Facile synthesis of monodisperse Ag@C@Ag core-double shell spheres for application in the simultaneous sensing of thymol and phenol

Article abstract of DOI:10.1039/c5nj00881f

A simple coupled synthesis and encapsulation route was developed to fabricate monodisperse, uniform Ag@C@Ag core-shell structured nanocomposites with excellent electrochemical and catalytic properties, in which Ag nanoparticles were first encapsulated in a carbonaceous shell through the catalyzed dehydration of glucose under hydrothermal conditions, and then the surface-activated Ag@C spheres were subsequently used to accumulate [Ag(NH₃)₂]⁺ or Ag⁺ ions through electrostatic attraction to anchor the Ag coating. The as-prepared nanocomposites were demonstrated to have a great potential for the simultaneous multiplexed detection of thymol and phenol, and exhibited high sensitivity and good reproducibility. In addition, the practical analytical application of the sensing platform was assessed by the determination of thymol and phenol in real honey samples, and satisfactory results were achieved.

Full text of DOI:10.1039/c5nj00881f

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Facile synthesis of monodisperse Ag@C@Ag core–double shell
DOI: 10.1039/C5NJ00881F
spheres for application in sensing thymol and phenol simultaneously†
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Tian Gan,ab Zhen Lv,a Yaping Deng,a Junyong Sun,a Zhaoxia Shi,a and Yanming Liua
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College of Chemistry and Chemical Engineering, Xinyang Normal University,
Xinyang 464000, P. R. China. E-mail: gantianxynu@163.com; Fax:
+86-376-6390597; Tel.: +86-376-6390702
b
State Key Laboratory of Lake Science and Environment, Nanjing Institute of
Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, P. R.
China
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† Electronic supplementary information (ESI) available.
A simple coupled synthesis and encapsulation route was developed to fabricate
monodisperse, uniform Ag@C@Ag core–shell structured nanocomposites with
excellent electrochemical and catalytic properties, in which Ag nanoparticles were
firstly encapsulated in a carbonaceous shell through the catalyzed dehydration of
glucose under hydrothermal condition, and the surface activated Ag@C spheres were
subsequently used to accumulate [Ag(NH3)2]+ or Ag+ ions through electrostatic
attraction to anchor Ag coating. The as–prepared nanocomposite was demonstrated to
have a great potential for the simultaneous multiplexed detection of thymol and
phenol, and exhibited high sensitivity and good reproducibility. In addition, the
practical analytical application of the sensing platform was assessed by determination
of thymol and phenol in real honey samples with satisfactory results.
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24 Introduction
DOI: 10.1039/C5NJ00881F
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Core–shell nanostructures, constructed by cores and shells of different chemical
compositions and properties, have been the subject of intensive research due to their
major advantages over simple component leading to the improvement of properties
such as increase in dispersibility, improved thermal and chemical stability, enhanced
resistance to catalyst poisoning, increased catalytic activity, and so on.1,2 Among them,
Ag nanoparticles (NPs) based core–shell nanostructures have received significant
5–7
attention.3 For example, Ag@fullerene4 and Ag@TiO2 core–shell structures have
shown excellent performance in plasmonic behavior and superior lithium storage,
respectively. In both cases, the Ag NPs functionalize as conductive metal NPs because
of their lower resistance as compared to other metal NPs.8 However, the weak
electrical conductivity as well as the nondispersibility of their coatings is not
beneficial for electrochemical and biological applications. Carbon materials possess
many advantages such as steady chemical properties, strong corrosion resistance, less
weight and low cost,9 which can be regarded as promising candidates to fabricate
functional core–shell composite structure with metal.
Templating is the most widely used method to synthesize core–shell nanostructures.
Typically, core–shell metal@carbon spheres are synthesized through multi–step
synthetic procedures including metal NPs preparation, surface functionalization or
modification, seeding and subsequent growing for complete carbon coating.10 These
are inherently time consuming, hence there remains a challenge to develop a simple
and cost–effective process to fabricate core–shell nanostructure. One–pot approach
represents a facile and green chemical strategy to simplify the synthesis process of
core–shell nanostructures.11 A recent example is the preparation of magnetic Fe3O4
core–carbon shell nanoparticles via the co–precipitation followed by the hydrothermal
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dehydrogenation of glucose. In addition, we recently reported a facile encapsulation
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approach to produce Fe@Fe3C–C core–shell nanostructure using glucose as the
reducer and carbon source.13 However, one drawback of this procedure is the
requirement of pre–synthesized core with suitable surface chemistry.
On the other hand, it is noticed that the chemical properties of carbon sphere are
usually neglected. Liu et al have confirmed that a large number of residual hydroxyl
groups lie on the surface of carbon after the hydrothermal treatment of glucose,14
which can be ionized into negative ions in alkali environment. The negative–charged
surface of carbon can be used as capturer for metal cations and another metal shell can
be coated around the carbon.
