Article pubs.acs.org/ac
Modification of Indium Tin Oxide with Dendrimer-Encapsulated Nanoparticles To Provide Enhanced Stable Electrochemiluminescence of Ru(bpy)32+/Tripropylamine While Preserving Optical Transparency of Indium Tin Oxide for Sensitive Electrochemiluminescence-Based Analyses Yeoju Kim and Joohoon Kim* Department of Chemistry, Research Institute for Basic Sciences, Kyung Hee University, 1 Hoegi-dong, Seoul 130-701, Republic of Korea S Supporting Information *
ABSTRACT: Here, we report highly enhanced stable electrogenerated chemiluminescence (ECL) of Ru(bpy)32+ (bpy = 2,2′bipyridyl) with tripropylamine (TPrA) coreactant on indium tin oxide (ITO) electrodes modified with amine-terminated dendrimers encapsulating catalytic nanoparticles while maintaining optical transparency of ITO and feasibility of the modified ITOs to sensitive ECL-based assays. As model systems, we prepared Pt and Au dendrimer-encapsulated nanoparticles (DENs) using amine-terminated sixth-generation poly(amido amine) dendrimers and subsequently immobilized the DENs onto ITO surfaces via electrooxidative grafting of the terminal amines of dendrimers to the surfaces. The resulting DENmodified ITOs preserved good optical transparency of ITO and exhibited highly catalyzed electrochemical oxidation of Ru(bpy)32+/TPrA, leading to significantly increased ECL emission. Especially, the Pt DEN-modified ITO electrode provides negligible transmittance drop, i.e., only ∼1.99% over the entire visible region, and exhibited not only much enhanced (i.e., ∼213-fold increase compared to ECL obtained from bare ITO) but also stable ECL emission under consecutive potential scans from 0.00 to 1.10 V for 10 cycles, which allowed ∼329 times more sensitive ECL-based analysis of nicotine using the Pt DEN-modified ITO compared with the use of bare ITO.
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low background and high sensitivity, good temporal and spatial controllability, robustness, versatility, and low cost.1−4 Thus, the ECL technique has been widely used in a variety of analytical applications such as immunoassays, DNA analyses, molecular diagnosis of clinically important compounds (e.g., steroid hormones, thyrotropin, digoxin, etc.), environmental assays for food and water testing, and detection of chemical/ biological warfare agents.5−8 The Ru(bpy)32+/TPrA system is one of the most frequently used ECL systems in analytical applications due to its good photochemical stability, high ECL quantum yield, and good water solubility.9,10 The use of an ITO electrode in the ECL technique is also attractive in the analytical applications because of its high optical transparency and good electrical conductivity.11,12 However, it has been reported that the ITO electrode exhibited sluggish kinetics for electrochemical reactions of many organic compounds including TPrA, leading to low ECL sensitivity when using
n this paper, we report the electrochemical modification of indium tin oxide (ITO) electrodes with dendrimerencapsulated nanoparticles (DENs), leading to highly enhanced stable electrogenerated chemiluminescence (ECL) of Ru(bpy)32+ (bpy = 2,2′-bipyridyl) with tripropylamine (TPrA) coreactant on the resulting modified ITOs, and the preliminary analytical application of the DEN-modified ITOs in the sensitive determination of nicotine. This suggests that the DEN-modified ITO electrodes can be promising analytical platforms for the sensitive ECL-based analysis. As model systems, we electrochemically immobilized two different types of DENs, Pt and Au DENs, on ITO electrodes and evaluated the ECL activities of the DEN-modified ITOs for the Ru(bpy)32+/TPrA system (Scheme 1). This is a significant study because it suggests a simple and versatile approach to modify ITO surfaces with various catalytic nanoparticles encapsulated inside dendrimers, which provides tunable properties of ITO electrodes for ECL-based analyses. The ECL technique involves electrogenerated species undergoing electron-transfer reactions to form excited states that emit light in the vicinity of electrode surfaces and has unique advantages over photoluminescence techniques, such as © 2014 American Chemical Society
Received: October 21, 2013 Accepted: January 8, 2014 Published: January 8, 2014 1654
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Scheme 1
the transparent ITO.13,14 Because of the slow kinetics of the electron-transfer process at ITO surfaces, the ITO electrodes have often been modified with various nanoparticles such as Au, Pt, SiO2, clay, and ITO nanoparticles for the facilitation of the electron-transfer process.15−19 The surface modification of ITO electrodes with nanoparticles also resulted in the increase of the surface area of electrodes leading to the enhancement of ECL signals.20 Several surface modification strategies have thus been applied to immobilize nanoparticles onto ITO surfaces. For example, while direct deposition of nanoparticles on ITO substrates has been achieved by electrodeposition or seed-mediated growth approaches,21−24 it has been shown that precise control in the composition and size of nanoparticles is challenging.15 Nanoparticles have been also immobilized onto ITO surfaces by chemical and physical methods, such as thiol (or amine)− gold binding and layer-by-layer (LbL) assembly techniques, which require intermediate adhesive layers on the surfaces.15,25−27 It is notable that the most frequent reports have concerned Au nanoparticle-modified ITO electrodes mainly because of the limited chemistry available to link nanoparticles to electrode surfaces. However, Au nanoparticle-modified electrodes have often been reported to show poor stability at high potentials due to the oxidation of thiol layers or the oxidation of Au nanoparticles.20,28 Recently, our group reported an electrochemical modification of ITO surfaces with amineterminated poly(amido amine) (PAMAM) dendrimers encapsulating nanoparticles.29 Since a variety of nanoparticles (for example, monometallic nanoparticles such as Pt, Au, Pd, Ni, Fe, and Cu and bimetallic alloy or core/shell nanoparticles30,31) can be encapsulated inside the dendrimers, the direct electrochemical method allows a unique and versatile means to decorate the ITO surfaces with various nanoparticles. In an effort to expand the scope of our preliminary study of the electrochemical decoration of ITO surfaces with DENs, we report here the enhanced stable ECL of the Ru(bpy)32+/TPrA system on DENs-modified ITO electrodes with well-preserved transparency and its feasibility to sensitive ECL-based assays. Specifically, we synthesized Pt DENs (diameter 1.94 ± 0.26 nm) and Au DENs (diameter 1.91 ± 0.26 nm) using amineterminated sixth-generation poly(amido amine) dendrimers (G6-NH2 PAMAM dendrimers) and subsequently immobilized the nanocomposites onto ITO electrodes via their electrooxidative coupling to ITO surfaces as we previously reported.29 The resulting DEN-modified ITOs exhibited highly improved catalytic activity for the electrochemical oxidation of Ru(bpy)32+/TPrA while preserving the good optical transparency of ITO electrodes, leading to significantly increased ECL emission. Especially, Pt DEN-modified ITO electrode exhibited not only much enhanced, i.e., ∼213-fold increase compared to
ECL obtained from bare ITO, but also stable ECL emission under consecutive potential scans from 0.00 to 1.10 V for 10 cycles, which allowed sensitive ECL-based analysis of nicotine using the Pt DEN-modified ITO.
