Catalytic synergy of Au@CeO2-rGO nanohybrids for the reductive transformation of antibiotics and dyes

Article abstract of DOI:10.1039/d1nj00180a

Au@CeO2-rGO nanohybrids (NHs) are synthesized using sucrose via a hydrothermal method. The interfacial interactions between gold encapsulated in CeO2 and rGO are observed by Raman and X-ray photoelectron spectroscopy (XPS). The optical properties and crystallinity of the nanohybrids are determined by using UV-Vis and X-ray diffraction (XRD), respectively. Elemental mapping using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) clearly demonstrates the Au@CeO2 core-shell sequence, in which the core of electron-dense Au NPs is encapsulated by the spherical CeO2 shell; these Au@CeO2 particles are then decorated on the reduced graphene oxide (rGO) nanosheets. As an application of the synthesized Au@CeO2-rGO nanohybrids, the reductive transformation of organic contaminants present in wastewater is accomplished. Ampicillin (AMP) gives 6-methyl-3-phenylpyrazin-2-ol and ciprofloxacin (CIP) affords 1-cyclopropyl-6-fluoro-3-(hydroxymethyl)-7-(piperazin-1-yl)-2,3-dihydroquinolin-4(1H)-one. Moreover, rhodamine B (RhB) forms (2-(3,6-bis(diethylamino)-9H-xanthen-9-yl)phenyl)methanol and Congo red (CR) produces benzidine and 3,4-diaminonaphthalene-1-sulfonic acid. The elimination of the antibacterial activities of AMP and CIP using reductive degradation and transformation shows the possibility of decreasing exposure of bacteria to antibacterial chemicals. The transformed by-product of CR is considered to be a potential anti-AIDS agent.

Full text of DOI:10.1039/d1nj00180a

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Catalytic synergy of Au@CeO2–rGO nanohybrids
for the reductive transformation of antibiotics and
dyes†
Cite this: DOI: 10.1039/d1nj00180a
a
Kanchan Mishra,a Samjhana Pradhan,a Muhammad Saeed Akhtar,
a
Won-Guen Yang,b Sung Hong Kimb and Yong Rok Lee
*
Au@CeO2–rGO nanohybrids (NHs) are synthesized using sucrose via a hydrothermal method. The inter-
facial interactions between gold encapsulated in CeO2 and rGO are observed by Raman and X-ray
photoelectron spectroscopy (XPS). The optical properties and crystallinity of the nanohybrids are deter-
mined by using UV-Vis and X-ray diffraction (XRD), respectively. Elemental mapping using high-angle
annular dark-field scanning transmission electron microscopy (HAADF-STEM) clearly demonstrates the
Au@CeO2 core–shell sequence, in which the core of electron-dense Au NPs is encapsulated by the
spherical CeO2 shell; these Au@CeO2 particles are then decorated on the reduced graphene oxide (rGO)
nanosheets. As an application of the synthesized Au@CeO2–rGO nanohybrids, the reductive transformation
of organic contaminants present in wastewater is accomplished. Ampicillin (AMP) gives 6-methyl-3-
phenylpyrazin-2-ol and ciprofloxacin (CIP) affords 1-cyclopropyl-6-fluoro-3-(hydroxymethyl)-7-(piperazin-1-yl)-2,
3-dihydroquinolin-4(1H)-one. Moreover, rhodamine B (RhB) forms (2-(3,6-bis(diethylamino)-9H-xanthen-9-
yl)phenyl)methanol and Congo red (CR) produces benzidine and 3,4-diaminonaphthalene-1-sulfonic acid. The
elimination of the antibacterial activities of AMP and CIP using reductive degradation and transformation shows
the possibility of decreasing exposure of bacteria to antibacterial chemicals. The transformed by-product of CR is
considered to be a potential anti-AIDS agent.
