Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis of silver nanocubes as a SERS substrate for the determination of pesticide paraoxon and thiram Bin Wang a,b, Li Zhang a,⇑, Xia Zhou a a b
School of Chemistry & Life Science, 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
Silver nanocubes as a SERS substrate
The uniforms of silver nanocubes were prepared using a polyol method in which AgNO3 was reduced to Ag by anhydrous ethylene glycol (EG) in the presence of poly(vinyl pyrrolidone) (PVP) and a trace amount of Na2S. The SERS substrates were prepared by adding 5 lL of silver nanocubes collosol suspensions to 5 lL different concentrations of analytes. Drop the mixture onto glasses, and natural drying at room temperature. We used the crystal violet (CV) dye as the probe molecules to detect the activity of sliver nanocubes substrates. And then use this substrate to detect paraoxon and thiram pesticides.
has high intensity and reproducibility. The pesticide residues of paraoxon and thiram are detected by SERS. The low concentrations of pesticide residues such as paraoxon and thiram have been detected.
a r t i c l e
i n f o
Article history: Received 10 July 2013 Received in revised form 3 October 2013 Accepted 5 October 2013 Available online 24 October 2013 Keywords: Surface-Enhanced Raman Scattering Silver nanocubes Pesticide detection
a b s t r a c t The silver cube-like nanostructure with uniform size and high yield have been synthesized through the rapid sulfide-mediated polyol method. The morphology, structure and optical properties of the asprepared silver nanocubes were characterized by UV–Visible spectroscopy, field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The Surface-Enhanced Raman Scattering (SERS) performance of the as-prepared Ag nanocubes was characterized by crystal violet (CV) as the probe molecules. Furthermore, the low levels of thiram and pesticide paraoxon can be detected by the SERS technique, which shows that the silver nanocubes as a SERS substrate have excellent sensitivity and reproducibility. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Surface-Enhanced Raman Scattering (SERS), an efficient trace analysis technique, can test molecular trace with remarkable ⇑ Corresponding author. Tel.: +86 557 2871006; fax: +86 557 2871003. E-mail address: [email protected] (L. Zhang). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.013
enhancement mechanization [1,2]. Therefore, it is widely applied in various fields such as biology [3], sensors [4], and chemical detection [5,6]. Silver nanoparticles are widely studied as target materials due to their better performance than traditional metallic materials [7]. Silver nanoparticles are highly reactive substrates for SERS. In the study of SERS, fabricating substrates is the key point, and it is difficult to prepare an excellent and reusable SERS
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B. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
substrate to enhance its sensitivity in the molecule detection [8]. In the past decade, much work in SERS research area has focused on the fabrication design of these SERS-active metal substrates [9]. Many approaches include colloidal chemistry, electrodeposition and template methods have been used to fabrication SERS-active metal substrates [10,11]. Chemical methods have been developed for the SERS-active substrates synthesis in recent years [12,13]. At present, about 2 million tons of chemical pesticides are produced every year. It is probable that over 1000 kinds of synthetic compounds are used as pesticide, fungicide, algaecide and deciduous agent. Among these pesticides, especially paraoxon and thiram which are widely utilized in agricultural production have caused serious pesticide pollution and therefore become a serious threat to human health. Although many researches have focused on fabrication of the SERS-active substrates, Studying mechanization of SERS-active analytes and their SERS enhancement factors, little attention has been paid to applying the SERS-active substrates to trace detection of pesticide residues. In this paper, the uniform Ag nanocubes have been prepared and used as a SERS substrate by a simple and efficient method, in which AgNO3 is reduced anhydrous ethylene glycol (EG) with poly(vinylpyrrolidone) as a dispersing agent. The mechanism and role of the poly(vinylpyrrolidone) (PVP) and Na2S in the growth of Ag nanocrystals are given and discussed. Furthermore, the SERS performance of the silver nanocubes is characterized by CV as the probe molecules. The different concentrations of thiram and paraoxon pesticides are detected by a series of experiments. The results show that the detect limit of paraoxon and thiram is low to 5 108 mol/L, which indicates that the silver nanocubes as a SERS substrate have excellent sensitivity and reproducibility.
