Anchoring of Cu (II)-Schiff base complex on magnetic mesoporous silica nanoparticles: catalytic efficacy in one-pot synthesis of 5-substituted-1H-tetrazoles, antibacterial activity evaluation and immobilization of α-amylase

Article abstract of DOI:10.1002/aoc.5572

Magnetic mesoporous silica nanocomposite, Fe₃O₄?MCM-41, was prepared and functionalized with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPS). Then Schiff base grafted nanoparticles were synthesized by the condensation of 5,5'-

Full text of DOI:10.1002/aoc.5572

Received: 18 October 2019
DOI: 10.1002/aoc.5572
Revised: 6 December 2019
Accepted: 29 December 2019
F U L L P A P E R
Anchoring of Cu (II)-Schiff base complex on magnetic
mesoporous silica nanoparticles: catalytic efficacy in one-
pot synthesis of 5-substituted-1H-tetrazoles, antibacterial
activity evaluation and immobilization of α-amylase
Ameneh Ahmadi1 | Tahereh Sedaghat1
| Hossein Motamedi2,3 | Roya Azadi1
1Department of Chemistry, Faculty of
Abstract
Science, Shahid Chamran University of
Ahvaz, Ahvaz, Iran
Magnetic mesoporous silica nanocomposite, Fe3O4@MCM-41, was prepared
and functionalized with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane
(AEAPS). Then Schiff base grafted nanoparticles were synthesized by
the condensation of 5,5'-methylene bis (salicylaldehyde) and then
benzhydrazide with Fe3O4@MCM-41-AEAPS. Finally, by adding Cu
(CH3COOH)2.H2O, the magnetic nanoparticles (MNPs) functionalized with
Cu (II) Schiff base complex were synthesized. The new organic–inorganic
hybrid nanocomposite was characterized by FT-IR, PXRD, AAS, BET, TGA,
VSM, FE-SEM, HRTEM and EDX techniques. Then, the performance of this
copper based magnetic nanocatalyst was investigated for the synthesis of
5-substituted 1H-tetrazole derivatives using one pot three-component reac-
tions of various aldehydes, hydroxyl amine hydrochloride and sodium azide.
The catalyst can be easily isolated from the reaction mixture by applying an
external magnet and reused for at least 5 times without significant loss in cat-
alytic activity. Also, the antibacterial activity of the streptomycin loaded mag-
netic nanoparticles against Gram-positive (S. aureus) and Gram-negative
(E. coli) bacteria in the presence and absence of a magnetic field were studied.
Results revealed that when these materials exposed to the magnetic field, bac-
teriostatic activity of nanocomposites was increased. Furthermore, the
enzyme immobilization ability of the synthesized compounds was investi-
gated and the results showed that these nanoparticles efficiently immobilized
amylase enzyme.
2Department of Biology, Faculty of
Science, Shahid Chamran University of
Ahvaz, Ahvaz, Iran
3Biotechnology and Biological Science
Research Center, Shahid Chamran
University of Ahvaz, Ahvaz, Iran
Correspondence
Tahereh Sedaghat, Department of
Chemistry, Faculty of Science, Shahid
Chamran University of Ahvaz, Ahvaz,
Iran.
Email: tsedaghat@scu.ac.ir; tsedaghat@
yahoo.com
Funding information
Shahid Chamran University of Ahvaz,
Grant/Award Number: 1396
K E Y W O R D S
antibacterial activity, Cu(II), enzyme immobilization, magnetic mesoporous material, Schiff Base
1 | INTRODUCTION
applications in different fields, such as drug delivery,[1]
magnetic resonance imaging (MRI),[2] cancer treatments
In recent years, magnetic nanoparticles (MNPs) have
become an attractive research area in material chemistry
because of their unique properties and potential
through hyperthermia,[3] biomedical diagnosis,[4] biomol-
ecule detection,[5] extraction of DNA from agarose gel[6]
and catalytic applications.[7] Since the MNPs can be
Appl Organometal Chem. 2020;e5572.
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© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5572

