Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 411–416
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis of silver nanowires as a SERS substrate for the detection of pesticide thiram Li Zhang a,⇑, Bin Wang a,b, Guang Zhu a, Xia Zhou a a b
School of Biological and Chemical Engineering, Anhui Key Laboratory of Spin Electron and Nanomaterials, Suzhou University, Suzhou 234000, PR China School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The silver nanomaterials with
The single-crystal silver nanowires with good draw ratios and smooth surfaces have been synthesized by a simple sulfide-mediated polyol method and used as a SERS substrate for the detection of pesticide thiram. The values of RSD are lower than 20%. The result shows as-prepared Ag nanowires SERS substrate has high sensitivity and reproducibility for thiram (sulfur-containing pesticide).
different structures were synthesized by a simple method. The pesticide residues of thiram are detected by SERS. The lower concentration of thiram was detected.
a r t i c l e
i n f o
Article history: Received 21 December 2013 Received in revised form 24 May 2014 Accepted 1 June 2014 Available online 16 June 2014 Keywords: Surface-Enhanced Raman Scattering Ag nanowires Crystal violet Thiram
a b s t r a c t Silver nanowires with uniform diameters have been synthesized via the rapid sulfide mediated polyol method. The morphology, structure, and properties of as-prepared samples are characterized by UV–Visible spectroscopy, field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD), respectively. The mechanism of as-prepared Ag nanowires is provided and discussed. Moreover, as-prepared Ag nanowires are used as a Surface-Enhanced Raman Scattering (SERS) substrate to detect thiram pesticide. The results show that this substrate based on Ag nanowires exhibits high sensitivity and reproducibility for the thiram detection. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Surface-Enhanced Raman Scattering (SERS) is an ultrasensitive trace analysis technique that can detect various analyte (single ⇑ Corresponding author. Tel.: +86 557 2871006. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.saa.2014.06.054 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
molecule or biomolecules) near or on the surface of plasmonic nanostructures of noble metals such as Au, Ag, Cu and transition metals, which can greatly extend the application of Raman spectroscopy [1–4]. Among these noble metals, Ag nanomaterials are widely studied and used as a SERS-active substrate due to their better properties of surface plasmon resonances (SPR) than traditional metallic materials [5]. One-dimensional (1D) Ag nanowires can provide better
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model for researching the dependence of photon–plasmon interaction and electromagnetic scattering properties due to their panning from nanometer to (sub)micrometer scale in length regions [6,7]. For instance, finite length nanowires are useful plasmonic structures owing to the strong enhancement of their polarizability. In addition, the resonance frequencies can be changed by adjusting the length or diameter of nanowires. Therefore, 1D nanowires can make them ideal SERS substrates for large-scale sensing applications [8–10]. Recently, some methods have been developed for fabrication of the SERS substrates based on Ag nanowires [11,12]. Ding et al. reported that the Ni/Ag nanocomposites were fabricated in situ by a simple and rapid redox-transmetalation reaction [13]. The as-synthesized Ni/Ag nanocomposites were reproducible and stable for use as a SERS substrate because of their magnetic properties. The results indicated that as-prepared nanocomposites exhibit strong SERS effects for the detection of R6G and 4-ATP. Hu et al. reported a simple and effective approach for the aqueous-phase synthesis of crystalline silver nanowires [14]. These Ag nanowires were prepared by a template-less and non-seed process with sodium dodecyl sulfonate (SDSN) as a capping agent, and the diameters and aspect ratios of asprepared nanowires could be effectively controlled via this process. Guo et al. reported a polyol method for large-scale synthesis of rectangular silver nanorods and nonowires in the presence of a directing agent and seeds [15]. A liquid–liquid assembly strategy was employed to construct uniform rectangular silver nanorod arrays with high activity, stability, and reproducibility as SERS substrates for the detection of R6G and 4-ATP. Bao et al. reported on room-temperature synthesis of Ag Nanoparticles (NPs) decorated silver molybdate nanowires by using a solution-based chemical reaction method, and the complex substrate had a good SERS performance in detecting organic molecules due to the formation of a plenty of plasmonic ‘‘hot’’ spots on the nanowire surface [16]. Despite the above process, the exploration on 1D Ag nano-structure as SERS substrates for the detection of pesticide thiram is not nearly enough so