Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490

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Journal of Phys...
S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490

atom of the NH moiety of piperazine gr...
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S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490

Fig. 2. Chemical formula of CF.
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S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490

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Fig. 4. (a) SEM images of Au-NPs...
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S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490

high uniformity in the resultant...
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Fig. 8. CVs of the Au-NPs/GCE in...
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S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490

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Synthetic antibacterial agent assisted synthesis of gold nanoparticles characterization and application studies

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In this study, we report synthesis of water-soluble gold nanoparticles (Au-NPs), having an average
diameter of ca. 20 nm, using ciprofloxacin (CF) as a reducing/stabilizing agent. The synthesized
Au-NPs have been characterized by scanning electron microscopy (SEM), EDX, TEM, UV–visible
spectroscopy (UV–vis), X-ray diffraction and cyclic voltammetry. TEM and SEM combined with EDX
analysis confirmed that spherical-shaped Au-NPs were formed. UV–vis spectra of the Au-NPs showed
two absorption bands corresponding to the capping agent ciprofloxacin and surface plasmon absorption
bands at 274 and 527 nm, respectively. The synthesized Au-NPs are used to modify a glassy carbon
electrode (GCE) and its electrochemical and electrocatalytic properties are investigated. The Au-NPs
modified electrode showed excellent electrocatalytic activity towards the oxidation of methanol at
+0.33 V in alkaline solution, which was not observed on the unmodified GCE. Further, electrocatalytic
reduction of oxygen was also studied using the Au-NPs modified electrode at lower potential. Here, CF
was used as a reducing agent for the preparation of Au-NPs dispersion. This Au-NPs dispersion is highly
stable, and can be stored for more than three months in air at room temperature.

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Synthetic antibacterial agent assisted synthesis of gold nanoparticles characterization and application studies

  1. 1. Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490 Contents lists available at ScienceDirect Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs Synthetic antibacterial agent assisted synthesis of gold nanoparticles: Characterization and application studies S. Ashok Kumar a,n, Yu-Tsern Chang b, Sea-Fue Wang a,n, His-Chuan Lu a a b Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, No.1, Section 3, Chung-Hsiao E. Road, Taipei, Taiwan Department of Chemical and Materials Engineering, Nanya Institute of Technology, Jhongli 32091, Taiwan a r t i c l e in f o a b s t r a c t Article history: Received 19 February 2010 Received in revised form 5 July 2010 Accepted 15 July 2010 In this study, we report synthesis of water-soluble gold nanoparticles (Au-NPs), having an average diameter of ca. $ 20 nm, using ciprofloxacin (CF) as a reducing/stabilizing agent. The synthesized Au-NPs have been characterized by scanning electron microscopy (SEM), EDX, TEM, UV–visible spectroscopy (UV–vis), X-ray diffraction and cyclic voltammetry. TEM and SEM combined with EDX analysis confirmed that spherical-shaped Au-NPs were formed. UV–vis spectra of the Au-NPs showed two absorption bands corresponding to the capping agent ciprofloxacin and surface plasmon absorption bands at 274 and 527 nm, respectively. The synthesized Au-NPs are used to modify a glassy carbon electrode (GCE) and its electrochemical and electrocatalytic properties are investigated. The Au-NPs modified electrode showed excellent electrocatalytic activity towards the oxidation of methanol at +0.33 V in alkaline solution, which was not observed on the unmodified GCE. Further, electrocatalytic reduction of oxygen was also studied using the Au-NPs modified electrode at lower potential. Here, CF was used as a reducing agent for the preparation of Au-NPs dispersion. This Au-NPs dispersion is highly stable, and