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AHMADI ET AL.
easily separated from the reaction mixture by magnetic
process, so their use as catalyst has recently received con-
synthesis of 5-substituted 1H-tetrazoles is via [3 + 2]
cycloaddition of azide anion to the corresponding nitriles
in the presence of a catalyst. Many homogeneous and
heterogeneous catalysts were used to prepare tetrazoles,
some of these catalysts include, InCl3,[33] ZnO[34] Fe
(OAc)2,[35] CdCl2,[36] ZrOCl2,[37] CuFe2O4,[38] natural
zeolite,[39] Cu2O[40] copper triflates[41] AgNPs[42] and
WAlPO-5 microspheres.[43] Although many of these cata-
lysts are efficient in the synthesis of tetrazole derivatives,
many of them have one or more of the following difficul-
ties: long reaction time, hard reaction conditions, hard-
ship in separation and recycling of the catalyst, the
application of costly and toxic metal catalysts, low yield,
formation of side product such as stable metal-tetrazole
complexes and tiresome workup of the reaction mixture.
Thus, the development of simple, comfortable, and envi-
ronment friendly benign methods for the synthesis of
tetrazoles that reduces drawbacks and gives good result
remains necessary. The design of heterogeneous magneti-
cally catalyst systems, especially organic–inorganic
hybrid catalysts owing to selectivity, excellent yield, short
reaction time and easy catalyst recycling have gained an
increasing interest.
Herein, we report the synthesis of a nanocomposite,
Fe3O4@MCM-41-SB-Cu, which contains a magnetic core
and silica mesoporous shell that functionalized with
Schiff base and its copper complex. This work is continu-
ing of previous researches and shows once again the
importance of modified magnetite nanoparticles in catal-
ysis and the biological activity. We used a dialdehyde and
synthesized a bis-Schiff base ligand on magnetic meso-
porous silica nanoparticles. Therefore several good coor-
dination sites are available that enhance chelating
potency to load appropriate amounts of copper and there-
fore improve catalytic activity also effectively immobilize
enzyme. It also seems that the use of a long linker lead to
better access of reactants to catalyst active sites. This
recoverable and efficient hybrid nanocatalyst was used
for the synthesis of 5-substituted 1H–tetrazoles from alde-
hydes. It is worthwhile due to easy approachability and
low toxicity of aldehydes compared to nitriles as
starting material and also great performance and no
toxicity of Cu catalysts in [3 + 2] cycloaddition
reactions. Also, this nanocomposite was tested for
enzyme immobilization due to the tunable and uniform
pores of silica mesoporous shell and its potential applica-
tion as biocatalyst center. In view of biological activity of
copper, the antibacterial activity of the synthesized mate-
rials conjugated with streptomycin (str@nanocomposite)
against Gram-positive (Staphylococcus-aureus) and
Gram-negative (Escherichia-coli) bacteria in the presence
and as well as absence of a magnetic field was also
investigated.
siderable
attention.[8–10]
Among
the
magnetic
nanoparticles, magnetite (Fe3O4) is containing unique
properties such as low toxicity, superparamagnetism,
high surface area, biocompatibility and chemically modi-
fiable surface.[11–13]
Although homogeneous catalysts contains many
advantages, such as high catalytic activity and efficient
selectivity, but there are drawbacks in the separation
and recycling of catalyst from the product such as
tedious and time consuming work-up procedures,
which prevent their practical applications in an indus-
trial scale. In order to overcome these difficulties, the
immobilization of homogeneous catalysts on a large
variety of organic or inorganic supports such as zeo-
lites, aluminas, organosilica materials, polymers, carbon
materials and silica materials to create heterogeneous
catalysts, is one of the most effective strategies that has
been developed in recent years.[14–17] These heteroge-
neous catalysts can be easily recovered from the reac-
tion medium, but the activity and selectivity of catalysts
decreases due to limited active sites on the surface.
Nano-scale supports can be used to solve this problem,
and in fact the immobilization of homogeneous cata-
lysts on nano-sized supports leads to the creation of a
system that combines the properties of homogenous
and heterogeneous catalysts.[18]
Among various supports, mesoporous silica mate-
rials, because of their various advantages such as high
surface area, nanometer sized pore, tunable pore size,
controlled size, and great chemical and mechanical sta-
bility have attracted much attention.[19–22] However,
the process of separating of these nanoparticles from
the reaction mixture using filtration or centrifugation
is time consuming and boring. This could be solved
using magnetic nanoparticles (MNPs), which can be
easily isolated from the reaction medium by applying
an external magnetic field. In fact, the integration of
MNPs with mesoporous silica nanoparticles was
formed a novel support that possesses the properties
and benefits of both mesoporous silica and magnetic
nanoparticles.[23–26]
Tetrazoles are synthetic nitrogen-rich compounds
which have attracted a great deal of attention because of
their wide applications, for example they are important
in coordination chemistry as ligand, in agriculture as fun-
gicides and herbicides, in photography and photo-
imaging as stabilizers and in medicinal chemistry due to
their anticonvulsants, antihypertensive, antiallergic, anti-
cancer and antibiotic activities.[27–31] Hence, preparation
of these heterocyclic compounds is of much current
importance.[32] The most comfortable way for the

