Synthesis, characterization, fluorescence, photocatalytic and antibacterial activity of CdS nanoparticles using Schiff base

Article abstract of DOI:10.1007/s10895-015-1639-5

Cadmium sulfide nanoparticles (CdS NPs) were successfully prepared using sonochemical method by employing Schiff-base, (2-[(4-methoxy-phenylimino)-methyl]-4-nitro phenol) as a complexing agent. Here, SB is used as a ligand to control the morphology of NPs

Full text of DOI:10.1007/s10895-015-1639-5

J Fluoresc
DOI 10.1007/s10895-015-1639-5
ORIGINAL ARTICLE
Synthesis, Characterization, Fluorescence, Photocatalytic
and Antibacterial Activity of CdS Nanoparticles Using Schiff Base
Dasari Ayodhya1 & M. Venkatesham1 & A. Santoshi Kumari1 & G. Bhagavanth Reddy1
D. Ramakrishna1 & G. Veerabhadram1
&
Received: 28 April 2015 /Accepted: 3 August 2015
#
Springer Science+Business Media New York 2015
Abstract Cadmium sulfide nanoparticles (CdS NPs) were
successfully prepared using sonochemical method by
employing Schiff-base, (2-[(4-methoxy-phenylimino)-meth-
yl]-4-nitro phenol) as a complexing agent. Here, SB is used
as a ligand to control the morphology of NPs. XRD patterns
and TEM images show that the synthesized CdS NPs have
cubic structures with a diameter of about 2–10 nm. The for-
mation of CdS NPs and their optical, structure, thermal and
morphologies were studied by means of UV-vis DRS, fluo-
rescence, FTIR, zeta potential, XRD, SEM and TEM. The
interactions between CdS NPs and SB were investigated in
an aqueous solution using fluorescence spectroscopy. The
fluorescence quenching studies suggest that SB quenches the
fluorescence of CdS NPs effectively. The degradation kinetics
of methyl red (MR) by the photocatalyst was followed by
Langmuir-Hinshelwood model. The results revealed that pho-
tocatalytic degradation of MR by SB capped CdS NPs could
be considered as a practical and reliable technique for the
removal of environmental pollutants. The antibacterial activi-
ty of samples was evaluated against E. coli, S. aureus and P.
aeruginosa and the results were compared. SB and SB capped
CdS NPs could be a potential antibacterial compounds after
further investigation.
Introduction
Among the chalcogenide compounds belonging to the II–VI
family, CdS has received much attention due to the fact that it
has a wide band gap of 2.42 eV at room temperature. It pos-
sesses many excellent physical and chemical properties which
have promising applications in multiple technical fields in-
cluding lasers, light-emitting diodes, sensors, solar cells and
in photochemical catalysis [1–6]. Several methods have been
explored towards the synthesis of CdS NPs, such as
microwave-solvothermal method, [7] sonochemical method
[8] and the surfactant–ligand co-assisting solvothermal meth-
od [9]. The methods listed above are in vogue to control the
size and growth of CdS nanostructures. These are done by
using coating agents, such as surfactants, ligands or polymers
to limit the growth of the nanomaterial structure. In recent
years, sonochemical methods have been greatly investigated
to prepare different nanostructures, including rods [10], wires
[11], tubes [12], particles [13], and porous spheres [14]. Dur-
ing sonochemical method, ultrasonic sound wave radiations
cause short required time and low temperature to carry out
chemical reaction. The sound wave radiations are beneficial
to the morphology and size of the samples, too.
Most dyes, used in the pigmentation of textiles, paper,
leather, ceramics, cosmetics and food-processing products,
are derived from azo dyes. These are characterised by the
presence of one or more azo groups (-N = N-) in their structure
[15]. Approximately 15 % of the dyes produced worldwide
are lost within waste water during synthesis and processing
[16]. This waste represents an enormous hazard to human and
environmental health due to the toxicity of azo dyes [17].
