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

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

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.

1486


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


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

1488


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


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.

1490


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