far. Through the adjustment of reaction conditions, the single-crystalline Ag nanowires have been prepared by a rapid sulfide-mediated polyol method. Compared with other literature, as-prepared substrates have some clear advantages, including simplicity, high quality, and ease of mass production. In addition, growth mechanism of Ag nanowires is explored in detail. The SERS performance of as-prepared Ag nanowires is characterized by using crystal violet (CV) as the probe molecules, and the as-prepared Ag nanowires SERS substrates are employed to detect pesticide thiram. The results show that the detection limit of pesticide residues reaches to 107 M, which indicates that the SERS substrates with as-prepared silver nanowires have a high sensitivity. Experimental section Materials Silver Nitrate (AgNO3), Acetone (C3H6O) and anhydrous ethyl alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Poly(vinylpyrrolidone) (PVP, K30, Mw 55,000), sodium sulfide (Na2S9H2O) and anhydrous ethylene alcohol (EG) were obtained from Shanghai Chemical Reagent Co., Ltd. All chemicals are analytical grade and used without further purification. Synthesis of Ag nanowires Ag nanowires were prepared by a modified Guo’s mothed [15]. Briefly, 30 mL of EG was added into a 250 mL three-neck flask under magnetic stirring at 150 °C for 1 h. The 420 lL of Na2S
(3 mM) EG solution was quickly added into flask. After 8 min, 9 mL of PVP (0.18 M) ethylene glycol solution and 3 mL AgNO3 (0.29 M) ethylene glycol solution were injected to the mixture. The reaction solution was quenched with ice water when the color of the solution changed to white. The as-prepared samples were washed with acetone 3 times and ultrapure water 3 times, respectively. The obtained samples were finally stored and dispersed in ultrapure water. Preparation of substrates and detection of thiram pesticides The detecting substrate was obtained by adding 5 lL of Ag nanowires solution suspensions to 5 lL different concentrations of thiram. In a typical process, the mixture was dropped onto glasses, and then natural dried at room temperature. In this work, a series of thiram solutions with different concentrations from 1 105 to 1 108 M were prepared, respectively. Subsequently, the activity of the as-prepared substrates was detected by the CV dye and the thiram pesticides. Characterization The UV–Visible absorption spectrum was measured by a Shimadzu UV-2550 spectrometer. The morphology and structure of asprepared samples were characterized by an FEI Sirion 200 fieldemission scanning electronic microscopy (FESEM; Eindhoven, The Netherlands), high resolution transmission electron microscopy (HRTEM JEOL 2010) and X-ray diffraction (XRD, Japan MAC Science Co.), respectively. Raman spectra were carried out on a LabRAM HR800 confocal microscope Raman system (Horiba Jobin Yvon) with a 532 nm excitation source. Results and discussion Characterizations of Ag nanowires Two characteristic UV–Visible absorption peaks are observed in Fig. 1 a, respectively. The sharp absorbance peak at about 350 nm is an optical characteristic of bulk silver assigned, and the broad and intense absorbance peak at about 385 nm is the transverse SPR peak of the Ag nanowires [14,17]. Fig. 1b shows the TEM image of as-prepared Ag nanowires, which indicates that as-prepared Ag nanowires have smooth surfaces and average diameter of ca 70 nm. The corresponding selected area electron diffraction (SAED) pattern in an upper left inset in Fig. 1a indicates that silver nanowires have the single-crystalline structure. Fig. 1 c and d are HRTEM images of as-prepared Ag nanowires. The lattice fringes of Ag nanowires can be observed clearly, which further confirms that the Ag nanowires have the single-crystalline structure. The lattice spacing measured of longitudinal axis for the crystal line plane is 0.29 nm, corresponding to the (1 1 0) interplanar distance of ace-centered cubic (fcc) silver [14], which indicates that longitudinal direction of Ag nanowires grows along the [1 0 0] crystallographic direction. The lattice spacing measured of transverse axis is 0.25 nm (as shown in Fig. 1c and d). Based on (1/3) (4 2 2) interplanar distance of silver, it indicates that transverse direction of Ag nanowires grows along the [1 0 1] crystallographic direction [18]. To further study the mechanism of Ag nanowires, the morphologies of samples with different growth conditions have been discussed. Fig. 1e is the SEM image of the as-synthesized Ag nanowires with synthesis time of 2 h, which shows that the diameter of as-prepared Ag nanowires is about 45 nm, and length is in the range of 5–10 lm. Fig. 1f is the SEM image of the as-synthesized Ag nanowires with synthesis time of 3 h, which shows that the
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Fig. 1. (a) UV–Visible absorption spectrum, (b–d) TEM and HRTEM images, and (e and f) SEM images of the as-synthesized Ag nanowires. The inset is corresponding selected area electron diffraction (SAED) pattern.