Received 12th January 2021,
Accepted 26th March 2021
DOI: 10.1039/d1nj00180a
rsc.li/njc
nanoparticles, which can enhance the catalytic activity.9
In addition to electronic interactions between the oxide support
Introduction
Supported noble metals have garnered considerable attention and the noble metal, the size, charge, and porosity of the metal
over the past few decades due to their versatility, which has nanoparticles can influence catalytic performance, as can the
been demonstrated by their application in various catalytic properties and morphology of the oxide support.10
protocols for the synthesis of diverse chemicals.1,2 Gold nano-
While several inorganic oxides as catalysts have been
particles supported on high-surface-area oxides have been reported, including CeO2,11 Fe2O3,12 SiO2,13 SnO2,14 TiO2,15
used in remarkable heterogeneous catalysis research,3,4 and and ZrO2,16 there has been significant interest in CeO2 due to
supported gold nanoparticles have been applied in many areas its high oxygen storage capacity, ample oxygen vacancy defects,
such as chemical processing, sensors, fuel cells, and pollution and relative ease of transition between the oxidation states III
control.5–7
and IV, all of which enhance the catalytic efficiency of the
The widespread use of supported gold in catalysis is material. However, there have only been a few reports on the
attributed to its high chemoselectivity.8 The catalytic properties fabrication of noble metal NPs@CeO2 core–shell nano-
of the heterogeneous system are quite different from those of composites decorated on reduced graphene oxide (rGO).17–19
both gold and the metal oxide support due to characteristic
From the last couple of decades, organic pollutants are
synergistic effects between the oxide support and the gold crucial to the contamination of water resources. Above all, the
pollution created by antibiotics and organic dyes has become a
a School of Chemical Engineering, Yeungnam University, Gyeongsan, 38541,
significant threat to the environment and human health.20,21
Antibiotics like ampicillin (AMP) and ciprofloxacin (CIP) have
been found in surface and underground water due to waste-
water leakages.22,23 This can have undesirable consequences,
such as contributing to antibiotic resistance in bacteria and
endocrine disruption in the human body.24,25 Current methods
Republic of Korea. E-mail: yrlee@yu.ac.kr
b Analysis Research Division, Daegu Center, Korea Basic Science Institute,
Daegu 41566, Republic of Korea
† Electronic supplementary information (ESI) available: Materials and methods,
characterization procedures, instrumentation, and data, and 1H NMR and 13C
NMR spectra for synthesized compounds. See DOI: 10.1039/d1nj00180a
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of removing antibiotics from wastewater include membrane Preparation of GO
separation,26 photocatalysis,27 adsorption,28 advanced oxidation
GO was prepared using
a
slightly modified version of
processes (AOPs),29 and biological approaches.30 Despite their
merits, such methods still have shortcomings, like the use of
expensive and hazardous chemicals. Thus, simpler and more
efficient methods of minimizing antibiotic resistance are needed,
such as removing the antibacterial functions of pollutants.
Among the various water contaminants, organic dyes are
toxic even at lower concentrations. Besides their prominent
effect on health, organic dyes cause harmful effects, like
alteration of the oxygen demand, consumption of oxygen
molecules, and reduction of water transparency, and ultimately
deteriorate the aquatic system.31 Most of the azo dyes are
reported to be very toxic to both the environment and human
health,32 and are considered a major contributor to environ-
mental pollution. The presence of dyes in the wastewater
generated by textile factories is of great environmental concern
not only because of the release of dye molecules into the water,
but also because other toxic substances can be produced
through hydrolysis and oxidation reactions.33–35 Therefore,
development of a facile, sensitive and fast protocol for the
reductive transformations of antibiotics and dyes in aqueous
media is of high interest and in demand.
Hummers’ method.41 Concentrated sulfuric acid (10 mL) was
cooled, and graphite powder (0.4 g) was mixed and stirred
vigorously for 10 min. During stirring, KMnO4 (0.8 g) was slowly
added to the suspension. The suspension was agitated for
15 min below 20 1C, and the temperature was increased to
35 1C for 30 min. After completion of the reaction, effervescence
decreased and the mixture slowly thickened, changing to a
brownish-gray color. Eventually, 100 mL of DI water was
gradually added to the mixture and the temperature was raised
to 98 1C for 15 min. The obtained suspension was centrifuged,
and the residue was thoroughly washed with 5% hydrochloric
acid (5 mL) until the aqueous layer no longer produced salt
with barium chloride. The final GO residue was washed with
acetone, and the obtained solid was dried in a vacuum oven for
5 h at 80 1C and stored.
