scan rate of 100 mV/s. Error bars show standard deviation for at least three different ECL measurements.
Fig. 6. Calibration curve for TPA obtained at the ECL sensor based on BMImPF6/sol– gel titania/Nafion composite film.
(100 mV/s) at a given concentration. The Ru(bpy)2+ 3 ECL sensor based on the ionic liquid/sol–gel titania/Nafion composite gave a linear response (R2 = 0.99924) for TPA ranging from 1.0 nM to 1.0 mM with a detection limit (S/N = 3) of 0.48 nM. The detection
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J. Jang, W.-Y. Lee / Journal of Electroanalytical Chemistry 736 (2015) 55–60 Table 1 Performance comparison of the ECL sensors based on different immobilization composite. Immobilization composite Nafion Silica/Nafion TiO2/Nafion V2O5/Nafion ZrO/Nafion CNT/Nafion CNT/TiO2/Nafion CNT/ZrO/Nafion Pt–CNT/Zirconia/Nafion Graphene/Nafion Ionic liquid/TiO2/Nafion
Linear dynamic range (M) 6
3
1.0 10 –1.0 10 1.0 107–1.0 103 1.0 107–1.0 103 5.0 108–1.0 103 5.0 108–1.0 103 1.0 107 – 1.0 103 5.0 108–1.0 103 1.0 108–1.0 103 3.0 109–1.0 103 1.0 107–1.0 104 1.0 109–1.0 103
Detection limit (M) 6
1.0 10 1.0 107 1.0 106 1.0 108 5.0 108 5.0 108 1.0 108 5.0 109 1.0 109 5.0 108 4.8 1010
Long-term stability
References
Less than 1 day Less than 2 weeks 60% in 3 weeks 60% in 3 weeks 83% in 3 weeks 90% in 2 months Not changed in 4 months 92% in 4 weeks 94% in 5 weeks 85% in 1 months Not changed in 2 months
[9,19] [18] [19] [20] [23] [21,22] [22] [23] [23] [25] This study
response for TPA [50]. These selectivity test results strongly indicate that the hydrophobicity of the ionic liquid has significant impact on the resulting ECL intensity. As the hydrophobicity of the ionic liquid increases, the ECL intensity increases for hydrophobic analyte (NADH) but decreases for hydrophilic analytes (oxalate and ascorbic acid) as shown in Fig. 7. In addition to the effect of hydrophobicity, the enhanced ECL intensity for NADH could be attributed to the strong p–p interaction between aromatic rings of NADH and ionic liquids [53]. The present selectivity test suggests that the selective detection for a variety of analytes could be realized through the proper tuning of the hydrophobicity of ionic liquid added in the sol–gel titania/Nafion composite film. 4. Conclusions
limit is lower than that obtained from the pure Nafion, sol–gel/Nafion, CNT/sol–gel ceramics/Nafion [18,19,22,23,34] and graphene/ Nafion [25] composite-modified ECL sensors. The present Ru(bpy)2+ 3 ECL sensor based on the ionic liquid/sol–gel titania/Nafion composite films showed good reproducibility and long-term storage stability in the determination of TPA. The relative standard deviation of the ECL intensity for 20 CV scans in 0.05 M phosphate buffer at pH 7.0 containing 500 lM TPA was 4.34%. The stability of the present Ru(bpy)2+ 3 ECL sensor was superior than that obtained from the pure Nafion, sol–gel/Nafion, and graphene/Nafion composite-modified ECL sensors, and comparable to the ECL sensor based on CNT/sol–gel titania/Nafion composite. Performance comparison of the ECL sensors based on different immobilization matrices is summarized in Table 1.
The incorporation of hydrophobic ionic liquid of 1-butyl-3methylimidazolium hexafluorophosphate (BMImPF6) within the sol–gel titania/Nafion composite films renders the compositemodified GC electrode surface more hydrophobic, which leads to not only the increased amount of Ru(bpy)2+ 3 immobilized on the ECL sensor but also the increased oxidation rate of a hydrophobic ECL co-reactant, tripropylamine (TPA). Due to the high conductivity of ionic liquid, the electron transfer resistance has been significantly decreased in the composite-modified GC electrode. Therefore, the present ECL sensor based on the BMImPF6/sol–gel titania/Nafion composite film showed improved ECL sensitivity, linear dynamic range, long-term stability and limit of detection for TPA compared to those obtained at the ECL sensors based on the other sol–gel ceramics/Nafion composite films and even carbon nanomaterials (e.g. CNT, graphene)/Nafion composite films. In addition, the present study implies that the selectivity of the ECL sensor can be tuned by proper tuning of the hydrophobicity of ionic liquid added in the sol–gel titania/Nafion composite film.
3.3. Selectivity
Conflict of interest
Fig. 7. Selectivity studies of the ECL sensor based on ionic liquids/sol–gel titania/ Nafion composite film for various analytes.
Ionic liquid is an important factor that determines the character of the composite films. Selectivity tests of the ECL sensor based on ionic liquid/sol–gel titania/Nafion composite were carried out with a number of different ionic liquids for various analytes. When hydrophilic analytes such as ascorbic acid and oxalate are used as co-reactants, the sequence of the resulting ECL intensities was EMIm+ > HMIm+ > BnMIm+ as shown in Fig. 7(a) and (b). The observed ECL sequence was the opposite to the sequence of the hydrophobicity of the ionic liquids used in the fabrication of the composite-based ECL sensor [56,57]. In contrast, the sequence of the observed ECL intensities for a hydrophobic analyte such as NADH was BnMIm+ > HMIm+ > EMIm+ as shown in Fig. 7(c), which accords closely with the sequence of the hydrophobicity of the ionic liquids used in the composite film. It is well known that the surface hydrophobicity of an electrode surface strongly affect the ECL
There is no conflict of interest. Acknowledgements Financial support for this work has been provided by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-R1A1A2006994). References [1] [2] [3] [4] [5]
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