diameter of as-prepared Ag nanowires is about 90 nm, and the length is more than 10 lm, respectively. The results show that the diameter and length of as-prepared samples increase with the increase of synthesis time. Moreover, as-prepared Ag nanowires have smooth surfaces and good draw ratio, which indicates the nanowires can be used as built platforms for SERS-active substrates [19]. In order to examine the presence of Ag nanowires, XRD analysis is conducted. The typical XRD pattern of as-prepared Ag nanowires is shown in Fig. 2a. Four characteristic diffraction peaks are observed. The positions of the characteristic diffraction peaks (37.9°, 44.1°, 64.2°, 77.3°) are ascribed to [1 1 1], [2 0 0], [2 2 0] and [3 1 1] planes of face-centered cubic (fcc) silver (JCPDS, No. 04-0783) [20], respectively. In addition, the morphologies of samples based on different growth conditions have been discussed. The samples without Na2S have a rod-shape, and the length of the rods is about 2 lm (as shown in Fig. 2c). When the mole ratio of PVP and AgNO3 is 1.91, the samples with cube shape are obtained (as shown in Fig. 2d). The Ag seeds grow along surface [1 0 0], and then Ag nanocubes are formed [21]. However, when the mole ratio of PVP and AgNO3 is 0.95, the Ag seeds grow along surface [1 0 0] and surface [1 0 1], and Ag nanowires are observed (as shown in Fig. 2b). PVP can selectively grow along Ag [1 0 1] and [1 0 0] surfaces, leading to growth of Ag nanowires along surface [1 0 1] and surface [1 0 0]. Therefore, the Ag nanocrystals can grow preferentially along these facets due to PVP selected growth. In addition, the surface free energies of different facets can become the same, and start to appear on the surface [22,23], leading to the formation of different structures (as shown in Fig. 2b–d). Sensitivity and reproducibility of Ag nanowires SERS substrate In order to evaluate the SERS performance of as-prepared Ag nanowires substrate, the SERS behavior of the dye crystal violet is studied. Fig. 3 shows the normal Raman and SERS Raman spectra of as-prepared samples. The curve a is a normal Raman measurement with a 0.01 M CV solution without Ag nanowires. The Ag nanowires as tags (curve b) can be detected the CV concentration
of 1.0 108 M, which indicates that SERS substrates with Ag nanowires have a higher sensitivity. In order to study the influence the different Ag nanostructures on SERS activity, Ag nanocubes (curve c) is also used as a substrate. Compared with SERS substrate with Ag nanowires, some weaker peaks at 667 cm1, 1433 cm1 and 1537 cm1 can be observed for the SERS substrate based on Ag nanocubes. However, the stronger Raman peaks at 913 cm1, 1178 cm1, 1374 cm1, 1589 cm1 and 1621 cm1 are observed for the SERS substrate with Ag nanocubes, respectively. Compared with nanocubes, Ag nanowires can provide obviously better SERS enhancement in main SERS peaks. The sensitivity of the SERS detection can be calculated from the analytical enhancement factor (EF). The EF for CV adsorbed on Ag nanowires is calculated according to the following quantification [24]:
EF ¼
ISERS Nv ol IRS N Surf
where Nvol and IRS are the corresponding to the average number of molecules in the scattering volume and Raman signal for the regular Raman measurement with CV solution on Si wafers, respectively. And NSurf and ISERS are the average number of absorbed molecules in the scattering volume molecular and Raman signal of CV molecules taken from the SERS substrate, respectively. If the number of molecules is below a single-molecule layer, the average intensity of SERS is proportional to the number (or concentration) of molecules, and this calculation can be written as C 0 AEF ¼ IISERS [25,26], where C0 and I0 are the corresponding to con0 C Surf centration of molecules and peak intensity for the normal Raman measurement with 0.01 M CV solution on Si wafers, respectively. And CSurf and ISERS are the molecular concentration with 1.0 108 M and Raman peak intensity of CV molecules taken from the SERS substrate, respectively. The EF of Ag nanowires as a SERS substrate is calculated by comparing a and b with Raman peak at 1621 cm1. The value of EF is estimated to be around 1.2 107, which indicates that the Ag nanowires can be used as a active substrate.
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Fig. 2. (a) XRD pattern of Ag nanowires, (b) SEM image of Ag nanowires with Na2S (420 lL, 3 mM), (c) SEM image of Ag nanorods without Na2S, and (d) SEM image of the Ag nanocubes at a molar ratio (PVP/AgNO3) of 1.91.
Fig. 3. Normal Raman spectra (curve a) of crystal violet, and SERS Raman (108 M) spectra of crystal violet on the SERS substrates (curve b: Ag nanowires, curve c: Ag nanocubes). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
To confirm the reproducibility of the as-prepared samples under a low concentration of target molecules, SERS spectra of CV molecules with a concentration of 108 M from 50 different selected spots on the substrate are given as shown in Fig. 4a. The relative standard deviation (RSD) curves of 50 spots SERS spectra are calculated. The RSD values of Raman vibrations at 1178, 1589, and 1621 cm1 are 19.63%, 18.23% and 12.79%, and all are lower than 20% (as shown in Fig. 4b–d). These results indicate that as-prepared Ag nanowires substrates have good enhancement effect [27]. Detection of thiram pesticide Thiram, as a kind of pesticides, has been widely used as protective fungicides on field crops, tea, vegetables, and fruits and the harm to