Herein, on the basis of above statements and our previous studies, we designed a
one–pot method for the preparation of monodispersed Ag@C spheres and their
one–pot coating of another compact Ag NPs shell, where metal NPs preparation,
surface modification, seeding, and the subsequent shell growing steps could be
achieved in twice hydrothermal reactions. The highlights or advantages of this method
could be attributed to the following two factors: (1) The Ag@C was constructed in a
one–pot solvothermal synthesis to produce an Ag NP as a core and amorphous carbon
with abundant dangling hydroxyl groups as the shell. This route was “green” by
design as no toxic reagents were added between core and shell syntheses. (2) The
Ag@C colloidal microsphere as the core template was activated all through the
reaction process, which avoided prior surface modification and seeding steps naturally,
thus simplified the silver coating process to a great extent. Compared with the tenuous
Ag NPs on Ag/C surface in previous work by Chen et al,15 this new configuration here
could ensure the uniform and complete coverage of Ag shell on Ag@C colloidal
microspheres reproducibly. Additionally, the electrochemistry experimental results
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showed that these composite microspheres with Ag NP cores and Ag NP–assembled
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shells supported by carbon exhibited great potential for simultaneous detecting trace
of thymol and phenol with very high sensitivity and satisfactory results for honey
analysis.
78 Experimental
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Chemicals and apparatus
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Silver nitrate (AgNO3), glucose, polyvinylpyrrolidone (PVP–K30), ammonia
(NH3·H2O), thymol and phenol were purchased from Aldrich Chemicals with
analytical purity. All chemicals were used as received without further purification.
Thymol and phenol were dissolved into absolute ethanol to prepare standard solutions,
and stored at 4 °C in dark. The water used was re–distilled.
Electrochemical measurements were performed on a CHI 660D electrochemical
workstation (CH Instruments Inc. Shanghai). A conventional three–electrode system,
consisting of an Ag@C@Ag modified glassy carbon electrode (GCE), a saturated
calomel reference electrode (SCE) and a platinum wire auxiliary electrode, was
employed. Field emission scanning electron microscopy (SEM) was conducted with a
Hitachi S–4800 microscope (Japan). Field emission transmission electron microscope
(TEM) images were measured using a Tecnai G220S–TWIN microscope (FEI
Company, Netherlands). X–ray diffraction (XRD) pattern was recorded on a Rigaku
MiniFlex600 X–ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) operating at
40 kV, 15 mA. The data were collected from 5 to 80° with the scan rate of 2° min–1
and steps of 0.02°. UV–vis absorption spectrum was recorded on a UV–vis 2550
spectroscope (Shimadzu, Ltd., Tokyo, Japan). Fourier transform infrared (FTIR)
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measurements (Perkin Elmer FTIR spectrophotometer) were collected in theDsOpI:e1c0t.1r0a3l9/C5NJ00881F
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range of 4004000 cm1 with a resolution better than 0.1 cm1.
Liquid chromatography (LC) detection of thymol and phenol was carried out with
Agilent 1100, coupled with a fluorescence detector.16 The Discovery RP–AmideC16
analytical column (150 mm×4.6 mm) was used. The mobile phase was acetonitrile
and water mixture, filtered through 0.45–m Millipore filter prior to use. The elution
was programmed with an initial isocratic step with a 20/80 (v/v) acetonitrile/water
mixture for 3 min, a gradient to 40/60 (v/v) acetonitrile/water in 0.01 min, and then,
maintaining this mixture for 8 min. The initial conditions were finally re–established
in 1 min and held for 15 min. The flow–rate was 1 mL min–1. And the sample
injection volume was 100 L. The detection was operated at wavelengths of 274/590
nm (excitation and emission).
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Synthesis of Ag@C@Ag
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The core–shell structured Ag@C spheres were synthesized by hydrothermal method
modified from previous report.17 In a typical synthesis, 0.0102 g of AgNO3 and
3.4230 g of glucose were respectively dissolved in 2 mL and 38 mL of distilled water
under vigorous stirring. When the two kinds of solution were fully mixed, the
resultant mixture was placed in a 50 mL Teflon–lined stainless steel autoclave and
heated at 180 °C for 4 h. The black products were then collected, washed five times
with water and ethanol, and dried at 80 °C for 5 h.
