Electrochemical Sensor Constructed Using a Carbon Paste Electrode Modified with Mesoporous Silica Encapsulating PANI Chains Decorated with GNPs for Detection of Ascorbic Acid

Article abstract of DOI:10.1016/j.electacta.2017.04.021

In this work, an electrochemical sensor constructed using a carbon paste electrode (CPE), modified with mesoporous silica encapsulating polyaniline (PANI) decorated with gold nanoparticles (GNPs) for the detection of ascorbic acid (AA), was presented. First of all, PANI latex was synthesized by emulsion polymerization. Then, the acid-catalyzed sol–gel solutions of tetraethyl silicate (TEOS) was prepared in the presence of D-glucose, followed by encapsulation of previously prepared PANI latex into as-prepared sol-gel solution. Upon formation of silica frameworks, electroactive composite powder was obtained. Subsequently, the as-prepared composite powder was subjected to Soxhlet extraction to remove D-glucose and the final product was denoted as PM silica. On the other hand, PGM silica was obtained by immersing PM silica in 5?mL of 0.10?mM HAuCl₄ solution for 6?h, which reduced the gold nanoparticles (GNPs) onto the surface of PANI chains located inside the nanochannels of wormhole-like mesoporous silica framework. The redox reactions that mediated between EB form of PANI and aqueous HAuCl₄ solution were monitored and confirmed by FTIR spectra. The appearance of GNPs inside the PGM silica was characterized by TEM, XPS, XRD, and BET analyses. PGM silica was found to reveal higher redox capability as compared to that of PM silica based on the electrochemical CV studies. It should be noted that PGM silica-modified CPE was found to exhibit high sensitivity, a relatively low limit of detection, excellent sensing repeatability and a broader linear dynamic range for AA detection than those of bare CPE or PM silica-modified CPE. Moreover, A electrochemical sensor constructed from PGM silica-modified CPE was found to reveal better selectivity for a tertiary mixture of AA/DA/UA as compared to that of PM silica-modified CPE based on a series of studies of differential pulse voltammograms. In this work, the fabricated sensor showed reliable data in detecting AA in the tested commercially available products.

Full text of DOI:10.1016/j.electacta.2017.04.021

Accepted Manuscript
Title: Electrochemical Sensor Constructed by Carbon Paste
Electrode Modified with Mesoporous Silica Encapsulating
PANI Chains Decorated with GNPs for Detection of Ascorbic
Acid
Authors: Sheng-Chieh Hsu, Hsiao-Tung Cheng, Pei-Xuan
Wu, Chang-Jian Weng, Karen S. Santiago, Jui-Ming Yeh
PII:
S0013-4686(17)30764-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2017.04.021
EA 29276
To appear in:
Electrochimica Acta
Received date:
Revised date:
Accepted date:
25-1-2017
29-3-2017
4-4-2017
Please cite this article as: Sheng-Chieh Hsu, Hsiao-Tung Cheng, Pei-Xuan Wu, Chang-
Jian Weng, Karen S.Santiago, Jui-Ming Yeh, Electrochemical Sensor Constructed
by Carbon Paste Electrode Modified with Mesoporous Silica Encapsulating PANI
Chains Decorated with GNPs for Detection of Ascorbic Acid, Electrochimica
Actahttp://dx.doi.org/10.1016/j.electacta.2017.04.021
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Electrochemical Sensor Constructed by Carbon Paste Electrode Modified
with Mesoporous Silica Encapsulating PANI Chains Decorated with
GNPs for Detection of Ascorbic Acid
a
a
a
a
Sheng-Chieh Hsu , Hsiao-Tung Cheng , Pei-Xuan Wu , Chang-Jian Weng
b
,a
Karen S. Santiago , Jui-Ming Yeh*
aDepartment of Chemistry and Center for Nanotechnology, Chung-Yuan Christian
University, Chung-Li 32023, Taiwan, R. O. C.
bDepartment of Chemistry, University of Santo Tomas, Espana, Manila, 1015 Philippines
*
To whom correspondence should be addressed.
Corresponding author. Tel.: +886-3-2653341; fax: +886-3-2653399
E-mail: juiming@cycu.edu.tw
1

Abstract
In this work, an electrochemical sensor constructed by carbon paste electrode (CPE) modified with
mesoporous silica encapsulating polyaniline (PANI) decorated with gold nanoparticles (GNPs) was
presented and applied to detect ascorbic acid (AA). First of all, PANI latex was synthesized by
emulsion polymerization. Moreover, the acid-catalyzed sol–gel solutions of tetraethyl silicate (TEOS)
was prepared in the presence of D-glucose, followed by encapsulation of previous PANI latex into as-
prepared sol-gel solution. Upon formation of silica frameworks, electroactive composite powders were
obtained. Subsequently, the as-prepared composite powders were subjected to Soxhlet extraction to
remove D-glucose and the final product denoted by PM silica. Furthermore, PGM silica was obtained
by immersing PM silica in 5 mL of 0.10 mM HAuCl solution for 6 h, which reduce the gold
4
nanoparticles (GNPs) onto the surface of PANI chains inside the nanochannels of wormhole-like
mesoporous silica framework. The redox reactions that mediated between EB form of PANI and
aqueous HAuCl solution were monitored and confirmed by FTIR spectra. The appearance of GNPs
4
inside the PGM silica was characterized by TEM, XPS, XRD, and BET analyses. PGM silica was
found to reveal higher redox capability as compared to that of PM silica based on the electrochemical
CV studies.
It should be noted that PGM silica-modified CPE was found to exhibit high sensitivity, a relatively
low limit of detection, excellent sensing repeatability and a broader linear dynamic range for AA
detection than those of bare CPE or PM silica-modified CPE. Moreover, A electrochemical sensor
constructed from PGM silica-modified CPE was found to reveal better selectrivity for a tertiary mixture
of AA/DA/UA as compared to that of PM silica-modified CPE based on a series of studies of
2

differential pulse valtammograms. The real commercial product for AA detection was also investigated
in this work.
Keywords: electroactive; polyaniline; mesoporous; gold; ascorbic acid; sensor.
1
. Introduction
Ascorbic acid (AA) is found in many foods/drinks and is well known for its anti-oxidant properties.
In addition, it plays a significant role in many human metabolic processes because of its redox
properties and can be used in the prevention and treatment of diseases[1] such as infertility, mental
illness, AIDS, cancer, and the common cold. Thus, the development of a simple, rapid, highly selective,
and sensitive method for the determination of AA is desirable, having diagnostic medical and food
safety applications. Several techniques have been explored for the determination of AA, including
spectroscopic,[2] chromatographic,[3] enzymatic,[4] and electroanalytical[5] methods. Among these
various methods, electrochemical methods using various chemically modified electrodes (CMEs) are
considered the best because of their simplicity and high sensitivity for the determination of AA. In
addition, for the development of electrochemical sensors, materials with redox capabilities are desired.
In recent years, many researches have focused on electrochemical biosensing because of ease of
tailoring properties of conducting polymers, such as their reversible redox capabilities. Polyaniline
(PANI) is an electroactive polymer with outstanding electrochemical properties toward biologically
important molecules; consequently, it is quite suitable for use in electrochemical sensors.[6-14]
Moreover, several research studies[15-26] have reported the encapsulation of polymers and/or
oligomers within the channels of mesoporous hosts in the past decades. For example, in 1994, Bein et
3

