2
H. Vidya et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
example, the modification of electrodes can significantly increase their
analytical applicability [22–31].
Our research group previously published methods for the determi-
nation of DA, AA and UA using various modified carbon paste electrodes
3
1 mM AgNO solution. The reaction mixtures were subjected to micro-
wave radiation for different time intervals (0 to 100 s). A change in col-
our from light yellow to dark brown indicated the formation of AgNPs.
Trace amounts of AgNO
tion. The cessation of a white precipitate forming signalled that most of
the AgNO was consumed. The final washing of the AgNPs with plenty
of water ensured removal of any unreacted AgNO . The product was
3
were removed by adding NaCl to the solu-
(CPEs). The modifications included surfaces containing carbon nanopar-
ticles, hydrogen double salt, CuO, and reduced graphene oxide nanopar-
ticles for the detection of DA [32–35]. Other work used pretreated/
carbon paste electrodes for the detection of DA [36], SDS/R-GO/MCPE
employed for the detection of DA [37], a polyglycine modified CPE
used for simultaneous detection of DA and AA [38], and a poly (naphthol
green B)-film modified CPE utilized for simultaneous detection of DA
and UA [39].
3
3
dried in a vacuum oven at 70–80 °C for about 12 h. The final product
was in the powder form. The synthesis of AgNPs is compared with re-
ported methods tabulated in Table 1. The data reveal that, for the pres-
+
ent method, reduction of Ag → Ag° takes less time.
In the present work, the improvement of the sensitivity and selectiv-
ity of a dopamine sensor is achieved by using AgNPs as modifiers of a
CPE. The modifiers are synthesized by a biological method. This work il-
lustrates that such a sensor can be used for the selective determination
of DA even in the presence of high concentrations of interfering mate-
rials such as AA and UA. A comparison with literature values shows
that the electrodes modified with AgNPs exhibit the lowest detection
limit for DA.
2.4. Preparation of the CPE and the AgNPs/MCPE
The bare carbon paste electrode was prepared by mixing graphite
powder and silicon oil at a ratio of 70:30 (w/w) in an agate mortar
until a homogenous paste was obtained. The prepared carbon paste
was tightly packed into a PVC homemade cavity (3 mm in diameter)
and then smoothed on a weighing paper. The carbon paste electrode
modified with AgNPs (AgNPs/MCPE) was prepared by immobilizing
the AgNPs solution on the CPE surface using a micropipette. The elec-
trode was then incubated for various times at room temperature. The
electrode was later thoroughly rinsed with water to remove unabsorbed
modifier. It was dried in air at room temperature. The electrical contact
was provided by a copper wire connected to the paste in the end of the
tube.
2
. Experimental
2
.1. Reagents
Banyan leaf (Ficus tinctoria) was collected from Shankaraghata,
3
Shimoga a town in Karnataka, India. AgNO , DA, AA, UA, and the surfac-
tant sodium dodecyl sulphate(SDS) were obtained from Himedia
Chemicals. Sodium hydroxide, perchloric acid, sodium dihydrogen or-
thophosphate dihydrate, and di-sodium hydrogen phosphate anhy-
drous were obtained from Merck. Graphite powder was acquired from
Lobo Chemical. All chemicals were analytical grade. Stock solution of
3. Results and discussion
3.1. Characterization of AgNPs
The prominent peaks in the XRD pattern of AgNPs are shown in
Fig. 1. The peaks are at 2θ = 38.1°, 43.3°, 64.4° and 77.6° corresponding
to (111), (200) (220) and (311) respectively, which can be indexed to
the face centred cubic structure of Ag metal (JCPDS 03-0931). The aver-
age crystallite size d of the AgNPs was calculated to be about 17 nm
using Scherer formula, which is given by d = Kλ/bcosθ, where K is the
shape factor which is 0.9, λ is the incident X-ray wavelength (Cu
Kα = 1.542 A°), b is the full width at half maximum in radians of the
prominent line (111), and θ is the position of that line in the pattern.
The data indicates the green synthesis of AgNPs was achieved here
using Banyan leaf (F. tinctoria) extract under microwave radiation.
UV–vis spectrophotometry was used to evaluate the optical proper-
ties of the AgNPs. Fig. 2 shows the UV–vis absorption spectrum of the
aqueous AgNPs suspension. The absorption band observed at 424 nm
is the absorption band that results from the formation of spherical
shaped AgNPs [40]. The time in Fig. 2 is the time that the samples
were subjected to microwave radiation. The longer the time the more
silver is reduced. The samples that have low reduction exhibit low-
intensity absorption bands, i.e. the intensity of the AgNPs absorption
spectra increases with increasing AgNPs reduction. This observation
allows the prediction of AgNPs yields from the UV–vis spectrum.
Variation in the particle size of AgNPs can produce a spectral blue-
shift, by which Henglein [41] exploited to estimate the particle size. An-
ionic surfactants such as SDS give a spectral shift towards shorter wave-
length (blue-shift) as can be seen in Fig. 3 where absorption bands are
observed at 416 nm. The shift is larger at lower surfactant concentra-
tions. At sufficiently large concentrations the surfactants saturate the
system and local changes in concentration does not produce a shift.
−
4
−3
−4
2
5 × 10 M DA, 25 × 10 M AA, and 25 × 10 M UA were prepared
by dissolving the solute in 0.1 M perchloric acid solution, double dis-
tilled water, and 0.1 M NaOH respectively. All reagent solutions were
prepared in double distilled water.
2
.2. Apparatus
Electrochemical measurements were carried out with a CHI model
60c Electrochemical Workstation connected to a personal computer
6
for control and data storage. All electrochemical experiments were per-
formed in a standard three-electrode cell. The bare carbon paste elec-
trode or the carbon paste electrode modified with the AgNPs was used
as working electrodes. The counter electrode was a platinum wire. The
reference electrode was a saturated calomel electrode (SCE). All poten-
tials reported are with respect to the SCE. A Microwave oven (ONIDA-
MO 17SJP1W, 2.45 GHz) was used for extracting phytochemicals from
the Banyan leaf in water and subsequently in the synthesis of AgNPs.
X-ray diffraction (XRD) patterns of AgNPs samples were obtained
using a PHILIPS PW3710 diffracto metre (Cu, kα radiation) with a step
scan at 0.2° in a 2θ range from 30°–80°. Optical absorption spectra
were recorded by using a UV–visible spectrophotometer (UV-1650PC/
SHIMADZU). Samples were loaded in a quartz cell and measurements
were taken in the wavelength range 200–800 nm.
2
.3. Synthesis of AgNPs
Ten grammes of chopped Banyan leaf (F. tinctoria) were added to
200 ml distilled water. The mixture was kept under microwave irradia-
tion for about 120 s to extract the phytochemicals from the leaf. The so-
lution was filtered through a 0.2 μm membrane filter under hot
conditions to remove fibrous impurities. Two aqueous solutions were
made for the synthesis. One solution contained only leaf broth and the
other contained leaf broth and SDS. The concentration of surfactants
used was 3 mM. The stock-solution 10 ml, was added to 50 ml of
3.2. Mechanism of reduction of silver
A probable reduction mechanism involving Banyan leaf (F. tinctoria)
broth and silver nitrate that produces AgNPs involves the reaction of the
components of the leaf. Phyochemical investigations have demonstrat-
ed that tannins, polyphenols, triterphenoids, flavonoids and saponins