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Gold nanoparticles directly modified glassy carbon electrode for non-enzymatic detection of glucose
Applied Surface Science 288 (2014) 524–529
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Gold nanoparticles directly modified glassy carbon electrode for non-enzymatic detection of glucose Gang Chang a , Honghui Shu a , Kai Ji a , Munetaka Oyama b , Xiong Liu a , Yunbin He a,∗ a Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Faculty of Materials Science and Engineering, Hubei University, No. 368 Youyi Avenue, Wuchang, Wuhan 430062, China b Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan
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Article history: Received 21 July 2013 Received in revised form 28 September 2013 Accepted 11 October 2013 Available online 19 October 2013 Keywords: Gold nanoparticles Glassy carbon Glucose Nonenzymatic biosensor Seed-mediated growth method
a b s t r a c t This work describes controllable preparation of gold nanoparticles on glassy carbon electrodes by using the seed mediated growth method, which contains two steps, namely, nanoseeds attachment and nanocrystals growth. The size and the dispersion of gold nanoparticles grown on glassy carbon electrodes could be easily tuned through the growth time based on results of field-emission scanning electron microscopy. Excellent electrochemical catalytic characteristics for glucose oxidation were observed for the gold nanoparticles modified glassy carbon electrodes (AuNPs/GC), resulting from the extended active surface area provided by the dense gold nanoparticles attached. It exhibited a wide linear range from 0.1 mM to 25 mM with the sensitivity of 87.5 A cm−2 mM−1 and low detection limit down to 0.05 mM for the sensing of glucose. The common interfering species such as chloride ion, ascorbic acid, uric acid and 4-acetamidophenol were verified having no interference effect on the detection of glucose. It is demonstrated that the seed mediated method is one of the facile approaches for fabricating Au nanoparticles modified substrates, which could work as one kind of promising electrode materials for the glucose nonenzymatic sensing. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Extensive studies have been devoted to nanoparticles modified surfaces in the recent decades because of their potential applications in fuel cells, biosensors, new photonic materials, and so on [1–10]. Unique optical, electronic, catalytic and magnetic properties of nanoparticles could be easily introduced into devices through the nanoparticles attachment. Therefore, various approaches for nanoparticles modification have been developed including electrochemical deposition, sputtering, bridging agent assistant, layer by layer, etc. [3,11,12]. Noble metal nanoparticles are convenient to modify the substrates by using bridging agents such as aminopropyltrimethoxysilane (APTES), mercaptopropyltrimethoxysilane (MPTMS) or some macromolecules (e.g. poly-l-lysine) [11,13–17]. However the adoption of bridging agent might block the electron transferring, which would seriously affect the sensitivity when they are used in the electroanalytical or electrocatalytical fields [18,19]. In our previous reports, it was possible to directly attach the noble metal nanoparticles onto conductive surfaces such as ITO without any linker molecules by using the modified seed mediated method,
∗ Corresponding author. Tel.: +86 278 8661803; fax: +86 278 8661803. E-mail addresses: [email protected], [email protected] (Y. He). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.064
which originated from the preparation of one-dimension nanomaterials in aqueous solution. In this method, metal nano-seeds with a diameter around 4 nm were first in situ attached to the substrate due to the strong adsorption capability of nanoparticles and defects present on the substrate surface. And then the seeds could grow up to nanoparticles with different sizes and morphology such as nanospheres and nanorods by immersing the nanoseeds attached surface into the growth solution. Through this method, Au, Ag and Pd nanoparticles have been successfully prepared on the ITO surface [18–21]. Attractive electrochemical and electrocatalytical properties were observed for these nanoparticles modified ITO surface, which should be attributed to the acceleration of interface electrons transfer resulted from the direct attachment of noble metal nanoparticles without linker molecules. Therefore it is reasonable to believe that this method could be extended to other substrates such as glassy carbon, which can find further electrocatalytical and biosensor applications for their higher conductivity. Especially, gold nanoparticles show the interesting activity for glucose oxidation in neutral and alkaline solution. Gold nanoparticles modified surfaces for enzyme-free electrochemical glucose biosensors have attracted enormous interests because it overcomes the drawback in the traditional glucose biosensor, the instability of glucose oxidase. It might bring great changes in the detection and treatment of diabetes [6,10,22–25]. Due to lacking of the unique selectivity of the glucose oxidase in the enzymatic biosensor, it
G. Chang et al. / Applied Surface Science 288 (2014) 524–529
has much higher quality requests for the nonenzymatic glucose biosensors in the abilities of electrocatalysis, anti-interferences and poison-resistance, which are much dependent on the morphology and structure of gold nanoparticles on the electrodes. Up to now, gold nanoparticles and their nanocomposites with different morphology were achieved by different approaches and applied into the glucose sensing without enzyme. Electrodepositing is one of the main methods to modify gold nanoparticles on the electrode surface [12,26,27]. However facile tuning and controlling of the size and morphology of gold nanoparticles on the surfaces remain still challenging. In the present work, we prepared Au nanoparticles modified glassy carbon electrodes by the seed mediated growth method. The sizes and morphologies of Au nanoparticles grown on the glassy carbon surfaces were systematically investigated through the field-emission scanning electron microscopy (FE-SEM). The electrochemical properties of the Au nanoparticles modified glassy carbon (AuNPs/GC) electrodes were studied in comparison to those of Au bulk electrodes and pure glassy carbon electrodes by the electrochemical measurements. Cyclic and linear voltammograms were applied to evaluate the oxidation of glucose on the AuNPs/GC electrodes prepared for 24 h. The AuNPs/GC electrodes exhibit excellent catalytical activity and selectivity for the glucose oxidation, which is promising for the application in nonenzymatic glucose biosensors. 2. Material and methods 2.1. Materials Cetyltrimethylammonium bromide (CTAB) and HAuCl4 ·3H2 O were purchased from Aldrich Co., Ltd. Trisodium citrate, ascorbic acid, NaBH4 , ethanol, acetone and NaOH were obtained from Sinopharm Chemical Regent Co. Ltd. Glassy carbon plates (size 10 mm × 10 mm × 1.0 mm) and gold bulk electrodes (diameter, 1.8 mm) were ordered from BAS. Co. Ltd. The glassy carbon plates were firstly polished to mirror-like by using polishing paper and alumina polishing powders, respectively. Then they were washed sequentially in acetone, ethanol and pure water with sonication each for 15 min and dried with nitrogen gas. In all the procedures, we used pure water prepared with Kertone Ultrapure Water System P60-CY (Kertone Water Treatment Co. Ltd, resistivity > 18 M cm). 2.2. Apparatuses The size and morphology of the Au nanoparticles attached to the glassy carbon plates were characterized with field-emission scanning electron microscopy (FE-SEM, Carl Zeiss Merlin). All electrochemical measurements were carried out with a 550 electrochemical workstation (Gaoss Union Instrument Company, Wuhan, China) in a conventional three-electrode cell. 2.3. Procedures A seed solution of 4 nm Au colloid was prepared by the procedure reported by Oyama and co-workers with small modification [19–21]. That is, 0.5 ml cooled pure water solution of 0.01 M NaBH4 was added into 20 mL pure water solution containing 0.25 mM HAuCl4 and 0.25 mM trisodium citrate while vigorously stirring. Then the solution was placed statically for 2 h in air. A piece of glassy carbon plate was then immersed into the seed solution, and left for 2 h in order to attach the Au seed particles to the surface. After removing it from the seed solution, the glassy carbon surface was washed by pure water for several times, and then dried with nitrogen gas.