can be stored for more than three months in air at room temperature. & 2010 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures A. Thin films B. Chemical synthesis C. X-ray diffraction 1. Introduction Synthesis of stable gold nanoparticles (Au-NPs) and their applications has been the subject of great interest, due to their remarkable physical and chemical properties. Au-NPs have uniform structures with promising stability, and their size-related electronic, magnetic, and optical properties [1,2] make them promising in important research areas such as catalysis [3], biosensors [4], drug delivery [5], energy-related applications [6], and biological applications [7]. Recently, researchers have studied the electronic and photonic properties of nanoparticles doped with different materials, and they expect potential applications of nanomaterials in building quantum computers [8–11]. Controlling the size of nanoparticles (NPs) has always been one of the challenges in colloidal science. Changing the size of NPs can result in modulation of their physical and chemical properties. Since the discovery of various reducing agents for the gold compounds to form the Au-NPs, like sodium citrate, sodium borohydride, phosphorus, alcohols, and tannic acid/citrate mixtures, the synthesis and applications of Au-NPs of different sizes have flourished [12]. Recently, gold nanoparticles capped with novel n Corresponding authors. Tel.: + 886 2 27712171 Â 2735; fax: + 886 2 27317185. E-mail addresses: sakumar80@gmail.com (S.A. Kumar), seafuewang@yahoo.com (S.-F. Wang). 0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.07.015 zwitterionic disulfide ligands was reported which showed remarkable stability in saline media [13]. Cetyltrimethylammonium bromide/silver bromide complex as the capping agent of gold nanorods [14], and preparation of gold nanoparticles using ascorbic acid as a reducing agent in reverse micelles [15] were also reported. Moreover, Au-NPs in the range of 5–6 nm were synthesized using sodium borohydride in the presence of newly synthesized mono-6-deoxy-6-pyridinium-b-cyclodextrin chloride [16]. In those methods, different reducing and capping agents were used together to prepare stable Au-NPs dispersion. In few studies, some reagents were used as a reducing agent as well as a stabilizing agent for Au-NPs. Gold nanoflowers were obtained by a one-pot synthesis using N-2-hydroxyethylpiperazine-N-2-ethanesulphonic acid as a reducing/stabilizing agent and their electrocatalytic effect towards oxidation of methanol was studied [17]. Zhang et al. [18] reported synthesis of various gold nanostructures on glassy carbon electrode in a low concentration of HAuCl4 solution (5 mM), and enzyme-free sensor was developed for the detection of glucose in pH 7.4 phosphate buffer solutions. Tom et al. [19] have used the antibacterial drug ciprofloxacin (CF) to protect gold nanoparticles of two different diameters, 4 and 20 nm. In their study, the trisodium citrate acted as a capping agent and sodium borohydride as the reducing agent. Further, they have investigated the nature of binding between gold nanoparticle and CF by several analytical techniques, and proposed that the nitrogen
  2. 2. S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490 atom of the NH moiety of piperazine group binds on the gold surface, as revealed by voltammetric and spectroscopic studies. In this study, we demonstrate a simple method to the preparation of stable and water-soluble Au-NPs by ciprofloxacin [1-cyclopropyl-6-fluoro-1, 4-dihydro-4-oxo-7-piperazinylquinolone-3-carboxylic acid] without using another reagent. Ciprofloxacin belongs to quinolones, which is a large and constantly expanding group of synthetic antibacterial agents [20–22], and ciprofloxacin is one of the most popular members of this family. Here, we examined a templateless, surfactantless chemical approach to the preparation of Au-NPs in a HAuCl4 solution using lower concentration of ciprofloxacin (2 mM). Synthesized Au-NPs were characterized by UV–vis spectroscopy (UV–vis), Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction pattern (XRD), and cyclic voltammetry (CV). These studies show that the spherical Au-NPs obtained, and exhibited high electrocatalytic activities towards the oxidation of methanol, and reduction of oxygen. 