Synthesis of symmetrical N,N′-alkylidene bis-amides catalyzed by silica coated magnetic NiFe2O4 nanoparticle supported polyphosphoric acid (NiFe2O4@SiO2-PPA) and its application toward silver nanopart

Article abstract of DOI:10.1039/c5ra16481h

A green and efficient procedure for the synthesis of N,N′-alkylidene bis-amide derivatives has been developed by one-pot condensation of aldehydes and amides in the presence of a magnetic supported acid catalyst NiFe₂O₄@SiO₂

Full text of DOI:10.1039/c5ra16481h

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Graphical Abstract
Synthesis of symmetrical N,N′-alkylidene bis-amides catalyzed by silica coated magnetic NiFe O4
2
nanoparticles supported polyphosphoric acid (NiFe O @SiO -PPA) and its application toward silver
2
4
2
nanoparticle synthesis for electrochemical detection of glucose
Behrooz Maleki, Mehdi Baghayeri
►
For the first time, the synthesized N,N′ꢀ(phenylmethylene)ꢀbisꢀbenzamide (PMꢀbꢀB) has been used as a base for
electrochemical deposition of silver nanoparticles to fabricate Ag@PMꢀbꢀB nanocomposite.
►
►
The Ag@PMꢀbꢀB nanocomposite showed high affinity to sensitive detection of glucose as a novel sensor.
The supported catalyst could be recycled and reused with the simple application of an external magnetic field,
while retaining similar catalytic activity for six successive reactions.

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Synthesis of symmetrical N,N′-alkylidene bis-amides catalyzed by
silica coated magnetic NiFe O4 nanoparticles supported
2
polyphosphoric acid (NiFe O @SiO -PPA) and its application
2
4
2
toward silver nanoparticle synthesis for electrochemical detection of
glucose
Behrooz Maleki*, Mehdi Baghayeri
Department of Chemistry, Hakim Sabzevari University, Sabzevar, 96179ꢀ76487, Iran
Eꢀmail:b.maleki@hsu.ac.ir, Tel: +98ꢀ44013324 Fax: +98ꢀ514401300
Abstract
A green and efficient procedure for the synthesis of N,N′ꢀalkylidene bisꢀamide derivatives has
been developed by oneꢀpot condensation of aldehydes and amides in the presence of a magnetic
supported acid catalyst NiFe O @SiO ꢀPPA (NFSꢀPPA) as a highly effective heterogeneous
2
4
2
catalyst under solventꢀfree conditions in refluxing methanol. The magnetically recoverable
catalyst was easily recycled at least six times without significant loss of catalytic activity. We
report on the modification of a glassy carbon electrode with N,N′ꢀ(phenylmethylene)ꢀbisꢀ
benzamide (PMꢀbꢀB), and how this electrode can serve as a platform for the construction of Ag
nanoparticles as a novel electrochemical sensor for glucose detection.
Keywords: Nanomagnetic catalyst, N,N′ꢀAlkylidene bisꢀamide, Heterogeneous catalyst,
Electrochemical sensor, Glucose
1