Synthesis of Au@CeO2–rGO NHs
GO (50 mg) was diffused in water in a Teflon cup. The mixture
was further sonicated to disperse the GO layers. Au@CeO2
(50 mg) was added to the dispersed GO along with 10 mg of
sucrose, and the pH was adjusted to make it alkaline.
The Teflon cup was sealed and heated for 10 h at 180 1C. The
Au@CeO2–rGO NHs were collected as a black solid and dried in
an oven for 12 h at 80 1C, and then processed for further use.
Ceria has been widely used as a promising material for
the elimination of organic pollutants from wastewater.36,37
Nanosized CeO2 has high electronic conductivity38 and is used
as UV-protection material due to its strong surface plasmon
resonance.39 Ceria-based nanocomposites can be used as
photocatalysts in visible light.40
Reductive transformation of AMP, CIP, RhB, and CR
The reductive transformations of AMP, CIP, RhB, and CR were
monitored using UV-Vis spectroscopy, 1H, and 13C NMR
spectroscopy, and gas chromatography-mass spectroscopy.
Au@CeO2–rGO NHs (2 mg) were added to solutions of AMP
(0.025 mM in H2O), CIP (0.025 mM in H2O/DMSO), RhB
(0.025 mM in H2O/DMSO), or CR (0.025 mM in H2O/DMSO)
at room temperature. NaBH4 (37.83 mg, 1.0 mmol) was added
to the above mixture, which was further heated for 1 h at
100 1C. The reaction mixture was then isolated by extracting
with ethyl acetate and evaporated using a reduced pressure
evaporator. The residue was isolated by column chromatography
using 30% ethyl acetate in hexane on silica gel to obtain the
products, the structures of which were determined by spectro-
scopic analysis (see the ESI† for details).
The water contamination caused by antibiotics and dyes is
of great concern due to the significant threat for environment
and human health. Therefore, it is quite necessary to transform
the contaminants like antibiotics and dyes from wastewater
before they get released into nature. Considering the importance
and usefulness of metal NPs/oxide core–shell nanocomposites in
transformation of water contaminants, we herein describe a
new approach to the preparation of Au@CeO2–rGO NHs
using sucrose via a hydrothermal route. We further report
on the catalytic synergy of these NHs for the reductive
transformation of antibiotics (AMP and CIP) and dyes (RhB
and CR) as a model study for removing undesirable contaminants
from wastewater.
Elimination of antibacterial activity of AMP and CIP
Experimental section
Synthesis of Au@CeO2
The antibacterial activity of the reduced compounds 2 and 4
was screened against Gram-positive bacteria, Staphylococcus
Sucrose (10 mg) was dissolved in water (20 mL), 0.2 mL of a aureus (KCTC-1916) and Gram-negative bacteria, Escherichia
10 mM aqueous solution of HAuCl4 was then added dropwise coli (KCTC-1924) using a modified version of the Kirby–Bauer
while stirring over 20 min. CeCl3ꢀ7H2O (70 mg) was added to disk diffusion method.42 Bacterial strains were purchased from
the above mixture and stirred intensely until a clear yellow the Korean Collection for Type Cultures (KCTC) and cultivated
solution was formed. The solution was then kept in a stainless- in 10 mL of fresh DifcoTM nutrient agar for 24 h. Then, precisely
steel Teflon-sealed autoclave and kept for 10 h at 180 1C. The 100 mL of a fresh batch of bacterial suspension was distributed
resulting deep brown precipitate was centrifuged and washed in this DifcoTM nutrient agar. The optical density of the freshly
thoroughly with water. Finally, the brown residue was calcined prepared suspension of bacteria was found to be 0.7 at 595 nm.
in a muffle furnace at 600 1C for 5 h.
