- Email: [email protected]
Electrogenerated chemiluminescence sensing platform using Ru(bpy)32+ doped silica nanoparticles and carbon nanotubes
Electrochemistry Communications 8 (2006) 1687–1691 www.elsevier.com/locate/elecom
Electrogenerated chemiluminescence sensing platform using RuðbpyÞ2þ 3 doped silica nanoparticles and carbon nanotubes Lihua Zhang, Shaojun Dong
*
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, China Received 13 June 2006; received in revised form 14 July 2006; accepted 17 July 2006 Available online 28 August 2006
Abstract An effective electrogenerated chemiluminescence (ECL) sensor was developed by coimmobilization of the RuðbpyÞ2þ 3 -doped silica (RuDS) nanoparticles and carbon nanotubes (CNTs) on glassy carbon electrode through hydrophobic interaction. The uniform RuDS nanoparticles were prepared by a water-in-oil (W/O) microemulsion method and RuðbpyÞ2þ 3 doped inside could still maintain its high ECL efficiency. With such unique immobilization method, a great deal of RuðbpyÞ2þ was immobilized three-dimensionally on the elec3 trode, which could greatly enhance the ECL response and result in the increased sensitivity. On the other hand, CNTs played dual roles as matrix to immobilize RuDS nanoparticles and promoter to accelerate the electron transfer between RuðbpyÞ2þ 3 and the electrode. The as-prepared ECL sensor displayed good sensitivity and stability. 2006 Elsevier B.V. All rights reserved. Keywords: Electrogenerated chemiluminescence (ECL) sensor; RuðbpyÞ2þ 3 doped silica (RuDS) nanoparticles; Carbon nanotubes (CNTs)
1. Introduction Electrogenerated chemiluminescence (ECL) involves the generation of species at electrode surfaces and then undergo electron-transfer reactions to form excited states that emit light [1]. Combining the simplicity of electrochemistry with the inherent sensitivity of chemiluminescence, ECL proved to be a useful tool [2–5] and a remarkable detector [6–9] in various applications. Among many ECL reagents, RuðbpyÞ2þ is the most studied and 3 exploited one because of its stability and capability of undergoing ECL at room temperature in aqueous buffered solution [10]. Moreover, unlike some other ECL reagents which are consumed in the ECL reaction, 2þ RuðbpyÞ3 is regenerated during the ECL process [11]. Extensive efforts have been directed towards the RuðbpyÞ2þ immobilization methods so as to fabricate 3
*
Corresponding author. Tel.: +86 431 5262101; fax: +86 431 5689711. E-mail address: [email protected] (S. Dong).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.07.025
regenerative ECL sensors. For example, with Langmuir–Blodgett film or self-assembled monolayer methods, 2þ the derivatives of RuðbpyÞ3 have been immobilized on the electrode surface [12,13]. But such films are easily desorbed in organic solution or with positively biased 2þ potential. The RuðbpyÞ3 could also be derived to immobilize on the modified electrode surface by covalent bond. Although such method produces stable film, the process of derivation is quite complex and arduous [14,15]. The most accessible matrix to immobilize RuðbpyÞ2þ is Naf3 ion, a cation exchanger. Many new methods were developed by mixing Nafion with some other substances (such as sol–gel, CNTs) to improve its sensitivity and stability [16–18]. Though some of above methods could attain good results, one inherent limitation lies in all of these 2þ methods is the limited amount of RuðbpyÞ3 immobilized on the electrode. Recently, Tan’s group developed a new strategy for the 2þ preparation of RuðbpyÞ3 doped silica (RuDS) nanoparticles by microemulsion method [19]. They were extensively used as photostable biomarker in spectrofluorometric
1688
L. Zhang, S. Dong / Electrochemistry Communications 8 (2006) 1687–1691
measurements [20]. However, we found that as a typical 2þ ECL reagent, RuðbpyÞ3 could still retain their ECL property even after doping inside the silica nanoparticles, and the exterior nano-silica prevented the luminophor from leaching out into the aqueous solution due to the electro2þ static interaction. Since a large quantity of RuðbpyÞ3 molecules were immobilized with such unique method, the corresponding ECL response enhanced significantly. Therefore, we attempt to extend the application of RuDS nanoparticles from the spectrofluorimetry into the ECL field. Although the as-prepared RuDS nanoparticles have so many outstanding advantages when applied in the field of ECL, there still exists certain limitation. Since the 2þ luminophor RuðbpyÞ3 is coated with the nonconductive silica, the transfer of the electrons between electrode and RuðbpyÞ2þ 3 could be retarded. Carbon nanotubes (CNTs) represent a new class of nanomaterial, composed of graphite carbon with one or several concentric tubules. The unique properties of CNTs make them possess high mechanical strength, high chemical stability and high electrical conductivity, that extremely attractive for electrochemical application [21]. Therefore, the combination of the RuDS nanoparticles and CNTs could be a reasonable solution to the above-mentioned problem. Here, CNTs play dual significant roles in both the RuDS nanoparticles immobilization and electron transfer promotion. Firstly, CNTs could be a proper matrix to immobilize