al.[15] reported the preparation of encapsulation of PANI chains in MCM-41 hosts with a channel
diameter of ~ 3 nm. This channel size permitted encapsulation of several PANI chains, and the
resulting filaments showed microwave conductivity. In an effort to apply PANI in the field of
electrorheological (ER) fluids, Choi et al.[16] synthesized a PANI/mesoporous SBA-15 nanocomposite.
Moreover, Yeh et al.[22] explored PANI encapsulated within mesoporous silica, prepared by a non-
surfactant templating route, and found that the materials having electroactive and had a high surface
area simultaneously.
Several types of nanoparticles, including metals, oxides, and semiconductor nanoparticles, have also
been applied in biosensing. For example, gold and silver nanoparticles or silver–silica hybrid
nanostructures have been used as biosensor substrates.[27-29] Among the nanomaterials used as
components in biosensors, gold nanoparticles (GNPs) have received the greatest interest because they
have several interesting properties.[30, 31] GNPs with diameters of 1 - 100 nm have a high surface-to-
volume ratio and a high surface energy, providing stable immobilization of a large number of
biomolecules, thus retaining their bioactivity. Moreover, GNPs permit the fast and direct electron
transfer between a wide range of electroactive species and electrode materials.
In this paper, we attempts to construct an electrochemical sensor for AA detection by preparing
sensing materials integrating both conducting polymer and metallic nanoparticles. First of all,
mesoporous silica encapsulating PANI chains (denoted by PM silica) was prepared by a non-surfactant-
template route with D-glucose as pore-forming agent. Subsequently, reducing the GNPs onto the
surface of PANI chains inside the nanochannels of mesoporous silica through the redox reactions
between the emeraldine base (EB) form of PANI and an aqueous HAuCl
4
solution. The materials
(denoted by PGM silica) with reduction of GNPs upon the surface of PANI inside the nanochannels of
4

PM silica was found to promote electrocatalytic properties of PM silica, indicating that the PGM silica
can be potentially applied in electrochemical sensor. The appearance of GNPs onto the surface of PANI
chains inside the nanochannels of PGM silica was characterized by transmission electron microscopy
(
TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-
Teller (BET) analyses. The oxidation transition of the EB form toward the pernigraniline base (PB)
form of PANI, accompanied by the reduction of the aqueous HAuCl solution, forming GNPs, was
confirmed by the observation of Fourier-transformation infrared (FTIR) spectroscopy. Redox
capability of PM silica and PGM silica was further investigated by electrochemical cyclic voltammetry
CV) studies. The electrochemical snsor for the sensitivity studies of AA detection constructed by
4
(
PGM-modified CPE, PM silica-modified CPE and bare-CPE was performed and compared. The
selectivity studies of electrochemical sensor of PGM silica and PM silica- modified CPE was also
investigated by differential pulse voltammograms (DPV) for the tertiary mixture of AA/UA/DA.
Moreover, the detection of AA in real commercial products of vitamin C tablets was also carried out in
this work.
2
2
. Experimental section
.1 Chemicals
Aniline (Fluka, 99%) was purified prior to use. Gold(III) chloride trihydrate (HAuCl
4
∙3H O; Alfa Aesar,
2
9
9%), tetraethyl orthosilicate (TEOS; Acros, 97%), D-(+)-glucose (Acros, 99%), sodium dodecyl
benzene sulfonate (DBSA; Fluka, 98%), dimethylformamide (Macron, 99%), ammonium
peroxydisulfate (APS; J.T. Baker, 99%), n-butyl alcohol (Sigma-Aldrich, 99.5%), N,N-diphenyl-p-
phenylenediamine (Merck, 99%), 4,4-diaminodiphenylamine sulfate (Aldrich, 99%), L-Ascorbic acid
5

(
Sigma, 99%), Dopamine hydrochloride (Sigma, 99%), Uric acid (Sigma, 99%), Vitamin C Tablet
Standard Chem. & Pharm. Co. & Yungshin Pharm. Ind. Co.) and hydrochloric acid (HCl; Riedel-de
(
Haën, 37%) were used without purification. Methanol (MeOH, Macron, 98%) was used as a solvent for
Soxhlet extraction. All reagents were reagent grade unless otherwise stated.
2
.2 Instrumentation
FTIR spectroscopy was performed using an FT/IR-4100 instrument (JASCO) using KBr pellets of
the samples. N adsorption–desorption isotherms were recorded at 77 K on a Micromeritics ASAP 2010
2
automated analyzer. The surface area was obtained by the BET method, and the pore size distributions
were calculated by applying the Barrett–Joyner–Halenda (BJH) model to the adsorption data. XPS
analysis was performed on an ESCALAB MK II X-ray photoelectron spectrometer. XRD diffraction
patterns were recorded with Cu Ka (λ = 0.15418 nm) using a Philips X’pert Pro. The morphology of the
PGM silica was observed using TEM (JEOL JEM-1400). Cyclic voltammetry (CV) was conducted on a
VoltaLab 40 (PGZ 301) electrochemical workstation in 20 mL of 0.1 M phosphate buffer solution
(PBS). A three-electrode system was used, consisting of PM silica and PGM silica-modified carbon
paste electrodes (CPEs) as the working electrodes. A platinum foil served as the auxiliary electrode and
a Ag/AgCl (3 M NaCl solution) electrode was used as the reference electrode, respectively.
2
.3 Preparation of PANI latex and aniline pentamers
In this work, PANI latex was synthesized by emulsion polymerization with APS as chain initiator.
Moreover, an aniline pentamer (AP) was also synthesized as control experiment. A typical procedure to
prepare PANI latex is given as follows: DBSA (0.5 g) and 10 mL of n-butyl alcohol were mixed with
6

2
2
2.5 g of aniline in 100 mL of distilled H
7.5 g of APS was added dropwisely. After 24 h, polymerization of PANI was complete, and a stable
) and weight
) of the as-prepared PANI obtained using emulsion polymerization were
2
O for 1 h. Then, 100 mL of an aqueous solution containing
green-colored PANI latex was obtained. The number-average molecular weight (M
n
average molecular weight (M
w
determined to be ~ 6600 and ~ 8600, respectively, based on gel permeation chromatography (GPC) with
polymethylmethacrylate (PMMA) as the standard. A representative procedure to synthesize the AP
used in the control experiment is given as follows: first of all, 2.6 g of N,N-diphenyl-p-
phenylenediamine and 3.5 g of 4,4-diaminodiphenylamine sulfate were dissolved in a solvent mixture
containing 15 mL of a 1.0 M HCl aqueous solution, 20 mL of distilled H O and 100 mL of
2
dimethylformamide, followed by stirring at -5 °C for 30 min. Subsequently, a solution of 2.28 g of APS
in 50 mL of a 1.0 M HCl solution was added via a dropping funnel into the as-prepared mixture to give
a AP-containing solution. The reaction mixture was further stirred for 1 h at -5 °C prior to adding
3
00 mL of distilled H
obtained precipitate was subsequently washed with 200 mL of 1.0 M aqueous HCl solution to remove
any unreacted monomer. For the de-doping process, the as-received solid product was washed
thoroughly with 100 mL of a 10 wt-% of NH OH solution, followed by washing with excess amount of
distilled H O. The final product was then dried in a vacuum oven at 50 °C for 24 h. The yield of
obtained AP fine green powder was calculated at ~ 57%.
2
O. The resulting solution containing precipitate of AP was filtered out. The
4
2
2
.4 Preparation of PM silica
The conventional preparation process for PM silica was given as follows: 5.20 g of TEOS, 3.45 g of
EtOH, and 1.8 g of 1 M HCl aqueous solution were mixed in a 50-mL flask at 85 °C for 2 h. The
7

