Materials Science and Engineering C 38 (2014) 39–45
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Preparation of novel silver nanoplates/graphene composite and their application in vanillin electrochemical detection Linhong Huang a,b,c, Keyu Hou b,c, Xiao Jia b,c, Haibo Pan a,b,c,⁎, Min Du a a b c
Fujian Key Lab of Medical Instrument & Pharmaceutical Technology, Yishan Campus, Fuzhou University, Fuzhou, Fujian 350002, China Institute of Research for Functional Materials, Fuzhou University, Yishan Campus, Fuzhou University, Fuzhou, Fujian 350002, China College of Chemistry and Chemical Engineering, Qishan Campus, Fuzhou University, Fuzhou, Fujian 350108, China
a r t i c l e
i n f o
Article history: Received 15 November 2013 Received in revised form 4 January 2014 Accepted 21 January 2014 Available online 29 January 2014 Keywords: Ag nanoplates Graphene Electrocatalysis Vanillin
a b s t r a c t Hexagonal Ag nanoplates (NPs) were synthesized by polyvinylpyrrolidone (PVP) and trisodium citrate (TSC) which selectively absorbed to Ag (100) and Ag (111) surfaces, then were anchored to graphene (GN) to form novel Ag NPs/GN composite. The thickness of Ag NPs is ~4 nm and the length is 18–66 nm. Transmission electron microscopy (TEM) image shows that the plates are f-c-c crystals containing {111} facets on their two planar surfaces. Zeta potential indicated that the surface of Ag NPs/GN is negatively charged while vanillin is positively charged. Thus Ag NPs/GN modified on glass carbon electrodes (GCE) allowed abundant adsorption for vanillin and electron transfer between vanillin and Ag NPs/GN/GCE. Square wave voltammetry (SWV) results indicated that the over potential on Ag NPs/GN/GCE negatively shifts 52 mV than that on Ag NPs/GCE. Ag NPs/GN with enhanced surface area and good conductivity exhibited an excellent electrocatalytic activity toward the oxidation of vanillin. The corresponding linear range was estimated to be from 2 to 100 μM (R2 = 0.998), and the detection limit is 3.32 × 10−7 M (S/N = 3). The as-prepared vanillin sensor exhibits good selectivity and potential application in practical vanillin determination. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Colloidal noble-metals, especially of Pd, Ag, Au, and Pt, have been studied intensively because of their application in catalysis [1], surfaceenhanced Raman scattering (SERS) [2] and sensors [3]. Silver, a comparatively inexpensive catalyst and conductive metal, has been developed to many interesting structures, such as spheres, cubes, polyhedrons, wires [4], plates [5,6], and dendrite, because of its strong shape-dependent chemical and physical properties [7,8]. Note that Ag nanoplates (Ag NPs) have been researched owing to their fascinating catalytic and optical properties, large specific surface and effective/geometric area [9,10]. So the Ag NPs are expected to be synthesized and used as electrochemical sensing materials. The facet-specific capping has emerged as a facile route to the synthesis of Ag NPs, such as citrate and polyvinylpyrrolidone (PVP) which selectively bind to Ag (111) and Ag (100) surfaces, respectively. Further, the growth rate of silver atoms is distinct along the different facets to promote the plate structure forming [11]. In this field, the research on morphology control about Ag plates is well considered, but reports about composite based on the Ag NPs and their application are quite few. Graphene (GN) with high electron mobility (~200,000 cm2 V s−1) [12] and high specific surface area (2600 m2 g− 1) [13] provides an ⁎ Corresponding author at: College of Chemistry and Chemical Engineering, Qishan Campus, Fuzhou University, Fuzhou, Fujian 350108, China. Tel.: +86 591 83759450/ 22866127; fax: +86 591 22866127. E-mail address: [email protected] (H. Pan). 0928-4931/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2014.01.037
avenue for fabricating electrochemical devices because it can facilitate electron transfer between electro-active species and electrodes [14,15]. The negatively charged graphene can attach free metal ions and oxygen groups at GO would act as crystal growth sites for the formation of composite. The oxygen-containing groups on graphene oxide (GO) would not only load metal ions but also afford targets for further covalent functionalization of GO [16,17]. The GN composite with the second component in layers can prevent the stacking of single-layer GN caused by π–π strong interaction [18,19]. Among various GN composites, Wang et al. indicated that the oxygen functional GO adsorbs trisodium citrate (TSC) on Ag triangle plates to drive the morphology modification [20]. In this work, GN with good conductivity will be added into pure Ag NPs to improve the electrical and catalytic performance. Vanillin (4-hydroxy-3-methoxybenzaldehyde) is widely used as flavor in milk powder, confectionery, beverages, food and perfumery [21]. However, excess vanillin is ingested into the human body, causing headaches and nausea and affecting liver or kidney functions. The maximum usage of vanillin is 7 mg/100 g (FAO/WHO 1992) while in the infant food vanillin is forbidden [22]. Hence, the determination of vanillin contents plays an important role in food fields. Several methods for determining vanillin have been researched including UV spectrophotometer [23], molecular imprinting [22], and electro-analysis [24,25]. However, electrochemical sensor based on nano-materials can be used for efficient detecting vanillin which contains electro-active groups. In this work, we present the application of Ag NPs/GN/GCE to detect vanillin by square wave voltammetry (SWV). By controlling the
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Scheme 1. The assembly process of Ag NPs/GN/GCE used in the detection of vanillin.
proportion of PVP and TSC, nano-scale hexagonal Ag NPs were synthesized and first anchored to GN through TSC. The Ag NPs/GN with enhanced surface area and good conductivity can lower over potential and increase peak current in electrochemical detection. Furthermore, we obtain the optimal condition for vanillin electro-oxidation and the analysis of vanillin in a real sample, biscuit. The sensor to vanillin exhibits high sensitivity, rapid response and good selectivity. 2. Experimental 2.1. Reagents Natural graphite (FP, 99.95% pure), silver nitrate (AgNO3), TSC (≥ 99%), sodium borohydride (NaBH4, ≥ 99%), PVP (average Mw ∼ 111) and vanillin were obtained from Sinopharm Chemical Regent Co., Ltd. Hydrogen peroxide (H2O2, 30 wt.%) was purchased from Tianjin Chemical Co., Ltd. All chemicals were used as received and without any further purification.