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EXPERIMENTAL SECTION Chemicals and Materials. Amine-terminated sixth-generation PAMAM (G6-NH2 PAMAM) dendrimers, HAuCl4· 3H2O, K2PtCl4, NaBH4, LiClO4, Ru(bpy)3Cl2·6H2O, tri-npropylamine, (−)-nicotine, phosphate-buffered saline (0.15 M PBS containing 2.07 M NaCl and 40.5 mM KCl), and cellulose dialysis sacks (MW cutoff of 12 000) were purchased from Sigma-Aldrich, Inc. (U.S.A.). HCl, H2SO4, ethanol, and acetone were obtained from Daejung, Inc. (Korea). NaOH was purchased from Duksan, Inc. (Korea). ITO-coated glass slides were received from Delta Technologies (U.S.A.). Deionized (DI) 18 MΩ·cm water was used in the preparation of aqueous solutions (Ultra370, Younglin Co., Korea). Synthesis of DENs. Pt and Au DENs were synthesized similarly to that previously reported.32−36 Briefly, 200 mol equiv of an aqueous 200 mM K2PtCl4 or 10 mM HAuCl4 was added to an aqueous 10 μM G6-NH2 PAMAM dendrimer solution. The mixture solutions were stirred for complete complexation between the metal ions and the interior amines of the dendrimers. Especially, the mixture (pH 5) of Pt ions and dendrimers was stirred for 76 h to ensure the complexation of the Pt ions to the intradendrimer tertiary amines. A stoichiometric excess of aqueous NaBH4 was then added to the complex solution under vigorous stirring. Specifically, a 20fold excess of NaBH4 was added to the Pt ion−dendrimer complex solution. We kept the mixture solution (pH 7−8) in a closed vial for 24 h to ensure complete reduction of the complexed Pt ions. Similarly, a 5-fold excess of NaBH4 in 0.3 M NaOH was added to the Au ion−dendrimer complex solution, which resulted in the reduction of the complexed Au ions to zero-valent Au nanoparticles inside the dendrimers. Finally, the synthesized DEN solutions were dialyzed for 24 h using cellulose dialysis sacks to remove impurities. Modification of ITO Electrodes with DENs. The electrochemical grafting of DENs on ITO electrodes was carried out using our previously reported method.29 In brief, an ITO electrode was ultrasonically cleaned with acetone, ethanol, and water subsequently and dried under a stream of N2. The ITO electrode was cleaned further in a plasma cleaner/sterilizer (PDC-32G, Harrick Scientific, U.S.A.) at medium power for 2 min. Immediately after the plasma treatment, the electrode (area: 0.096 cm2) was exposed to an aqueous 10 μM DEN solution containing 0.1 M LiClO4 and the potential of the electrode was cycled three times between 1.20 and 1.75 V (vs Ag/AgCl) at a scan rate of 10 mV·s−1. After the electrooxidative 1655
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grafting of DENs, the modified ITO was rinsed with DI water, ultrasonicated in DI water thoroughly for 10 min, and then blown until dry. Instrumentation and Measurements. Transmission electron microscope (TEM) images were obtained using a Tecnai G2 F30 ST instrument (FEI Co., U.S.A.) operating at 300 kV. TEM samples were prepared by dropping aqueous DEN solutions on 200 mesh carbon-coated copper grids (Ted Pella Inc., U.S.A.) followed by drying in air. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a PHI 5800 spectrometer (Physical Electronics Inc., U.S.A.) using Al Kα radiation (hν = 1486.6 eV). UV−vis transmission spectra of ITO slides were recorded on an Agilent 8453 UV−vis spectrometer (Agilent Tech., U.S.A.) using a cell holder (model CH/2049, Starna Scientific Ltd., U.K.). All electrochemical experiments were carried out with a model 440 potentiostat (CH Instruments, U.S.A.) using a conventional three-electrode cell. A Pt wire and a Ag/AgCl (3 M Cl−) electrode were used as a counter and a reference electrode, respectively. The three-electrode cell was connected to the slit of a monochromator (Acton Standard SP2150, Princeton Instruments, U.S.A.) equipped with a charge-coupled device camera (PIXIS 100B, Princeton Instruments, U.S.A.) for ECL measurements. Prior to the ECL measurements, the electrode potential of DEN-modified ITO was subjected to be cycled 10 times in 0.5 M H2SO4 between −0.20 and 1.20 V, and 0.10 and 1.55 V (vs Ag/AgCl) for Pt DEN-modified and Au DENmodified ITO, respectively. Pt DEN-modified ITOs were also subjected to repetitive potential cycling in a PBS solution (pH 7) containing 100 μM Ru(bpy)32+ and 100 mM TPrA until stable ECL emission was obtained. We found that the ECL emission increased and reached a stable maximum value upon the repetitive cycles of the electrode potential on Pt DENmodified ITOs between 0.00 and 1.70 V (vs Ag/AgCl) (Supporting Information, Figure S1).
Figure 1. (a and b) TEM images of as-synthesized Pt and Au DENs, respectively. (c and d) Particle size distribution histograms of assynthesized Pt and Au DENs, respectively.
The Pt DEN-modified ITO exhibited platinum-characteristic redox waves, i.e., hydrogen adsorption/desorption, Pt surface oxidation, and the corresponding oxide reduction, which were not found on a bare ITO. Similarly, we observed goldcharacteristic redox waves with a Au DEN-modified ITO (solid line in Figure 2b) rather than with a bare ITO (dotted line in Figure 2b). These electrochemical measurements suggest the presence of Pt or Au DENs on the ITO surfaces after the electrooxidative grafting process. Furthermore, they indicate that the immobilized DENs were electrochemically active via good electrochemical communication with the underlying ITO electrodes.29,39 The XPS measurements further verified the existence of DENs on the ITOs after the electrochemical grafting of the DENs. Parts c and d of Figure 2 show the XPS spectra of a Pt DEN-modified and a Au DEN-modified ITO, respectively. The characteristic N(1s) peak confirms the presence of amine-terminated PAMAM dendrimers grafted on the ITO surfaces, while the characteristic Pt [i.e., Pt(4p), Pt(4d), and Pt(4f)] and Au [i.e., Au(4p), Au(4d), and Au(4f)] peaks indicate the existence of Pt and Au nanoparticles encapsulated inside the grafted dendrimers, respectively. The observed surface coverage of Pt and Au DENs immobilized on ITOs were estimated by taking the ratio of atomic percentage values of Pt and In or Au and In and found to be 2.9 ± 0.8 and 0.7 ± 0.1 (sample number, n = 3), respectively. Note that the surface coverage of Pt DENs is substantially greater (∼4 times) than that of Au DENs, which agrees with our previous XPS investigations of DEN-modified ITOs29 and is probably a consequence of the high catalytic activity of encapsulated Pt nanoparticles for grafting the dendrimers onto the ITO surfaces.40,41 The immobilization of DENs on the ITO surfaces was also estimated by measuring the changes in the optical properties of the ITO substrates. Figure 3 shows the influence of immobilized DENs on ITO transparency. For comparison purposes, the transmittance of only dendrimer (but without encapsulated nanoparticle)-modified ITO is also presented in Figure 3 (see the dashed orange line). The immobilization of
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RESULTS AND DISCUSSION Immobilization of DENs on ITO Electrodes. We synthesized two different DENs (i.e., Pt and Au DENs) using previously reported methods (see the Experimental Section for details).32−36 Parts a and b of Figure 1 show TEM images of the as-synthesized Pt and Au DENs, respectively. The TEM images indicate that the nanoparticles were rarely aggregated and nearly monodisperse in size, which suggested stabilization of the nanoparticles via their encapsulation inside dendrimers.30 The average diameters of the Pt and Au nanoparticles were 1.94 ± 0.26 and 1.91 ± 0.26 nm, respectively (Figure 1, parts c and d). The measured values are very close to the theoretical values (1.81 and 1.87 nm), which are calculated by assuming that the nanoparticles are spherical in shape for nanoparticles containing 200 Pt or Au atoms.37,38 The synthesized Pt or Au DENs were then immobilized onto ITO surfaces via the electrooxidative grafting of the terminal amines of dendrimers to the surfaces, as we reported previously.29 Briefly, the potential of the ITO electrodes was scanned three times between 1.20 and 1.75 V (vs Ag/AgCl) in aqueous 10 μM DEN solutions containing 0.1 M LiClO4. After the electrografting process, the resulting electrodes were rinsed, ultrasonicated for 10 min, and then examined by electrochemical and XPS measurements to confirm the presence of Pt or Au DENs grafted on ITOs. Figure 2a shows the cyclic voltammograms (CVs) of a Pt DEN-modified (solid line) and a bare ITO (dotted line) electrode obtained in 0.5 M H2SO4. 1656
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Figure 2. (a and b) CVs of a Pt DEN-modified and a Au DEN-modified ITO, respectively, in 0.5 M H2SO4. For comparison purposes, corresponding CVs of bare ITOs are represented as dotted lines. Scan rate: 100 mV·s−1. (c and d) XPS spectra of a Pt DEN-modified and a Au DEN-modified ITO, respectively.