In order to coat Ag@C with Ag NPs, PVP was used as weak reducer.18 0.03125 g of
Ag@C, 33.75 mL of ethanol and 2.5 g of PVP–K30 were mixed step–by–step under
vigorous stirring in a reagent bottle. To the homogeneous solution obtained, 5 mL
re–distilled water containing 0.25 g of AgNO3 and 0.4 mL NH3·H2O was added and
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allowed to stir for another 10 min. After further dispersion in an ultrasonic bath for 10
DOI: 10.1039/C5NJ00881F
min, the resultant dark transparent solution was then transferred to a 50 mL
Teflon–lined stainless steel autoclave, tightly sealed and heated at a temperature of
120 °C for 6 h. After the reaction is complete, the autoclave was allowed to cool to
room temperature and the resultant precipitate was centrifuged and washed
completely with water and ethanol. The powder sample obtained was air–dried at 50
°C.
For comparison, carbon spheres (CS) without the Ag core were also prepared
according to the procedure for Ag@C, but exclude the addition of AgNO3.
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Electrode preparation
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Firstly, a 20.0–mg Ag@C@Ag was dispersed into 10.0 mL of re–distilled water by
1–h ultrasonic agitation to give a homogeneous Ag@C@Ag suspension. Secondly, the
GCE (diameter of 3 mm) was polished with 0.05 m alumina slurry on silk, and then
washed with ethanol/water (1:1, v/v) and water in an ultrasonic bath, each for 1 min.
10.0 L of the obtained Ag@C@Ag solution was coated onto the surface of GCE and
allowed to dry under an infrared lamp in the air.
Similarly, the CS film–modified GCE was prepared by coating 10.0 L of 2.0 mg
mL–1 CS suspension on GCE surface. And the Ag@C film–modified GCE was
prepared by coating 10.0 L of 2.0 mg mL–1 Ag@C suspension on GCE surface.
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Analytical procedure
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Unless otherwise stated, 0.1 M H2SO4 was used as determining medium for thymol
and phenol analysis. The analytical procedure mainly contains two steps:
accumulation step and determining step. Firstly, thymol and phenol were
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preconcentrated onto the Ag@C@Ag film–modified GCE surface under open–circuit
DOI: 10.1039/C5NJ00881F
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for 60–s stirring. After that, the differential pulse voltammograms from 0.5 to 1.3 V
were recorded, and the oxidation peak currents at 0.86 V and 1.03 V were measured
as the analytical signals for thymol and phenol, respectively.
148 Results and discussion
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Characterizations
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The dispersibility and stability of CS, Ag@C and Ag@C@Ag in water were tested
(Fig. 1). It can be seen that all of these three materials can be dispersed into water due
to the oxygen–containing groups on carbon surface that formed during the
carbonization of glucose.14 The color of Ag@C suspension is much similar to that of
CS because the Ag NPs are encapsulated into carbon shell, so both show the
characteristic appearance of CS. However, with the in situ growth of Ag NPs on
carbon surface, the color changes from yellowish–brown to black, indicating the
coating of another shell outside of carbon.
The crystalline structures of the prepared materials were studied by XRD (Fig. 1).
In XRD pattern of CS (curve a), the broad peak in the 2θ range of 10–30° is attributed
to the amorphous carbon.19 In contrast, four new diffraction peaks at 2θ = 38.1°, 44.2°,
64.5° and 77.3° appear on Ag@C (curve b) represent the (111), (200), (220) and (311)
planes of the face–centered–cubic structure of metallic Ag, respectively (JCPDS
870597). Furthermore, the diffraction peak corresponding to amorphous carbon still
exists. Therefore, Ag NP coexists with carbon in this hybrid. After the further coating
of Ag NPs (curve c), the intensity of diffraction peaks from Ag enhances significantly,
demonstrating an increasing amount of Ag crystals. Only diffraction peaks of the Ag
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appear and there is almost no carbon diffraction peak anymore, which indicateDtOhI:a1t0.t1h03Ve9ie/wCA5rNticJl0e0O8n8lin1Fe
Ag NPs successfully and densely deposit on the surface of carbon.20 According to the
Scherrer formula, the diameters of Ag NPs corresponding to inner core and outer shell
are respectively calculated to be 35.7 and 25.6 nm, which means that the encapsulated
and coated Ag are in nanoscale.
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Fig. 1 Appearance and XRD pattern of CS (a), Ag@C (b) and Ag@C@Ag (c).
