# Silver Nanoparticles Confined in SBA-15 Mesoporous Silica and the Application as a Sensor for Detecting Hydrogen Peroxide

Authors: Dong-Hai Lin; Yan-Xia Jiang; Ying Wang; Shi-Gang Sun
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Silver nanoparticles within the pore channels of selectively grafted mesoporous silica SBA-15 were synthesized. Silanols on the external surface of as-SBA-15 were first capped by –Si(CH3)3 groups. After removal of the template of capped SBA-15 by calcination, silanols on the internal surface of SBA-15 were modified by 3-aminopropyltrimethoxysilane (APTMS), and then formaldehyde was grafted by amino groups of APTMS, and further Ag(NH3)2NO3SBA-15). High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), nitrogen adsorption/desorption isotherms,  and UV-vis spectra confirm that the silver nanoparticles have been confined inside the channels of SBA-15. In addition, the Ag-mSBA-15 modified electrode (Ag-mSBA-15/GC) exhibited an excellent electrocatalytic activity toward the reduction of hydrogen peroxide (H2O2). The proposed H2O2 sensor exhibits a linear range of 48.5 μM–0.97 M with a detection limit of 12 μM (S/N=3
) and analytical time of 10 seconds per sample.


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## Body

## 1. Introduction

Recently, the discovery of
mesoporous silicas, such as M41s [1] and SBA-15 [2], has stimulated intensive studies
of “host-guest” chemistry inside the channels of mesoporous silicas [3–6],
which have potential applications in catalysis, selective adsorbents, medical [7],
sensors [8, 9], and nanomaterials fabrications. Thanks to their uniform
mesostructures, high surface areas, and tunable pore sizes [2], these ordered
mesoporous silicas have been used as the promising templates to control the
shape and size of metal nanoparticles [10–16]. Many published works gave the
relation between nanoparticles confined in various molecular sieves and their
properties [17–19]. Moreover, the surface of these ordered mesoporous silicas
are also modified for many potential applications. For modifying the mesoporous
materials through covalent linkage between functional groups and silica
framework, two major methods, grafting (post synthesis) and cocondensation
(direct incorporation), have been traditionally explored [20]. Grafting is one
of the modification methods for presynthesized mesoporous silica, in which the
organic functional groups can be introduced by direct reaction of organosilanes
to silica surface. It is up to the application of thus formed materials, other
functionalities can be fixed to the previously introduced functional groups including
amino, thiol, and alkyl groups through covalent bonding and/or molecular
recognition [20–23]. The distribution and concentration of functional groups
are influenced by reactivity of the organosilane and their accessibility to
surface silanols, which are limited by diffusion and steric factors. Chao et al. [6, 11] prepared SBA-15 functionalized with (CH3O)3Si(CH2)3N(CH3)3Cl
(TPTAC) and further synthesized metal nanoparticles by anion exchange between
grafted SBA-15 and metal precursors inside the channels as well as upon
reduction of precursors. The amount of metal loading as well as the morphology of
metal in host SBA-15 can be rationally controlled through repeating
ion-exchange/reduction cycles in the TPTAC-SBA-15 silica host. They used the
same method to prepare Au nanoparticles and found that the size and morphology
of Au nanoparticles in mesoporous SBA-15 are controllable by the preparation methods.However, above-mentioned grafting
methods basically allow introduction of functionalization at both intrapore and
extrapore media, which led the nanoparticles form at both surfaces. Therefore,
large metal particles aggregate would form on the external surface of the host
materials. To overcome this disadvantage, Shi et al. [4] synthesized Pt
nanoclusters within the pore channels of selectively modified mesoporous silica
SBA-15 by a new in situ reduction process. The silanols on the external surface
of SBA-15 were capped with –Si(CH3)3 groups, thus
effectively avoiding the formation of large particles outside the channels. On
the other hand, the inner surface of the channel was functionalized with highly
reducing Si–H bonds. Pt nanoclusters were formed inside the channels of SBA-15
from H2PtCl6 by in situ reduction with Si–H bonds. Recently,
Sun et al. [3] have developed a novel in situ autoreduction route to synthesize
monodispersed silver nanoparticles inside the channels of SBA-15. It was
demonstrated for the first time by the 13C CP/MAS NMR spectroscopy that amino
groups of aminopropyltriethoxyl silane (APTS) modified mesoporous silica can be
used to anchor formaldehyde to form NHCH2OH species, on which Ag(NH3)2NO3 could be in situ reduced. The silver nanoparticles confined inside the channels
of SBA-15 resulted in an unusual thermal stability.Nanoparticles confined in SBA-15
silica possess high catalytic activity and stability thanks to the
stabilizer-free and confined. Jiang et al. [24] found that the Pt nanoparticles
confined in SBA-15 exhibit a high electrocatalytic activity toward the
oxidation of carbon monoxide (CO) and methanol, and the linearly adsorbed CO
species is the only intermediate derived from dissociative adsorption of
methanol, which is more readily oxidized to form CO2 in the aid of the active oxide in SBA-15.Rapid, accurate, reliable, and
reagentless determination of hydrogen peroxide (H2O2) is
of great importance in food, clinical, pharmaceutical, industrial, and
environmental analysis. Many analytical techniques have been reported for determination
of hydrogen peroxide, for example, chemiluminescence [25], titrimetry [26], and
spectrophotometry [25, 27]. Electrochemistry is an inexpensive and effective
way to examine the reactions of many substances [27, 28]. Amperometric sensors
are especially attractive because of their simplicity and high sensitivity. By
now horseradish peroxidase (HRP), one of the most studied members of the family
of heme enzymes, was shown to catalyze the reaction of nonmediated cathodic reduction
of H2O2 resulting from direct electron transfer from the
electrode to the heme containing active site of HRP when immobilized at the electrode
surface [29, 30]. Nevertheless, these systems were found to be too complex and
the linear range for H2O2 detection is narrow.In this paper, the
stabilizer-free and confined silver nanoparticles inside the channels of
selectively grafted mesoporous silica SBA-15 by an in situ reduction process were
synthesized (see Scheme1). The catalytic activity of silver nanoparticles
confined in the mesoporous silica SBA-15 to the reduction of hydrogen peroxide
(H2O2) was studied for sensing application. The Ag-mSBA-15/GC
modified electrode, thus allowed highly sensitive amperometric detection of H2O2,
low applied potential, and a broad linear relationship with the concentration
of H2O2 over a wide range of 4.8×10−5– 9.7×10−1 M.
The new application of the stabilizer-free silver nanoparticles confined inside
the channel of SBA-15 mesoporous silica, an attractive electrocatalytic
nanomaterial for preparation of an amperometric sensor is proposed.Scheme 1
Schematic representation of synthesis of Ag-mSBA-15.


## 2. Experimental

### 2.1. Synthesis of Ag-mSBA-15

As illustrate by Scheme1, mesoporous silica SBA-15 was synthesized following the published procedure [2] using
the triblock copolymer Pluronic P123 as a template in acid conditions.
Typically, a 6.4 g Pluronic P123 template was dissolved with stirring in a solution of 250 mL of 2 M HCl at 313 K, and 13.6 g of tetraethyl orthosilicate (TEOS) was
then added. The resulting mixture was stirred at 313 K for 20 hours, and then
aged at 373 K for 24 hours under static condition. The recovered solid was
extensively washed with deionized water and drying at 353 K for 12 hours yielded
as-SBA-15. To get calcinated SBA-15 (cal-SBA-15), the surfactant template of as-SBA-15 was removed by calcination in air at 823 K for 6 hours.Ag-mSBA-15 was prepared
according to the literature procedure [3]. 2.0 g as-SBA-15 was dispersed
in 150 mL dry toluene under flowing N2, and then 10 mL trimethylchlorosilane
(TMCS) was added dropwise under stirring. The mixture was filtered with toluene
and ethanol after stirring continuously at 353 K for 8 hours. After that, the
surfactant template was removed by calcination in air at 823 K for 6 hours.
Thus, SBA-15 with the external surface capped with –Si(CH3)3 (named cal-TMCS-SBA-15) was collected.After having been outgassed at 353
K for 12 hours. 2.0 g cal-TMCS-SBA-15
was suspended in 150 mL dry toluene, and then 6.0 mL 3-aminopropyltrimethoxysilane
(APTMS) was added under stirring. The mixture obtained was stirred for another
12 hours at room temperature and refluxed at 353 K for 8 hours. The solid was obtained
after washing with toluene and then with ethanol intensively to eliminate the
physically adsorbed APTMS and toluene. The selectively modified sample after
being vacuum-dried at 353 K for 6 hours was labeled APTMS-TMCS-SBA-15.To introduce reducer of
formaldehyde into the channels, 1.0 g APTMS-TMCS-SBA-15 was soaked in a 105 mL mixture of formaldehyde, ethanol and
water (formaldehyde/ethanol/water, 5 : 20 : 80, v/v/v), and the suspension was
stirred at 313 K for 30 minutes. The product was filtered, rinsed with deionized
water and dried at 323 K for 12 hours and denoted as HCHO-APTMS-TMCS-SBA-15.For Ag incorporation, 1.0 g HCHO-APTMS-TMCS-SBA-15 was added into a
mixture of ethanol and 0.01 M Ag(NH3)2NO3 (aq) (1 : 4, v/v), and then the mixture
was stirred at 313 K for 30 minutes. The product was filtered and rinsed thoroughly
with deionized water and dried under vacuum at 323 K overnight, and thus formed
confined silver nanoparticles were noted as Ag-mSBA-15.In the control experiment, an
aqueous solution of 1 mM NaBH4 (70 mL) was cooled with ice and then 1 mM AgNO3 aqueous solution (100 mL) was added to it under vigorous
stirring, resulting in the light-brown Ag colloidal solution [31]. The product
was filtered, rinsed with deionized water, dried and denoted as nm-Ag.