RuDS nanoparticles on the electrode by hydrophobic interaction. It is well known that CNTs modified electrode could be constructed simply by casting the CNTs suspension on the electrode surface. The strong hydrophobic interaction between CNTs and RuDS nanoparticles could make the RuDS nanoparticles coimmobilize with CNTs on the electrode firmly. Secondly, as the CNTs merit the high electrical conductivity and electron transfer accelerating capability, the addition could compensate for the non-conductivity of the silica to some extent. Moreover, the high surface-volume ratio of CNTs provides more electroactive area, which could lead to the enhanced response. Here, we fabricated a new kind of ECL sensor by combination of RuDS nanoparticles with CNTs. The RuDS– CNTs composite film was characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), cyclic voltammetry (CV) and ECL. Such sensor displays high sensitivity and good stability. 2. Experimental 2.1. Materials Tris(2,2 0 -bipyridyl)dichlororuthenium(II) hexahydrate (Ru(bpy)3Cl2 Æ 6H2O) and tripropylamine (TPA) were purchased from Aldrich, while Triton X-100 (TX-100) was purchased from Sigma. The multi-wall carbon nanotubes with 80% purity were purchased from Shenzhen Nanotech. Port. Co. Ltd. (China). It was purified by the pub-
lished procedure [22]. Tetraethylorthosilicate (TEOS) was obtained from Beijing Yili Chemical Reagent Factory (Beijing, China), n-hexanol, cyclohexane and ammonium hydroxide (25 wt%) were purchased from Beijing Chemical Reagent Factory (Beijing, China). All other chemicals were of analytical grade and the aqueous solutions were prepared with doubly distilled water. 2.2. Instrument Cyclic voltammetric experiments were performed using a CH Instruments 832 voltammetric analyzer. A three-electrode configuration was employed. A glassy carbon electrode coated with RuDS–CNTs composite film served as working electrode. A platinum wire was the counter electrode, and an Ag/AgCl (saturated KCl) worked as a reference electrode. All the potentials were measured and reported according to this reference electrode. The ECL signal produced in the electrolytic cell was detected and recorded by a flow injection chemiluminescence analyzer (IFFD, Xi’an Remax electronic Science Tech. Co. Ltd. China), and the photomultiplier tube (PMT) was operated in current mode. Unless noted, otherwise the PMT was biased at 800 V. The size of RuDS nanoparticles were measured using the JEOL 2010 transmission electron microscope (TEM) operated at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were determined with a Philips XL-30 ESEM. The accelerating voltage was 20 kV. For SEM imaging, the RuDS–CNTs composite film was dropped on the ITO electrode. 2.3. Preparation of the modified electrode The RuDS nanoparticles were prepared as previously described with a little modification [23]. The W/O microemulsion was prepared first by mixing 1.77 mL of TX100, 7.5 mL of cyclohexane, 1.8 mL of n-hexanol, and 340 lL of water. Then 0.1 M RuðbpyÞ2þ solution was 3 added into the mixture. In the presence of 100 lL of Tetraethylorthosilicate (TEOS), a polymerization reaction was initiated by adding 60 lL of NH4OH [24]. The reaction was allowed to stir for 24 h. After the reaction was completed, the RuDS nanoparticles were isolated by acetone, followed by centrifuging and washing with ethanol and water several times to remove any surfactant molecules. Finally, the required yellow RuDS nanoparticles were obtained. The CNTs were dispersed in N,N-dimethylformamide(DMF) with different concentrations. The experiments were performed with different CNTs concentrations in order to optimize the results. Then certain amount of RuDS nanoparticles was dissolved in the CNTs solution with the aid of ultrasonication. Finally a homogeneous solution was resulted. Five microlitres of RuDS–CNTs mixture was placed on a clean glassy carbon electrode and allowed to dry at room temperature. When not in
L. Zhang, S. Dong / Electrochemistry Communications 8 (2006) 1687–1691
1689
use, the modified electrode was kept in the dry state at room temperature. The experiments were carried out at room temperature about 20 C. 3. Results and discussion 3.1. Composite film characterization In a typical W/O microemulsion system, water droplets are stabilized by surfactant molecules and retain dispersed in bulk oil. The low polarity of the microemulsion water 2þ pool allows luminophore RuðbpyÞ3 particles to aggregate while the silica network is being formed. The RuDS nanoparticles prepared by this method are uniform in size, about 40 nm as characterized by TEM in the inset of Fig. 1. As seen from Fig. 1, the composite film is homogenous and porous. The uniformity of the film formed may be attributed to the strong interaction forces between them. The main driving force for film assembly was hydrophobic interaction between CNTs and RuDS nanoparticles.
Fig. 2. Cyclic voltammograms of RuðbpyÞ2þ 3 immobilized in RuDS–CNTs composite film electrode in the absence (dotted line) and presence (solid line) of 1.8 · 104 MTPA in PBS (pH 7.5) at the scan rate of 100 mV/s. Inset: ECL–potential curve for RuDS–CNTs composite film electrode in PBS (pH 7.5) containing 6 · 105 M TPA at the scan rate of 10 mV/s.