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Introduction
Amides are widely used not only in chemical industries but also in organic synthesis.
Specifically, bisamides are key fragments for the introduction of gemꢀdiaminoalkyl residues in
1
,2
retroꢀinverse pseudopeptide derivatives. These compounds are synthesized by the condensation
3
of aldehydes and amides using catalysts such as sulfuric acid, trifluoromethanesulfonic acid
4
5
6
(
CF SO H), sulfamic acid (NH SO H), pꢀtoluenesulfonic acid (pꢀTSA), silicaꢀsupported
3 3 2 3
7
8
9
barium chloride (SiO ꢀBaCl ), boric acid [B(OH) ], phosphotungstic acid (PWA), silicaꢀ
2
2
3
10
11
supported perchloric acid (HClO ꢀSiO ), silicaꢀsupported polyphosphoric acid (SiO ꢀPPA),
4
2
2
12
silicaꢀsupported magnesium chloride (SiO ꢀMgCl ),
and tris(hydrogensulfato)boron
2
2
13
14
15
[
B(HSO ) ], nanoꢀSnCl .SiO , and H [NaP W MoO ]. However, several of these
4
3
4
2
14
5
29
110
methods suffer from some drawbacks such as prolonged reactions times, unsatisfactory yield,
utilization of hazardous organic solvents, tedious workꢀup conditions, use of microwave
irradiation, employment of large amounts of catalyst and harsh reaction conditions. The present
work reports the use of polyphosphoric acid supported on silicaꢀcoated NiFe O nanoparticle
2
4
(
NiFe O @SiO –PPA) as a catalyst for the preparation of symmetrical N,N′ꢀalkylidene bisꢀ
2 4 2
amides.
In recent years, there has been a rapid growth in the development of novel supported
compounds such as supported catalysts, reagents and scavengers. Preparing heterogeneous
catalysts by immobilizing the homogenous precursors on solid support is one of the important
routes for developing novel heterogeneous catalysts. In most of these cases, the immobilized
catalysts so prepared could provide advantages over their unsupported counterparts in terms of
1
6ꢀ18
easy separation, low toxicity, moisture resistance, air tolerance, easy handling, reusability.
On a different note, magnetic nanoparticles (MNPs) have gained an increasing interest because
2

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1
9
of their potential applications such as their uses for cell separation, magnetic resonance
2
0
21
22
imaging, drug delivery systems, protein separation and cancer treatments through
2
3
hyperthermia. Also, magnetic nanoparticles (MNPs) can be a good candidate as a support
material for heterogeneous catalysts, because of easy synthesis, high surface area, facile
2
4
separation by magnetic forces, and low toxicity and cost. According to these attractive
properties, many MNPs supported catalysts have been designed and widely applied as novel
2
5
26
magnetically separated catalysts in traditional metal catalysis, organocatalysis, and even
2
7
enzyme catalysis.
Material selection process is of paramount importance to achieve successful lowꢀcost
engineering design that can fit functional requirements as well as customer satisfaction attributes
for various sensing applications. This is usually carried out considering various conflicting
criteria and utilizing decision making tools to reveal the potential of new materials in expanding
the possibilities of new applications. In recent years, MNPs have been a center of scientific
interest and active multidisciplinary research because of their excellent potential for widespread
2
8
applications such as rechargeable batteries, sensors, electroꢀcatalyst and so on. The ability to
precisely prepare MNPs on a large scale with tailored morphology and geometry is an important
goal of the material science. In the last decade, various methods have been developed to prepare
highꢀdensity and wellꢀaligned MNPs. The conventional techniques used to fabricate nanowires
array include sol–gel method, chemical vapor deposition, direct electrodeposition and so forth.
Among these methods, carrierꢀassisted electrodeposition has been established to be an attractive
route for the synthesis of MNPs because of its simplicity, costꢀeffectiveness, low temperature
2
9ꢀ30
fabrication, etc.
Also, the morphology, nanostructure, and composition of MNPs could be
tuned through selectively controlling deposition parameters. Among them, the functional groups
3

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31
on carriers play a crucial role in enhancing the formation of MNPs and its further applications.
Amino, carbonyl, carboxylic and thiol groups are the four types of functional groups most
commonly used during the nanoparticle synthesis process; these functional groups can form
stable recognition sites and cavities within their matrix. In this study, we have combined the
advantages of the functional groups from the N,N′ꢀ(phenylmethylene)ꢀbisꢀbenzamide (3a) and
the high ability of the electrodeposition technique to synthesis silver NPs. Beside this purpose, a
glucose sensor based on a glassy carbon electrode coated with an N,N′ꢀ(phenylmethylene)ꢀbisꢀ
benzamide and Ag NPs (Ag@PMꢀbꢀB/GCE) with a satisfactory selective recognition ability
toward glucose have been prepared.
Results and Discussion
As a continuation of our work on the development of efficient and environmentally benign
3
2
reagents and catalysts, we now report that NiFe O @SiO –PPA is an excellent catalyst for the
2
4
2
synthesis of symmetrical N,N′ꢀalkylidene bisꢀamide derivatives (Scheme 1).
NiFe O @SiO -PPA
O
2
4
2
CH OH/Reflux
Aryl
Alkyl
3
H N
2
O
2
Aryl
HN
O
Alkyl
Aryl
Alkyl
NH
Aryl
Alkyl
O
3
a-r
Aryl
Alkyl
Aryl
Alkyl
NH2
OHC
CH OH/Reflux
3
1
a-r
2
Scheme 1. Synthesis of symmetrical N,N′ꢀalkylidene bisꢀamide.
4