After the injection, the color of the solution immediately became light yellow and maintained 30 min, then the color of the solution changed into dark yellow and quickly tuned into red, green, and blue gradually. GO was synthesized from natural graphite powder using the modified Hummers' method [27]. In brief, 1 g of graphite dispersed in 46 mL H2SO4 was slowly mixed with 1 g of NaNO3 and 6 g KMnO4 in an ice bath under strong stirring for 2 h. Then the mixture was transferred to a water bath and stirred for 2 h at 35 °C. The temperature was raised to 90 °C after the addition of 280 mL H2O and kept at 90 °C for 1 h. The KMnO4 was removed by the addition of 20 mL of H2O2. The light brown graphite oxide was purified by washing with 10 wt.% HCl and washed using warm water (30 °C) for several times. Exfoliation was accomplished by sonicating graphite oxide aqueous solution for 120 min and then centrifuged at 5000 rpm for 10 min to obtain GO solution. GO was reduced to GN by NaBH4 in water bath for 1 h followed by addition of TSC. The composite was assembled by injecting Ag NPs into GN under vigorous stirring for 40 min at the ice bath. The route to construct vanillin sensor is illustrated in Scheme 1. The sufficient citrate ions in GN can act as complex sites and were purified by dialysis finally.
2.2. Apparatus UV–vis spectra were recorded with a spectrophotometer (Perkin-Elmer Lambda 900 USA). The morphologies of samples were observed by AFM (Agilent 5500 USA). High-resolution transmission electron microscope (HRTEM) image was obtained using Tecnai G2 F20 S-TWIN, 200 kV (FEI Company, USA). Zeta-potential value measurement was performed on Zeta Sizer 3000 Laser Particle Size and Zeta Potential Tester (Malvern Corporation, UK), and deionized water was used as a dispersant here. Electrochemical experiments were performed with a CHI660D electrochemical workstation (CH Instrument Company, Shanghai, China) with a conventional three-electrode cell. A Pt wire and a Ag/AgCl electrode were used as the auxiliary electrode and reference electrode, respectively. Glass carbon electrode (8 mm diameter) loading different materials was used as working electrode. The surface of GCE was polished with 0.3 and 0.05 μm alumina slurries, ultrasonicated in water and ethanol, respectively. Then the bare GCE was dried under pure N2. Finally, different materials were dropped on the surface of bare GCE, and dried under room temperature to form working electrode.
3. Results and discussion 3.1. Physicochemical characterization Fig. 1 depicts the UV–vis absorption spectra of Ag NPs, GN, and Ag NPs/GN dispersed in water for reference. The as-prepared Ag NPs
2.3. Preparation of Ag NPs/GN and the vanillin sensor The synthesis of Ag NPs was referred to previous work [26]. Typically, 50 μL of AgNO3 (0.05 M) was added into 24.15 mL deionized water followed by 0.5 mL of TSC (75 mM), 60 μL of H2O2 (30 wt.%), and 0.1 mL of PVP (17.5 mM) under vigorous stirring at room temperature. Then, 0.25 mL NaBH4 (100 mM) was injected into the above solution.
Fig. 1. UV–vis absorption spectra of (a) Ag NPs, (b) GN and (c) Ag NPs/GN. Inset: photographic images of Ag NPs, GN and Ag NPs/GN dispersed in H2O.
L. Huang et al. / Materials Science and Engineering C 38 (2014) 39–45
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Fig. 2. AFM images of (a) Ag NPs, (b) GN and (c) Ag NPs/GN.
show blue color and its corresponding UV–vis absorption spectra (Fig. 1a) displays two distinct plasmon bands at 331 nm and 620 nm which are assigned to the out-of-plane quadruple and in-plane dipole plasmon resonance of Ag hexagonal plates, respectively. After introducing GN, the in-plane dipole plasmonic bond of Ag NPs blue-shifts from 620 nm to 602 nm as seen in curve c and the inset
of Fig. 1. This phenomenon usually results from a little reduction of the size and the aspect ratio of the NPs. The out-of-plane band in 331 nm slightly red-shifted to 333 nm in Fig. 1c, which may stem from the interaction between Ag NPs and GN [10]. The characteristic peak at 262 nm attributes to the restored electronic conjugation within GN.
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Fig. 3. (a) HRTEM images of Ag NPs/GN, (b) high-resolution TEM image displaying the lattice fringes of Ag NPs, (c) high-resolution lateral view of Ag NPs and (d) inset: diffraction patterns of Ag NP.
Atomic force microscopy (AFM) is a direct method to observe the thickness and surface morphology. Ag NPs dispersed well have a thickness of ~4 nm as shown in Fig. 2a. Fig. 2b reveals that GN has uniform thickness and lateral dimensions ranging from several hundreds nanometers to a few micrometers. The thickness (~ 0.98 nm) of exfoliated GN is somewhat larger than Van der Waals thickness of 0.34 nm suggesting GN sheets may be bilayers or still contain oxygen functional groups. The composite appears to be uniform and typical sheets are
shown in Fig. 2c. After the self-assembly of Ag NPs, the height of composite is ~ 4 nm, illustrating that it can prevent the stacking of singlelayer GN by introducing Ag NPs, retaining the excellent property of GN. To further characterize the exact structures of this composite, HRTEM analysis was performed. The transparent GN sheets which can increase the electronic delivery exhibit flexible two-dimensional structure. Fig. 3a shows that the diameters of Ag NPs are in the distribution of 18–66 nm. The shapes of Ag NPs distributed randomly on GN are almost in a hexagonal form. From the unique profile drawn based on the different brightnesses in TEM, we can presume that hexagonal Ag NPs
Fig. 4. CVs of the Ag NPs/GN modified GCE in the (a) absence and (b) presence of 10−5 M vanillin in 0.1 M PBS (pH 6.98).