### 2.2. Characterization Methods

High-resolution transmission
electron microscopy (HRTEM) images were obtained on instruments of FEI
Tecnai-F30 electron microscopy operating at
300 KV. The powder samples were characterized by powder X-ray diffraction (XRD)
using a Panalytical X'pert PRO diffractormeter (Tokyo, Japan)
equipped with graphite monochromatized Cu Kalpha radiation. Surface
functionalization was monitored by Fourier transformation infrared spectroscopy
(FTIR) using a Nicolet FTIR 340 spectrometer. The pore diameter, pore volume,
and surface area of the samples were derived from the nitrogen sorption
isotherm at 77 K using a Micromeritics TriStar 3000 system. Prior to measurements,
the sample was evacuated at 393 K for 5 hours. The UV-vis absorption was
recorded on a Shimadzu UV-2100 spectrometer.The material Ag-mSBA-15
was dispersed through ultrasonic vibration in a solution of dichlorethane containing
polyvinyl chloride (PVC) to form a suspension. A defined quantity of the
suspension was applied to a clean surface of glassy carbon (GC) substrate to
form a thin film electrode, noted as Ag-mSBA-15/GC, then dried in
the air for about 30 minutes. The solutions were prepared with Millipore water
and chemicals of analytical grade. A saturated calomel electrode was used as
reference electrode, and all electrochemical experiments were carried out at
room temperature around 293 K. A CHI-660C potentiostat/galvanostat (Chenhua Instruments, Inc., Shanghai, China)
was used in electrochemical studies.


## 2.1. Synthesis of Ag-mSBA-15

As illustrate by Scheme1, mesoporous silica SBA-15 was synthesized following the published procedure [2] using
the triblock copolymer Pluronic P123 as a template in acid conditions.
Typically, a 6.4 g Pluronic P123 template was dissolved with stirring in a solution of 250 mL of 2 M HCl at 313 K, and 13.6 g of tetraethyl orthosilicate (TEOS) was
then added. The resulting mixture was stirred at 313 K for 20 hours, and then
aged at 373 K for 24 hours under static condition. The recovered solid was
extensively washed with deionized water and drying at 353 K for 12 hours yielded
as-SBA-15. To get calcinated SBA-15 (cal-SBA-15), the surfactant template of as-SBA-15 was removed by calcination in air at 823 K for 6 hours.Ag-mSBA-15 was prepared
according to the literature procedure [3]. 2.0 g as-SBA-15 was dispersed
in 150 mL dry toluene under flowing N2, and then 10 mL trimethylchlorosilane
(TMCS) was added dropwise under stirring. The mixture was filtered with toluene
and ethanol after stirring continuously at 353 K for 8 hours. After that, the
surfactant template was removed by calcination in air at 823 K for 6 hours.
Thus, SBA-15 with the external surface capped with –Si(CH3)3 (named cal-TMCS-SBA-15) was collected.After having been outgassed at 353
K for 12 hours. 2.0 g cal-TMCS-SBA-15
was suspended in 150 mL dry toluene, and then 6.0 mL 3-aminopropyltrimethoxysilane
(APTMS) was added under stirring. The mixture obtained was stirred for another
12 hours at room temperature and refluxed at 353 K for 8 hours. The solid was obtained
after washing with toluene and then with ethanol intensively to eliminate the
physically adsorbed APTMS and toluene. The selectively modified sample after
being vacuum-dried at 353 K for 6 hours was labeled APTMS-TMCS-SBA-15.To introduce reducer of
formaldehyde into the channels, 1.0 g APTMS-TMCS-SBA-15 was soaked in a 105 mL mixture of formaldehyde, ethanol and
water (formaldehyde/ethanol/water, 5 : 20 : 80, v/v/v), and the suspension was
stirred at 313 K for 30 minutes. The product was filtered, rinsed with deionized
water and dried at 323 K for 12 hours and denoted as HCHO-APTMS-TMCS-SBA-15.For Ag incorporation, 1.0 g HCHO-APTMS-TMCS-SBA-15 was added into a
mixture of ethanol and 0.01 M Ag(NH3)2NO3 (aq) (1 : 4, v/v), and then the mixture
was stirred at 313 K for 30 minutes. The product was filtered and rinsed thoroughly
with deionized water and dried under vacuum at 323 K overnight, and thus formed
confined silver nanoparticles were noted as Ag-mSBA-15.In the control experiment, an
aqueous solution of 1 mM NaBH4 (70 mL) was cooled with ice and then 1 mM AgNO3 aqueous solution (100 mL) was added to it under vigorous
stirring, resulting in the light-brown Ag colloidal solution [31]. The product
was filtered, rinsed with deionized water, dried and denoted as nm-Ag.


## 2.2. Characterization Methods

High-resolution transmission
electron microscopy (HRTEM) images were obtained on instruments of FEI
Tecnai-F30 electron microscopy operating at
300 KV. The powder samples were characterized by powder X-ray diffraction (XRD)
using a Panalytical X'pert PRO diffractormeter (Tokyo, Japan)
equipped with graphite monochromatized Cu Kalpha radiation. Surface
functionalization was monitored by Fourier transformation infrared spectroscopy
(FTIR) using a Nicolet FTIR 340 spectrometer. The pore diameter, pore volume,
and surface area of the samples were derived from the nitrogen sorption
isotherm at 77 K using a Micromeritics TriStar 3000 system. Prior to measurements,
the sample was evacuated at 393 K for 5 hours. The UV-vis absorption was
recorded on a Shimadzu UV-2100 spectrometer.The material Ag-mSBA-15
was dispersed through ultrasonic vibration in a solution of dichlorethane containing
polyvinyl chloride (PVC) to form a suspension. A defined quantity of the
suspension was applied to a clean surface of glassy carbon (GC) substrate to
form a thin film electrode, noted as Ag-mSBA-15/GC, then dried in
the air for about 30 minutes. The solutions were prepared with Millipore water
and chemicals of analytical grade. A saturated calomel electrode was used as
reference electrode, and all electrochemical experiments were carried out at
room temperature around 293 K. A CHI-660C potentiostat/galvanostat (Chenhua Instruments, Inc., Shanghai, China)
was used in electrochemical studies.


## 3. Results and Discussion

### 3.1. Characterization

#### 3.1.1. HRTEM Studies

In our synthesis procedures: (1)
before removing the template, the external surface of the SBA-15 was firstly
capped by trimethylchlorosilane (TMCS); (2) after removing the template, 3-aminopropyltrimethoxysilane
(APTMS) molecules were introduced into the channels of SBA-15, which leads to
that the amino functional groups were grafted to the internal walls of SBA-15 channels;
(3) the reducers of formaldehyde were then introduced into the channels of
SBA-15 and grafted on the amino groups inside the channel; (4) the metal precursors
of Ag(NH3)2NO3 were finally introduced into the channels of SBA-15 by diffusion
when the modified SBA-15 is put in a solution containing Ag(NH3)2NO3. The
silver (0) nanoparticles were, therefore, formed through the reduction of Ag (I)
ions by reducers previously fixed on the internal walls of the SBA-15 channels.
Theoretically, Ag nanoparticles prepared by this method were located inside the
channel between the walls in the method. The
silver nanoparticles assembled inside the channels of SBA-15 can be further
confirmed by HRTEM (Figure 1). Figure
1(a) shows the HRTEM of calcinated SBA-15 (cal-SBA-15), in which well-ordered
channels are illustrated and are characteristic of mesoporous materials. Figure
1(b) depicts the HRTEM of Ag-mSBA-15, where the highly ordered pore
structure of SBA-15 is still preserved. We can observe clearly that silver
nanoparticles appear as dark spherical objects between the walls of SBA-15 and
are homogeneously distributed inside the channels of SBA-15. It can be also seen
that the size of Ag is slightly larger than the channel, which leads to
the distortion of the channels.HRTEM images of (a) cal-SBA-15, (b) Ag-mSBA-15.
(a)(b)


#### 3.1.2. XRD Studies

Figure2(a) gives a small-angle
XRD pattern of calcinated SBA-15(cal-SBA-15), three diffraction peaks
appear in the spectrum, which are attributed to the characteristic diffraction
peaks of (100), (110), and (200) for SBA-15, respectively, due to typical
hexangular phase [2, 24]. After reaction with APTMS, the small-angle XRD pattern of APTMS-TMCS-SBA-15 (Figure
2(b)) still shows the characteristic diffraction peaks, which exhibits that the
grafting of amino groups inside the channels did not affect the long-range
ordering of the mesostructures. Compared with cal-SBA-15, the peaks shift to
lower angles, and such shift may imply the enlargement of the frameworks
[32]. The little negative shifting may be aroused
by the covalent linkage between APTMS and hydroxy in the channels. We know from
BET data in Table 1  the DBJH value is almost constant during the
covalent linkage, but DBJH value has an obviously decrease after Ag
nanoparticles were assembled. In addition, after the loading of Ag into APTMS-TMCS-SBA-15 (assigned
as Ag-mSBA-15), the peaks (Figure 2(c)) have shifted positively
comparing with that of APTMS-TMCS-SBA-15, the intensity of the peaks has
decreased to a certain extent, while the full width at half-maximum (FWHM) of
the peaks that is normalized by height has increased. These three features are
attributed to contraction
of framework during the support treatment, implying that Ag nanoparticles have
been introduced into the channel of SBA-15 successfully as discussed elsewhere
[24, 33]. The inset of Figure 2 is wide-angle XRD of Ag-mSBA-15. The
broadening of the Ag diffraction peaks suggests that the size of Ag
nanoparticles is in the nanometer range [8]. The average Ag particle size was
estimated to be 8.0 nm, from the peak width of Ag (220) reflection by using Scherrer’s
equation for approximation [34].Table 1
Physicochemical properties of the samples.SBET, BET specific surface area; Vt, total pore
volume; DBJH, pore diameter calculated using BJH method; d100, periodicity of host
SBA-15 derived from XRD. The wall thicknesses, t, was calculated as A0-pore size (A0 = 2d(100)/3).
SampleSBET/m2g−1Vt/cm3g−1DBJH/nmd100/nmA0/nmt/nmas-SBA-152620.416.0010.9212.616.61cal-SBA-157031.026.5410.1011.695.15cal-TMCS-SBA-157051.046.5710.1311.705.13APTMS-TMCS-SBA-154190.686.4910.1511.725.34HCHO-APTMS-TMCS-SBA-153350.485.869.7911.305.44Ag-mSBA-152320.305.0510.1111.676.62Figure 2
Small-angle XRD patterns of (a) cal-SBA-15, (b) APTMS-TMCS-SBA-15,
and (c) Ag-mSBA-15. Inset is the wide-angle XRD pattern of Ag-mSBA-15.