3.2. Electrochemistry and ECL behavior We investigated the cyclic voltammograms (CVs) changes before and after the presence of 1.8 · 104 M TPA in pH 7.5 phosphate buffer solution (PBS) with the 2þ scan rate of 100 mV/s, since RuðbpyÞ3 TPA system has been well studied and gave much higher ECL compared with other generally used reductants [25]. As demon2þ strated in the Fig. 2, the RuðbpyÞ3 oxidation current increases obviously while its reduction current decreases once the TPA is added. Since TPA molecule is quite small, it could permeate into the silica nanoparticle through the pores of the nanoparticles to react with the oxidized
RuðbpyÞ2þ 3 . It is noted that compared with our first attempt to immobilize RuDS nanoparticles in chitosan film [26], both the oxidation and reduction currents increase a lot using present CNTs immobilization method. This could be attributed to the superior properties of CNTs such as electron transfer promotion, electrocatalytic capability and high surface-to-area ratio with high electroactivity. Moreover, CNTs could act as electronic wire to connect the RuDS nanoparticles with the electrode. Although the silica is electronically nonconductive, the combination with CNTs could balance this disadvantage to a certain extent. 2þ The corresponding ECL behavior of RuðbpyÞ3 in RuDS–
Fig. 1. SEM of the RuDS–CNTs composite film. Inset: TEM image of RuDS nanoparticles.
1690
L. Zhang, S. Dong / Electrochemistry Communications 8 (2006) 1687–1691
CNTs composite film in the presence of 6 · 105 M TPA in PBS (pH 7.5) at the scan rate of 10 mV/s is illustrated in the inset of Fig. 2. The luminescence starts at about 0.9 V, and then rises steeply until it reaches a maximum near 1.15 V, which is coincident with the oxidation poten2þ tial of RuðbpyÞ3 . This means the oxidation of immobilized 2þ RuðbpyÞ3 plays an important role in the process of ECL. We investigated the effect of CNTs concentration on the ECL intensity with 1 mg/mL RuDS nanoparticles in the presence of 104 M TPA in PBS (pH 7.5) at the scan rate of 100 mV/s. As seen from Fig. 3, the ECL intensity increases accordingly with the increase of CNTs concentration at first, which may be attributed to the fact that more CNTs could immobilize more RuDS nanoparticles on the electrode by hydrophobic interaction. But when the CNTs concentration is higher than 1 mg/mL, ECL begins to fall. This could be explained as the increased CNTs amount might absorb and scatter the ECL emission within the films. Therefore, we choose 1 mg/mL CNTs in our experiment. We studied the effect of pH on the ECL response, since 2þ RuðbpyÞ3 TPA is a pH-dependent reaction. At first, the ECL intensity increases gradually with the increasing pH. But when pH becomes higher than 7.5, ECL begins to decrease. This is consistent with the literature [27]. The ECL signal increases from 6.0 to 7.5, implying that deprotonation of TPA is required during ECL process. While as the pH increases continuously, some decomposition of species would be expected, leading to a diminished ECL reagent available for ECL reaction. Therefore, the corresponding ECL intensity decreases. Besides, ECL also has certain relationship with the scan rate. The ECL intensity decreases with the increasing of scan rate in the range from 20 to 200 mV/s. It is reported that ECL signal changes with the scan rate depends on two factors: the chemical kinetics of the system and the rate of TPA diffusion in the film [18]. Because the ECL signal increases with the scan rate
decreasing, the chemical kinetics of the system plays a major role in this process.
Fig. 3. Effect of CNTs concentration on the ECL intensity in PBS (pH 7.5) containing 104 M TPA with the scan rate of 100 mV/s.
Fig. 4. Calibration of TPA at the RuDS–CNTs composite film electrode in PBS (pH 7.5) at the scan rate of 100 mV/s.
3.3. Sensitivity and stability Calibration curves for TPA have been constructed using the present ECL sensor based on the RuDS–CNTs composite films. Each point as shown in Fig. 4 is a mean of three ECL signals obtained by consecutive cyclic potential scans (100 mV/s) at a given concentration in PBS (pH 7.5). ECL intensity has good linearity with the TPA concentration ranging from 8.5 · 109 M to 7.9 · 104 M with a distinguished detection limit (S/N = 3) of 2.8 nM. Compared 2þ with the effective RuðbpyÞ3 preconcentration medium Nafion, the detection limit of RuDS–CNTs composite films is three orders of magnitude lower. The remarkable sensitivity could be attributed to the following two points: a 2þ large amount of RuðbpyÞ3 is three-dimensionally immobilized in the composite film which greatly increases ECL response; CNTs are highly electrically conductive and able to accelerate electron transfer efficiently. We investigated both the operational and storage stability of the ECL sensor prepared. When the potential scanning continuously for 10 cycles in PBS (pH 7.5) containing 8.8 · 105 M TPA with the scan rate of 100 mV/s (Fig. 5), there is almost no change concerning the ECL intensity. The long-term storage stability of the present sensor was studied over a 20-day period by monitoring its ECL response to 5 · 106 M TPA in PBS (pH 7.5) with intermittent detection (every 2–3 days) and store in the air at room temperature when not in use. The coating of the RuDS–CNTs composite film does not come off during this period, implying that the RuDS–CNTs composite film is well adhered to the glassy carbon electrode. It is observed that the response of this ECL sensor gradually decreases to almost 85% of its initial value after this 20-day period.