Removal of the colour of dyes and complete mineralization
.
.
Keywords CdS nanoparticles Schiff base Fluorescence
.
.
studies Photodegradation Antibacterial activity
* G. Veerabhadram
gvbhadram@gmail.com
+
2−
of C, N and S hetero atoms into CO2, NH4 , NO3− and SO4
,
1
respectively has also been studied by them. Rajeshwar et al.
[18] focused on the heterogeneous photocatalytic treatment of
Department of Chemistry, Osmania University,
Hyderabad, Telangana State 500007, India

J Fluoresc
organic dyes present in air and water. They have used TiO2,
ZnO, CdS, WO3 and Fe2O3 for decolorizing and decomposing
the organic dye to mineralized products. UV spectrophotom-
etry was used to measure the bleaching efficiency. The min-
eralization efficiency was calculated from total organic carbon
(TOC) measurements. Herrmann et al. reported, in the MR
degradation, TOC elimination is fastest and most easy [19].
The CdS NPs suspension prepared in this study has an ex-
tremely strong adsorption towards MR. Although pHZPC for
MR is estimated to be 5.4, it was found that repulsive electro-
static interactions between the CdS surface play an important
role. Guindo et al. [20] reported even lower values of pHzpc
between 1 and 1.5 for spherical CdS particles prepared by the
method of Matijevic and Wilhemy [21]. At pH higher than
pHZPC (for CdS), as the photogenerated electrons cannot eas-
ily overcome the negative charged surface, the holes may be
prevented from reacting with the dye, thus ultimately
recombining with the electrons.
Experimental
Materials
All the chemicals used for synthesis purposes were of AR
grade, purchased from Sigma Aldrich chemicals, and for spec-
tral analysis spectral grade solvents were used. Double dis-
tilled water was used for preparing solutions. The test strains,
Escherichia coli (E. coli) gram-negative, Staphylococcus au-
reus (S. aureus) gram-positive bacteria and Pseudomonas
aeruginosa (P. aeruginosa) were purchased from IMTECH,
Chandigarh, India. Yeast extract, tryptophan and bacterial-
grade agar–agar were purchased from Hi-media Laboratories,
Mumbai, India.
Synthesis of 2-[(4-methoxy-phenyl)iminomethyl]
-4-nitrophenol (SB)
The use of semiconductors as fluorescence probes has
increased in recent years along with their application in
different areas such as medicine, chemistry and engi-
neering. This interest is related to their broad absorption
spectra and strong and tunable fluorescence [1, 22–24].
It is important to point out the presence of the amine
moieties in some of these structures. We are interested
in understanding the interaction between simple amines
and NPs in order to correlate possible behaviours in
more complex structures. CdS NPs are also fluorescent
due to the radiative recombination of the electron–hole
pair. These light emitting properties have been among
the most studied in quantum dots due to an interest in
using them as fluorescent probes in bioimaging [25], as
well as sensor applications [26, 27]. Fluorescence
quenching refers to any process that decreases the fluo-
rescence intensity of a sample. A variety of molecular
interactions can result in quenching.
Schiff bases are considered as a very important class of
organic compounds which have wide application in many
biological aspects. Some Schiff bases were reported to possess
antibacterial, antifungal and antitumor activities [28, 29]. Syn-
thesis and crystal structure of 2-[(4-methoxy-
phenyl)iminomethyl]-4-nitrophenol was reported earlier
[30]. We report in this paper, the synthesis of CdS NPs by
Schiff base using sonochemical bath. The quenching studies
have been investigated using fluorescence emission spectros-
copy technique. The average particle size of the synthesized
sample is about 15 nm as determined by XRD. The photocat-
alytic activity of the prepared samples was evaluated by the
photocatalytic decolouration of MR under sunlight irradiation.
The heterogeneous photocatalysis of MR by CdS NPs shows
effective degradation by using small amount of catalyst. In
addition, the antibacterial activity of CdS NPs was evaluated
and the results were compared with SB.