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For the Au nanocrystals growth on the glassy carbon plate, the growth solution was prepared by mixing relevant solutions as follows: 0.5 ml of 10 mM HAuCl4 solution, 18 ml of 0.10 M CTAB solution, 0.1 ml of 1.0 M NaOH solution, and typically 0.10 ml of 0.10 M ascorbic acid solution. Then the previous Au nanoseeds attached glassy carbon plate was immersed into the growth solution and left for different growth time, 4, 12, 24 h, respectively. Au nanoparticle-attached GC was taken out from the growth solution, and rinsed several times with pure water then dried for FE-SEM characterizations and electrochemical measurements. 2.4. Fabrication of AuNPs/GC electrodes For the electrochemical measurements, AuNPs/GC electrodes were prepared as follows. Firstly, the AuNPs attached GC plate was connected with a copper tape (3 M copper tape 1181, 50 mm × 6 mm). Then, a piece of tape (3 M filament tape 898, 50 mm × 18 mm) with a 4 mm-diameter hole was molded and sealed the surface of the GC plate for preventing the water penetration. 3% Nafion solution was dropped in the 4 mm-diameter hole. The exposed 4 mm-diameter hole acted as the working surface to contact with the sample solution. Before electrochemical measurements, AuNPs/GC electrodes were pretreated by cycling between −0.2 V and 1.2 V in 0.5 M H2 SO4 until a stable voltammograms was obtained. 3. Results and discussion 3.1. FE-SEM observation of Au nanoparticles-attached GC surfaces with different growth time Fig. 1a shows a typical FE-SEM image of Au nanoseeds attached on the GC plate. Au nanoseeds corresponding to bright spots are clearly seen with fine dispersion, while the black background represents the surface of glassy carbon. The average size of nanoseeds is around 4 nm, which is consistent with the size of Au nanoparticles prepared in the solution through NaBH4 reduction [28]. It illustrates that Au nanoseeds could attach to the surface of GC plate through the defects on its surface without special binder reagent (APTES, MPTMS, etc.) similar to their behaviors on ITO [18,19]. The effects of the immersion time and the concentration of Au seeds on the attachment of Au nanoseeds to the GC surface have also been investigated and the results are shown in Figures S1 and S2, respectively. It is clearly seen that the immersion time has no obvious effect on the attachment of Au nanoseeds to the GC surface (Figure S1). However higher concentration of Au nanoseeds could result in denser attachment of Au nanoseeds (Figure S2b) to the GC surface compared to lower Au seeds concentrations (Figure S2a). Some agglomeration of Au seeds occurred in the meantime, which might decrease the electrochemical activity of the later grown Au nanoparticles in sensing glucose. We therefore chose an immersion time of 2 h and Au precursor solution of 0.25 mM for the Au seeds attachment process in this work. Fig. 1b–d shows the Au nanoparticles growth after immersing the Au nanoseeds attached GC plate into the growth solution for different time, 4 h, 12 h and 24 h, respectively. It is apparently observed that, while keeping well dispersed with sphere shape, the size of AuNPs increases with the growth time. AuNPs are around 26 nm in diameter after 4 h of growth, and the average size increases to 35 nm after 12 h. And Au nanoparticles have slightly grown up to 40 nm after 24 h growth. We did not see any nanorods appearing during the growth in this study, which is quite different from Au nanorods growth on ITO surfaces as we reported previously [18,19]. The different surface characteristics such as structure and defects of GC vs. ITO might be the origin responsible for the different growth models of the
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Fig. 1. Typical FE-SEM images of Au nanoparticles modified GC plates for different growth time (a) 0 h (Au nanoseeds before immersing into the growth solution), (b) 4 h growth, (c) 12 h growth, (d) 24 h growth (zoom-in image), (e) 24 h growth (zoom-out image) and (f) XPS survey-scan spectrum of Au nanoparticles modified GC plates (24 h), the inset is the fine spectrum of Au 4f.