solution was combined with an appropriate quantity of a solution containing 0.75 g D-glucose to a
prehydrolyzed solution of TEOS. After 0.5 h of magnetic stirring, 6 g of PANI latex was added to the
as-prepared sol–gel solution, followed by stirring for an additional 1 h. The obtained solution was then
covered with a layer of paraffin film pierced with ~15 - 20 pinholes to allow the evaporation of solvent
and reaction by-products. After ~ 2 weeks, an electroactive silica composite including pore-forming
agent (i.e., D-glucose) was produced. To remove D-glucose, the as-synthesized composite sample was
grounded into fine powders and followed by extracting the remaining components with excess
MeOH/H
2
O (1:2 v/v) at 120 °C for 7 days. After dynamic drying at 100 °C for 24 h, fine powder of
composite in dark green color, denoted PM silica, was obtained.
2
.5 Synthesis of PGM silica
A representative procedure to prepare PGM silica is given as follows: the PGM silica sample was
prepared by immersing as-synthesized PM silica material into aqueous solution of 0.10 mM
HAuCl ∙3H O at room temperature for 6 h. After centrifugal filtration and washing with excess amount
of distilled H O, the PGM silica was collected and dried overnight under vacuum at 50 °C.
4
2
2
2
.6 Preparation of PM silica/PGM silica-modified CPE
A modified carbon paste electrode (CPE) for the electrochemical sensor was fabricated by
homogenization followed by mixing using agate pestle and mortar. First of all, carbon paste (75 mg)
was prepared by thoroughly mixing graphite powder (50 mg), PM silica or PGM silica (15 mg) and
paraffin oil (10 mg). A Teflon tube, open at one end (3 mm diameter), was filled with the prepared
carbon paste and served as electrode body. Electrical contact was made with a copper wire through the
8

center of tube. Suitable packing was obtained compact electrode surface toward bond paper until
surface was smooth.
3
. Results and Discussion
The base form of PANI can be schematically illustrated in the following manner:[32]
The y value ranges from 0 for a fully oxidized polymer (so-called pernigraniline) to 0.5 for the
half-oxidized polymer (emeraldine) to 1 for the fully reduced polymer (leucoemeraldine).
3
.1 Redox behavior of PANI and AP
In this study, the redox capability of as-prepared PANI and AP was determined by cyclic
voltammetry (CV) using a 1.0 M H
2
SO aqueous solution, a scan range from -0.2 to +1.0 V, and a scan
4
-1
rate of 50 mV·s . Cyclic voltammogram of neat CPE contains no redox peaks, as shown in Figure 1(a).
Moreover, the voltammogram of PANI-modified CPE contains a pair of redox peaks at 192 and 778
mV, as shown in Figgure 1(b). The pair of redox peak that appeared in PANI-modified CPE was similar
to that of AP, which exhibited a pair of redox peaks at 261 and 542 mV, as shown in Figure 1(c). The
as-prepared PANI having longer conjugated length as compared to that of AP, implying that superior
electroactivity of PANI as compared to that of AP. For example, the PANI was found to exhibit a
-2
-2
higher current density of 917 A cm in the first oxidation wave compared to 316 A cm for AP.
9

Therefore, for the subsequent experiments, PANI was chosen as the potential candidate for the
following electrochemical sensor for AA detection.
3
.2 Characterization of PM and PGM silica
PM and PGM silica materials were prepared by acid-catalyzed sol–gel process in the presence of
PANI latex using a non-surfactant template, as shown in Scheme 1. For the preparation of PGM silica,
the PM silica was immersed in 5 mL of a 0.1 mM HAuCl ∙3H O aqueous solution for 6 h at room
temperature. The GNPs can be obtained after the reduction of AuCl -
, mediated by the redox change
4
2
4
of the PANI chains encapsulated in the nanochannels of wormhole-like mesoporous silica
frameworks from the EB to PB form.
The redox reactions occurring simultaneously between the EB form of PANI and the
HAuCl
4
∙3H O aqueous solution were confirmed by the FTIR spectra of PM silica and PGM silica,
2
as shown in Figure 2. In Figure 2(a), the benzenoid (B) diamine and quinoid (Q) diimine units of
the EB form of the PANI chains encapsulating in the PM silica are indicated by the transmission
-
1
bands appeared at wavenumber of 1503 and 1596 cm , respectively. After immersing PM silica
into aqueous HAuCl ∙3H O solution for 6 h, the spectrum [Figure 2(b)] of the as-obtained PGM
silica showed a characteristic band corresponding to the increase in Q unit of EB form of PANI at
4
2
-
1
wavenumber of 1596 cm , indicating that EB form of PANI had been partially oxidized toward the
PB form. The spectral changes also implied that GNPs were reduced and formed upon the surface
of PANI chains inside the nanochannels of mesoporous silica frameworks at the same time. In
addition, the appearance of GNPs inside PGM silica was further confirmed by transmission electron
1
0

microscopy (TEM), X-ray diffraction pattern (XRD), X-ray photoelectron spectroscopy (XPS) and
Brunaeur-Emmett-Teller (BET) analyses.
Morphology of GNPs onto the surface of PANI chains inside the nanochannels of PGM silica was
observed by TEM at magnification of 40k, as shown in Figure 3. For example, the electronic images of
TEM with scale bar of 50 nm was found to show black spots with irregular shape of ~ 1 nm in diameter
can be identified as GNPs, as shown in Figure 3 (a). Moreover, the electron diffraction pattern of GNPs
in PGM silica was shown in Figure 3 (b). The XRD diffraction pattern of PM silica and PGM silica
was illustrated in Figure 4. For example, the XRD of PM silica (i.e., without incorporating GNPs) was
shown in Figure 4(a). A clear broad diffraction pattern appeared at the location of 2θ = 25.04° was
ascribed to the amorphous structures of PANI polymer chains.[33] However, after immersing the PM
silica in 5 mL of a 0.1 mM aqueous HAuCl
4
∙3H O solution, the as-obtained PGM silica showed four
2
additional clear diffraction peaks at the position of 2θ = 38.01, 43.95, 64.52, and 77.43°, indicating
Bragg reflections from the Au planes of (111), (200), (220), and (311), respectively, as revealed in
Figure 4(b).[34] The formation of GNPs inside the nanochannels of PGM silica was also confirmed by
analysis of XPS spectra, as shown in Figure 5. For example, the binding energies of doublet observed
0
for Au 4f7/2 and Au 4f5/2 were consistent with those of Au oxidation state, located at the position of 83.4
and 86.9 eV, respectively, as shown in Figure 5(A).[35] These results clearly indicated that the
0
reduction of Au(III) to Au was achieved.
As shown in Figure 5(B), three characteristic peaks for
nitrogen of PANI are present, representing three different electronic states. Binding energy of nitrogen
of benzenoid was observed at the value of 399.5 eV, whereas that of quinoid was seen at 398.6 eV, and
+
that of e cationic radical (N ) was observed at 400.6 eV.[36]
1
1