The Schiff-base, 2-[(4-methoxy-phenyl)iminomethyl]-4-ni-
trophenol, (C14H12N2O4) was synthesized by using p-
Anisidine in methanol and this was slowly added to 5-
Nitrosalicylaldehyde in methanol as shown in Scheme 1.
The mixture was refluxed and stirred for 3 h at room temper-
ature. The completion of the reaction was monitored through
TLC for the disappearance of the starting compounds. Then,
the solvent was evaporated yielding yellow precipitation of
2-[(4-methoxy-phenylimino)-methyl]-4-nitrophenol. The
yield was about 85 %. The solid thus obtained was dried in
oven.
Analytical Data of SB
Colour: Yellow, Yield: 85 %, M.P. 162 °C. FTIR (υ, cm−1):
3618 (υ-O-H), 3072 (υ-C-H), 1621 (υ-C = N), 1516 (υ-C =
C), 1336 (υ-N-O), 1258 (υ-C-O), 887 (υ-C-N). UV (λmax
/
1
nm): 236, 352. H-NMR (CDCl3, δ, ppm): 14.8 s 1H, 8.7 s
1H, 8.2–8.4 d 2H, 7.2–7.4 d 2H, 6.9–7.1 m 3H, 3.9 s 3H. ESI-
MS: m/z=273.2 (M+1).
Synthesis of CdS NPs
In a typical procedure, 0.035 M of Cd(CH3COO)2.2H2O was
mixed with appropriate concentration of Na2S.9H2O solution
Scheme 1 The synthetic procedure of the formation of SB

J Fluoresc
under stirring. In a separate beaker, 10−3 M of synthesized SB
was dissolved in 10 ml DMF. The two solutions were
mixed together under stirring and resulting yellow pre-
cipitate solution was transferred to a sonochemical bath.
After 60 min of sonochemical treatment, the resulting
CdS precipitate was collected, filtered, washed with
double distilled water and absolute ethanol several times
to remove the unreacted chemicals, and finally dried in
an oven at 80 °C for 5 h. Similar procedure was
adopted to synthesize uncapped CdS NPs.
Characterizations
The morphology of CdS NPs was observed by SEM
(scanning electron microscopy, Zeiss evo18) and TEM
(transmission electron microscopy, TechnaiG2). Elemental
mapping over the selected regions of the CdS NPs was
conducted by Energy-dispersive X-ray spectroscopy
(EDX). The UV–visible diffuse reflectance spectra (UV-
vis DRS) were obtained on the spectrophotometer of
Shimadzu UV-3600 equipped with an integrating sphere
accessory (BaSO4 was used as a reference). Fluorescence
spectra were recorded on a Shimadzu RF-5301PC spectro-
fluorometer. The FT-IR spectra were recorded on a
Shimadzu spectrophotometer. The powder X-ray diffraction
(XRD) analysis was made with an X’pert Pro diffractom-
eter. Zeta potential measurements were determined with
the Zeta sizer Nano Z (Malvern, UK). 1H-NMR spectra
was recorded on Avance-III 400 MHz Fourier Transform
Digital NMR Spectrometer at 25 °C (BRUKER BIOSPIN,
Switzerland). The Mass spectra were recorded by ESI
technique on Shimadzu mass spectrometer.