Au nanoparticles. As shown in the zoom-in image (Fig. 1d), some Au nanocrystals appear to stick and fuse together, which can be explained by the high activity and the growth of the nanocrystals. In the zoom-out image (Fig. 1e), Au nanoparticles keep the sphere structure and well dispersed in a larger area on the glassy carbon surface. After 24 h growth (36 and 48 h), no more growth of Au nanoparticles could be observed due to the exhausting of Au ion precursor in the solution (not shown). The attachment of dense and well-dispersed Au nanoparticles might be effective to improve the electrochemical and electrocatalytical activities of the GC electrodes. Thus we have investigated the electrochemical properties of the AuNPs/GC electrodes in the following sections. The formation of Au nanoparticles prepared for 24 h was further characterized by X-ray photoelectron spectroscopy (XPS, Fig. 1f). In the survey-scan spectrum, the main peak centered at 284.6 eV is associated with C C bonds (C 1s) due to the glassy carbon basal plane, and we can also see the peak of O 1s (534.2 eV), which is attributable to the adsorbed hydroxyl groups in the air. Furthermore, it shows significant Au 4f signals from the Au nanoparticles. As seen in the narrow-scan spectrum of Au 4f signals (inset), two peaks are clearly resolved at 84.0 eV and 87.6 eV, corresponding to metallic Au 4f7/2 and Au 4f5/2 , respectively. From these XPS results, it can be concluded that Au nanoparticles have been effectively
assembled on the surface of glassy carbon. Moreover, the surfaces of the AuNPs/GC plates are free of any other interferential elements, which is very important for the Au nanoparticles to electro-oxidize the glucose. 3.2. Electrochemical characterizations of the AuNPs/GC electrodes For evaluating the electrochemical properties of AuNPs modified glassy carbon plates, cyclic voltammograms of the AuNPs/GC electrodes (4 and 24 h growth) were carried out in 0.5 M sulfuric acid aqueous solution compared with both the pure gold bulk electrode and glassy carbon electrode (Fig. 2). The AuNPs/GC electrodes show similar electrochemical response to the bulk Au electrode. The typical anodic oxidation peak starting from 1.1 V and the cathodic reduction peak appearing at 0.95 V are attributed to the formation of gold oxide and subsequently its reductive reaction, respectively. No response is observed on the pure glassy carbon electrode. This comparison confirms the AuNPs attachment on the glassy carbon surface. Moreover, the gradually raising anodic peak of AuNPs/GC for different growth time (4 h and 24 h) reflects the increasing electrochemical area due to the attachment of Au nanoparticles, consistent with the observation of FE-SEM stated above.
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Fig. 2. Cyclic voltammograms obtained with (a) pure Au bulk electrode, (b) pure glassy carbon electrode, (c) AuNPs/GC (4 h growth) and (d) AuNPs/GC (24 h growth) electrodes in the solution containing 0.5 M H2 SO4 . Scan rate: 100 mV s−1 .
Fig. 4. Linear voltammograms of the AuNPs/GC (24 h growth) electrode in 0.1 M NaOH with concentrations of glucose varying from 0.1 mM to 30 mM. Scan rate: 100 mV s−1 .
3.3. Electrochemical oxidation of glucose on AuNPs/GC electrodes For investigating the electrocatalytical properties after the attachment of Au nanoparticles, we examined the electrocatalytical oxidation of the glucose on AuNPs/GC electrodes. Fig. 3 shows the cyclic voltammograms of 10 mM glucose solution with different electrodes, glassy carbon electrode, pure gold bulk electrode and AuNPs/GC electrodes (4, 12, 24 h growth), respectively. The potential sweep was performed from −0.4 V to 0.7 V. While no response appearing on the pure glassy carbon electrode, weak oxidation peak is observed for the pure gold electrode in the anodic scanning. Similar to the electrocatalytic oxidation of glucose on the pure Au bulk electrode, one broad oxidation peak appears from −0.3 V to 0.4 V on the AuNPs/GC electrode. The shoulder peak around −0.1 V could be ascribed to the adsorption of glucose on the AuNPs/GC surface and the formation of gluconolactone, the reaction of two-electron transferring [22]. With the increase of potential, enhanced oxidation peak appears at about 0.3 V in the anodic direction, which is assigned to the continuous oxidation of gluconolactone. This indicates that the dense attachment of Au nanoparticles on GC electrode enhances the catalytic activity for the glucose oxidation compared to the bulk Au. The gold oxide is formed on the surface after 0.4 V and passivates the active surface of Au, which causes the decrease
of the oxidation peak. In the cathodic scanning, fresh Au surface is released by the reduction of Au oxide. Thus another oxidation peak appears at the potential of about 0.05 V [22,24,27,29]. In addition, the proper increasing of the size of Au nanoparticles contributes to the enhancement of the electrocatalytical activity, as evidenced by the increasing peak currents from AuNPs/GC electrodes with Au nanoparticles grown for longer periods. The enhanced electrocatalytical activity results from the extended active surface area provided by the dense gold nanoparticles as shown in our previous study [18–21]. Based on this we determine that the appropriate growth time for Au nanoparticles in the growth solution is 24 h. The relationship of the oxidation current with the glucose concentration was investigated through the linear voltammograms measurement from 0.1 mM to 30 mM (Fig. 4). The oxidation peak rises with increasing concentration of the glucose. The glucose oxidation peaks located around 0.24 V as derived from Fig. 4 are linearly correlated to the concentrations of glucose (Fig. 5). It possesses a high coefficient of 0.999 with a sensitivity factor of 87.5 A cm−2 mM−1 and a wide linear range from 0.1 to 25 mM, which covers the normal physiological range for the detection of blood sugar. The detection limit is down to 0.05 mM at a signal to noise ratio of three. This could act as a calibration for the nonenzymatic biosensors application. The AuNPs/GC electrodes derived in
Fig. 3. Cyclic voltammograms of different electrodes in 0.1 M NaOH solution containing 10 mM glucose. Working electrodes: (a) pure glassy carbon, (b) pure Au bulk and (c–e) AuNPs/GC (4, 12, 24 h growth, respectively). Scan rate: 100 mV s−1 .
Fig. 5. Dependence of the peak currents on the glucose concentrations (from 0.1 mM to 30 mM), which is derived from Fig. 4.