Fig. 5. SWV of (a) bare GCE, (b) Ag NPs/GCE, (c) GN/GCE and (d) Ag NPs/GN/GCE in 0.1 M PBS (pH 6.98) containing 1.0 × 10−5 M vanillin.
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Fig. 8. Amperometric responses of Ag NPs/GN/GCE upon subsequent additions of 10 μM vanillin, 100 μM TSC, 1 mM glucose and 1 mM L-cysteine in 0.1 M PBS (pH 6.98).
thickness of ~ 4 nm. The facet of the part sides indicated in TEM is (111) based on a spacing of 0.231 nm corresponding to the value in Fig. 3b. A morphological model of Ag NPs can be illustrated in the inset of Fig. 3c. The illustration in Fig. 3d gives a typical electron diffraction pattern taken by focusing the electron beam along the [111] zone of an individual NP. The strongest intensity spot (see the triangle in the inset) can be indexed to the Bragg diffraction from the {220} lattice planes of an f-c-c single crystal oriented in the [111], corresponding to the lattice spacing of 0.144 nm, proving that the hexagonal facets of the Ag NPs are composed of {111} planes. The inner spots (circle in the inset) could be ascribed to the 1/3{422} reflection indicating a number of parallel {111} twin planes and possibly atomically flat surface [28]. 3.2. Electrochemical characterization Fig. 6. (a) CVs of Ag NPs/GN/GCE in PBS (pH 6.98) containing 1 × 10−5 M vanillin at scan rates of 50, 100, 150, 200, 250, and 300 mV s−1. (b) The relationship of the oxidation current with the scan rate.
with two-dimensional growth have a certain thickness. The flatexposed crystal facets are {111}, corresponding to the lattice spacing of 0.238 nm as shown in Fig. 3b. Due to citrate selectively binding to Ag (111), it favors to form Ag NPs enclosed preferentially by {111} facets. The coexisting PVP adsorbs on Ag (100) and impels the formation of hexagonal NPs [10]. Fig. 3c shows the side view of Ag NPs with a
Current/µA
80
60
100 90 80 70 60 50 40 30 20 10 0 0.4
Current/ µA
100
100µM 100µ
2µM 0.5
0.6 E/V
0.7
0.8
40
I=0.871+0.959C R2=0.998
20
0 0
20
40
60
80
100
Concentration/µM Fig. 7. Calibration curves between vanillin concentration and peak current. Inset: SWV curves of Ag NPs/GN/GCE in vanillin solution with concentrations of 2–100 μM.
The electro-catalytic properties of Ag NPs/GN modified electrodes were mainly performed in a CHI660D electrochemical workstation based on SWV since it is more sensitive than cyclic voltammetry (CV). Fig. S1 displays CVs of different electrodes in 1 mM [Fe (CN) 6]3− containing 0.1 M KCl at 100 mVs−1 scan rate. After GCE was modified with Ag NPs (curve b), the peak current of [Fe (CN) 6]3− decreased, indicating that a small barrier to interfacial electron transfer appears by immobilizing Ag NPs. The peak current increased after modified Ag NPs/GN than pure Ag NPs, showing that the introduction of GN plays an important role in the increase of electron delivery. GN not only provides the conducting bridges for the electron-transfer of [Fe (CN) 6]3− but also enlarges the electro-active surface area. Fig. 4 shows the CV curves using the Ag NPs/GN/GCE in the absence (curve a) and presence (curve b) of 10− 5 M vanillin in 0.1 M PBS (pH 6.98). The sensor shows no response in pure PBS while exhibiting a typical reduction/oxidation at 0.62 V in the presence of 10−5 M vanillin. These results indicate that Ag NPs/GN can effectively catalyze vanillin oxidation on the modified electrode surface. The proposed electrochemical mechanism of vanillin is shown as the following mechanism (Scheme 1 and Fig. S2.), which involved two electrons and two protons oxidation [29]. Thus the electrons transfer fast through Ag NPs/GN composite, leading to a larger peak current. To better study the electro-catalytic activity of the same amount of Ag NPs, GN and Ag NPs/GN toward the vanillin oxidation in PBS (pH 6.98), typical SWVs are displayed in Fig. 5. Compared to GCE, Ag NPs/GCE exhibited a little higher current response due to their energetic sharp corners and a broad oxidation peak. This is because their sizes do not exceed the dimension of the diffusion layer; they should not contribute to increasing the electro-active area of modified GCE [9,30]. In the case of GN/GCE, an obvious peak as vanillin oxidation appears at around + 0.61 V, indicating that GN exhibits excellent catalytic
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Table 1 Determination of vanillin in real samples (n = 5). Samples
Quality/g
Determination/mg
Added/mg
Found/mg
Recovery/%
RSD/%
1 2
2.0002 2.0003
0 0
0.1 0.2
0.098 0.195
98.0 97.5
2.0 2.4
performance towards the vanillin oxidation. While the peak (+0.60 V) current of the Ag NPs/GN/GCE (Fig. 5d) is much larger and the over potential is 52 mV more negative than that of Ag NPs/GCE (+ 0.65 V) (Fig. 5b). This attributes to the enhanced surface area and excellent electro-catalytic activity carried out by both Ag NPs and GN. Zeta potential indicated that the surface of Ag NPs/GN is negatively charged (−2 mV) while vanillin is positively charged (+1.7 mV). Thus Ag NPs/ GN/GCE allowed abundant adsorption and electron transfer of vanillin. 3.3. Effect of scan rate Fig. 6a shows an increase in oxidation peak currents and a positive shift in peak position as scan rate increases for 10−5 M vanillin. The linear relationship between the anodic peak currents and scan rate from 50 to 300 mV S−1 is shown in Fig. 6b, which reveals that the oxidation of vanillin is an adsorption-controlled process. The Ag NPs/GN/GCE after detecting vanillin also shows oxidation peak for vanillin in blank PBS (pH 6.98) with the peak current decreasing gradually (depicted in Fig. S3). This phenomenon also agrees with the adsorption-controlled mechanism. 