Fig. 1 UV-vis diffuse absorption spectra of CdS NPs
Photocatalytic Activity
The photodegradation of MR solution by varying the
amount of CdS NPs was studied. In a typical procedure,
0, 10, 20, 30 and 50 mg amount of the catalyst is added
to the 100 ml of the 2×10−5 M of MR solution. The
reaction suspensions were prepared by adding CdS NPs
to the above mentioned MR solution. The suspensions
were ultrasonically sonicated for 20 min and stirred in
dark for 45 min to ensure an adsorption/desorption equi-
librium. The reaction suspensions containing MR and
photocatalyst were irradiated by sunlight with continuous
stirring. After irradiation the reaction solution aliquots
were taken from the reaction solution at regular intervals
and centrifuged immediately to remove the catalyst. The
absorbance of MR solution was determined at 394 nm
(pH=5) for absence and 430 nm (pH=4) for the presence
Fluorescence Measurements
Fluorescence signal was detected in a right angle viewing
mode of the incident light and that of emission. Excitation
and emission slits were set both at 5.0 nm band-pass. In a
typical procedure, the measurement of emission wave-
length of SB and SB capped CdS NPs at room tempera-
ture. Fluorescence of the mixture was recorded in a quartz
cuvette at a fixed excitation wavelength of 350 nm and
fluorescence emission wavelengths of SB and SB capped
CdS NPs was measured. Subsequently, quenching mea-
surements, the appropriate volume of CdS NPs and a se-
ries of SB standard solutions were added to the 10 ml
comparison colour tube, diluted by DMF and homoge-
nized for determination. The detection spectra were obtain-
ed in the range of 350–650 nm with the excitation wave-
length of 350 nm. The whole process was carried out at
the room temperature.
Fig. 2 The FTIR spectrum of CdS NPs

J Fluoresc
(100 ml). Then the agar medium was sterilized by autoclaving
at a pressure of 15 psi and 120 °C temperature for 1 h. This
medium was transferred into sterilized Petri dishes in a lami-
nar air flow. After solidification of media, overnight culture of
E. coli (100 μL), S. aureus (100 μL) and P. aeruginosa
(100 μL) was spread separately on the solid surface of the
media. Sterile discs were kept on these inoculated plates
with the help of sterile forceps. Sample (10 μL) solutions
were placed on these discs and were incubated at 35 °C
for 24 h in a bacterial incubator. The inhibition zone that
appeared around the disc was measured and recorded as
the antibacterial effect of SB, uncapped and SB capped
CdS NPs.
Fig. 3 The XRD spectrum of CdS NPs
Results and Discussion
UV-vis DRS Analysis
of catalyst in photodegradation of MR using an UV-visible
spectrophotometer.
UV-vis diffuse absorption edge of CdS NPs is obtained from
the plots of absorbance against wavelength. The interception
of the tangent on the descending part of the absorption peak of
the wavelength axis gives the value of diffuse absorption edge
(nm). The UV–vis DRS of uncapped and SB capped CdS NPs
were presented in Fig. 1. The absorption peak of CdS NPs can
be easily found in the visible region. The wavelength for pure
Antibacterial Activity
Luria–Bertani (LB) agar medium was prepared by adding
yeast extract (0.5 g), tryptophan (1 g), sodium chloride (1 g)
and bacterial grade agar (2.5 g) in double distilled water
Fig. 4 a–b SEM micrographs of SB capped and uncapped CdS NPs c–d corresponding EDX images

J Fluoresc
Fig. 5 The TEM images of a
100 nm, b 50 nm, c 20 nm
magnifications and d
corresponding SAED pattern of
CdS NPs
CdS NPs was about 590 nm, corresponding to a band gap of
2.10 eV, which was smaller than the reported in literature
(CdS, 2.42 eV) [31]. The band gap of the material can be
estimated by using the formula: Eg=1240/λ, where Eg is the
band gap energy and λ is the wavelength of the absorption
edge. A blue shift was observed for the uncapped and SB
capped CdS NPS, the absorption edge shifted to 576 nm
(2.14 eV) and 570 nm (2.17 eV) respectively. It may be due
to the size quantization effect, the grain size decrease leads to
the energy gap wider and a blue shift was found [32].
absorption peaks observed at 3427, 1562, 1412, 1009 and
672 cm−1 are attributed to form CdS NPs.