The paper discs (8 mm in diameter) were autoclaved and then
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thoroughly soaked with 20 mL of the test sample
(100.00 mg mLꢁ1), transferred to the agar plate and incubated
at 37 1C. The inhibition zones observed on the discs were
analyzed at the end of the incubation period, with the agar
depth in the plate being an evaluation factor. Filter paper discs
saturated with ciprofloxacin and ampicillin were used as
positive controls, whereas negative control was prepared with
DMSO saturated on discs.
Results and discussion
Role of sucrose in the synthesis of Au@CeO2–rGO NHs
The Au@CeO2–rGO NHs were fabricated by using sucrose as a
green protocol. Scheme 1 indicates a general approach for the
synthesis of Au@CeO2–rGO NHs. While sucrose is a non-
reducing sugar, chloroauric acid hydrolysis of sucrose produces
fructose and glucose, which are responsible for the bioreduction
of the metal salts to nanoparticles.43 Thus, the Au3+ ions were
first reduced to Au0 under hydrothermal conditions by glucose
or fructose to form the cores. The glucose was carbonized during
the hydrothermal process due to dehydration. The carbonized
glucose or fructose formed carbon dioxide reacting with oxygen
and diffused in the air after calcined at 600 1C for 5 h. At the
same time, the CeO2 shells formed from Ce3+ in two steps, with
the first being the formation of cerium hydroxide during the
hydrothermal treatment in the presence of NH4OH, followed by
dehydration and oxidation to form Ce4+, as confirmed by XPS.44
Fig. 1 (a) XRD patterns of Au (JCPDS No. 34-0394), CeO2 (JCPDS No.
34-0394), Au@CeO2, and Au@CeO2–rGO NHs. (b) Raman spectra of
Au@CeO2 and Au@CeO2–rGO NHs, inset: CeO2 peaks. (c) N2
adsorption–desorption isotherm of Au@CeO2–rGO NHs. (d) Pore size
distribution of Au@CeO2–rGO NHs.
synergy between Au, CeO2, and rGO when forming Au@CeO2–rGO
NHs.49
XRD was used to determine the structure of the (Fig. 1a). The
XRD peaks at 2y = 37.751, 43.981, 64.241, and 77.301 were
indexed as (111), (200), (220), and (311), respectively, and
correspond to face-centered cubic gold reflective planes (JCPDS
No. 34-0394).11 In addition, the peaks at 2y = 28.101, 32.651,
47.101, 55.981, 58.731, 69.041, 76.351, 78.751, and 88.151 are
attributed to the (111), (200), (220), (311), (222), (400), (331),
(420), and (422) reflection planes, respectively, of the fluorite
CeO2 (JCPDS No. 34-0394).14 The XRD pattern of the Au@CeO2–
rGO NHs is dominated by the CeO2 reflections, as the CeO2
shell wraps around the Au core.50 The presence of a broad peak
at about 251 corresponds to the (002) plane of the reduced
graphene due to its reduced oxygen-rich functionality, which
clearly suggests formation of Au@CeO2–rGO.51
The interfaces between the components were analyzed by
comparing their Raman spectra (Fig. 1b). The Raman spectrum
of CeO2 shows a distinct peak at about 454 cmꢁ1, which is due
to the F2g vibrational mode of the fluorite phase,52,53 while the
broad peak at a 600 cmꢁ1 is correlated with the oxygen
vacancies introduced into the ceria lattice.54 The shifting of
the F2g mode of CeO2 in the spectrum of Au@CeO2 was due to
strain effects resulting from the core–shell interactions between
Au and CeO2.55 However, the Au bands were not clearly
detectable in the spectrum of Au@CeO2 as their intensities
were very low compared to those of the CeO2 bands, as the Au
core was shielded by the CeO2 shell.56 The Raman spectrum of
the nanohybrid also showed the presence of rGO, with two D
and G graphene bands being visible. The D-band is related to
the 1st order sp3 carbons scattering, while the G-band refers to
the sp2-hybridized carbons vibration. The ID/IG ratio for rGO in
the nanohybrids was 1.02, which is associated with a rise in the
Characterization of Au@CeO2–rGO NHs
The optical properties of Au@CeO2–rGO NHs were preliminarily
examined using UV-Visible spectroscopy (Fig. S1, ESI†). A well-
defined absorption spectrum was obtained, with a distinct peak
at about 300 nm being attributed to the characteristics of the
CeO2 nanoparticles,45 while that at 210 nm was due to the rGO.
