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

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

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Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

n Corr

E-m

seafuew

journal homepage: www.elsevier.com/locate/jpcs

Synthetic antibacterial agent assisted synthesis of goldnanoparticles: Characterization and application studies

S. Ashok Kumar a,n, Yu-Tsern Chang b, Sea-Fue Wang a,n, His-Chuan Lu a

a Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, No.1, Section 3, Chung-Hsiao E. Road, Taipei, Taiwanb Department of Chemical and Materials Engineering, Nanya Institute of Technology, Jhongli 32091, Taiwan

a r t i c l e i n f o

Article history:

Received 19 February 2010

Received in revised form

5 July 2010

Accepted 15 July 2010

Keywords:

A. Nanostructures

A. Thin films

B. Chemical synthesis

C. X-ray diffraction

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016/j.jpcs.2010.07.015

esponding authors. Tel.: +886 2 27712171�

ail addresses: [email protected] (S.A. Ku

[email protected] (S.-F. Wang).

a b s t r a c t

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.

1. Introduction

Synthesis of stable gold nanoparticles (Au-NPs) and theirapplications has been the subject of great interest, due to theirremarkable physical and chemical properties. Au-NPs have uni-form structures with promising stability, and their size-relatedelectronic, magnetic, and optical properties [1,2] make thempromising in important research areas such as catalysis [3],biosensors [4], drug delivery [5], energy-related applications [6],and biological applications [7]. Recently, researchers have studiedthe electronic and photonic properties of nanoparticles dopedwith different materials, and they expect potential applications ofnanomaterials in building quantum computers [8–11]. Control-ling the size of nanoparticles (NPs) has always been one of thechallenges in colloidal science. Changing the size of NPs can resultin modulation of their physical and chemical properties. Since thediscovery of various reducing agents for the gold compounds toform the Au-NPs, like sodium citrate, sodium borohydride,phosphorus, alcohols, and tannic acid/citrate mixtures, thesynthesis and applications of Au-NPs of different sizes haveflourished [12]. Recently, gold nanoparticles capped with novel

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mar),

zwitterionic disulfide ligands was reported which showedremarkable stability in saline media [13]. Cetyltrimethylammo-nium bromide/silver bromide complex as the capping agent ofgold nanorods [14], and preparation of gold nanoparticles usingascorbic acid as a reducing agent in reverse micelles [15] werealso reported. Moreover, Au-NPs in the range of 5–6 nm weresynthesized using sodium borohydride in the presence of newlysynthesized mono-6-deoxy-6-pyridinium-b-cyclodextrin chloride[16]. In those methods, different reducing and capping agentswere used together to prepare stable Au-NPs dispersion. In fewstudies, some reagents were used as a reducing agent as well as astabilizing agent for Au-NPs.

Gold nanoflowers were obtained by a one-pot synthesisusing N-2-hydroxyethylpiperazine-N-2-ethanesulphonic acid asa reducing/stabilizing agent and their electrocatalytic effecttowards oxidation of methanol was studied [17]. Zhang et al.[18] reported synthesis of various gold nanostructures on glassycarbon electrode in a low concentration of HAuCl4 solution(5 mM), and enzyme-free sensor was developed for the detectionof glucose in pH 7.4 phosphate buffer solutions. Tom et al. [19]have used the antibacterial drug ciprofloxacin (CF) to protectgold nanoparticles of two different diameters, 4 and 20 nm. Intheir study, the trisodium citrate acted as a capping agent andsodium borohydride as the reducing agent. Further, they haveinvestigated the nature of binding between gold nanoparticle andCF by several analytical techniques, and proposed that the nitrogen

S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490 1485

atom of the NH moiety of piperazine group binds on the goldsurface, as revealed by voltammetric and spectroscopic studies.

In this study, we demonstrate a simple method to thepreparation of stable and water-soluble Au-NPs by ciprofloxacin[1-cyclopropyl-6-fluoro-1, 4-dihydro-4-oxo-7-piperazinylquino-lone-3-carboxylic acid] without using another reagent. Ciproflox-acin belongs to quinolones, which is a large and constantlyexpanding group of synthetic antibacterial agents [20–22], andciprofloxacin is one of the most popular members of this family.Here, we examined a templateless, surfactantless chemicalapproach to the preparation of Au-NPs in a HAuCl4 solution usinglower concentration of ciprofloxacin (2 mM). Synthesized Au-NPswere characterized by UV–vis spectroscopy (UV–vis), Scanningelectron microscopy (SEM), transmission electron microscopy(TEM), X-ray diffraction pattern (XRD), and cyclic voltammetry(CV). These studies show that the spherical Au-NPs obtained, andexhibited high electrocatalytic activities towards the oxidation ofmethanol, and reduction of oxygen.