#### 3.1.3. FTIR Studies

FTIR spectroscopy can provide surface information of materials for identification of
chemical groups. Figure3(a)
presents the transmission FTIR spectrum of as-SBA-15, in which the bands in
2900–3000 cm−1 were attributed to template P123. After calcination
in air at 823 K for 6 hours
(cal-SBA-15), the bands in 2900–3000 cm−1 were disappeared (Figure
3(b)). This indicates that the template P123 was completely removed. After
modification of the outer surface of as-SBA-15 with trimethylchlorosilane (TMCS),
the sample was named as-TMCS-SBA-15. Figure 3(c) shows the FTIR spectrum of as-TMCS-SBA-15,
in which the bands in 2900–3000 cm−1 attributed to template P123 and
TMCS can be seen clearly. However, when the temple of the as-TMCS-SBA-15 has been
removed by calcination in air at 823 K for 6 hours as it was done in Figure 2(b),
we can still observe the bands in 2900–3000 cm−1 from Figure 3(d),
which can be attributed to the C–H stretching modes of –CH3 in cal-TMCS-SBA-15 [6]. This result
confirmed that the methyl groups are still present on the material cal-TMCS-SBA-15
when template P123 was completely removed at 823 K. Such stability comes from the
covalent linkage between –CH3 groups and the hydroxy in the outer
surface of silica framework.Figure 3
FTIR spectra of (a) cal-SBA-15, (b) cal-TMCS-SBA-15.The modification process of
external surface of as-SBA-15 does not affect the internal surface of the
channel because the template molecules occupy these channels during the
external surface modification. The incorporation of APTMS inside the channels of
the SBA-15 has been confirmed by transmission FTIR spectra as shown in Figure4.
The curve c is the spectrum of APTMS, and we can see the bands in the range
2900–3000 cm−1, which are attributed to the C–H stretching modes of –CH2 in APTMS. The curve a is the spectrum of cal-TMCS-SBA-15, in which we can
observe the characteristic absorption of Si–O–Si at 1082, 797, 465 cm−1 assigned to asymmetric stretching, symmetric stretching, and bend stretching
[35]. In addition, the well-known IR adsorption bands which due to the
stretching vibrational mode of surface silanol groups and water in the range 3500–3740 cm−1 [36, 37], and the C–H stretching modes of –CH3 in
the range 2900–3000 cm−1 [6] were also seen. The curve b showed the
spectrum that APTMS was introduced into the channel of cal-TMCS-SBA-15, in
which the intensity of IR absorption decrease for silanol group at 3500–3740 cm−1,
and the intensity of IR absorption increase for the C–H group at 2900–3000 cm−1.
The ratio of integrated intensity of the Si–OH band (ISi–OH)
to integrated intensity of the C–H band (IC–H), that is, ISi–OH/IC–H, can be used to evaluate the increase of –CH2 and –CH3 and decrease of silanol due to the introduction of APTMS inside the channel of
SBA-15. And the change of ISi–OH/IC–H can be used to monitor
the reaction process between Si–OH and APTMS, since the intensity of the Si–OH band
decreases and the intensity of the C–H band increases with the reaction
progressing. Here, ISi–OH/IC–H is reduced from 38.1 at cal-TMCS-SBA-15 to 16.0 at APTMS-TMCS-SBA-15. The decrease of integrated
intensity of silanol groups demonstrates an anchoring mechanism (see Scheme 1) involving
reaction between Si–OH and APTMS [38].Figure 4
FTIR spectra of (a) cal-TMCS-SBA-15, (b) APTMS-TMCS-SBA-15, and (c) APTMS.


#### 3.1.4. Nitrogen Adsorption/Desorption Isotherms

Nitrogen adsorption/desorption
isotherms for as-SBA-15, cal-SBA-15, cal-TMCS-SBA-15, APTMS-TMCS-SBA-15,
HCHO-APTMS-TMCS-SBA-15, and Ag-mSBA-15 are found to be type IV
isotherm curves with distinct hysteresis loops and steep adsorption/desorption
steps were recorded that indicate a narrow pore size distribution. This suggests
that the host silica was still maintained during the modification. Table 1
summarizes the results of N2 desorption analyses. The specific surface
area (SBET) of as-SBA-15 is 262 m2/g. After silanols on
the external surface of as-SBA-15 were capped by –Si(CH3)3 groups, and the template was removed by calcination, SBET increases
to 705 m2/g. The total pore volume (Vt), and the average
pore size (DBJH) also show an increased value. After silanols on the
internal surface of SBA-15 were grafted by 3-aminopropyltrimethoxysilane
(APTMS), SBET and Vt of APTMS-TMCS-SBA-15 decrease from
705 to 419 m2/g and from 1.04 to 0.68 cm3/g,
respectively, with a slight decrease of the DBJH from 6.57 to 6.49 nm.
A considerable decrease in the SBET, Vt, and
DBJH was measured in Ag-mSBA-15 due to the incorporation
of Ag nanoparticles. The values of SBET, Vt, and
DBJH for HCHO-APTMS-TMCS-SBA-15 are 335 m2/g, 0.48 cm3/g,
and 5.86 nm, whereas they are 232 m2/g, 0.30 cm3/g, and 5.05 nm
for Ag-mSBA-15. All these, together with the increased thickness
of pore walls (t), can be attributed to the pore-filling effect [14, 39] and
the fact demonstrated that the Ag nanoparticles have been confined inside the
channel of SBA-15 [4].The silver nanoparticles were synthesized in the
inner surface of
silica framework by the “ship-in-a-bottle” approach,
in which the channel of SBA-15 is used as a microreactor to prepare the
nanoparticles. Precursors were introduced into the channels one by one, and were assembled
into Ag nanoparticles in the microreactor. This resulted in the Ag
nanoparticles were confined inside the channel of SBA-15. Silanols on the external surface
of as-SBA-15 were first capped by –Si(CH3)3 groups. After removal of the template of capped SBA-15 by
calcination, silanols on the internal surface of SBA-15 were modified by 3-aminopropyltrimethoxysilane
(APTMS), and then formaldehyde was grafted by amino groups of APTMS to form
NHCH2OH species, on which Ag(NH3)2NO3 could
be in situ reduced into Ag nanoparticles. Without silanols on the surface of SBA-15, there will be impossible to form
NHCH2OH species, and further to produce Ag nanoparticles. So there will be no Ag nanoparticles
on the external surface of SBA-15 thanks to silanols on the
external surface first capped by –Si(CH3)3 groups to decrease its activity. DBJH of APTMS-TMCS-SBA-15
is 6.49 nm from BET data, and the average Ag nanoparticle size
was estimated to be 8.0 nm from XRD analysis. HRTEM image has also demonstrated
that the size of Ag is slightly larger than the
channel, which leads to the distortion of the channels. Therefore, we infer that the
channels are slightly distorted during the nanoparticles synthesized in the
microreactor, which makes the size of nanoparticles prepared inside the channel
of SBA-15 larger than the size of the channel diameter. As
a result, the problem of catalyst leaking is much lessened
or eliminated. Similar results have been also
reported in literature [40].


#### 3.1.5. UV-vis Absorption Spectroscopic Studies

UV-vis absorption spectra of the Ag-mSBA-15
samples may be used to provide additional evidence of the formation of Ag
nanostructures inside the channels of SBA-15 powder and reveal unique optical
properties. Figure 5(a) (line 1) shows the UV-vis absorption spectrum of HCHO-APTMS-TMCS-SBA-15, in which a spectrum with a nearly linear was recorded. Figure 5(a) (line 2) shows the
absorption spectrum of Ag-mSBA-15, in which only the peak of 420 nm which
assigned to the surface plasmon resonance (SPR) of Ag nanoparticles [16] is
observed and the peak of 345 nm assigned to small silver clusters from 2 to 8
atoms formed in micropores of silica walls [41] is absent. SPR is a characteristic
feature of metal nanoparticles between the sizes of 2 and 50 nm [42]. This
result indicates that the reduction of silver (I) ions in the micropores of the
SBA-15 does not exist, since all the reducers were introduced into the
mesopores of the SBA-15 in our experiment.(a) UV-vis spectra of (line 1) APTMS-TMCS-SBA-15 and (line 2) Ag-mSBA-15
immerged in 1 M NaOH solution, (b) Successive UV-vis spectra (taken every 1 minute) of methylene blue (MB) dye reduction, using
silica SBA-15 containing silver nanoparticles (Ag-mSBA-15) as the catalyst and NaBH4 as the reducing
agent. UV-vis spectrum of MB dye with pure silica SBA-15, using NaBH4 as the reducing
agent (dash line).
(a)(b)Moreover, silver nanoparticles
have been studied as a catalyst in reduction reactions of dyes like methylene
blue (MB) [43]. Here, reduction of MB by NaBH4 is used as a standard
for determining the catalytic activity of Ag-mSBA-15. The
preliminary catalytic testing for Ag-mSBA-15 was carried out by
reduction of MB (2 mL 10−5 M) in water using Ag-mSBA-15
(1 mg) as catalyst and NaBH4 (1 mL 10−4 M) as the
reducing agent. The progression of the catalytic reduction of MB can be easily
followed by the change of absorbance intensity at 665 nm that is absorbance
maximum (λmax) of MB.
The dot line in Figure 4(b) is the UV-vis
spectrum of the mixture containing a mixture of MB dye plus NaBH4 reducing agent, with pure silica SBA-15, which
gives a strong peak at 665 nm attributing
to λmax of MB, illustrating that MB have not been reduced. When 1 mg
Ag-mSBA-15 was added into above solution, it can be seen the band at
420 nm attributing to the SPR of Ag nanoparticles Figure 5(b) (solid line). The
absorbance at λmax of MB gradually decreases with the reaction time,
which suggests MB begin to reduce, and meanwhile we can observe the color of
solution changes from blue to colorless. Above investigations have illustrated
that silver nanoparticles exhibit a high catalytic activity to the reduction of MB.


