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Synthesis and SERS activity of super-multibranched AuAg nanostructure via silver coating-induced aggregation of nanostars
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 204 (2018) 380–387
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Synthesis and SERS activity of super-multibranched Au\\Ag nanostructure via silver coating-induced aggregation of nanostars Jian-Jun Li, Chen Wu, Jing Zhao, Guo-Jun Weng, Jian Zhu, Jun-Wu Zhao ⁎ The Key Laboratory of Biomedical Information Engineering of Ministry of Education, School of Life Science and Technology, Xi'an Jiaotong University, Xi'an 710049, China
a r t i c l e
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Article history: Received 3 April 2018 Received in revised form 19 June 2018 Accepted 21 June 2018 Available online 22 June 2018 Keywords: Aggregation Multi-branched nanostructure Bimetallic SERS Thiram Detection
a b s t r a c t The super-multibranched Au\\Ag bimetallic nanostructures are synthesized due to the aggregation of Au nanostars in the process of silver coating. The super-multibranched bimetallic nanostructures with different silver coating thickness are obtained by changing the concentration of silver nitrate and ascorbic acid. It has been found that the formation of these nanostructures is due to the stacking of several nanostars during the process of silver coating. By comparing the silver coating process of gold nanostars with different branch lengths, we found that the nanostars with longish branches are easy to aggregate and form the super-multibranched nanostructures in the process of silver coating. In the Au\\Ag bimetallic nanostructures, the silver layer is mainly covered on the surface of the cores and the thickness increases with the increasing of the AgNO3, which leads to the change of the surface-enhanced Raman scattering (SERS) activity. It has been found that the SERS activity is stronger when the silver layer is thin and the Au branches are still exposed to the outside of the Ag shell. The sample with the strongest SERS activity has been used to detect thiram with different concentrations. The Raman intensity increases linearly with the logarithmic concentration of thiram ranging from 10−3 to 10−7 M with a detection limit of 6.3 × 10−7 M. These experimental results show that the super-multibranched bimetallic nanostructures have a broad application prospect in molecular detection and biologic sensing based on SERS. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Surface enhanced Raman scattering (SERS) is an important analytical technique and has been widely applied in life sciences, archaeology, forensic medicine, environmental protection, etc. due to its high sensitivity, selectivity and nondestructive detection. It is generally believed that SERS includes two major contributors that are the electromagnetic enhancement (EM) and the chemical enhancement (CM). EM results from the increase in local electromagnetic field due to the excitation of localized surface plasmon resonances (LSPRs), while CM is mainly due to charge transfer [1, 2]. It is well acknowledged that in most circumstances EM is the major contributor of the total enhancement factor and can be N1010, while the chemical enhancement factor is usually no N102 and highly molecule specific [3]. The SERS enhancement factor (EF), especially the electromagnetic enhancement factor, is closely related to the composition, size, morphology and interparticle distance of the SERS substrate which usually consist of gold and silver nanostructures. For anisotropic nanostructures and nanostructure assemblies, the electromagnetic enhancement factor at the “hot spots” such as the tips, crevices and interparticle nanogaps are much larger than the rest of the ⁎ Corresponding author at: School of Life Science and Technology, Xi'an Jiaotong University, Xi'an 710049, China. E-mail address: [email protected] (J.-W. Zhao).
https://doi.org/10.1016/j.saa.2018.06.080 1386-1425/© 2018 Elsevier B.V. All rights reserved.