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33
In order to explore the catalytic activity of NiFe O @SiO –PPA, the condensation of 3ꢀ
2
4
2
nitrobenzaldehyde (1 mmol) with benzamide (2 mmol) was selected as a model reaction
Scheme 2).
(
O
O
HN
CHO
NH2 Conditions
NH
+
2
O
NO2
NO2
1d
2
3d
Scheme
2.
The
selected
model
reaction
for
synthesis
of
N,N′ꢀ(3ꢀ
nitroophenylmethylene)dibenzamide (3d)
The effect of various parameters, such as the influence of solvents, concentration of the
catalyst and temperature, was evaluated to optimize the reaction conditions (Table 1, entries 1ꢀ
12). Additionally, this reaction took place in the absence of solvent using 0.1 g of catalyst at 110
0
C and the yield (64%) in 50 min was achieved (entry 8). As can be seen from Table 1, the
shortest time and best yield were achieved using 0.1 g of NiFe O @SiO –PPA (entry 2). It was
2
4
2
found that by decreasing the catalyst amount from 0.1 to 0.08 and 0.05 g, the yield of the
reaction decreased from 92 to 84 and 80 % (entries 9ꢀ10). When the amount of the catalyst
increased from 0.1 to 0.12 and 0.24 g, there was no prominent change in the yield (entries 11ꢀ
1
2). It is noteworthy that, in the absence of NiFe O @SiO –PPA, no product was detected in
2 4 2
long reaction time (entry 13).
5

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Table
1
Various conditions used for the synthesis of N,N′ꢀ(3ꢀ
nitroPhenylmethylene)dibenzamide (3d)
Entry
Catalyst (g)
Conditions
Ethanol/reflux
Time (min)
Yield (%)
83
1
2
3
4
5
6
7
8
9
NiFe O @SiO –PPA (0.1 g)
50
50
50
50
50
50
50
50
50
50
50
50
180
2
4
2
NiFe O @SiO –PPA (0.1 g)
Methanol/reflux
92
2
4
2
NiFe O @SiO –PPA (0.1 g)
Ethyl acetate/reflux
Acetonitrile/Reflux
Toluene/reflux
50
2
4
2
NiFe O @SiO –PPA (0.1 g)
72
2
4
2
NiFe O @SiO –PPA (0.1 g)
83
2
4
2
NiFe O @SiO –PPA (0.1 g)
Dichloromethane/reflux
1,4ꢀDioxane/reflux
32
2
4
2
NiFe O @SiO –PPA (0.1 g)
84
2
4
2
0
NiFe O @SiO –PPA (0.1 g)
Solventꢀfree/110 C
64
2
4
2
NiFe O @SiO –PPA (0.08 g)
Methanol/reflux
Methanol/reflux
Methanol/reflux
Methanol/reflux
Methanol/reflux
84
2
4
2
1
1
1
1
0
1
2
3
NiFe O @SiO –PPA (0.05 g)
80
2
4
2
NiFe O @SiO –PPA (0.12 g)
92
2
4
2
NiFe O @SiO –PPA (0.24 g)
92
2
4
2
a
b
ꢀꢀ
ꢀꢀ
a
b
In the absence of any catalyst. No reaction.
To assess the efficiency and the scope of the nanomagnetic catalyst in the preparation of
symmetrical N,N′ꢀalkylidene bisꢀamides, the condensation of benzamide with a wide variety of
aromatic aldehydes containing both electronꢀwithdrawing and electronꢀdonating groups and of
aliphatic aldehydes was examined in the presence of NiFe O @SiO –PPA in methanol under
2
4
2
reflux condition (Table 2, entries 1ꢀ15). The method was also successfully worked when
acetamide was used instead of benzamide in the reaction (Table 2, entries 16ꢀ18).
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Table 2 Synthesis of symmetrical N,N′ꢀalkylidene bisꢀamide derivatives in the presence of
NiFe O @SiO –PPA
2
4
2
O
O
R1
HN
O
NiFe O @SiO -PPA
2
4
2
RCHO + 2
R1
NH2
NH
CH OH/Reflux
R
3
R1
1a-r
2
3a-r
a
0
Entry
1
Products (3a-r)
Yield
%)
Time
(min)
mp ( C)
(
Found
Lit.
O
86
80
213ꢀ215 214ꢀ216 (Ref. 14)
HN
NH
O
3
a
2
O
92
40
257ꢀ259
259ꢀ261(Ref. 14)
HN
NH
O N
2
O
3b
3
O
93
40
234ꢀ236
230ꢀ234 (Ref. 8)
HN
O
NH
NC
3
c
7

