Detoxification of organophosphates using imidazole-coated Ag, Au and AgAu nanoparticles

Article abstract of DOI:10.1039/c7ra07059d

Organophosphate (OP) detoxification is a worldwide problem due to the high stability of P-O bonds. Here, we designed several imidazole-coated nanocatalysts targeted towards the cleavage of OP and thus their detoxification. Specifically, Ag, Au and bimetallic AgAu nanoparticles (NPs) supported on SiO₂ were functionalized with methimazole (MTZ), which comprises thiol and imidazole groups, foreseeing enabling freely available imidazole groups. Raman analyses indicate that MTZ interacts preferably via its sulfur atom over Au NPs and via the nitrogen of the imidazole ring over Ag NPs. The MTZ-derived nanocatalysts were effective towards OP cleavage. For instance, rate enhancements of 10⁸ fold were obtained for the toxic pesticide Paraoxon, compared to the uncatalyzed reaction. Interestingly, Au-derived nanocatalysts were significantly more effective since the imidazole group is free to react with the OP, which is not possible when Ag-N interactions take place.

Full text of DOI:10.1039/c7ra07059d

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Detoxification of organophosphates using
imidazole-coated Ag, Au and AgAu nanoparticles†
Cite this: RSC Adv., 2017, 7, 40711
b
b
Valmir B. Silva,a Thenner S. Rodrigues,
Pedro H. C. Camargo
a
*
and Elisa S. Orth
Organophosphate (OP) detoxification is a worldwide problem due to the high stability of P–O bonds. Here,
we designed several imidazole-coated nanocatalysts targeted towards the cleavage of OP and thus their
detoxification. Specifically, Ag, Au and bimetallic AgAu nanoparticles (NPs) supported on SiO2 were
functionalized with methimazole (MTZ), which comprises thiol and imidazole groups, foreseeing
enabling freely available imidazole groups. Raman analyses indicate that MTZ interacts preferably via its
sulfur atom over Au NPs and via the nitrogen of the imidazole ring over Ag NPs. The MTZ-derived
nanocatalysts were effective towards OP cleavage. For instance, rate enhancements of 108 fold were
obtained for the toxic pesticide Paraoxon, compared to the uncatalyzed reaction. Interestingly, Au-
derived nanocatalysts were significantly more effective since the imidazole group is free to react with the
OP, which is not possible when Ag–N interactions take place.
Received 25th June 2017
Accepted 11th August 2017
DOI: 10.1039/c7ra07059d
rsc.li/rsc-advances
the reaction can be feasibly followed. For example, dimethyl
methylphosphonate (DMMP), p-nitrophenyl diphenylphosphate
(PNPDPP),5 and diethyl-2,4-dinitrophenyl phosphate (DEDNPP)8–10
Introduction
Organophosphates (OP) are intrinsically related to many
fundamental biological functions, such as biosynthesis,
signaling, regulation, and energy transduction.1 Moreover, they
are responsible for the preservation of genetic information
since they constitute DNA and RNA strands.2 Actually, one-third
of all biological proteins are phosphorylated at some time,
evidencing the importance of phosphoryl transfer. OP are
mostly known for their high stability.3 For example, the hydro-
lysis of DNA is estimated take in over 100 000 years.4 Since
enzymes are very successful in promoting phosphorylation in
biological systems,2 they have bioinspired many mimicking
studies. The downside of OP's stability is that they have been
used as a scaffold for many chemical warfare and pesticides
(Scheme 1), knowingly to have high toxicity, sometimes even
being lethal.5 Not surprising, much effort has been directed
towards the development of efficient methods of destruction
and detection of these toxic OP, focusing on deactivating
unused (probably prohibited) stockpiles and monitoring
abusive use of agrochemical and/or chemical attacks5,6
have been explored, as shown in Scheme 1.
In order to pursue the detoxication of OP, many bio-
inspired models have been proposed as detoxifying agent or
articial enzymes targeting the phosphoryl cleavage.5 In this
context, imidazole-derived catalysts have attracted great
It is noteworthy that poisoning by pesticides is a serious
worldwide public concern.7 As a matter of security, many studies in
this eld have used structural mimics of pesticides and chemical
warfare that usually have lower toxicity and higher reactivity so that
a
´
´
Departamento de Quımica, Universidade Federal do Parana, CP 19032, 81531-980
Curitiba, PR, Brazil. E-mail: elisaorth@ufpr.br
b
´
´
˜
Instituto de Quımica, Departamento de Quımica Fundamental, Universidade de Sao
˜
Paulo, Av. Prof. Lineu Prestes 748, SP 05508-900, Sao Paulo, Brazil
Scheme 1 The structure of some relevant organophosphates used as
chemical warfare and pesticides, along with some mimics of toxic OP.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra07059d
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 40711–40719 | 40711

View Article Online
RSC Advances
Paper
Synthesis of Ag NPs29
Ag seeds were prepared by the polyol process. In a typical
experiment, 5 g of polyvinylpyrrolidone (PVP, 10 000 g molꢁ1
)
was dissolved in 37.5 mL of ethylene glycol. Then, AgNO3
(200 mg, 1.2 mmol) was added and mixed until the complete
dissolution. The resulting solution was heated to 125 ꢂC for 2.5
hours, leading to the appearance of a greenish-yellow color,
allowed to cool down to room temperature, and diluted to 125
mL of water. Aer this step, 46 mL of the resulting suspension
(0.2197 mg mLꢁ1, determined by ICP) were washed twice with
ethanol and three times with water by successive rounds of
centrifugation at 15 000 rpm and removal of the supernatant.