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agglomeration of some nanoparticles with highly active surfaces. The durability of AuNPs/GC electrode was studied after storing the electrode in air for four weeks. No obvious variation could be found which assured the long-term stability of the electrode for biosensors. 3.5. Conclusions
Fig. 6. Effects of the interfering species (0.1 mM ascorbic acid, 0.2 mM uric acid, 0.1 mM p-acetaminophen and 0.15 M NaCl) toward the electrocatalytic oxidation of glucose with the AuNPs/GC electrode (24 h growth) in the 0.1 M NaOH solution containing 5 mM glucose. Scan rate: 100 mV s−1 .
this study exhibit a broader linear detection range and a comparable sensing sensitivity compared to the biosensors reported in the literature [24,27,30,31]. In addition, the in situ and direct growth of nanoparticles on the GC substrates provides a more convenient means for making clean electrode materials for the oxidation of glucose with high catalytic activity. 3.4. Effect of the interfering species and stability of the electrode on the detection of glucose Au nanoparticles dense attachment on the glassy carbon provides more active surface area and thus enhanced electrocatalytic performances for the glucose oxidation as seen in Fig. 4. However, the oxidation of interfering species on the electrode, which is mainly controlled by the diffusion, might be less affected by the extended active surface area of the gold nanoparticles [24]. These would improve the selectivity of the electrode, which is of great importance for the nonenzymatic glucose sensors. Hence, it was of both fundamental and application interests to evaluate effects of the interfering species such as ascorbic acid, uric acid and pacetaminophen on the AuNPs/GC electrode toward electrocatalytic oxidation of glucose in the alkaline condition through the linear voltammograms (Fig. 6). Additionally, the chloride ions existing naturally in the physiological environment often have serious poison effects for the electrocatalysis of glucose on the noble metals because of the formation of complex intermediates [1,2,5]. Therefore, the interfering effect of the chloride ions was also investigated for the AuNPs/GC electrode. As seen in Fig. 6, adding of the typical interfering molecules in blood such as AA, AP, UA, etc. and the chloride ions could not bring apparent change for the oxidation of glucose on the AuNPs/GC electrode. The glassy carbon electrode attached with dense and well-dispersed Au nanoparticles shows excellent catalytic ability and selectivity even with existence of complicated interfering species, implying its great potential in the fabrication of nonenzymatic glucose biosensors. The stability of AuNPs/GC electrode on the electrochemical oxidation of glucose was also evaluated through comparing SEM images of the electrode before and after glucose sensing test (Figure S3). No obvious changes could be found in the size and morphology of Au nanoparticles before and after the electrooxidation test, suggesting a good stability of Au nanoparticles in the harsh electrochemical process. The number of Au particles was slightly decreased from 510/m2 to 480/m2 , which might result from
Au nanoparticles were successfully prepared on the surface of the glassy carbon plates by the modified seed mediated growth method. The size and density of the gold nanoparticles could be easily tuned through the immersion time in the growth solution. The attachment of dense and well-dispersed Au nanoparticles on the electrodes could significantly promote the electrocatalytic ability toward the oxidation of glucose for the electrodes. This confirms that Au nanoparticles could work as excellent electrode materials for the sensing of glucose. The AuNPs/GC electrodes show a wide linear range from 0.1 mM to 25 mM with a sensitivity of 87.5 A cm−2 mM−1 and a detection limit down to 0.05 mM for the detection of glucose. Common interfering species naturally present in the physiological environment have no obvious effects for the oxidation of glucose on AuNPs/GC electrodes. No obvious changing was observed after one month storage of the AuNPs/GC electrode in air, indicating an excellent stability of the electrode. The good electrocatalytic ability, high selectivity and excellent stability all together imply a promising perspective of AuNPs/GC electrodes in the fabrication of nonenzymatic glucose biosensors. Furthermore, the present approach might be extendable to other metal nanoparticles immobilization on certain substrates with different sensing and catalysis applications. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 51102085, 50972041, 61274010), Program for New Century Excellent Talents in University, Ministry of Education of China (NCET-09-0135), Natural Science Foundation of Hubei Province (Nos. 2011CDB057, 2011CDA81) and the science foundation from Hubei Provincial Department of Education (No. Q20111002), which are gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2013. 10.064. References [1] K.E. Toghill, R.G. Compton, Electrochemical non-enzymatic glucose sensors: a perspective and an evaluation, Int. J. Electrochem. Sci. 5 (2010) 1246–1301. [2] S. Park, H. Boo, T.D. Chung, Electrochemical non-enzymatic glucose sensors, Anal. Chim. Acta 556 (2006) 46–57. [3] S. Zhang, Y. Shao, G. Yin, Y. Lin, Electrostatic self-assembly of a Pt-around-Au nanocomposite with high activity towards formic acid oxidation, Angew. Chem Int. Ed. 49 (2010) 2211–2214. [4] C. Xia, W. Ning, A novel non-enzymatic electrochemical glucose sensor modified with FeOOH nanowire, Electrochem. Commun. 12 (2010) 1581–1584. [5] J. Wang, Electrochemical glucose biosensors, Chem. Rev. 108 (2008) 814–825. [6] A. Heller, B. Feldman, Electrochemical glucose sensors and their applications in diabetes management, Chem. Rev. 108 (2008) 2482–2505. [7] L. Meng, J. Jin, G. Yang, T. Lu, H. Zhang, C. Cai, Nonenzymatic electrochemical detection of glucose based on palladium-single-walled carbon nanotube hybrid nanostructures, Anal. Chem. 81 (2009) 7271–7280. [8] F. Xiao, F. Zhao, D. Mei, Z. Mo, B. Zeng, Nonenzymatic glucose sensor based on ultrasonic-electrode position of bimetallic PtM (M = Ru, Pd and Au) nanoparticles on carbon nanotubes-ionic liquid composite film, Biosens. Bioelectron. 24 (2009) 3481–3486. [9] S.J. Guo, S.J. Dong, E.W. Wang, Three-dimensional Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheet: facile synthesis and used as
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Gold nanoparticles directly modified glassy carbon electrode for non-enzymatic detection of glucose Gang Chang a , Honghui Shu a , Kai Ji a , Munetaka Oyama b , Xiong Liu a , Yunbin He a,∗ a Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Faculty of Materials Science and Engineering, Hubei University, No. 368 Youyi Avenue, Wuchang, Wuhan 430062, China b Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan
a r t i c l e
i n f o
Article history: Received 21 July 2013 Received in revised form 28 September 2013 Accepted 11 October 2013 Available online 19 October 2013 Keywords: Gold nanoparticles Glassy carbon Glucose Nonenzymatic biosensor Seed-mediated growth method