3.4. Optimization of experimental conditions and calibration curve The effect of solution pH on the electrochemical response of vanillin at Ag NPs/GN/GCE was investigated with PBS solution in the pH range from 5.6 to 8.0 (Fig. S4); the maximum peak current appeared at pH 6.98, so this value was selected throughout the experiments. Since the electrode process was adsorption-controlled, the accumulation potential and time for vanillin adsorbing on the Ag NPs/GN/GCE were also investigated. Accumulation process was studied under open potential condition. As shown in Fig. S5, the peak current increased first and then decreased with the accumulation potential increasing which showed that + 0.4 V is the best choice for accumulation potential for vanillin detection. Fig. S6 shows the best accumulation time is 10 s. The deposition amount of Ag NPs/GN was also studied in Fig. S7. The response increased rapidly with composite amounts increasing to 5 μL and subsequently decreasing. Because of hindrance of the electron transfer in a thick membrane at high amounts, the optimal amount selected to modify GCE was 5 μL. Finally, these optimized parameters were applied as test conditions for vanillin microanalysis. The calibration curve obtained with the oxidation peak currents under the optimal conditions is shown in Fig. 7. In the range of 2 μM to 100 μM, the peak current response is proportional to the vanillin concentration with the linear regression equation, I (μA) = 0.871 + 0.959C (μM) (R2 = 0.998). The detection limit is estimated to be 3.32 × 10−7 M at a signal-to-noise ratio of 3.
competitive adsorption of methyl blue or p-toluenesulfonic acid on graphene which in turn proves the possible π–π adsorption of vanillin. The vanillin solution was also measured by the modified electrodes stored in air for 7 days; the oxidation peak potential of vanillin is not shifted and the peak current has no apparent decrease, with a relative standard deviation (RSD) of 4.2 %, indicating that the vanillin sensor has good stability and might be applied to determine vanillin in real samples. In order to test the practical application of the proposed sensor, the Ag NPs/GN/GCE was used to determine vanillin in commercial biscuits. A certain amount of biscuits (2 g) were immersed in 100 mL deionized water for 6 h. The mixture was ultrasonicated for half hour and then filtered. No vanillin in this biscuits was detected under the optimized conditions. Finally, the standard addition method was used to inspect the average recovery of vanillin added in the biscuits. The results obtained were shown in Table 1. The average recovery of the method was 97.8% under critical condition (5 mg (vanillin)/100 g (biscuit)) compared with FAO/WHO (1992) (7 mg/100 g) [20]. Thus, the results prove that the vanillin sensor by Ag NPs/GN/GCE could satisfy to identify the maximum usage of vanillin in the foods. Also, the low RSD value indicated that the Ag NPs/GN/GCE has good reproducibility and potential application in practical vanillin determination.
4. Conclusions In summary, hexagonal Ag NPs were synthesized by PVP and TSC which selectively bind to Ag (111) and Ag (100) surfaces. The novel Ag NPs/GN composite was simply synthesized using TSC as bridging agent. Ag NPs have a thickness of ~ 4 nm and 18–66 nm in length. Their two planar surfaces contained single {111} facets and zeta potential indicated that the surface of Ag NPs/GN is negatively charged while vanillin is positively charged. Thus Ag NPs/GN/GCE allowed abundant adsorption and electron transfer of vanillin. The application for silver NPs has been broadened after assembling composite with GN. The Ag NPs/GN with enhanced surface area and good conductivity exhibits an excellent electro-catalytic activity toward the oxidation of vanillin. The composite can facilitate electron transfer between electro-active species and electrodes producing lower over-potential and increasing the peak current. The constructed vanillin electrochemical sensor has high sensitivity, a low detection limit of 3.32 × 10−7 M and a wide linear range from 2 μM to 100 μM. The introduction of GN to Ag NPs can broaden their application providing a promising platform to develop excellent electrochemical sensors.
Acknowledgements 3.5. Interference test and analytical application The possible interference for the detection of vanillin was investigated by successively adding foreign chemical species co-existing in real samples, such as Ca2+, Na+, K+, Fe3+, NH4HCO3, TSC, glucose, and amino acid. The results show that no interference is observed in the presence of 100-fold of metal ions. Other foreign species also do not interfere with the oxidation of vanillin with a relative current change less than 5% (Fig. 8). In addition, p-toluenesulfonic acid and methyl blue as interfering substances were also tested out in vanillin oxidation. The SWVs indicate that 10-fold of methyl blue and 100-fold of p-toluenesulfonic acid interfere with the oxidation of vanillin. This may attribute to the
The authors gratefully acknowledge the financial support from Project for International S & T Cooperation of China (2012DFM30040), National Science Fund of China (NSFC) (21201035, 61201397, J1103303 (J2013-004)) and the Fujian Department of Science and Technology (2012J01204).