XRD Analysis
The structural analysis of the synthesized CdS NPs has been
carried out using XRD. The XRD pattern in Fig. 3 consists of
three diffraction peaks located at 2 =26.10, 43.80 and 51.50
FTIR Analysis
Figure 2 shows the FTIR spectrum of synthesized CdS NPs at
room temperature in the range of 4000–250 cm−1. In the FTIR
spectrum of SB capped CdS NPs, the strong absorption peaks
at 3435 cm−1, 1556 cm−1, 1412 cm−1, 1330 cm−1 and
1016 cm−1 are assigned to O-H stretching in phenols, N = O
stretching in nitro compounds, C-C stretching in aromatics, C-
N stretching and C-O stretching respectively. For the SB
capped CdS NPs, when compared to SB, the increase in the
intensity is followed by the shifting of the peaks are observed
in all the cases. The new weak bands observed at 674, 582 and
481 cm−1 which are not seen in the spectrum of SB can be
attributed to υ(Cd–O), υ(Cd–N) and υ(Cd-S). This also tes-
tifies the interaction of Cd with SB, in other words, CdS NPs
are modified by SB. In uncapped CdS NPs, the strong
Fig. 6 The fluorescence emission spectra of SB and SB capped CdS NPs

J Fluoresc
SEM & EDX Study
SEM is a suitable technique for study of the morphology of
materials. SEM micrograph with EDX spectra recorded for
CdS NPs are shown in Fig. 4a–d. From the SEM image it is
noticed that the surface morphology is in the form of assem-
blies of nanoparticles having different shapes. The chemical
composition of CdS NPs was measured through the EDX
spectrum. According to EDX, the particles consist of O, C,
N, Cd and S. The peaks of C, N and O were mainly generated
by SB and the peaks of Cd and S were generated by CdS NPs.
TEM Study
Fig. 7 Fluorescence spectra of CdS NPs quenched by SB in the
concentration range of 0–7×10–5 M
Figure 5a–c shows the typical TEM image of prepared SB
capped CdS NPs at room temperature. NPs exhibit
monodispersity and nearly spherical shape with a narrow size
distribution. The average particle size is in the range of 2–
10 nm. The selected area electron diffraction (SAED,
Fig. 5d) clearly gives three diffraction rings of (111), (220)
and (311) planes from inner to outer, respectively, which iden-
tifies the cubic phase. Some nanoparticles get aggregated due
to large specific surface area and high surface energy. The
aggregation occurred probably during the process of drying.
which can also be indexed as (111), (220) and (311) crystal-
lographic planes, respectively, indicating again that, as-
prepared CdS NPs belong to the face-centred cubic phase with
the zinc blende structure. The particle size of CdS NPs was
estimated based on the full width at half-maximum (FWHM)
of the (111) peak according to the Debye-Scherrer equation
shown as follows:
Kλ
D ¼
Electrophoretic Light Scattering Study
βcosθ
The variation in zeta potential of uncapped and SB capped
CdS NPs in their aqueous dispersion was recorded. The ob-
served value of ζ, -2.04 and 2.98 mV for uncapped and SB
capped CdS NPs respectively reveal that their aqueous sus-
pensions are quite stable because of the electrostatic repul-
sions between the same charged species. When CdS NPs dis-
persed in water then the surface becomes anionic in nature and
Where K is a constant (shape factor, about 0.89), λ is the X-
ray wavelength, β is the FWHM of the diffraction line, and is
the diffraction angle. It can be observed that the average par-
ticle sizes were in the range of 5–15 nm based on (111) crystal
plane. The lattice parameters of SB capped and uncapped CdS
NPs found to be 5.98 and 5.79 Ao respectively.
Fig. 8 a Stern-Volmer plot and b modified stern-Volmer plot of CdS NPs in presence of SB

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Fig. 9 Photocatalytic degradation of MR dye in the absence of catalyst
under sunlight irradiation
Fig. 11 The variation of degradation efficiency with amount of catalyst
Fig. 10 Photocatalytic degradation of MR dye using a 10 mg, b 20 mg, c 30 mg and d 50 mg of catalyst under sunlight irradiation

J Fluoresc
and the surface quality is related to their fluorescence
emission.