areas and hence larger than the average EF of the entire substrate [4, 5]. By deliberately measuring the SERS at the “hot spots” such as the nanogap of a silver nano-dumbbell, single molecule detection can be achieved [6–8]. However, the application of single-molecule surfaceenhanced Raman spectroscopy (SM-SERS) has its own difficulties such as fluctuation, unrepeatability, and uncertainty of the location [9–11]. In addition, Raman background from the photoluminescence of metals, contaminant molecules, in-band emission of metals, etc. may further reduces the signal-to-noise ratio [12, 13]. An alternative approach is to increase the density of hot spots in the SERS substrate so as to improve the overall EF and reduce its spatial variation. The multi-branched gold nanoparticles contain a large number of branches and hence high density of hot spots. Rodriguez et al. [14] found that multi-branched gold nanoparticles have very high Raman enhancement factor and can be used for molecular detection with an ultralow detection limit of 10−21 M. Zhu et al. [15] studied the Raman spectral properties of Rhodamine 6G in the presence of gold nanoparticles. The maximum Raman enhancement factor of Rhodamine 6G was EF = 1.17 × 105. Yuan et al. [16] found that the SERS on nanostars was several orders of magnitude greater than that of nanospheres. In their study, 4 unique nanostars were used as SERS probes in both in-vivo solutions and ex-vivo tissue samples under near infrared ray excitation. In order to realize quantitative multiplex detection, Liu et al. [17] created a pHsensing nanoprobe based on SERS using nanostars. Li et al. [18] used
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polydopamine-coated gold nanostars (Au NSs) for computed tomography imaging and enhanced photothermal therapy of tumors. Bimetallic nanoparticles have attracted wide attentions due to their optical, chemical and electrical properties depending on the shell thickness and composition ratio [19–24]. The formation of gold and silver bimetallic nanoparticles is mainly formed by coating a layer of silver on the surface of various morphologies of gold nanoparticles to form gold silver core-shell nanoparticles, such as core-shell structure Au\\Ag nanorods [20], Au\\Ag nanospheres [21], Au\\Ag triangle plate [22], etc. Different from the morphology of nanospheres and nanorods with gold silver core-shell structure, the branched nanoparticles such as nanostars and nanoflowers will form bimetallic nanoparticles by coating a layer of silver on the surface. The feature of this kind of particles is that the silver layer is mainly covered on the gold cores, and the branches of gold will expose different lengths out of the silver layer. Thus there are two kinds of metals on the surface of the nanoparticles at the same time. For non-branched nanoparticles, there is only silver element on the surface of the nanoparticles after the silver coating. This feature has significant advantages for SERS and other biosensors. For example, the multibranched Au\\Ag bimetallic nanostructures can also be used to detect two kinds of tumor markers at same time based on surface enhanced Raman scattering. By different biologic or chemical modifications on gold and silver surface, different kinds of biologic molecules could be specifically bound on the gold branch or silver shell. Therefore, we can use its bimetallic character and good SERS properties to detect two kinds of tumor markers based on SERS. Thiram, a pesticide widely used in vegetables, fruits and crops, is difficult to estimate from the skin and mucous membranes of the human body. Therefore, the SERS detection and determination of thiram is more urgent. At present, the main methods to detect thiram are UV– Vis spectrophotometry, chemiluminescence analysis, gas chromatography/mass spectrometry, and high-performance liquid chromatography, but the traditional method for detection of thiram is time consuming, complex and expensive [25–27]. In recent years, SERS has attracted much attention because of its excellent sensitivity. What's more, the molecules of thiram could disconnect S\\S bond to adsorb to the surface of metallic nanoparticles, and form the strongest SERS characteristic peak at 1386 cm−1 [28]. Therefore, thiram was selected in this study for determination of SERS activity of super-multibranched metallic nanoparticles. In this study, we use AgNO3 to induce the aggregation of nanostars and synthesize super-multibranched Au\\Ag bimetallic nanoparticles for the first time. What's more, the effects of the concentration of AgNO3 and AA on the morphology of the multibranching have been studied. Then we discussed the effect of different branching length on the formation of the super-multibranched bimetallic nanoparticles. In the experiment, the changes of SERS activity under different silver layer thickness were analyzed by using rhodamine 6G as Raman probe molecule and the synthesized super-multibranched bimetallic nanoparticles as the substrate. Finally, thiram with different concentrations was detected by using this kind of super-multibranched nanostructure.
2. Experimental 2.1. Materials and Chemicals Polyethylene glycol octyl phenyl ether (TX100) and silver nitrate (AgNO3, N99%) were purchased from Sigma-Aldrich. Chloroauric acid tetrahydrate (HAuCl4.4H2O) was achieved from Sinopharm Chemical Reagent Co. Ltd., China. Sodium borohydride (NaBH4, 98%) and ascorbic-acid (AA, 99.99%) were obtained from Aladdin Industrial Corporation. Ammonium hydroxide (25%) was purchased from Tianjin Fu Yu Chemical Reagent Co. Ltd., China. The ultrapure water was obtained from Millipore water purification system (Millipore, USA).