our kin and mucous membranes is also under estimated [28]. Therefore, SERS detection and identification of thiram is extremely urgent. So far, various methods have been developed for the determination of thiram, particularly, gas chromatography/mass spectrometry, UV–visible spectrophotometry, high-performance liquid chromatography, chemiluminescence analysis, and SERS spectroscopy [29–33]. Among them, SERS study of thiram detection has attracted much attention due to its excellent sensitivity. Yuan et al. reported the self-assembled clusters of Ag nanoparticles were composed of thousands of densely packed Ag nanoparticles in an emulsion and generated multiple active sites or hot spots in a single cluster, showing significant SERS activity for thiram [34]. Zhang et al. fabricated novel Au@Ag NPs/GO/Au@Ag NPs sandwich nanostructure as a SERS substrate. This sandwich nanostructured substrate have good sensitivity, reproducibility and reliability for the detection of thiram detection [35]. Zheng et al. reported the fabrication and characterization of the Au/Ag (core/shell) bipyramids. The influence of experimental parameters, such as the thickness of Ag shell of the bipyramids, sodium chloride concentration, and pH value on SERS of thiram were examined and optimized. The results showed this metallic substrates have high SERS performance for the thiram detection and are very suitable for the analytical sensors [36]. SERS spectra of the thiram with different SERS substrates are shown in Fig. 5, and the inset is the a schematic diagram of the conjecture process of chemical coordination adsorption on Ag nanowires surface. Compared to the normal Raman spectrum of thiram powder, the vibrational peak at 564 cm1 are decreased, and a new peak at 1517 cm1 are appeared. In the meantime, peaks at 1150 cm1 and 1386 cm1 are enhanced for Ag nanowires substrate. The reason is formation of the resonated radical structure to thiram molecule easily when interacting with the metal surface, leading to the SAS bond cleavage of thiram, which gives rise to two dimethyl residues by strongly chemical coordination adsorption on Ag nanowires surface [37]. To evaluate the SERS signals of thiram pesticides, SERS spectra of the thiram with different concentrations from 1 107 M to
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Fig. 4. (a) A series of SERS spectra of CV (108 M) molecules from 50 different spots on the SERS-active substrates, (b–d) the main Raman vibrational intensities of CV (108 M) (b: 1178 cm1, c: 1589 cm1, d: 1621 cm1).
Fig. 5. SERS spectra of the thiram with different SERS substrates (curve 1 is Ag nanowires, and curve 2 is Ag nanocubes, curve 3 is Raman spectra of the thiram powder). The inset is the conjecture process of chemical coordination adsorption on Ag nanowires surface.
5 106 are measured (as shown in Fig. 6). The prominent bands of thiram can be observed, which are ascribed to characteristic peaks of thiram. The main Raman peaks include: 446 cm1 attributed to r(CH3NC) or t(C@S), 560 cm1 attributed to t(SAS), 870 cm1 to t(CH3N), 928 cm1 to t(CH3N) or t(C@S), 1150 cm1 to q(CH3) or t(CAN), 1386 cm1 to rs(CH3) or t(CAN), 1440 cm1 to ras(CH3), 1517 cm1 to t(CAN), r(CH3) or q(CH3), respectively [38,39]. The intensities of variations peaks increase concomitantly with the increase of the concentration of sulfurcontaining pesticide. Obviously variational peaks at 560, 928, 1150, 1386, and 1517 cm1 are recorded at different concentrations of thiram ranging from 1 107 to 5 106 M, respectively.
Fig. 6. SERS spectra of the thiram with different concentrations from 1 107 M to 5 106.
In particular, the CAN stretching Raman peak at 1386 cm1 is used as a quantitative evaluation of thiram pesticides. The detection limit of thiram reaches to 1 107 M, indicating the high sensitivity of the SERS sensor for thiram pesticide. Fig. 7a shows a series of SERS spectra of thiram from 52 different spots on the Ag nanowires substrate. The results show that the SERS substrates have a high reproducibility. To get a statistically meaningful result, the RSD of the Raman intensity is calculated. RSD results from 52 spots are shown in Fig. 7b–d. The values at 1150, 1386, and 1517 cm1 are 16.40%, 12.06% and 18.29%, respectively. The results further conform that the as-prepared Ag nanowires substrate has good enhancement effect and high reproducibility for the detection of thiram pesticide.
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Fig. 7. (a) A series of SERS spectra of thiram (106 M) molecules from 52 different spots on the Ag nanowires substrate, (b–d) the main Raman vibrational intensities of thiram (106 M) (b: 1150 cm1, c: 1386 cm1, d: 1517 cm1).
In summary, single-crystalline Ag nanowires with uniform diameter and high yield have been synthesized via a rapid sulfide-mediated polyol method. The as-synthesized Ag nanowires have a good draw ratio as a built platform for SERS substrates. The EF of Ag nanowires is more than 1.2 107 for CV probe molecules. Meanwhile, the thiram pesticide is detected by SERS, and the detection limit of thiram reaches to 1 107 M. The values of RSD are lower than 20%. These results show that as-prepared Ag nanowires SERS substrates have high sensitivity and reproducibility for the detection of thiram (sulfur-containing pesticide).