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2014.01.037.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Preparation of novel silver nanoplates/graphene composite and their application in vanillin electrochemical detection Linhong Huang a,b,c, Keyu Hou b,c, Xiao Jia b,c, Haibo Pan a,b,c,⁎, Min Du a a b c
Fujian Key Lab of Medical Instrument & Pharmaceutical Technology, Yishan Campus, Fuzhou University, Fuzhou, Fujian 350002, China Institute of Research for Functional Materials, Fuzhou University, Yishan Campus, Fuzhou University, Fuzhou, Fujian 350002, China College of Chemistry and Chemical Engineering, Qishan Campus, Fuzhou University, Fuzhou, Fujian 350108, China
a r t i c l e
i n f o
Article history: Received 15 November 2013 Received in revised form 4 January 2014 Accepted 21 January 2014 Available online 29 January 2014 Keywords: Ag nanoplates Graphene Electrocatalysis Vanillin
a b s t r a c t Hexagonal Ag nanoplates (NPs) were synthesized by polyvinylpyrrolidone (PVP) and trisodium citrate (TSC) which selectively absorbed to Ag (100) and Ag (111) surfaces, then were anchored to graphene (GN) to form novel Ag NPs/GN composite. The thickness of Ag NPs is ~4 nm and the length is 18–66 nm. Transmission electron microscopy (TEM) image shows that the plates are f-c-c crystals containing {111} facets on their two planar surfaces. Zeta potential indicated that the surface of Ag NPs/GN is negatively charged while vanillin is positively charged. Thus Ag NPs/GN modified on glass carbon electrodes (GCE) allowed abundant adsorption for vanillin and electron transfer between vanillin and Ag NPs/GN/GCE. Square wave voltammetry (SWV) results indicated that the over potential on Ag NPs/GN/GCE negatively shifts 52 mV than that on Ag NPs/GCE. Ag NPs/GN with enhanced surface area and good conductivity exhibited an excellent electrocatalytic activity toward the oxidation of vanillin. The corresponding linear range was estimated to be from 2 to 100 μM (R2 = 0.998), and the detection limit is 3.32 × 10−7 M (S/N = 3). The as-prepared vanillin sensor exhibits good selectivity and potential application in practical vanillin determination. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Colloidal noble-metals, especially of Pd, Ag, Au, and Pt, have been studied intensively because of their application in catalysis [1], surfaceenhanced Raman scattering (SERS) [2] and sensors [3]. Silver, a comparatively inexpensive catalyst and conductive metal, has been developed to many interesting structures, such as spheres, cubes, polyhedrons, wires [4], plates [5,6], and dendrite, because of its strong shape-dependent chemical and physical properties [7,8]. Note that Ag nanoplates (Ag NPs) have been researched owing to their fascinating catalytic and optical properties, large specific surface and effective/geometric area [9,10]. So the Ag NPs are expected to be synthesized and used as electrochemical sensing materials. The facet-specific capping has emerged as a facile route to the synthesis of Ag NPs, such as citrate and polyvinylpyrrolidone (PVP) which selectively bind to Ag (111) and Ag (100) surfaces, respectively. Further, the growth rate of silver atoms is distinct along the different facets to promote the plate structure forming [11]. In this field, the research on morphology control about Ag plates is well considered, but reports about composite based on the Ag NPs and their application are quite few. Graphene (GN) with high electron mobility (~200,000 cm2 V s−1) [12] and high specific surface area (2600 m2 g− 1) [13] provides an ⁎ Corresponding author at: College of Chemistry and Chemical Engineering, Qishan Campus, Fuzhou University, Fuzhou, Fujian 350108, China. Tel.: +86 591 83759450/ 22866127; fax: +86 591 22866127. E-mail address: [email protected] (H. Pan). 0928-4931/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2014.01.037
avenue for fabricating electrochemical devices because it can facilitate electron transfer between electro-active species and electrodes [14,15]. The negatively charged graphene can attach free metal ions and oxygen groups at GO would act as crystal growth sites for the formation of composite. The oxygen-containing groups on graphene oxide (GO) would not only load metal ions but also afford targets for further covalent functionalization of GO [16,17]. The GN composite with the second component in layers can prevent the stacking of single-layer GN caused by π–π strong interaction [18,19]. Among various GN composites, Wang et al. indicated that the oxygen functional GO adsorbs trisodium citrate (TSC) on Ag triangle plates to drive the morphology modification [20]. In this work, GN with good conductivity will be added into pure Ag NPs to improve the electrical and catalytic performance. Vanillin (4-hydroxy-3-methoxybenzaldehyde) is widely used as flavor in milk powder, confectionery, beverages, food and perfumery [21]. However, excess vanillin is ingested into the human body, causing headaches and nausea and affecting liver or kidney functions. The maximum usage of vanillin is 7 mg/100 g (FAO/WHO 1992) while in the infant food vanillin is forbidden [22]. Hence, the determination of vanillin contents plays an important role in food fields. Several methods for determining vanillin have been researched including UV spectrophotometer [23], molecular imprinting [22], and electro-analysis [24,25]. However, electrochemical sensor based on nano-materials can be used for efficient detecting vanillin which contains electro-active groups. In this work, we present the application of Ag NPs/GN/GCE to detect vanillin by square wave voltammetry (SWV). By controlling the
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Scheme 1. The assembly process of Ag NPs/GN/GCE used in the detection of vanillin.
proportion of PVP and TSC, nano-scale hexagonal Ag NPs were synthesized and first anchored to GN through TSC. The Ag NPs/GN with enhanced surface area and good conductivity can lower over potential and increase peak current in electrochemical detection. Furthermore, we obtain the optimal condition for vanillin electro-oxidation and the analysis of vanillin in a real sample, biscuit. The sensor to vanillin exhibits high sensitivity, rapid response and good selectivity. 2. Experimental 2.1. Reagents Natural graphite (FP, 99.95% pure), silver nitrate (AgNO3), TSC (≥ 99%), sodium borohydride (NaBH4, ≥ 99%), PVP (average Mw ∼ 111) and vanillin were obtained from Sinopharm Chemical Regent Co., Ltd. Hydrogen peroxide (H2O2, 30 wt.%) was purchased from Tianjin Chemical Co., Ltd. All chemicals were used as received and without any further purification.