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2.2. The Preparation of Gold Nanostars Gold nanostars were synthesized as previously reported [29]. The gold seed was prepared by adding 5 mL 0.5 mM HAuCl4 to 5 mL 0.2 M TX100 solution. Then 0.6 mL ice NaBH4 was added to the mixture solution with gently string. At this point, the color of the solution changed from light yellow to reddish brown. After the color of the solution was stabilized, the solution was placed at 4 °C for several hours until use. To prepare the growth solution, 1.25 mL 4 mM AgNO3 was added to 2.5 mL 0.2 M TX100 solution, followed by the addition of 2.2 mL 1 mM HAuCl4. At present, the solution appears yellowish. Then adding 85 μL 0.078 M of ascorbic acid solution into the mixed solution, the color of the mixture gradually changed from yellow to colorless transparent liquid. Finally, adding 5 μL of seed solution to the mixed liquid and slightly shake, wherein the mixed solution was rendered pink immediately and gradually changed into blue-green, the mixed solution was taken as dark green solution. The gold nanostars can be obtained after being placed in a water bath environment at 27 °C for 1 h. 2.3. Replacement of Surfactants Because TX-100 is a nonionic surfactant, its combination with gold nanoparticle is a physical adsorption, and the binding is so weak that it cannot be centrifuged. Therefore, it is necessary to replace TX-100 with a more stable surfactant. CTAB is a relatively stable surfactant, and it is easy to replace the TX100 on the surface of gold nanoparticles. Therefore, 0.1 M CTAB is chosen to mix with gold nanoparticles in the ratio of 2:1. 2.4. Silver Coating and Aggregation of Gold Nanostars Silver was coated onto the nanostars using an established protocol [30]. Added 1 μL 0.1 M AgNO3 solution and equivalent ascorbic acid into 1 mL prepared gold nanostars solution, and then adding 3 μL NH4OH. The color of the solution changed from turquoise to light purple, then deepened gradually, and finally changed to purplish red. When the color of the solution no longer changed, it indicated that silver had been coated on the gold nanostars. 2.5. The Preparation of SERS Samples To study the SERS characteristics of Ag coated gold nanostars (Au@Ag nanostars), we used rhodamine 6G as Raman probe molecule. 250 μL 10 −6 M rhodamine 6G solution was added to 1 mL Au@Ag nanostars solution. After mixing for 1 h, the solution were centrifuged down at 6000 rpm for 10 min and then resuspended in 50 μL water. Took 20 μL of the solution, dropped it on the clean glass, and measured the Raman spectrum after drying. 2.6. Equipment and Characterization The measurement of absorption spectrum was carried out by a Shimadzu UV-3600 UV/Vis/near-IR spectrometers. H-600 (Hitachi, Japan) transmission electron microscope (TEM) under an acceleration voltage of 75 kV was used to obtain TEM images. The Raman spectra were recorded using HORIBA JOBIN YVON Raman spectrometer (HORIBA, France) equipped with 633 nm laser and 10× objective. HRTEM images and energy dispersive X-ray spectroscopy (EDS) were obtained by a JEM-2100Plus transmission electron microscope (JEOL, Japan). 3. Results and Discussions 3.1. Plasmonic Absorption Spectrum Properties of Au Nanostars and Agcoated Au Nanostars The absorption spectra and TEM of gold nanostars are shown in Fig. 1. From the TEM, one can find the number of gold branches is 4–6
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Fig. 1. (a) Absorption spectra and (b) TEM image of gold nanostars.
and the gold core size is about 10 nm. From the core to the tip of the branch, the branches get thin gradually. What is more, there are two branches on the same line. From the absorption spectrum in Fig. 1a, we can see that there are three plasmon resonance absorption peaks. For instance, when the amount of AA is 60 μL, the first absorption peak at 510 nm is own to the LSPR of gold core, the second absorption peak at 700 nm is due to the LSPR of the gold branches, and the third absorption peak at 1250 nm (in the near infrared region) is the result of the aligned branches. Thus, the transverse oscillations of valence electrons form the shorter LSPR at 510 nm. The longitudinal oscillation of electrons along the aligned branches is the reason for the long LSPR in the near infrared region. On the other hand, at least one branch forms
an angle with all branches. So the rest of the branches are not coupled and the intermediate LSPR are generated [29]. It can also be seen from the absorption spectrum that the second absorption peak gradually red shifts by increasing the amount of AA, which indicates that the length of branch become longer with the increase of AA. The absorption spectra and TEM of Ag-coated Au nanostars (Au@Ag nanostars) are shown in Fig. 2. The gold nanostars with 80 μL AA (the corresponding second LSPR peak is located at 940 nm) was used in Ag coating. Fig. 2a is the absorption spectrum of Au@Ag nanostars with different silver layer thickness. It can be seen that the second absorption peak blue shifts to 510 nm, the third absorption peak gradually blue shifts and the absorption peak of 510 nm was gradually increased
Fig. 2. (a) Absorption spectra of Ag coated Au nanostars (80 μL AA) with different Ag coating thicknesses; TEM images of (b) dispersion and (c) aggregation of Au@Ag nanostars.