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Acknowledgment
[24]
Conclusions
This work is supported by the National Science Foundation of China (Nos. 20871089, 21271136), the Important Project of Anhui Provincial Education Department (KJ2010ZD09) and the Program of Innovative Research Team of Suzhou University (2013kytd02). References [1] F.J. Li, Y.F. Huang, Y. Ding, et al., Nature 464 (2010) 392–395. [2] Z.Q. Tian, B. Ren, D.Y. Wu, J. Phys. Chem. B 106 (2002) 9463–9483. [3] L. Yang, H. Liu, J. Wang, F. Zhou, Z. Tian, J. Liu, Chem. Commun. 47 (2011) 3583– 3585. [4] Y. Ye, H. Liu, L. Yang, J. Liu, Nanoscale 4 (2012) 6442–6448. [5] L. Zhang, Appl. Surf. Sci. 270 (2013) 292–294. [6] J.P. Kottmann, O.J.F. Martin, D.R. Smith, S. Schultz, Phys. Rev. B 64 (2001) 235402. [7] Z. Li, F. Hao, Y. Huang, Y. Fang, P. Nordlander, H. Xu, Nano Lett. 9 (2009) 4383– 4386.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis of silver nanowires as a SERS substrate for the detection of pesticide thiram Li Zhang a,⇑, Bin Wang a,b, Guang Zhu a, Xia Zhou a a b
School of Biological and Chemical Engineering, Anhui Key Laboratory of Spin Electron and Nanomaterials, Suzhou University, Suzhou 234000, PR China School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The silver nanomaterials with
The single-crystal silver nanowires with good draw ratios and smooth surfaces have been synthesized by a simple sulfide-mediated polyol method and used as a SERS substrate for the detection of pesticide thiram. The values of RSD are lower than 20%. The result shows as-prepared Ag nanowires SERS substrate has high sensitivity and reproducibility for thiram (sulfur-containing pesticide).
different structures were synthesized by a simple method. The pesticide residues of thiram are detected by SERS. The lower concentration of thiram was detected.
a r t i c l e
i n f o
Article history: Received 21 December 2013 Received in revised form 24 May 2014 Accepted 1 June 2014 Available online 16 June 2014 Keywords: Surface-Enhanced Raman Scattering Ag nanowires Crystal violet Thiram
a b s t r a c t Silver nanowires with uniform diameters have been synthesized via the rapid sulfide mediated polyol method. The morphology, structure, and properties of as-prepared samples are characterized by UV–Visible spectroscopy, field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD), respectively. The mechanism of as-prepared Ag nanowires is provided and discussed. Moreover, as-prepared Ag nanowires are used as a Surface-Enhanced Raman Scattering (SERS) substrate to detect thiram pesticide. The results show that this substrate based on Ag nanowires exhibits high sensitivity and reproducibility for the thiram detection. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Surface-Enhanced Raman Scattering (SERS) is an ultrasensitive trace analysis technique that can detect various analyte (single ⇑ Corresponding author. Tel.: +86 557 2871006. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.saa.2014.06.054 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
molecule or biomolecules) near or on the surface of plasmonic nanostructures of noble metals such as Au, Ag, Cu and transition metals, which can greatly extend the application of Raman spectroscopy [1–4]. Among these noble metals, Ag nanomaterials are widely studied and used as a SERS-active substrate due to their better properties of surface plasmon resonances (SPR) than traditional metallic materials [5]. One-dimensional (1D) Ag nanowires can provide better
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model for researching the dependence of photon–plasmon interaction and electromagnetic scattering properties due to their panning from nanometer to (sub)micrometer scale in length regions [6,7]. For instance, finite length nanowires are useful plasmonic structures owing to the strong enhancement of their polarizability. In addition, the resonance frequencies can be changed by adjusting the length or diameter of nanowires. Therefore, 1D nanowires can make them ideal SERS substrates for large-scale sensing applications [8–10]. Recently, some methods have been developed for fabrication of the SERS substrates based on Ag nanowires [11,12]. Ding et al. reported that the Ni/Ag nanocomposites were fabricated in situ by a simple and rapid redox-transmetalation reaction [13]. The as-synthesized Ni/Ag nanocomposites were reproducible and stable for use as a SERS substrate because of their magnetic properties. The results indicated that as-prepared nanocomposites exhibit strong SERS effects for the detection of R6G and 4-ATP. Hu et al. reported a simple and effective approach for the aqueous-phase synthesis of crystalline silver nanowires [14]. These Ag nanowires were prepared by a template-less and non-seed process with sodium dodecyl sulfonate (SDSN) as a capping agent, and the diameters and aspect ratios of asprepared nanowires could be effectively controlled via this process. Guo et al. reported a polyol method for large-scale synthesis of rectangular silver nanorods and nonowires in the presence of a directing agent and seeds [15]. A liquid–liquid assembly strategy was employed to construct uniform rectangular silver nanorod arrays with high activity, stability, and reproducibility as SERS substrates for the detection of R6G and 4-ATP. Bao et al. reported on room-temperature synthesis of Ag Nanoparticles (NPs) decorated