Fig. 1. UV–vis spectra of (a) Au-NPs/CF solution, (b) HAuCl4 �3H2O, (c) CF solution

and (d) separated Au-NPs dispersed in distilled water.

2. Experimental

2.1. Reagents and chemicals

HAuCl4 �3H2O and ciprofloxacin were purchased from Sigma-Aldrich. Potassium ferricyanide, potassium ferrocyanide, anddisodium hydrogen phosphate were received from J.T. Baker.Methanol (99.9%) was purchased from ECHO, Taiwan. All otherreagents used were analytical grade.

2.2. Instruments and apparatus

The morphological features and element compositions weremeasured by a field-emission scanning electron microscope andenergy-dispersive X-ray (EDX) (HITACHI S-4700), respectively.Absorption spectra were recorded using a UV–vis spectrophot-ometer (PerkinElmer Lambda 900). Electrochemical experimentswere performed with a CH Instruments (Chi611c, USA). Allelectrochemical experiments were carried out with a conven-tional three-electrode system. The Au-NPs modified glassy carbonelectrode (GCE) or unmodified GCE was used as a workingelectrode, and indium tin oxide coated (ITO) glass was used forthe 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-purityargon gas for 10 min prior to each electrochemical experiment.X-ray diffraction (XRD, Rigaku DMX-2200) with Cu-Ka radiationwas conducted to reveal the nanostructures of the Au-NPs.

2.3. Preparation of Au-NPs

Aqueous solution of tetrachloroauric acid (HAuCl4 �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 wasstirred using a magnetic bar at 1000 rpm, and the bathtemperature was maintained at 60 1C. After the addition ofHAuCl4 �3H2O into CF solution, immediately a yellow colorsolution was formed. The pH of the resulting mixture wasincreased to �8.0 using 0.5 M NaOH with constant stirring. Inhigh pH, yellow colored solution completely disappeared. Subse-quently, wine red solution was obtained and stored at roomtemperature. The obtained Au-NPs solution was centrifuged, andthe precipitate was washed five times with 0.1 M HCl and, thenthe washing process is repeated with de-ionized water foranother three times to remove any unadsorbed or unreacted

reagents. The obtained precipitate can be easily re-dispersed inde-ionized water by sonication. This Au-NPs solution was stablefor 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 threedistinct absorption bands at 276, 314, and 327 nm (curve c). Theabsorption maximum at 276 nm corresponds to the p–pn

transition of the fluorobenzene moiety, and other twocorrespond to n–pn as well as p–pn transitions of the quinolonering [19,23–25]. HAuCl4 did not show any absorption peaks in therange from 200 to 400 nm (curve b). However, the absorptionspectrum of synthesized Au-NPs solution shows four distinctbands at 270, 321, 333, and 527 nm (curve a). After the generationof Au-NPs in CF solution, the absorption peak at 276 nm is shiftedto shorter wavelength and the other two bands also shifted tolonger wave regions. In addition, a new band was appeared at527 nm, which is ascribed to the surface plasmon absorption ofthe spherical Au-NPs. These observations were corroborating theformation of Au(0) nanoparticles [26].

To remove unreacted reagents, as-prepared Au-NPs solutionwas centrifuged, and the resulting CF-capped Au-NPs werewashed with 0.1 M HCl, and subsequently with de-ionized water.Next, the absorption spectrum of the separated Au-NPs wasmeasured after re-dispersion in distilled water, and it showed twostrong absorption bands at 274 and 527 nm (Fig. 1 curve d). It wasvouched that Au-NPs were prepared and the synthesized particleswere covered by CF molecules. In the control experiment,synthesis procedure of Au-NPs was repeated without using CFsolution. We have not observed any color change in HAuCl4

solution. By this experiment, it was confirmed that CF moleculesworked as a reducing as well as a capping agent for Au-NPs.The CF-protected Au-NPs can be easily re-dispersed in waterby sonication. Ciprofloxacin with the piperazinyl group in the7-position contains two relevant ionizable functional groups. Theprotolytic equilibria of fluoroquinolone analogues are expressedas shown in Fig. 2 [27].