The 420 lL of Na2S (3 mM) ethylene glycol solution was quickly added into flask. After 8 min, 7.5 mL of PVP (20 mg/mL) ethylene glycol solution and 2.5 mL AgNO3 (48 mg/mL) ethylene glycol solution were injected to the mixture [15]. The reaction solution was quenched with ice water when the color of the solution changed into yellowish brown. Substrates preparation and detection of pesticides The detecting substrate was obtained by adding 5 lL of silver nanocubes collosol suspensions to 5 lL different concentrations of analytes. The mixture was dropped onto glasses, and then natural dried at room temperature. The crystal violet (CV) dye was used as the probe molecules to detect the activity of sliver nanocubes substrates. Subsequently, these substrates were used to detect paraoxon and thiram pesticides. Characterization The UV–Visible absorption spectrum of silver nanocubes was obtained by a Shimadzu UV-2550 spectrometer. The morphology and structure of silver nanocubes was observed by an FEI Sirion 200 field-emission scanning electronic microscopy (FESEM; Eindhoven, The Netherlands). Raman spectra were carried out on a LabRAM HR800 confocal microscope Raman system (Horiba Jobin Yvon). The excitation wavelength is 532 nm from a He–Ne laser. X-ray diffraction (XRD) patterns obtained on a Map 18AHF
Experimental section Materials Silver nitrate (AgNO3), acetone and anhydrous ethyl alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Poly(vinylpyrrolidone) (PVP, K30), Na2S9H2O and Anhydrous ethylene glycol (EG) were obtained from shanghai Chemical Reagent Co., Ltd. All chemicals were analytical grade and used without further purification. Ultrapure water (Milli-Q) with an average resistivity of 18.25 MX cm was used for all experiments. Synthesis of monodisperse silver nanocubes The uniform silver nanocubes were synthesized by a modified Xia’s method [14]. Briefly, 30 mL of ethylene glycol was added into a 250 mL three-neck flask under magnetic stirring at 152 °C for 1 h.
Fig. 2. UV–Visible absorption spectra of the samples with different react times of 1 h, 2 h, 3 h and 4 h, and the insert image show the shape and the size of the silver nanocubes correspond to different react times.
Fig. 1. (a) UV–Visible absorption spectra and (b) SEM image of the as-prepared silver nanocubes.
B. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
65
Fig. 3. (a and b) Different magnification SEM images of the SERS substrate, (c) typical XRD pattern of as-prepared silver nanocubes, and (d) photo of the as-prepared Ag nanocubes solution.
instrument (Japan MAC Science Co.) operated at 35 kV (20 mA) with Cu KR radiation (k = 0.154 nm). Results and discussion Fig. 1a shows the UV–Visible absorption spectra of the Ag nanocubes. Three representative SPR peaks of 345 nm, 398 nm and 496 nm are observed, respectively. There is a broad and intense absorbance peak at about 496 nm, which is assigned to the major surface plasma resonance (SPR) of the (1 0 0) plane of face-centered cubic (fcc) silver metal. Two sharp absorbance peaks at about 345
Fig. 4. SERS spectra of CV from 106 mol/L to 1010 mol/L in aqueous Ag nanocubes colloidal suspension.
and 398 nm were assigned to the SPR peaks of the corners and edges of the Ag nanocubes, respectively, which can conform that the formation of silver nanocubes [16]. The morphology and size of monodisperse silver nanocubes were characterized by an FEI Sirion 200 field-emission scanning electronic microscopy. Fig. 1b shows the SEM image of the as-prepared silver nanocubes with treatment for 4 h, which indicates that the monodisperse silver nanocubes have a edge length of 110 ± 5 nm. Ag nanocubes have been prepared at 152 °C for different times of 1 h, 2 h, 3 h and 4 h, respectively. The absorption spectrum of the sample with the synthesis time of 1 h (as shown in Fig. 2, curve a)
Fig. 5. Normal and SERS Raman spectra of crystal violet at 532 nm laser wavelength (excitation intensity is 25 mW, and data acquisition time is 5 s).