silver molybdate nanowires by using a solution-based chemical reaction method, and the complex substrate had a good SERS performance in detecting organic molecules due to the formation of a plenty of plasmonic ‘‘hot’’ spots on the nanowire surface [16]. Despite the above process, the exploration on 1D Ag nano-structure as SERS substrates for the detection of pesticide thiram is not nearly enough so far. Through the adjustment of reaction conditions, the single-crystalline Ag nanowires have been prepared by a rapid sulfide-mediated polyol method. Compared with other literature, as-prepared substrates have some clear advantages, including simplicity, high quality, and ease of mass production. In addition, growth mechanism of Ag nanowires is explored in detail. The SERS performance of as-prepared Ag nanowires is characterized by using crystal violet (CV) as the probe molecules, and the as-prepared Ag nanowires SERS substrates are employed to detect pesticide thiram. The results show that the detection limit of pesticide residues reaches to 107 M, which indicates that the SERS substrates with as-prepared silver nanowires have a high sensitivity. Experimental section Materials Silver Nitrate (AgNO3), Acetone (C3H6O) and anhydrous ethyl alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Poly(vinylpyrrolidone) (PVP, K30, Mw 55,000), sodium sulfide (Na2S9H2O) and anhydrous ethylene alcohol (EG) were obtained from Shanghai Chemical Reagent Co., Ltd. All chemicals are analytical grade and used without further purification. Synthesis of Ag nanowires Ag nanowires were prepared by a modified Guo’s mothed [15]. Briefly, 30 mL of EG was added into a 250 mL three-neck flask under magnetic stirring at 150 °C for 1 h. The 420 lL of Na2S
(3 mM) EG solution was quickly added into flask. After 8 min, 9 mL of PVP (0.18 M) ethylene glycol solution and 3 mL AgNO3 (0.29 M) ethylene glycol solution were injected to the mixture. The reaction solution was quenched with ice water when the color of the solution changed to white. The as-prepared samples were washed with acetone 3 times and ultrapure water 3 times, respectively. The obtained samples were finally stored and dispersed in ultrapure water. Preparation of substrates and detection of thiram pesticides The detecting substrate was obtained by adding 5 lL of Ag nanowires solution suspensions to 5 lL different concentrations of thiram. In a typical process, the mixture was dropped onto glasses, and then natural dried at room temperature. In this work, a series of thiram solutions with different concentrations from 1 105 to 1 108 M were prepared, respectively. Subsequently, the activity of the as-prepared substrates was detected by the CV dye and the thiram pesticides. Characterization The UV–Visible absorption spectrum was measured by a Shimadzu UV-2550 spectrometer. The morphology and structure of asprepared samples were characterized by an FEI Sirion 200 fieldemission scanning electronic microscopy (FESEM; Eindhoven, The Netherlands), high resolution transmission electron microscopy (HRTEM JEOL 2010) and X-ray diffraction (XRD, Japan MAC Science Co.), respectively. Raman spectra were carried out on a LabRAM HR800 confocal microscope Raman system (Horiba Jobin Yvon) with a 532 nm excitation source. Results and discussion Characterizations of Ag nanowires Two characteristic UV–Visible absorption peaks are observed in Fig. 1 a, respectively. The sharp absorbance peak at about 350 nm is an optical characteristic of bulk silver assigned, and the broad and intense absorbance peak at about 385 nm is the transverse SPR peak of the Ag nanowires [14,17]. Fig. 1b shows the TEM image of as-prepared Ag nanowires, which indicates that as-prepared Ag nanowires have smooth surfaces and average diameter of ca 70 nm. The corresponding selected area electron diffraction (SAED) pattern in an upper left inset in Fig. 1a indicates that silver nanowires have the single-crystalline structure. Fig. 1 c and d are HRTEM images of as-prepared Ag nanowires. The lattice fringes of Ag nanowires can be observed clearly, which further confirms that the Ag nanowires have the single-crystalline structure. The lattice spacing measured of longitudinal axis for the crystal line plane is 0.29 nm, corresponding to the (1 1 0) interplanar distance of ace-centered cubic (fcc) silver [14], which indicates that longitudinal direction of Ag nanowires grows along the [1 0 0] crystallographic direction. The lattice spacing measured of transverse axis is 0.25 nm (as shown in Fig. 1c and d). Based on (1/3) (4 2 2) interplanar distance of silver, it indicates that transverse direction of Ag nanowires grows along the [1 0 1] crystallographic direction [18]. To further study the mechanism of Ag nanowires, the morphologies of samples with different growth conditions have been discussed. Fig. 1e is the SEM image of the as-synthesized Ag nanowires with synthesis time of 2 h, which shows that the diameter of as-prepared Ag nanowires is about 45 nm, and length is in the range of 5–10 lm. Fig. 1f is the SEM image of the as-synthesized Ag nanowires with synthesis time of 3 h, which shows that the
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Fig. 1. (a) UV–Visible absorption spectrum, (b–d) TEM and HRTEM images, and (e and f) SEM images of the as-synthesized Ag nanowires. The inset is corresponding selected area electron diffraction (SAED) pattern.