### 3.2. Application of Ag-mSBA-15 as a Sensor for Detecting Hydrogen Peroxide

#### 3.2.1. Electrochemical Response of Ag-mSBA-15/GC Electrode to Reduction of H2O2

Figure6(a) displayed the cyclic
voltammetry (CV) of glassy carbon (GC) in 0.2 M HAc-NaAc buffer solution (pH 5)
containing 3 mM H2O2, in which a featureless CV was
observed, and the current may be mainly ascribed to the double-layer charging
of the electrode. Figure 6(b) is CV of Ag-mSBA-15/GC in 0.2 M HAc-NaAc
buffer solution (pH 5), in which a small reduction current appears that may be attributing
to reduction of support electrolyte. Figures 6(c), 6(d),
and 6(e) show the CVs of
Ag-mSBA-15/GC, nm-Ag/GC, and bulk Ag, respectively, in 0.2 M HAc-NaAc
+ 3 mM H2O2. It can be seen a wave at −0.42 V for Ag-mSBA-15/GC,
−0.65 V for nm-Ag/GC, and −0.53 V for bulk Ag appear. Ag-mSBA-15/GC,
nm-Ag/GC, and bulk Ag electrode on the initial reduction potential of H2O2 is −0.12 V, −0.15 V, and −0.17 V,
respectively. The reduction potential of H2O2 on Ag-mSBA-15/GC was 140 mV, 230 mV,
and 110 mV more negative than
that on silver nanoparticles assembles supported on GC [44], nm-Ag/GC, and bulk
Ag, indicating a lower overpotential for H2O2 reduction
at the Ag-mSBA-15/GC. Figure 7 depicts the CVs of Ag-mSBA-15/GC
in 0.2 M HAc-NaAc + 3 mM H2O2 with different scan rate. It
can be seen that the peak currents of the H2O2 reduction
increase in Ag-mSBA-15/GC in 0.2 M HAc-NaAc + 3 mM H2O2 with an increasing scan
rate. In the scan rate range from 10 mV·s−1 to 500 mV·s−1, the reduction current of H2O2 is proportional to the square root of scan rate (inset of Figure 7), which
suggests that the rate of electrochemical reaction is rather fast and the
electrode process is controlled by the diffusion of H2O2 from
solution to electrode surface. In order to determine the optimal working
potential for the H2O2 sensing, the electrochemical
response of the H2O2 was researched in different
potentials. The relationship between the steady-state current and the operating
potential in 0.2 M HAc-NaAc + 3 mM H2O2 at Ag-mSBA-15/GC
is showed in Figure 8. Considering the economy of energy, the sensitivity and
the steadiness of Ag-mSBA-15/GC, −0.45 V were chosen as optimal
working potential for H2O2 sensing with amperometry of
constant potential.Figure 6
The voltammetric response of (a) GC to 3 mMH2O2,
(b) Ag-mSBA-15/GC to 0 mM H2O2, (c) Ag-mSBA-15/GC
to 3 mM H2O2, (d) nm-Ag/GC to 3 mM H2O2,
and (e) bulk Ag to 3 mM H2O2. All experiments were
carried out in 0.2 M HAc-NaAc buffer (pH = 5) electrolyte, scan rate: 50 mV·s−1.Figure 7
The cyclic voltammograms of Ag-mSBA-15/GC
in 0.2 M HAc-NaAc buffer (pH = 5) + 3 mM H2O2, at different
scan rates (from upper to lower): 10, 25, 50, 75, 100, 150, 200, 250, 300, 350, 400,
500 mV·s−1, respectively. Inset is plot of peak
current versus v1/2.Figure 8
Influence of applied potential on amperometric
response of Ag-mSBA-15/GC in 0.2 M HAc-NaAc buffer (pH = 5)
+ 3 mM H2O2.


#### 3.2.2. Detection ofH2O2

Figure9(a) shows the dynamic
response of the Ag-mSBA-15/GC modified electrode at a working
potential of −0.45 V with successive injections of 3 mM H2O2 in 0.2 M HAc-NaAc buffer solution (pH 5). The calibration curve of Ag-mSBA-15/GC
modified electrode under the optimal working potential of −0.45 V is showed in
Figure 9(b). Under an exactly same condition, the Ag-mSBA-15/GC
modified electrode for H2O2 determination exhibits a
linear range of 4.8×10−5–9.7×10−1 M with a detection
limit of 1.2×10−5 M (S/N = 3), and analytical time of 10 seconds
per sample. The relative standard deviation (RSD) is 2.8 % for ten repetitive
measurements of 3 mM H2O2 solution. While the amperometric
response of the bulk Ag electrode to H2O2 shows a narrower linear relation in the range of 1.2×10−4–1.1×10−1 M
with a detection limit of 6.8×10−5 M, and analytical
time of 10 seconds per sample.(a) Typical amperometric response of sensor at −0.45 V to
successive addition of 3 mMH2O2 to 0.2 M HAc-NaAc buffer (pH = 5),
(b) calibration plot between the steady-state current and H2O2 concentration. Inset is Eadie-Hofstee plot between the steady-state current and
H2O2 concentration obtained from the upper limit of the linear range.
(a)(b)The reproducibility and storage
stability of the sensor were examined. When Ag-mSBA-15/GC modified
electrode was stored in air and subjected to the day-by-day calibrations at
room temperature, the electrode can maintain over 97% of the initial value in the response to 3.0
mM H2O2 after 100 days, while the bulk Ag electrode decayed
quickly to 92% after 24 hours. Since the size of silver nanoparticles synthesized
inside the channel of SBA-15 is larger than the channel diameter, the
problem of catalyst leaking is much lessened or eliminated. In addition, silver
nanoparticles were confined inside the channel of SBA-15, which made the
catalyst stable and anticontaminated.As a comparison, we have modified also the synthesis procedures of Ag-mSBA-15
simply skipping the step TMCS treatment. In this case, both inside and outside
of the SBA-15 channels were grafted by reducers, and Ag (0) nanoparticles were
produced both inside and outside of the SBA-15 channels. We have tested the
stability of this sample; and the results demonstrated that its activity forH2O2 reduction was declined at the first 15 days, then stabilized for long time
as the sample of Ag-mSBA-15. The results confirmed that the Ag (0)
nanoparticles inside the SBA-15 channels are stable, while those outside the
SBA-15 channels are less stable.In the experimental, it may be
always difficult to attain an entire consistent with the theoretical design of preparation
of the catalyst. So, a small portion of sites in external surface of the SBA-15
may not have been capped by trimethylchlorosilane (TMCS), and lead to form some Ag (0) nanoparticles outside the
SBA-15. However, as reported in UV-vis spectra, we have not observed the peak
at 345 nm in UV-vis spectra of samples, which indicated that the quantity of
clusters from 2 to 8 atoms could be neglected. In addition, if a small quantity
of Ag (0) nanoparticles were formed outside the SBA-15 channels, they are not
stable. As a consequence, they will not affect the stability of the sensor that
uses the Ag (0) nanoparticles catalysts confined in the SBA-15 channels.


## 3.1. Characterization

### 3.1.1. HRTEM Studies

In our synthesis procedures: (1)
before removing the template, the external surface of the SBA-15 was firstly
capped by trimethylchlorosilane (TMCS); (2) after removing the template, 3-aminopropyltrimethoxysilane
(APTMS) molecules were introduced into the channels of SBA-15, which leads to
that the amino functional groups were grafted to the internal walls of SBA-15 channels;
(3) the reducers of formaldehyde were then introduced into the channels of
SBA-15 and grafted on the amino groups inside the channel; (4) the metal precursors
of Ag(NH3)2NO3 were finally introduced into the channels of SBA-15 by diffusion
when the modified SBA-15 is put in a solution containing Ag(NH3)2NO3. The
silver (0) nanoparticles were, therefore, formed through the reduction of Ag (I)
ions by reducers previously fixed on the internal walls of the SBA-15 channels.
Theoretically, Ag nanoparticles prepared by this method were located inside the
channel between the walls in the method. The
silver nanoparticles assembled inside the channels of SBA-15 can be further
confirmed by HRTEM (Figure 1). Figure
1(a) shows the HRTEM of calcinated SBA-15 (cal-SBA-15), in which well-ordered
channels are illustrated and are characteristic of mesoporous materials. Figure
1(b) depicts the HRTEM of Ag-mSBA-15, where the highly ordered pore
structure of SBA-15 is still preserved. We can observe clearly that silver
nanoparticles appear as dark spherical objects between the walls of SBA-15 and
are homogeneously distributed inside the channels of SBA-15. It can be also seen
that the size of Ag is slightly larger than the channel, which leads to
the distortion of the channels.HRTEM images of (a) cal-SBA-15, (b) Ag-mSBA-15.
(a)(b)