small sizes of less than 2 nm. Aggregates of surface bound Pt or Au DENs must increase surface plasmon resonance (SPR) absorbance significantly in the corresponding spectral regions,42 resulting in substantial transmittance drop of the ITO substrates. However, the DEN-modified ITOs preserved the good optical transparency of ITOs due to the well-surfacedispersed DENs, of which the diameters were less than 2 nm. Enhanced ECL of the Ru(bpy)32+/TPrA System on DEN-Modified ITOs. After characterizing the as-synthesized DENs and the DEN-modified ITOs, we investigated the electrochemical and ECL features of the Ru(bpy)32+/TPrA system on the DEN-modified ITOs. Parts a and b of Figure 4 show the representative CV and ECL responses, respectively, of the Ru(bpy)32+/TPrA system on a Pt DEN-modified, a dendrimer-modified, and a bare ITO. The Pt DEN-modified ITO electrode displayed significant anodic current starting from ∼0.9 V attributable to facile oxidation of Ru(bpy)32+/TPrA (solid violet line in Figure 4a) while the bare ITO revealed relatively small anodic current only occurring beyond ∼1.2 V (dotted dark cyan line in Figure 4a) similar to the dendrimermodified ITO (dashed orange line in Figure 4a). The Pt DENmodified ITO thus exhibited much enhanced ECL emission starting from ∼0.9 V (solid violet line in Figure 4b) compared to the emission obtained with the bare and dendrimer-modified ITOs (dotted dark cyan and dashed orange lines in Figure 4b). It is notable that the dendrimer-modified ITO displayed even smaller ECL emission than the bare ITO (inset in Figure 4b), which suggested that the grafted dendrimers themselves retarded the access, and thus the electrochemical oxidation of Ru(bpy)32+/TPrA on the ITO surface.36 These results clearly demonstrate that the ECL emission was significantly enhanced by the encapsulated Pt nanoparticles inside the dendrimers grafted on the ITO surface. This is an interesting finding since it was previously reported that a hydrophilic electrode such as
Figure 3. Transmittance spectra of a bare (dotted dark cyan line), a dendrimer-modified (dashed orange line), a Au DEN-modified (dashed-dotted navy line), and a Pt DEN-modified ITO (solid violet line).
dendrimer itself did not change the transmittance of the ITO substrate, but the immobilization of DENs slightly decreased the transmittance over the visible region due to the reflection, scattering, and absorption of the immobilized DENs. Especially, Pt DEN-modified ITO exhibited lower transmittance over the visible region than Au DEN-modified ITO; this was ascribed to higher surface coverage of Pt DENs as discussed in the above XPS studies of DEN-modified ITOs. However, it is notable that the decrease in the transmittance of Pt DEN-modified ITOs (sample number, n = 4) was only ∼1.99% over the entire visible region and ∼2.42% at 610 nm (the wavelength where the emission of Ru(bpy)32+ is maximum in water). The decrease in the transmittance of Au DEN-modified ITOs (sample number, n = 4) was also negligible, i.e., ∼1.39% over the visible region (∼2.16% at 610 nm). These slight transmittance drops of the DEN-modified ITOs indicate that the immobilized DENs were well-dispersed on ITO surfaces while maintaining their original 1657
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electrochemical surface area of the Pt DEN-modified ITO electrode, calculated from its capacitive current (sample number, n = 4), was only ∼3.3 times larger than that of the bare ITO (Supporting Information, Figure S2), which suggested that the ∼213-fold enhancement in ECL intensity could not be solely attributed to the increased surface area. Therefore, the enhanced electrocatalytic activity of the Pt DENmodified ITO for oxidation of Ru(bpy)32+/TPrA substantially contributed to the ECL improvement.19 The Pt DEN-modified ITOs were robust and stable, providing satisfactory reproducibility on the ECL emissions of Ru(bpy)32+/TPrA. Multiple ECL spectra were obtained from three independent Pt DENmodified ITO replicates treated identically to that shown in Figure 4c, which suggested steady (192 ± 19)-fold enhancement in ECL intensity. Figure 5 also shows the steady and
Figure 5. ECL emissions of 100 μM Ru(bpy)32+ and 100 mM TPrA in 0.15 M PBS solution (pH 7) upon the application of repetitive potential cycles between 0.00 and 1.10 V (vs Ag/AgCl) on a Pt DENmodified ITO. Scan rate: 100 mV·s−1. Integration time: 0.1 s.
coincident ECL emissions with a RSD (the relative standard deviation; the standard deviation expressed as a percentage of the mean value) of 1.73% upon the applications of repetitive potential cycles between 0.00 and 1.10 V (vs Ag/AgCl) on the Pt DEN-modified ITOs, indicating the good stability and reliability of the Pt DEN-modified ITOs for ECL measurements. Similarly, both CV and ECL behaviors of Ru(bpy)32+/TPrA on a Au DEN-modified ITO electrode were compared with those on a bare ITO (Supporting Information, Figure S3, parts a and b). Concurring with the previous reports, the ECL emission on the Au DEN-modified ITO emerged earlier than a bare ITO or even than a Pt DEN-modified ITO and also peaked at lower potential of ∼0.7 V due to the low oxidation potential of TPrA on Au.15 However, the Au surface was oxidized beyond ∼0.7 V, resulting in the formation of a surface oxide layer that suppressed TPrA oxidation,45 and thus the integrated ECL intensity obtained with the Au DEN-modified ITO was only ∼0.03-fold that of the Pt DEN-modified ITO (Supporting Information, Figure S3c). It is also worth noting that we observed a significant drop in the ECL intensity of the Ru(bpy)32+/TPrA system even after the first cycle when applying repetitive potential cycles between 0.00 and 1.10 V (vs Ag/AgCl) on the Au DEN-modified ITO (Supporting Information, Figure S4), which is consistent with previous studies reporting poor stability of Au nanoparticle-modified electrodes.20,28 In contrast, the Pt DEN-modified ITO provided steady and reliable ECL emissions under the same conditions as discussed above.