After the injection, the color of the solution immediately became light yellow and maintained 30 min, then the color of the solution changed into dark yellow and quickly tuned into red, green, and blue gradually. GO was synthesized from natural graphite powder using the modified Hummers' method [27]. In brief, 1 g of graphite dispersed in 46 mL H2SO4 was slowly mixed with 1 g of NaNO3 and 6 g KMnO4 in an ice bath under strong stirring for 2 h. Then the mixture was transferred to a water bath and stirred for 2 h at 35 °C. The temperature was raised to 90 °C after the addition of 280 mL H2O and kept at 90 °C for 1 h. The KMnO4 was removed by the addition of 20 mL of H2O2. The light brown graphite oxide was purified by washing with 10 wt.% HCl and washed using warm water (30 °C) for several times. Exfoliation was accomplished by sonicating graphite oxide aqueous solution for 120 min and then centrifuged at 5000 rpm for 10 min to obtain GO solution. GO was reduced to GN by NaBH4 in water bath for 1 h followed by addition of TSC. The composite was assembled by injecting Ag NPs into GN under vigorous stirring for 40 min at the ice bath. The route to construct vanillin sensor is illustrated in Scheme 1. The sufficient citrate ions in GN can act as complex sites and were purified by dialysis finally.
2.2. Apparatus UV–vis spectra were recorded with a spectrophotometer (Perkin-Elmer Lambda 900 USA). The morphologies of samples were observed by AFM (Agilent 5500 USA). High-resolution transmission electron microscope (HRTEM) image was obtained using Tecnai G2 F20 S-TWIN, 200 kV (FEI Company, USA). Zeta-potential value measurement was performed on Zeta Sizer 3000 Laser Particle Size and Zeta Potential Tester (Malvern Corporation, UK), and deionized water was used as a dispersant here. Electrochemical experiments were performed with a CHI660D electrochemical workstation (CH Instrument Company, Shanghai, China) with a conventional three-electrode cell. A Pt wire and a Ag/AgCl electrode were used as the auxiliary electrode and reference electrode, respectively. Glass carbon electrode (8 mm diameter) loading different materials was used as working electrode. The surface of GCE was polished with 0.3 and 0.05 μm alumina slurries, ultrasonicated in water and ethanol, respectively. Then the bare GCE was dried under pure N2. Finally, different materials were dropped on the surface of bare GCE, and dried under room temperature to form working electrode.
3. Results and discussion 3.1. Physicochemical characterization Fig. 1 depicts the UV–vis absorption spectra of Ag NPs, GN, and Ag NPs/GN dispersed in water for reference. The as-prepared Ag NPs
2.3. Preparation of Ag NPs/GN and the vanillin sensor The synthesis of Ag NPs was referred to previous work [26]. Typically, 50 μL of AgNO3 (0.05 M) was added into 24.15 mL deionized water followed by 0.5 mL of TSC (75 mM), 60 μL of H2O2 (30 wt.%), and 0.1 mL of PVP (17.5 mM) under vigorous stirring at room temperature. Then, 0.25 mL NaBH4 (100 mM) was injected into the above solution.
Fig. 1. UV–vis absorption spectra of (a) Ag NPs, (b) GN and (c) Ag NPs/GN. Inset: photographic images of Ag NPs, GN and Ag NPs/GN dispersed in H2O.
L. Huang et al. / Materials Science and Engineering C 38 (2014) 39–45
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Fig. 2. AFM images of (a) Ag NPs, (b) GN and (c) Ag NPs/GN.
show blue color and its corresponding UV–vis absorption spectra (Fig. 1a) displays two distinct plasmon bands at 331 nm and 620 nm which are assigned to the out-of-plane quadruple and in-plane dipole plasmon resonance of Ag hexagonal plates, respectively. After introducing GN, the in-plane dipole plasmonic bond of Ag NPs blue-shifts from 620 nm to 602 nm as seen in curve c and the inset
of Fig. 1. This phenomenon usually results from a little reduction of the size and the aspect ratio of the NPs. The out-of-plane band in 331 nm slightly red-shifted to 333 nm in Fig. 1c, which may stem from the interaction between Ag NPs and GN [10]. The characteristic peak at 262 nm attributes to the restored electronic conjugation within GN.
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Fig. 3. (a) HRTEM images of Ag NPs/GN, (b) high-resolution TEM image displaying the lattice fringes of Ag NPs, (c) high-resolution lateral view of Ag NPs and (d) inset: diffraction patterns of Ag NP.
Atomic force microscopy (AFM) is a direct method to observe the thickness and surface morphology. Ag NPs dispersed well have a thickness of ~4 nm as shown in Fig. 2a. Fig. 2b reveals that GN has uniform thickness and lateral dimensions ranging from several hundreds nanometers to a few micrometers. The thickness (~ 0.98 nm) of exfoliated GN is somewhat larger than Van der Waals thickness of 0.34 nm suggesting GN sheets may be bilayers or still contain oxygen functional groups. The composite appears to be uniform and typical sheets are
shown in Fig. 2c. After the self-assembly of Ag NPs, the height of composite is ~ 4 nm, illustrating that it can prevent the stacking of singlelayer GN by introducing Ag NPs, retaining the excellent property of GN. To further characterize the exact structures of this composite, HRTEM analysis was performed. The transparent GN sheets which can increase the electronic delivery exhibit flexible two-dimensional structure. Fig. 3a shows that the diameters of Ag NPs are in the distribution of 18–66 nm. The shapes of Ag NPs distributed randomly on GN are almost in a hexagonal form. From the unique profile drawn based on the different brightnesses in TEM, we can presume that hexagonal Ag NPs
Fig. 4. CVs of the Ag NPs/GN modified GCE in the (a) absence and (b) presence of 10−5 M vanillin in 0.1 M PBS (pH 6.98).