2. Experimental 2.1. Reagents and chemicals HAuCl4 Á 3H2O and ciprofloxacin were purchased from SigmaAldrich. Potassium ferricyanide, potassium ferrocyanide, and disodium hydrogen phosphate were received from J.T. Baker. Methanol (99.9%) was purchased from ECHO, Taiwan. All other reagents used were analytical grade. 2.2. Instruments and apparatus The morphological features and element compositions were measured by a field-emission scanning electron microscope and energy-dispersive X-ray (EDX) (HITACHI S-4700), respectively. Absorption spectra were recorded using a UV–vis spectrophotometer (PerkinElmer Lambda 900). Electrochemical experiments were performed with a CH Instruments (Chi611c, USA). All electrochemical experiments were carried out with a conventional three-electrode system. The Au-NPs modified glassy carbon electrode (GCE) or unmodified GCE was used as a working electrode, and indium tin oxide coated (ITO) glass was used for the preparation of dry films. Platinum wire and Ag/AgCl (3 M KCl) were used as the counter electrode and the reference electrode, respectively. Electrolyte solution was purged with high-purity argon gas for 10 min prior to each electrochemical experiment. X-ray diffraction (XRD, Rigaku DMX-2200) with Cu-Ka radiation was conducted to reveal the nanostructures of the Au-NPs. 1485 reagents. The obtained precipitate can be easily re-dispersed in de-ionized water by sonication. This Au-NPs solution was stable for several months at room temperature. 3. Results and discussion 3.1. Characterization of Au-NPs Fig. 1 shows the UV–vis spectra of CF solution (curve c), HAuCl4 solution (curve b) and as-prepared Au-NPs (curve a) colloidal solution. The absorption spectra of CF showed three distinct absorption bands at 276, 314, and 327 nm (curve c). The absorption maximum at 276 nm corresponds to the p–pn transition of the fluorobenzene moiety, and other two correspond to n–pn as well as p–pn transitions of the quinolone ring [19,23–25]. HAuCl4 did not show any absorption peaks in the range from 200 to 400 nm (curve b). However, the absorption spectrum of synthesized Au-NPs solution shows four distinct bands at 270, 321, 333, and 527 nm (curve a). After the generation of Au-NPs in CF solution, the absorption peak at 276 nm is shifted to shorter wavelength and the other two bands also shifted to longer wave regions. In addition, a new band was appeared at 527 nm, which is ascribed to the surface plasmon absorption of the spherical Au-NPs. These observations were corroborating the formation of Au(0) nanoparticles [26]. To remove unreacted reagents, as-prepared Au-NPs solution was centrifuged, and the resulting CF-capped Au-NPs were washed with 0.1 M HCl, and subsequently with de-ionized water. Next, the absorption spectrum of the separated Au-NPs was measured after re-dispersion in distilled water, and it showed two strong absorption bands at 274 and 527 nm (Fig. 1 curve d). It was vouched that Au-NPs were prepared and the synthesized particles were covered by CF molecules. In the control experiment, synthesis procedure of Au-NPs was repeated without using CF solution. We have not observed any color change in HAuCl4 solution. By this experiment, it was confirmed that CF molecules worked as a reducing as well as a capping agent for Au-NPs. The CF-protected Au-NPs can be easily re-dispersed in water by sonication. Ciprofloxacin with the piperazinyl group in the 7-position contains two relevant ionizable functional groups. The protolytic equilibria of fluoroquinolone analogues are expressed as shown in Fig. 2 [27]. 2.3. Preparation of Au-NPs Aqueous solution of tetrachloroauric acid (HAuCl4 Á 3H2O, 2 mL, 2 mM) was mixed with the solution of ciprofloxacin (dissolved in 0.1 M HCl) (2 mL, 2 mM) and the mixture was stirred using a magnetic bar at 1000 rpm, and the bath temperature was maintained at 60 1C. After the addition of HAuCl4 Á 3H2O into CF solution, immediately a yellow color solution was formed. The pH of the resulting mixture was increased to $ 8.0 using 0.5 M NaOH with constant stirring. In high pH, yellow colored solution completely disappeared. Subsequently, wine red solution was obtained and stored at room temperature. The obtained Au-NPs solution was centrifuged, and the precipitate was washed five times with 0.1 M HCl and, then the washing process is repeated with de-ionized water for another three times to remove any unadsorbed or unreacted Fig. 1. UV–vis spectra of (a) Au-NPs/CF solution, (b) HAuCl4 Á 3H2O, (c) CF solution and (d) separated Au-NPs dispersed in distilled water.