Fluorescence Interaction Studies of CdS NPs with SB
The fluorescence interaction spectra of CdS NPs in the pres-
ence of SB are shown in Fig. 7. The fluorescence intensity of
CdS NPs was significantly decreased without any change in
the emission maximum and spectral shape as the concentra-
tion of SB was increased, this suggests that the SB quenched
the fluorescence effectively [34]. Sonochemically synthesized
SB capped CdS NPs and SB were soluble in DMF. The SB
solutions of 1–7×10−5 M concentrations are prepared in DMF
and subsequently were added to the 2×10−5 M CdS NPs so-
lution. Then the fluorescence intensity of mixed solutions was
measured. The fluorescence band of CdS NPs at 465 nm grad-
ually decreased from a concentration of 1×10−5 M and
vanished when 7×10−5 M SB was added, which may proba-
bly be due to a stronger interaction between SB and CdS NPs.
The normalized quenching intensities (F/F0; F and F0 is the
fluorescence intensities of CdS NPs in the presence and ab-
sence of the SB, respectively) versus concentration of SB
suggests that it quenches CdS NPs more efficiently. To ana-
lyze the dependence of the fluorescence intensity on the SB
concentration, Stern–Volmer relationship has been used.
Fig. 12 The percentage of degradation efficiency of MR dye under
sunlight irradiation
the surface area increases that would render the more coverage
of hydroxyl groups from water [33].
Fluorescence Study
Fluorescence Measurement
Fluorescence spectra (Fig. 6) indicate that the SB and
SB capped CdS NPs exhibited emission peaks with
slightly different full-width-at-half-maximum (FWHM)
values of 58 nm (SB), 46 nm (SB-CdS NPs). Emission
maximum of SB was found to be at around 512 nm
(2.41 eV), while that of SB capped CdS NPs was
shifted to around 465 nm (2.65 eV). The fluorescence
emission spectra of the CdS NPs solutions showed a
significant bathochromic shift of the emission maxima
by complexation of SB, in comparison with fluores-
cence of SB. It is well known that the size of the
nanoparticles is related to their absorption maximum
Fo
¼ 1 þ Ksv½QŠ
F
Where, KSV is the Stern–Volmer constant which is a mea-
sure of the efficiency of quenching and [Q] is the concentra-
tion of the SB [35]. By plotting (F0/F) versus [Q], KSV can be
calculated from the slopes of the linear Stern–Volmer plots. In
Fig. 8a, the Stern–Volmer plot of CdS NPs was shown. From
the slope of the linear plot, the Stern–Volmer quenching con-
stant KSV (0.12×105 L mol−1) was calculated. The results of
the fluorescence study indicated that the quenching effect of
SB on the fluorescence emission of CdS NPs is found to be
HCOOH
Scheme 2 The possible
degradation mechanism of MR
dye using ESI-MS
OH
OH
O
O
N
+
COOH
COOH
+
CO2 H2O
(3)
NH2
(1)
+
H2C COOH
OH
N
N N
HOOC
COOH
COOH
HO
+
CO2 H2O
-CO2
(2)
(4)

J Fluoresc
Fig. 13 a At/Ao curves and b kinetic plots for photocatalytic degradation of MR under sunlight irradiation
concentration dependent. The quenching data was analyzed
degradation of MR was investigated systematically under sun-
according to the modified Stern–Volmer equation:
light irradiation.