Aer washing, the product was suspended in 10 mL of deion-
ized water.
Fig. 1 Functionalization of AuSiO2, AgSiO2 or AgAuSiO2 with MTZ.
attention due to their presence in many enzymatic active sites
and their chemical versatility,11 i.e., they can react as acid–base
or nucleophilic catalysts under considerably mild conditions
(pH ꢀ 7).12,13 Another promising class of catalysts for OP
cleavage involve nanoparticles (NPs),14 although there are still
only a few reports in this eld,5,15–20 in contrast to the many
studies regarding NPs-based sensors for various OP.21
Synthesis of Au NPs30
The synthesis of Au nanospheres was performed by a seeded
growth approach. In the rst step, Au seeds were synthesized by
adding 150 mL of a 2.2 mmol Lꢁ1 sodium citrate aqueous
solution to a 250 mL round-bottom ask under magnetic stir-
ring. This system was heated to 100 ꢂC for 15 minutes. Then, 1
mL of a 25 mmol Lꢁ1 AuCl4ꢁ(aq) solution was added, and the
reaction mixture was kept at 100 ꢂC under vigorous stirring for
30 minutes. The obtained Au nanoparticles were employed as
seeds for the synthesis of Au NPs having larger sizes by two
successive steps of Au deposition. For the rst deposition step, 1
mL of a 60 mmol Lꢁ1 sodium citrate solution was added to the
same 250 mL round-bottom ask containing the Au NPs seeds
The physicochemical features of several NPs, such as shape
and size, can be decisive for catalysis.22–24 A previous study
showed that the reduction of 4-nitrophenol can be catalyzed by
Ag and AgAu NPs alone, with higher reactivity for the later
attributed to Au content and the increased surface area due to
the hollow interiors.25 In another study, bimetallic nanoshells of
AgAu, AgPd, and AgPt NPs were used as bio-metallo catalysts
with dual activity for transesterication; and silane oxidation.26
Insofar, to the best of our knowledge, there are no studies
correlating NPs features with its effect on OP degradation.
Herein, we coated silver (AgSiO2), gold (AuSiO2) and bime-
tallic nanoshells AgAuSiO2 NPs (supported on SiO2) with a thiol-
substituted imidazole (methimazole, MTZ) in order to obtain
nanocatalysts with freely available imidazole moieties. We
choose MTZ due to the extraordinary catalytic activity known for
imidazole groups (vide supra) and the thiol group that interacts
favorably with NPs,20,27 hence anchoring efficiently imidazole
moieties on the NPs surface. For this purpose, the NPs were
rstly obtained with controlled shape and size following
a previously reported procedure.28,29 Then, the NPs were func-
tionalized with MTZ (Fig. 1). Finally, the nanocatalysts were
evaluated toward OP degradation, using the simulant DEDNPP
and the pesticide Paraoxon as model substrates.
ꢂ
under magnetic stirring at 100 C for 5 minutes. Aerward, 1
mL of a 25 mmol Lꢁ1 AuCl4ꢁ(aq) solution was added to the
reaction mixture containing the Au NPs seeds and the reaction
mixture was carried out for another 30 min. Similarly, a second
deposition step could be performed by adding another 1 mL of
a 60 mmol Lꢁ1 sodium citrate solution and 1 mL of a 25 mmol
Lꢁ1 AuCl4ꢁ(aq) solution to the reaction mixture obtained aer
the rst deposition step, in which the Au NPs produced aer the
rst deposition step served as seeds for further growth. Aer the
reaction, the resulting suspension was allowed to cool down to
room temperature. Aer this step, 88 mL of the resulting
suspension (0.1146 mg mLꢁ1, determined by ICP) were washed
three times with water by successive rounds of centrifugation at
15 000 rpm and removal of the supernatant. Aer washing, the
product was suspended in 10 mL of deionized water.
Synthesis of AgAu nanoshells29
The synthesis of AgAu nanoshells was based on the galvanic
replacement reaction between Ag nanospheres and AuCl4ꢁ(aq).
In a typical procedure, a mixture containing 50 mL of PVP
Experimental
Materials
MTZ Analytical grade silver nitrate (AgNO3, 99%, Sigma-Aldrich), aqueous solution (0.1 wt%, M.W. 55 000 g molꢁ1) and 10 mL of
polyvinylpyrrolidone (PVP, Sigma-Aldrich, M.W. 10 000 g molꢁ1), the as-prepared suspension containing the Ag nanospheres was
polyvinylpyrrolidone (PVP, Sigma-Aldrich, M.W. 55 000 g molꢁ1), stirred at 100 C for 10 min in a 250 mL round-bottom ask.
ꢂ
ethylene glycol (EG, 99.8%, Sigma-Aldrich), tetrachloroauric(III) Then, 20 mL of AuCl4ꢁ(aq) (0.6 mmol Lꢁ1) was added dropwise
ꢂ
acid trihydrate (HAuCl4$3H2O, $99.9%, Sigma-Aldrich), silica and the reaction allowed to proceed at 100 C for another 1 h.
(pore size 22 A, 800 m gꢁ1, CAS number 112926-00-8, Sigma- Aer that, the suspension was allowed to cool down to room
2
˚
ˆ
Aldrich), ethanol (Exodo) were used as received.
temperature and 28 mL (0.0458 mg mLꢁ1, determined by ICP)
40712 | RSC Adv., 2017, 7, 40711–40719
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
were washed twice with a supersaturated NaCl solution and
three times with water by successive rounds of centrifugation at
15 000 rpm and removal of the supernatant. Aer washing, the
product was suspended in 10 mL of deionized water.