### 3.1.2. XRD Studies

Figure2(a) gives a small-angle
XRD pattern of calcinated SBA-15(cal-SBA-15), three diffraction peaks
appear in the spectrum, which are attributed to the characteristic diffraction
peaks of (100), (110), and (200) for SBA-15, respectively, due to typical
hexangular phase [2, 24]. After reaction with APTMS, the small-angle XRD pattern of APTMS-TMCS-SBA-15 (Figure
2(b)) still shows the characteristic diffraction peaks, which exhibits that the
grafting of amino groups inside the channels did not affect the long-range
ordering of the mesostructures. Compared with cal-SBA-15, the peaks shift to
lower angles, and such shift may imply the enlargement of the frameworks
[32]. The little negative shifting may be aroused
by the covalent linkage between APTMS and hydroxy in the channels. We know from
BET data in Table 1  the DBJH value is almost constant during the
covalent linkage, but DBJH value has an obviously decrease after Ag
nanoparticles were assembled. In addition, after the loading of Ag into APTMS-TMCS-SBA-15 (assigned
as Ag-mSBA-15), the peaks (Figure 2(c)) have shifted positively
comparing with that of APTMS-TMCS-SBA-15, the intensity of the peaks has
decreased to a certain extent, while the full width at half-maximum (FWHM) of
the peaks that is normalized by height has increased. These three features are
attributed to contraction
of framework during the support treatment, implying that Ag nanoparticles have
been introduced into the channel of SBA-15 successfully as discussed elsewhere
[24, 33]. The inset of Figure 2 is wide-angle XRD of Ag-mSBA-15. The
broadening of the Ag diffraction peaks suggests that the size of Ag
nanoparticles is in the nanometer range [8]. The average Ag particle size was
estimated to be 8.0 nm, from the peak width of Ag (220) reflection by using Scherrer’s
equation for approximation [34].Table 1
Physicochemical properties of the samples.SBET, BET specific surface area; Vt, total pore
volume; DBJH, pore diameter calculated using BJH method; d100, periodicity of host
SBA-15 derived from XRD. The wall thicknesses, t, was calculated as A0-pore size (A0 = 2d(100)/3).
SampleSBET/m2g−1Vt/cm3g−1DBJH/nmd100/nmA0/nmt/nmas-SBA-152620.416.0010.9212.616.61cal-SBA-157031.026.5410.1011.695.15cal-TMCS-SBA-157051.046.5710.1311.705.13APTMS-TMCS-SBA-154190.686.4910.1511.725.34HCHO-APTMS-TMCS-SBA-153350.485.869.7911.305.44Ag-mSBA-152320.305.0510.1111.676.62Figure 2
Small-angle XRD patterns of (a) cal-SBA-15, (b) APTMS-TMCS-SBA-15,
and (c) Ag-mSBA-15. Inset is the wide-angle XRD pattern of Ag-mSBA-15.


### 3.1.3. FTIR Studies

FTIR spectroscopy can provide surface information of materials for identification of
chemical groups. Figure3(a)
presents the transmission FTIR spectrum of as-SBA-15, in which the bands in
2900–3000 cm−1 were attributed to template P123. After calcination
in air at 823 K for 6 hours
(cal-SBA-15), the bands in 2900–3000 cm−1 were disappeared (Figure
3(b)). This indicates that the template P123 was completely removed. After
modification of the outer surface of as-SBA-15 with trimethylchlorosilane (TMCS),
the sample was named as-TMCS-SBA-15. Figure 3(c) shows the FTIR spectrum of as-TMCS-SBA-15,
in which the bands in 2900–3000 cm−1 attributed to template P123 and
TMCS can be seen clearly. However, when the temple of the as-TMCS-SBA-15 has been
removed by calcination in air at 823 K for 6 hours as it was done in Figure 2(b),
we can still observe the bands in 2900–3000 cm−1 from Figure 3(d),
which can be attributed to the C–H stretching modes of –CH3 in cal-TMCS-SBA-15 [6]. This result
confirmed that the methyl groups are still present on the material cal-TMCS-SBA-15
when template P123 was completely removed at 823 K. Such stability comes from the
covalent linkage between –CH3 groups and the hydroxy in the outer
surface of silica framework.Figure 3
FTIR spectra of (a) cal-SBA-15, (b) cal-TMCS-SBA-15.The modification process of
external surface of as-SBA-15 does not affect the internal surface of the
channel because the template molecules occupy these channels during the
external surface modification. The incorporation of APTMS inside the channels of
the SBA-15 has been confirmed by transmission FTIR spectra as shown in Figure4.
The curve c is the spectrum of APTMS, and we can see the bands in the range
2900–3000 cm−1, which are attributed to the C–H stretching modes of –CH2 in APTMS. The curve a is the spectrum of cal-TMCS-SBA-15, in which we can
observe the characteristic absorption of Si–O–Si at 1082, 797, 465 cm−1 assigned to asymmetric stretching, symmetric stretching, and bend stretching
[35]. In addition, the well-known IR adsorption bands which due to the
stretching vibrational mode of surface silanol groups and water in the range 3500–3740 cm−1 [36, 37], and the C–H stretching modes of –CH3 in
the range 2900–3000 cm−1 [6] were also seen. The curve b showed the
spectrum that APTMS was introduced into the channel of cal-TMCS-SBA-15, in
which the intensity of IR absorption decrease for silanol group at 3500–3740 cm−1,
and the intensity of IR absorption increase for the C–H group at 2900–3000 cm−1.
The ratio of integrated intensity of the Si–OH band (ISi–OH)
to integrated intensity of the C–H band (IC–H), that is, ISi–OH/IC–H, can be used to evaluate the increase of –CH2 and –CH3 and decrease of silanol due to the introduction of APTMS inside the channel of
SBA-15. And the change of ISi–OH/IC–H can be used to monitor
the reaction process between Si–OH and APTMS, since the intensity of the Si–OH band
decreases and the intensity of the C–H band increases with the reaction
progressing. Here, ISi–OH/IC–H is reduced from 38.1 at cal-TMCS-SBA-15 to 16.0 at APTMS-TMCS-SBA-15. The decrease of integrated
intensity of silanol groups demonstrates an anchoring mechanism (see Scheme 1) involving
reaction between Si–OH and APTMS [38].Figure 4
FTIR spectra of (a) cal-TMCS-SBA-15, (b) APTMS-TMCS-SBA-15, and (c) APTMS.


### 3.1.4. Nitrogen Adsorption/Desorption Isotherms

Nitrogen adsorption/desorption
isotherms for as-SBA-15, cal-SBA-15, cal-TMCS-SBA-15, APTMS-TMCS-SBA-15,
HCHO-APTMS-TMCS-SBA-15, and Ag-mSBA-15 are found to be type IV
isotherm curves with distinct hysteresis loops and steep adsorption/desorption
steps were recorded that indicate a narrow pore size distribution. This suggests
that the host silica was still maintained during the modification. Table 1
summarizes the results of N2 desorption analyses. The specific surface
area (SBET) of as-SBA-15 is 262 m2/g. After silanols on
the external surface of as-SBA-15 were capped by –Si(CH3)3 groups, and the template was removed by calcination, SBET increases
to 705 m2/g. The total pore volume (Vt), and the average
pore size (DBJH) also show an increased value. After silanols on the
internal surface of SBA-15 were grafted by 3-aminopropyltrimethoxysilane
(APTMS), SBET and Vt of APTMS-TMCS-SBA-15 decrease from
705 to 419 m2/g and from 1.04 to 0.68 cm3/g,
respectively, with a slight decrease of the DBJH from 6.57 to 6.49 nm.
A considerable decrease in the SBET, Vt, and
DBJH was measured in Ag-mSBA-15 due to the incorporation
of Ag nanoparticles. The values of SBET, Vt, and
DBJH for HCHO-APTMS-TMCS-SBA-15 are 335 m2/g, 0.48 cm3/g,
and 5.86 nm, whereas they are 232 m2/g, 0.30 cm3/g, and 5.05 nm
for Ag-mSBA-15. All these, together with the increased thickness
of pore walls (t), can be attributed to the pore-filling effect [14, 39] and
the fact demonstrated that the Ag nanoparticles have been confined inside the
channel of SBA-15 [4].The silver nanoparticles were synthesized in the
inner surface of
silica framework by the “ship-in-a-bottle” approach,
in which the channel of SBA-15 is used as a microreactor to prepare the
nanoparticles. Precursors were introduced into the channels one by one, and were assembled
into Ag nanoparticles in the microreactor. This resulted in the Ag
nanoparticles were confined inside the channel of SBA-15. Silanols on the external surface
of as-SBA-15 were first capped by –Si(CH3)3 groups. After removal of the template of capped SBA-15 by
calcination, silanols on the internal surface of SBA-15 were modified by 3-aminopropyltrimethoxysilane
(APTMS), and then formaldehyde was grafted by amino groups of APTMS to form
NHCH2OH species, on which Ag(NH3)2NO3 could
be in situ reduced into Ag nanoparticles. Without silanols on the surface of SBA-15, there will be impossible to form
NHCH2OH species, and further to produce Ag nanoparticles. So there will be no Ag nanoparticles
on the external surface of SBA-15 thanks to silanols on the
external surface first capped by –Si(CH3)3 groups to decrease its activity. DBJH of APTMS-TMCS-SBA-15
is 6.49 nm from BET data, and the average Ag nanoparticle size
was estimated to be 8.0 nm from XRD analysis. HRTEM image has also demonstrated
that the size of Ag is slightly larger than the
channel, which leads to the distortion of the channels. Therefore, we infer that the
channels are slightly distorted during the nanoparticles synthesized in the
microreactor, which makes the size of nanoparticles prepared inside the channel
of SBA-15 larger than the size of the channel diameter. As
a result, the problem of catalyst leaking is much lessened
or eliminated. Similar results have been also
reported in literature [40].


### 3.1.5. UV-vis Absorption Spectroscopic Studies

UV-vis absorption spectra of the Ag-mSBA-15
samples may be used to provide additional evidence of the formation of Ag
nanostructures inside the channels of SBA-15 powder and reveal unique optical
properties. Figure 5(a) (line 1) shows the UV-vis absorption spectrum of HCHO-APTMS-TMCS-SBA-15, in which a spectrum with a nearly linear was recorded. Figure 5(a) (line 2) shows the
absorption spectrum of Ag-mSBA-15, in which only the peak of 420 nm which
assigned to the surface plasmon resonance (SPR) of Ag nanoparticles [16] is
observed and the peak of 345 nm assigned to small silver clusters from 2 to 8
atoms formed in micropores of silica walls [41] is absent. SPR is a characteristic
feature of metal nanoparticles between the sizes of 2 and 50 nm [42]. This
result indicates that the reduction of silver (I) ions in the micropores of the
SBA-15 does not exist, since all the reducers were introduced into the
mesopores of the SBA-15 in our experiment.(a) UV-vis spectra of (line 1) APTMS-TMCS-SBA-15 and (line 2) Ag-mSBA-15
immerged in 1 M NaOH solution, (b) Successive UV-vis spectra (taken every 1 minute) of methylene blue (MB) dye reduction, using
silica SBA-15 containing silver nanoparticles (Ag-mSBA-15) as the catalyst and NaBH4 as the reducing
agent. UV-vis spectrum of MB dye with pure silica SBA-15, using NaBH4 as the reducing
agent (dash line).
(a)(b)Moreover, silver nanoparticles
have been studied as a catalyst in reduction reactions of dyes like methylene
blue (MB) [43]. Here, reduction of MB by NaBH4 is used as a standard
for determining the catalytic activity of Ag-mSBA-15. The
preliminary catalytic testing for Ag-mSBA-15 was carried out by
reduction of MB (2 mL 10−5 M) in water using Ag-mSBA-15
(1 mg) as catalyst and NaBH4 (1 mL 10−4 M) as the
reducing agent. The progression of the catalytic reduction of MB can be easily
followed by the change of absorbance intensity at 665 nm that is absorbance
maximum (λmax) of MB.
The dot line in Figure 4(b) is the UV-vis
spectrum of the mixture containing a mixture of MB dye plus NaBH4 reducing agent, with pure silica SBA-15, which
gives a strong peak at 665 nm attributing
to λmax of MB, illustrating that MB have not been reduced. When 1 mg
Ag-mSBA-15 was added into above solution, it can be seen the band at
420 nm attributing to the SPR of Ag nanoparticles Figure 5(b) (solid line). The
absorbance at λmax of MB gradually decreases with the reaction time,
which suggests MB begin to reduce, and meanwhile we can observe the color of
solution changes from blue to colorless. Above investigations have illustrated
that silver nanoparticles exhibit a high catalytic activity to the reduction of MB.