Figure 10a–d shows the absorption spectra of MR after
addition of 10, 20, 30 and 50 mg of catalyst under sunlight
irradiation. The intensity of the peak decreases with increase
in irradiation time due to photocatalytic degradation of MR. It
indicates that the photocatalytic efficiency in the presence of
CdS NPs was higher than in the absence of catalyst. It was
observed that there is an enhancement of photocatalytic deg-
radation efficiency of MR dye with the 30 mg of catalyst
among 10, 20, 30 and 50 mg of catalyst (Fig. 11). The rate
of MR degradation was found to be faster in 30 mg amount of
catalyst. Moreover, the photocatalytic activity of a
photocatalyst depends on the amount of catalyst and its per-
centage content on the surface of the catalyst. The amount of
catalyst is also likely to affect the rate of dye bleaching and
hence photocatalytic bleaching was observed using different
amounts of photocatalyst. The degradation efficiencies in the
absence and presence of various amounts catalyst were calcu-
lated in photodegradation of MR dye. Figure 12 shows the
detailed photocatalytic degradation efficiency of MR dye in
the absence and presence of CdS NPs with increasing reaction
irradiation time under solar irradiations. Finally, the
Fo
1
1
1
¼
þ
Fo−F
faka ½QŠ fa
Where, fa is the fraction of the initial fluorescence and Ka is
the effective quenching constant. A plot of (F0−F)/F versus
1/[Q] (Fig. 8b) gives a straight line where the values of Ka of
CdS NPs has been found to be 0.13×104 M−1, from slope and
fa was found to be 5.102 from intercept of the plot.
Photocatalytic Degradation Study
The photocatalytic degradation of MR was studied in the pres-
ence of varying amounts of CdS NPs. The position of absorp-
tion wavelength changes in the optical absorption spectrum to
394 nm in the presence of CdS NPs. The reaction vessel was
exposed to sunlight irradiation for 4 h in the absence of CdS
NPs. Only a very small change in the intensity of the maxi-
mum absorption was observed after 4 h irradiation in the ab-
sence of CdS NPs and the UV-vis absorption spectra was
shown in Fig. 9. The effect of amount of catalyst on the
Table 1 The calculated values of
S. no
Amount of catalyst
% of degradation
Rateconstant, k (min−1
)
R2
degradation efficiency and rate
constants of photo degradation of
MR dye
1
2
3
4
5
0 mg
5.07
65.61
78.29
82.59
80.43
0.012
0.254
0.379
0.431
0.415
0.98
0.98
0.99
0.98
0.99
10 mg
20 mg
30 mg
50 mg

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Fig. 14 Antibacterial test results
of E. coli, S. aureus and
P. aeruginosa after 24 h
incubation by SB capped CdS
NPs. Ampicillin used as positive
control
significant photocatalytic performance of the CdS NPs was
observed with the commercially available Degussa P25 and
TiO2. The superior performance of Degussa P25 over TiO2
samples of a different origin has often been reported in the
literature with a number of postulations being suggested in
order to explain its high photo activity, with regards to both
its surface and bulk properties [36–41].
Scheme 2. The intermediates (1 and 2) are formed on the CdS
NPs surface due to the attack of hydroxyl radicals at the site of
the azo bond. During irradiation the degradation products of
intermediate (1) proceeds through the formation of hydroxyl-
ated aromatic derivatives like 1, 4-dihydroxy benzene and
hydroquinone. After 2 h of irradiation mass spectrum shows
corresponding peaks, which were due to the formation of sim-
ple aliphatic carboxylic acids like oxalic acid (3), formic acid
and glyoxalic acid. Ring opening reaction of hydroquinone
results in the formation of the above intermediates. The inter-
mediate (2) further undergoes to formation of benzoic acid (4).
After 4 h of solar irradiation, the mass spectrum showed no
characteristic peaks belonging to any functional group
confirming the complete mineralization of the dye.