It should be noted that the formation of uniform, ultrafine GNP distribution in the PGM silica plays a
vital role in exposing AuCl −
to the PM silica. Scheme 2 shows a possible mechanism for the reduction
of GNPs onto the surface of PANI chains inside the nanochannels of mesoporous silica frameworks. In
this mechanism, the AuCl −
ions are selectively adsorbed onto the surface of PANI chains inside the
4
4
nanochannels of PM silica and the protonated imine sites of PANI by electrostatic attraction, facilitating
0
complexation [37]. These complexes serve as the specific active sites for the formation of Au nuclei
because of electron transfer from the imine sites to AuCl −
,while PANI inside the nanochannels of the
4
PM silica matrix is oxidized, simultaneously reducing AuCl −
[38]. Subsequently, AuCl
−
ions are
4
4
0
further reduced on the surface of Au nuclei by autocatalytic surface reduction, leading to the growth of
0
Au nuclei and a uniform and ultrafine GNP distribution.
After the identification of the EB form of PANI chains and characterization of GNPs, the surface area,
pore volume, and pore size of the as-prepared mesoporous silica (MS), PM silica and PGM silica were
also investigated. The adsorption isotherms of the samples show type-IV-like isotherms, with a sharp
increase in the absorbed volume of N
2
at the relative pressure range of P/P
0
= 0.4–0.6. The N
2
adsorption–desorption isotherms for the MS, PM silica, and PGM silica samples at an operating
temperature of 77 K are shown in Figure 6. The data of composition, pore parameters of MS, PM silica
and PGM silica obtained from BET studies are summarized in Table 1. For example, BET surface area
2
-1
(
SA) of mesoporous silica (MS) was found to be 1145 m g , whereas its pore volume and pore
3
-1
diameter were 0.941 cm g and 2.9 nm, respectively. Moreover, BET SA of PM silica was found to
decrease due to the encapsulation of PANI chains. For example, SA of PM silica decreased to 630 m2
g-1
and the corresponding pore volume and pore diameter of PM silica was found to decrease to 0.275
,
3
-1
cm g and 2.19 nm, respectively. After formation of GNPs inside the nanochannels of PM silica, the
1
2

2
-1
SA, pore volume, and pore size of PGM silica was found to be further decreased to ~624 m g , 0.272
3
-1
cm g , and 2.13 nm, respectively. These results indicated that the GNPs were formed onto the PANI
chains inside the nanochannels of wormhole-like frameworks of PM silica.
3
.3 Cyclic voltammetry of PM and PGM silica
After the characterization of as-prepared materials, the important physical property of materials used
in electrochemical sensor application need to examined is redox capability. In this study, the redox
capability of MS, PM silica and PGM silica was identified by electrochemical cyclic voltammetry (CV)
studies. Based on the proposed redox mechanism of PANI,[39] electrochemical CV measurements
-1
were performed in a 1.0 M H
2
SO solution at a scan rate of 50 mV·s . The voltammograms of MS,
4
PM silica and PGM silica using a Ag/AgCl electrode as reference electrode were shown in Figure 7.
As shown in Figure 7(a), the curve of raw MS without incorporating PANI is a straight line without
any redox peak, indicating that no redox behavior occurred in the MS-modified electrode. Moreover,
the curve of PM silica revealed a pair of oxidation peaks at the potential of 243 and 512 mV,
corresponding to the change in oxidation state from LB to EB form of PANI, as well as from EB form
to PB form of PANI,[20, 40] as shown in the right-hand side of Figure 7. This observation indicated
that encapsulation of PANI chains into the nanochannels of wormhole-like mesoporous silica
framework may introduce redox capability to MS sample. For example, PM silica exhibited first
-2
oxidation peak at current density of 337 A cm , as shown in Figure 7(b). Furthermore, by reduction of
tiny number of GNPs onto the surface of PANI chains inside the nanochannels of PM silica, the
voltammogram of PGM silica was found to display first oxidation peak at current density of 834 μA cm-
2,
as shown in Figure 7(c). This result indicated that the reduction of GNPs onto the surface of PANI
1
3

chains inside the worm-like channels of PM silica may facilitate the oxidation of PANI from EB form
towards PB form, which was encapsulated in PGM silica.
3
.4 Electrocatalytic activity of PM silica/PGM silica CPE for the detection of AA
Electrocatalytic activity of as-prepared materials was used to detect AA by CV and amperometry.
Figure 8 shows the voltammograms corresponding to bare CPE, PM silica, and PGM silica-modified
-1
CPE in 0.1 M PBS (pH = 7) in the absence and presence of 2 mM AA, with a scan rate at 50 mVs .
Platinum foil and Ag/AgCl were used as the counter and reference electrodes, respectively. When there
was no AA appeared in PBS solution, no oxidation current appeared in voltammograms.
However, In Figure 8 (A), in the presence of 2 mM AA in PBS, a small oxidation current of bare
CPE occurred at a more positive potential of 670 mV, as shown in Figure 8 (A) (a), as compared to that
of the PM silica (E = 390 mV) and PGM silica-modified CPE (E = 390 mV), as shown in Figure 8 (A)
(b) and Figure 8 (A) (c), respectively. For example, the voltammogram of PM silica has a maximum
-2
current density for first oxidation transition at 193 A cm , which was lower than that of PGM silica,
-2
which was 338 A cm .
Furthermore, the repeatability of constructed electrochemical sensor for AA detection was also
investigated, as shown in Figure 8 (B). The long-term stability of PGM silica-modified CPE and PM
silica modified CPE was studied by storing the electrode at room temperature for 2 weeks in 0.1 M PBS
solution at pH = 7.0. It should be noted that, by detection of 2 mM of AA, PGM-modified CPE was
2
found to reveal good repeatability at the stable current density of ~ 340 A/cm with the mininum
relative standard derivatives of 0.43%, as shown in 8(B) (b) However, the PM silica modified CPE was
2
found to exhibit stable current density of ~ 190 A/cm with the mininum relative standard derivatives
1
4