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4
5
6
7
O
92
50
70
60
70
226ꢀ228 232ꢀ235 (Ref. 13)
HN
O
NH
NO2
3
HN
O
d
O
80
86
83
257ꢀ259 257ꢀ259 (Ref. 12)
250ꢀ252 251ꢀ254 (Ref. 12)
198ꢀ200 195ꢀ197 (Ref. 13)
NO2
NH
3e
O
HN
NH
Cl
O
3
f
O
Cl HN
NH
Cl
O
3g
8

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8
O
82
80
219ꢀ221 221ꢀ222 (Ref. 15)
HN
O
NH
Cl
3h
9
O
90
84
75
60
254ꢀ256 252ꢀ254 (Ref. 13)
224ꢀ226 226ꢀ228 (Ref. 15)
220ꢀ222 223ꢀ225 (Ref. 15)
HN
NH
Br
O
3i
1
0
O
80
HN
O
NH
Br
3j
1
1
O
100
Me HN
NH
O
3
k
9

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1
2
O
79
90
242ꢀ244 242ꢀ243 (Ref. 13)
HN
O
NH
Me
3l
1
3
O
90
50
192ꢀ193 194ꢀ196 (Ref. 15)
HN
O
NH
O
3m
1
4
O
71
100
217ꢀ219 216ꢀ218 (Ref. 10)
HN
H
NH
O
3n
1
5
O
52
130
200ꢀ202 199ꢀ201 (Ref. 14)
HN
NH
O
3
o
1
0

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1
1
1
6
7
8
O
90
60
80
60
270ꢀ272 270ꢀ272 (Ref. 10)
HN
O
Me
NH
O N
2
Me
3
HN
O
p
O
82
243ꢀ245 244ꢀ246 (Ref. 10)
Me
NH
Br
Me
3q
O
85
230ꢀ232
231ꢀ233 (Ref. 6)
HN
O
Me
NH
Me
NO2
3r
a
Isolated yields.
The recycling of NiFe O @SiO –PPA was investigated using the synthesis of 3d. After
2
4
2
completion of the reaction, the nanomagnetic catalyst was separated from the reaction mixture by
employing an external magnetic field, washed with acetone and dried at 100 °C for 2h. It was reꢀ
used directly in the model reaction to give 3d in yields of 92%, 91%, 91%, 90%, 88%, 85%,
85%, 83%, 82%, and 80% for ten consecutive runs after 50 min.
1
1

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The FTꢀIR spectra of catalyst before use (fresh) and after reuse ten times (recovered) were
studied. As shown in Fig. 1, the FTꢀIR spectrum of the recovered nanocatalyst showed which the
structure of catalyst remained almost the same after tenꢀrun reuse.
Fig. 1. FTꢀIR spectrum of (a) fresh and (b) recovered (after ten times) NiFe O @SiO –PPA.
2
4
2
As shown in Table 2, the aromatic aldehydes with electronꢀwithdrawing groups reacted faster
than the aromatic aldehydes with electrondontaing groups. This observation can be rationalized
on the basis of the mechanism in Scheme 3. The aldehyde is first activated by NiFe O @SiO –
2
4
2
PPA. Nucleophilic addition of amides to activated aldehyde followed by loss of H O generates
2
intermediate I, which is further activated NiFe O @SiO –PPA. Then, nucleophilic addition of a
2
4
2
1
2