Synthesis of the SiO2 supported NPs28
The incorporation of Ag, Au, and AgAu nanostructures onto
a silica matrix (1% wt, metal basis) were performed based on
a wet impregnation approach. In a typical run, 10 mL of the
suspensions containing the respective nanoparticles washed
and concentrated were added dropwise to a beaker containing
1 g of commercial silica suspended in 20 mL of deionized water
under magnetic stirring. The resulting mixture was stirred at
room temperature for 24 h. Aer this step, the resulting
suspensions were washed three times with distilled water and
twice with ethanol by successive rounds of centrifugation at
7000 rpm and removal of the supernatant. The resulting cata-
Scheme 2 Cleavage reaction of DEDNPP and Paraoxon in the pres-
ence of the nanocatalysts.
Catalytic studies of OP detoxication
For the catalytic studies, two OP were employed: DEDNPP and
Paraoxon. Pseudo-rst order conditions were maintained,
where 30 mg of MTZ-derived nanocatalyst was added to 3 mL of
a buffered solution (bicarbonate 0.2 mol Lꢁ1). The reaction
started upon addition of 15 mL of DEDNPP or Paraoxon stock
solution (5 ꢃ 10ꢁ3 mol Lꢁ1 in acetonitrile) and the medium was
maintained under magnetic stirring (4 h and 15 days for
DEDNPP and Paraoxon, respectively). The suspension was
centrifugated (12 000 rpm) and UV-vis spectra of the superna-
tant were recorded. The catalytic activity was calculated based
on the product concentration, hence OP degradation by
measuring the absorbance variation at 400 nm related to the
products 4-nitrophenolate (PNP) or 2,4 dinitrophenolate (DNP),
that originated from Paraoxon and DEDNPP, respectively
(Scheme 2). The molar absorptivity (3) used were 18 380 and
12 100 L molꢁ1 cmꢁ1 for PNP and DNP, respectively.31,32
Considering pseudo-rst order kinetics, already corroborated
for these reactions in a separate study, the integrated rate law
was used to obtain the pseudo-rst order constant.
ꢂ
lysts were then treated at 120 C for 2 h under air.
Coating of the NPs with MTZ
The MTZ derived nanocatalysts were prepared as follows:
500 mg of the SiO2 supported AgSiO2, AuSiO2 or AgAuSiO2 NPs
was added to a becker along with MTZ (26.0, 15.0 and 21.0 mg
for AgSiO2, AuSiO2 and AgAuSiO2, respectively) in 50 mL of
ethanol. The mixture was stirred during 24 h followed by
centrifugation (12 000 rpm) and separated. In the following, the
material was washed with ethanol (three times), centrifugated,
and the supernatant was discarded. The ethanol was evaporated
at room temperature. The samples were denominated AgSiO2–
MTZ, AuSiO2–MTZ and AgAuSiO2–MTZ, originated from
AgSiO2, AuSiO2, and AgAuSiO2 NPs, respectively.
Characterizations
For the NPs, the scanning electron microscopy (SEM) images
were obtained using a JEOL eld emission gun electron
microscope JSM6330F operated at 5 kV. The samples were
prepared by drop-casting an aqueous suspension containing
the nanostructures over a silicon wafer, followed by drying
under ambient conditions. Transmission electron microscopy
(TEM) images were obtained with a JEOL JEM1010 microscope
operated at 80 kV. Samples for TEM were prepared by drop-
casting an aqueous suspension of the nanostructures over
a carbon-coated copper grid, followed by drying under ambient
conditions. UV-vis spectra were obtained from aqueous
suspensions containing the nanostructures with a Shimadzu
UV-1700 spectrophotometer. The Ag and Au atomic percentages
were measured by inductively coupled plasma optical emission
spectrometry (ICP-OES) using a Spectro Arcos equipment. The
X-ray diffraction (XRD) data were obtained using a Rigaku-
Miniex equipment, CuKa radiation. The diffraction pattern
was measured in the range of 10–90ꢂ 2q with a 1ꢂ minꢁ1 angular
speed scan. For the MTZ-derived NPs, Raman spectra were ob-
tained with a Renishaw spectrophotometer using the 632.8 nm
excitation laser, with 25 accumulations and 25–50% of the laser
intensity.
Fig. 2 (A–C) SEM images of Ag nanospheres (A), Au nanospheres (B),
and AgAu nanoshells (C). The insets correspond to their corresponding
TEM images (the scale bars in the insets correspond to 20 nm). (D–F)
SEM images of (D) AgSiO2, (E) AuSiO2, and (F) AgAuSiO2 materials that
were obtained from wet impregnation of the samples (A–C) onto
a commercial SiO2 support.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 40711–40719 | 40713

View Article Online
RSC Advances
Paper
Results and discussion
Characterization of the SiO2 supported NPs
Our studies started with the synthesis of Ag nanospheres, Au
nanospheres, and AgAu nanoshells as shown in Fig. 2A–C,
respectively. Ag nanospheres (Fig. 2A) were obtained by a one-
step polyol approach and were 35 ꢄ 4 nm in diameter.33 On
the other hand, Au nanospheres (Fig. 2B) were obtained by
a seeded growth approach, in which Au nanospheres obtained
by citrate reduction of AuCl4ꢁ(aq) acted as template for addi-
tional Au deposition and growth leading to the formation of
nanoparticles displaying larger diameters (36 ꢄ 3 nm) that were
similar to the Ag nanospheres.30 For the AgAu nanoshells, the
obtained Ag nanospheres were employed as sacricial
templates in the galvanic replacement reaction using AuCl4-
ꢁ(aq), which represents an effective strategy for the synthesis of
hollow nanostructures displaying controllable properties in
a single-step.29,34 As depicted in Fig. 2C, the formation of the
hollow interiors is conrmed by the brighter mass-thickness
contrast at the center of the AgAu nanoshells, which were 36
ꢄ 4 nm in diameter, in agreement with the use of Ag nano-
spheres as templates. Interestingly, all obtained nanostructures
displayed spherical shapes and relatively monodisperse sizes,
which allowed us systematically investigate their catalytic
performances as a function of the compositions and structures.