of 0.59%, as shown in Fig. 8 (B) (a). After this testing period, the constructed electrochemical sensor
was found to retain ~ 93 - 94 % of its initial response, indicating the both constructed electrochemical
sensors reveal excellent long-term stability.
Effect of scan rate on performance of PGM silica modified electrode towards oxidation of AA was
-1
also studied for kinetic parameters. The different of scan rate (10 to 500 mV s ) was studied at PGM
silica-modified CPE in 0.1 M PBS (pH = 7) in the 400 µM AA, and their results are shown in Figure 9.
-1
The oxidation current of AA was increased when the scan rate rises from 10 to 500 mV s . Additionally,
1
/2
the linear relationship was plotted between peak current (I
observed linear regression equation is; I (µA) = 0.248 ν (Vs ) + 16.084.951, R = 0.9976.
Meaning that the electrode reaction of AA oxidation is controlled by diffusion based electron transfer
p
) and square roots of scan rates (ν ), the
1
/2
-1 1/2
2
p
reaction, as presented in the Figure 9 (B). Moreover, the plots of peak potential (E
p
) versus logν
2
(
Figure 9(C)) , the linear equation of this straight line is obtained as ; E
p
= 0.1077 log (ν) 0.5662, R =
0
.9955, was use to further investigate the mechanism of electrocatalysis of AA on the modified
electrode, Using Eq. (1), for a fully irreversible diffusion-controlled system. The anodic peak potential
can be evidenced by [41]:
2
.303RT
퐸푝 =
푙표푔휈 + 푘
2
(1 − 훼)푛 퐹
푎
−
1
−1
Where, α is the electron transfer coefficient, R (R = 8.314 J K mol ) is the rate constant, F (F =
−1
9
6485 C mol ) is Faraday constant and T (T = 298 K) is room temperature and n is the number of
a
electrons involved in the rate-determining step. Herein, the Tafel slope of 0.215 V/decade was
calculated from Figure 9(C), which indicates that single-electron transfer was involved in the rate-
limiting step assuming a transfer coefficient of α = 0.73.
CV responses of PGM silica-modified CPE with various concentrations of AA in PBS were shown in
1
5

Figure 10. The responses indicate that an increase in the concentration of AA corresponds to a rise in
the oxidation current. The concentration of AA in the buffer varied from 0.0 to 10.0 mM. The inset
shows the peak current plotted against the amount of AA added. A linear plot of oxidation current
-1
2
density versus the concentration of AA revealed a slope of 70.81 A·mM (R = 0.929), suggesting the
efficient electrocatalytic oxidation of AA by the PGM silica-modified CPE[42, 43].
Figure 11 (a) showed the typical amperometric responses to AA by bare CPE, PM silica-, and PGM
silica-modified CPE at an applied potential of 390 mV in a 30 mL PBS solution (pH = 7) under constant
stirring. The current response increased gradually, accompanied by an increase in AA concentration. It
should be noted that the GNPs formed/reduced onto the surface of PANI chains inside the nanochannels
of wormhole-like mesoporous silica framework resulted in a sudden increase in current response of AA.
-1
For example, sensitivity of PGM silica-modified CPE was calculated to be 95.95 A·mM , which is
.4-fold and 34.6-fold higher than those of PM silica-modified CPE and bare CPE, respectively, as
shown in Table 2.
At low concentrations, the bare CPE, PM silica-, and PGM silica-modified CPE all displayed a
direct linear relationship between the current response and AA concentration at a constant potential of
90 mV (Figure 11(b)). However, slope for PGM silica-modified CPE was steep compared to those of
6
3
other electrodes. On the gradual injection of different quantities of AA into a strongly stirred PBS
solution equipped with PGM silica-modified CPE, AA was found to detect with high sensitivity
-1
(
95.95 A mM ) and a high correlation coefficient (0.995). The defined sensitivity of PGM silica-
-1
modified CPE was 95.95 A·mM , which was found to be 6.4-fold higher than that of PM silica-
-1
-1
modified CPE (15.13 A·mM ) and almost 34.6-fold higher than that of bare CPE (2.76 A·mM ).
The linear dynamic range (LDR) and limit of detection (LOD) (S/N = 3) of the PGM silica-modified
1
6

CPE were found to be 0.005–13.3 mM and 0.97 M, respectively. This electrode can be seen easily in
the magnified image figure in the upper left corner of Figure 11(A).
In contrast, at higher AA concentrations (0.1–57.4 mM), the relationship between the responses and
the concentration of AA can be described by a two-parameter sigmoidal equation:[44]
j = jmax[1- exp(-c/b)],
where j represents the oxidation current, jmax is the maximum value obtained at a sufficiently high
concentration of AA, c is the concentration of AA, and b is an empirical coefficient characterizing the
-2
steepness of j, c transient, where jmax = 1421.52 A cm and b = 16.05 mM with a correlation
coefficient of r = 0.9937. However, such a fixed equation for j vs. c is inconvenient to use; therefore, the
logarithm of the abscissa of the calibration curve was used to generate a linear relationship. The slope
of the linear curve represents the sensitivity of the electrode. The defined sensitivity of PGM silica-
-2
-1
-2
modified CPE , PM silica-modified CPE and bare CPE were 62.94 A cm (lg(mM)) ,17.16 A cm
-1
-2
-1
(
lg(mM)) and 3.22 A cm (lg(mM)) , respectively. Moreover, the oxidation current increased with
the addition of AA and achieved a steady state after 2 s. This result indicated that the proposed PGM
silica-modified CPE sensor is a rapid-response sensor with high sensitivity, wide LDR, and low LOD. A
comparison of the sensitivity, LOD, and LDR for the other samples is given in Table 2. As seen in this
comparison, the designed sensor exhibited high sensitivity, a relatively LOD, and a broad LDR.
Moreover, the corresponding calibration plots for current density vs. concentration of AA of Bare CPE,
PM silica-modified CPE and PGM silica-modified CPE was also studied, as shown in Figure 11 (B).
3
.5 Differential pulse voltammetric responses for tertiary mixture of AA, DA and UA
1
7

For the selectivity studies of constructed electrochemical sensor, a tertiary mixture of 20 M
ascorbic acid (AA)/dopamine (DA)/uric acid (UA) was employed. Differential pulse voltammograms
(DPV) were recorded for a mixture of 20 M of DA, 20 M of AA and 20 M of UA for bare CPE, PM
silica-modified CPE and PGM silica- modified CPE in 0.1 M PBS (pH = 7), the experiments were
carried out using DPV in the range of - 0.3 to 0.7 V, as shown in Figure 12. The Figure 12 (a) - (c) were
graphs containing a tertiary mixture of AA/DA/UA by different modified CPEs and the Figure 12 (d) -
(
f) were three individual peaks for AA, DA and UA combined into one graph by PGM silica- modified
CPE. It should be noted that the the selectivity of bare CPE (Figure 12(a)) and PM silica-modified CPE
Figure 12 (b)) is poor for the tertiary mixture of AA/DA/UA. In contrast, the PGM silica-modified
(
CPE exhibited excellent selectivity and three distinct voltammetric peaks can be resolved at the
potential of - 0.01, 0.11, and 0.33 V (Figure 12 (d) - (f)), which corresponds to the oxidation of AA, DA,
and UA, respectively.
3
.6 Real sample of vitamin C tablets for the detection of AA
To validate the reliability of PGM silica-modified CPE in real samples, the proposed method was
applied to directly detect AA in real commercial products of vitamin C tablets. The results of tests
using two kinds of commercial vitamin C tablets spiked with 20 -50 M of AA were summarized in
Table 3, and these data were found in good agreement with those added with quantitative recoveries
from 95.7-104.7%, indicating that proposed method is quite reliable for detection of AA in real
commercial samples.
4
. Concluding remarks
1
8