Furthermore, the presence of gold in the Au@CeO2–rGO NHs
was evidenced by a typical surface plasmon resonance band at
about 540 nm.46 The red shift compared to the peak of Au NPs at
525 nm indicates the presence of the surrounding medium and
its morphological characteristics.47 Thus, the stable nanostructure
of Au@CeO2–rGO NHs and its prominent absorption bands are
due to the interactions when gold is encapsulated by CeO2 and
then, decorated on rGO.48 This phenomenon demonstrates the
Scheme 1 General strategy for the fabrication of Au@CeO2–rGO NHs.
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extent of disorder and defects, and a diminishing the average
sp2 domain size.57
The nitrogen adsorption–desorption isotherms were measured
to analyze the porosities of the synthesized materials, including
their specific surface areas and pore diameters. The nitrogen
adsorption–desorption isotherm and the corresponding pore size
distribution graph of the Au@CeO2–rGO NHs are shown in Fig. 1c
and d, respectively. The specific area of a surface offers ample
active sites that are critical for enhancing catalytic activity.58
The BET specific surface area of the Au@CeO2–rGO NHs was
found to be 47.62 m2 gꢁ1, and the average pore size was estimated
to be 3.80 nm.
XPS was conducted to analyze the surface chemistry of the
Au@CeO2–rGO NHs, providing information about the elemental
composition and oxidation states. The Ce 3d XPS spectrum was
dominated by the Ce4+ state, but the two peaks at around 884.0
and 894.5 eV correspond with the Ce3+ valence state (Fig. 2a).59
The unsymmetrical O 1s peak could be deconvoluted into two
sections (Fig. 2b), with the peak at 529.4 eV confirming the
surface lattice oxygen species from cerium oxide, while the other
high binding energy peak at about 531.1 eV is known to be
related to surface-absorbed oxygen species.60 The auger peak at
about 533.4 eV was attributed to hydroxide or dissociated
molecular oxygen.61 Likewise, the XPS spectra for the Au 4f
electrons showed binding energies of 84.7 and 87.7 eV, due to
4f7/2 and 4f5/2, respectively, with a peak distance of 3.87 eV
(Fig. 2c). The prominent peak at about 84.7 eV (Au 4f7/2) is due
to Au0, although it is slightly shifted due to the core Au 4f7/2
electrons being shielded by CeO2.62,63 The XPS spectra were
calibrated at 284.9 eV using the C 1s value (Fig. 2d).
Fig. 3 (a) HAADF-STEM image and (b) HRTEM image of Au@CeO2.
(c) HAADF-STEM image and (d) TEM image of CeO2.
The morphology of CeO2 was observed to change from a
spindled shape (Fig. 3c and d) to a mesoporous form once Au
was encapsulated by CeO2. This can be attributed to the
synergistic effects of the gold on the ceria in Au@CeO2
nanocomposites decorated over rGO. The structural changes
observed by introducing multimetallic system includes the
mixing of one type of metal by another one on the surfaces,
electron charge transfer between metal atoms, and interfacial
interactions between metal phases.64
The size and morphology of the Au@CeO2–rGO nanohybrids
were analyzed by TEM images. TEM images are shown in
Fig. 4a–d, with scale bars of 50 and 20 nm. The size of Au was
found to be 20 nm. EDS analysis exhibited the characteristic EDS
peaks for Au, Ce, O, and C, together with Cu observed from the
Lacey copper grid (Fig. S2, ESI†).