The metal at% in the nanostructures were determined by ICP-
OES analyses and corresponded to 100% of Ag, 100% of Au,
and 49% of Ag + 51% of Au for Ag, Au, and AgAu, respectively.
Aer the synthesis of the Ag, Au, and AgAu nanostructures, we
turned our attention to their incorporation onto the surface of
a commercial SiO2 in order to produce powdered catalysts, which
are strongly desired in terms of heterogeneous catalysis due to
their relatively easy separation, purication, and reuse. In addi-
tion, the uniform incorporation of NPs over solid supports have
also been demonstrated as a promising strategy to prevent the
particle agglomeration during the catalytic processes.28 Fig. 2D–F
show SEM images for the produced catalysts aer the NPs
incorporation onto the SiO2 support. Interestingly, for all nano-
materials, an excellent dispersion of metal constituents was
Fig. 4 XRD profiles of (A) Ag nanospheres, (B) Au nanospheres, and (C)
AgAu nanoshells.
observed over the entire SiO2 surface and no signicant
agglomeration and changes on the morphologies were detected,
indicating the robustness of our employed wet impregnation
method to produce solid metal/SiO2 catalysts. Here, the metal
loading corresponded to 1 wt% (Fig. 2F). The solid catalysts were
denoted AgSiO2 (Fig. 2D), AuSiO2 (Fig. 2E), and AgAuSiO2.
Fig. 3 shows the UV-vis extinction spectra for the obtained Ag
and Au nanospheres and AgAu nanoshells. The Ag and Au
nanospheres displayed clear extinction peaks centered at z 410
and 520 nm assigned to the dipolar mode of their surface plas-
mon resonance excitation29,30 On the other hand, a red-shi and
broadening of the SPR in the AgAu nanoshells was observed
compared to the Ag nanospheres employed as templates. This
behavior can be associated with the Au deposition as well as the
formation of the hollow interiors in during the galvanic reaction.25
The formation of Ag, Au, and AgAu was also monitored by
XRD analyses (Fig. 4), in which the XRD patterns also conrmed
the formation of Ag (Fig. 4A), Au (Fig. 4B) and AgAu (Fig. 4C)
nanostructures without the presence of any signicant crystalline
impurities. However, as both Ag and Au has very similar lattice
constants, the XRD diffraction peaks for these two species could
not be resolved under our experimental conditions.
Characterization of the MTZ-derived NPs coated material
Fig. 3 UV-vis extinction spectra recorded from aqueous suspensions
containing Ag nanospheres (black trace), Au nanospheres (red trace),
and AgAu nanoshells (blue trace).
Regarding the MTZ-derived NPs, to the best of our knowledge,
this represents the rst report using MTZ on NPs supported on
40714 | RSC Adv., 2017, 7, 40711–40719
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Scheme 3 Tautomeric equilibrium for MTZ.
SiO2. There are other studies that report other thiol-substituted
imidazole for coating NPs, evidencing that the thiol group
interacts with the NPs, enabling freely available imidazole
moieties. For example, in a previous study, authors report
a dipeptide-functionalized thiol with terminal histidine and
carboxylate groups that was used to passivate Au NPs. The
material was successfully prepared and used as catalysts in acyl
transfer reactions, although the NPs nature was not the focus of
study.27 In another report, N-methylimidazole derivative func-
tionalized Au NPs were obtained similarly via thiol anchoring
on NPs surface, which aerward was employed in the cleavage
of 2,4-dinitrophenyl acetate.20 It is noteworthy, that we also
attempted to anchor MTZ on NPs (Ag, Au, and AgAu) not sup-
ported on SiO2, but functionalization was followed by aggrega-
tion of the particles.
MTZ is a known drug (commercially Tapazol) used for the
treatment of hyperthyroidism.35 MTZ has a tautomeric equi-
librium of thiol–thione as shown in Scheme 3. The concentra-
tion of each species depends on the pH, although in general, the
thione species predominates. For example, at pH 9, the
percentage of thione is 65%.36 Another important feature of
MTZ is its pKa of 11.38.35
The MTZ-derived NPs were characterized by Raman spec-
troscopy. Indeed, Raman analysis of MTZ is quite difficult due
to its tautomeric and ionic equilibrium, but overall results
evidence that the NPs were successfully functionalized. Raman
spectra are presented in Fig. 5 including spectrum of pure MTZ
for comparison and additionally all spectra all plotted together
in Fig. S1.† The Raman spectra for the NPs prior to the MTZ
functionalization (red trace on Fig. 5) show bands that were
characteristic of PVP (in the case of AgSiO2 and AgAuSiO2),
citrate (in the case of AuSiO2) and SiO2 (spectra for pure SiO2
and PVP are shown in the ESI†).37,38 These species are present at
the surface of the NPs as they were employed as stabilizers
during the synthesis of each respective material. It is important
to note that all the nanoparticles were washed by successive
rounds of centrifugation and removal of the supernatant aer
their synthesis and isolation, material. Although this process
removes the reducing agents, unreacted precursors, and the
excess of stabilizers that are employed during the synthesis,
stabilizer molecules remain at the surface of the NPs. Their
detection in the Raman experiments is due to the surface-
enhanced Raman scattering (SERS) effect as enabled by Ag,
Au, and AgAu NPs.