L. Zhang, S. Dong / Electrochemistry Communications 8 (2006) 1687–1691
1691
Acknowledgement This work was supported by National Natural Science Foundation of China (Nos. 20427003, 20575064). References
Fig. 5. ECL intensity of RuDS–CNTs composite film electrode in PBS (pH 7.5) containing 8.8 · 105 M TPA under continuous CVs for 10 cycles with the scan rate of 100 mV/s.
4. Conclusion 2þ
RuðbpyÞ3 is three-dimensionally immobilized in the silica nanoparticles with the microemulsion method and it still maintains its electrogenerated chemiluminescence property. Since a great deal of RuðbpyÞ2þ 3 is doped inside the silica nanoparticles, the corresponding ECL response is enhanced a lot. By coimmobilization of the CNTs and RuDS nanoparticles on glassy carbon electrode through hydrophobic interaction, we constructed an effective ECL sensor to detect TPA. The calibration curve for TPA is linear in the concentration range from 8.5 · 109 M to 7.9 · 104 M, and the as-prepared ECL sensor could maintain 85% of its initial ECL response after 20 days. Furthermore, both the silica and CNTs are biocompatible and the silica nanoparticles are easily modified with functional groups, so such ECL sensor has great potential in the application of bioanalysis.
[1] M.M. Richter, Chem. Rev. 104 (2004) 3003. [2] A.W. Knight, Trends Anal. Chem. 18 (1999) 47. [3] R.C. Engstrom, K.W. Johnson, S. Desjarlais, Anal. Chem. 59 (1987) 670. [4] V.G. Kozlov, P.E. Burrows, S.R. Forrest, Nature 389 (1977) 362. [5] F.R. Fan, D. Cliffel, A.J. Bard, Anal. Chem. 70 (1998) 2941. [6] Y. Chi, J. Duan, S. Lin, G. Chen, Anal. Chem. 78 (2006) 1568. [7] J.B. Noffsinger, N.D. Danielson, J. Chromatogr. 387 (1987) 520. [8] J.F. Liu, W.D. Cao, H.B. Qiu, X.H. Sun, X.R. Yang, E.K. Wang, Clinic. Chem. 48 (2002) 1049. [9] Y. Du, H. Wei, J. Kang, J. Yan, X. Yin, X.R. Yang, E.K. Wang, Anal. Chem. 77 (2005) 7993. [10] X.B. Yin, S.J. Dong, E.K. Wang, Trends Anal. Chem. 23 (2004) 432. [11] I. Rubinstein, A.J. Bard, J. Am. Chem. Soc. 103 (1981) 512. [12] S.J. Miller, P. McCord, A.J. Bard, Langmuir 7 (1991) 2781. [13] Y.S. Obeng, A.J. Bard, Langmuir 7 (1991) 195. [14] J.K. Lee, S.H. Lee, M. Kim, H. Kim, D.H. Kim, W.Y. Lee, Chem. Commun. 13 (2003) 1602. [15] G.M. Greenway, A. Greenwood, P. Watts, C. Wiles, Chem. Commun. 1 (2006) 85. [16] A.N. Khramov, M.M. Collinson, Anal. Chem. 72 (2000) 2943. [17] H.N. Choi, S.H. Cho, W.Y. Lee, Anal. Chem. 75 (2003) 4250. [18] Z.H. Guo, S.J. Dong, Anal. Chem. 76 (2004) 2683. [19] R.P. Bagwe, C.Y. Yang, L.R. Hilliard, W.H. Tan, Langmuir 20 (2004) 8336. [20] S. Santra, P. Zhang, K. Wang, R. Tapec, W.H. Tan, Anal. Chem. 73 (2001) 4988. [21] P.M. Ajayan, Chem. Rev. 99 (1999) 1787. [22] Z. Shi, Y. Lian, X. Zhou, Z. Gu, Y. Zhang, S. Iijima, H. Li, K.T. Yue, S.L. Zhang, J. Phys. Chem. B 103 (1999) 8698. [23] L. Wang, C.Y. Yang, W.H. Tan, Nanoletters 5 (2005) 37. [24] W. Stober, A. Fink, E. Bohn, J. Colloid Interf. Sci. 26 (1968) 62. [25] W.Y. Lee, T.A. Nieman, Anal. Chem. 67 (1995) 1789. [26] L.H. Zhang, S.J. Dong, Anal. Chem. 78 (2006) 5119. [27] Z.H. Guo, Y. Shen, F. Zhao, M.K. Wang, S.J. Dong, Analyst 129 (2004) 657.