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The formation of core–shell structured Ag@C@Ag spheres was also observed from
SEM and TEM images. The SEM study depicted in Fig. 2a indicates that the
morphology of hydrothermally grown Ag@C is sphere shaped. Fig. 2d shows typical
TEM image of Ag@C spheres with an average diameter of about 228 nm, with
smooth surfaces. It seems that most spheres exhibit perfect core–shell nanostructures
with single Ag core (~36 nm) and uniform shell thickness (~95 nm). Figs. 2b, c, e and
f show the SEM and TEM images of Ag coated Ag@C. Low–magnification SEM
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image of Ag@C@Ag nanocomposite is illustrated in Fig. 2b, where Ag@CDsOpI:h10e.r10e3Vs9ie/wCA5rNticJl0e0O8n8lin1Fe
are uniformly coated with Ag NPs. The uniform coating is basically composed of a
large number of spherical Ag NPs with different sizes, ranging from 20 to 30 nm. The
average diameter of the Ag NP is about 26 nm, which is in agreement with the above
XRD observation. High–magnification SEM image shows that the surface of the
Ag@C sphere becomes rough after the growth of Ag NPs shell (Fig. 2c), and the
Ag@C spheres are covered with Ag NPs completely, which indicates the excellent
interfacial interaction between carbon and Ag NPs. The TEM image shown in Fig. 2e
further confirms the shell growth of Ag NPs. The apparent contrast in the TEM image
between Ag@C core and Ag NPs shell offers evidence for the formation of
Ag@C@Ag spheres. The periodicity of the lattice fringes for Ag obtained from high
resolution–TEM image of Ag@C@Ag indicates that d = 0.236 nm (Fig. 2f),
corresponding to the (111) planes of the metallic Ag with face–centered–cubic
structure.21
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Fig. 2 SEM (a–c) and TEM (d–f) images of Ag@C (a, d) and Ag@C@Ag (b, c, e, f).
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A UV–vis spectrum study of Ag@C@Ag was further performed (Fig. 3DAO)I:.10T.1h03e9/C5NJ00881F
extinction peak corresponding to typical extinction spectrum of Ag NPs red–shifts to
460 nm, this is resulted from the coverage of Ag NPs by carbonaceous shells.20
Furthermore, the absorption in a wide range from 200 to 600 nm is generated from the
intermediary carbonaceous shell.
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Fig. 3 (A) UV–vis spectrum of Ag@C@Ag. (B) FTIR spectrum of Ag@C.
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Growth mechanism of Ag@C@Ag core–double shell spheres
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The schematic illustration in Scheme 1 depicts the growth process of Ag@C@Ag
core–double shell spheres: Firstly, nucleation of inner Ag NPs (step a). After being
heated to 180 °C in autoclaves, Ag+ will be deoxidized by glucose in their
homogeneous mixed solution, and then Ag NPs nucleate gradually. Secondly, growth
of carbonaceous shell (step b). The small satellite Ag NPs are evenly dispersed in
solution, while exposing reactive surfaces outside, which catalyze following
carbonization of glucose and lead to in situ deposition of carbonaceous products
around the Ag NP surfaces to form carbonaceous shells. The FTIR spectrum (Fig. 3B)
clearly shows that a large amount of functional groups such as CO (10001450
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cm1), OH (10001450 cm1), C=O (1701 cm1), C=C (1620 and 1510 cm1), CH
(2926 cm1) and OH (3404 cm1) covalently bond onto the carbon framework in the
carbonaceous component. These surface functional groups can be broken or form
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ionized sites (O groups) by hydroxyl anions present in alkaline conditions, which
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imply that a large surface charge will be present. Consequently, positivecharged
complex ions [Ag(NH3)2]+ or Ag+ will be accumulated around the Ag@C
microspheres through electrostatic attraction. Thirdly, seeding of outer Ag NPs (step
c). The captured [Ag(NH3)2]+ or Ag+ are gradually deoxidized by PVP as gentle
reducer, and Ag NPs are formed in situ as seeds. Finally, growth of Ag shell (step d).
The initialformed Ag seeds can act as growing centers for the subsequent shell
growth until complete Ag coverage is realized. All the reaction steps mentioned above
are integrated and achieved in twostep hydrothermal reaction in our work.
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Scheme 1 Schematic illustration of the formation of core–shell structured Ag@C@Ag
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spheres.
Voltammetric behaviors of thymol and phenol
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The electrochemical behaviors of thymol and phenol at the bare GCE and Ag@C@Ag
film–modified GCE were investigated using cyclic voltammetry (CV). Fig. 4a shows
the cyclic voltammograms of 0.1 mM thymol on the bare GCE at scan rate of 100 mV
s–1. In 0.1 M H2SO4, an oxidation peak is observed at 0.98 V during the anodic sweep
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from 0.5 to 1.3 V. On the following reverse scan, no corresponding reduction peak iVsiew Article Online
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observed, suggesting that the electrochemical oxidation of thymol on the bare GCE is
irreversible. The cyclic voltammogram of Ag@C@Ag film–modified GCE in 0.1 mM
thymol is depicted as Fig. 4b. During the anodic potential sweep, a well–defined and
sensitive oxidation peak appears at 0.95 V. On the reverse scan, a negligible reduction
peak is observed at 0.83 V. From the comparison between curves (a) and (b), it is very
clear that the oxidation peak current of thymol obviously increases at the Ag@C@Ag
film–modified GCE. This can be attributed to the following two reasons: on one hand,
the carbon shell outside effectively protects the Ag NPs from corrosion. On the other
hand, the core–shell architecture maximizes the Ag–support interaction through the
three–dimensional contact between the Ag NPs and the carbon, thereby facilitating the
electron transfer rate.