Fig. 5 Ordinary Raman and SERS spectra (l ¼ 632.8 nm) for pure MTZ,
bare NPs, and MTZ-functionalized Ag, Au and AgAu NPs.
3130 cmꢁ1 due to ring C–H stretching; (ii) at 1521 cmꢁ1 due to
C–C and C–N stretching, along with ring C–H bending; (iii) at
1450 cmꢁ1 attributed to C–S and ring C–N stretching, in addi-
tion to ring C–H bending; (iv) at 1360 cmꢁ1 (most intense band)
referent to C–N ring stretching with contribution of C–H
Upon functionalization (AgAuSiO2–MTZ, AgSiO2–MTZ and
AuSiO2–MTZ in Fig. 5) it is possible identify some characteristic
bands of MTZ that are identied in the rst spectrum:17,35,36 (i) at
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 40711–40719 | 40715

View Article Online
RSC Advances
Paper
bending; (v) at 1320 cmꢁ1 attributed to ring C–N stretching, ring
bending and ring CH–NH bending; (vi) 1142 cmꢁ1 referent to
C–S and ring C–N stretching, along with ring C–H bending; (vii)
at 1086 cmꢁ1 due to ring C–N stretching and ring C–H bending;
(viii) at 1034 cmꢁ1 attributed to ring bending, ring C–H
bending, methylene C–H bending (ix) at 936 cmꢁ1 referent to
ring bending, ring C–N stretching and ring CH–NH bending; (x)
at 620 cmꢁ1 due to out-of-plane ring and CH–NH bending; (xi)
at 500 and 430 cmꢁ1 due to out-of-plane ring bending. It is
noticeable that spectrum for pure MTZ presents more bands
than discussed above and in addition some bands of MTZ are
shied when comparing coated materials to pure MTZ. Actu-
ally, these detected differences for the SERS spectra from the
functionalized materials and ordinary Raman from pure MTZ
are characteristic of many molecules and indicate that the
functionalization was successful as previously observed.35,36,39
Moreover, the bands observed herein indicate that MTZ is
present as thione and thiol/thiolate,35,36 although it should be
noted that despite the high pKa for MTZ, previous studies show
that the thiolate species is preferable for interacting with Ag and
Au NPs.35
Fig. 6 pH effect rate constant for the reaction of Au-derived NPs with
DEDNPP at 20 ꢂC. Reaction with SiO2 and in water is shown for
comparison.42
that the MTZ is anchored solely on the nanoparticle, the overall
amount of MTZ is extremely low to detect or differentiate by
FTIR.
Regarding the nature of the interaction between NPs and
MTZ, previous studies report that MTZ interacts with Au NPs
preferably via the thiol group.35 On the other hand, with Ag NPs,
Catalytic activity on the detoxication of OP
there are some reports that claim a preferable interaction via The catalytic activity of the MTZ-derived materials was evalu-
the nitrogen atom of the imidazole group,36 although another ated in the cleavage reactions of OP, thus its detoxication.
study evidences that Ag NPs can also interact with thiol group35 Firstly, as a proof-of-concept, we choose one class of material,
or with both groups concomitantly.39 In fact, when analyzing derived from Au, to react with the model OP (i.e. simulant)
Raman spectrum for AuSiO2–MTZ, there is a band at 198 cmꢁ1
,
DEDNPP. This is a common strategy before applying with real
not present in the non-coated material. Despite the typical life OP due to their higher toxicity. In addition, DEDNPP is more
bands of SiO2 in this region, they don't seem to overlap. This reactive than Paraoxon, hence its reaction can be followed on
new band has been attributed in the literature to Au–S a shorter time scale. Upon conrmation of the catalytic activity
stretching, indicating that for this material, MTZ interacts with DEDNPP, the nanocatalysts are then evaluated in the
preferably via the thiolate group.35 In the case of AgSiO2–MTZ, reaction with Paraoxon. Fig. 6 presents the rate constants at
the band at 202 cmꢁ1, inexistent without MTZ, can be attributed different pH values for the cleavage of DEDNPP with AuSiO2-
to Ag–N stretching,36 indicating that for this material MTZ derived catalysts. Constants solely with SiO2 under the same
interacts preferably via the imidazole ring through the lone pair conditions is also shown along with the reaction in water
of the nitrogen atom. Finally, for AgAuSiO2–MTZ, the spectrum (spontaneous).42,43
is noisier, with a broad band in the region of 220–190 cmꢁ1 that
could be attributed to both Ag–S and Au–N interactions.