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with the increasing of AgNO3. It indicates that the length of exposed gold branches is shorter with the increasing of silver layer thickness. The TEM image of Fig. 2b shows that the silver layer is mainly coated on the gold core and the total size of the core is gradually increasing, which leads to the exposed branches of gold become shorter gradually. When the volume of AgNO3 reaches 12 μL (the corresponding concentration of AgNO3 in the sample is 1.1 mM), the absorption peak of 510 nm red shifts slightly, while the third absorption peak still blue shifts. This spectral change indicates the formation of supermultibranched bimetallic nanostars. Fig. 2c shows the TEM of aggregated Au@Ag nanostars. Compared with the dispersed ones, the aggregation state has the following characteristics: 1. Branches are more numerous and dense; 2. The core size is larger; 3. The length of branches is shorter. The analysis of elements is shown in Fig. 3. It is found that the main elements of the core are gold and silver, and the main elements of branches are gold, which further verifies that the silver layer is actually coated on the cores, which also indicates that the formation of this kind of multi-branching is due to aggregation instead of the re-growth. In order to further understand the property of super-multibranched Au@Ag nanostars, the amount of AgNO3 is increased continuously and
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the TEM images are shown in Fig. S1. It can be seen from Fig. S1a, when the amount of AgNO3 is small, branches can be longer and the number of branches increases. One can also find the aggregation is due to the accumulation of several gold nanostars after the addition of AgNO3. When the amount of AgNO3 increases, the exposed branches of the dispersed Au@Ag nanostars are relatively short. When the amount of AgNO3 is increased to 18 μL, the majority of branches have been completely coated with silver, and form a variety of irregular polygons. 3.2. The Effect of Different Branch Length on the Formation of Bimetallic Nanoparticles In order to investigate the effect of different branch length of gold nanostars on the formation of super-multibranched bimetallic nanoparticles in the process of silver coating, we selected gold nanostars synthesized with different concentrations of AA for silver coating. Fig. S2a shows the absorption spectra of Au@Ag nanostars with different silver layer thickness, the initial gold nanostars have the second absorption peak at 690 nm. From the absorption spectra, it can be seen that the
Fig. 3. EDS elemental mapping of aggregated Au@Ag nanostars. (a) the SEM of aggregated Au@Ag nanostars; (b) the mapping of Ag element; (c) the mapping of Au element and (d) comprehensive distribution map of Ag and Au element.
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third absorption peak continuously blue shifts with the increase of silver nitrate concentration. When the amount of AgNO3 reaches 4 μL (the corresponding concentration of AgNO3 in the sample is 0.39 mM), the second absorption peak blue shifts to 510 nm, and no longer shifts when the AgNO3 amount is further increased. When the amount of silver nitrate is 7 μL, as shown in Fig. S2b, most of the Au@Ag nanostars still have exposed branches. When the amount of AgNO3 reaches 10 μL (the corresponding concentration of AgNO3 in the sample is 0.99 mM), as shown in Fig. S2c, the gold nanostars are completely covered by silver layer, and no aggregation and super-multibranched bimetallic nanoparticles are produced. In Fig. S3, the absorption spectra and TEM images of Ag coated gold nanostars (70 μL AA) with different Ag coating thicknesses are studied. In this case, the second LSPR peak is located at 880 nm. Similar to the results in Fig. S2, with the increase of AgNO3 content, the second absorption peak blue shifts to 510 nm firstly and then fixed at the position, whereas the third absorption peak blue shifts continuously. When the amount of AgNO3 is increased to 12 μL, as shown in Fig. S3b, some of the tips of branches are not completely covered by the silver layer. When the amount of AgNO3 increases to 18 μL, the corresponding TEM is shown in Fig. S3c, it can be seen that the branches of gold are completely coated by the silver layer, forming irregular silver nanoparticles. From the above results, we can see that when the branches