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Fig. 4 Cyclic voltammograms of 0.1 mM thymol and phenol at bare GCE (a,c) and
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Ag@C@Ag/GCE (b,d) in 0.1 M H2SO4. Scan rate: 100 mV s–1.
The effectiveness of Ag@C@Ag film–modified GCE for the oxidation of phenol
was also assessed by CV in 0.1 M H2SO4. Compared with bare GCE (Fig. 4c), sharp
and well–resolved oxidation peak of phenol is found on the Ag@C@Ag
film–modified GCE (Fig. 4d), suggesting that the Ag@C@Ag has a strong
enhancement ability for the electric signal of phenol. The electrochemical reaction of
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phenol on the Ag@C@Ag film–modified GCE is a totally irreversible process.
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Ag@C@Ag can be used for the simultaneous determination of thymol and phenol
because this nanocomposite has the ability to enhance the determination sensitivity of
thymol and phenol.
The electrochemical responses of low concentration of thymol and phenol on the
surface of bare GCE, CS/GCE, Ag@C/GCE and Ag@C@Ag/GCE were compared
using differential pulse voltammetry (DPV). As seen in Fig. 5, two oxidation peaks at
0.85 V (O1) and 0.99 V (O2) corresponding to thymol and phenol are observed on the
bare GCE surface, and the peak currents are relatively low (curve a). The weak
response signals suggest that the oxidation activities of thymol and phenol on bare
GCE surface are low. Under the identical conditions, the oxidation peak currents
increase at the CS film–coated GCE (curve b). CS with good electric conductivity can
form a perfect thin film on GCE surface. Therefore, the CS film–coated GCE has
better conductivity and exhibits higher accumulation ability to thymol and phenol.
Without a doubt, the oxidation peak currents of thymol and phenol increase at the CS
film–coated GCE. For the Ag@C/GCE, the presence of Ag core leads to the
augmented current responses of thymol and phenol (curve c). This indicates better
electrocatalytic oxidation of thymol and phenol by the Ag NPs. In comparison with
the Ag@C/GCE, thymol and phenol at the Ag@C@Ag/GCE exhibit more sensitive
electrochemical responses (curve d). The obvious peak current enlargements indicate
that Ag@C@Ag exhibit strong signal enhancement for the oxidation of thymol and
phenol. SEM and TEM measurements tell us that the obtained Ag@C@Ag samples
possess excellent monodispersity, rough and textured surfaces. As a result, the
accumulation efficiency is improved remarkably, and the oxidation signals enhance
greatly. In addition, the DPV response of Ag@C@Ag/GCE in the absence of thymol
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and phenol is also studied, and the curve is shown as curve e. It is virtually featurelesVsiew Article Online
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and no oxidation wave appears. So the observed oxidation peak at 0.86 and 1.03 V are
attributed to the oxidation of thymol and phenol. In summary, the prepared
Ag@C@Ag spheres are more active and greatly increase the detection sensitivity of
thymol and phenol.
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Fig. 5 DPV graphs of bare GCE (a), CS/GCE (b), Ag@C/GCE (c) and
Ag@C@Ag/GCE (d) in 0.1 M H2SO4 containing 1.0 M thymol and 5.0 M phenol.
Curve e corresponds to DP voltammogram of Ag@C@Ag/GCE in blank solution.
Accumulation time = 60 s, pulse amplitude = 50 mV, pulse width = 0.2 s, pulse period
= 0.5 s.
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Effect of scan rate
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The influences of scan rate on electrochemical behaviors of 0.1 mM thymol and
phenol at the Ag@C@Ag/GCE in 0.1 M H2SO4 were also investigated (Fig. 6). Good
linearity between the anodic peak current (Ip) and the scan rate (ν) with r2 = 0.994 and
r2 = 0.995 are obtained within the range from 50 to 300 mV s–1 for thymol (inset A of
Fig. 6) and phenol (inset B of Fig. 6), respectively. The linear regression equations can
be expressed as Ip (A) = –1.111+0.0803 ν (mV s–1) (for thymol) and Ip (A) =
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7.307+0.173 ν (mV s ) (for phenol). The results indicate that the electrochemical
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oxidation of thymol and phenol at the Ag@C@Ag/GCE are adsorption–controlled
processes. The peak potentials (Ep) of thymol and phenol shift to more positive values
with the increase of scan rate, which further indicates the irreversibility of the
oxidation processes.