Firstly, it should be noted that reactions with solely MTZ is
not shown, since its comparison is not straightforward because
It should be noted that despite the similarities in the bands of its homogenous reaction nature (herein is heterogeneous)
attributed to Au–S and Ag–N, the other possible interactions and to the difficulty of estimating the amount of MTZ coating
Au–N and Ag–S are known to occur at higher wavenumbers (Au– the NPs. Therefore we cannot obtain the rate constant of pure
N ꢀ225 cmꢁ1 and Ag–S ꢀ240 cmꢁ1).40,41 Taken together, Raman MTZ in the same concentration coating the NPs, hence would
analysis conrms that MTZ was anchored on the NPs and that lead to erroneous underestimated comparison. In addition, we
for AuSiO2–MTZ the interaction seems to occur through the could not evaluate the reactions of bare NPs since they aggre-
imidazole ring (Ag–N). For AgSiO2–MTZ, this occurs via the gate. Results show that AuSiO2–MTZ is, in fact, effective in
thiolate group (Au–S). For AgAuSiO2–MTZ, there could be cleaving DEDNPP, specially at pH 9.0, since it differentiates
a combination of these interactions. In addition, the following from the non-coated material (AuSiO2). At lower pH, the MTZ-
catalytic studies agree with the interaction proposed since due derived material shouldn't exhibit any activity due to the lower
to the lower catalytic activity of AgSiO2–MTZ.
reactivity of imidazole moiety.42 On the other hand, at pH 10,
FTIR analyses were also carried out (shown in ESI†) but the reaction is very effective in all cases due to the contribution
didn't show any evident differences, probably due to the low of the alkaline hydrolysis (reaction in water). Reactions solely
degree of functionalization and also to the known low intensity with SiO2 evidenced for pH 8 and 9, similar constants to the
of the bands of imidazole and thiol moieties. The MTZ molecule spontaneous reaction, although at pH 10, its higher activity can
is only detected by Raman spectroscopy as a result of the SERS be attributed to hydroxide species on the surface.44 Insofar, the
effect. Due to the low amount of metal (ꢀ1% in mass) and given pH of interest to carry out further studies with the pesticide was
40716 | RSC Adv., 2017, 7, 40711–40719
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Table 1 Rate constants for the cleavage reaction of Paraoxon with the
nanocatalystsa
Rate constant, gꢁ1 hꢁ1
AgAuSiO2
AgSiO2
AuSiO2
AgAuSiO2–MTZ
AgSiO2–MTZ
AuSiO2–MTZ
4.10
4.03
4.58
13.10
3.76
9.52
a
pH 9.0, 20 ꢂC. Rate constants are given as a function of the mass of
metal in the nanocatalysts. Reactions solely with SiO2, where similar
to the analogous spontaneous reaction at pH 9.0.
9.0, since the MTZ-derived material showed higher reactivity
(although discrete) than their non-functionalized analogous.
Considering the positive results with DEDNPP, the nano-
catalysts were then evaluated in the reactions with the highly
toxic pesticide Paraoxon at pH 9. Table 1 presents the rate
constants obtained given as a function of the mass of metal for
the reactions of Paraoxon with all the materials: coated and
non-coated with MTZ. In general, catalytic rate constants are
given as a function of the reactive group of the catalysts,
although in this case, it was not possible to calculate the
concentration of MTZ in the materials obtained, low values are
expected. One approach is to obtain the ratio of the rate
constants and the total amount of catalysts, but since in the
present study it was impregnated with SiO2, we found more
convenient to consider the amount of metal that is responsible
for interacting with the MTZ molecules. Nevertheless, not all
NPs surface may be covered by MTZ, hence the rate constant
considering solely the amount of MTZ reacting should be even
higher. Overall, Table 1 evidences high rate constants for Par-
aoxon were among the highest reported in the literature. For
example, previous studies with thiol-based nanocatalysts
derived from graphene oxide show rate constants of 2.13 gꢁ1
hꢁ1 (considering the amount of the reactive group thiolate) with
Paraoxon.10 In another study, Ag, and cobalt (Co) NPs were
coated with imidazole derived fatty acids that give rate
constants for Co-derived material ꢀ1.3 gꢁ1 hꢁ1 (considering the
amount of metal).16 A magnetic nanocatalyst of iron oxide with
supported amino acids (Asp–His) was also used to cleave Para-
oxon with a rate constant of 0.30 gꢁ1 hꢁ1 (considering the
amount of Fe2O3–Asp–His).18
Fig. 7 The mechanism proposed for the reaction of the OP with the
nanocatalysts and interaction of MTZ with Au and Ag-derived
materials.
contrast, with AgSiO2, functionalization slightly decreases the
rate constant. This behavior agrees with previous Raman anal-
yses that indicate that for Ag-derived materials, MTZ interacts
via the nitrogen while Au-based materials prefer the sulfur
atom. Hence, the effect of MTZ on Au- and AgAu-based nano-
catalysts is different from that of Ag-based materials. This can
be explained by the expected mechanism for reactions of
imidazole with OP, shown in Fig. 7. Based on previous mecha-
nistic studies,45 the nitrogen of the imidazole ring attacks the
phosphorus atom, thus cleaving the OP and leading to a phos-
phorylated intermediate and the phenolic product –OR. In the
following, the unstable intermediate is hydrolyzed giving inor-
ganic phosphate and regenerating the MTZ-derived nano-
catalyst. This mechanism has been broadly accepted for
imidazole mediated reactions and occurs analogously in many
enzymatic active sites, therefore the nanocatalysts can be
considered as articial enzymes. Further, the proposed mech-
anism is consistent with the recycling of the nanocatalyst.
Indeed, the UV-vis spectra taken aer the reactions don't show
any lixiviation of MTZ or the NPs, since the only band that
appears in solution is of the products.
While only discrete enhancements were observed for the
MTZ-derived materials towards DEDNPP in comparison to its
non-functionalized material, the difference was substantially
different with Paraoxon (for Au- and AgAu-based materials).
This is not unprecedented since Paraoxon is much less reactive
than DEDNPP. In fact, as mentioned, DEDNPP is a simulant of
OP such as Paraoxon and is used to verify possible catalytic
effects, since the response is fast.