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B. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
Fig. 6. A series of SERS spectra of Crystal violet (108 M) molecules.
shows only a SPR absorption peak at around 399 nm, which indicates that the average size of as-prepared silver nanoparticles is about 40 nm. The spectrum of the synthesis time of 2 h shows two relatively sharp SPR peaks at about 350 and 432 nm, indicating the formation of the silver nanocubes. However, when the synthesis time is increased to more than 3 h, six sharp SPR peaks at about 346, 392, 474 nm for sample with synthesis time of 3 h (Fig. 2, curve c) and at 346, 396, 489 nm for the sample with synthesis time of 4 h (Fig. 2, curve d) are observed, which shows a typical UV–Visible absorption spectra of the Ag nanocubes in aqueous solution. Based on the UV–Visible absorption spectra (Fig. 2, curves a–d), an obvious evolution process of the silver nanocubes’ size and
shape can be provided. The absorption bands of the samples gradually become sharper with a wider bandwidth and exhibit an obvious red-shift with the increase of the synthesis time [17]. This peak changes from wide to sharp with the increase of reaction time, indicating that the corners of the Ag nanocubes are sharpened gradually with further growth. Therefore, these silver nanoparticles can grow into cubes with larger sizes with the increasing of the reaction time. SEM images of the samples with different reaction times also conform this result. In addition, the number of peaks and relative positions are consistent with theoretical calculations [18]. This theoretical explanation of structure feature is quite common for the different shape metal, such as silver rods [19], silver disks [20], gold [21,22], and Ag@Au [23]. The UV–Visible absorption spectra (Fig. 2) also show that the increase of the nanoparticles diameter leads to a red-shift of LSPR wavelength. It should be noticed that the SPR absorption band at around 496 nm is close to excitation wavelength of Raman scattering (532 nm), and the diameters of silver nanocube are about 110 ± 10 nm. This enhancement factor (EF) improvement is due to the acute coffin-corner, and the acute coffin-corner has the tip-enhance and the larger spatial in SERS measurements [24]. Fig. 3a and b shows the different magnification SEM images of the SERS substrate. Fig. 3d is a photograph of as-prepared Ag nanocubes solution. In order to further examine the presence of silver nanocubes, XRD analysis was conducted. The typical XRD pattern of as-prepared silver nanocubes is shown in Fig. 3c. Four characteristic diffraction peaks are observed in the Ag pattern. The positions of the characteristic diffraction peaks (37.9°, 44.1°, 64.2°, 77.2°) are correspond to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of silver (JC PDS, No. 04-0783), respectively [25]. The overwhelmingly intensive diffraction located at 2h (37.9°, 44.1°), this shown that the basal plane, i.e., the top crystal plane of the nanocubes, corresponds
Fig. 7. The intensities of the main Raman vibrations of crystal violet (108 M) solution from the 37 spots SERS line-scan spectra.
B. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
67
Fig. 8. SERS spectra of the paraoxon (1 105 mol/L) and thiram (1 106 mol/L) in silver aqueous solution.
to the (1 1 1) and (2 0 0) lattice plane of face-centered cubic (fcc) silver [26]. The favor formation of Ag nanocrystals enclosed preferentially by (1 1 1) or (1 0 0) facets owing to PVP selectively bind to these surfaces, respectively [27]. Therefore, the growth will finally cause the concentration of free PVP in the solution, and the surface free energies of (1 0 0) facets and (1 1 1) facets will become the same. So, the (1 1 1) facets will start to appear on the surface, leading to the formation of a cube with slight truncation at corners. In order to evaluate the SERS performance of the substrate, the SERS behavior of the dye crystal violet, a well known SERS target analyte, is studied [28]. It is mentioned that SERS can be separated into two comprehensive sensitivity theorems: the low sensitivity theorem and the high sensitivity (single molecule) theorems. Since it is an ensemble averaged signal and composed of contributions
from numerous molecules, the observed spectrum is stable and reproducible in the low sensitivity regime. Therefore, the high sensitivity (single molecule) regime could be obtained when substrates have very high enhancement factors, and the observed signal originates from a small number of scattering molecules are situated in ‘‘hot spots’’ [29]. Fig. 4 shows the characteristic crystal violet SERS spectra for concentrations ranging from 106 to 1010 mol/L, and the detect limit is 109 mol/L. These Raman spectra are obtained under the same experimental conditions. The main Raman bands include: 339 cm1 attributed to c(CNC) or qc(CH3), 425 cm1 attributed to r(CNC) or r(CCcenterC), 442 and 526 cm1 to r(CNC), 561 cm1 to c(CCC), r(CNC) or r(CCcenterC), 726 cm1 to t(CN), 761 cm1 to t(CN) or ts (CCcenterC), 913 cm1 to r(CCcenterC), 941 cm1 to
Fig. 9. SERS spectra of (a) paraoxon (106–5 108 mol/L), (b) sulfur-containing thiram (106–5 108 mol/L), and (c) the 3D SERS spectra of thiram solution (106 mol/L) from 8 spots.