a b s t r a c t This work describes controllable preparation of gold nanoparticles on glassy carbon electrodes by using the seed mediated growth method, which contains two steps, namely, nanoseeds attachment and nanocrystals growth. The size and the dispersion of gold nanoparticles grown on glassy carbon electrodes could be easily tuned through the growth time based on results of field-emission scanning electron microscopy. Excellent electrochemical catalytic characteristics for glucose oxidation were observed for the gold nanoparticles modified glassy carbon electrodes (AuNPs/GC), resulting from the extended active surface area provided by the dense gold nanoparticles attached. It exhibited a wide linear range from 0.1 mM to 25 mM with the sensitivity of 87.5 A cm−2 mM−1 and low detection limit down to 0.05 mM for the sensing of glucose. The common interfering species such as chloride ion, ascorbic acid, uric acid and 4-acetamidophenol were verified having no interference effect on the detection of glucose. It is demonstrated that the seed mediated method is one of the facile approaches for fabricating Au nanoparticles modified substrates, which could work as one kind of promising electrode materials for the glucose nonenzymatic sensing. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Extensive studies have been devoted to nanoparticles modified surfaces in the recent decades because of their potential applications in fuel cells, biosensors, new photonic materials, and so on [1–10]. Unique optical, electronic, catalytic and magnetic properties of nanoparticles could be easily introduced into devices through the nanoparticles attachment. Therefore, various approaches for nanoparticles modification have been developed including electrochemical deposition, sputtering, bridging agent assistant, layer by layer, etc. [3,11,12]. Noble metal nanoparticles are convenient to modify the substrates by using bridging agents such as aminopropyltrimethoxysilane (APTES), mercaptopropyltrimethoxysilane (MPTMS) or some macromolecules (e.g. poly-l-lysine) [11,13–17]. However the adoption of bridging agent might block the electron transferring, which would seriously affect the sensitivity when they are used in the electroanalytical or electrocatalytical fields [18,19]. In our previous reports, it was possible to directly attach the noble metal nanoparticles onto conductive surfaces such as ITO without any linker molecules by using the modified seed mediated method,
∗ Corresponding author. Tel.: +86 278 8661803; fax: +86 278 8661803. E-mail addresses: [email protected], [email protected] (Y. He). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.064
which originated from the preparation of one-dimension nanomaterials in aqueous solution. In this method, metal nano-seeds with a diameter around 4 nm were first in situ attached to the substrate due to the strong adsorption capability of nanoparticles and defects present on the substrate surface. And then the seeds could grow up to nanoparticles with different sizes and morphology such as nanospheres and nanorods by immersing the nanoseeds attached surface into the growth solution. Through this method, Au, Ag and Pd nanoparticles have been successfully prepared on the ITO surface [18–21]. Attractive electrochemical and electrocatalytical properties were observed for these nanoparticles modified ITO surface, which should be attributed to the acceleration of interface electrons transfer resulted from the direct attachment of noble metal nanoparticles without linker molecules. Therefore it is reasonable to believe that this method could be extended to other substrates such as glassy carbon, which can find further electrocatalytical and biosensor applications for their higher conductivity. Especially, gold nanoparticles show the interesting activity for glucose oxidation in neutral and alkaline solution. Gold nanoparticles modified surfaces for enzyme-free electrochemical glucose biosensors have attracted enormous interests because it overcomes the drawback in the traditional glucose biosensor, the instability of glucose oxidase. It might bring great changes in the detection and treatment of diabetes [6,10,22–25]. Due to lacking of the unique selectivity of the glucose oxidase in the enzymatic biosensor, it
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has much higher quality requests for the nonenzymatic glucose biosensors in the abilities of electrocatalysis, anti-interferences and poison-resistance, which are much dependent on the morphology and structure of gold nanoparticles on the electrodes. Up to now, gold nanoparticles and their nanocomposites with different morphology were achieved by different approaches and applied into the glucose sensing without enzyme. Electrodepositing is one of the main methods to modify gold nanoparticles on the electrode surface [12,26,27]. However facile tuning and controlling of the size and morphology of gold nanoparticles on the surfaces remain still challenging. In the present work, we prepared Au nanoparticles modified glassy carbon electrodes by the seed mediated growth method. The sizes and morphologies of Au nanoparticles grown on the glassy carbon surfaces were systematically investigated through the field-emission scanning electron microscopy (FE-SEM). The electrochemical properties of the Au nanoparticles modified glassy carbon (AuNPs/GC) electrodes were studied in comparison to those of Au bulk electrodes and pure glassy carbon electrodes by the electrochemical measurements. Cyclic and linear voltammograms were applied to evaluate the oxidation of glucose on the AuNPs/GC electrodes prepared for 24 h. The AuNPs/GC electrodes exhibit excellent catalytical activity and selectivity for the glucose oxidation, which is promising for the application in nonenzymatic glucose biosensors. 2. Material and methods 2.1. Materials Cetyltrimethylammonium bromide (CTAB) and HAuCl4 ·3H2 O were purchased from Aldrich Co., Ltd. Trisodium citrate, ascorbic acid, NaBH4 , ethanol, acetone and NaOH were obtained from Sinopharm Chemical Regent Co. Ltd. Glassy carbon plates (size 10 mm × 10 mm × 1.0 mm) and gold bulk electrodes (diameter, 1.8 mm) were ordered from BAS. Co. Ltd. The glassy carbon plates were firstly polished to mirror-like by using polishing paper and alumina polishing powders, respectively. Then they were washed sequentially in acetone, ethanol and pure water with sonication each for 15 min and dried with nitrogen gas. In all the procedures, we used pure water prepared with Kertone Ultrapure Water System P60-CY (Kertone Water Treatment Co. Ltd, resistivity > 18 M cm). 2.2. Apparatuses The size and morphology of the Au nanoparticles attached to the glassy carbon plates were characterized with field-emission scanning electron microscopy (FE-SEM, Carl Zeiss Merlin). All electrochemical measurements were carried out with a 550 electrochemical workstation (Gaoss Union Instrument Company, Wuhan, China) in a conventional three-electrode cell. 2.3. Procedures A seed solution of 4 nm Au colloid was prepared by the procedure reported by Oyama and co-workers with small modification [19–21]. That is, 0.5 ml cooled pure water solution of 0.01 M NaBH4 was added into 20 mL pure water solution containing 0.25 mM HAuCl4 and 0.25 mM trisodium citrate while vigorously stirring. Then the solution was placed statically for 2 h in air. A piece of glassy carbon plate was then immersed into the seed solution, and left for 2 h in order to attach the Au seed particles to the surface. After removing it from the seed solution, the glassy carbon surface was washed by pure water for several times, and then dried with nitrogen gas.