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second molecule of amide to activated intermediate I, affords the symmetrical N,N′ꢀalkylidine
bisamide products (3a-r).
NiFe O @SiO -PPA
2
4
2
O
R
O
R
O
OH R1
H+
H
NH2
H+
H
O
R1
R1
HO
N
H
N
-
H O
2
H
R
R
[
I]
1
a-r
O
H+
R1
NH2
O
R
O
R1
N
H
N
R1
H
3a-r
Scheme 3. Proposed mechanism
To test the worth of the present work in comparison with results in the literature, we
compared results of NiFe O @SiO –PPA with other applied Lewis and Bronsted acids in
2
4
2
synthesis of N,N′ꢀalkylidene bisꢀamides. These data, which are shown in Table 3, revealed that
NiFe O @SiO –PPA is a better catalyst than most of the conventional catalysts mentioned with
2
4
2
respect to reaction times and yields of the obtained products.
1
3

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Table 3 Comparison of catalytic activity of NiFe O @SiO –PPA with known catalysts in the synthesis of
2
4
2
N,N′ꢀalkylidene bisꢀamides
Entry N,N′ꢀAlkylidene bisꢀAmides Conditions
Time
Yield
(%)
(
min)
90
0
7
O
SiO ꢀBaCl /100 C/solventꢀfree
91
65
71
92
2
2
8
B(OH) /toluene/reflux
3
HN
O
1
1
800
200
9
PWA//toluene/reflux
3
b
NH
O N
2
Present work
4
0
8
O
B(OH)
SiO
3
/toluene/reflux
2880
120
70
65
74
82
86
0
12
HN
2
ꢀMgCl
2
/100 C/solventꢀfree
3
f
15
NH
H [NaP W MoO110]/methanol/reflux
14 5 29
Cl
O
Present work
60
0
5
O
Sulfamic acid/125 C/SolventꢀFree
29MoO110]/methanol/reflux
Present work
1080
70
68
82
85
15
3
r
HN
H14[NaP W
5
Me
Me
NH
60
O
NO2
Although the use of NiFe O @SiO ꢀPPA clearly shows that reaction times are better than
2
4
2
some of the other methods in literature, the novelty and value of our results are principally
related to comparison between homogeneous polyphosphoric acid (PPA) with heterogeneous
novel NiFe O @SiO –PPA. Polyphosphoric acid (PPA) suffers from drawbacks such as a) nonꢀ
2
4
2
reusability and difficult separation, b) necessity to neutralize the reaction mixtures prior to
1
4