CU(II)-SCHIFF BASE COMPLEX ON MAGNETIC MESOPOROUS SILICA NANOPARTICLE
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2 | EXPERIMENTAL
absorption spectrometer model AnalytikjenacontrAA
700. Bacterial growth was measured through reading
the absorption of bacterial suspension by ELISA reader
(Bio-Rad 680 Microplate Reader, California, USA) and
growth curve analysis. Also, to create temporary
nanomagnets, an external magnetic field was used made
by three Neodymium Block Magnets, NdFeB, Grade N42,
5.08 × 2.54 × 0.317 cm thick, model: BY0X02, K&J
Magnetics, Inc.
2.1 | Materials
All chemicals including Iron (III) chloride hexahydrate
(FeCl3.6H2O),
iron
(II)
chloride
tetrahydrate
(FeCl2.4H2O), tetraethylorthosilicate (TEOS), hexadecylt-
rimethyl ammonium bromide (CTAB, 99%), N-
(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPS),
salicylaldehyde, 1,3,5-trioxane, benzhydrazide, Cu
(OAc)2ꢀH2O, ammonia (25 wt.%), various aldehydes,
hydroxyl amine hydrochloride (NH2OH.HCl), sodium
azide (NaN3), sulphuric acid (98%), acetic acid (glacial),
ethanol, DMF, toluene, n-hexane, ethyl acetate and
hydrochloric acid (HCl) were purchased from Merck and
used without further purification. 5,50-methylene-bis-
salicylaldehyde was synthesized according to the previ-
ous method using 1,3,5-trioxane as starting material, in
presence of catalytic amount of conc. H2SO4 and in gla-
cial CH3COOH as a solvent.[44] Fe3O4 superparamagnetic
nanoparticles, Fe3O4@MCM-41 and its amine and alde-
hyde functionalized nanocomposites (Fe3O4@MCM-
41-AEAPS and Fe3O4@MCM-41-ald, respectively) were
synthesized according to the method we reported ear-
lier.[45] Bacterial species were Staphylococcus aureus
(ATCC 6538) and Escherichia coli (ATCC 25922).
2.3 | Synthesis of Schiff base-
functionalized Fe3O4@MCM-41
nanoparticles (Fe3O4@MCM-41-SB)
Fe3O4@MCM-41-ald nanoparticles (1 g) were dispersed
in absolute ethanol (20 ml) for 30 min. Then, benzhy-
drazide (2 mmol, 0.272 g) was dissolved in ethanol
(20 ml) and added into the above suspension. The mixed
solution was refluxed at 80 ꢁC for 24 hr. Schiff base
functionalized mesoporous materials were separated
using an external magnet, washed with ethanol for sev-
eral times, and dried in oven at 60 ꢁC.
2.4 | Synthesis of Fe3O4@MCM-41-SB-Cu
nanoparticles (Fe3O4@MCM-41-SB-Cu)
2.2 | Characterization methods
For the synthesis of Fe3O4@MCM-41-SB-Cu nanoca-
talyst, 0.5 g of Fe3O4@MCM-41-SB was sonicated in
ethanol (20 ml), and a solution of Cu (CH3COOH)2.H2O
(1 mmol, 0.199 g) in ethanol (20 ml) was added to this
mixture. The mixture was refluxed for 24 hr with nitro-
gen protection. The resulting product was collected using
an external magnet, washed with ethanol and finally
dried in oven at 60 ꢁC.
Morphology and size of nanoparticles were examined
using field emission scanning electron microscopy (FE-
SEM) equipped with EDS analyzer model Mira 3-XMU
and with an accelerating voltage of 15 kv. High resolution
Transmission electron microscopy (HRTEM) images were
taken using a HR-TEM FEI TEC9G20 (200 kV). Fourier
transform infrared spectra (FT-IR) were recorded on a
Perkin-Elmer spectrum two Fourier transforminfrared
spectrometer in the range of 400–4000 cm−1 using pressed
KBr pellets. Powder X-ray diffraction (PXRD) patterns
were recorded with a PW 1730 X-ray diffractometer
model X Pert Pro using CuKα irradiation at 40 kv and
30 mA. Brunauer–Emmett–Teller (BET) surface area,
pore size distribution and pore volume were determined
using a Belsorp mini II instrument at liquid N2 tempera-
2.5 | General procedure for synthesis of
5-substitutd-1H-tetrazole derivatives
catalyzed by Fe3O4@MCM-41-SB-Cu
In a 50 ml round-bottom flask, 30 mg of the nanocatalyst
(Fe3O4@MCM-41-SB-Cu) was dispersed in DMF (5 ml)
by sonication for 20 min at room temperature. To this
suspension, aldehyde (1 mmol), hydroxylamine hydro-
chloride (1 mmol) and sodium azide (1.5 mmol) were
ꢁ
ture. Samples were degassed at 120 C for 15 hr prior to
analysis. Magnetic properties of the synthesized
nanoparticles were measured by means of vibrating sam-
ple magnetometer (VSM) of Meghnatis DaghighKavir
Company. Thermogravimetric analysis (TGA) of the sam-
ples were carried out using a TA Q600 insꢁtrument under
flowing argon with a heating rate of 10 C min−1. The
amount of Cu nanocatalyst was measured by atomic
ꢁ
added and the reaction mixture was refluxed at 120 C
for the appropriate time. Progress of the reaction was
monitored by TLC (n-hexane/EtOAc). After completion
of the reaction, the mixture was cooled to the room tem-
perature and subsequently the catalyst was recovered
using an external magnet. Then HCl (10 ml, 1 M) and

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AHMADI ET AL.
ethyl acetate (10 ml) was added to the solution and
stirred strongly. The organic phase was isolated and the
aqueous phase extracted several times with ethyl acetate.
The organic layer was dried over anhydrous sodium sul-
fate and the product was obtained after recrystallization
in ethanol.
times with PBS. These nanocomposites were dispersed in
500 μL of PBS and then sterile paper disks were saturated
with this suspension and placed on staꢁrch agar medium.
The plates were then incubated at 37 C for 24 hr and
the amylase enzyme activity was assessed after addition
of iodine solution on the surface of agar. The area of clear
zone around the disc was measured and the mean values
recorded as the α-amylase activity.
2.6 | Enzyme immobilization
In this procedure, 20 mg of each of the synthesized mate-
rials was combined with 1 ml of phosphate buffer saline
(PBS) containing 2 mg of α-amylase enzyme and incu-
2.7 | Antibacterial activity of the
streptomycin loaded magnetic
nanoparticles (str@nanocomposite)
ꢁ
bated at continuous shaking (75 rpm) at 37 C for 24 hr.
Then, the enzyme conjugated samples were isolated by
centrifuging at 10000 rpm for 5 min and washed three
In order to conjugation of streptomycin with the as-
synthesized nanoparticles, 20 mg of each compound was
FIGURE 1 The route for synthesis
of the functionalized magnetic
mesoporous nanoparticles,
Fe3O4@MCM-41-SB-Cu