  3. 3. 1486 S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490 Fig. 2. Chemical formula of CF. CF molecules can exist in four possible forms: an acidic cation H2Q + , a neutral nonionized species HQ, an intermediate zwitter ion HQ 7 , and a basic anion Q À , depending on the pH. At low pH values, both the 7-piperazinyl group and 3-carboxyl group are protonated, whereas at high pH values, neither is protonated. The carboxyl group is normally a stronger acid than the ammonium group, the reason being that the neutral nonionic form is spontaneously rearranged to the zwitter ion. The pKa1 of CF was found at around 6 and the pKa2, which is due to the presence of an ionizable proton on the external piperazinyl nitrogen, was found at around 8.5. As in our experimental condition, upon heating, chloroauric acid liberates hydrogen chloride, giving gold(III) chloride. In aqueous solution, chloroauric acid consists of the À square planar AuCl4 ion, which is a common precursor to other gold coordination complexes [28]. The possible reaction mechanism for the formation of ciprofloxacin-capped gold nanoparticle is shown in Eqs. (1)–(3). Previously, CF molecules have been used as a capping agent for Au-NPs. Further, it was stated that piperazinyl ring is modified by the adsorption on gold nanoparticles, possibly through nitrogen whereas pyridone moiety is unaffected [19]. 2HAuCl4 2Au2 Cl6 þ2HCl pH o 6 ½AuCl4 ŠÀ þ HQ ÀÀ!½AuðHQ ÞCl2 Š þ2Cl pH 4 6 ð1Þ À ½AuðHQ ÞCl2 þ2NaOHÀÀ!Au0 þ 2NaCl þ Q À þH2 Oþ OHÀ ð2Þ ð3Þ 3.2. Electrochemical properties Next, synthesized Au-NPs were dispersed in de-ionized water by sonication. 10 mL of the Au-NPs dispersion was casted onto a pre-cleaned GCE surface and the electrode was dried in an air oven at 60 1C. Thereafter, Au-NPs modified GCE was used to record cyclic voltammograms (CVs) in 0.1 M KCl containing 5 mM [Fe(CN)6]3 À /4 À for five cycles at a scan rate of 20 mV s À 1 (Fig. 3, curve a). Further, CVs of a bare GCE in the same condition (Fig. 3 curve b) was compared with the CVs of the Au-NPs modified GCE, which showed an enhanced redox peak currents (Fig. 3 curve a). In addition, peak-to-peak separation for [Fe(CN)6]3 À /4 À redox peak was 0.09 V, which is lower than observed at a bare GCE (0.20 V). This observation elucidated that Au-NPs were attached onto the electrode surface and increased the real surface area. The area of the unmodified GCE and Au-NPs/GCE were calculated to be 0.0707 and 0.1139 cm2, respectively. To find out the fouling of the electrode surface, CVs were recorded in 0.1 M KCl using the Au-NPs modified GCE, which was previously used for the measurement of CVs in [Fe(CN)6]3 À /4 À + 0.1 M KCl. The Au-NPs modified electrode did not show any signal for adsorbed [Fe(CN)6]3 À /4 À in the blank Fig. 3. CVs were recorded for five cycles using the Au-NPs/GCE in 5 mM [Fe(CN)6]3 À /4 À + 0.1 M KCl (curve a). CVs of the bare-GCE in the same condition (curve b). After running continues CVs for five cycles in 5 mM [Fe(CN)6]3À /4À +0.1 M KCl, the Au-NPs/GCE was transferred into a blank 0.1 M KCl, and CVs were recorded in À1 the same potential window (curve c). Scan rate¼20 mV s . electrolyte; it is confirmed that the negatively charged redox probe molecules are not retained on the modified electrode surface (Fig. 3 curve c). 3.3. SEM, EDX, and TEM investigations Field-emission SEM measurements were made to investigate the surface morphology and the shape of the Au-NPs. Fig. 4a shows the representative SEM image obtained for the Au-NPs. It vouched that nanoparticles with spherical structure were formed. The average size of the nanoparticles was calculated to be $ 20 nm. Brinas et al. [29] reported that pH value of solution influenced on the size of the nanoparticles during the synthesis of Au-NPs by reduced glutathione (GSH). Au-NPs coated with GSH were synthesized at various pH values and they reported that higher pH decreased the sizes from 6 to 2 nm. In the present study, Au-NPs were synthesized with an average size of 20 nm using CF by adjusting pH of the precursors. As can be seen in Fig. 4a, the synthesized nanoparticles do not cluster together, likely due to the presence of CF molecules on their surface. Obviously, CF molecules not only stabilize the nanoparticles when