The representative HAADF-STEM image of Au@CeO2 clearly
shows the core–shell pattern of the nanocomposites, in which
an electron-dense core Au NP was encapsulated by a spherical
CeO2 shell (Fig. 3a). The HRTEM image clearly shows core
gold nanoparticles surrounded by mesoporous CeO2 (Fig. 3b).
The thermal stability of Au@CeO2–rGO NHs was confirmed
by the TG–DTA analysis (Fig. S3, ESI†). The initial weight loss of
2.57% at around 100 1C was due to the evaporation of adsorbed
moisture and other volatile matters contained in the nanohybrids.
This feature was further reflected by the endothermic peak at
approximately 50 1C. The second weight loss of 3.80% can be
attributed to the loss of some oxygen containing functional
groups present in the rGO.65 This showed that the Au@CeO2–
rGO NHs were stable at high temperature, leaving the residual
mass of 93.63%. The high temperature stability of Au@CeO2–rGO
NHs leads to synergism of Au, CeO2 and rGO.66
Reductive transformation of AMP, CIP, RhB, and CR
The reductions of AMP, CIP, RhB, and CR (Scheme 2) were
monitored by UV-Vis, FTIR, NMR, and GC-MS analyses (see the
ESI† for details). A prominent absorption peak at around
Fig. 2 XPS for (a) Ce 3d electrons and (b) O 1s electrons, (c) XPS for Au 4f
electrons, and (d) C 1s electrons at 284.8 eV.
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organic pollutants in wastewater can be sufficiently
transformed using the Au@CeO2–rGO NHs.
Plausible mechanism for the reductive transformation of AMP
Scheme 3 shows a feasible mechanism for the reduction of
AMP. Metal nanohybrids activate the carbonyl group of 1 to give
complex 10. The nucleophilic addition of water to 10 gives
intermediate A through the ring opening of the b-lactam ring of
AMP,67 which undergoes C–S bond cleavage of the thiazolidine
ring by a retro-Michael-type reaction to afford intermediate B.
The Michael-type addition of water to B affords another
intermediate C, and is followed by breaking of the C–N bond
to give D and E. Intermolecular cyclization of D furnishes
intermediate F, and the fragmented molecule E affords G by
reduction of carboxylic acid by hydrogen on the catalyst surface.
Elimination of water followed by enolization and aromatization
produces intermediate H, which undergoes a reduction of the
carboxylic acid moiety at the benzylic position by hydrogen on
the catalyst to give the final product, 2. The compound 2
structure was confirmed by its spectral data analysis, and
compound G was confirmed in the crude reaction mixture by
GC-MS.
Fig. 4 TEM images of Au@CeO2–rGO at 50 (a and b) and 20 nm (c and d).
Plausible mechanism for the reductive transformation of CIP
Scheme 4 shows a feasible mechanism for the reduction of CIP.
The liberated H2 from NaBH4 is activated through the
electronically supported Au in the Au@CeO2–rGO NHs. The
hydrogenation of the double bond and subsequent reduction of
the carboxylic acid moiety of CIP (3) provides the product 4,
whose structure was confirmed by analyzing its spectral data.
Plausible mechanism for the reductive transformation of RhB
Scheme 5 demonstrates a plausible mechanism for the
reduction of RhB. First, the hydride anion from NaBH4 attacks
5 to afford LeucoRhB (6), which is reduced by hydrogenation on
the catalyst to give product 7.68–70 The isolated compound 7 was
identified by its spectral data.
Scheme 2 Reductive transformation of AMP, CIP, RhB, and CR using
Au@CeO2–rGO NHs.
345 nm in the UV-vis spectrum indicated the transformation of
AMP, as AMP itself displayed no absorption peak (Fig. S4a,
ESI†). The CIP peak at about 280 nm vanished and a new
peak emerged at around 225 nm, showing the transformation
of CIP to its reduced product (Fig. S4c, ESI†). The peak
disappearance at 550 nm confirmed the transformation of
RhB (Fig. S5a, ESI†). In the presence of NaBH4 and
Au@CeO2–rGO NHs, the absorption peak of CR at 500 nm
disappeared, which supported the transformation of this mate-
rial into its product (Fig. S5c, ESI†). These results show that
Scheme 3 Plausible mechanism for the reductive transformation of AMP.