In this paper, an electrochemical sensor, constructed by CPE modified with PGM silica, with high
sensitivity, excellent sensing repeatability, a relatively low limit of detection, a broader linear dynamic
range and high selectivity for the detection of AA was successfully presented. First of all, the PANI
with M
w
= 8,600 and M = 6,600, respectively, was prepared by emulsion polymerization based on the
n
studies of gel permeation chromatography (GPC). The PM silica was prepared by performing the acid-
catalyzed sol-gel reactions of TEOS in the presence of suitable amount of D-glucose (used as pore-
forming agent) and previous PANI latex, followed by removal of D-glucose through Soxhlet extraction.
Subsequently, PGM silica was obtained by immersing suitable amount of PM silica in 5 mL of 0.1 mM
HAuCl
4
solution for 6 h. The redox reactions that mediated between EB form of PANI and HAuCl
4
aqueous solution were monitored and confirmed by FTIR spectroscopy. The appearance of GNPs onto
surface of PANI chains inside the nanochannels of PGM silica was characterized by TEM, XRD, XPS
and BET analyses. The appearance of GNPs in the nanochannels of PGM silica was clearly observed
in TEM image. Moreover, XRD pattern of PGM silica was found to exhibit four additional diffraction
peaks as compared to that of PM silica. The four diffraction peaks found at the position of 2θ = 38.01,
4
3.95, 64.52, and 77.43°, which are characteristic Bragg reflections from the plane of Au (111), (200),
(220), and (311) planes, respectively. Furthermore, the XPS spectrum of PGM silica was found to
show binding energies of Au 4f7/2 and Au 4f5/2 doublet, which were consistent with oxidation state of
0
Au located at the position of 83.4 and 86.9 eV, respectively, indicating the appearance of GNPs.
After reduction/formation of GNPs inside the nanochannels of PM silica, the corresponding surface
area, pore volume, and pore size of PGM silica was found to decrease as compared to that of PM silica
based by the studies of BET. The redox capability of PGM was found to be higher than that of PM
silica based on the electrochemical CV studies. The higher redox capability of PGM silica as compared
1
9

to that of PM silica implied that the electrochemical sensor constructed by CPE modified with PGM
silica may reveal better sensitivity.
The as-prepared PGM silica-modified carbon paste electrode (CPE) was found to exhibit higher
sensitivity, a better sensing stability, a relatively low limit of detection, and a broader linear dynamic
range for AA detection as compared to that of bare CPE and PM silica-modified CPE. For example, a
electrochemical sensor constructed by PGM silica-modified CPE was found to demonstrate ~ 34.6-fold
and ~ 6.4-fold increase of AA detection as compared to that of bare CPE and PM silica-modified CPE,
respectively. Moreover, the PGM silica-modified CPE revealed an excellent electrochemical response
toward AA at a lower oxidative potential of 390 mV, with excellent stability and sensitivity of 95.95
-1

A·mM , a relatively low detection limit of 0.97 M, and a wide linear dynamic range from 5 M to
1
3.3 mM. The electrochemical sensor constructed by CPE modified with PGM silica was found to
reveal excellent selectivity for the tertiary mixture of 20 M AA/DA/UA as compared to that of bare
CPE and CPE modified with PM silica. The electrochemical sensor constructed by this work for AA
detection of real commercial samples of vitamin C tablets was investigated and the results was quite
reliable.
Acknowledgements
The authors acknowledge financial support from the Ministry of Science and Technology, Taiwan,
R.O.C. (MOST 104-2113-M-033-001-MY3 and MOST 104-2923-M-033-001-MY2) as well as the
Department of Chemistry, Center for Nanotechnology at Chung-Yuan Christian University and
Department of Chemistry at University of Santo Tomas.
2
0

Reference
[
1] L. Hu, L. Deng, S. Alsaiari, D. Zhang, N.M. Khashab, "Light-on" sensing of
antioxidants using gold nanoclusters, Anal. Chem. , 86 (2014) 4989-4994.
2] H. Bi, A.C. Fernandes, S. Cardoso, P. Freitas, Interference-blind microfluidic sensor
for ascorbic acid determination by UV/vis spectroscopy, Sens. Actuator B-Chem., 224
[
(2016) 668-675.
[3] C. Lou, W. Liu, X. Liu, Quantitative analysis of arsenic speciation in guano and
ornithogenic sediments using microwave-assisted extraction followed by high-
performance liquid chromatography coupled to hydride generation atomic fl uorescence
spectrometry, J. Chromatogr. B, 969 (2014) 29-34.
[4] N. Xia, L. Liu, R. Wu, H. Liu, S.J. Li, Y. Hao, Ascorbic acid-triggered
electrochemical–chemical–chemical redox cycling for design of enzyme-amplified
electrochemical biosensors on self-assembled monolayer-covered gold electrodes, J.
Electroanal. Chem., 731 (2014) 78-83.
[5] G.P. Keeley, N. McEvoy, H. Nolan, M. Holzinger, S. Cosnier, G.S. Duesberg,
Electroanalytical sensing properties of pristine and functionalized multilayer graphene,
Chem. Mat., 26 (2014) 1807-1812.
[6] D. Micić, B. Šljukić, Z. Zujovic, J. Travas-Sejdic, G. Ćirić-Marjanović,
Electrocatalytic activity of carbonized nanostructured polyanilines for oxidation
reactions: sensing of nitrite ions and ascorbic acid, Electrochim. Acta, 120 (2014) 147-
1
58.
[7] S. Pakapongpan, J.P. Mensing, D. Phokharatkul, T. Lomas, A. Tuantranont, Highly
selective electrochemical sensor for ascorbic acid based on a novel hybrid graphene-
copper phthalocyanine-polyaniline nanocomposites, Electrochim. Acta, 133 (2014) 294-
3
01.
[8] U. Rana, N.D. Paul, S. Mondal, C. Chakraborty, S. Malik, Water soluble polyaniline
coated electrode: a simple and nimble electrochemical approach for ascorbic acid
detection, Synth. Met., 192 (2014) 43-49.
[9] M.M. Sk, C.Y. Yue, Synthesis of polyaniline nanotubes using the self-assembly
behavior of vitamin C: a mechanistic study and application in electrochemical
supercapacitors, J. Mater. Chem. A, 2 (2014) 2830-2838.
[10] C. Xue, X. Wang, W. Zhu, Q. Han, C. Zhu, J. Hong, X. Zhou, H. Jiang,
Electrochemical serotonin sensing interface based on double-layered membrane of
reduced graphene oxide/polyaniline nanocomposites and molecularly imprinted polymers
embedded with gold nanoparticles, Sens. Actuator B-Chem., 196 (2014) 57-63.
[11] C. Zhou, Y. Shi, J. Luo, L. Zhang, D. Xiao, Diameter-controlled synthesis of
polyaniline microtubes and their electrocatalytic oxidation of ascorbic acid, J. Mater.
Chem. B, 2 (2014) 4122-4129.
[12] T. Wu, L.Y. Wang, S. Du, W.J. Guo, M.S. Pei, Micro/nanostructures of PANI
obtained in the presence of water soluble polymers and their electrochemical sensing
properties, RSC Adv., 5 (2015) 69067-69074.
2
1