A more detailed analysis of Table 1 shows that upon func-
tionalization of AuSiO2 with MTZ, the rate constant nearly
doubles. In the case of AgAuSiO2, the analogous MTZ nano-
catalyst increases more signicantly the rate constant. In
Correlating the mechanism with the proposed interaction
for AuSiO2–MTZ (or AgAuSiO2–MTZ) and AgSiO2–MTZ (vide
Raman), we can observe that, while Au–S guarantees freely
available imidazole groups, Ag–N, doesn't (shown in Fig. 7).
Therefore, AgSiO2–MTZ doesn't exhibit signicant catalytic
activity in contrast to AuSiO2 due to the type of interaction that
directly disables the mechanistic preference.
Regarding the catalytic activity for the NPs without MTZ, this
can be attributed to the Lewis acidity of the metal which
interacts favorably with the phosphoryl oxygen, leading to
a more electrophilic phosphorus center, as observed for ester
cleavage (phosphate or acyl).15,16
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 40711–40719 | 40717

View Article Online
RSC Advances
Paper
The higher activity of Au-derived nanocatalysts was attrib-
uted to the imidazole group that is free to react with the OP,
since in this case Au–S interactions is preferable. When Ag–N
interactions takes place, the reactivity of imidazole is reduced.
Overall, we were able to detoxify OP efficiently which is
promising destroying toxic stocks and also to design moni-
toring devices for these compounds. These perspectives are
aligned with the importance of the issue as evidenced by the
Peace Nobel Prize in 2013 awarded by the Organization for the
Prohibition of Chemical Weapons “for its extensive efforts to
eliminate chemical weapons”.46
Conflicts of interest
Fig. 8 Rate enhancements of the nanocatalysts with Paraoxon (kcat in
There are no conicts to declare.
gꢁ1
h
ꢁ1) compared to the reaction with water (kH O in gꢁ1 hꢁ1).45
2
Acknowledgements
Fig. 8 presents the rate enhancements of the nanocatalysts
Authors acknowledge the nancial support from UFPR, CNPq,
with Paraoxon when compared to its spontaneous reactions
(kH O ¼ 1.20 ꢃ 10ꢁ7 hꢁ1 or 4.0 ꢃ 10ꢁ8 gꢁ1 hꢁ1),45 evidencing that
˜
´
´
CAPES, Fundaçao Araucaria, L'Oreal-UNESCO-ABC National,
Institute of Science and Technology of Carbon Nanomaterials
(INCT-Nanocarbon).
2
the materials proposed are highly reactive towards OP detoxi-
cation. Interestingly, AgAuSiO2–MTZ is the most efficient
nanocatalyst. This could be related to another previous study
that concluded that bimetallic AgAu NPs (without SiO2) are
better catalysts toward 4-nitrophenol reduction than Au or Ag
NPs. This effect was attributed to a possible higher surface area,
resulting from the formation of porous/hollow walls.25
References
1 S. C. L. Kamerlin, P. K. Sharma, R. B. Prasad and A. Warshel,
Q. Rev. Biophys., 2013, 46, 1–132.
2 F. H. Westheimer, Science, 1987, 235, 1173–1178.
3 A. J. Kirby, B. S. Souza and F. Nome, Can. J. Chem., 2015, 93,
422–427.
4 S. C. Kamerlin, P. K. Sharma, R. B. Prasad and A. Warshel, Q.
Rev. Biophys., 2013, 46, 1.
5 K. Kim, O. G. Tsay, D. A. Atwood and D. G. Churchill, Chem.
Rev., 2011, 111, 5345–5403.
6 Y. C. Yang, J. A. Baker and J. R. Ward, Chem. Rev., 1992, 92,
1729–1743.
7 R. T. Delno, T. S. Ribeiro and J. D. Figueroa-Villar, J. Braz.
Chem. Soc., 2009, 20, 407–428.
8 A. M. Manfredi, W. Demos, E. H. Wanderlind, B. V. Silva,
A. C. Pinto, B. S. Souza and F. Nome, J. Phys. Org. Chem.,
2016, 29, 600–603.
9 E. S. Orth, J. E. S. Fonsaca, T. G. Almeida, S. H. Domingues,
J. G. L. Ferreira and A. J. G. Zarbin, Chem. Commun., 2014, 50,
9891–9894.
Conclusions
Herein, we functionalized Ag, Au and bimetallic AgAu NPs
(supported on SiO2) with MTZ, in order to obtain nanocatalysts
comprising freely available imidazole groups. Results evidence
that the NPs were successfully coated and that MTZ interacts
preferably via sulfur atom over Au NPs and via the imidazole
nitrogen over Ag NPs. The nanocatalysts were highly effective
towards OP cleavage. For example, Fig. 9 illustrates the effi-
ciency of detoxication of the toxic OP Paraoxon with the
nanocatalysts, that is given as the time in days taken to degrade
50% of the OP. Normally in water, the lifetime of Paraoxon is
extremely high and the estimated time to degrade 50% is over
600 years.45 In the presence of the nanocatalysts, this time is
reduced to 7 days with the most reactive AgAuSiO2–MTZ.
10 J. Fonsaca, L. Hostert, E. S. Orth and A. J. Zarbin, J. Mater.
Chem. A, 2017, 5, 9591–9603.
11 P. V. Attwood, M. J. Piggott, X. L. Zu and P. G. Besant, Amino
Acids, 2007, 32, 145–156.
12 K. Hofmann, Imidazole and its derivatives, Interscience
Publishers, New York, 1953.
13 J. G. Ferreira and E. S. Orth, J. Braz. Chem. Soc., 2017, 28,
1760–1767.