Figure 4. (a and b) CVs and corresponding ECL curves of 100 μM Ru(bpy)32+ and 100 mM TPrA in 0.15 M PBS solution (pH 7), respectively, obtained on a Pt DEN-modified (solid violet line), a dendrimer-modified (dashed orange line), and a bare ITO (dotted dark cyan line). Inset in panel b shows an expanded view of the ECL curves obtained on the dendrimer-modified and bare ITOs. Scan rate: 100 mV·s−1. Integration time: 0.1 s. (c) ECL spectra of 100 μM Ru(bpy)32+ and 100 mM TPrA in 0.15 M PBS solution (pH 7) obtained on a Pt DEN-modified (solid violet line) and a bare ITO (dotted dark cyan line). Inset in panel c shows the expanded view of the ECL spectrum obtained on the bare ITO. Applied potentials: 1.00 and 1.20 V (vs Ag/AgCl) for the Pt DEN-modified and the bare ITO, respectively. Integration time: 30 s.
Pt revealed sluggish kinetics toward oxidation of TPrA.15,43 Even though Pt nanoparticle itself on the Pt DEN-modified ITO was hydrophilic, the Pt nanoparticle was encapsulated inside the hydrophobic core of the dendrimer44 and thus could facilitate the oxidation of TPrA, leading to enhanced ECL on the Pt DEN-modified ITO. Fan and Bard observed a similar Pt nanoparticle-induced ECL enhancement during collisions of individual Pt nanoparticles, which catalyze the oxidation of TPrA as well as Ru(bpy)32+, on an ITO electrode.16 Figure 4c also indicates that the ECL intensity integrated over wavelength, obtained with the Pt DEN-modified ITO at 1.00 V, was ∼213 times larger than that of the bare ITO obtained even at higher potential, i.e., 1.20 V. Interestingly, the effective 1658
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Sensitive ECL-Based Analysis of Nicotine. Taking advantage of the enhanced stable ECL emission on the Pt DEN-modified ITO, we demonstrated the feasibility of the modified ITO as an analytical platform for sensitive ECL-based analysis of nicotine. Nicotine is a natural alkaloid representing 98% of the total alkaloids in tobacco, and thus responsible for tobacco addiction, and is suspected to contribute to various human diseases such as lung cancer, cardiovascular, Alzheimer’s, and Parkinson’s diseases.46−48 ECL-based analysis of nicotine using Ru(bpy)32+ is feasible because nicotine could act as a coreactant in the oxidative−reductive ECL process of Ru(bpy)32+ due to the tertiary amine structure of nicotine.47−49 As shown in Figure 6, we obtained a calibration plot with Pt
increased ECL emission. Especially, the Pt DEN-modified ITO electrode exhibited not only much enhanced (i.e., ∼213-fold increase compared to ECL obtained from bare ITO) but also stable ECL emission under consecutive potential scans, which allowed sensitive ECL-based analysis of nicotine using the Pt DEN-modified ITO. We envision that we can modify the ITO surface with a wide variety of catalytic nanoparticles encapsulated inside dendrimers (for example, monometallic nanoparticles and bimetallic alloy or core/shell nanoparticles30,31), which provides tunable properties of ITO electrodes for sensitive ECL-based analyses.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Phone: 82-2-961-9384. Fax: 82-2966-3701. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (No. 2012R1A1A1014408).
Figure 6. Calibration curves obtained with Pt DEN-modified (solid violet line) and bare (dotted dark cyan line) ITOs for ECL-based analysis of nicotine in 0.15 M PBS (pH 7) solutions containing 100 μM Ru(bpy)32+. Applied potential: 1.20 V. Integration time: 3 s. Inset shows an expanded view of the low-concentration region. All experiments were carried out with at least three independent replicates to collect statistical data with error bars confirming satisfactory reproducibility in the ECL-based analysis of nicotine.
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REFERENCES
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DEN-modified ITOs (solid violet line) for the ECL-based analysis of nicotine in PBS solutions (pH 7) containing 100 μM Ru(bpy)32+, representing a good linear relationship between the ECL intensity and the nicotine concentration ranging from 10−1 to 102 μM with a correlation coefficient of 0.997. The limit of detection (LOD) of nicotine was also determined to be 69 nM using the IUPAC recommendation (k = 3),50 which is superior or at least comparable to other conventional methods for the determination of nicotine.51−53 From the slope of the straight line, we determined that the sensitivity of the ECLbased analysis of nicotine with the Pt DEN-modified ITOs was 631 au/μM, which was ∼329 times higher than that obtained with bare ITOs (dotted dark cyan line), consistent with the ∼213-fold enhanced ECL of Ru(bpy)32+/TPrA on the Pt DENmodified ITO electrodes as discussed above.
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CONCLUSION In summary, we have described the highly enhanced ECL of the Ru(bpy)32+/TPrA system on ITO electrodes modified with amine-terminated dendrimers encapsulating Pt or Au nanoparticles and the feasibility of the modified ITOs to sensitive ECL-based assays. The modification of ITOs has been realized through the electrooxidative grafting of the terminal amines of dendrimers encapsulating the catalytic nanoparticles to the surfaces. The resulting DEN-modified ITOs exhibited highly catalyzed electrochemical oxidation of Ru(bpy)32+/TPrA with well-preserved transparency of the ITOs, leading to significantly 1659
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(19) Huang, R.; Wei, M.-Y.; Guo, L.-H. J. Electroanal. Chem. 2011, 656, 136−139. (20) Yin, X.-B.; Qi, B.; Sun, X.; Yang, X.; Wang, E. Anal. Chem. 2005, 77, 3525−3530. (21) Li, Y.; Shi, G. J. Phys. Chem. B 2005, 109, 23787−23793. (22) Dai, X.; Compton, R. G. Anal. Sci. 2006, 22, 567−570. (23) Zhang, J.; Kambayashi, M.; Oyama, M. Electrochem. Commun. 2004, 6, 683−688. (24) Zhang, J.; Oyama, M. Anal. Chim. Acta 2005, 540, 299−306. (25) Aziz, M. A.; Patra, S.; Yang, H. Chem. Commun. 2008, 4607− 4609. (26) Kim, J.; Lee, S. W.; Hammond, P. T.; Shao-Horn, Y. Chem. Mater. 2009, 21, 2993−3001. (27) Kim, J. M.; Ju, H.; Choi, H. S.; Lee, J.; Kim, J.; Kim, J.; Kim, H. D.; Kim, J. Bull. Korean Chem. Soc. 2010, 31, 491−494. (28) Qi, H.; Peng, Y.; Gao, Q.; Zhang, C. Sensors 2009, 9, 674−695. (29) Lee, S. B.; Ju, Y.; Kim, Y.; Koo, C. M.; Kim, J. Chem. Commun. 2013, 49, 8913−8915. (30) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181−190. (31) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B 2005, 109, 692−704. (32) Kim, Y.-G.; Oh, S.-K.; Crooks, R. M. Chem. Mater. 2004, 16, 167−172. (33) Zhao, M.; Crooks, R. M. Adv. Mater. 1999, 11, 217−220. (34) Ju, H.; Koo, C. M.; Kim, J. Chem. Commun. 2011, 47, 12322− 12324. (35) Kim, J. M.; Kim, J.; Kim, J. Chem. Commun. 2012, 48, 9233− 9235. (36) Kim, T. H.; Choi, H. S.; Go, B. R.; Kim, J. Electrochem. Commun. 2010, 12, 788−791. (37) Shao-Horn, Y.; Sheng, W. C.; Chen, S.; Ferreira, P. J.; Holby, E. F.; Morgan., D. Top. Catal. 2007, 46, 285−305. (38) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036−7041. (39) Yancey, D. F.; Carino, E. V.; Crooks, R. M. J. Am. Chem. Soc. 2010, 132, 10988−10989. (40) Ye, H.; Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 4930−4934. (41) Deutsch, D. S.; Lafaye, G.; Liu, D.; Chandler, B.; Williams, C. T.; Amiridis, M. D. Catal. Lett. 2004, 97, 139−143. (42) Blakey, I.; Merican, Z.; Thurecht, K. J. Langmuir 2013, 29, 8266−8274. (43) Zu, Y.; Bard, A. J. Anal. Chem. 2001, 73, 3960−3964. (44) Shao, N.; Su, Y.; Hu, J.; Zhang, J.; Zhang, H.; Yiyun, C. Int. J. Nanomed. 2011, 6, 3361−3372. (45) Zu, Y.; Bard, A. J. Anal. Chem. 2000, 72, 3223−3232. (46) Benowitz, N. L. Prev. Med. 1997, 26, 412−417. (47) Lin, M. S.; Wang, J. S.; Lai, C. H. Electrochim. Acta 2008, 53, 7775−7780. (48) Sun, J.; Du, H.; You, T. Electrophoresis 2011, 32, 2148−2154. (49) Chang, P.-L.; Lee, K.-H.; Hu, C.-C.; Chang, H.-T. Electrophoresis 2007, 28, 1092−1099. (50) Analytical Methods Committee. Analyst 1987, 112, 199−204 (51) Al-Tamrah, S. A. Anal. Chim. Acta 1999, 379, 75−80. (52) Oddoze, C.; Pauli, A. M.; Pastor, J. J. Chromatogr., B 1998, 708, 95−101. (53) Matysik, F.-M. J. Chromatogr., A 1999, 853, 27−34.