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B. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
qc(CH3) or t(CN), 1123 cm1 to t(CN) or r(CCcenterC), 1178 cm1 to tas (CCcenterC), 1298 cm1 to tas (CCcenterC), r(CCC)ring and r(CH), 1336 cm1 and 1377 cm1 to r(CCC)ring, tas (CCcenterC) and r(CH), 1448 cm1 and 1474 cm1 attributed to ras, 1535 to ras or t(CringN), respectively [30]. The silver nanocubes can detect CV as low as 1.0 109 mol/L (as shown in Fig. 5). The regular Raman measurement (curve b) with 0.01 mol/L crystal violet solution on Si wafers has been obtained. The sensitivity of the SERS detection can be calculated from the enhancement factor (EF). The EF for crystal violet on SERS substrate was calculated according to the following quantification [31]:
EF ¼
ISERS Nv ol IRS NSurf
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 [32], where C0 and I0 are the corresponding concentra0 C Surf
tion of molecules and peak intensity for the regular Raman measurement with 0.01 mol/L CV solution on Si wafers, respectively. CSurf and ISERS are the molecular concentration with 1.0 109 mol/L and Raman peak intensity of CV molecules taken from the SERS substrate, respectively. The EF of Ag nanocubes as SERS substrate is calculated by comparing a and b at 1620 cm1. And the AEF of monodisperse silver nanoparticles is estimated to be around 5.06 107, which indicates that the Ag nanocubes can be used as 7 15720:01 . active substrate [33]. AEF ¼ 310:610 9 5:06 10 The reproducibility of signals from SERS spectra is important for the use of SERS as a routine analytical tool [34]. To confirmation
whether the as-prepared samples are able to give reproducible SERS signals under a low concentration of target molecules, SERS spectra of CV molecules with a concentration of 108 M from 37 randomly selected spots on the substrates have been provided (as shown in Fig. 6). The relative standard deviation (RSD) curve of 37 SERS spectra, which are used to estimate the reproducibility of SERS signals, is calculated by the method reported previously [35]. The values of RSD of Raman vibrations at 913, 1178, 1587, and 1620 cm1 are all
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis of silver nanocubes as a SERS substrate for the determination of pesticide paraoxon and thiram Bin Wang a,b, Li Zhang a,⇑, Xia Zhou a a b
School of Chemistry & Life Science, 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
Silver nanocubes as a SERS substrate
The uniforms of silver nanocubes were prepared using a polyol method in which AgNO3 was reduced to Ag by anhydrous ethylene glycol (EG) in the presence of poly(vinyl pyrrolidone) (PVP) and a trace amount of Na2S. The SERS substrates were prepared by adding 5 lL of silver nanocubes collosol suspensions to 5 lL different concentrations of analytes. Drop the mixture onto glasses, and natural drying at room temperature. We used the crystal violet (CV) dye as the probe molecules to detect the activity of sliver nanocubes substrates. And then use this substrate to detect paraoxon and thiram pesticides.
has high intensity and reproducibility. The pesticide residues of paraoxon and thiram are detected by SERS. The low concentrations of pesticide residues such as paraoxon and thiram have been detected.