diameter of as-prepared Ag nanowires is about 90 nm, and the length is more than 10 lm, respectively. The results show that the diameter and length of as-prepared samples increase with the increase of synthesis time. Moreover, as-prepared Ag nanowires have smooth surfaces and good draw ratio, which indicates the nanowires can be used as built platforms for SERS-active substrates [19]. In order to examine the presence of Ag nanowires, XRD analysis is conducted. The typical XRD pattern of as-prepared Ag nanowires is shown in Fig. 2a. Four characteristic diffraction peaks are observed. The positions of the characteristic diffraction peaks (37.9°, 44.1°, 64.2°, 77.3°) are ascribed to [1 1 1], [2 0 0], [2 2 0] and [3 1 1] planes of face-centered cubic (fcc) silver (JCPDS, No. 04-0783) [20], respectively. In addition, the morphologies of samples based on different growth conditions have been discussed. The samples without Na2S have a rod-shape, and the length of the rods is about 2 lm (as shown in Fig. 2c). When the mole ratio of PVP and AgNO3 is 1.91, the samples with cube shape are obtained (as shown in Fig. 2d). The Ag seeds grow along surface [1 0 0], and then Ag nanocubes are formed [21]. However, when the mole ratio of PVP and AgNO3 is 0.95, the Ag seeds grow along surface [1 0 0] and surface [1 0 1], and Ag nanowires are observed (as shown in Fig. 2b). PVP can selectively grow along Ag [1 0 1] and [1 0 0] surfaces, leading to growth of Ag nanowires along surface [1 0 1] and surface [1 0 0]. Therefore, the Ag nanocrystals can grow preferentially along these facets due to PVP selected growth. In addition, the surface free energies of different facets can become the same, and start to appear on the surface [22,23], leading to the formation of different structures (as shown in Fig. 2b–d). Sensitivity and reproducibility of Ag nanowires SERS substrate In order to evaluate the SERS performance of as-prepared Ag nanowires substrate, the SERS behavior of the dye crystal violet is studied. Fig. 3 shows the normal Raman and SERS Raman spectra of as-prepared samples. The curve a is a normal Raman measurement with a 0.01 M CV solution without Ag nanowires. The Ag nanowires as tags (curve b) can be detected the CV concentration
of 1.0 108 M, which indicates that SERS substrates with Ag nanowires have a higher sensitivity. In order to study the influence the different Ag nanostructures on SERS activity, Ag nanocubes (curve c) is also used as a substrate. Compared with SERS substrate with Ag nanowires, some weaker peaks at 667 cm1, 1433 cm1 and 1537 cm1 can be observed for the SERS substrate based on Ag nanocubes. However, the stronger Raman peaks at 913 cm1, 1178 cm1, 1374 cm1, 1589 cm1 and 1621 cm1 are observed for the SERS substrate with Ag nanocubes, respectively. Compared with nanocubes, Ag nanowires can provide obviously better SERS enhancement in main SERS peaks. The sensitivity of the SERS detection can be calculated from the analytical enhancement factor (EF). The EF for CV adsorbed on Ag nanowires is calculated according to the following quantification [24]:
EF ¼
ISERS Nv ol IRS N Surf
where Nvol and IRS are the corresponding to the average number of molecules in the scattering volume and Raman signal for the regular Raman measurement with CV solution on Si wafers, respectively. And NSurf and ISERS are the average number of absorbed molecules in the scattering volume molecular and Raman signal of CV molecules taken from the SERS substrate, respectively. If the number of molecules is below a single-molecule layer, the average intensity of SERS is proportional to the number (or concentration) of molecules, and this calculation can be written as C 0 AEF ¼ IISERS [25,26], where C0 and I0 are the corresponding to con0 C Surf centration of molecules and peak intensity for the normal Raman measurement with 0.01 M CV solution on Si wafers, respectively. And CSurf and ISERS are the molecular concentration with 1.0 108 M and Raman peak intensity of CV molecules taken from the SERS substrate, respectively. The EF of Ag nanowires as a SERS substrate is calculated by comparing a and b with Raman peak at 1621 cm1. The value of EF is estimated to be around 1.2 107, which indicates that the Ag nanowires can be used as a active substrate.
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Fig. 2. (a) XRD pattern of Ag nanowires, (b) SEM image of Ag nanowires with Na2S (420 lL, 3 mM), (c) SEM image of Ag nanorods without Na2S, and (d) SEM image of the Ag nanocubes at a molar ratio (PVP/AgNO3) of 1.91.