## 3.1.1. HRTEM Studies

In our synthesis procedures: (1)
before removing the template, the external surface of the SBA-15 was firstly
capped by trimethylchlorosilane (TMCS); (2) after removing the template, 3-aminopropyltrimethoxysilane
(APTMS) molecules were introduced into the channels of SBA-15, which leads to
that the amino functional groups were grafted to the internal walls of SBA-15 channels;
(3) the reducers of formaldehyde were then introduced into the channels of
SBA-15 and grafted on the amino groups inside the channel; (4) the metal precursors
of Ag(NH3)2NO3 were finally introduced into the channels of SBA-15 by diffusion
when the modified SBA-15 is put in a solution containing Ag(NH3)2NO3. The
silver (0) nanoparticles were, therefore, formed through the reduction of Ag (I)
ions by reducers previously fixed on the internal walls of the SBA-15 channels.
Theoretically, Ag nanoparticles prepared by this method were located inside the
channel between the walls in the method. The
silver nanoparticles assembled inside the channels of SBA-15 can be further
confirmed by HRTEM (Figure 1). Figure
1(a) shows the HRTEM of calcinated SBA-15 (cal-SBA-15), in which well-ordered
channels are illustrated and are characteristic of mesoporous materials. Figure
1(b) depicts the HRTEM of Ag-mSBA-15, where the highly ordered pore
structure of SBA-15 is still preserved. We can observe clearly that silver
nanoparticles appear as dark spherical objects between the walls of SBA-15 and
are homogeneously distributed inside the channels of SBA-15. It can be also seen
that the size of Ag is slightly larger than the channel, which leads to
the distortion of the channels.HRTEM images of (a) cal-SBA-15, (b) Ag-mSBA-15.
(a)(b)


## 3.1.2. XRD Studies

Figure2(a) gives a small-angle
XRD pattern of calcinated SBA-15(cal-SBA-15), three diffraction peaks
appear in the spectrum, which are attributed to the characteristic diffraction
peaks of (100), (110), and (200) for SBA-15, respectively, due to typical
hexangular phase [2, 24]. After reaction with APTMS, the small-angle XRD pattern of APTMS-TMCS-SBA-15 (Figure
2(b)) still shows the characteristic diffraction peaks, which exhibits that the
grafting of amino groups inside the channels did not affect the long-range
ordering of the mesostructures. Compared with cal-SBA-15, the peaks shift to
lower angles, and such shift may imply the enlargement of the frameworks
[32]. The little negative shifting may be aroused
by the covalent linkage between APTMS and hydroxy in the channels. We know from
BET data in Table 1  the DBJH value is almost constant during the
covalent linkage, but DBJH value has an obviously decrease after Ag
nanoparticles were assembled. In addition, after the loading of Ag into APTMS-TMCS-SBA-15 (assigned
as Ag-mSBA-15), the peaks (Figure 2(c)) have shifted positively
comparing with that of APTMS-TMCS-SBA-15, the intensity of the peaks has
decreased to a certain extent, while the full width at half-maximum (FWHM) of
the peaks that is normalized by height has increased. These three features are
attributed to contraction
of framework during the support treatment, implying that Ag nanoparticles have
been introduced into the channel of SBA-15 successfully as discussed elsewhere
[24, 33]. The inset of Figure 2 is wide-angle XRD of Ag-mSBA-15. The
broadening of the Ag diffraction peaks suggests that the size of Ag
nanoparticles is in the nanometer range [8]. The average Ag particle size was
estimated to be 8.0 nm, from the peak width of Ag (220) reflection by using Scherrer’s
equation for approximation [34].Table 1
Physicochemical properties of the samples.SBET, BET specific surface area; Vt, total pore
volume; DBJH, pore diameter calculated using BJH method; d100, periodicity of host
SBA-15 derived from XRD. The wall thicknesses, t, was calculated as A0-pore size (A0 = 2d(100)/3).
SampleSBET/m2g−1Vt/cm3g−1DBJH/nmd100/nmA0/nmt/nmas-SBA-152620.416.0010.9212.616.61cal-SBA-157031.026.5410.1011.695.15cal-TMCS-SBA-157051.046.5710.1311.705.13APTMS-TMCS-SBA-154190.686.4910.1511.725.34HCHO-APTMS-TMCS-SBA-153350.485.869.7911.305.44Ag-mSBA-152320.305.0510.1111.676.62Figure 2
Small-angle XRD patterns of (a) cal-SBA-15, (b) APTMS-TMCS-SBA-15,
and (c) Ag-mSBA-15. Inset is the wide-angle XRD pattern of Ag-mSBA-15.


## 3.1.3. FTIR Studies

FTIR spectroscopy can provide surface information of materials for identification of
chemical groups. Figure3(a)
presents the transmission FTIR spectrum of as-SBA-15, in which the bands in
2900–3000 cm−1 were attributed to template P123. After calcination
in air at 823 K for 6 hours
(cal-SBA-15), the bands in 2900–3000 cm−1 were disappeared (Figure
3(b)). This indicates that the template P123 was completely removed. After
modification of the outer surface of as-SBA-15 with trimethylchlorosilane (TMCS),
the sample was named as-TMCS-SBA-15. Figure 3(c) shows the FTIR spectrum of as-TMCS-SBA-15,
in which the bands in 2900–3000 cm−1 attributed to template P123 and
TMCS can be seen clearly. However, when the temple of the as-TMCS-SBA-15 has been
removed by calcination in air at 823 K for 6 hours as it was done in Figure 2(b),
we can still observe the bands in 2900–3000 cm−1 from Figure 3(d),
which can be attributed to the C–H stretching modes of –CH3 in cal-TMCS-SBA-15 [6]. This result
confirmed that the methyl groups are still present on the material cal-TMCS-SBA-15
when template P123 was completely removed at 823 K. Such stability comes from the
covalent linkage between –CH3 groups and the hydroxy in the outer
surface of silica framework.Figure 3
FTIR spectra of (a) cal-SBA-15, (b) cal-TMCS-SBA-15.The modification process of
external surface of as-SBA-15 does not affect the internal surface of the
channel because the template molecules occupy these channels during the
external surface modification. The incorporation of APTMS inside the channels of
the SBA-15 has been confirmed by transmission FTIR spectra as shown in Figure4.
The curve c is the spectrum of APTMS, and we can see the bands in the range
2900–3000 cm−1, which are attributed to the C–H stretching modes of –CH2 in APTMS. The curve a is the spectrum of cal-TMCS-SBA-15, in which we can
observe the characteristic absorption of Si–O–Si at 1082, 797, 465 cm−1 assigned to asymmetric stretching, symmetric stretching, and bend stretching
[35]. In addition, the well-known IR adsorption bands which due to the
stretching vibrational mode of surface silanol groups and water in the range 3500–3740 cm−1 [36, 37], and the C–H stretching modes of –CH3 in
the range 2900–3000 cm−1 [6] were also seen. The curve b showed the
spectrum that APTMS was introduced into the channel of cal-TMCS-SBA-15, in
which the intensity of IR absorption decrease for silanol group at 3500–3740 cm−1,
and the intensity of IR absorption increase for the C–H group at 2900–3000 cm−1.
The ratio of integrated intensity of the Si–OH band (ISi–OH)
to integrated intensity of the C–H band (IC–H), that is, ISi–OH/IC–H, can be used to evaluate the increase of –CH2 and –CH3 and decrease of silanol due to the introduction of APTMS inside the channel of
SBA-15. And the change of ISi–OH/IC–H can be used to monitor
the reaction process between Si–OH and APTMS, since the intensity of the Si–OH band
decreases and the intensity of the C–H band increases with the reaction
progressing. Here, ISi–OH/IC–H is reduced from 38.1 at cal-TMCS-SBA-15 to 16.0 at APTMS-TMCS-SBA-15. The decrease of integrated
intensity of silanol groups demonstrates an anchoring mechanism (see Scheme 1) involving
reaction between Si–OH and APTMS [38].Figure 4
FTIR spectra of (a) cal-TMCS-SBA-15, (b) APTMS-TMCS-SBA-15, and (c) APTMS.