Fig. 2. Chemical formula of CF.

Fig. 3. CVs were recorded for five cycles using the Au-NPs/GCE in 5 mM

[Fe(CN)6]3�/4� +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]3� /4� +0.1 M

KCl, the Au-NPs/GCE was transferred into a blank 0.1 M KCl, and CVs were recorded in

the same potential window (curve c). Scan rate¼20 mV s�1

.

S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–14901486

CF molecules can exist in four possible forms: an acidic cationH2Q+, a neutral nonionized species HQ, an intermediate zwitterion HQ7 , and a basic anion Q� , depending on the pH. At low pHvalues, both the 7-piperazinyl group and 3-carboxyl group areprotonated, whereas at high pH values, neither is protonated. Thecarboxyl group is normally a stronger acid than the ammoniumgroup, the reason being that the neutral nonionic form isspontaneously rearranged to the zwitter ion. The pKa1 of CF wasfound at around 6 and the pKa2, which is due to the presence of anionizable proton on the external piperazinyl nitrogen, was foundat 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 thesquare planar AuCl4

� ion, which is a common precursor to othergold coordination complexes [28]. The possible reaction mechan-ism for the formation of ciprofloxacin-capped gold nanoparticle isshown in Eqs. (1)–(3). Previously, CF molecules have been used asa capping agent for Au-NPs. Further, it was stated that piperazinylring is modified by the adsorption on gold nanoparticles, possiblythrough nitrogen whereas pyridone moiety is unaffected [19].

2HAuCl42Au2Cl6þ2HCl ð1Þ

½AuCl4��þHQ��!

pHo6½AuðHQÞCl2�þ2Cl� ð2Þ

½AuðHQ ÞCl2þ2NaOH��!pH46

Au0þ2NaClþQ�þH2OþOH� ð3Þ

3.2. Electrochemical properties

Next, synthesized Au-NPs were dispersed in de-ionized waterby sonication. 10 mL of the Au-NPs dispersion was casted onto apre-cleaned GCE surface and the electrode was dried in an airoven at 60 1C. Thereafter, Au-NPs modified GCE was used torecord cyclic voltammograms (CVs) in 0.1 M KCl containing 5 mM[Fe(CN)6]3�/4� for five cycles at a scan rate of 20 mV s�1

(Fig. 3, curve a). Further, CVs of a bare GCE in the samecondition (Fig. 3 curve b) was compared with the CVs of theAu-NPs modified GCE, which showed an enhanced redox peakcurrents (Fig. 3 curve a). In addition, peak-to-peak separation for[Fe(CN)6]3�/4� redox peak was 0.09 V, which is lower thanobserved at a bare GCE (0.20 V). This observation elucidatedthat Au-NPs were attached onto the electrode surface andincreased the real surface area. The area of the unmodified GCEand Au-NPs/GCE were calculated to be 0.0707 and 0.1139 cm2,respectively. To find out the fouling of the electrode surface, CVswere 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�/4� +0.1 M KCl. The Au-NPs modified electrode did notshow any signal for adsorbed [Fe(CN)6]3�/4� in the blank

electrolyte; it is confirmed that the negatively charged redoxprobe molecules are not retained on the modified electrodesurface (Fig. 3 curve c).

3.3. SEM, EDX, and TEM investigations

Field-emission SEM measurements were made to investigatethe surface morphology and the shape of the Au-NPs. Fig. 4ashows the representative SEM image obtained for the Au-NPs. Itvouched 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 solutioninfluenced on the size of the nanoparticles during the synthesis ofAu-NPs by reduced glutathione (GSH). Au-NPs coated with GSHwere synthesized at various pH values and they reported thathigher pH decreased the sizes from 6 to 2 nm. In the presentstudy, Au-NPs were synthesized with an average size of 20 nmusing CF by adjusting pH of the precursors. As can be seen inFig. 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

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.

S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490 1487

suspended in water, but also prevent them from aggregatingwhen dried in air as well. The CF attached Au-NPs are visible inthe SEM image (Fig. 4a). Thereafter, synthesized Au-NPs werecharacterized by EDX. Fig. 4b shows the EDX spectrum of theAu-NPs for the selected area as shown in SEM image of Au-NPs(Fig. 4a). EDX analysis confirms that the major peaks of Au presentin the spectrum with the considerable amounts of C, Si, and In.These minor peaks (C, Si, and In) came from the ITO surface, whichis used as a platform for EDX analysis.