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Modification of Indium Tin Oxide with Dendrimer-Encapsulated Nanoparticles To Provide Enhanced Stable Electrochemiluminescence of Ru(bpy)32+/Tripropylamine While Preserving Optical Transparency of Indium Tin Oxide for Sensitive Electrochemiluminescence-Based Analyses Yeoju Kim and Joohoon Kim* Department of Chemistry, Research Institute for Basic Sciences, Kyung Hee University, 1 Hoegi-dong, Seoul 130-701, Republic of Korea S Supporting Information *
ABSTRACT: Here, we report highly enhanced stable electrogenerated chemiluminescence (ECL) of Ru(bpy)32+ (bpy = 2,2′bipyridyl) with tripropylamine (TPrA) coreactant on indium tin oxide (ITO) electrodes modified with amine-terminated dendrimers encapsulating catalytic nanoparticles while maintaining optical transparency of ITO and feasibility of the modified ITOs to sensitive ECL-based assays. As model systems, we prepared Pt and Au dendrimer-encapsulated nanoparticles (DENs) using amine-terminated sixth-generation poly(amido amine) dendrimers and subsequently immobilized the DENs onto ITO surfaces via electrooxidative grafting of the terminal amines of dendrimers to the surfaces. The resulting DENmodified ITOs preserved good optical transparency of ITO and exhibited highly catalyzed electrochemical oxidation of Ru(bpy)32+/TPrA, leading to significantly increased ECL emission. Especially, the Pt DEN-modified ITO electrode provides negligible transmittance drop, i.e., only ∼1.99% over the entire visible region, and exhibited not only much enhanced (i.e., ∼213-fold increase compared to ECL obtained from bare ITO) but also stable ECL emission under consecutive potential scans from 0.00 to 1.10 V for 10 cycles, which allowed ∼329 times more sensitive ECL-based analysis of nicotine using the Pt DEN-modified ITO compared with the use of bare ITO.
I
low background and high sensitivity, good temporal and spatial controllability, robustness, versatility, and low cost.1−4 Thus, the ECL technique has been widely used in a variety of analytical applications such as immunoassays, DNA analyses, molecular diagnosis of clinically important compounds (e.g., steroid hormones, thyrotropin, digoxin, etc.), environmental assays for food and water testing, and detection of chemical/ biological warfare agents.5−8 The Ru(bpy)32+/TPrA system is one of the most frequently used ECL systems in analytical applications due to its good photochemical stability, high ECL quantum yield, and good water solubility.9,10 The use of an ITO electrode in the ECL technique is also attractive in the analytical applications because of its high optical transparency and good electrical conductivity.11,12 However, it has been reported that the ITO electrode exhibited sluggish kinetics for electrochemical reactions of many organic compounds including TPrA, leading to low ECL sensitivity when using
n this paper, we report the electrochemical modification of indium tin oxide (ITO) electrodes with dendrimerencapsulated nanoparticles (DENs), leading to highly enhanced stable electrogenerated chemiluminescence (ECL) of Ru(bpy)32+ (bpy = 2,2′-bipyridyl) with tripropylamine (TPrA) coreactant on the resulting modified ITOs, and the preliminary analytical application of the DEN-modified ITOs in the sensitive determination of nicotine. This suggests that the DEN-modified ITO electrodes can be promising analytical platforms for the sensitive ECL-based analysis. As model systems, we electrochemically immobilized two different types of DENs, Pt and Au DENs, on ITO electrodes and evaluated the ECL activities of the DEN-modified ITOs for the Ru(bpy)32+/TPrA system (Scheme 1). This is a significant study because it suggests a simple and versatile approach to modify ITO surfaces with various catalytic nanoparticles encapsulated inside dendrimers, which provides tunable properties of ITO electrodes for ECL-based analyses. The ECL technique involves electrogenerated species undergoing electron-transfer reactions to form excited states that emit light in the vicinity of electrode surfaces and has unique advantages over photoluminescence techniques, such as © 2014 American Chemical Society
Received: October 21, 2013 Accepted: January 8, 2014 Published: January 8, 2014 1654
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Scheme 1
the transparent ITO.13,14 Because of the slow kinetics of the electron-transfer process at ITO surfaces, the ITO electrodes have often been modified with various nanoparticles such as Au, Pt, SiO2, clay, and ITO nanoparticles for the facilitation of the electron-transfer process.15−19 The surface modification of ITO electrodes with nanoparticles also resulted in the increase of the surface area of electrodes leading to the enhancement of ECL signals.20 Several surface modification strategies have thus been applied to immobilize nanoparticles onto ITO surfaces. For example, while direct deposition of nanoparticles on ITO substrates has been achieved by electrodeposition or seed-mediated growth approaches,21−24 it has been shown that precise control in the composition and size of nanoparticles is challenging.15 Nanoparticles have been also immobilized onto ITO surfaces by chemical and physical methods, such as thiol (or amine)− gold binding and layer-by-layer (LbL) assembly techniques, which require intermediate adhesive layers on the surfaces.15,25−27 It is notable that the most frequent reports have concerned Au nanoparticle-modified ITO electrodes mainly because of the limited chemistry available to link nanoparticles to electrode surfaces. However, Au nanoparticle-modified electrodes have often been reported to show poor stability at high potentials due to the oxidation of thiol layers or the oxidation of Au nanoparticles.20,28 Recently, our group reported an electrochemical modification of ITO surfaces with amineterminated poly(amido amine) (PAMAM) dendrimers encapsulating nanoparticles.29 Since a variety of nanoparticles (for example, monometallic nanoparticles such as Pt, Au, Pd, Ni, Fe, and Cu and bimetallic alloy or core/shell nanoparticles30,31) can be encapsulated inside the dendrimers, the direct electrochemical method allows a unique and versatile means to decorate the ITO surfaces with various nanoparticles. In an effort to expand the scope of our preliminary study of the electrochemical decoration of ITO surfaces with DENs, we report here the enhanced stable ECL of the Ru(bpy)32+/TPrA system on DENs-modified ITO electrodes with well-preserved transparency and its feasibility to sensitive ECL-based assays. Specifically, we synthesized Pt DENs (diameter 1.94 ± 0.26 nm) and Au DENs (diameter 1.91 ± 0.26 nm) using amineterminated sixth-generation poly(amido amine) dendrimers (G6-NH2 PAMAM dendrimers) and subsequently immobilized the nanocomposites onto ITO electrodes via their electrooxidative coupling to ITO surfaces as we previously reported.29 The resulting DEN-modified ITOs exhibited highly improved catalytic activity for the electrochemical oxidation of Ru(bpy)32+/TPrA while preserving the good optical transparency of ITO electrodes, leading to significantly increased ECL emission. Especially, Pt DEN-modified ITO electrode exhibited not only much enhanced, i.e., ∼213-fold increase compared to
ECL obtained from bare ITO, but also stable ECL emission under consecutive potential scans from 0.00 to 1.10 V for 10 cycles, which allowed sensitive ECL-based analysis of nicotine using the Pt DEN-modified ITO.