Electrogenerated chemiluminescence sensing platform using RuðbpyÞ2þ 3 doped silica nanoparticles and carbon nanotubes Lihua Zhang, Shaojun Dong
*
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, China Received 13 June 2006; received in revised form 14 July 2006; accepted 17 July 2006 Available online 28 August 2006
Abstract An effective electrogenerated chemiluminescence (ECL) sensor was developed by coimmobilization of the RuðbpyÞ2þ 3 -doped silica (RuDS) nanoparticles and carbon nanotubes (CNTs) on glassy carbon electrode through hydrophobic interaction. The uniform RuDS nanoparticles were prepared by a water-in-oil (W/O) microemulsion method and RuðbpyÞ2þ 3 doped inside could still maintain its high ECL efficiency. With such unique immobilization method, a great deal of RuðbpyÞ2þ was immobilized three-dimensionally on the elec3 trode, which could greatly enhance the ECL response and result in the increased sensitivity. On the other hand, CNTs played dual roles as matrix to immobilize RuDS nanoparticles and promoter to accelerate the electron transfer between RuðbpyÞ2þ 3 and the electrode. The as-prepared ECL sensor displayed good sensitivity and stability. 2006 Elsevier B.V. All rights reserved. Keywords: Electrogenerated chemiluminescence (ECL) sensor; RuðbpyÞ2þ 3 doped silica (RuDS) nanoparticles; Carbon nanotubes (CNTs)
1. Introduction Electrogenerated chemiluminescence (ECL) involves the generation of species at electrode surfaces and then undergo electron-transfer reactions to form excited states that emit light [1]. Combining the simplicity of electrochemistry with the inherent sensitivity of chemiluminescence, ECL proved to be a useful tool [2–5] and a remarkable detector [6–9] in various applications. Among many ECL reagents, RuðbpyÞ2þ is the most studied and 3 exploited one because of its stability and capability of undergoing ECL at room temperature in aqueous buffered solution [10]. Moreover, unlike some other ECL reagents which are consumed in the ECL reaction, 2þ RuðbpyÞ3 is regenerated during the ECL process [11]. Extensive efforts have been directed towards the RuðbpyÞ2þ immobilization methods so as to fabricate 3
*
Corresponding author. Tel.: +86 431 5262101; fax: +86 431 5689711. E-mail address: [email protected] (S. Dong).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.07.025
regenerative ECL sensors. For example, with Langmuir–Blodgett film or self-assembled monolayer methods, 2þ the derivatives of RuðbpyÞ3 have been immobilized on the electrode surface [12,13]. But such films are easily desorbed in organic solution or with positively biased 2þ potential. The RuðbpyÞ3 could also be derived to immobilize on the modified electrode surface by covalent bond. Although such method produces stable film, the process of derivation is quite complex and arduous [14,15]. The most accessible matrix to immobilize RuðbpyÞ2þ is Naf3 ion, a cation exchanger. Many new methods were developed by mixing Nafion with some other substances (such as sol–gel, CNTs) to improve its sensitivity and stability [16–18]. Though some of above methods could attain good results, one inherent limitation lies in all of these 2þ methods is the limited amount of RuðbpyÞ3 immobilized on the electrode. Recently, Tan’s group developed a new strategy for the 2þ preparation of RuðbpyÞ3 doped silica (RuDS) nanoparticles by microemulsion method [19]. They were extensively used as photostable biomarker in spectrofluorometric
1688
L. Zhang, S. Dong / Electrochemistry Communications 8 (2006) 1687–1691
measurements [20]. However, we found that as a typical 2þ ECL reagent, RuðbpyÞ3 could still retain their ECL property even after doping inside the silica nanoparticles, and the exterior nano-silica prevented the luminophor from leaching out into the aqueous solution due to the electro2þ static interaction. Since a large quantity of RuðbpyÞ3 molecules were immobilized with such unique method, the corresponding ECL response enhanced significantly. Therefore, we attempt to extend the application of RuDS nanoparticles from the spectrofluorimetry into the ECL field. Although the as-prepared RuDS nanoparticles have so many outstanding advantages when applied in the field of ECL, there still exists certain limitation. Since the 2þ luminophor RuðbpyÞ3 is coated with the nonconductive silica, the transfer of the electrons between electrode and RuðbpyÞ2þ 3 could be retarded. Carbon nanotubes (CNTs) represent a new class of nanomaterial, composed of graphite carbon with one or several concentric tubules. The unique properties of CNTs make them possess high mechanical strength, high chemical stability and high electrical conductivity, that extremely attractive for electrochemical application [21]. Therefore, the combination of the RuDS nanoparticles and CNTs could be a reasonable solution to the above-mentioned problem. Here, CNTs play dual significant roles in both the RuDS nanoparticles immobilization and electron transfer promotion. Firstly, CNTs could be a proper matrix to immobilize RuDS nanoparticles on the electrode by hydrophobic interaction. It is well known that CNTs modified electrode