Fig. 5. SWV of (a) bare GCE, (b) Ag NPs/GCE, (c) GN/GCE and (d) Ag NPs/GN/GCE in 0.1 M PBS (pH 6.98) containing 1.0 × 10−5 M vanillin.
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Fig. 8. Amperometric responses of Ag NPs/GN/GCE upon subsequent additions of 10 μM vanillin, 100 μM TSC, 1 mM glucose and 1 mM L-cysteine in 0.1 M PBS (pH 6.98).
thickness of ~ 4 nm. The facet of the part sides indicated in TEM is (111) based on a spacing of 0.231 nm corresponding to the value in Fig. 3b. A morphological model of Ag NPs can be illustrated in the inset of Fig. 3c. The illustration in Fig. 3d gives a typical electron diffraction pattern taken by focusing the electron beam along the [111] zone of an individual NP. The strongest intensity spot (see the triangle in the inset) can be indexed to the Bragg diffraction from the {220} lattice planes of an f-c-c single crystal oriented in the [111], corresponding to the lattice spacing of 0.144 nm, proving that the hexagonal facets of the Ag NPs are composed of {111} planes. The inner spots (circle in the inset) could be ascribed to the 1/3{422} reflection indicating a number of parallel {111} twin planes and possibly atomically flat surface [28]. 3.2. Electrochemical characterization Fig. 6. (a) CVs of Ag NPs/GN/GCE in PBS (pH 6.98) containing 1 × 10−5 M vanillin at scan rates of 50, 100, 150, 200, 250, and 300 mV s−1. (b) The relationship of the oxidation current with the scan rate.
with two-dimensional growth have a certain thickness. The flatexposed crystal facets are {111}, corresponding to the lattice spacing of 0.238 nm as shown in Fig. 3b. Due to citrate selectively binding to Ag (111), it favors to form Ag NPs enclosed preferentially by {111} facets. The coexisting PVP adsorbs on Ag (100) and impels the formation of hexagonal NPs [10]. Fig. 3c shows the side view of Ag NPs with a
Current/µA
80
60
100 90 80 70 60 50 40 30 20 10 0 0.4
Current/ µA
100
100µM 100µ
2µM 0.5
0.6 E/V
0.7
0.8
40
I=0.871+0.959C R2=0.998
20
0 0
20
40
60
80
100
Concentration/µM Fig. 7. Calibration curves between vanillin concentration and peak current. Inset: SWV curves of Ag NPs/GN/GCE in vanillin solution with concentrations of 2–100 μM.
The electro-catalytic properties of Ag NPs/GN modified electrodes were mainly performed in a CHI660D electrochemical workstation based on SWV since it is more sensitive than cyclic voltammetry (CV). Fig. S1 displays CVs of different electrodes in 1 mM [Fe (CN) 6]3− containing 0.1 M KCl at 100 mVs−1 scan rate. After GCE was modified with Ag NPs (curve b), the peak current of [Fe (CN) 6]3− decreased, indicating that a small barrier to interfacial electron transfer appears by immobilizing Ag NPs. The peak current increased after modified Ag NPs/GN than pure Ag NPs, showing that the introduction of GN plays an important role in the increase of electron delivery. GN not only provides the conducting bridges for the electron-transfer of [Fe (CN) 6]3− but also enlarges the electro-active surface area. Fig. 4 shows the CV curves using the Ag NPs/GN/GCE in the absence (curve a) and presence (curve b) of 10− 5 M vanillin in 0.1 M PBS (pH 6.98). The sensor shows no response in pure PBS while exhibiting a typical reduction/oxidation at 0.62 V in the presence of 10−5 M vanillin. These results indicate that Ag NPs/GN can effectively catalyze vanillin oxidation on the modified electrode surface. The proposed electrochemical mechanism of vanillin is shown as the following mechanism (Scheme 1 and Fig. S2.), which involved two electrons and two protons oxidation [29]. Thus the electrons transfer fast through Ag NPs/GN composite, leading to a larger peak current. To better study the electro-catalytic activity of the same amount of Ag NPs, GN and Ag NPs/GN toward the vanillin oxidation in PBS (pH 6.98), typical SWVs are displayed in Fig. 5. Compared to GCE, Ag NPs/GCE exhibited a little higher current response due to their energetic sharp corners and a broad oxidation peak. This is because their sizes do not exceed the dimension of the diffusion layer; they should not contribute to increasing the electro-active area of modified GCE [9,30]. In the case of GN/GCE, an obvious peak as vanillin oxidation appears at around + 0.61 V, indicating that GN exhibits excellent catalytic
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Table 1 Determination of vanillin in real samples (n = 5). Samples
Quality/g
Determination/mg
Added/mg
Found/mg
Recovery/%
RSD/%
1 2
2.0002 2.0003
0 0
0.1 0.2
0.098 0.195
98.0 97.5
2.0 2.4
performance towards the vanillin oxidation. While the peak (+0.60 V) current of the Ag NPs/GN/GCE (Fig. 5d) is much larger and the over potential is 52 mV more negative than that of Ag NPs/GCE (+ 0.65 V) (Fig. 5b). This attributes to the enhanced surface area and excellent electro-catalytic activity carried out by both Ag NPs and GN. Zeta potential indicated that the surface of Ag NPs/GN is negatively charged (−2 mV) while vanillin is positively charged (+1.7 mV). Thus Ag NPs/ GN/GCE allowed abundant adsorption and electron transfer of vanillin. 3.3. Effect of scan rate Fig. 6a shows an increase in oxidation peak currents and a positive shift in peak position as scan rate increases for 10−5 M vanillin. The linear relationship between the anodic peak currents and scan rate from 50 to 300 mV S−1 is shown in Fig. 6b, which reveals that the oxidation of vanillin is an adsorption-controlled process. The Ag NPs/GN/GCE after detecting vanillin also shows oxidation peak for vanillin in blank PBS (pH 6.98) with the peak current decreasing gradually (depicted in Fig. S3). This phenomenon also agrees with the adsorption-controlled mechanism. 