14 P. Xu, S. Guo, H. Yu and X. Li, Small, 2014, 10, 2404–2412.
15 M. H. Kuchma, C. B. Komanski, J. Colon, A. Teblum,
A. E. Masunov, B. Alvarado, S. Babu, S. Seal, J. Summy and
C. H. Baker, Nanomedicine, 2010, 6, 738–744.
Fig. 9 Efficiency for the detoxification of Paraoxon by the nano-
catalysts, given as the time in days required to degrade 50%.
40718 | RSC Adv., 2017, 7, 40711–40719
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
16 L. Bromberg, L. Chen, E. P. Chang, S. Wang and T. A. Hatton, 31 G. N. Bowers, R. B. McComb, R. Christensen and R. Schaffer,
Chem. Mater., 2010, 22, 5383–5391. Clin. Chem., 1980, 26, 724–729.
17 L. Bromberg and T. A. Hatton, Ind. Eng. Chem. Res., 2007, 46, 32 L. J. Daumann, L. Marty, G. Schenk and L. R. Gahan, Dalton
3296–3303. Trans., 2013, 42, 9574–9584.
18 Y. Zheng, C. Duanmu and Y. Gao, Org. Lett., 2006, 8, 3215– 33 K. TekaiaaElhsissen, J. Mater. Chem., 1996, 6, 573–577.
3217. 34 Y. Sun, B. Mayers and Y. Xia, Adv. Mater., 2003, 15, 641–646.
19 L. Y. Kuo, A. K. Bentley, Y. A. Shari'ati and C. P. Smith, 35 M. Muniz-Miranda, F. Muniz-Miranda and A. Pedone, Phys.
Organometallics, 2012, 31, 5294–5301. Chem. Chem. Phys., 2016, 18, 5974–5980.
20 L. Pasquato, F. Rancan, P. Scrimin, F. Mancin and C. Frigeri, 36 N. Biswas, S. Thomas, A. Sarkar, T. Mukherjee and S. Kapoor,
Chem. Commun., 2000, 2253–2254. J. Phys. Chem. C, 2009, 113, 7091–7100.
¨¨
´
21 L. Rassaei, F. Marken, M. Sillanpaa, M. Amiri, C. M. Cirtiu 37 D. De Faria, H. Gil and A. De Queiroz, J. Mol. Struct., 1999,
¨¨
and M. Sillanpaa, TrAC, Trends Anal. Chem., 2011, 30,
478, 93–98.
´
´
´
1704–1715.
38 J. Garcıa-Aguilar, D. Cazorla-Amoros and A. Berenguer-
Murcia, Appl. Catal., A, 2017, 538, 139–147.
39 T. A. Saleh, M. M. Al-Shalalfeh and A. A. Al-Saadi, Sci. Rep.,
2016, 6, 32185.
22 J. Zeng, Q. Zhang, J. Chen and Y. Xia, Nano Lett., 2009, 10,
30–35.
23 C. Jin, J. Han, F. Chu, X. Wang and R. Guo, Langmuir, 2017,
33, 4520–4527.
24 T. S. Rodrigues, A. G. da Silva, A. B. de Moura, I. G. Freitas
and P. H. Camargo, RSC Adv., 2016, 6, 62286–62290.
25 M. V. Petri, R. A. Ando and P. H. Camargo, Chem. Phys. Lett.,
2012, 531, 188–192.
40 B. Pergolese, M. Muniz-Miranda and A. Bigotto, J. Phys.
Chem. B, 2004, 108, 5698–5702.
41 P. Galinetto, A. Taglietti, L. Pasotti, P. Pallavicini, G. Dacarro,
E. Giulotto and M. Grandi, J. Appl. Spectrosc., 2016, 82, 1052–
1059.
26 C. M. Kisukuri, D. J. Palmeira, T. S. Rodrigues, 42 E. S. Orth, E. H. Wanderlind, M. Medeiros, P. S. M. Oliveira,
P. H. Camargo and L. H. Andrade, ChemCatChem, 2016, 8,
171–179.
B. G. Vaz, M. N. Eberlin, A. J. Kirby and F. Nome, J. Org.
Chem., 2011, 76, 8003–8008.
´
´
27 P. Pengo, S. Polizzi, L. Pasquato and P. Scrimin, J. Am. Chem. 43 E. H. Wanderlind, Cinetica e mecanismo de hidrolise do
´
Soc., 2005, 127, 1616–1617.
triester dietil 2,4-dinitrofenil fosfato, Master thesis, Federal
28 T. S. Rodrigues, A. H. da Silva, A. G. da Silva, D. G. Ceara,
University of Santa Catarina, 2014.
J. F. Gomes, J. M. Assaf and P. H. Camargo, Catal. Sci. 44 A. Torrents and A. T. Stone, Environ. Sci. Technol., 1991, 25,
Technol., 2016, 6, 2162–2170. 143–149.
29 T. S. Rodrigues, A. G. da Silva, A. Macedo, B. W. Farini, 45 E. S. Orth, T. G. Almeida, V. B. Silva, A. R. M. Oliveira,
R. d. S. Alves and P. H. Camargo, J. Mater. Sci., 2015, 50,
5620–5629.
F. M. M. Ocampos and A. Barison, J. Mol. Catal. A: Chem.,
2015, 403, 93–98.
30 A. G. da Silva, T. S. Rodrigues, A. Macedo, R. T. da Silva and 46 https://www.nobelprize.org/nobel_prizes/peace/laureates/
P. H. Camargo, Quim. Nova, 2014, 37, 1716–1720.
2013/opcw-facts.html, accessed in May, 5, 2017.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 40711–40719 | 40719