could be constructed simply by casting the CNTs suspension on the electrode surface. The strong hydrophobic interaction between CNTs and RuDS nanoparticles could make the RuDS nanoparticles coimmobilize with CNTs on the electrode firmly. Secondly, as the CNTs merit the high electrical conductivity and electron transfer accelerating capability, the addition could compensate for the non-conductivity of the silica to some extent. Moreover, the high surface-volume ratio of CNTs provides more electroactive area, which could lead to the enhanced response. Here, we fabricated a new kind of ECL sensor by combination of RuDS nanoparticles with CNTs. The RuDS– CNTs composite film was characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), cyclic voltammetry (CV) and ECL. Such sensor displays high sensitivity and good stability. 2. Experimental 2.1. Materials Tris(2,2 0 -bipyridyl)dichlororuthenium(II) hexahydrate (Ru(bpy)3Cl2 Æ 6H2O) and tripropylamine (TPA) were purchased from Aldrich, while Triton X-100 (TX-100) was purchased from Sigma. The multi-wall carbon nanotubes with 80% purity were purchased from Shenzhen Nanotech. Port. Co. Ltd. (China). It was purified by the pub-
lished procedure [22]. Tetraethylorthosilicate (TEOS) was obtained from Beijing Yili Chemical Reagent Factory (Beijing, China), n-hexanol, cyclohexane and ammonium hydroxide (25 wt%) were purchased from Beijing Chemical Reagent Factory (Beijing, China). All other chemicals were of analytical grade and the aqueous solutions were prepared with doubly distilled water. 2.2. Instrument Cyclic voltammetric experiments were performed using a CH Instruments 832 voltammetric analyzer. A three-electrode configuration was employed. A glassy carbon electrode coated with RuDS–CNTs composite film served as working electrode. A platinum wire was the counter electrode, and an Ag/AgCl (saturated KCl) worked as a reference electrode. All the potentials were measured and reported according to this reference electrode. The ECL signal produced in the electrolytic cell was detected and recorded by a flow injection chemiluminescence analyzer (IFFD, Xi’an Remax electronic Science Tech. Co. Ltd. China), and the photomultiplier tube (PMT) was operated in current mode. Unless noted, otherwise the PMT was biased at 800 V. The size of RuDS nanoparticles were measured using the JEOL 2010 transmission electron microscope (TEM) operated at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were determined with a Philips XL-30 ESEM. The accelerating voltage was 20 kV. For SEM imaging, the RuDS–CNTs composite film was dropped on the ITO electrode. 2.3. Preparation of the modified electrode The RuDS nanoparticles were prepared as previously described with a little modification [23]. The W/O microemulsion was prepared first by mixing 1.77 mL of TX100, 7.5 mL of cyclohexane, 1.8 mL of n-hexanol, and 340 lL of water. Then 0.1 M RuðbpyÞ2þ solution was 3 added into the mixture. In the presence of 100 lL of Tetraethylorthosilicate (TEOS), a polymerization reaction was initiated by adding 60 lL of NH4OH [24]. The reaction was allowed to stir for 24 h. After the reaction was completed, the RuDS nanoparticles were isolated by acetone, followed by centrifuging and washing with ethanol and water several times to remove any surfactant molecules. Finally, the required yellow RuDS nanoparticles were obtained. The CNTs were dispersed in N,N-dimethylformamide(DMF) with different concentrations. The experiments were performed with different CNTs concentrations in order to optimize the results. Then certain amount of RuDS nanoparticles was dissolved in the CNTs solution with the aid of ultrasonication. Finally a homogeneous solution was resulted. Five microlitres of RuDS–CNTs mixture was placed on a clean glassy carbon electrode and allowed to dry at room temperature. When not in
L. Zhang, S. Dong / Electrochemistry Communications 8 (2006) 1687–1691
1689
use, the modified electrode was kept in the dry state at room temperature. The experiments were carried out at room temperature about 20 C. 3. Results and discussion 3.1. Composite film characterization In a typical W/O microemulsion system, water droplets are stabilized by surfactant molecules and retain dispersed in bulk oil. The low polarity of the microemulsion water 2þ pool allows luminophore RuðbpyÞ3 particles to aggregate while the silica network is being formed. The RuDS nanoparticles prepared by this method are uniform in size, about 40 nm as characterized by TEM in the inset of Fig. 1. As seen from Fig. 1, the composite film is homogenous and porous. The uniformity of the film formed may be attributed to the strong interaction forces between them. The main driving force for film assembly was hydrophobic interaction between CNTs and RuDS nanoparticles.
Fig. 2. Cyclic voltammograms of RuðbpyÞ2þ 3 immobilized in RuDS–CNTs composite film electrode in the absence (dotted line) and presence (solid line) of 1.8 · 104 MTPA in PBS (pH 7.5) at the scan rate of 100 mV/s. Inset: ECL–potential curve for RuDS–CNTs composite film electrode in PBS (pH 7.5) containing 6 · 105 M TPA at the scan rate of 10 mV/s.