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EXPERIMENTAL SECTION Chemicals and Materials. Amine-terminated sixth-generation PAMAM (G6-NH2 PAMAM) dendrimers, HAuCl4· 3H2O, K2PtCl4, NaBH4, LiClO4, Ru(bpy)3Cl2·6H2O, tri-npropylamine, (−)-nicotine, phosphate-buffered saline (0.15 M PBS containing 2.07 M NaCl and 40.5 mM KCl), and cellulose dialysis sacks (MW cutoff of 12 000) were purchased from Sigma-Aldrich, Inc. (U.S.A.). HCl, H2SO4, ethanol, and acetone were obtained from Daejung, Inc. (Korea). NaOH was purchased from Duksan, Inc. (Korea). ITO-coated glass slides were received from Delta Technologies (U.S.A.). Deionized (DI) 18 MΩ·cm water was used in the preparation of aqueous solutions (Ultra370, Younglin Co., Korea). Synthesis of DENs. Pt and Au DENs were synthesized similarly to that previously reported.32−36 Briefly, 200 mol equiv of an aqueous 200 mM K2PtCl4 or 10 mM HAuCl4 was added to an aqueous 10 μM G6-NH2 PAMAM dendrimer solution. The mixture solutions were stirred for complete complexation between the metal ions and the interior amines of the dendrimers. Especially, the mixture (pH 5) of Pt ions and dendrimers was stirred for 76 h to ensure the complexation of the Pt ions to the intradendrimer tertiary amines. A stoichiometric excess of aqueous NaBH4 was then added to the complex solution under vigorous stirring. Specifically, a 20fold excess of NaBH4 was added to the Pt ion−dendrimer complex solution. We kept the mixture solution (pH 7−8) in a closed vial for 24 h to ensure complete reduction of the complexed Pt ions. Similarly, a 5-fold excess of NaBH4 in 0.3 M NaOH was added to the Au ion−dendrimer complex solution, which resulted in the reduction of the complexed Au ions to zero-valent Au nanoparticles inside the dendrimers. Finally, the synthesized DEN solutions were dialyzed for 24 h using cellulose dialysis sacks to remove impurities. Modification of ITO Electrodes with DENs. The electrochemical grafting of DENs on ITO electrodes was carried out using our previously reported method.29 In brief, an ITO electrode was ultrasonically cleaned with acetone, ethanol, and water subsequently and dried under a stream of N2. The ITO electrode was cleaned further in a plasma cleaner/sterilizer (PDC-32G, Harrick Scientific, U.S.A.) at medium power for 2 min. Immediately after the plasma treatment, the electrode (area: 0.096 cm2) was exposed to an aqueous 10 μM DEN solution containing 0.1 M LiClO4 and the potential of the electrode was cycled three times between 1.20 and 1.75 V (vs Ag/AgCl) at a scan rate of 10 mV·s−1. After the electrooxidative 1655
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grafting of DENs, the modified ITO was rinsed with DI water, ultrasonicated in DI water thoroughly for 10 min, and then blown until dry. Instrumentation and Measurements. Transmission electron microscope (TEM) images were obtained using a Tecnai G2 F30 ST instrument (FEI Co., U.S.A.) operating at 300 kV. TEM samples were prepared by dropping aqueous DEN solutions on 200 mesh carbon-coated copper grids (Ted Pella Inc., U.S.A.) followed by drying in air. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a PHI 5800 spectrometer (Physical Electronics Inc., U.S.A.) using Al Kα radiation (hν = 1486.6 eV). UV−vis transmission spectra of ITO slides were recorded on an Agilent 8453 UV−vis spectrometer (Agilent Tech., U.S.A.) using a cell holder (model CH/2049, Starna Scientific Ltd., U.K.). All electrochemical experiments were carried out with a model 440 potentiostat (CH Instruments, U.S.A.) using a conventional three-electrode cell. A Pt wire and a Ag/AgCl (3 M Cl−) electrode were used as a counter and a reference electrode, respectively. The three-electrode cell was connected to the slit of a monochromator (Acton Standard SP2150, Princeton Instruments, U.S.A.) equipped with a charge-coupled device camera (PIXIS 100B, Princeton Instruments, U.S.A.) for ECL measurements. Prior to the ECL measurements, the electrode potential of DEN-modified ITO was subjected to be cycled 10 times in 0.5 M H2SO4 between −0.20 and 1.20 V, and 0.10 and 1.55 V (vs Ag/AgCl) for Pt DEN-modified and Au DENmodified ITO, respectively. Pt DEN-modified ITOs were also subjected to repetitive potential cycling in a PBS solution (pH 7) containing 100 μM Ru(bpy)32+ and 100 mM TPrA until stable ECL emission was obtained. We found that the ECL emission increased and reached a stable maximum value upon the repetitive cycles of the electrode potential on Pt DENmodified ITOs between 0.00 and 1.70 V (vs Ag/AgCl) (Supporting Information, Figure S1).
Figure 1. (a and b) TEM images of as-synthesized Pt and Au DENs, respectively. (c and d) Particle size distribution histograms of assynthesized Pt and Au DENs, respectively.