CU(II)-SCHIFF BASE COMPLEX ON MAGNETIC MESOPOROUS SILICA NANOPARTICLE
5 of 17
added to 1 ml of streptomycin solution (10 mg/ml Milli-Q
water). The mixture was then stirred at room tempera-
ture for 24 hr and the streptomycin loaded magnetic
composites (str@nanocomposite) were separated by cen-
trifugation at 10000 rpm for 5 min. Also, to remove the
surplus streptomycin, str@nanocomposites were washed
with deionized water for three times. The antibacterial
activity of the str@nanocomposites was checked against
a Gram positive bacterium, i.e. Staphylococcus aureus
and a Gram negative species, i.e. Escherichia coli in the
presence and absence of a magnetic field. For this pur-
pose, 20 mg of each compound in Milli-Q water (1 ml)
was inoculated with Mueller-Hinton broth (Merck,
Darmstadt, Germany) containing bacterial suspension
(E. coli or S. aureus) with ꢁ0.5 McFarland. The cultures
were then incubated at 37 C, and bacterial growth was
monitored every 4 hr through measuring the bacterial
suspension absorbance at 655 nm using ELISA reader.
Simultaneously controls including bacterial culture with-
out nanocomposite as well as uninoculated culture
medium containing nanocomposite were regarded.
Fe3O4@MCM-41-SB-Cu are shown in Figure 2. In the
spectrum of Fe3O4, the bands at 3000–3700 and
1626 cm−1 were attributed to the vibrational modes of
physically adsorbed water molecules. The strong absorp-
tion peak at 582 cm−1 is assigned to the Fe–O–Fe
stretching vibration and is observable in all spectra.[46]
Three new bands at 1066, 794 and 449 cm−1 observed in
the Figures 2b to 2g, are ascribed to the asymmetric
stretching, symmetric stretching and bending vibration
modes, respectively, of the Si–O–Si bonds in SiO2.[47–50]
In FT-IR spectrum of Fe3O4@ MCM-41-CTAB
nanoparticles (Figure 2b), the absorption bands at 2920
and 2851 cm−1 are ascribed to the C − H stretching vibra-
tions of –(–CH2–)3– group of surfactant. In the spectrum
of Fe3O4@ MCM-41, the absence of any characteristic
peaks of alkyl group, confirms the template of CTAB
were completely removed. Appearance of the new bands
in the spectrum of Fe3O4@MCM-41-AEAPS at 2933, 2880
and 1630 cm−1 can be related to the asymmetric and sym-
metric C-H stretching modes and NH (NH2) bending
vibrations, respectively, of AEAPS (Figure 2d). In the IR
spectrum of Fe3O4@MCM-41-ald and Fe3O4@MCM-
41-SB, presence of a sharp band at 1635 cm−1 is assigned
to the imine group (C=N) and clearly confirms that dia-
ldehyde and benzhydrazide are successfully grafted onto
the surface of modified Fe3O4@ MCM-41, after the reac-
tion between carbonyl groups of dialdehyde with
anchored amino group of Fe3O4@MCM-41-AEAPS and
then with amino group of benzhydrazide. In the FT-IR
spectrum of Fe3O4@MCM-41-SB-Cu (Figure 2g), the
C=N stretching frequency is shifted to lower wavelength
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and characterization
A
pictorial representation for the synthesis of
Fe3O4@MCM-41-SB-Cu nanocatalyst is shown in
Figure 1. The superparamagnetic nanoparticles of Fe3O4
were prepared through co-precipitation procedure and
then silica was coated on the surface using tetra ethyl
orthosilicate (TEOS) in the presence of CTAB during a
sol–gel process. After removing of the CTAB, the synthe-
sized Fe3O4@MCM-41 nanoparticles were functionalized
with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane
(AEAPS). Then Fe3O4@MCM-41-AEAPS nanocomposite
was condensed with the carbonyl groups of dialdehyde
and after that with amino group of benzhydrazide to pro-
duce the Schiff base grafted magnetic nanocomposite.
Finally, Fe3O4@MCM-41-SB-Cu was synthesized by reac-
tion of the Fe3O4@MCM-41-SB nanoparticles with Cu
(CH3COOH)2.H2O. In this reaction, the supported Schiff
base can be coordinated to Cu2+ via oxygen and nitrogen
atoms (Figure 1). The concentration of copper loading
determined by AAS was 12.3 wt% (0.116 mmol.g−1). The
structure and morphology of the newly synthesized
nanocatalyst was carried out using various methods such
as FT-IR, XRD, FE-SEM, HRTEM, EDS, BET, AAS, VSM
and TGA.
FIGURE 2 FT-IR spectra of synthesized nanocomposites
(a) Fe3O4, (b) Fe3O4@MCM-41-CTAB, (c) Fe3O4@MCM-41,
(d) Fe3O4@ MCM-41- AEAPS, (e) Fe3O4@MCM-41-ald (f)
Fe3O4@MCM-41-SB and (g) Fe3O4@MCM-41-SB-Cu
The FT-IR spectra of the Fe3O4, Fe3O4@MCM-
41-CTAB, Fe3O4@ MCM-41, Fe3O4@MCM-41-AEAPS,
Fe3O4@MCM-41-ald,
Fe3O4@MCM-41-SB
and

6 of 17
AHMADI ET AL.
and appeared at 1627 cm−1. This observation confirms
the coordination of imine nitrogen with metal ion and
show that the catalyst has been successfully synthesized.
The size and morphology of Fe3O4, Fe3O4@ MCM-41
and Fe3O4@MCM-41-SB-Cu nanoparticles can be
explained by FE-SEM and HRTEM images (Figure 3).
These images show that the catalyst has spherical shape
similar to the parent Fe3O4@ MCM-41 and Fe3O4. Some
aggregation occurs due to magnetic property of
nanoparticles. The size of these nanoparticles is less than
100 nm. The HRTEM images of Fe3O4@MCM-41-SB-Cu
are shown in Figure 3(d-f). As it is shown, the Fe3O4
FIGURE 3 FE-SEM images of the
magnetic nanoparticles: (a) Fe3O4
(b) Fe3O4@ MCM-41 (c) Fe3O4@MCM-
41-SB-Cu, (d-f) HRTEM images of
Fe3O4@MCM-41-SB-Cu

CU(II)-SCHIFF BASE COMPLEX ON MAGNETIC MESOPOROUS SILICA NANOPARTICLE
7 of 17
particles (core) are black spots and the mesoporous silica
shell functionalized with Cu Schiff base complex is
located around of magnetite with a white color.
The EDX pattern and mapping analysis of
Fe3O4@MCM-41-SB-Cu are shown in Figures 4 and 5,
respectively. This analysis shows the presence of Fe, Si,
Cu, N, C, and O in the structure of the nanocomposite,
therefore confirms the functionalization of the
Fe3O4@MCM-41 and immobilization of Schiff base cop-
per complex on silica coated Fe3O4 nanoparticles. The
elemental mapping reveals good distribution of Cu on
nanocatalyst.
The crystalline structures of the Fe3O4, Fe3O4@
MCM-41, Fe3O4@MCM-41-SB and Fe3O4@MCM-41-SB-
Cu were investigated by XRD (Figures 6 and 7). The XRD
FIGURE 4 The Elemental composition of Fe3O4@MCM-
41-SB-Cu determined by EDS analysis
FIGURE 5 Elemental mapping of
Fe3O4@MCM-41-SB-Cu; carbon (white),
nitrogen (orange), oxygen (yellow), silicon
(green), iron (blue) and copper (red)