Degradation Mechanism of MR dye
The degradation mechanism of MR was carried out by the
ESI-MS analysis. The degradation products may be more
harmful and persistent than the parent compound [42]. There-
fore, it is necessary to identify the degradation products. The
mass spectrum recorded at the time of decolorisation of the
dye for this process shows m/z peaks at 136 and 138 attributed
to the formation of 4,4-N,N-dimethyl aniline (1) and 2-
hydroxy benzoic acid (2). Based on the intermediates obtain-
ed, probable degradation mechanisms have been proposed in
Kinetic Studies of Photocatalytic Degradation of MR
The Langmuir-Hinshelwood method can be used to describe
the rates of the photocatalytic degradation of MR in the
Table 2 Zone of inhibition study
S. no
Sample
Zone of inhibition in mm
on E. coli, S. aureus and
P. aeruginosa bacterias with
prepared samples against
ampicillin (control)
E.coli
S.aureus
P.aeruginosa
Ampicillin
1
2
3
SB capped CdS NPs
Uncapped CdS NPs
SB
18.04
6.09
21.31
7.62
25.6
27.4
26.6
27.1
10.92
20.02
18.11
17.27

J Fluoresc
presence of CdS NPs as a function of irradiation time [43].
The ratio of absorbances with increasing reaction irradiation
time and initial absorbance (At/Ao) was plotted against irradi-
ation time as shown in Fig. 13a. It shows a gradual decrease
with reaction time. Plotting the natural logarithm of the ratio
between the original absorbance of MR and the absorbance
after photocatalytic degradation [ln(Ao/At)] against the irradi-
ation time yields a linear relationship as shown in Fig. 13b.
Therefore, the photocatalytic degradation reaction of MR by
CdS NPs follows the pseudo-first order reaction kinetics. The
adsorption rate constant and degradation efficiency values of
MR with amount of SB capped CdS NPs under sunlight irra-
diation are summarised in Table 1.
CdS NPs and SB were investigated using fluorescence stud-
ies. The study of fluorescence of CdS NPS with the increasing
SB concentration indicates some static fluorescence
quenching seems to be involved in the quenching mechanism.
Synthesized NPs have been used for photocatalytic degrada-
tion of MR under sunlight irradiation. The results of this study
prove that the amount of catalyst and sunlight are significantly
affects the photocatalytic degradation of dye. The Langmuir-
Hinshelwood kinetic analyses revealed that pseudo-first order
kinetics is followed for the degradation of dye. The prepared
samples were screened for their antibacterial activity against
bacteria. The results showed that the SB capped CdS NPs are
biologically active.
Acknowledgments The authors would like to acknowledge Head, De-
partment of Chemistry, Osmania University for providing necessary fa-
cilities. One of the authors, D. Ayodhya wishes to thank the UGC, New
Delhi for award of SRF which supported this work.
Antibacterial Activity Study
The details of the antibacterial experiment were provided in
the experimental section. In present study antibacterial activ-
ities of prepared samples were tested by paper disc method
using E. coli, S. aureus and P. aeruginosa and shown in
Fig. 14. The SB and CdS NPs are very promising in exhibiting
their ability to inhibit/destroy both gram positive and gram
negative pathogenic bacteria. The zone of inhibition was mea-
sured for uncapped and SB capped CdS NPs as well as SB
against the bacteria E. coli, S. aureus and P. aeruginosa at
1 mg/ml concentration by the paper disc method using nutri-
ent agar as the medium and shown in Table 2. An increase in
the lipophilic character of the complex favours its permeation
through the lipid layer of the bacterial membrane, and there-
fore shows higher activity. The higher activity of the com-
plexes compared to free ligand may be attributed to chelation
[44] which reduces polarity of the metal ion by partial sharing
of the positive charge with donor atoms of the ligand. This
increases the lipophilic character, favouring the permeation
through lipid layers of the bacterial membrane. The SB can
penetrate the bacterial cell membrane by coordination of metal
ion through oxygen or nitrogen donor atom to lipopolysaccha-
ride which leads to the damage of outer cell membrane and
consequently inhibits the growth of the bacteria. Therefore,
the SB capped CdS NPs exhibited more antibacterial activity
against all bacteria than with SB or CdS NPs alone.
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