  4. 4. S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490 1487 Fig. 4. (a) SEM images of Au-NPs coated ITO electrode and (b) EDX spectra of the Au-NPs. (c and d) TEM images of the synthesized Au-NPs. suspended in water, but also prevent them from aggregating when dried in air as well. The CF attached Au-NPs are visible in the SEM image (Fig. 4a). Thereafter, synthesized Au-NPs were characterized by EDX. Fig. 4b shows the EDX spectrum of the Au-NPs for the selected area as shown in SEM image of Au-NPs (Fig. 4a). EDX analysis confirms that the major peaks of Au present in the spectrum with the considerable amounts of C, Si, and In. These minor peaks (C, Si, and In) came from the ITO surface, which is used as a platform for EDX analysis. Thus, it was confirmed that the desired element Au-NPs have been effectively synthesized by CF. Further, surface morphology and shape of the nanoparticles were investigated by a highresolution transmission electron microscopy (TEM). Fig. 4c and d show the TEM images of the Au-NPs synthesized by CF and the average sizes of the spherical particles were $20 nm, which is in good agreement with SEM results. In addition, as shown in TEM images, sizes of the Au-NPs are not uniform. The reduction in HAuCl4 occurs due to transfer of electrons from the –NH of piperazine group to the metal ion, resulting in the formation of Au0, with the subsequent formation of Au-NPs. The CF is a weak reducing agent, and incapable of reducing the gold salt without the gold seeds. In this study, by addition of 0.5 M NaOH to the CF–Au(III) solution, the growth solution pH raised to 8.0, and resulted in a dramatic increase in the relative formation of nanoparticles. It is generally believed that the slow reaction rates for Turkevitch and Brust-Schiffrin process is responsible for the
  5. 5. 1488 S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490 high uniformity in the resultant nanoparticles [1]. This method of synthesis is comparatively fast in the presence of NaOH, so it results in non-uniform nanoparticles. 3.4. XRD studies As can be seen in Fig. 5a, the diffraction patterns of the Au-NPs modified ITO shows several Bragg-like features than bare surface (Fig. 5b). Their position and relative intensity match those of the sharp Bragg peaks observed in the diffraction pattern of bulk crystalline gold [30]. As shown in Fig. 5a, XRD pattern of the Au-NPs reveals that nanoparticles have fcc structure corresponding to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) gold crystalline facets [30]. 3.5. Electrocatalytic oxidation of methanol Fig. 6 (curve a) shows cyclic voltammetric data of the Au-NPs modified GCE in 0.1 M NaOH. A broad oxidation current flow was evident at +0.38 V on the positive sweep, which was accompanied by a large and sharp reduction wave at 0.15 V on the reverse sweep. These two waves are attributed to the formation and reduction of surface Au oxide (AuOx) or Au(OH)x on the nanoparticles [17,31,32]. Fig. 6 shows two typical sets of cyclic voltammetric data obtained using the Au-NPs/GCE and unmodified GCE in alkaline solution containing methanol. A large oxidation peak was observed at + 0.34 V for methanol oxidation on the Au-NPs modified electrode (Fig. 6 curve b). This kind of catalytic effect for methanol oxidation was not observed at the unmodified GCE