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For the Au nanocrystals growth on the glassy carbon plate, the growth solution was prepared by mixing relevant solutions as follows: 0.5 ml of 10 mM HAuCl4 solution, 18 ml of 0.10 M CTAB solution, 0.1 ml of 1.0 M NaOH solution, and typically 0.10 ml of 0.10 M ascorbic acid solution. Then the previous Au nanoseeds attached glassy carbon plate was immersed into the growth solution and left for different growth time, 4, 12, 24 h, respectively. Au nanoparticle-attached GC was taken out from the growth solution, and rinsed several times with pure water then dried for FE-SEM characterizations and electrochemical measurements. 2.4. Fabrication of AuNPs/GC electrodes For the electrochemical measurements, AuNPs/GC electrodes were prepared as follows. Firstly, the AuNPs attached GC plate was connected with a copper tape (3 M copper tape 1181, 50 mm × 6 mm). Then, a piece of tape (3 M filament tape 898, 50 mm × 18 mm) with a 4 mm-diameter hole was molded and sealed the surface of the GC plate for preventing the water penetration. 3% Nafion solution was dropped in the 4 mm-diameter hole. The exposed 4 mm-diameter hole acted as the working surface to contact with the sample solution. Before electrochemical measurements, AuNPs/GC electrodes were pretreated by cycling between −0.2 V and 1.2 V in 0.5 M H2 SO4 until a stable voltammograms was obtained. 3. Results and discussion 3.1. FE-SEM observation of Au nanoparticles-attached GC surfaces with different growth time Fig. 1a shows a typical FE-SEM image of Au nanoseeds attached on the GC plate. Au nanoseeds corresponding to bright spots are clearly seen with fine dispersion, while the black background represents the surface of glassy carbon. The average size of nanoseeds is around 4 nm, which is consistent with the size of Au nanoparticles prepared in the solution through NaBH4 reduction [28]. It illustrates that Au nanoseeds could attach to the surface of GC plate through the defects on its surface without special binder reagent (APTES, MPTMS, etc.) similar to their behaviors on ITO [18,19]. The effects of the immersion time and the concentration of Au seeds on the attachment of Au nanoseeds to the GC surface have also been investigated and the results are shown in Figures S1 and S2, respectively. It is clearly seen that the immersion time has no obvious effect on the attachment of Au nanoseeds to the GC surface (Figure S1). However higher concentration of Au nanoseeds could result in denser attachment of Au nanoseeds (Figure S2b) to the GC surface compared to lower Au seeds concentrations (Figure S2a). Some agglomeration of Au seeds occurred in the meantime, which might decrease the electrochemical activity of the later grown Au nanoparticles in sensing glucose. We therefore chose an immersion time of 2 h and Au precursor solution of 0.25 mM for the Au seeds attachment process in this work. Fig. 1b–d shows the Au nanoparticles growth after immersing the Au nanoseeds attached GC plate into the growth solution for different time, 4 h, 12 h and 24 h, respectively. It is apparently observed that, while keeping well dispersed with sphere shape, the size of AuNPs increases with the growth time. AuNPs are around 26 nm in diameter after 4 h of growth, and the average size increases to 35 nm after 12 h. And Au nanoparticles have slightly grown up to 40 nm after 24 h growth. We did not see any nanorods appearing during the growth in this study, which is quite different from Au nanorods growth on ITO surfaces as we reported previously [18,19]. The different surface characteristics such as structure and defects of GC vs. ITO might be the origin responsible for the different growth models of the
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Fig. 1. Typical FE-SEM images of Au nanoparticles modified GC plates for different growth time (a) 0 h (Au nanoseeds before immersing into the growth solution), (b) 4 h growth, (c) 12 h growth, (d) 24 h growth (zoom-in image), (e) 24 h growth (zoom-out image) and (f) XPS survey-scan spectrum of Au nanoparticles modified GC plates (24 h), the inset is the fine spectrum of Au 4f.