8 of 17
AHMADI ET AL.
MCM-41 and Fe3O4@MCM-41-SB-Cu were also investi-
gated as depicted in Figure 7. The XRD pattern of the
Fe3O4@ MCM-41 shows an intense peak at 2θ = 2.13 and
two weak peaks at 2θ = 3.54 and 4.35ꢁ, corresponding to
the (1 0 0), (1 1 0) and (2 0 0) planes, respectively, which
are characteristic of a hexagonal pore system.[52,53,56–60]
Fe3O4@MCM-41-SB-Cu gives only one broad peak at low
angle corresponding to (1 0 0) plane. This broadening
and disappearing of XRD peaks are due to func-
tionalization that reduces the order of mesos-
tructure.[56,61] Hence, the decrease in the intensity of the
(1 0 0) peak for Fe3O4@MCM-41-SB-Cu nanocatalyst
confirms that functionalization occurs also within the
porous channels.[62]
Figure 8 shows the nitrogen adsorption–desorption
isotherms of Fe3O4@ MCM-41, Fe3O4@MCM-41-AEAPS,
Fe3O4@MCM-41-SB and Fe3O4@MCM-41-SB-Cu. The
diagrams exhibited a type IV isotherm according to the
IUPAC classification, which is characteristic of meso-
porous material. According to the results reported in
Table 1, the Fe3O4@ MCM-41 nanocomposite possesses
high specific surface area (as, 481.13 m2/g) and large
cumulative pore volume (Vp, 0.5 cm3/g). The grafting of
organic fragments onto the Fe3O4@ MCM-41 nanoparti-
cle strongly reduces the surface area and pore volume. As
shown in Table 1, after functionalization, the surface area
decreases to 33.75, 34.86, and 21.14 m2/g for
FIGURE 6 Wide angle X-ray diffraction patterns of bare
Fe3O4 nanoparticles, Fe3O4@ MCM-41, Fe3O4@MCM-41-SB,
Fe3O4@MCM-41-SB-Cu
Fe3O4@MCM-41-AEAPS,
Fe3O4@MCM-41-SB
and
Fe3O4@MCM-41-SB-Cu, respectively. Also, a consider-
able decrease in the pore volume of functionalized
nanocomposites compared to free Fe3O4@MCM-41 was
observed. These evidences show that the Cu (II)-Schiff
base complexes are successfully loaded onto the surface
of the Fe3O4@MCM-41nanocomposite and inside the
pore channels.[56,61,63,64]
The magnetic measurements of all the prepared mate-
rials were carried out using a vibrating sample magne-
tometer (VSM) at room temperature and in the applied
magnetic field sweeping from −12000 to 12000
(Figure 9). The saturation magnetization (Ms) value of
FIGURE 7 Low angle X-ray diffraction patterns of Fe3O4@
MCM-41 and Fe3O4@MCM-41-SB-Cu
pattern of the Fe3O4 nanoparticles (Figure 6) include
peaks at 2θ = 30.50ꢁ, 35.87ꢁ,43.51ꢁ, 53.88ꢁ, 57.43ꢁ, 63.00ꢁ
and 74.55ꢁ which are assigned to the (2 2 0), (3 1 1), (4 0
0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3) phases of Fe3O4,
respectively, and agrees with the crystalline cubic spinel
structure of standard Fe3O4 nanoparticles.[51–53] The dif-
fraction peaks appearing in the XRD patterns of
Fe3O4@MCM-41, Fe3O4@MCM-41-SB and Fe3O4@
MCM-41-SB-Cu (Figure 5), suggest that the crystallinity
has been preserved during the immobilization process.
Also, the XRD pattern of the Fe3O4@MCM-41-SB-Cu
nanocatalyst (Figure 6) shows the new peaks at
2θ = 66.40ꢁand 72.36ꢁ which are attributed to the differ-
ent copper phases of Cu(331) and Cu(220), respec-
tively.[54,55] The low-angle XRD patterns of the Fe3O4@
Fe3O4,
Fe3O4@MCM-41,
Fe3O4@MCM-41-AEAPS,
Fe3O4@MCM-41-SB and Fe3O4@MCM-41-SB-Cu are
80.5, 55.8, 34.63, 30.66 and 27.80 emu/g, respectively. The
Ms value of Fe3O4 after coating with SiO2 and Cu (II)-
Schiff base complex decreases significantly from about
80.5 emu/g to 27.80 emu/g. In fact, silica shell that sur-
rounds the Fe3O4 nanoparticles decreases the interactions
between these particles. However, despite the reduction
of the saturation magnetization, the catalyst still can be
rapidly and easily separated from solution media by
applying an external magnetic field.
Thermal stability of the Fe3O4@MCM-41, Fe3O4@
MCM-41-AEAPS, Fe3O4@MCM-41-SB and Fe3O4@MCM