in the potential range used (Fig. 6 curve c), indicating that Au-NPs were involved in the catalytic oxidation of methanol. In the absence of methanol, CVs responsible for the formation and reduction of surface Au oxides were retained as shown in Fig. 6 curve a. The voltammetric response observed in the presence of methanol is solely due to methanol oxidation by the Au-NPs. In addition, the reduction peak of Au-NPs at +0.15 V disappeared in the presence of methanol in alkaline solution, suggesting that electrogenerated Au-oxide species are involved in the oxidation of methanol [17]. It is well documented that the Fig. 5. XRD patterns of (a) the Au-NPs modified ITO and (b) bare-ITO. Fig. 6. CVs of the Au-NPs modified electrode in the absence (curve a) and in the presence of 800 mM methanol (curve b) in 0.1 M NaOH. Curve c shows the CVs of the oxidation of methanol at the unmodified electrode in the same conditions. Scan rate¼ 20 mV s À 1. surface oxides of Au-NPs can function as an electron-transfer mediator in the oxidation process of methanol [31,32]. The role of CF on the electrocatalytic oxidation of methanol was tested by base-catalyzed desorption of CF from the Au-NPs [19]. The Au-NPs modified GCE was dipped in 5 mL of 0.1 M NaOH solution for few hours, and then using UV–vis spectra, it was confirmed that CF molecules were desorbed from the Au-NPs to some extent. Next, we have tested electrocatalytic oxidation of methanol in 0.1 M NaOH using a freshly prepared Au-NPs/GCE and basic solution treated Au-NPs/GCE. There was no any difference in the oxidation current of methanol on these electrodes. We believe that CF molecules do not have considerable effect on the methanol oxidation. It could then suppose that the protective agent (CF) plays a fundamental role in determining the stability of the colloidal systems. We also concerned about the stability of the Au-NPs after the removal of CF. As described in the above experiment, CF molecules are not completely removed by NaOH treatment. However, there should be some difference in the layers of CF between freshly prepared Au-NPs/GCE and basic solution treated Au-NPs/GCE. By this experiment, we have not found considerable effect of CF on methanol oxidation. However, the detailed stability mechanism of Au-NPs stabilized by CF, and also in tuning the catalytic activity of Au-NPs is under investigation. Fig. 7A shows the voltammetric dependence on scan rate (v) for Au-NPs in 0.1 M NaOH containing methanol. The oxidation current of methanol was approximately linear with v1/2 at scan rates of 20–200 mV s À 1 on the Au-NPs/GCE, indicating that the current of methanol oxidation may be controlled by the diffusion of methanol (inset of Fig. 7A). All these results show that the oxidation of methanol is mediated by the surface oxide redox species. Further to support it, voltammetric responses were recorded for different concentrations of methanol using the AuNPs coated GCE (Fig. 7B). The gradual decrease in the cathodic peak corresponding to the reduction of surface oxide on increasing the concentration of methanol in the solution demonstrates the involvement of surface oxides in the catalytic reaction (Fig. 7B). Moreover, stability of the Au-NPs modified GCE was tested by potential sweeping in 0.1 M NaOH between À0.1 and 0.6 V for 100 cycles. Initially, oxidation and reduction peak currents of the electrode was decreased about 3%, and then peak current decrease was stopped and remained almost constant after