a r t i c l e
i n f o
Article history: Received 10 July 2013 Received in revised form 3 October 2013 Accepted 5 October 2013 Available online 24 October 2013 Keywords: Surface-Enhanced Raman Scattering Silver nanocubes Pesticide detection
a b s t r a c t The silver cube-like nanostructure with uniform size and high yield have been synthesized through the rapid sulfide-mediated polyol method. The morphology, structure and optical properties of the asprepared silver nanocubes were characterized by UV–Visible spectroscopy, field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The Surface-Enhanced Raman Scattering (SERS) performance of the as-prepared Ag nanocubes was characterized by crystal violet (CV) as the probe molecules. Furthermore, the low levels of thiram and pesticide paraoxon can be detected by the SERS technique, which shows that the silver nanocubes as a SERS substrate have excellent sensitivity and reproducibility. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Surface-Enhanced Raman Scattering (SERS), an efficient trace analysis technique, can test molecular trace with remarkable ⇑ Corresponding author. Tel.: +86 557 2871006; fax: +86 557 2871003. E-mail address: [email protected] (L. Zhang). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.013
enhancement mechanization [1,2]. Therefore, it is widely applied in various fields such as biology [3], sensors [4], and chemical detection [5,6]. Silver nanoparticles are widely studied as target materials due to their better performance than traditional metallic materials [7]. Silver nanoparticles are highly reactive substrates for SERS. In the study of SERS, fabricating substrates is the key point, and it is difficult to prepare an excellent and reusable SERS
64
B. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
substrate to enhance its sensitivity in the molecule detection [8]. In the past decade, much work in SERS research area has focused on the fabrication design of these SERS-active metal substrates [9]. Many approaches include colloidal chemistry, electrodeposition and template methods have been used to fabrication SERS-active metal substrates [10,11]. Chemical methods have been developed for the SERS-active substrates synthesis in recent years [12,13]. At present, about 2 million tons of chemical pesticides are produced every year. It is probable that over 1000 kinds of synthetic compounds are used as pesticide, fungicide, algaecide and deciduous agent. Among these pesticides, especially paraoxon and thiram which are widely utilized in agricultural production have caused serious pesticide pollution and therefore become a serious threat to human health. Although many researches have focused on fabrication of the SERS-active substrates, Studying mechanization of SERS-active analytes and their SERS enhancement factors, little attention has been paid to applying the SERS-active substrates to trace detection of pesticide residues. In this paper, the uniform Ag nanocubes have been prepared and used as a SERS substrate by a simple and efficient method, in which AgNO3 is reduced anhydrous ethylene glycol (EG) with poly(vinylpyrrolidone) as a dispersing agent. The mechanism and role of the poly(vinylpyrrolidone) (PVP) and Na2S in the growth of Ag nanocrystals are given and discussed. Furthermore, the SERS performance of the silver nanocubes is characterized by CV as the probe molecules. The different concentrations of thiram and paraoxon pesticides are detected by a series of experiments. The results show that the detect limit of paraoxon and thiram is low to 5 108 mol/L, which indicates that the silver nanocubes as a SERS substrate have excellent sensitivity and reproducibility.
The 420 lL of Na2S (3 mM) ethylene glycol solution was quickly added into flask. After 8 min, 7.5 mL of PVP (20 mg/mL) ethylene glycol solution and 2.5 mL AgNO3 (48 mg/mL) ethylene glycol solution were injected to the mixture [15]. The reaction solution was quenched with ice water when the color of the solution changed into yellowish brown. Substrates preparation and detection of pesticides The detecting substrate was obtained by adding 5 lL of silver nanocubes collosol suspensions to 5 lL different concentrations of analytes. The mixture was dropped onto glasses, and then natural dried at room temperature. The crystal violet (CV) dye was used as the probe molecules to detect the activity of sliver nanocubes substrates. Subsequently, these substrates were used to detect paraoxon and thiram pesticides. Characterization The UV–Visible absorption spectrum of silver nanocubes was obtained by a Shimadzu UV-2550 spectrometer. The morphology and structure of silver nanocubes was observed by an FEI Sirion 200 field-emission scanning electronic microscopy (FESEM; Eindhoven, The Netherlands). Raman spectra were carried out on a LabRAM HR800 confocal microscope Raman system (Horiba Jobin Yvon). The excitation wavelength is 532 nm from a He–Ne laser. X-ray diffraction (XRD) patterns obtained on a Map 18AHF
Experimental section Materials Silver nitrate (AgNO3), acetone and anhydrous ethyl alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Poly(vinylpyrrolidone) (PVP, K30), Na2S9H2O and Anhydrous ethylene glycol (EG) were obtained from shanghai Chemical Reagent Co., Ltd. All chemicals were analytical grade and used without further purification. Ultrapure water (Milli-Q) with an average resistivity of 18.25 MX cm was used for all experiments. Synthesis of monodisperse silver nanocubes The uniform silver nanocubes were synthesized by a modified Xia’s method [14]. Briefly, 30 mL of ethylene glycol was added into a 250 mL three-neck flask under magnetic stirring at 152 °C for 1 h.
Fig. 2. UV–Visible absorption spectra of the samples with different react times of 1 h, 2 h, 3 h and 4 h, and the insert image show the shape and the size of the silver nanocubes correspond to different react times.
Fig. 1. (a) UV–Visible absorption spectra and (b) SEM image of the as-prepared silver nanocubes.
B. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
65
Fig. 3. (a and b) Different magnification SEM images of the SERS substrate, (c) typical XRD pattern of as-prepared silver nanocubes, and (d) photo of the as-prepared Ag nanocubes solution.
instrument (Japan MAC Science Co.) operated at 35 kV (20 mA) with Cu KR radiation (k = 0.154 nm). Results and discussion Fig. 1a shows the UV–Visible absorption spectra of the Ag nanocubes. Three representative SPR peaks of 345 nm, 398 nm and 496 nm are observed, respectively. There is a broad and intense absorbance peak at about 496 nm, which is assigned to the major surface plasma resonance (SPR) of the (1 0 0) plane of face-centered cubic (fcc) silver metal. Two sharp absorbance peaks at about 345
Fig. 4. SERS spectra of CV from 106 mol/L to 1010 mol/L in aqueous Ag nanocubes colloidal suspension.
and 398 nm were assigned to the SPR peaks of the corners and edges of the Ag nanocubes, respectively, which can conform that the formation of silver nanocubes [16]. The morphology and size of monodisperse silver nanocubes were characterized by an FEI Sirion 200 field-emission scanning electronic microscopy. Fig. 1b shows the SEM image of the as-prepared silver nanocubes with treatment for 4 h, which indicates that the monodisperse silver nanocubes have a edge length of 110 ± 5 nm. Ag nanocubes have been prepared at 152 °C for different times of 1 h, 2 h, 3 h and 4 h, respectively. The absorption spectrum of the sample with the synthesis time of 1 h (as shown in Fig. 2, curve a)
Fig. 5. Normal and SERS Raman spectra of crystal violet at 532 nm laser wavelength (excitation intensity is 25 mW, and data acquisition time is 5 s).
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B. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
Fig. 6. A series of SERS spectra of Crystal violet (108 M) molecules.
shows only a SPR absorption peak at around 399 nm, which indicates that the average size of as-prepared silver nanoparticles is about 40 nm. The spectrum of the synthesis time of 2 h shows two relatively sharp SPR peaks at about 350 and 432 nm, indicating the formation of the silver nanocubes. However, when the synthesis time is increased to more than 3 h, six sharp SPR peaks at about 346, 392, 474 nm for sample with synthesis time of 3 h (Fig. 2, curve c) and at 346, 396, 489 nm for the sample with synthesis time of 4 h (Fig. 2, curve d) are observed, which shows a typical UV–Visible absorption spectra of the Ag nanocubes in aqueous solution. Based on the UV–Visible absorption spectra (Fig. 2, curves a–d), an obvious evolution process of the silver nanocubes’ size and
shape can be provided. The absorption bands of the samples gradually become sharper with a wider bandwidth and exhibit an obvious red-shift with the increase of the synthesis time [17]. This peak changes from wide to sharp with the increase of reaction time, indicating that the corners of the Ag nanocubes are sharpened gradually with further growth. Therefore, these silver nanoparticles can grow into cubes with larger sizes with the increasing of the reaction time. SEM images of the samples with different reaction times also conform this result. In addition, the number of peaks and relative positions are consistent with theoretical calculations [18]. This theoretical explanation of structure feature is quite common for the different shape metal, such as silver rods [19], silver disks [20], gold [21,22], and Ag@Au [23]. The UV–Visible absorption spectra (Fig. 2) also show that the increase of the nanoparticles diameter leads to a red-shift of LSPR wavelength. It should be noticed that the SPR absorption band at around 496 nm is close to excitation wavelength of Raman scattering (532 nm), and the diameters of silver nanocube are about 110 ± 10 nm. This enhancement factor (EF) improvement is due to the acute coffin-corner, and the acute coffin-corner has the tip-enhance and the larger spatial in SERS measurements [24]. Fig. 3a and b shows the different magnification SEM images of the SERS substrate. Fig. 3d is a photograph of as-prepared Ag nanocubes solution. In order to further examine the presence of silver nanocubes, XRD analysis was conducted. The typical XRD pattern of as-prepared silver nanocubes is shown in Fig. 3c. Four characteristic diffraction peaks are observed in the Ag pattern. The positions of the characteristic diffraction peaks (37.9°, 44.1°, 64.2°, 77.2°) are correspond to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of silver (JC PDS, No. 04-0783), respectively [25]. The overwhelmingly intensive diffraction located at 2h (37.9°, 44.1°), this shown that the basal plane, i.e., the top crystal plane of the nanocubes, corresponds
Fig. 7. The intensities of the main Raman vibrations of crystal violet (108 M) solution from the 37 spots SERS line-scan spectra.
B. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
67
Fig. 8. SERS spectra of the paraoxon (1 105 mol/L) and thiram (1 106 mol/L) in silver aqueous solution.
to the (1 1 1) and (2 0 0) lattice plane of face-centered cubic (fcc) silver [26]. The favor formation of Ag nanocrystals enclosed preferentially by (1 1 1) or (1 0 0) facets owing to PVP selectively bind to these surfaces, respectively [27]. Therefore, the growth will finally cause the concentration of free PVP in the solution, and the surface free energies of (1 0 0) facets and (1 1 1) facets will become the same. So, the (1 1 1) facets will start to appear on the surface, leading to the formation of a cube with slight truncation at corners. In order to evaluate the SERS performance of the substrate, the SERS behavior of the dye crystal violet, a well known SERS target analyte, is studied [28]. It is mentioned that SERS can be separated into two comprehensive sensitivity theorems: the low sensitivity theorem and the high sensitivity (single molecule) theorems. Since it is an ensemble averaged signal and composed of contributions
from numerous molecules, the observed spectrum is stable and reproducible in the low sensitivity regime. Therefore, the high sensitivity (single molecule) regime could be obtained when substrates have very high enhancement factors, and the observed signal originates from a small number of scattering molecules are situated in ‘‘hot spots’’ [29]. Fig. 4 shows the characteristic crystal violet SERS spectra for concentrations ranging from 106 to 1010 mol/L, and the detect limit is 109 mol/L. These Raman spectra are obtained under the same experimental conditions. The main Raman bands include: 339 cm1 attributed to c(CNC) or qc(CH3), 425 cm1 attributed to r(CNC) or r(CCcenterC), 442 and 526 cm1 to r(CNC), 561 cm1 to c(CCC), r(CNC) or r(CCcenterC), 726 cm1 to t(CN), 761 cm1 to t(CN) or ts (CCcenterC), 913 cm1 to r(CCcenterC), 941 cm1 to
Fig. 9. SERS spectra of (a) paraoxon (106–5 108 mol/L), (b) sulfur-containing thiram (106–5 108 mol/L), and (c) the 3D SERS spectra of thiram solution (106 mol/L) from 8 spots.
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B. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 63–69
qc(CH3) or t(CN), 1123 cm1 to t(CN) or r(CCcenterC), 1178 cm1 to tas (CCcenterC), 1298 cm1 to tas (CCcenterC), r(CCC)ring and r(CH), 1336 cm1 and 1377 cm1 to r(CCC)ring, tas (CCcenterC) and r(CH), 1448 cm1 and 1474 cm1 attributed to ras, 1535 to ras or t(CringN), respectively [30]. The silver nanocubes can detect CV as low as 1.0 109 mol/L (as shown in Fig. 5). The regular Raman measurement (curve b) with 0.01 mol/L crystal violet solution on Si wafers has been obtained. The sensitivity of the SERS detection can be calculated from the enhancement factor (EF). The EF for crystal violet on SERS substrate was calculated according to the following quantification [31]:
EF ¼
ISERS Nv ol IRS NSurf
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 [32], where C0 and I0 are the corresponding concentra0 C Surf
tion of molecules and peak intensity for the regular Raman measurement with 0.01 mol/L CV solution on Si wafers, respectively. CSurf and ISERS are the molecular concentration with 1.0 109 mol/L and Raman peak intensity of CV molecules taken from the SERS substrate, respectively. The EF of Ag nanocubes as SERS substrate is calculated by comparing a and b at 1620 cm1. And the AEF of monodisperse silver nanoparticles is estimated to be around 5.06 107, which indicates that the Ag nanocubes can be used as 7 15720:01 . active substrate [33]. AEF ¼ 310:610 9 5:06 10 The reproducibility of signals from SERS spectra is important for the use of SERS as a routine analytical tool [34]. To confirmation
whether the as-prepared samples are able to give reproducible SERS signals under a low concentration of target molecules, SERS spectra of CV molecules with a concentration of 108 M from 37 randomly selected spots on the substrates have been provided (as shown in Fig. 6). The relative standard deviation (RSD) curve of 37 SERS spectra, which are used to estimate the reproducibility of SERS signals, is calculated by the method reported previously [35]. The values of RSD of Raman vibrations at 913, 1178, 1587, and 1620 cm1 are all