Thus, it was confirmed that the desired element Au-NPs havebeen effectively synthesized by CF. Further, surface morphologyand shape of the nanoparticles were investigated by a high-resolution transmission electron microscopy (TEM). Fig. 4c and d

show the TEM images of the Au-NPs synthesized by CF and theaverage sizes of the spherical particles were �20 nm, which is ingood agreement with SEM results. In addition, as shown in TEMimages, sizes of the Au-NPs are not uniform. The reduction inHAuCl4 occurs due to transfer of electrons from the –NH ofpiperazine group to the metal ion, resulting in the formation ofAu0, with the subsequent formation of Au-NPs. The CF is a weakreducing agent, and incapable of reducing the gold salt withoutthe gold seeds. In this study, by addition of 0.5 M NaOH to theCF–Au(III) solution, the growth solution pH raised to 8.0, andresulted in a dramatic increase in the relative formation ofnanoparticles. It is generally believed that the slow reaction ratesfor Turkevitch and Brust-Schiffrin process is responsible for the

S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–14901488

high uniformity in the resultant nanoparticles [1]. This method ofsynthesis is comparatively fast in the presence of NaOH, so itresults in non-uniform nanoparticles.

3.4. XRD studies

As can be seen in Fig. 5a, the diffraction patterns of the Au-NPsmodified ITO shows several Bragg-like features than bare surface(Fig. 5b). Their position and relative intensity match those of thesharp Bragg peaks observed in the diffraction pattern of bulkcrystalline gold [30]. As shown in Fig. 5a, XRD pattern of theAu-NPs reveals that nanoparticles have fcc structurecorresponding to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) goldcrystalline facets [30].

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 rate¼20 mV s�1.

3.5. Electrocatalytic oxidation of methanol

Fig. 6 (curve a) shows cyclic voltammetric data of the Au-NPsmodified GCE in 0.1 M NaOH. A broad oxidation current flow wasevident at +0.38 V on the positive sweep, which wasaccompanied by a large and sharp reduction wave at 0.15 V onthe reverse sweep. These two waves are attributed to theformation and reduction of surface Au oxide (AuOx) or Au(OH)x

on the nanoparticles [17,31,32]. Fig. 6 shows two typical sets ofcyclic voltammetric data obtained using the Au-NPs/GCE andunmodified GCE in alkaline solution containing methanol. A largeoxidation peak was observed at +0.34 V for methanol oxidationon the Au-NPs modified electrode (Fig. 6 curve b). This kind ofcatalytic effect for methanol oxidation was not observed at theunmodified GCE in the potential range used (Fig. 6 curve c),indicating that Au-NPs were involved in the catalytic oxidation ofmethanol. In the absence of methanol, CVs responsible for theformation and reduction of surface Au oxides were retained asshown in Fig. 6 curve a. The voltammetric response observed inthe presence of methanol is solely due to methanol oxidation bythe Au-NPs. In addition, the reduction peak of Au-NPs at +0.15 Vdisappeared in the presence of methanol in alkaline solution,suggesting that electrogenerated Au-oxide species are involved inthe 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.

surface oxides of Au-NPs can function as an electron-transfermediator in the oxidation process of methanol [31,32].

The role of CF on the electrocatalytic oxidation of methanolwas 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 NaOHsolution for few hours, and then using UV–vis spectra, it wasconfirmed that CF molecules were desorbed from the Au-NPs tosome extent. Next, we have tested electrocatalytic oxidation ofmethanol in 0.1 M NaOH using a freshly prepared Au-NPs/GCEand basic solution treated Au-NPs/GCE. There was no anydifference in the oxidation current of methanol on theseelectrodes. We believe that CF molecules do not have considerableeffect on the methanol oxidation. It could then suppose that theprotective agent (CF) plays a fundamental role in determiningthe stability of the colloidal systems. We also concerned about thestability of the Au-NPs after the removal of CF. As described in theabove experiment, CF molecules are not completely removed byNaOH treatment. However, there should be some difference in thelayers of CF between freshly prepared Au-NPs/GCE and basicsolution treated Au-NPs/GCE. By this experiment, we havenot found considerable effect of CF on methanol oxidation.However, the detailed stability mechanism of Au-NPs stabilizedby CF, and also in tuning the catalytic activity of Au-NPs is underinvestigation.