of the nanoparticles are relatively short, the particle aggregation is not easy to take place, and the supermultibranched bimetallic nanoparticles cannot be obtained in the process of silver coating. Fig. S4 shows the absorption spectra and TEM images of Ag coated gold nanostars (90 μL AA) with different Ag coating thicknesses. In this case, the second LSPR peak is located at 1030 nm. With the increase of the silver layer thickness, the third absorption peak is continuously blue shifted. The second absorption peak firstly blue shifts to 485 nm, and then red shifts when the amount of silver nitrate reaches 12 μL (the corresponding concentration of AgNO 3 in the sample is 1.1 mM). Fig. S4b shows TEM image when the amount of silver nitrate is 12 μL. As can be seen from the figure, the thickness of the silver layer is not uniform, and a small number of nanoparticles aggregate to form multi-branched nanoparticle, but most of the nanoparticles are still in the dispersed state. When the amount of silver nitrate increases to 20 μL, as shown in Fig. S4c, the silver layer thickness increases and the thickness is relatively uniform. Only a few nanoparticles aggregate to form multi-branched bimetallic nanoparticles. Compared with the results in Fig. S1, the AA amount is 80 μL, the effect of agglomeration is not obvious, and the thickness of silver layer is not uniform.
From the above results, it can be seen that the length of branches does have an effect on the formation of super-multibranched bimetallic nanoparticles. When the length of branches is short, there is no aggregation. Whereas when the length of branches is longer, the aggregation effect is also not obvious. When the AA content is 80 μL and the second absorption peak is about 900 nm, it is easy to aggregate in the process of silver coating and form super-multibranched bimetallic nanoparticles. Based on the above experimental results, we speculate that the formation of the multi-branched bimetallic nanoparticles is due to the following reasons. In the longer branched gold nanostars, the AA is excessive, which increases the negative charge in the solution. After the replacing of surfactant on the gold nanostars with CTAB, the interaction of positive and negative charge reduces the effect of CTAB. When silver nitrate and AA are added, a part of AA is served as reducing agent to reduce silver ion to silver. The presence of excess AA causes the solution to be negatively charged. The combination of CTAB and nanoparticles is weakened, which results in the aggregation of nanoparticles to form super-branched bimetallic nanoparticles. In the process of silver coating, the second and third absorption peaks are constantly shifting blue and the intensity of the absorption peaks at about 510 nm is increased continuously. However, the absorption peaks of the gold branches have not completely disappeared, but constantly blue shift. So there are still bare branches of gold, which could be demonstrated by the absorption spectrum in Figs. S3 and S4. EDS technology is used to analyze the distribution of elements. In this paper, the main purpose is to analyze the distribution of silver elements, mainly to explain whether the branches of nanostructure are gold or silver. From the analysis of EDS, the elements on the branches are mainly gold, and the silver is distributed on the nucleus. The results indicate that the formation of the nanoparticles is due to aggregation instead of growing silver branches again. 3.3. Improve the SERS Activity of the Super-multibranched Nanoparticles by Tuning the Ag Coating In order to study the SERS activity of the super-multibranched nanostars, rhodamine 6G was used as Raman active molecule and 633 nm laser as excitation light to measure the Raman spectra. Fig. 4a shows the absorption spectra of Ag coated gold nanostars (80 μL AA) with different Ag coating thicknesses. The blue shift of the longer wavelength peak and the increase of the peaks in the visible region indicate the silver is coated on the gold nanostars. The Raman spectra are shown in Fig. 4b. The characteristic peak of rhodamine 6G is represented by SERS spectrum, and the characteristic peak at 613 cm−1 exhibits the vibrational mode of C\\C\\C ring. The characteristic peak at 1312 cm−1
Fig. 4. (a) Absorption spectra and (b) SERS spectra of Ag coated gold nanostars (80 μL AA) with different Ag coating thicknesses and aggregated Au@Ag nanostars.