  6. 6. S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490 1489 Fig. 8. CVs of the Au-NPs/GCE in O2-saturated (curve b) and in O2-free (curve a) pH 7.2 buffer solution. Electrocatalytic reduction of oxygen at the unmodified electrode (curve c). Cyclic voltammetric response of the Au-NPs/GCE in O2-saturated solution containing 1.8 Â 10 À 3 M H2O2 (curve d). Scan rate¼20 mV s À 1. Fig. 7. (A) CVs for the oxidation of methanol (82 mM) at the Au-NPs/GCE in 0.1 M NaOH. Scan rate (in mV s À 1): (from inner to outer) (a) 20, (b) 40, (c) 50, (d) 60, (e) 70, (f) 80, (g) 90, (h) 100, (i) 120, (j) 140, (k) 160, (l) 180 and (m) 200. Inset shows the corresponding plot of peak current against v1/2. (B) CVs for the oxidation of methanol at the Au-NPs/GCE in 0.1 M NaOH containing different concentrations of methanol. Each addition increased the concentration of methanol by 160 mM. Scan rate¼ 20 mV s À 1. 100 cycles. This good stability of the modified electrode may arise due to the hydrophobic interaction between CF (attached with the Au-NPs) and GCE surface [33]. 3.6. Electrocatalytic reduction of oxygen Fig. 8 shows the cyclic voltammetric response of the Au-NPs modified GCE in O2-free (curve a) and in O2-saturated (curve b) phosphate buffer solution. In the presence of O2 a significant increase in the cathodic peak at about À0.33 V was observed, indicating electrocatalytic effect of the attached Au-NPs. However, at the unmodified GCE, such characteristic reduction wave was not observed in O2-saturated buffer solution in the potential range used (Fig. 8 curve c). The reduction peak observed at À0.33 V corresponds to the reduction of oxygen to H2O. The mechanism for the reduction of oxygen at the Au-NPs modified electrode in pH 7.2 buffer solution was believed to follow a 4-electron reduction of O2 to H2O (or OH À ) [34,35]. To support this conclusion, a known amount of H2O2 (1.8 Â 10 À 3 M H2O2) was added into the O2-saturated buffer solution and the cyclic voltammetric response was recorded. It was found that cathodic current was increased at À 0.33 V (Fig. 8 curve d), confirming that final product of O2 reduction was H2O. This observation further supported by Jena and Raj, who observed that spherical gold nanoparticles showed a single reduction peak for catalytic reduction of oxygen and H2O2 [17]. In addition, Au-NPs modified GCE showed a cathodic peak for O2 reduction at the same potential ( À 0.33 V), suggesting that spherical Au-NPs were successfully synthesized by CF and they have good catalytic effect towards reduction of oxygen. Further, effect of CF molecules on O2 reduction was also studied using a newly prepared Au-NPs/GCE, and a basic solution treated Au-NPs/GCE in O2-saturated (pH 7.2) buffer solution. However, we have not found any difference in the reduction current of oxygen. 4. Conclusions Selecting the right reducing agent for NPs synthesis is very important in forming Au-NPs with desirable sizes. In this study, we had chosen the synthetic antibiotic ciprofloxacin as a reducing/capping agent. To the best of author’s knowledge, this is the first report for the synthesis of water-soluble Au-NPs using CF as a reducing agent. Because of the favorable properties such as the presence of –C ¼O, carboxylic acid, and imino groups, water solubility at relevant biological pH and biological compatibility, CF is a very attractive ligand in making water-soluble Au-NPs for catalysis applications. Spectral and electrocatalytic studies of synthesized Au-NPs were reported. TEM, SEM combined with EDX showed spherical-shaped nanoparticles with an average size of $ 20 nm. Electrocatalytic oxidation of methanol and reduction of oxygen were observed at the Au-NPs modified GCE. The Au-NPs modified electrode shows a single voltammetric peak for the reduction of oxygen at a less-negative potential in neutral pH, which is not observed at the unmodified electrode. Here, we have described a simple method for the preparation of Au-NPs, and expected to have major applications in the electrocatalysis of methanol and oxygen.
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