3.2. Electrochemistry and ECL behavior We investigated the cyclic voltammograms (CVs) changes before and after the presence of 1.8 · 104 M TPA in pH 7.5 phosphate buffer solution (PBS) with the 2þ scan rate of 100 mV/s, since RuðbpyÞ3 TPA system has been well studied and gave much higher ECL compared with other generally used reductants [25]. As demon2þ strated in the Fig. 2, the RuðbpyÞ3 oxidation current increases obviously while its reduction current decreases once the TPA is added. Since TPA molecule is quite small, it could permeate into the silica nanoparticle through the pores of the nanoparticles to react with the oxidized
RuðbpyÞ2þ 3 . It is noted that compared with our first attempt to immobilize RuDS nanoparticles in chitosan film [26], both the oxidation and reduction currents increase a lot using present CNTs immobilization method. This could be attributed to the superior properties of CNTs such as electron transfer promotion, electrocatalytic capability and high surface-to-area ratio with high electroactivity. Moreover, CNTs could act as electronic wire to connect the RuDS nanoparticles with the electrode. Although the silica is electronically nonconductive, the combination with CNTs could balance this disadvantage to a certain extent. 2þ The corresponding ECL behavior of RuðbpyÞ3 in RuDS–
Fig. 1. SEM of the RuDS–CNTs composite film. Inset: TEM image of RuDS nanoparticles.
1690
L. Zhang, S. Dong / Electrochemistry Communications 8 (2006) 1687–1691
CNTs composite film in the presence of 6 · 105 M TPA in PBS (pH 7.5) at the scan rate of 10 mV/s is illustrated in the inset of Fig. 2. The luminescence starts at about 0.9 V, and then rises steeply until it reaches a maximum near 1.15 V, which is coincident with the oxidation poten2þ tial of RuðbpyÞ3 . This means the oxidation of immobilized 2þ RuðbpyÞ3 plays an important role in the process of ECL. We investigated the effect of CNTs concentration on the ECL intensity with 1 mg/mL RuDS nanoparticles in the presence of 104 M TPA in PBS (pH 7.5) at the scan rate of 100 mV/s. As seen from Fig. 3, the ECL intensity increases accordingly with the increase of CNTs concentration at first, which may be attributed to the fact that more CNTs could immobilize more RuDS nanoparticles on the electrode by hydrophobic interaction. But when the CNTs concentration is higher than 1 mg/mL, ECL begins to fall. This could be explained as the increased CNTs amount might absorb and scatter the ECL emission within the films. Therefore, we choose 1 mg/mL CNTs in our experiment. We studied the effect of pH on the ECL response, since 2þ RuðbpyÞ3 TPA is a pH-dependent reaction. At first, the ECL intensity increases gradually with the increasing pH. But when pH becomes higher than 7.5, ECL begins to decrease. This is consistent with the literature [27]. The ECL signal increases from 6.0 to 7.5, implying that deprotonation of TPA is required during ECL process. While as the pH increases continuously, some decomposition of species would be expected, leading to a diminished ECL reagent available for ECL reaction. Therefore, the corresponding ECL intensity decreases. Besides, ECL also has certain relationship with the scan rate. The ECL intensity decreases with the increasing of scan rate in the range from 20 to 200 mV/s. It is reported that ECL signal changes with the scan rate depends on two factors: the chemical kinetics of the system and the rate of TPA diffusion in the film [18]. Because the ECL signal increases with the scan rate
decreasing, the chemical kinetics of the system plays a major role in this process.
Fig. 3. Effect of CNTs concentration on the ECL intensity in PBS (pH 7.5) containing 104 M TPA with the scan rate of 100 mV/s.
Fig. 4. Calibration of TPA at the RuDS–CNTs composite film electrode in PBS (pH 7.5) at the scan rate of 100 mV/s.
3.3. Sensitivity and stability Calibration curves for TPA have been constructed using the present ECL sensor based on the RuDS–CNTs composite films. Each point as shown in Fig. 4 is a mean of three ECL signals obtained by consecutive cyclic potential scans (100 mV/s) at a given concentration in PBS (pH 7.5). ECL intensity has good linearity with the TPA concentration ranging from 8.5 · 109 M to 7.9 · 104 M with a distinguished detection limit (S/N = 3) of 2.8 nM. Compared 2þ with the effective RuðbpyÞ3 preconcentration medium Nafion, the detection limit of RuDS–CNTs composite films is three orders of magnitude lower. The remarkable sensitivity could be attributed to the following two points: a 2þ large amount of RuðbpyÞ3 is three-dimensionally immobilized in the composite film which greatly increases ECL response; CNTs are highly electrically conductive and able to accelerate electron transfer efficiently. We investigated both the operational and storage stability of the ECL sensor prepared. When the potential scanning continuously for 10 cycles in PBS (pH 7.5) containing 8.8 · 105 M TPA with the scan rate of 100 mV/s (Fig. 5), there is almost no change concerning the ECL intensity. The long-term storage stability of the present sensor was studied over a 20-day period by monitoring its ECL response to 5 · 106 M TPA in PBS (pH 7.5) with intermittent detection (every 2–3 days) and store in the air at room temperature when not in use. The coating of the RuDS–CNTs composite film does not come off during this period, implying that the RuDS–CNTs composite film is well adhered to the glassy carbon electrode. It is observed that the response of this ECL sensor gradually decreases to almost 85% of its initial value after this 20-day period.