CU(II)-SCHIFF BASE COMPLEX ON MAGNETIC MESOPOROUS SILICA NANOPARTICLE
9 of 17
FIGURE 8 N2 adsorption–
desorption isotherms of (a) Fe3O4@
MCM-41, (b) Fe3O4@MCM-41-AEAPS,
(c) Fe3O4@MCM-41-SB and
(d) Fe3O4@MCM-41-SB-Cu
TABLE 1 BET and BJH data of Fe3O4@ MCM-41,
Fe3O4@MCM-41-AEAPS, Fe3O4@MCM-41-SB and
Fe3O4@MCM-41-SB-Cu
BET
surface
area
(m2 g−1
Pore
volume
(cm3 g−1
Pore
size
(nm)
Sample
)
)
Fe3O4@MCM-41
481.13
0.5
3.22
Fe3O4@MCM-41-AEAPS 33.75
0.19
0.17
0.12
23.08
19.67
23.21
Fe3O4@MCM-41-SB
34.86
21.14
Fe3O4@MCM-41-SB-Cu
-41-SB-Cu was investigated by TGA under N2 atmosphere
at the heating rate of 10 ꢁC/min (Figure 10). For all
samples, the weight loss below 200 C can be attributed
FIGURE 9 Magnetization curves at 300 K for Fe3O4,
Fe3O4@MCM-41, Fe3O4@MCM-41-AEAPS, Fe3O4@MCM-41-SB
and Fe3O4@MCM-41-SB-Cu
ꢁ
to the loss of physically adsorbed water on the surface or
in the nanochannels of the mesoporous silica.[65] In com-
parison with Fe3O4@MCM-41, the TGA curves of the
Fe3O4@MCM-41-AEAPS, Fe3O4@MCM-41-SB and Fe3O4
@MCM-41-SB-Cu demonstrated significant decomposi-
tionꢁ(15.68, 17.04 and 26.42% respectively) around 200 to
800 C corresponds to the decomposition of organic moi-
ety grafted on the Fe3O4@MCM-41surface.
3.2 | Catalytic study
The catalytic performance of Fe3O4@MCM-41-SB-Cu
was evaluated through the [3 + 2] cycloaddition for the
synthesis of 5-substituted 1H-tetrazoles. Initially, in order
to achieve optimal conditions, the reaction of

10 of 17
AHMADI ET AL.
in water, ethanol and mixtures of H2O–EtOH (1:1)
(Table 2, entries 8–10), the 1-phenyl-1H-tetrazole prod-
uct is detected in a low yield. Hence the best perfor-
mances were achieved in DMF as solvent and in the
presence of 30 mg Fe3O4@MCM-41-SB-Cu at 120 ꢁC.
Thereafter, the above optimized reaction condition was
applied for the synthesis of 5-substituted-1H-tetrazole
derivatives using various aldehydes bearing a wide
range of electron-withdrawing or electron-donating
groups and the results are summarized in Table 3. The
results show in all cases, 5-substituted-1H-tetrazole
derivatives were obtained in excellent yields. The alde-
hydes having electron-withdrawing groups compared to
those that have electron-donating groups, gave desired
products in higher yield and shorter reaction time. For
example, 4-bromobenzaldhyde reacts in shorter reaction
time (2 hr), 90% yield and 0.13 h−1 TOF (Table 3, entry
2) whereas 4-methoxybenzaldhyde reacts in longer time
(4.5 hr) with 75% yield and 0.05 h−1 TOF (Table 3,
entry 5).
FIGURE 10 The TGA curve of the Fe3O4@MCM-41 support,
Fe3O4@MCM-41-AEAPS, Fe3O4@MCM-41-SB and Fe3O4@MCM-
41-SB-Cu nanocatalyst
benzaldehyde, hydroxylamine hydrochloride and sodium
azide was chosen as a model and the effect of various
parameters on the catalytic efficiency of Fe3O4@MCM-
41-SB-Cu was examined. The reactions were followed
by TLC and the yields were obtained after isolation of
the product. The details of the optimization conditions
were given in Table 2. Firstly, to optimize the catalyst
amount, the model reaction was carried out in the pres-
ence of various amounts of the catalyst. As shown in
Table 2 (entries 1–4), the reaction did not show any
progress in the absence of catalyst and the optimum
amount of the catalyst was chosen to be 30 mg,
3.48 mol% Cu. According to the results of Table 2
(entries 5–7), the efficiency of the reaction increases
Catalytic activity of Fe3O4@MCM-41-SB-Cu is com-
pared with some other catalyst systems reported in the
literature for the production of 5-phenyl 1H-tetrazole
(Table 4).[33,66–74] The results show that the mentioned
magnetic nanocatalyst provides higher yield of the prod-
uct in shorter reaction time. Furthermore, the catalyst is
easily removed from the reaction mixture using an exter-
nal magnetic field.
Catalyst recovery and reusability was investigated
for the synthesis of 5-phenyl 1H-tetrazole under the
optimized reaction conditions. At the end of each run,
the reaction mixture was cooled at room temperature
and the catalyst was separated using an external
magnet, washed several times with ethanol and water
ꢁ
with temperature rising up to 120 C and the best result
ꢁ
ꢁ
was obtained at 120 C. When the reaction is performed
for removing surplus, dried at 60 C and reused for the
TABLE 2 Optimization of conditions for preparation of 5-substituted-1H-tetrazoles using Fe3O4@MCM-41-SB-Cua
Entry
Amount of catalyst (mg)
Cu content (mol%)
Solvent
DMF
Temperature (ꢁC)
Time (hr)
Yield (%)b
1
2
0
0
120
120
120
120
rt
24
1/5
1
0
20
30
40
30
30
30
30
30
30
2.32
3.48
4.64
3.48
3.48
3.48
3.48
3.48
3.48
DMF
86
3
DMF
95
4
DMF
1
95
5
DMF
24
1
Trace
80
6
DMF
100
120
90
7
DMF
1
95
8
H2O
1
Trace
45
9
EtOH
H2O/EtOH
80
1
10
90
1
40
aReaction conditions: benzaldehyde (1 mmol), NH2OHꢀHCl (1 mmol), sodium azide (1.5 mmol) and solvent (5 ml).
bIsolated yield.