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product isolation c) low stability, d) higher cost, e) higher toxicity and f) sensitivity to moisture
and air. NiFe O @SiO ꢀPPA is easily separated by employing an external magnet and may be reꢀ
2
4
2
used without loss of activity. It is very stable, highly efficient due to its greater surface area and
inexpensive because of the reꢀusability. Additionally, for the first time, a nanomagnetically
separable catalyst (NiFe O @SiO ꢀPPA) has been used for the synthesis of N,N′ꢀalkylidene bisꢀ
2
4
2
amides. Finally, we believe that the above points are the major advantages of our work compared
to previous reports.
The electrochemical characteristics of Ag@PM-b-B/GCE
The oxidation–reduction behavior of Ag NPs on the surface of the GCE was estimated to attain
direct and certain evidence for the formation of these nanoparticles. Cyclic voltammograms were
recorded at the bare GCE, PMꢀbꢀB/GCE, and Ag@PMꢀbꢀB/GCE in phosphate buffer solution
(PBS, pH 7.0), respectively, as shown in Fig. 2. As expected, no redox peaks were observed in
the absence of Ag NPs. In contrast, when the Ag NPs were electrochemically deposited on the
PMꢀbꢀB modified glassy carbon electrode surface, a clear anodic peak with a peak potential of
+0.08 V vs. Ag|AgCl|KCl (3 M), which was the characteristic oxidation peak of Ag
nanoparticles and a small reduction peak at potential of about ꢀ0.1 V vs. Ag|AgCl|KCl (3 M),
+
which was related to the reduction of Ag cations, appeared due to the excellent behavior of Ag
NPs for the electronꢀtransfer process.
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Fig. 2. Cyclic voltammogram of bare GCE (a), PMꢀbꢀB/GCE (b) and Ag@PMꢀbꢀB/GCE (c) in
ꢀ1
0.1 M PBS (pH 7.0) at scan rate 100 mV s
The microꢀmorphology of Ag@PMꢀbꢀB film has been visualized by SEM images. As shown
in Fig. 3a, the PMꢀbꢀB on the substrate presents a smooth and featureless morphology. After the
electrodeposition of Ag NPs, it is obviously seen that many Ag nanoparticles are distributed and
attached to the surface of the PMꢀbꢀB (Fig. 3b). As can be seen, the Ag film was composed of
about 40 nm Ag NPs, and some Ag NPs assembled to large microspheres, which proves that the
Ag NPs can definitely deposit on the PMꢀbꢀB modified surface and the Ag@PMꢀbꢀB interface is
favorable for electron transfer and signal amplification.
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Fig. 3. The SEM images of (a) PMꢀbꢀB/GCE and (b) Ag@PMꢀbꢀB/GCE.
Electrocatalytic oxidation of glucose
The electrocatalytic performance of the bare GCE, PMꢀbꢀB/GCE and Ag@PMꢀbꢀB/GCE toward
the oxidation of glucose was investigated by CV. Fig. 4 describes the cyclic voltammograms of
glucose (1 mM) on the bare GCE, PMꢀbꢀB/GCE and Ag@ PMꢀbꢀB/GCE in 0.1 M NaOH,
respectively. The bare GCE (curve a) showed no electrochemical response in the presence of
glucose, while a typical board oxidation peak was observed for PMꢀbꢀB/GCE (curve b) in the
potential range from 0.4 to 1.15 V. However, after adding 1 mM glucose, Ag@PMꢀbꢀB/GCE
(curve c) shows a notable catalytic current peak about 187 µA in intensity at 0.685 V. The
excellent anodic peak current of glucose at surface of Ag@PMꢀbꢀB nanocomposite confirm that
Ag@PMꢀbꢀB have high catalytic ability for glucose oxidation. The porous structure of Ag NPs
on PMꢀbꢀB provide extra proper surface for improvement of their catalytic performance. All the
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above explanations prove that Ag NPs contained in the nanocomposites present a remarkable
catalytic performance for glucose oxidation.
Fig. 4. Cyclic voltammogram of Ag@PMꢀbꢀB/GCE in absence of glucose (a) and cyclic
voltammograms of 1 mM glucose in 0.1 M NaOH at the surface of different electrodes: bare
ꢀ1
GCE (b), PMꢀbꢀB/GCE (c) and Ag@PMꢀbꢀB/GCE (d) at scan rate of 100 mV s .
Amperometric response of the Ag@PM-b-B/GCE to glucose
Fig. 5A displays typical amperometric response of the Ag@PMꢀbꢀB/GCE to subsequent addition
of glucose in alkaline solution at +0.7 V. It is perceived that the modified electrode responds
quickly to the change of glucose concentration. With the successive additions of glucose, the