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is the bending mode in N\\H bond plane. The bands at about 1362 and 1510 cm−1 are assigned to the C\\C stretching modes [31]. The SERS activity of bimetallic gold and silver nanoparticles was obviously enhanced compared with the bare gold nanostars without Ag coating. In the multibranched Ag coated gold nanostars, the Raman enhancement is mainly resulted from the physical origin based on local field effect. The location of the hot-spots induced from the local electric field enhancement may appear at the tips of the Au branches and the surfaces of the Ag shell. In the study of Atta et al., local electric field distributions for multibranched Au nanostars have been calculated using finite element simulations. One can find the hot-spots of the electric field usually take place at the tips of the branches [32]. Similar results have also been reported by Kumar et al. [33]. In the report of Pande et al., the three-dimensional finite difference time domain (3D-FDTD) method has been applied to calculate the local electric field on the spherical Au@Ag nanoparticles for various core-shell ratios [34]. One can find the hot-spots of the electric field appear at the outer surfaces of the Ag shell of the bimetallic nanoparticles. Similar results have also been observed from the Ag-coated Au nanorods [35, 36], and nanodumbbells [37]. Based on these previous reports, we believe the hot-spots may appear at the tips of the Au branches and the outer surfaces of the Ag shell. And the hot spot size and number could be tuned by the Ag coating thickness and exposed Au branches length. However, with the increase of silver layer thickness, the Raman activity decreased gradually. When the silver layer thickness is thin, the SERS activity is the highest. The reason is that when the silver layer is thin, the exposed gold branches are longer which leads to more hot spots at the tip of the branches and the stronger electromagnetic field. When most branches were covered with silver layer, the tip effect was weakened which results in the decrease of SERS activity. After the branches were completely coated by Ag shell, the SERS activity would not change again. It can also be seen from the SERS spectra that the SERS activity of bimetallic nanostructures is stronger than that of pure silver spheres. The SERS enhancement factor used in this study is defined as G ¼ I SERS =cSERS I Raman =cRaman .
[38]. The ISERS and IRaman are the intensity of Raman and
SERS signal, the cSERS and cRaman are the concentration of R6G used for the Raman and SERS measurements, respectively. According to this formula, the SERS enhancement factor of the super-multibranched nanoparticles for rhodamine 6G is 1.2 × 107. The comparison of SERS enhancement factor of rhodamine 6G with different morphological nanoparticles is shown in Table S1. The results show that the SERS activity of the synthesized super-multibranched nanoparticles is better than others. Although some nanoparticles have higher SERS enhancement factors, the preparation process of the nanostructure is too complicated.
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While, the super-multibranched nanoparticles synthesized in this paper is simple and does not require modification.
3.4. The Detection of Thiram Based on the Substrate of Supermultibranched Nanoparticles From the SERS activity of the multi-branched Au@Ag nanostars shown in Fig. 4b, it is found when the amount of silver nitrate reaches 10 μL (the corresponding concentration of AgNO3 in the sample is 0.99 mM), the multi-branched Au@Ag nanostars have the strongest SERS enhancement. Therefore, this sample was used as a Raman substrate to detect thiram with different concentrations. As shown in Fig. 5a, the Raman spectra of thiram with different concentrations were detected. The strongest Raman characteristic peak of thiram is located at 1376 cm−1, which is related to the symmetric CH3 deformation model and the CN stretching mode. While the characteristic peak at 558 cm−1 is caused by the S\\S stretching mode. The characteristic peak at 926 cm−1 is related to the CH3-N stretching pattern. The stretching mode of CN and the swaying mode of CH3 lead to the formation of characteristic peaks at 1142 cm−1 and 1498 cm−1 [39]. The characteristic peak at 1376 cm−1 is usually regarded as the characteristic peak of thiram with the largest intensity and the most obvious change. It can be seen from the Fig. 5a that the activity of SERS is enhanced with the increase of the concentration of thiram. The detectable range is 10−3–10−7 M. Fig. 5b is the linear fitting of the characteristic peak at 1376 cm−1 with the change of the logarithm of the thiram concentration. The regression equation is Y = 2055.49 + 281.82 X, where the Y is the Raman intensity of the Raman peak, and X is the logarithm of the thiram concentration. The correlation coefficient is 0.9972 and the limit of detection (LOD) of thiram is 6.3 × 10−7 M. The result shows that this method has a wide linear range and high sensitivity to detect thiram. In order to test the selectivity of thiram by this sensing method, we selected several different pesticides such as chlorothalonil, parathionmethyl, chlorpyrifos, phosmet and imidacloprid as interference factors to measure the Raman spectrum. The concentration of the interference factors is 100 μg/mL, whereas the concentration of thiram is 24 μg/mL. The result of interference experiment is shown in Fig. 6. The Raman intensity of thiram is N10 times higher than other interference factors. The result shows that the detection of thiram by this substrate based on super-multibranched Au\\Ag bimetallic nanostructures has good selectivity and specificity. The mechanism could be illuminated as that thiram molecules will disconnect S\\S bond to adsorb to the surface of super-multibranched Au\\Ag nanostructures. But other pesticides
Fig. 5. (a) SERS spectra of thiram with different concentration dropped on aggregated Au@Ag nanostars with 10 μL AgNO3; (b) The relationship between Raman peak intensity and thiram concentration.