[13] K. Shanmugasundaram, G. Sai Anand, A.I. Gopalan, H.G. Lee, H.K. Yeo, S.W.
Kang, K.P. Lee, Direct electrochemistry of cytochrome c with three-dimensional
nanoarchitectured multicomponent composite electrode and nitrite biosensing, Sens.
Actuator B-Chem., 228 (2016) 737-747.
[
14] H. Zhao, H. Yin, Y. Yang, Label-free electrochemical detection of DNA
methyltransferase activity via a DNA tetrahedron-structured probe, RSC Adv., 6 (2016)
9624-29628.
15] C.G. Wu, T. Bein, Polyaniline wires in oxidant-containing mesoporous channel
hosts, Chem. Mat. , 6 (1994) 1109-1112.
16] M.S. Cho, H.J. Choi, K.Y. Kim, W.S. Ahn, Synthesis and characterization of
polyaniline/ mesoporous SBA-15 nanocomposite, Macromol. Rapid Commun., 23 (2002)
13–716.
17] Q. Cheng, V. Pavlinek, C. Li, A. Lengalova, Y. He, P. Saha, Synthesis and
2
[
[
7
[
characterization of new mesoporous material with conducting polypyrrole confined in
mesoporous silica, Mater. Chem. Phys., 98 (2006) 504-508.
[
18] M. Nandi, R. Gangopadhyay, A. Bhaumik, Mesoporous polyaniline having high
conductivity at room temperature, Microporous Mesoporous Mat., 109 (2008) 239-247.
19] F.F. Fang, H.J. Choi, W.S. Ahn, Electroactive response of mesoporous silica and its
nanocomposites with conducting polymers, Compos. Sci. Technol., 69 (2009) 2088-
092.
20] S. Weng, Z. Lin, Y. Zhang, L. Chen, J. Zhou, Facile synthesis of SBA-
5/polyaniline nanocomposites with high electrochemical activity under neutral and
acidic conditions, React. Funct. Polym., 69 (2009) 130-136.
21] F.F. Fang, H.J. Choi, W.S. Ahn, Electrorheology of a mesoporous silica having
conducting polypyrrole inside expanded pores, Microporous Mesoporous Mat., 130
[
2
[
1
[
(2010) 338-343.
[22] C.J. Weng, W.I. Hung, C.W. Peng, C.H. Chang, Y.H. Chang, Y. Wei, K.C. Chiu,
S.J. Tang, J.M. Yeh, Preparation of electroactive silica mesopores by encapsulating
polyaniline chains into silica framework via nonsurfactant templating sol–gel route, J.
Non-Cryst. Solids, 357 (2011) 227-231.
[23] C.C.S. Pedroso, V. Junqueira, C.P.L. Rubinger, T.S. Martins, R. Faez, Preparation,
characterization and electrical conduction mechanism of polyaniline/ordered mesoporous
silica composites, Synth. Met., 170 (2013) 11-18.
[24] J. Cheng, S.J. Rathi, P. Stradins, G.L. Frey, R.T. Collins, S.K.R. Williams, Free
standing silica thin films with highly ordered perpendicular nanopores, RSC Adv., 4
(2014) 7627-7633.
[25] L. Ding, B. Su, A non-enzymatic hydrogen peroxide sensor based on platinum
nanoparticle–polyaniline nanocomposites hosted in mesoporous silica film, J.
Electroanal. Chem., 736 (2015) 83-87.
[
26] J. Noh, C.M. Yoon, J. Jang, Enhanced electrorheological activity of polyaniline
coated mesoporous silica with high aspect ratio, J. Colloid Interface Sci., 470 (2016)
37-244.
2
2
2

[
27] Y. Xiao, F. Patolsky, E. Katz, J.F. Hainfeld, I. Willner, Plugging into Enzymes
Nanowiring of Redox Enzymes by a Gold Nanoparticle, science, 199 (2003) 1877-1881.
28] M. Schierhorn, S.J. Lee, S.W. Boettcher, G.D. Stucky, M. Moskovits, Metal–Silica
Hybrid Nanostructures for Surface-Enhanced Raman Spectroscopy, Adv. Mater., 18
[
(2006) 2829-2832.
[29] H. Cai, Y. Xu, N. Zhu, P. He, Y. Fang, An electrochemical DNA hybridization
detection assay based on a silver nanoparticle label, Analyst, 127 (2002) 803-808.
30] J. Wang, R. Polsky, D. Xu, Silver-Enhanced Colloidal Gold Electrochemical
Stripping Detection of DNA Hybridization, Langmuir, 17 ( 2001) 5739-5741.
31] J. Wang, D. Xu, R. Polsky, Magnetically-Induced Solid-State Electrochemical
Detection of DNA Hybridization, J. Am. Chem. Soc., 124 (2002) 4208-4209.
32] J.-M. Yeh, S.-J. Liou, C.-Y. Lai, P.-C. Wu, Enhancement of Corrosion Protection
[
[
[
Effect in Polyaniline via the Formation of Polyaniline-Clay Nanocomposite Materials,
Chem. Mater., 13 ( 2001) 1131-1136.
[33] M. Kim, S. Cho, J. Song, S. Son, J. Jang, Controllable synthesis of highly
conductive polyaniline coated silica nanoparticles using self-stabilized dispersion
polymerization, ACS Appl. Mater. Interfaces, 4 (2012) 4603-4619.
[
34] S. Mondal, U. Rana, R.R. Bhattacharjee, S. Malik, One pot green synthesis of
polyaniline coated gold nanorods and its applications, RSC Adv., 4 (2014) 57282-57289.
35] X. Feng, C. Mao, G. Yang, W. Hou, J.J. Zhu, Polyaniline/Au composite hollow
[
spheres: synthesis, characterization, and application to the detection of dopamine,
Langmuir 22 (2006) 4384-4389.
[
36] X.L. Wei, M. Fahlman, A.J. Epstein, XPS study of highly sulfonated polyaniline,
Macromolecules, 32 (1999) 3114-3117.
37] D.W. Hatchett, M. Josowicz, J. Janata, D.R. Baer, Electrochemical formation of Au
clusters in polyaniline, Chem. Mat., 11 (1999) 2989-2994.
38] M.M. Nobrega, V.L. Martins, R.M. Torresi, M.L.A. Temperini, One-step synthesis,
[
[
characterization, and properties of emeraldine salt nanofibers containing gold
nanoparticles, J. Phys. Chem. C, 118 (2014) 4267-4274.
[
39] E. Song, J.W. Choi, Conducting Polyaniline Nanowire and Its Applications in
Chemiresistive Sensing, Nanomaterials, 3 (2013) 498-523.
40] S.B. Yoon, E.H. Yoon, K.B. Kim, Electrochemical properties of leucoemeraldine,
[
emeraldine, and pernigraniline forms of polyaniline/multi-wall carbon nanotube
nanocomposites for supercapacitor applications, J. Power Sources, 196 (2011) 10791-
1
[
0797.
41] A.J. Bard, L.R. Faulkner, Electrochemical Methods- Fundamentals and
Applications, Wiley, New York, (2001).
42] A.M. Pisoschi, A. Pop, A.I. Serban, C. Fafaneata, Electrochemical methods for
ascorbic acid determination, Electrochim. Acta, 121 (2014) 443-460.
43] A.M. Pisoschi, A. Pop, G.P. Negulescu, A. Pisoschi, Determination of ascorbic acid
[
[
content of some fruit juices and wine by voltammetry performed at pt and carbon paste
electrodes, Molecules, 16 (2011) 1349-1365.
2
3