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through the fragmentation of the quinolone moieties or the
piperazine ring.72,73 These results confirmed the possibility of
removing the antibacterial activities of AMP and CIP.
Synergy of Au@CeO2–rGO NHs
Scheme 4 Plausible mechanism for the reductive transformation of CIP.
We have performed the control experiment. No product was
detected without the use of Au@CeO2–rGO nanohybrids as the
catalyst, confirming that Au@CeO2–rGO nanohybrids play a
catalytic role in the reductive transformations of organic dyes
and antibiotics. Moreover, the reaction rate of Au@CeO2–rGO
NHs ternary catalysts was greater than that of CeO2–rGO and
Au@CeO2 nanocomposites. The significant catalytic activity of
the ternary Au@CeO2–rGO NHs catalysts was due to the highly
dispersed Au nanoparticles, which serve as an electron relay in
the reaction mixture.74 The integration of graphene nanosheets
with metal oxide and noble metal offer massive opportunities
toward potent utilities of graphene based composites.75,76
There is a strong synergistic interaction among graphene
nanosheets, metal oxide and noble metal, which can significantly
enhance the catalytic properties of noble metal nanostructures.77
Moreover, the synergistic interactions and support effect between
ternary components play a vital role in their catalytic activity and
stability.78 Thus, the synergistic effect comprising the combi-
nation of graphene, metal oxide, and noble metal is attributed
to the enhanced catalytic performance.
Scheme 5 Plausible mechanism for the reductive transformation of RhB.
Plausible mechanism for the reductive transformation of CR
Scheme 6 shows a plausible mechanism for the reductive
degradation of CR. The reduction of 8 by hydrogen on the
catalyst gives intermediate 9, which is subjected to N–N bond
cleavage to furnish 10 and 11. These two compounds
were verified by the spectral data analysis. The transformed
by-product, 3,4-diaminonaphthalene-1-sulfonic acid (11), is
considered a potential anti-AIDS agent.71
Conclusions
Au@CeO2–rGO NHs were fabricated by using sucrose as a green
hydrothermal approach. The optical and surface electronic
properties of Au@CeO2–rGO NHs revealed the synergistic
effects between the different materials in the catalytic active
site of the Au@CeO2–rGO NHs. The synthesized nanohybrids
were used as effective catalysts to reduce antibiotics and dyes
using very mild NaBH4 as a hydrogen source. The transformed
by-product of CR (11) is considered to be a potential anti-AIDS
agent. The elimination of the antibacterial activities of AMP
and CIP using reductive degradation and transformation
showed the possibility of decreasing exposure of bacteria to
antibacterial chemicals, which would decrease the potential
for the development of antibacterial resistance. This facile
experimental protocol for the reductive transformation of
antibiotics and dyes using the Au@CeO2–rGO NHs as a nano-
catalyst is considered to be used in industrial application.
Elimination of antibacterial activity of AMP and CIP
Antibacterial activity tests were conducted using reference
species of Escherichia coli and Staphylococcus aureus. Fig. S5
(ESI†) shows the changes in the inhibition zones of AMP and
CIP solutions after their reduction. The zone of inhibition of
AMP showed that it possessed antibacterial activity; however,
the transformed product 2 obtained from AMP did not show
any zone of inhibition (Fig. S6a and b, ESI†). The CIP solution
also showed antibacterial activity with a zone of inhibition, but
the transformed product of CIP (4) did not show any antibacterial
activity (Fig. S6c and d, ESI†). However, previous reports have
demonstrated the elimination of the antibacterial activity of CIP
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea Government (MSIT)
(2018R1A2B2004432) and by the Korean Ministry of Education,
Science, and Technology (2012M3A7B4049677).
Scheme 6 Plausible mechanism for the reductive transformation of CR.
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