L. Zhang, S. Dong / Electrochemistry Communications 8 (2006) 1687–1691
1691
Acknowledgement This work was supported by National Natural Science Foundation of China (Nos. 20427003, 20575064). References
Fig. 5. ECL intensity of RuDS–CNTs composite film electrode in PBS (pH 7.5) containing 8.8 · 105 M TPA under continuous CVs for 10 cycles with the scan rate of 100 mV/s.
4. Conclusion 2þ
RuðbpyÞ3 is three-dimensionally immobilized in the silica nanoparticles with the microemulsion method and it still maintains its electrogenerated chemiluminescence property. Since a great deal of RuðbpyÞ2þ 3 is doped inside the silica nanoparticles, the corresponding ECL response is enhanced a lot. By coimmobilization of the CNTs and RuDS nanoparticles on glassy carbon electrode through hydrophobic interaction, we constructed an effective ECL sensor to detect TPA. The calibration curve for TPA is linear in the concentration range from 8.5 · 109 M to 7.9 · 104 M, and the as-prepared ECL sensor could maintain 85% of its initial ECL response after 20 days. Furthermore, both the silica and CNTs are biocompatible and the silica nanoparticles are easily modified with functional groups, so such ECL sensor has great potential in the application of bioanalysis.
[1] M.M. Richter, Chem. Rev. 104 (2004) 3003. [2] A.W. Knight, Trends Anal. Chem. 18 (1999) 47. [3] R.C. Engstrom, K.W. Johnson, S. Desjarlais, Anal. Chem. 59 (1987) 670. [4] V.G. Kozlov, P.E. Burrows, S.R. Forrest, Nature 389 (1977) 362. [5] F.R. Fan, D. Cliffel, A.J. Bard, Anal. Chem. 70 (1998) 2941. [6] Y. Chi, J. Duan, S. Lin, G. Chen, Anal. Chem. 78 (2006) 1568. [7] J.B. Noffsinger, N.D. Danielson, J. Chromatogr. 387 (1987) 520. [8] J.F. Liu, W.D. Cao, H.B. Qiu, X.H. Sun, X.R. Yang, E.K. Wang, Clinic. Chem. 48 (2002) 1049. [9] Y. Du, H. Wei, J. Kang, J. Yan, X. Yin, X.R. Yang, E.K. Wang, Anal. Chem. 77 (2005) 7993. [10] X.B. Yin, S.J. Dong, E.K. Wang, Trends Anal. Chem. 23 (2004) 432. [11] I. Rubinstein, A.J. Bard, J. Am. Chem. Soc. 103 (1981) 512. [12] S.J. Miller, P. McCord, A.J. Bard, Langmuir 7 (1991) 2781. [13] Y.S. Obeng, A.J. Bard, Langmuir 7 (1991) 195. [14] J.K. Lee, S.H. Lee, M. Kim, H. Kim, D.H. Kim, W.Y. Lee, Chem. Commun. 13 (2003) 1602. [15] G.M. Greenway, A. Greenwood, P. Watts, C. Wiles, Chem. Commun. 1 (2006) 85. [16] A.N. Khramov, M.M. Collinson, Anal. Chem. 72 (2000) 2943. [17] H.N. Choi, S.H. Cho, W.Y. Lee, Anal. Chem. 75 (2003) 4250. [18] Z.H. Guo, S.J. Dong, Anal. Chem. 76 (2004) 2683. [19] R.P. Bagwe, C.Y. Yang, L.R. Hilliard, W.H. Tan, Langmuir 20 (2004) 8336. [20] S. Santra, P. Zhang, K. Wang, R. Tapec, W.H. Tan, Anal. Chem. 73 (2001) 4988. [21] P.M. Ajayan, Chem. Rev. 99 (1999) 1787. [22] Z. Shi, Y. Lian, X. Zhou, Z. Gu, Y. Zhang, S. Iijima, H. Li, K.T. Yue, S.L. Zhang, J. Phys. Chem. B 103 (1999) 8698. [23] L. Wang, C.Y. Yang, W.H. Tan, Nanoletters 5 (2005) 37. [24] W. Stober, A. Fink, E. Bohn, J. Colloid Interf. Sci. 26 (1968) 62. [25] W.Y. Lee, T.A. Nieman, Anal. Chem. 67 (1995) 1789. [26] L.H. Zhang, S.J. Dong, Anal. Chem. 78 (2006) 5119. [27] Z.H. Guo, Y. Shen, F. Zhao, M.K. Wang, S.J. Dong, Analyst 129 (2004) 657.