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Fig. 6 Cyclic voltammograms of 0.1 mM thymol and phenol in 0.1 M H2SO4 at the
Ag@C@Ag/GCE with different scan rates including 50 (a,a′), 100 (b,b′), 150 (c,c′),
200 (d,d′), 250 (e,e′) and 300 (f,f′) mV s–1. Inset: linear relationship of Ip vs. ν (A,B)
and Ep vs. lnν (C,D).
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For an irreversible and adsorption–controlled oxidation process, the relationship
between Ep and ν is defined by the following equation according to Laviron′s theory:22
RT
RTk0 RT
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Ep  E0' 
ln

ln
(1)
nF nF nF
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where E0′ is the formal potential, k0 is the standard heterogeneous electron transfer
rate constant, T is the temperature, F is the Faraday constant, α is the electron transfer
coefficient, and n is the number of transferred electrons. In this work, the Ep of thymol
and phenol against logarithm of scan rate (lnν) is also examined under the same
conditions and show linear relationships (insets C and D of Fig. 6), following the
regression equations:
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Ep = 0.762+0.0426 lnν, r2 = 0.995 (thymol)
Ep = 0.894+0.0434 lnν, r2 = 0.995 (phenol)
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Combining equation (1), we can calculate the value of αn to be 0.603 and 0.592 for
thymol and phenol, respectively. And the calculation has been given in details in ESI†.
Generally, α is assumed as 0.5 for a totally irreversible electrode process. Hence one
electron is involved in the oxidation of thymol and phenol, which is consistent with
the reported results.23,24 The oxidation mechanisms for thymol and phenol on
Ag@C@Ag film modified GCE are presented in Scheme 2.
327
328
Scheme 2 Electrochemical reaction mechanisms of thymol and phenol.
329
Optimization of experimental conditions
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331
332
333
334
335
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338
The effects of several parameters, such as the electrolyte, amount of Ag@C@Ag
suspension, accumulation potential and time on the voltammetric responses of thymol
and phenol using this method were investigated.
The electrochemical responses of 1.0 M thymol and phenol at the Ag@C@Ag
film modified electrode in different supporting electrolytes such as 0.2 M disodium
hydrogen phosphate–citric acid buffer (pH 2.2–8.0), 0.2 M acetic acid–sodium acetate
buffer (pH 2.6–5.8), 0.2 M boric acid–borax buffer (pH 7.4–9.0), 0.1 M disodium
hydrogen phosphate–sodium dihydrogen phosphate buffer (pH 5.8–8.0),
Britton–Robinson buffer (pH 1.81–11.92) and 0.1 M HCl, H2SO4, HClO4, NaOH
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solutions were examined by DPV. In alkaline electrolytes, the Ag NPs onDOAI:g10@.10C39/C5NJ00881F
microspheres will react with OH– and the final products Ag2O can be formed. So the
electrochemical activity of the modified electrode is weakened seriously and no redox
peaks attributing to thymol and phenol can be observed. However, two small
oxidation peaks are found in neutral and weak acid environment. But evident peaks
can be obtained in strong acid solutions, like 0.1 M HCl, H2SO4, and HClO4, and the
best voltammogram shape in 0.1 M H2SO4. Therefore, 0.1 M H2SO4 was selected as
the electrocatalytic medium for the simultaneous determination of thymol and phenol.
It is well clear that Ag@C@Ag spheres can remarkably improve the oxidation peak
current of thymol and phenol from Fig. 4 and Fig. 5. However, the electron transfer
will be blocked and the background current may increase if the modifier film is too
thick on electrode surface. The experimental results show that as gradually improving
the amount of Ag@C@Ag suspension from 0 to 10 L, the oxidation peak currents of
thymol and phenol increase dramatically because the sites for adsorption of thymol
and phenol also increase, resulting in much higher accumulation efficiency, and thus
the obvious enhancement of oxidation peak current. However, the oxidation peak
current of thymol and phenol decrease when the amount of Ag@C@Ag suspension on
electrode surface exceeds 10 L, and the very thick film tends to fall off the electrode
surface when dipped in electrolyte. In this work, the optimized amount of Ag@C@Ag
suspension was chosen as 10 L.
The effects of accumulation potential and time on the anodic peak currents of 1.0
M thymol and phenol were examined by DPV. The effects of accumulation potential
on the responses of thymol and phenol were investigated under different potentials.
The accumulation time was 2 min, and the studied accumulation potential changed
from 0.1 to 0.7 V. Moreover, the accumulation step was also conducted under
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open–circuit. It is found that the oxidation peak current of 1.0 M thymol and phenol
almost keep unchanged, indicating no influence of accumulation potential on the
detection of thymol and phenol. Herein, the accumulation step was performed under
open–circuit.