Au nanoparticles. As shown in the zoom-in image (Fig. 1d), some Au nanocrystals appear to stick and fuse together, which can be explained by the high activity and the growth of the nanocrystals. In the zoom-out image (Fig. 1e), Au nanoparticles keep the sphere structure and well dispersed in a larger area on the glassy carbon surface. After 24 h growth (36 and 48 h), no more growth of Au nanoparticles could be observed due to the exhausting of Au ion precursor in the solution (not shown). The attachment of dense and well-dispersed Au nanoparticles might be effective to improve the electrochemical and electrocatalytical activities of the GC electrodes. Thus we have investigated the electrochemical properties of the AuNPs/GC electrodes in the following sections. The formation of Au nanoparticles prepared for 24 h was further characterized by X-ray photoelectron spectroscopy (XPS, Fig. 1f). In the survey-scan spectrum, the main peak centered at 284.6 eV is associated with C C bonds (C 1s) due to the glassy carbon basal plane, and we can also see the peak of O 1s (534.2 eV), which is attributable to the adsorbed hydroxyl groups in the air. Furthermore, it shows significant Au 4f signals from the Au nanoparticles. As seen in the narrow-scan spectrum of Au 4f signals (inset), two peaks are clearly resolved at 84.0 eV and 87.6 eV, corresponding to metallic Au 4f7/2 and Au 4f5/2 , respectively. From these XPS results, it can be concluded that Au nanoparticles have been effectively
assembled on the surface of glassy carbon. Moreover, the surfaces of the AuNPs/GC plates are free of any other interferential elements, which is very important for the Au nanoparticles to electro-oxidize the glucose. 3.2. Electrochemical characterizations of the AuNPs/GC electrodes For evaluating the electrochemical properties of AuNPs modified glassy carbon plates, cyclic voltammograms of the AuNPs/GC electrodes (4 and 24 h growth) were carried out in 0.5 M sulfuric acid aqueous solution compared with both the pure gold bulk electrode and glassy carbon electrode (Fig. 2). The AuNPs/GC electrodes show similar electrochemical response to the bulk Au electrode. The typical anodic oxidation peak starting from 1.1 V and the cathodic reduction peak appearing at 0.95 V are attributed to the formation of gold oxide and subsequently its reductive reaction, respectively. No response is observed on the pure glassy carbon electrode. This comparison confirms the AuNPs attachment on the glassy carbon surface. Moreover, the gradually raising anodic peak of AuNPs/GC for different growth time (4 h and 24 h) reflects the increasing electrochemical area due to the attachment of Au nanoparticles, consistent with the observation of FE-SEM stated above.
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Fig. 2. Cyclic voltammograms obtained with (a) pure Au bulk electrode, (b) pure glassy carbon electrode, (c) AuNPs/GC (4 h growth) and (d) AuNPs/GC (24 h growth) electrodes in the solution containing 0.5 M H2 SO4 . Scan rate: 100 mV s−1 .
Fig. 4. Linear voltammograms of the AuNPs/GC (24 h growth) electrode in 0.1 M NaOH with concentrations of glucose varying from 0.1 mM to 30 mM. Scan rate: 100 mV s−1 .
3.3. Electrochemical oxidation of glucose on AuNPs/GC electrodes For investigating the electrocatalytical properties after the attachment of Au nanoparticles, we examined the electrocatalytical oxidation of the glucose on AuNPs/GC electrodes. Fig. 3 shows the cyclic voltammograms of 10 mM glucose solution with different electrodes, glassy carbon electrode, pure gold bulk electrode and AuNPs/GC electrodes (4, 12, 24 h growth), respectively. The potential sweep was performed from −0.4 V to 0.7 V. While no response appearing on the pure glassy carbon electrode, weak oxidation peak is observed for the pure gold electrode in the anodic scanning. Similar to the electrocatalytic oxidation of glucose on the pure Au bulk electrode, one broad oxidation peak appears from −0.3 V to 0.4 V on the AuNPs/GC electrode. The shoulder peak around −0.1 V could be ascribed to the adsorption of glucose on the AuNPs/GC surface and the formation of gluconolactone, the reaction of two-electron transferring [22]. With the increase of potential, enhanced oxidation peak appears at about 0.3 V in the anodic direction, which is assigned to the continuous oxidation of gluconolactone. This indicates that the dense attachment of Au nanoparticles on GC electrode enhances the catalytic activity for the glucose oxidation compared to the bulk Au. The gold oxide is formed on the surface after 0.4 V and passivates the active surface of Au, which causes the decrease
of the oxidation peak. In the cathodic scanning, fresh Au surface is released by the reduction of Au oxide. Thus another oxidation peak appears at the potential of about 0.05 V [22,24,27,29]. In addition, the proper increasing of the size of Au nanoparticles contributes to the enhancement of the electrocatalytical activity, as evidenced by the increasing peak currents from AuNPs/GC electrodes with Au nanoparticles grown for longer periods. The enhanced electrocatalytical activity results from the extended active surface area provided by the dense gold nanoparticles as shown in our previous study [18–21]. Based on this we determine that the appropriate growth time for Au nanoparticles in the growth solution is 24 h. The relationship of the oxidation current with the glucose concentration was investigated through the linear voltammograms measurement from 0.1 mM to 30 mM (Fig. 4). The oxidation peak rises with increasing concentration of the glucose. The glucose oxidation peaks located around 0.24 V as derived from Fig. 4 are linearly correlated to the concentrations of glucose (Fig. 5). It possesses a high coefficient of 0.999 with a sensitivity factor of 87.5 A cm−2 mM−1 and a wide linear range from 0.1 to 25 mM, which covers the normal physiological range for the detection of blood sugar. The detection limit is down to 0.05 mM at a signal to noise ratio of three. This could act as a calibration for the nonenzymatic biosensors application. The AuNPs/GC electrodes derived in
Fig. 3. Cyclic voltammograms of different electrodes in 0.1 M NaOH solution containing 10 mM glucose. Working electrodes: (a) pure glassy carbon, (b) pure Au bulk and (c–e) AuNPs/GC (4, 12, 24 h growth, respectively). Scan rate: 100 mV s−1 .
Fig. 5. Dependence of the peak currents on the glucose concentrations (from 0.1 mM to 30 mM), which is derived from Fig. 4.