CU(II)-SCHIFF BASE COMPLEX ON MAGNETIC MESOPOROUS SILICA NANOPARTICLE
11 of 17
TABLE 3 Synthesis of 5-substituted-1H-tetrazoles catalyzed by Fe3O4@MCM-41-SB-Cu nanocompositea
Entry
Aldehyde
Product
Time (hr)
Yield (%)b
TONc
TOF (h−1 d
)
1
1
95
0.27
0.27
0.13
0.17
0.06
2
3
4
2
90
88
79
0.26
0.25
0.23
1.5
4
5
6
4.5
75
80
0.22
0.23
0.05
0.11
2
7
8
3.5
69
83
0.20
0.24
0.06
0.08
3
(Continues)

12 of 17
AHMADI ET AL.
TABLE 3 (Continued)
Entry
Aldehyde
Product
Time (hr)
Yield (%)b
TONc
TOF (h−1 d
)
9
2.5
72
0.21
0.08
10
11
1.5
84
80
0.24
0.23
0.16
3
0.08
aReaction conditions: aldehyde (1 mmol), sodium azide (1.5 mmol), hydroxylamine hydrochloride (1 mmol), Fe3O4@MCM- 41-SB-Cu (30 mg, 3.48 mol% Cu),
DMF (5 ml), and 120 ꢁC. bIsolated yield. cTON, turnover number, moles of aryl halides converted per mole of Pd. dTOF, turnover frequencies, TON/time of
reaction.
TABLE 4 Comparison between Fe3O4@MCM-41-SB-Cu and other catalysts in the synthesis of 5-substituted 1H-tetrazoles
Entry
Catalyst
Solvent
H2O
Temp. (ꢁC)
Reflux
Reflux
140
Time (hr)
Yield (%)
Ref.
[66]
1
Fe3O4@SiO2-TCTPVA-Cu (II)
(NH4)2Ce(SO4)4.2H2O
Cu-MCM-41
4
96
72
90
96
87
90
92
96
91
92
95
[67]
[68]
[69]
[70]
[71]
[33]
[72]
[73]
[74]
2
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
5
3
12
12
24
4
4
Cu (OAc)2(25 mol%)
Bi (OTf)3
120
5
120
6
P2O5
Reflux
120
7
InCl3
15
1.5
7
8
Fe3O4–CNT–TEA–Cu (II)
Schiff base complex of Cu
nano-Cu2O–MFR
Fe3O4@MCM-41-SB-Cu
70
9
110
10
11
100
8
120
1
Present work
next run under the same conditions. After five cycles,
the immobilized catalyst is recoverable and preserves its
catalytic efficiency and selectivity with no significant
loss of activity (Figure 11). Comparison of the copper
measured by atomic absorption shows that metal
leaching was negligible. The FT-IR spectrum of the
recovered catalyst showed no change after five succes-
sive times (Figure 12). Furthermore, XRD pattern of
reused catalyst (Figure 13) confirms that the catalyst
can be recycled without any considerable change in
concentration before catalytic reaction (0.116 mmol.g−1
,
12.3 wt%) and after five run (0.099 mmol, 10.5 wt%)

CU(II)-SCHIFF BASE COMPLEX ON MAGNETIC MESOPOROUS SILICA NANOPARTICLE
13 of 17
FIGURE 11 Reusability of catalyst in the synthesis of
5-phenyl-1H-tetrazole
FIGURE 14 Comparison between the magnetization curve of
the fresh Fe3O4@MCM-41-SB-Cu composite (red) and the
magnetization curve of the Fe3O4@MCM-41-SB-Cu composite after
recycling five times (yellow)
the structure. The saturation magnetization (Ms) of
recovered Fe3O4@MCM-41-SB-Cu was determined as
19.21 emu/g and this reused catalyst still can be rapidly
and easily separated from solution by an external mag-
netic field (Figure 14).
A proposed mechanism for the synthesis of tetrazole
derivatives catalyzed by the Fe3O4@MCM-41-SB-Cu
nanocatalyst is represented in Figure 15. It is suggested
that the interaction of oxygen atom of aldehyde with
Cu (II) initially increases the electrophilicity of alde-
hyde (I). Then, the nucleophilic attack of nitrogen atom
of hydroxylammine to activated aldehyde causes the
losing of water and formation of oxime intermediate
(II). The oxime loses the water and gives the nitrile
intermediate (III). Interaction of Cu (II) with nitrogen
atom of nitrile increases the electrophilic character of
the nitrile. Subsequently, the activated nitril reacts with
sodium azide for transformation into respective
tetrazole. In fact, the reaction proceeds via [3 + 2]-
cycloaddition pattern which leads to compound IV after
acidic work up.
FIGURE 12 Comparison of the IR spectrum of
Fe3O4@MCM-41-SB-Cu composite (a) before catalytic reaction and
(b) after recycling five times
3.3 | Enzyme immobilization
In the process of immobilization of α-amylase enzyme, in
fact the ability of synthesized samples for binding to
enzyme was examined. The results of α-amylase enzyme
immobilization on Fe3O4, Fe3O4@MCM-41 and function-
alized Fe3O4@MCM-41 nanocomposites are shown in
Figure 16 and listed in Table 5. The results show that α-
Amylase was successfully immobilized on all compounds.
FIGURE 13 Comparison of XRD patterns of Fe3O4@MCM-
41-SB-Cu and reused Fe3O4@MCM-41-SB-Cu