## 3.1.4. Nitrogen Adsorption/Desorption Isotherms

Nitrogen adsorption/desorption
isotherms for as-SBA-15, cal-SBA-15, cal-TMCS-SBA-15, APTMS-TMCS-SBA-15,
HCHO-APTMS-TMCS-SBA-15, and Ag-mSBA-15 are found to be type IV
isotherm curves with distinct hysteresis loops and steep adsorption/desorption
steps were recorded that indicate a narrow pore size distribution. This suggests
that the host silica was still maintained during the modification. Table 1
summarizes the results of N2 desorption analyses. The specific surface
area (SBET) of as-SBA-15 is 262 m2/g. After silanols on
the external surface of as-SBA-15 were capped by –Si(CH3)3 groups, and the template was removed by calcination, SBET increases
to 705 m2/g. The total pore volume (Vt), and the average
pore size (DBJH) also show an increased value. After silanols on the
internal surface of SBA-15 were grafted by 3-aminopropyltrimethoxysilane
(APTMS), SBET and Vt of APTMS-TMCS-SBA-15 decrease from
705 to 419 m2/g and from 1.04 to 0.68 cm3/g,
respectively, with a slight decrease of the DBJH from 6.57 to 6.49 nm.
A considerable decrease in the SBET, Vt, and
DBJH was measured in Ag-mSBA-15 due to the incorporation
of Ag nanoparticles. The values of SBET, Vt, and
DBJH for HCHO-APTMS-TMCS-SBA-15 are 335 m2/g, 0.48 cm3/g,
and 5.86 nm, whereas they are 232 m2/g, 0.30 cm3/g, and 5.05 nm
for Ag-mSBA-15. All these, together with the increased thickness
of pore walls (t), can be attributed to the pore-filling effect [14, 39] and
the fact demonstrated that the Ag nanoparticles have been confined inside the
channel of SBA-15 [4].The silver nanoparticles were synthesized in the
inner surface of
silica framework by the “ship-in-a-bottle” approach,
in which the channel of SBA-15 is used as a microreactor to prepare the
nanoparticles. Precursors were introduced into the channels one by one, and were assembled
into Ag nanoparticles in the microreactor. This resulted in the Ag
nanoparticles were confined inside the channel of SBA-15. Silanols on the external surface
of as-SBA-15 were first capped by –Si(CH3)3 groups. After removal of the template of capped SBA-15 by
calcination, silanols on the internal surface of SBA-15 were modified by 3-aminopropyltrimethoxysilane
(APTMS), and then formaldehyde was grafted by amino groups of APTMS to form
NHCH2OH species, on which Ag(NH3)2NO3 could
be in situ reduced into Ag nanoparticles. Without silanols on the surface of SBA-15, there will be impossible to form
NHCH2OH species, and further to produce Ag nanoparticles. So there will be no Ag nanoparticles
on the external surface of SBA-15 thanks to silanols on the
external surface first capped by –Si(CH3)3 groups to decrease its activity. DBJH of APTMS-TMCS-SBA-15
is 6.49 nm from BET data, and the average Ag nanoparticle size
was estimated to be 8.0 nm from XRD analysis. HRTEM image has also demonstrated
that the size of Ag is slightly larger than the
channel, which leads to the distortion of the channels. Therefore, we infer that the
channels are slightly distorted during the nanoparticles synthesized in the
microreactor, which makes the size of nanoparticles prepared inside the channel
of SBA-15 larger than the size of the channel diameter. As
a result, the problem of catalyst leaking is much lessened
or eliminated. Similar results have been also
reported in literature [40].


## 3.1.5. UV-vis Absorption Spectroscopic Studies

UV-vis absorption spectra of the Ag-mSBA-15
samples may be used to provide additional evidence of the formation of Ag
nanostructures inside the channels of SBA-15 powder and reveal unique optical
properties. Figure 5(a) (line 1) shows the UV-vis absorption spectrum of HCHO-APTMS-TMCS-SBA-15, in which a spectrum with a nearly linear was recorded. Figure 5(a) (line 2) shows the
absorption spectrum of Ag-mSBA-15, in which only the peak of 420 nm which
assigned to the surface plasmon resonance (SPR) of Ag nanoparticles [16] is
observed and the peak of 345 nm assigned to small silver clusters from 2 to 8
atoms formed in micropores of silica walls [41] is absent. SPR is a characteristic
feature of metal nanoparticles between the sizes of 2 and 50 nm [42]. This
result indicates that the reduction of silver (I) ions in the micropores of the
SBA-15 does not exist, since all the reducers were introduced into the
mesopores of the SBA-15 in our experiment.(a) UV-vis spectra of (line 1) APTMS-TMCS-SBA-15 and (line 2) Ag-mSBA-15
immerged in 1 M NaOH solution, (b) Successive UV-vis spectra (taken every 1 minute) of methylene blue (MB) dye reduction, using
silica SBA-15 containing silver nanoparticles (Ag-mSBA-15) as the catalyst and NaBH4 as the reducing
agent. UV-vis spectrum of MB dye with pure silica SBA-15, using NaBH4 as the reducing
agent (dash line).
(a)(b)Moreover, silver nanoparticles
have been studied as a catalyst in reduction reactions of dyes like methylene
blue (MB) [43]. Here, reduction of MB by NaBH4 is used as a standard
for determining the catalytic activity of Ag-mSBA-15. The
preliminary catalytic testing for Ag-mSBA-15 was carried out by
reduction of MB (2 mL 10−5 M) in water using Ag-mSBA-15
(1 mg) as catalyst and NaBH4 (1 mL 10−4 M) as the
reducing agent. The progression of the catalytic reduction of MB can be easily
followed by the change of absorbance intensity at 665 nm that is absorbance
maximum (λmax) of MB.
The dot line in Figure 4(b) is the UV-vis
spectrum of the mixture containing a mixture of MB dye plus NaBH4 reducing agent, with pure silica SBA-15, which
gives a strong peak at 665 nm attributing
to λmax of MB, illustrating that MB have not been reduced. When 1 mg
Ag-mSBA-15 was added into above solution, it can be seen the band at
420 nm attributing to the SPR of Ag nanoparticles Figure 5(b) (solid line). The
absorbance at λmax of MB gradually decreases with the reaction time,
which suggests MB begin to reduce, and meanwhile we can observe the color of
solution changes from blue to colorless. Above investigations have illustrated
that silver nanoparticles exhibit a high catalytic activity to the reduction of MB.


## 3.2. Application of Ag-mSBA-15 as a Sensor for Detecting Hydrogen Peroxide

### 3.2.1. Electrochemical Response of Ag-mSBA-15/GC Electrode to Reduction of H2O2

Figure6(a) displayed the cyclic
voltammetry (CV) of glassy carbon (GC) in 0.2 M HAc-NaAc buffer solution (pH 5)
containing 3 mM H2O2, in which a featureless CV was
observed, and the current may be mainly ascribed to the double-layer charging
of the electrode. Figure 6(b) is CV of Ag-mSBA-15/GC in 0.2 M HAc-NaAc
buffer solution (pH 5), in which a small reduction current appears that may be attributing
to reduction of support electrolyte. Figures 6(c), 6(d),
and 6(e) show the CVs of
Ag-mSBA-15/GC, nm-Ag/GC, and bulk Ag, respectively, in 0.2 M HAc-NaAc
+ 3 mM H2O2. It can be seen a wave at −0.42 V for Ag-mSBA-15/GC,
−0.65 V for nm-Ag/GC, and −0.53 V for bulk Ag appear. Ag-mSBA-15/GC,
nm-Ag/GC, and bulk Ag electrode on the initial reduction potential of H2O2 is −0.12 V, −0.15 V, and −0.17 V,
respectively. The reduction potential of H2O2 on Ag-mSBA-15/GC was 140 mV, 230 mV,
and 110 mV more negative than
that on silver nanoparticles assembles supported on GC [44], nm-Ag/GC, and bulk
Ag, indicating a lower overpotential for H2O2 reduction
at the Ag-mSBA-15/GC. Figure 7 depicts the CVs of Ag-mSBA-15/GC
in 0.2 M HAc-NaAc + 3 mM H2O2 with different scan rate. It
can be seen that the peak currents of the H2O2 reduction
increase in Ag-mSBA-15/GC in 0.2 M HAc-NaAc + 3 mM H2O2 with an increasing scan
rate. In the scan rate range from 10 mV·s−1 to 500 mV·s−1, the reduction current of H2O2 is proportional to the square root of scan rate (inset of Figure 7), which
suggests that the rate of electrochemical reaction is rather fast and the
electrode process is controlled by the diffusion of H2O2 from
solution to electrode surface. In order to determine the optimal working
potential for the H2O2 sensing, the electrochemical
response of the H2O2 was researched in different
potentials. The relationship between the steady-state current and the operating
potential in 0.2 M HAc-NaAc + 3 mM H2O2 at Ag-mSBA-15/GC
is showed in Figure 8. Considering the economy of energy, the sensitivity and
the steadiness of Ag-mSBA-15/GC, −0.45 V were chosen as optimal
working potential for H2O2 sensing with amperometry of
constant potential.Figure 6
The voltammetric response of (a) GC to 3 mMH2O2,
(b) Ag-mSBA-15/GC to 0 mM H2O2, (c) Ag-mSBA-15/GC
to 3 mM H2O2, (d) nm-Ag/GC to 3 mM H2O2,
and (e) bulk Ag to 3 mM H2O2. All experiments were
carried out in 0.2 M HAc-NaAc buffer (pH = 5) electrolyte, scan rate: 50 mV·s−1.Figure 7
The cyclic voltammograms of Ag-mSBA-15/GC
in 0.2 M HAc-NaAc buffer (pH = 5) + 3 mM H2O2, at different
scan rates (from upper to lower): 10, 25, 50, 75, 100, 150, 200, 250, 300, 350, 400,
500 mV·s−1, respectively. Inset is plot of peak
current versus v1/2.Figure 8
Influence of applied potential on amperometric
response of Ag-mSBA-15/GC in 0.2 M HAc-NaAc buffer (pH = 5)
+ 3 mM H2O2.