The Pt DEN-modified ITO exhibited platinum-characteristic redox waves, i.e., hydrogen adsorption/desorption, Pt surface oxidation, and the corresponding oxide reduction, which were not found on a bare ITO. Similarly, we observed goldcharacteristic redox waves with a Au DEN-modified ITO (solid line in Figure 2b) rather than with a bare ITO (dotted line in Figure 2b). These electrochemical measurements suggest the presence of Pt or Au DENs on the ITO surfaces after the electrooxidative grafting process. Furthermore, they indicate that the immobilized DENs were electrochemically active via good electrochemical communication with the underlying ITO electrodes.29,39 The XPS measurements further verified the existence of DENs on the ITOs after the electrochemical grafting of the DENs. Parts c and d of Figure 2 show the XPS spectra of a Pt DEN-modified and a Au DEN-modified ITO, respectively. The characteristic N(1s) peak confirms the presence of amine-terminated PAMAM dendrimers grafted on the ITO surfaces, while the characteristic Pt [i.e., Pt(4p), Pt(4d), and Pt(4f)] and Au [i.e., Au(4p), Au(4d), and Au(4f)] peaks indicate the existence of Pt and Au nanoparticles encapsulated inside the grafted dendrimers, respectively. The observed surface coverage of Pt and Au DENs immobilized on ITOs were estimated by taking the ratio of atomic percentage values of Pt and In or Au and In and found to be 2.9 ± 0.8 and 0.7 ± 0.1 (sample number, n = 3), respectively. Note that the surface coverage of Pt DENs is substantially greater (∼4 times) than that of Au DENs, which agrees with our previous XPS investigations of DEN-modified ITOs29 and is probably a consequence of the high catalytic activity of encapsulated Pt nanoparticles for grafting the dendrimers onto the ITO surfaces.40,41 The immobilization of DENs on the ITO surfaces was also estimated by measuring the changes in the optical properties of the ITO substrates. Figure 3 shows the influence of immobilized DENs on ITO transparency. For comparison purposes, the transmittance of only dendrimer (but without encapsulated nanoparticle)-modified ITO is also presented in Figure 3 (see the dashed orange line). The immobilization of
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RESULTS AND DISCUSSION Immobilization of DENs on ITO Electrodes. We synthesized two different DENs (i.e., Pt and Au DENs) using previously reported methods (see the Experimental Section for details).32−36 Parts a and b of Figure 1 show TEM images of the as-synthesized Pt and Au DENs, respectively. The TEM images indicate that the nanoparticles were rarely aggregated and nearly monodisperse in size, which suggested stabilization of the nanoparticles via their encapsulation inside dendrimers.30 The average diameters of the Pt and Au nanoparticles were 1.94 ± 0.26 and 1.91 ± 0.26 nm, respectively (Figure 1, parts c and d). The measured values are very close to the theoretical values (1.81 and 1.87 nm), which are calculated by assuming that the nanoparticles are spherical in shape for nanoparticles containing 200 Pt or Au atoms.37,38 The synthesized Pt or Au DENs were then immobilized onto ITO surfaces via the electrooxidative grafting of the terminal amines of dendrimers to the surfaces, as we reported previously.29 Briefly, the potential of the ITO electrodes was scanned three times between 1.20 and 1.75 V (vs Ag/AgCl) in aqueous 10 μM DEN solutions containing 0.1 M LiClO4. After the electrografting process, the resulting electrodes were rinsed, ultrasonicated for 10 min, and then examined by electrochemical and XPS measurements to confirm the presence of Pt or Au DENs grafted on ITOs. Figure 2a shows the cyclic voltammograms (CVs) of a Pt DEN-modified (solid line) and a bare ITO (dotted line) electrode obtained in 0.5 M H2SO4. 1656
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Figure 2. (a and b) CVs of a Pt DEN-modified and a Au DEN-modified ITO, respectively, in 0.5 M H2SO4. For comparison purposes, corresponding CVs of bare ITOs are represented as dotted lines. Scan rate: 100 mV·s−1. (c and d) XPS spectra of a Pt DEN-modified and a Au DEN-modified ITO, respectively.
small sizes of less than 2 nm. Aggregates of surface bound Pt or Au DENs must increase surface plasmon resonance (SPR) absorbance significantly in the corresponding spectral regions,42 resulting in substantial transmittance drop of the ITO substrates. However, the DEN-modified ITOs preserved the good optical transparency of ITOs due to the well-surfacedispersed DENs, of which the diameters were less than 2 nm. Enhanced ECL of the Ru(bpy)32+/TPrA System on DEN-Modified ITOs. After characterizing the as-synthesized DENs and the DEN-modified ITOs, we investigated the electrochemical and ECL features of the Ru(bpy)32+/TPrA system on the DEN-modified ITOs. Parts a and b of Figure 4 show the representative CV and ECL responses, respectively, of the Ru(bpy)32+/TPrA system on a Pt DEN-modified, a dendrimer-modified, and a bare ITO. The Pt DEN-modified ITO electrode displayed significant anodic current starting from ∼0.9 V attributable to facile oxidation of Ru(bpy)32+/TPrA (solid violet line in Figure 4a) while the bare ITO revealed relatively small anodic current only occurring beyond ∼1.2 V (dotted dark cyan line in Figure 4a) similar to the dendrimermodified ITO (dashed orange line in Figure 4a). The Pt DENmodified ITO thus exhibited much enhanced ECL emission starting from ∼0.9 V (solid violet line in Figure 4b) compared to the emission obtained with the bare and dendrimer-modified ITOs (dotted dark cyan and dashed orange lines in Figure 4b). It is notable that the dendrimer-modified ITO displayed even smaller ECL emission than the bare ITO (inset in Figure 4b), which suggested that the grafted dendrimers themselves retarded the access, and thus the electrochemical oxidation of Ru(bpy)32+/TPrA on the ITO surface.36 These results clearly demonstrate that the ECL emission was significantly enhanced by the encapsulated Pt nanoparticles inside the dendrimers grafted on the ITO surface. This is an interesting finding since it was previously reported that a hydrophilic electrode such as
Figure 3. Transmittance spectra of a bare (dotted dark cyan line), a dendrimer-modified (dashed orange line), a Au DEN-modified (dashed-dotted navy line), and a Pt DEN-modified ITO (solid violet line).
dendrimer itself did not change the transmittance of the ITO substrate, but the immobilization of DENs slightly decreased the transmittance over the visible region due to the reflection, scattering, and absorption of the immobilized DENs. Especially, Pt DEN-modified ITO exhibited lower transmittance over the visible region than Au DEN-modified ITO; this was ascribed to higher surface coverage of Pt DENs as discussed in the above XPS studies of DEN-modified ITOs. However, it is notable that the decrease in the transmittance of Pt DEN-modified ITOs (sample number, n = 4) was only ∼1.99% over the entire visible region and ∼2.42% at 610 nm (the wavelength where the emission of Ru(bpy)32+ is maximum in water). The decrease in the transmittance of Au DEN-modified ITOs (sample number, n = 4) was also negligible, i.e., ∼1.39% over the visible region (∼2.16% at 610 nm). These slight transmittance drops of the DEN-modified ITOs indicate that the immobilized DENs were well-dispersed on ITO surfaces while maintaining their original 1657
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electrochemical surface area of the Pt DEN-modified ITO electrode, calculated from its capacitive current (sample number, n = 4), was only ∼3.3 times larger than that of the bare ITO (Supporting Information, Figure S2), which suggested that the ∼213-fold enhancement in ECL intensity could not be solely attributed to the increased surface area. Therefore, the enhanced electrocatalytic activity of the Pt DENmodified ITO for oxidation of Ru(bpy)32+/TPrA substantially contributed to the ECL improvement.19 The Pt DEN-modified ITOs were robust and stable, providing satisfactory reproducibility on the ECL emissions of Ru(bpy)32+/TPrA. Multiple ECL spectra were obtained from three independent Pt DENmodified ITO replicates treated identically to that shown in Figure 4c, which suggested steady (192 ± 19)-fold enhancement in ECL intensity. Figure 5 also shows the steady and
Figure 5. ECL emissions of 100 μM Ru(bpy)32+ and 100 mM TPrA in 0.15 M PBS solution (pH 7) upon the application of repetitive potential cycles between 0.00 and 1.10 V (vs Ag/AgCl) on a Pt DENmodified ITO. Scan rate: 100 mV·s−1. Integration time: 0.1 s.