14 of 17
AHMADI ET AL.
FIGURE 15 Plausible mechanism for the
synthesis of tetrazoles
TABLE 5 Starch hydrolysis zone diameter (mm) following the
effect of synthesized composites
Compound
Effect zone
Fe3O4
15
18
22
21
26
32
Fe3O4@MCM-41
Fe3O4@MCM-41-AEAPS
Fe3O4@MCM-41-dialdhyde
Fe3O4@MCM-41-SB
Fe3O4@MCM-41-SB-Cu
groups on their surface, which can interact with OH
groups of the amylase enzyme using hydrogen bonding.
Also, the amine, imine and carbonyl groups in the
functionalized nanocomposites are suitable for hydrogen
bonding or covalent bond formation with hydroxyl
groups of α-amylase. Also, the copper in Fe3O4@MCM-
41-SB-Cu possesses free coordination sites that may be
suitable for enzyme binding.
FIGURE 16 Starch hydrolysis zone diameter (mm) following
the effect of enzyme conjugated Fe3O4 (a), Fe3O4@MCM-41 (b),
Fe3O4@MCM-41-AEAPS (c), Fe3O4@MCM-41-ald (d),
Fe3O4@MCM-41-SB (e) and Fe3O4@MCM-41-SB-Cu (f)
3.4 | Antibacterial activity
Among these, Fe3O4@MCM-41-SB-Cu and Fe3O4@
MCM-41-SB more efficiently loaded the enzyme. In gen-
eral, the immobilization of enzymes on a support occurs
using various methods such as the encapsulation of
enzyme, cross-linking and binding to a support through
covalent or noncovalent interactions.[75–78] The nanopar-
ticles of Fe3O4 and Fe3O4@MCM-41 contain hydroxyl
The bacterial growth was inhibited in bacterial cultures
treated with the wash water rinsed from conjugated
nanocomposites. This finding confirms loading of strep-
tomycin onto synthesized compounds. The growth cur-
ves of treated bacteria were shown in Figures S1-S12.
The results reveal that Fe3O4 doesn't have any

CU(II)-SCHIFF BASE COMPLEX ON MAGNETIC MESOPOROUS SILICA NANOPARTICLE
15 of 17
inhibitory effect against E. coli while its conjugation
with streptomycin lead to bacterial growth inhibition
that was reflected through growth curve shift and
decreasing the bacterial suspension absorbance. This
inhibitory effect was more prominent in case of mag-
netic field application. This is also true in case of
S. aureus. Similarly, the Fe3O4@MCM-41conjugated
with streptomycin reduced the growth of both species,
i.e., S. aureus and E. coli but its inhibitory potential
was more than Fe3O4. This may be due to the struc-
tural properties of MCM-41 that can load more antibi-
otic than Fe3O4 and hence shows higher antibacterial
activity. In the case of Fe3O4@MCM-41, application of
magnetic field was promoted the inhibition potential of
this compound against both species. Interestingly,
Fe3O4@MCM-41-AEAPS exhibited much higher inhibi-
tory effect as it almost completely inhibited the growth
of E. coli and its growth curve was remained near the
baseline. In other word, this compound had bactericidal
potential against E. coli and its effect was increased
under magnetic field. This effect can be attributed to
the functionalization of nanocomposites with amine
which facilitates streptomycin binding and its conjuga-
tion with nanocomposites. So higher concentration of
this antibiotic can bind to nanocomposite and the mini-
mum bactericidal concentration (MBC) of antibiotic
will be provided following treatment of target bacte-
rium. In spite of E.coli, the Fe3O4@MCM-41-AEAPS
didn't have bactericidal effect against S. aureus, how-
ever it was significantly reduced the bacterial growth.
The structural differences between E. coli and S. aureus
are one of the possible reasons for this difference. The
peptidoglycan network in cell wall of S. aureus has
more cross linkages that limit diffusion of extracellular
compounds toward cell cytoplasm. As the main target
site of streptomycin is bacterial ribosome, this limita-
tion will reduces the antibacterial potential of synthe-
sized compound. In case of Fe3O4@MCM-41-SB-Cu,
this nanocomposite was completely inhibited the
help them to recover Fe from their environment and
hence these magnetic nanocomposites are absorbed effi-
ciently by them. Treating bacterial species with mag-
netic field accelerates nanocomposites absorption and
as a result the higher concentration of streptomycin
will be provided inside the cytoplasm and inhibit pro-
tein synthesis and bacterial growth and division.
4 | CONCLUSION
Magnetic mesoporous silica nanocomposites were synthe-
sized and characterized by various methods. The novel
and reusable magnetic nanocatalyst, Fe3O4@MCM-41-
SB-Cu, was successfully used in one pot three-component
reactions of various aldehydes, hydroxyl amine hydro-
chloride and sodium azide for the synthesis of
5-substituted 1H-tetrazole derivatives and obtained excel-
lent results. This catalyst can easily be separated from the
reaction environment using an external magnetic field
and reused several times without significant loss of cata-
lytic stability and activity. The study of antibacterial activ-
ity of streptomycin loaded mesoporous nanocomposites
against E.coli and S.aureus in the absent and present of
magnetic field showed the magnetic field increase the
antibacterial activity. The ability to amylase enzyme
immobilization was also investigated for the synthesized
compounds and achieved good results.
ACKNOWLEDGEMENT
Support of this work by Shahid Chamran University of
Ahvaz, Ahvaz, Iran (Grant No. 1396) is gratefully
acknowledged.
SUPPLEMENTARY MATERIAL
The effect of synthesized nanomaterials on growth curve
of bacteria in the presence and absence of a magnetic
field have been given in Figures S1-S12.
growth
of
both
E.
coli
and
S.
aureus,
ORCID
i.e., functionalization of Fe3O4@MCM-41 with SB and
Cu gives it bactericidal potential that successfully
inhibited the growth of both treated bacteria. This
effect was also more notable under magnetic field
Tahereh Sedaghat https://orcid.org/0000-0002-3352-
1932
Roya Azadi https://orcid.org/0000-0002-8580-0810
application.
For
the
remainder
synthesized
REFERENCES
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has increased the biological activity of conjugated com-
pounds. It means that the magnetic field facilitated the
diffusion of nanocomposites across the bacterial enve-
lope. Furthermore, as bacteria need Fe as an essential
growth factor they produce Fe binding proteins that
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