The influence of accumulation time on the oxidation peak currents of thymol and
phenol was also evaluated. The oxidation peak currents increase with the
accumulation time increase, and when the accumulation time is longer than 60 s, the
peak currents converge to a stable value, indicating that the absorption of thymol and
phenol onto the Ag@C@Ag film has reached saturation quickly. Therefore, the
accumulation time was set to 60 s.
374
Reproducibility and Interference
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After each measurement, the Ag@C@Ag film was carefully removed from the GCE
surface and another new Ag@C@Ag modified GCE was remade as above–mentioned
procedure. The reproducibility between multiple Ag@C@Ag modified electrodes was
estimated by comparing the oxidation peak currents of 1.0 M thymol and phenol.
The relative standard deviation (RSD) on ten Ag@C@Ag modified GCEs is 3.7% and
4.2% for thymol and phenol, respectively, revealing that this method possesses good
reproducibility.
The interferences of many foreign species (especially contained in honey sample)
on the determination of thymol and phenol were studied by DPV under the
above–optimized conditions. The oxidation peak currents of 1.0 M thymol and
phenol in the absence and presence of foreign species with various concentrations
were measured, respectively. Based on this, the peak current change can be achieved.
Herein, when the peak current change exceeds 8%, it is considered that this substance
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causes obvious interference, and the corresponding concentration is defined aVsiew Article Online
DOI: 10.1039/C5NJ00881F
tolerance level. The results are listed in Table 1, suggesting that 5000–fold
–
concentration of Al3+, 1000–fold concentration of Cu2+, NO3 , K+, Na+, Ca2+, Zn2+,
Mg2+, H2O2, glucose, 500–fold concentration of tartrazine, 250–fold concentration of
ascorbic acid, and 50–fold concentration of hydroquinone almost have no influence
on the determination of thymol and phenol.
Table 1 Interference on the determination of 1.0 M thymol and phenol
Tolerance level
(M)
Signal change for
Signal change for
Interferents
thymol (%)
3.5
phenol (%)
0.95
–2.6
8.0
Al3+
Cu2+
5.00×10–3
1.00×10–3
1.00×10–3
1.00×10–3
1.00×10–3
1.00×10–3
1.00×10–3
1.00×10–3
1.00×10–3
1.00×10–3
5.00×10–4
2.50×10–4
5.00×10–5
–6.4
5.1
–
NO3
K+
Na+
5.0
7.6
7.0
–3.1
–6.7
–5.0
1.3
Ca2+
–4.5
–1.7
7.5
Zn2+
Mg2+
H2O2
0.8
–4.6
7.2
Glucose
Tartrazine
Ascorbic acid
Hydroquinone
1.2
–6.2
–7.3
3.9
–5.5
1.8
3.4
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396
Simultaneous determination of thymol and phenol
For simultaneous and quantitative determination of thymol and phenol, DPV curves at
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different concentrations of thymol were recorded in Fig. 7A, whereDOpI:h1e0n.10o3l9/C5NJ00881F
concentration was kept at 3.0 M. The inset shows that the peak current varies
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linearly with thymol concentration between 0.1 and 10 M with r = 0.997.
Importantly, the anodic peak current of phenol is almost uninfluenced by the increase
of thymol concentration, suggesting that oxidation of thymol and phenol at the
Ag@C@Ag/GCE is independent of each other. With the DPV technique the detection
limit of thymol is 21.6 nM in the presence of 3.0 M phenol interference (S/N = 3).
Fig. 7B presents DPV responses at different concentrations of phenol while thymol is
kept constant at 1.0 M. Similar to the scenario in Fig. 7A, the anodic peak current of
thymol stays almost constant as phenol concentration is increased gradually, further
confirming that this modified electrode can be employed for simultaneous
determination of thymol and phenol. The inset in Fig. 7B illustrates that the peak
current increases linearly with phenol concentration between 0.5 and 50 M with r =
0.999. In the presence of thymol, the low detection limit is 41.5 nM for phenol (S/N =
3).
412
413
Fig. 7 DPVs of Ag@C@Ag/GCE in (A) 3.5 M phenol and different concentration of
thymol: 0.1, 0.3, 0.5, 0.8, 1.0, 3.0, 5.0, 8.0 and 10 M (from a to h), and (B) 1.0 M
phenol and different concentration of phenol: 0.5, 0.8, 1.0, 3.0, 5.0, 8.0, 10, 30 and 50
M (from a′ to h′). The insets show the calibration plots of thymol and phenol versus
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peak current. Other conditions are as in Fig. 5.