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agglomeration of some nanoparticles with highly active surfaces. The durability of AuNPs/GC electrode was studied after storing the electrode in air for four weeks. No obvious variation could be found which assured the long-term stability of the electrode for biosensors. 3.5. Conclusions
Fig. 6. Effects of the interfering species (0.1 mM ascorbic acid, 0.2 mM uric acid, 0.1 mM p-acetaminophen and 0.15 M NaCl) toward the electrocatalytic oxidation of glucose with the AuNPs/GC electrode (24 h growth) in the 0.1 M NaOH solution containing 5 mM glucose. Scan rate: 100 mV s−1 .
this study exhibit a broader linear detection range and a comparable sensing sensitivity compared to the biosensors reported in the literature [24,27,30,31]. In addition, the in situ and direct growth of nanoparticles on the GC substrates provides a more convenient means for making clean electrode materials for the oxidation of glucose with high catalytic activity. 3.4. Effect of the interfering species and stability of the electrode on the detection of glucose Au nanoparticles dense attachment on the glassy carbon provides more active surface area and thus enhanced electrocatalytic performances for the glucose oxidation as seen in Fig. 4. However, the oxidation of interfering species on the electrode, which is mainly controlled by the diffusion, might be less affected by the extended active surface area of the gold nanoparticles [24]. These would improve the selectivity of the electrode, which is of great importance for the nonenzymatic glucose sensors. Hence, it was of both fundamental and application interests to evaluate effects of the interfering species such as ascorbic acid, uric acid and pacetaminophen on the AuNPs/GC electrode toward electrocatalytic oxidation of glucose in the alkaline condition through the linear voltammograms (Fig. 6). Additionally, the chloride ions existing naturally in the physiological environment often have serious poison effects for the electrocatalysis of glucose on the noble metals because of the formation of complex intermediates [1,2,5]. Therefore, the interfering effect of the chloride ions was also investigated for the AuNPs/GC electrode. As seen in Fig. 6, adding of the typical interfering molecules in blood such as AA, AP, UA, etc. and the chloride ions could not bring apparent change for the oxidation of glucose on the AuNPs/GC electrode. The glassy carbon electrode attached with dense and well-dispersed Au nanoparticles shows excellent catalytic ability and selectivity even with existence of complicated interfering species, implying its great potential in the fabrication of nonenzymatic glucose biosensors. The stability of AuNPs/GC electrode on the electrochemical oxidation of glucose was also evaluated through comparing SEM images of the electrode before and after glucose sensing test (Figure S3). No obvious changes could be found in the size and morphology of Au nanoparticles before and after the electrooxidation test, suggesting a good stability of Au nanoparticles in the harsh electrochemical process. The number of Au particles was slightly decreased from 510/m2 to 480/m2 , which might result from
Au nanoparticles were successfully prepared on the surface of the glassy carbon plates by the modified seed mediated growth method. The size and density of the gold nanoparticles could be easily tuned through the immersion time in the growth solution. The attachment of dense and well-dispersed Au nanoparticles on the electrodes could significantly promote the electrocatalytic ability toward the oxidation of glucose for the electrodes. This confirms that Au nanoparticles could work as excellent electrode materials for the sensing of glucose. The AuNPs/GC electrodes show a wide linear range from 0.1 mM to 25 mM with a sensitivity of 87.5 A cm−2 mM−1 and a detection limit down to 0.05 mM for the detection of glucose. Common interfering species naturally present in the physiological environment have no obvious effects for the oxidation of glucose on AuNPs/GC electrodes. No obvious changing was observed after one month storage of the AuNPs/GC electrode in air, indicating an excellent stability of the electrode. The good electrocatalytic ability, high selectivity and excellent stability all together imply a promising perspective of AuNPs/GC electrodes in the fabrication of nonenzymatic glucose biosensors. Furthermore, the present approach might be extendable to other metal nanoparticles immobilization on certain substrates with different sensing and catalysis applications. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 51102085, 50972041, 61274010), Program for New Century Excellent Talents in University, Ministry of Education of China (NCET-09-0135), Natural Science Foundation of Hubei Province (Nos. 2011CDB057, 2011CDA81) and the science foundation from Hubei Provincial Department of Education (No. Q20111002), which are gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2013. 10.064. References [1] K.E. Toghill, R.G. Compton, Electrochemical non-enzymatic glucose sensors: a perspective and an evaluation, Int. J. Electrochem. Sci. 5 (2010) 1246–1301. [2] S. Park, H. Boo, T.D. Chung, Electrochemical non-enzymatic glucose sensors, Anal. Chim. Acta 556 (2006) 46–57. [3] S. Zhang, Y. Shao, G. Yin, Y. Lin, Electrostatic self-assembly of a Pt-around-Au nanocomposite with high activity towards formic acid oxidation, Angew. Chem Int. Ed. 49 (2010) 2211–2214. [4] C. Xia, W. Ning, A novel non-enzymatic electrochemical glucose sensor modified with FeOOH nanowire, Electrochem. Commun. 12 (2010) 1581–1584. [5] J. Wang, Electrochemical glucose biosensors, Chem. Rev. 108 (2008) 814–825. [6] A. Heller, B. Feldman, Electrochemical glucose sensors and their applications in diabetes management, Chem. Rev. 108 (2008) 2482–2505. [7] L. Meng, J. Jin, G. Yang, T. Lu, H. Zhang, C. Cai, Nonenzymatic electrochemical detection of glucose based on palladium-single-walled carbon nanotube hybrid nanostructures, Anal. Chem. 81 (2009) 7271–7280. [8] F. Xiao, F. Zhao, D. Mei, Z. Mo, B. Zeng, Nonenzymatic glucose sensor based on ultrasonic-electrode position of bimetallic PtM (M = Ru, Pd and Au) nanoparticles on carbon nanotubes-ionic liquid composite film, Biosens. Bioelectron. 24 (2009) 3481–3486. [9] S.J. Guo, S.J. Dong, E.W. Wang, Three-dimensional Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheet: facile synthesis and used as
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