response increases immediately and reaches 95% of the steady state value within 5 s. The
corresponding calibration curve for the glucose sensor is shown in Fig. 5B. The sensor presents a
linear range of 3ꢀ2000 µM with a detection limit of 1.4 µM (S/N = 3). Comparison of analytic
performance for Ag@PMꢀbꢀB/GCE with other reported nonenzymatic glucose sensors is given
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in Table 4. It can be observed that the performance of the present sensor is comparable or better
than the other reported sensors. This may be related to the higher specific surface area of the
deposited Ag nanoparticles which provides large surfaceꢀtoꢀvolume ratio and enhances active
sites for the oxidation of glucose.
Fig. 5. (A) Typical amperometric response of the Ag@PMꢀbꢀB/GCE on successive injection of
glucose into the stirring 0.1 M NaOH, applied potential: +0.7 V. (B) Calibration curve of glucose
versus its concentration.
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Table 4 Comparison of analytic performance for Ag@PMꢀbꢀB/GCE with other reported
nonenzymatic glucose sensors
Type of electrode
Detection limit (ꢁM)
190
Linear range (mM)
0.0002–2.5
Lit.
Pdꢀdoped copper oxide nanofibers
CuꢀCo NSs
Ref. 34
Ref. 35
Ref. 36
Ref. 37
Ref. 38
This work
10
5
0.015–1.21
0.010–3.2
N/A
NiꢀCu/TiO NTs
2
5% NiO@Ag NWs
1.01
4
Ti/TiO nanotube array/Ni
0.1–1.7
0.003–2
2
Ag@PMꢀbꢀB
1.4
NWs: nanowires, NSs: nanostructures, NTs: nanotube arrays.
Sensor stability and interference
The stability of Ag@PMꢀbꢀB/GCE was evaluated by amperometric measurement under a
constant potential of +0.7 V for 1000 s. Fig. 6A shows relationship of the current responses and
time for fabricated sensor. After 1000 s, Ag@PMꢀbꢀB/GCE keeps 76.44% quantity of initial
current values, representing the high electrocatalytic stability and excellent catalytic properties of
◦
Ag@PMꢀbꢀB/GCE. When Ag@PMꢀbꢀB/GCE was stored at 4 C, further repeated tests about the
stability of the sensor were carried out every ten days in one month. The remained current
response to 100 µM glucose is shown in Fig. 6B. The Ag@PMꢀbꢀB/GCE keeps at least 90% of
the initial response in the continuous tests in one month, proposing the longꢀterm stability of the
electrode. The reproducibility of the sensor was also examined. Five different modified
electrodes are established independently for the amperometric response of glucose, giving a
relative standard deviation (R.S.D.) less than 7%. These results approve that the prepared sensor
is highly reproducible and stable.
The effect of common interfering species on Ag@PMꢀbꢀB/GCE was studied. Fig. 6C shows
the amperometric response of the modified electrode towards the successive addition of 100 µM
glucose and this is followed by ascorbic acid, ethanol, glycine and uric acid into 0.1 M NaOH
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solution. As seen, the current of these interfering substances does not reveal an obvious
additional signal which indicates that proposed sensor has a good selectivity towards glucose.
Fig. 6. (A) The amperometric response for the oxidation of 100 µM glucose in 0.1 M NaOH
solution at +0.7 V at the surface of Ag@PMꢀbꢀB/GCE. (B) Stability of the fabricated sensor to
glucose tested every ten days by amperometric measurements in one month. (C) Amperometric
response of Ag@PMꢀbꢀB/GCE upon the successive addition of glucose (100 µM), ascorbic acid
(500 µM), ethanol (300 µM), glycine (300 µM) and uric acid (500 µM) into 0.1 M NaOH
solution with an applied potential +0.7 V under a stirring condition.
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Experimental Section
All reagents were obtained from commercial sources and were used without purification. IR
spectra were recorded as KBr pellets on a Shimadzu 435ꢀUꢀ04 spectrophotometer. SEM images
were recorded on a Hitachi S4160 instrument. Electrochemical experiment including cyclic
voltammetry (CV) was recorded with a Metrohm (797 VA Computrace, Switzerland) controlled
by a personal computer. The measurements were carried out using a conventional three electrode
cell using an Ag|AgCl|KCl (3 M) electrode as the reference and a Pt wire as the counter
2
(