[44] W. Shi, C. Liu, Y. Song, N. Lin, S. Zhou, X. Cai, An ascorbic acid amperometric
sensor using over-oxidized polypyrrole and palladium nanoparticles composites, Biosens.
Bioelectron., 38 (2012) 100-106.
[45] W. Li, L. Ding, Q. Wang, B. Su, Differential pulse voltammetry detection of
dopamine and ascorbic acid by permselective silica mesochannels vertically attached to
the electrode surface, Analyst, 139 (2014) 3926-3931.
[46] L. Ding, W. Li, Q. Sun, Y. He, B. Su, Gold nanoparticles confined in vertically
aligned silica nanochannels and their electrocatalytic activity toward ascorbic acid,
Chem. Eur. J., 20 (2014) 12777-12780.
[47] L. Ding, W. Li, Q. Wang, Q. Sun, Y. He, B. Su, Vertically oriented silica
mesochannels as the template for electrodeposition of polyaniline nanostructures and
their electrocatalytic and electroanalytical applications, Chem. Eur. J., 20 (2014) 1829-
1
833.
[48] F. Yan, B. Su, Tailoring Molecular Permeability of Nanochannel-Micelle
Membranes for Electrochemical Analysis of Antioxidants in Fruit Juices without Sample
Treatment, Anal. Chem., 88 (2016) 11001-11006.
[49] S. Sohrabnezhad, A. Pourahmad, New methylene blue (NMB) encapsulated in
mesoporous AlMCM-41 material and its application for amperometric determination of
ascorbic acid in real samples, Electroanalysis, 19 (2007) 1635-1641.
[50] J.B. Raoof, F. Chekin, V. Ehsani, Palladium-doped mesoporous silica SBA-15
modified in carbon-paste electrode as a sensitive voltammetric sensor for detection of
oxalic acid, Sens. Actuator B-Chem., 207 (2015) 291-296.
[51] A. Ambrosi, A. Morrin, M.R. Smyth, A.J. Killard, The application of conducting
polymer nanoparticle electrodes to the sensing of ascorbic acid, Anal. Chim. Acta, 609
(2008) 37-43.
[52] W. Kit-Anan, A. Olarnwanich, C. Sriprachuabwong, C. Karuwan, A. Tuantranont,
A. Wisitsoraat, W. Srituravanich, A. Pimpin, Disposable paper-based electrochemical
sensor utilizing inkjet-printed polyaniline modified screen-printed carbon electrode for
ascorbic acid detection, J. Electroanal. Chem., 685 (2012) 72-78.
[53] C.J. Weng, P.H. Hsu, S.C. Hsu, C.H. Chang, W.I. Hung, P.S. Wu, J.M. Yeh,
Synthesis of electroactive mesoporous gold–organosilica nanocomposite materials via a
sol–gel process with non-surfactant templates and the electroanalysis of ascorbic acid, J.
Mater. Chem. B, 1 (2013) 4983-4991.
[54] J. Huang, Y. Liu, H. Hou, T. You, Simultaneous electrochemical determination of
dopamine, uric acid and ascorbic acid using palladium nanoparticle-loaded carbon
nanofibers modified electrode, Biosens. Bioelectron., 24 (2008) 632-647.
[55] L. Xi, D. Ren, J. Luo, Y. Zhu, Electrochemical analysis of ascorbic acid using
copper nanoparticles/polyaniline modified glassy carbon electrode, J. Electroanal.
Chem., 650 (2010) 127-134.
[56] H. Zhang, F. Huang, S. Xu, Y. Xia, W. Huang, Z. Li, Fabrication of nanoflower-like
dendritic Au and polyaniline composite nanosheets at gas/liquid interface for
electrocatalytic oxidation and sensing of ascorbic acid, Electrochem. Commun., 30
(2013) 46-50.
2
4

Figure Captions
Fig. 1. Cyclic voltammograms of (a) CPE, (b) PANI and (c) aniline pentamer as measured in 0.1 M of
-1
H
2
SO aqueous solution with scan rate of 50 mV·s .
4
Fig. 2. Fourier-Transformation Infrared Spectroscopy of (a) PM silica and (b) PGM silica.
B: benzenoid diamine; Q: quinoid diimine
Fig. 3. TEM images of (a) PGM silica, (b) an electron diffraction pattern of GNPs inside PGM silica.
Fig. 4. Relative intensity of XRD diffraction patterns of (a) PM silica and (b) PGM silica.
Fig. 5. XPS spectrum of (a) PGM silica and (b) N(1S) spectra of PGM silica.
Fig. 6. (A) Nitrogen adsorption–desorption isotherms for (a) MS, (b) PM silica and (c) PGM silica. (B)
Pore-size distribution curves for (a) MS, (b) PM silica and (c) PGM silica.
Fig. 7. Cyclic voltammograms of (a) MS, (b) PM silica and (e) PGM silica as measured in 1.0 M H
2
SO
4
-1
aqueous solution with scan rate of 50 mV·s . The proposed reversible redox mechanism of
PANI was shown to the right-hand side.
Fig. 8. (A) Cyclic voltammograms of (a) bare CPE, (b) PM silica and (c) PGM silica modified CPE in 2
-1
mM of AA with scan rate of 50 mV·s . (B) The repeatability of (a) PM silica (b) PGM silica
modified CPE in proposed sensor for fourteen successive measurements in 2 mM AA.
Fig. 9. (A) Cyclic voltammograms obtained of PGM silica-modified CPE in PBS (pH~7) at different
-1
scan rates (curves a to g; 10, 20, 40, 100, 200, 300, 500 mVs ) in the presence of 400 µM AA.
1
/2
(
B) The plot between I
p
vs. square roots of scan rates (ν ) (C) The plot of E vs. logν.
p
Fig.10. Cyclic voltammograms of PGM silica-modified CPE in PBS (pH~7) with different
concentration of AA: (a) 0 mM, (b) 2 mM, (c) 4 mM, (d) 6 mM, (e) 8 mM and (f) 10 mM with
-1
scan rate of 50 mV·s . Inset shows the corresponding calibration curve.
2
5

Fig. 11. (A) Typical amperometric response obtained with (a) bare CPE, (b) PM silica and (c) PGM
silica-modified CPE at 390 mV with successive additions of AA into 30 mL of PBS. (B) The
corresponding calibration plot for current density vs. [AA] of the (a) bare CPE, (b) PM silica-
modified CPE and (c) PGM silica-modified CPE.
Fig. 12. Differential pulse voltammograms obtained with (a) bare-CPE, (b) PM silica and (c) PGM
silica-modified CPE for a tertiary mixture of 20 μM AA/DA/ UA in 0.1 M PBS. (d) 20 μM
AA, (e) 20 μM DA and (f) 20 μM UA were the individual peak obtained from PGM silica-
modified CPE in 0.1 M PBS, followed by combined into one graph.
Scheme 1. Flowchart for the synthetic route of PM silica and PGM silica.
Scheme 2. Possible mechanism for the formation of PGM silica.
2
6

scheme 1

scheme 2

Figure 1