Although a lot of electrochemical methods have been reported for the detection of
thymol or phenol,23–27 but none of them intends to analysis thymol and phenol
simultaneously. It is significant that Ag@C@Ag core–shell structured spheres used in
our study can realize the direct and simultaneous determination of thymol and phenol
with much lower detection limits (Table 2). Moreover, this is the first report for the
simultaneous determination of thymol and phenol based on electrochemical method as
far as we know.
Table 2 Comparison of the analytical performances of this work with the reports
Linear range
Limit of
Modified electrode
Analyte
Reference
(M)
detection (nM)
CeO2/graphene/GCE
Graphene oxide/GCE
Thymol
Thymol
0.1~18
2~200
50
65
23
25
21~292 and
357~922
1.0~750
Zincon/CPE
Phenol
Phenol
Phenol
900
500
500
24
26
27
ZnO/CNTsa/HPDBb/CPEc
Poly
9.8~80
diphenylamine/CNTs/GCE
Thymol
Phenol
0.1~10
0.5~50
21.6
41.5
This
Ag@C@Ag/GCE
work
427
428
429
a CNT: Carbon nanotube
b HPDB: N-(4-hydroxyphenyl)-3,5-dinitrobenzamide
c CPE: carbon paste electrode
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430
Honey samples analysis
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Honey is a naturally sweet and levorotatory carbohydrate rich product, which is
elaborated by honey bees from carbohydrate–containing exudates produced by plants,
and is largely consumed worldwide.28 According to EC norm 2001/110/EC (2002),
honey must be exempt of antibiotics, pesticides, atmospheric pollutants and heavy
metals. However, thymol and phenol can usually be found in honey as residues of
acaricide against Varroa destructor29 and bee repellent,30 respectively. Although
thymol residues in honey are not toxic, they can devalue honey quality.24 Contrarily,
phenol is known have adverse effects.31 Therefore, this newly–proposed method was
used to detect thymol and phenol in honey samples of different botanical origin
(chrysanthemum, jujube flower and Chinese milk velch) to evaluate its application in
the food control sciences. These honey samples were obtained from several
beekeepers from China. All samples were kept sealed in the absence of light. Next,
1.00 g of each honey sample was exactly weighed and respectively dissolved into 10
mL hot water with a temperature of about 50 °C, and then 500 L of the completely
dissolved sample solution was added into 0.1 M H2SO4, and then analyzed according
to the above–described procedure under the optimized conditions. Table 3 shows the
content of thymol and phenol in three honey samples, which is measured by the
standard addition method. Each sample undergoes six parallel measurements, and the
RSD is below 3%, suggesting that this method possesses good reproducibility. To
testify the accuracy of this method, the LC was also used to determine thymol and
phenol. The results are listed in Table 2, too. It is found that the results by Ag@C@Ag
film modified GCE and LC are in good agreement, indicative of good accuracy.
Otherwise, the recoveries of thymol and phenol standard that added into the samples
are also measured, and the results are in the range from 96.4% to 107%, indicating
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that this method has great potential in the practical sample analysis.
Table 3 Detection of thymol and phenol in Chrysanthemum honey (A), Jujube flower
honey (B) and Chinese milk velch honey (C) samples
Ag@C@
Ag/GCE
(M)
Rel
error
(%)
LC
Added Found
RSD
Recovery
(%)
Sample Analyte
thymol
(%) (M)
(M)
(M)
/
/
2.00
1.00
2.11
105
A
0.080
2.9
2
phenol
0.0831
3.62
0.964
96.4
–3.3
8
thymol
B
0.400
1.7 0.414
0.500 0.531
0.800 0.845
106
phenol
/
0.320
/
/
106
98.0
107
thymol
2.3 0.309 3.56 0.600 0.588
1.00 1.07
C
phenol
/
458 Conclusion
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467
In summary, we have successfully developed a facile one–pot approach to produce
uniform Ag@carbon with a core–shell structure by combining simultaneous Ag
reduction and carbonization of glucose. Subsequently, a template–activated strategy
toward uniform coating Ag nanoparticles on the Ag@carbon microspheres in one step
was presented, which greatly simplified the traditional coating process and may
provide a simple avenue for other metal encapsulation and coating. The uniform
Ag@carbon@Ag core–shell composite microspheres were also investigated as
electrochemical platform, which demonstrated an excellent detecting performance to
thymol and phenol. Armed with the remarkable advantages, such as facile one–pot
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synthesis, low analyte consumption, inexpensive cost and fast response time, the
DOI: 10.1039/C5NJ00881F
electrochemical sensing platform may hold great potential for the in–field food safety
monitoring.
471 Acknowledgment
472
473
474
The authors acknowledge funding support from the National Natural Science
Foundation of China (No. 61201091) and the open fund of State Key Laboratory of
Lake Science and Environment (No. 2014SKL013).
475 References
476
477
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