Fig. 3. Normal Raman spectra (curve a) of crystal violet, and SERS Raman (108 M) spectra of crystal violet on the SERS substrates (curve b: Ag nanowires, curve c: Ag nanocubes). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
To confirm the reproducibility of the as-prepared samples under a low concentration of target molecules, SERS spectra of CV molecules with a concentration of 108 M from 50 different selected spots on the substrate are given as shown in Fig. 4a. The relative standard deviation (RSD) curves of 50 spots SERS spectra are calculated. The RSD values of Raman vibrations at 1178, 1589, and 1621 cm1 are 19.63%, 18.23% and 12.79%, and all are lower than 20% (as shown in Fig. 4b–d). These results indicate that as-prepared Ag nanowires substrates have good enhancement effect [27]. Detection of thiram pesticide Thiram, as a kind of pesticides, has been widely used as protective fungicides on field crops, tea, vegetables, and fruits and the harm to
our kin and mucous membranes is also under estimated [28]. Therefore, SERS detection and identification of thiram is extremely urgent. So far, various methods have been developed for the determination of thiram, particularly, gas chromatography/mass spectrometry, UV–visible spectrophotometry, high-performance liquid chromatography, chemiluminescence analysis, and SERS spectroscopy [29–33]. Among them, SERS study of thiram detection has attracted much attention due to its excellent sensitivity. Yuan et al. reported the self-assembled clusters of Ag nanoparticles were composed of thousands of densely packed Ag nanoparticles in an emulsion and generated multiple active sites or hot spots in a single cluster, showing significant SERS activity for thiram [34]. Zhang et al. fabricated novel Au@Ag NPs/GO/Au@Ag NPs sandwich nanostructure as a SERS substrate. This sandwich nanostructured substrate have good sensitivity, reproducibility and reliability for the detection of thiram detection [35]. Zheng et al. reported the fabrication and characterization of the Au/Ag (core/shell) bipyramids. The influence of experimental parameters, such as the thickness of Ag shell of the bipyramids, sodium chloride concentration, and pH value on SERS of thiram were examined and optimized. The results showed this metallic substrates have high SERS performance for the thiram detection and are very suitable for the analytical sensors [36]. SERS spectra of the thiram with different SERS substrates are shown in Fig. 5, and the inset is the a schematic diagram of the conjecture process of chemical coordination adsorption on Ag nanowires surface. Compared to the normal Raman spectrum of thiram powder, the vibrational peak at 564 cm1 are decreased, and a new peak at 1517 cm1 are appeared. In the meantime, peaks at 1150 cm1 and 1386 cm1 are enhanced for Ag nanowires substrate. The reason is formation of the resonated radical structure to thiram molecule easily when interacting with the metal surface, leading to the SAS bond cleavage of thiram, which gives rise to two dimethyl residues by strongly chemical coordination adsorption on Ag nanowires surface [37]. To evaluate the SERS signals of thiram pesticides, SERS spectra of the thiram with different concentrations from 1 107 M to
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Fig. 4. (a) A series of SERS spectra of CV (108 M) molecules from 50 different spots on the SERS-active substrates, (b–d) the main Raman vibrational intensities of CV (108 M) (b: 1178 cm1, c: 1589 cm1, d: 1621 cm1).
Fig. 5. SERS spectra of the thiram with different SERS substrates (curve 1 is Ag nanowires, and curve 2 is Ag nanocubes, curve 3 is Raman spectra of the thiram powder). The inset is the conjecture process of chemical coordination adsorption on Ag nanowires surface.
5 106 are measured (as shown in Fig. 6). The prominent bands of thiram can be observed, which are ascribed to characteristic peaks of thiram. The main Raman peaks include: 446 cm1 attributed to r(CH3NC) or t(C@S), 560 cm1 attributed to t(SAS), 870 cm1 to t(CH3N), 928 cm1 to t(CH3N) or t(C@S), 1150 cm1 to q(CH3) or t(CAN), 1386 cm1 to rs(CH3) or t(CAN), 1440 cm1 to ras(CH3), 1517 cm1 to t(CAN), r(CH3) or q(CH3), respectively [38,39]. The intensities of variations peaks increase concomitantly with the increase of the concentration of sulfurcontaining pesticide. Obviously variational peaks at 560, 928, 1150, 1386, and 1517 cm1 are recorded at different concentrations of thiram ranging from 1 107 to 5 106 M, respectively.
Fig. 6. SERS spectra of the thiram with different concentrations from 1 107 M to 5 106.
In particular, the CAN stretching Raman peak at 1386 cm1 is used as a quantitative evaluation of thiram pesticides. The detection limit of thiram reaches to 1 107 M, indicating the high sensitivity of the SERS sensor for thiram pesticide. Fig. 7a shows a series of SERS spectra of thiram from 52 different spots on the Ag nanowires substrate. The results show that the SERS substrates have a high reproducibility. To get a statistically meaningful result, the RSD of the Raman intensity is calculated. RSD results from 52 spots are shown in Fig. 7b–d. The values at 1150, 1386, and 1517 cm1 are 16.40%, 12.06% and 18.29%, respectively. The results further conform that the as-prepared Ag nanowires substrate has good enhancement effect and high reproducibility for the detection of thiram pesticide.
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Fig. 7. (a) A series of SERS spectra of thiram (106 M) molecules from 52 different spots on the Ag nanowires substrate, (b–d) the main Raman vibrational intensities of thiram (106 M) (b: 1150 cm1, c: 1386 cm1, d: 1517 cm1).
In summary, single-crystalline Ag nanowires with uniform diameter and high yield have been synthesized via a rapid sulfide-mediated polyol method. The as-synthesized Ag nanowires have a good draw ratio as a built platform for SERS substrates. The EF of Ag nanowires is more than 1.2 107 for CV probe molecules. Meanwhile, the thiram pesticide is detected by SERS, and the detection limit of thiram reaches to 1 107 M. The values of RSD are lower than 20%. These results show that as-prepared Ag nanowires SERS substrates have high sensitivity and reproducibility for the detection of thiram (sulfur-containing pesticide).
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Acknowledgment
[24]
Conclusions
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