coincident ECL emissions with a RSD (the relative standard deviation; the standard deviation expressed as a percentage of the mean value) of 1.73% upon the applications of repetitive potential cycles between 0.00 and 1.10 V (vs Ag/AgCl) on the Pt DEN-modified ITOs, indicating the good stability and reliability of the Pt DEN-modified ITOs for ECL measurements. Similarly, both CV and ECL behaviors of Ru(bpy)32+/TPrA on a Au DEN-modified ITO electrode were compared with those on a bare ITO (Supporting Information, Figure S3, parts a and b). Concurring with the previous reports, the ECL emission on the Au DEN-modified ITO emerged earlier than a bare ITO or even than a Pt DEN-modified ITO and also peaked at lower potential of ∼0.7 V due to the low oxidation potential of TPrA on Au.15 However, the Au surface was oxidized beyond ∼0.7 V, resulting in the formation of a surface oxide layer that suppressed TPrA oxidation,45 and thus the integrated ECL intensity obtained with the Au DEN-modified ITO was only ∼0.03-fold that of the Pt DEN-modified ITO (Supporting Information, Figure S3c). It is also worth noting that we observed a significant drop in the ECL intensity of the Ru(bpy)32+/TPrA system even after the first cycle when applying repetitive potential cycles between 0.00 and 1.10 V (vs Ag/AgCl) on the Au DEN-modified ITO (Supporting Information, Figure S4), which is consistent with previous studies reporting poor stability of Au nanoparticle-modified electrodes.20,28 In contrast, the Pt DEN-modified ITO provided steady and reliable ECL emissions under the same conditions as discussed above.
Figure 4. (a and b) CVs and corresponding ECL curves of 100 μM Ru(bpy)32+ and 100 mM TPrA in 0.15 M PBS solution (pH 7), respectively, obtained on a Pt DEN-modified (solid violet line), a dendrimer-modified (dashed orange line), and a bare ITO (dotted dark cyan line). Inset in panel b shows an expanded view of the ECL curves obtained on the dendrimer-modified and bare ITOs. Scan rate: 100 mV·s−1. Integration time: 0.1 s. (c) ECL spectra of 100 μM Ru(bpy)32+ and 100 mM TPrA in 0.15 M PBS solution (pH 7) obtained on a Pt DEN-modified (solid violet line) and a bare ITO (dotted dark cyan line). Inset in panel c shows the expanded view of the ECL spectrum obtained on the bare ITO. Applied potentials: 1.00 and 1.20 V (vs Ag/AgCl) for the Pt DEN-modified and the bare ITO, respectively. Integration time: 30 s.
Pt revealed sluggish kinetics toward oxidation of TPrA.15,43 Even though Pt nanoparticle itself on the Pt DEN-modified ITO was hydrophilic, the Pt nanoparticle was encapsulated inside the hydrophobic core of the dendrimer44 and thus could facilitate the oxidation of TPrA, leading to enhanced ECL on the Pt DEN-modified ITO. Fan and Bard observed a similar Pt nanoparticle-induced ECL enhancement during collisions of individual Pt nanoparticles, which catalyze the oxidation of TPrA as well as Ru(bpy)32+, on an ITO electrode.16 Figure 4c also indicates that the ECL intensity integrated over wavelength, obtained with the Pt DEN-modified ITO at 1.00 V, was ∼213 times larger than that of the bare ITO obtained even at higher potential, i.e., 1.20 V. Interestingly, the effective 1658
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Sensitive ECL-Based Analysis of Nicotine. Taking advantage of the enhanced stable ECL emission on the Pt DEN-modified ITO, we demonstrated the feasibility of the modified ITO as an analytical platform for sensitive ECL-based analysis of nicotine. Nicotine is a natural alkaloid representing 98% of the total alkaloids in tobacco, and thus responsible for tobacco addiction, and is suspected to contribute to various human diseases such as lung cancer, cardiovascular, Alzheimer’s, and Parkinson’s diseases.46−48 ECL-based analysis of nicotine using Ru(bpy)32+ is feasible because nicotine could act as a coreactant in the oxidative−reductive ECL process of Ru(bpy)32+ due to the tertiary amine structure of nicotine.47−49 As shown in Figure 6, we obtained a calibration plot with Pt
increased ECL emission. Especially, the Pt DEN-modified ITO electrode exhibited not only much enhanced (i.e., ∼213-fold increase compared to ECL obtained from bare ITO) but also stable ECL emission under consecutive potential scans, which allowed sensitive ECL-based analysis of nicotine using the Pt DEN-modified ITO. We envision that we can modify the ITO surface with a wide variety of catalytic nanoparticles encapsulated inside dendrimers (for example, monometallic nanoparticles and bimetallic alloy or core/shell nanoparticles30,31), which provides tunable properties of ITO electrodes for sensitive ECL-based analyses.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Phone: 82-2-961-9384. Fax: 82-2966-3701. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (No. 2012R1A1A1014408).
Figure 6. Calibration curves obtained with Pt DEN-modified (solid violet line) and bare (dotted dark cyan line) ITOs for ECL-based analysis of nicotine in 0.15 M PBS (pH 7) solutions containing 100 μM Ru(bpy)32+. Applied potential: 1.20 V. Integration time: 3 s. Inset shows an expanded view of the low-concentration region. All experiments were carried out with at least three independent replicates to collect statistical data with error bars confirming satisfactory reproducibility in the ECL-based analysis of nicotine.
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REFERENCES
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DEN-modified ITOs (solid violet line) for the ECL-based analysis of nicotine in PBS solutions (pH 7) containing 100 μM Ru(bpy)32+, representing a good linear relationship between the ECL intensity and the nicotine concentration ranging from 10−1 to 102 μM with a correlation coefficient of 0.997. The limit of detection (LOD) of nicotine was also determined to be 69 nM using the IUPAC recommendation (k = 3),50 which is superior or at least comparable to other conventional methods for the determination of nicotine.51−53 From the slope of the straight line, we determined that the sensitivity of the ECLbased analysis of nicotine with the Pt DEN-modified ITOs was 631 au/μM, which was ∼329 times higher than that obtained with bare ITOs (dotted dark cyan line), consistent with the ∼213-fold enhanced ECL of Ru(bpy)32+/TPrA on the Pt DENmodified ITO electrodes as discussed above.
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CONCLUSION In summary, we have described the highly enhanced ECL of the Ru(bpy)32+/TPrA system on ITO electrodes modified with amine-terminated dendrimers encapsulating Pt or Au nanoparticles and the feasibility of the modified ITOs to sensitive ECL-based assays. The modification of ITOs has been realized through the electrooxidative grafting of the terminal amines of dendrimers encapsulating the catalytic nanoparticles to the surfaces. The resulting DEN-modified ITOs exhibited highly catalyzed electrochemical oxidation of Ru(bpy)32+/TPrA with well-preserved transparency of the ITOs, leading to significantly 1659
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