### 3.2.2. Detection ofH2O2

Figure9(a) shows the dynamic
response of the Ag-mSBA-15/GC modified electrode at a working
potential of −0.45 V with successive injections of 3 mM H2O2 in 0.2 M HAc-NaAc buffer solution (pH 5). The calibration curve of Ag-mSBA-15/GC
modified electrode under the optimal working potential of −0.45 V is showed in
Figure 9(b). Under an exactly same condition, the Ag-mSBA-15/GC
modified electrode for H2O2 determination exhibits a
linear range of 4.8×10−5–9.7×10−1 M with a detection
limit of 1.2×10−5 M (S/N = 3), and analytical time of 10 seconds
per sample. The relative standard deviation (RSD) is 2.8 % for ten repetitive
measurements of 3 mM H2O2 solution. While the amperometric
response of the bulk Ag electrode to H2O2 shows a narrower linear relation in the range of 1.2×10−4–1.1×10−1 M
with a detection limit of 6.8×10−5 M, and analytical
time of 10 seconds per sample.(a) Typical amperometric response of sensor at −0.45 V to
successive addition of 3 mMH2O2 to 0.2 M HAc-NaAc buffer (pH = 5),
(b) calibration plot between the steady-state current and H2O2 concentration. Inset is Eadie-Hofstee plot between the steady-state current and
H2O2 concentration obtained from the upper limit of the linear range.
(a)(b)The reproducibility and storage
stability of the sensor were examined. When Ag-mSBA-15/GC modified
electrode was stored in air and subjected to the day-by-day calibrations at
room temperature, the electrode can maintain over 97% of the initial value in the response to 3.0
mM H2O2 after 100 days, while the bulk Ag electrode decayed
quickly to 92% after 24 hours. Since the size of silver nanoparticles synthesized
inside the channel of SBA-15 is larger than the channel diameter, the
problem of catalyst leaking is much lessened or eliminated. In addition, silver
nanoparticles were confined inside the channel of SBA-15, which made the
catalyst stable and anticontaminated.As a comparison, we have modified also the synthesis procedures of Ag-mSBA-15
simply skipping the step TMCS treatment. In this case, both inside and outside
of the SBA-15 channels were grafted by reducers, and Ag (0) nanoparticles were
produced both inside and outside of the SBA-15 channels. We have tested the
stability of this sample; and the results demonstrated that its activity forH2O2 reduction was declined at the first 15 days, then stabilized for long time
as the sample of Ag-mSBA-15. The results confirmed that the Ag (0)
nanoparticles inside the SBA-15 channels are stable, while those outside the
SBA-15 channels are less stable.In the experimental, it may be
always difficult to attain an entire consistent with the theoretical design of preparation
of the catalyst. So, a small portion of sites in external surface of the SBA-15
may not have been capped by trimethylchlorosilane (TMCS), and lead to form some Ag (0) nanoparticles outside the
SBA-15. However, as reported in UV-vis spectra, we have not observed the peak
at 345 nm in UV-vis spectra of samples, which indicated that the quantity of
clusters from 2 to 8 atoms could be neglected. In addition, if a small quantity
of Ag (0) nanoparticles were formed outside the SBA-15 channels, they are not
stable. As a consequence, they will not affect the stability of the sensor that
uses the Ag (0) nanoparticles catalysts confined in the SBA-15 channels.


## 3.2.1. Electrochemical Response of Ag-mSBA-15/GC Electrode to Reduction of H2O2

Figure6(a) displayed the cyclic
voltammetry (CV) of glassy carbon (GC) in 0.2 M HAc-NaAc buffer solution (pH 5)
containing 3 mM H2O2, in which a featureless CV was
observed, and the current may be mainly ascribed to the double-layer charging
of the electrode. Figure 6(b) is CV of Ag-mSBA-15/GC in 0.2 M HAc-NaAc
buffer solution (pH 5), in which a small reduction current appears that may be attributing
to reduction of support electrolyte. Figures 6(c), 6(d),
and 6(e) show the CVs of
Ag-mSBA-15/GC, nm-Ag/GC, and bulk Ag, respectively, in 0.2 M HAc-NaAc
+ 3 mM H2O2. It can be seen a wave at −0.42 V for Ag-mSBA-15/GC,
−0.65 V for nm-Ag/GC, and −0.53 V for bulk Ag appear. Ag-mSBA-15/GC,
nm-Ag/GC, and bulk Ag electrode on the initial reduction potential of H2O2 is −0.12 V, −0.15 V, and −0.17 V,
respectively. The reduction potential of H2O2 on Ag-mSBA-15/GC was 140 mV, 230 mV,
and 110 mV more negative than
that on silver nanoparticles assembles supported on GC [44], nm-Ag/GC, and bulk
Ag, indicating a lower overpotential for H2O2 reduction
at the Ag-mSBA-15/GC. Figure 7 depicts the CVs of Ag-mSBA-15/GC
in 0.2 M HAc-NaAc + 3 mM H2O2 with different scan rate. It
can be seen that the peak currents of the H2O2 reduction
increase in Ag-mSBA-15/GC in 0.2 M HAc-NaAc + 3 mM H2O2 with an increasing scan
rate. In the scan rate range from 10 mV·s−1 to 500 mV·s−1, the reduction current of H2O2 is proportional to the square root of scan rate (inset of Figure 7), which
suggests that the rate of electrochemical reaction is rather fast and the
electrode process is controlled by the diffusion of H2O2 from
solution to electrode surface. In order to determine the optimal working
potential for the H2O2 sensing, the electrochemical
response of the H2O2 was researched in different
potentials. The relationship between the steady-state current and the operating
potential in 0.2 M HAc-NaAc + 3 mM H2O2 at Ag-mSBA-15/GC
is showed in Figure 8. Considering the economy of energy, the sensitivity and
the steadiness of Ag-mSBA-15/GC, −0.45 V were chosen as optimal
working potential for H2O2 sensing with amperometry of
constant potential.Figure 6
The voltammetric response of (a) GC to 3 mMH2O2,
(b) Ag-mSBA-15/GC to 0 mM H2O2, (c) Ag-mSBA-15/GC
to 3 mM H2O2, (d) nm-Ag/GC to 3 mM H2O2,
and (e) bulk Ag to 3 mM H2O2. All experiments were
carried out in 0.2 M HAc-NaAc buffer (pH = 5) electrolyte, scan rate: 50 mV·s−1.Figure 7
The cyclic voltammograms of Ag-mSBA-15/GC
in 0.2 M HAc-NaAc buffer (pH = 5) + 3 mM H2O2, at different
scan rates (from upper to lower): 10, 25, 50, 75, 100, 150, 200, 250, 300, 350, 400,
500 mV·s−1, respectively. Inset is plot of peak
current versus v1/2.Figure 8
Influence of applied potential on amperometric
response of Ag-mSBA-15/GC in 0.2 M HAc-NaAc buffer (pH = 5)
+ 3 mM H2O2.


## 3.2.2. Detection ofH2O2

Figure9(a) shows the dynamic
response of the Ag-mSBA-15/GC modified electrode at a working
potential of −0.45 V with successive injections of 3 mM H2O2 in 0.2 M HAc-NaAc buffer solution (pH 5). The calibration curve of Ag-mSBA-15/GC
modified electrode under the optimal working potential of −0.45 V is showed in
Figure 9(b). Under an exactly same condition, the Ag-mSBA-15/GC
modified electrode for H2O2 determination exhibits a
linear range of 4.8×10−5–9.7×10−1 M with a detection
limit of 1.2×10−5 M (S/N = 3), and analytical time of 10 seconds
per sample. The relative standard deviation (RSD) is 2.8 % for ten repetitive
measurements of 3 mM H2O2 solution. While the amperometric
response of the bulk Ag electrode to H2O2 shows a narrower linear relation in the range of 1.2×10−4–1.1×10−1 M
with a detection limit of 6.8×10−5 M, and analytical
time of 10 seconds per sample.(a) Typical amperometric response of sensor at −0.45 V to
successive addition of 3 mMH2O2 to 0.2 M HAc-NaAc buffer (pH = 5),
(b) calibration plot between the steady-state current and H2O2 concentration. Inset is Eadie-Hofstee plot between the steady-state current and
H2O2 concentration obtained from the upper limit of the linear range.
(a)(b)The reproducibility and storage
stability of the sensor were examined. When Ag-mSBA-15/GC modified
electrode was stored in air and subjected to the day-by-day calibrations at
room temperature, the electrode can maintain over 97% of the initial value in the response to 3.0
mM H2O2 after 100 days, while the bulk Ag electrode decayed
quickly to 92% after 24 hours. Since the size of silver nanoparticles synthesized
inside the channel of SBA-15 is larger than the channel diameter, the
problem of catalyst leaking is much lessened or eliminated. In addition, silver
nanoparticles were confined inside the channel of SBA-15, which made the
catalyst stable and anticontaminated.As a comparison, we have modified also the synthesis procedures of Ag-mSBA-15
simply skipping the step TMCS treatment. In this case, both inside and outside
of the SBA-15 channels were grafted by reducers, and Ag (0) nanoparticles were
produced both inside and outside of the SBA-15 channels. We have tested the
stability of this sample; and the results demonstrated that its activity forH2O2 reduction was declined at the first 15 days, then stabilized for long time
as the sample of Ag-mSBA-15. The results confirmed that the Ag (0)
nanoparticles inside the SBA-15 channels are stable, while those outside the
SBA-15 channels are less stable.In the experimental, it may be
always difficult to attain an entire consistent with the theoretical design of preparation
of the catalyst. So, a small portion of sites in external surface of the SBA-15
may not have been capped by trimethylchlorosilane (TMCS), and lead to form some Ag (0) nanoparticles outside the
SBA-15. However, as reported in UV-vis spectra, we have not observed the peak
at 345 nm in UV-vis spectra of samples, which indicated that the quantity of
clusters from 2 to 8 atoms could be neglected. In addition, if a small quantity
of Ag (0) nanoparticles were formed outside the SBA-15 channels, they are not
stable. As a consequence, they will not affect the stability of the sensor that
uses the Ag (0) nanoparticles catalysts confined in the SBA-15 channels.


## 4. Conclusions

The new application of the
stabilizer-free silver nanoparticles confined inside the channel of SBA-15
mesoporous silica (Ag-mSBA-15) is proposed. The uniform
mesostructures, high surface areas, and tunable pore sizes of SBA-15 facilitates
its manipulation for sensor preparation and sensing application. The resulting Ag-mSBA-15
modified electrode shows a very efficient electrocatalytic behavior toward the
reduction of H2O2 at a low overpotential. The sensor for H2O2 exhibits very good analytical performance with low cost, convenient
preparation, and sensitive and rapid detection. Thus, the Ag-mSBA-15/GC
is an attractive amperometric sensor for H2O2 and other
practical applications.



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*Source: 473791-2008-05-08.xml*