auxiliary) electrode. A glassy carbon electrode with a geometrical area of 0.0314 cm , bare or
1
13
modified, was used as working electrode. H and C NMR spectra were determined on a Bruker
DRXꢀ300 Avance spectrometer in DMSOꢀd or CDCl , and shifts are given in δ downfield from
6
3
tetramethylsilane (TMS) as an internal standard. Melting points were determined using an
Electrothermal 9200 apparatus and are uncorrected.
1. General Procedure for the Synthesis of Symmetrical N,N′-Alkylidene bis-Amides
The catalyst, NiFe O @SiO –PPA (0.1 g), was added to a mixture of the aldehyde (1 mmol), the
2
4
2
amide (2 mmol), and 1 mL of methanol in a 5 mL flask fitted with a reflux condenser. The
resulting mixture was heated to reflux (an oil bath) for the appropriate time (see Table 2) with
stirring (spin bar). After the completion of the reaction as determined by TLC (nꢀhexaneꢀethyl
acetate, 2:1), the nanomagnetic catalyst was separated from the reaction mixture by employing
an external magnetic field. The resulting crude product was poured into crushed ice and the solid
product, which separated was filtered and recrystallized from ethanol 96% (2 ml) to afford the
pure symmetrical N,N′ꢀalkylidene bisꢀamide (3a-r).
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2. Preparation of Ag@PM-b-B modified GCE (Ag@PM-b-B/GCE)
The glassy carbon electrode (GCE) was first successively polished using alumina slurries, rinsed
with doubly distilled water in ultrasonic bath for 3 min. Finally, the electrode was rinsed with 1:1
ethanol, 1:1 nitric acid, and doubly distilled water in ultrasonic bath, respectively. The cleaned
GCE was dried at ambient temperature before the modification. 0.5 mg PMꢀbꢀB was sonicated in
1
mL distilled water to obtain white suspension. 10 µL of the PMꢀbꢀB suspension was dropped
on the surface of the clean GCE. After being dried in air, the PMꢀbꢀB functionalized GCE (PMꢀ
bꢀB/GCE) was immersed in the 1 mM AgNO solution. The synthesis of Ag NPs was performed
3
by the electrodeposition on the PMꢀbꢀB/GCE at 0.0 V for 120 s in 1 mM AgNO solution
3
(deaerated by bubbling with nitrogen).
Analytical Data for Selected Compounds.
N,N′-(3-Bromophenylmethylene)dibenzamide (3j): IR: 3280, 1675, 1540, 1510, 1480, 1330,
1
1
280, 1080, 715, 690; H NMR (300 MHz, DMSOꢀd ): δ 6.94 (t, 1H, CH), 7.31ꢀ7.42 (m, 6H),
6
1
3
7
.51ꢀ7.60 (m, 5H), 8.09 (d, 2H), 7.82 (s, 1H), 7.94 (d, 2H, NH, D O exchangeable); C NMR
2
(
75 MHz, DMSOꢀd ): δ 59.31, 114.70, 127.65, 128.08, 128.71, 128.91, 129.91, 132.31, 133.72,
6
134.91, 159.31, 166.61.
N,N′-(3-Phenoxyphenylmethylene)dibenzamide (3m): IR: 3310, 1705, 1580, 1540, 1480,
1
1
340, 1210, 710; H NMR (300 MHz, DMSOꢀd ): δ 6.91 (t, 1H, CH), 6.97ꢀ7.14 (m, 5H), 7.22ꢀ
6
1
3
7
.24 (d, 2H), 7.33ꢀ7.57 (m, 9H), 7.86ꢀ7.98 (d, 4H), 9.00 (d, 2H, NH, D O exchangeable); C
2
NMR (75 MHz, DMSOꢀd ): δ δ 58.46, 116.49, 117.50, 118.71, 121.58, 123.54, 127.46, 128.28,
6
129.93, 129.99, 131.56, 133.73, 133.77, 142.53, 156.29, 156.70, 165.59, 165.67.
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Conclusion
In conclusion, NiFe O @SiO –PPA, was found to be an efficient magnetically catalyst for
2
4
2
the reaction aldehydes with amides to afford the corresponding product in high to excellent
yields. These catalysts have several advantages over homogeneous acid catalysts; they are not
corrosive, are environmentally benign, present fewer disposal problems, are removed easily and
are thermally stable. The catalyst can be easily isolated from the reaction mixture by simple
magnetic separation, and it can be used in six reaction cycles without any significant loss of
activity. Furthermore, a new and facile Ag@PMꢀbꢀB nanocomposite electrochemical sensor was
successfully developed for the detection of glucose, which used the advantages of the high
conductivity, large specific surface area and excellent electrochemical activity of Ag NPs. The
proposed sensor showed rapid electrochemical detection of glucose and will have the potential
applications in clinical diagnostics and biotechnology.
Acknowledgements. The author thanks the Research Council of Hakim Sabzevari University for
partial support of this work.
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