3.4. Optimization of experimental conditions and calibration curve The effect of solution pH on the electrochemical response of vanillin at Ag NPs/GN/GCE was investigated with PBS solution in the pH range from 5.6 to 8.0 (Fig. S4); the maximum peak current appeared at pH 6.98, so this value was selected throughout the experiments. Since the electrode process was adsorption-controlled, the accumulation potential and time for vanillin adsorbing on the Ag NPs/GN/GCE were also investigated. Accumulation process was studied under open potential condition. As shown in Fig. S5, the peak current increased first and then decreased with the accumulation potential increasing which showed that + 0.4 V is the best choice for accumulation potential for vanillin detection. Fig. S6 shows the best accumulation time is 10 s. The deposition amount of Ag NPs/GN was also studied in Fig. S7. The response increased rapidly with composite amounts increasing to 5 μL and subsequently decreasing. Because of hindrance of the electron transfer in a thick membrane at high amounts, the optimal amount selected to modify GCE was 5 μL. Finally, these optimized parameters were applied as test conditions for vanillin microanalysis. The calibration curve obtained with the oxidation peak currents under the optimal conditions is shown in Fig. 7. In the range of 2 μM to 100 μM, the peak current response is proportional to the vanillin concentration with the linear regression equation, I (μA) = 0.871 + 0.959C (μM) (R2 = 0.998). The detection limit is estimated to be 3.32 × 10−7 M at a signal-to-noise ratio of 3.
competitive adsorption of methyl blue or p-toluenesulfonic acid on graphene which in turn proves the possible π–π adsorption of vanillin. The vanillin solution was also measured by the modified electrodes stored in air for 7 days; the oxidation peak potential of vanillin is not shifted and the peak current has no apparent decrease, with a relative standard deviation (RSD) of 4.2 %, indicating that the vanillin sensor has good stability and might be applied to determine vanillin in real samples. In order to test the practical application of the proposed sensor, the Ag NPs/GN/GCE was used to determine vanillin in commercial biscuits. A certain amount of biscuits (2 g) were immersed in 100 mL deionized water for 6 h. The mixture was ultrasonicated for half hour and then filtered. No vanillin in this biscuits was detected under the optimized conditions. Finally, the standard addition method was used to inspect the average recovery of vanillin added in the biscuits. The results obtained were shown in Table 1. The average recovery of the method was 97.8% under critical condition (5 mg (vanillin)/100 g (biscuit)) compared with FAO/WHO (1992) (7 mg/100 g) [20]. Thus, the results prove that the vanillin sensor by Ag NPs/GN/GCE could satisfy to identify the maximum usage of vanillin in the foods. Also, the low RSD value indicated that the Ag NPs/GN/GCE has good reproducibility and potential application in practical vanillin determination.
4. Conclusions In summary, hexagonal Ag NPs were synthesized by PVP and TSC which selectively bind to Ag (111) and Ag (100) surfaces. The novel Ag NPs/GN composite was simply synthesized using TSC as bridging agent. Ag NPs have a thickness of ~ 4 nm and 18–66 nm in length. Their two planar surfaces contained single {111} facets and zeta potential indicated that the surface of Ag NPs/GN is negatively charged while vanillin is positively charged. Thus Ag NPs/GN/GCE allowed abundant adsorption and electron transfer of vanillin. The application for silver NPs has been broadened after assembling composite with GN. The Ag NPs/GN with enhanced surface area and good conductivity exhibits an excellent electro-catalytic activity toward the oxidation of vanillin. The composite can facilitate electron transfer between electro-active species and electrodes producing lower over-potential and increasing the peak current. The constructed vanillin electrochemical sensor has high sensitivity, a low detection limit of 3.32 × 10−7 M and a wide linear range from 2 μM to 100 μM. The introduction of GN to Ag NPs can broaden their application providing a promising platform to develop excellent electrochemical sensors.
Acknowledgements 3.5. Interference test and analytical application The possible interference for the detection of vanillin was investigated by successively adding foreign chemical species co-existing in real samples, such as Ca2+, Na+, K+, Fe3+, NH4HCO3, TSC, glucose, and amino acid. The results show that no interference is observed in the presence of 100-fold of metal ions. Other foreign species also do not interfere with the oxidation of vanillin with a relative current change less than 5% (Fig. 8). In addition, p-toluenesulfonic acid and methyl blue as interfering substances were also tested out in vanillin oxidation. The SWVs indicate that 10-fold of methyl blue and 100-fold of p-toluenesulfonic acid interfere with the oxidation of vanillin. This may attribute to the
The authors gratefully acknowledge the financial support from Project for International S & T Cooperation of China (2012DFM30040), National Science Fund of China (NSFC) (21201035, 61201397, J1103303 (J2013-004)) and the Fujian Department of Science and Technology (2012J01204).
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2014.01.037.
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