Fig. 7A shows the voltammetric dependence on scan rate (v)for Au-NPs in 0.1 M NaOH containing methanol. The oxidationcurrent of methanol was approximately linear with v1/2 at scanrates of 20–200 mV s�1 on the Au-NPs/GCE, indicating that thecurrent of methanol oxidation may be controlled by the diffusionof methanol (inset of Fig. 7A). All these results show that theoxidation of methanol is mediated by the surface oxide redoxspecies. Further to support it, voltammetric responses wererecorded for different concentrations of methanol using the Au-NPs coated GCE (Fig. 7B). The gradual decrease in the cathodicpeak corresponding to the reduction of surface oxide onincreasing the concentration of methanol in the solutiondemonstrates the involvement of surface oxides in the catalyticreaction (Fig. 7B). Moreover, stability of the Au-NPs modified GCEwas tested by potential sweeping in 0.1 M NaOH between �0.1and 0.6 V for 100 cycles. Initially, oxidation and reduction peakcurrents of the electrode was decreased about 3%, and then peakcurrent decrease was stopped and remained almost constant after

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�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 rate¼20 mV s�1.

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�10�3 M H2O2 (curve d). Scan rate¼20 mV s�1.

S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–1490 1489

100 cycles. This good stability of the modified electrode may arisedue to the hydrophobic interaction between CF (attached with theAu-NPs) and GCE surface [33].

3.6. Electrocatalytic reduction of oxygen

Fig. 8 shows the cyclic voltammetric response of the Au-NPsmodified GCE in O2-free (curve a) and in O2-saturated (curve b)phosphate buffer solution. In the presence of O2 a significantincrease in the cathodic peak at about �0.33 V was observed,indicating electrocatalytic effect of the attached Au-NPs. However,at the unmodified GCE, such characteristic reduction wave wasnot observed in O2-saturated buffer solution in the potentialrange used (Fig. 8 curve c). The reduction peak observed at�0.33 V corresponds to the reduction of oxygen to H2O. Themechanism for the reduction of oxygen at the Au-NPs modifiedelectrode in pH 7.2 buffer solution was believed to follow a4-electron reduction of O2 to H2O (or OH�) [34,35]. To supportthis conclusion, a known amount of H2O2 (1.8�10�3 M H2O2)was added into the O2-saturated buffer solution and the cyclicvoltammetric response was recorded. It was found that cathodiccurrent was increased at �0.33 V (Fig. 8 curve d), confirming that

final product of O2 reduction was H2O. This observation furthersupported by Jena and Raj, who observed that spherical goldnanoparticles showed a single reduction peak for catalyticreduction of oxygen and H2O2 [17]. In addition, Au-NPsmodified GCE showed a cathodic peak for O2 reduction at thesame potential (�0.33 V), suggesting that spherical Au-NPs weresuccessfully synthesized by CF and they have good catalytic effecttowards 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 thereduction current of oxygen.

4. Conclusions

Selecting the right reducing agent for NPs synthesis is veryimportant in forming Au-NPs with desirable sizes. In this study,we had chosen the synthetic antibiotic ciprofloxacin as areducing/capping agent. To the best of author’s knowledge, thisis the first report for the synthesis of water-soluble Au-NPs usingCF as a reducing agent. Because of the favorable properties such asthe presence of –C¼O, carboxylic acid, and imino groups, watersolubility at relevant biological pH and biological compatibility,CF is a very attractive ligand in making water-soluble Au-NPs forcatalysis applications. Spectral and electrocatalytic studies ofsynthesized Au-NPs were reported. TEM, SEM combined with EDXshowed spherical-shaped nanoparticles with an average size of�20 nm. Electrocatalytic oxidation of methanol and reduction ofoxygen were observed at the Au-NPs modified GCE. The Au-NPsmodified electrode shows a single voltammetric peak for thereduction of oxygen at a less-negative potential in neutral pH,which is not observed at the unmodified electrode. Here, we havedescribed a simple method for the preparation of Au-NPs, andexpected to have major applications in the electrocatalysis ofmethanol and oxygen.

S.A. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1484–14901490

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