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4. Conclusions
Fig. 6. SERS intensity of thiram and other interference factors (100 μg/mL), including chlorothalonil, parathion-methyl, chlorpyrifos, imidacloprid and phosmet.
This paper mainly introduces a kind of multi-branched bimetallic nanoparticles produced by silver ion induced aggregation. At the same time, the thickness of silver layer is adjusted by changing the amount of AgNO3 and AA. The properties and applications of the bimetallic multi-branched nanoparticles in SERS detection are described. The effect of branch length on the aggregation and formation of supermultibranched Au\\Ag bimetallic nanostructures is studied, and the result indicates the aggregation effect is the best when the nanostars with 80 μL AA was used for silver coating. In the SERS activity test, we used the bimetallic nanostructures as the substrate and rhodamine 6G as the Raman signal molecule. It has been found that the nanostructures have the strongest SERS activity when the silver layer was thin. Finally, for the first time, we used this new kind of super-multibranched nanoparticles to detect thiram with different concentrations. The detection range is 10−3–10−7 M, and the detection limit is 6.3×10−7 M. These results indicate that the bimetallic super-multibranched nanoparticles have a broad prospect for the application in molecular detection and biosensor. Acknowledgement
cannot absorb to the surface without the S\\S bond. Therefore, the detection of thiram has good specificity and selectivity. The comparison of the super-multibranched Au\\Ag nanostructures with other different morphological nanoparticles for the SERS thiram detection is shown in Table S2. Compared with these nanoparticles, one can find that the detection of thiram by using the super-multibranched Au\\Ag nanostructures has wider detection range. Compared with other nanoparticles, the super-multibranched nanoparticles have many sharp branches which make it produce more hot spots, thus making SERS characteristics superior to others. Thus more thiram molecules are needed to cover the whole particles. Therefore, the multi-branched bimetallic nanoparticles used as substrate for the detection of thiram have a wide range of detection. Other nanoparticles, such as Ag dendritic nanostructures [40], have a lower detection limit, but their detection range is small and the preparation process is complex. However, the preparation of the super-multibranched Au\\Ag nanostructures is simple and does not require any modification. Therefore, it is a good choice to use this super-multibranched Au\\Ag nanostructure as substrate to detect thiram.
3.5. Detection of Thiram on Tomato Peels In practical application, thiram is mainly used to prevent diseases and pests of vegetables and fruits. Therefore, we choose the peels of tomato as the sample for the detection of thiram. First, the tomato was washed by ultra-pure water and fixed on the glass plate. Then 10 μL thiram with different concentrations were dropped on the peels. After the solution was evaporated and dried, 10 μL ethanol solution was added, and then dropped it with 10 μL colloidal nanoparticles. The experimental results are shown in Table 1. For the detection of thiram on the tomato peels, the recovery rate is in the range of 94–99%. The experimental result shows that this method can be used to detect the thiram on pericarp.
Table 1 Detection of thiram on tomato peels. Sample
Added
Found
Recovery
RSD (%, n = 3)
Tomato peels
240 μg/mL 2.4 μg/mL 24 μg/mL
238.35 μg/mL 2.29 μg/mL 22.50 μg/mL
99% 96% 94%
4.04 6.03 7.64
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis and SERS activity of super-multibranched Au\\Ag nanostructure via silver coating-induced aggregation of nanostars Jian-Jun Li, Chen Wu, Jing Zhao, Guo-Jun Weng, Jian Zhu, Jun-Wu Zhao ⁎ The Key Laboratory of Biomedical Information Engineering of Ministry of Education, School of Life Science and Technology, Xi'an Jiaotong University, Xi'an 710049, China
a r t i c l e
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Article history: Received 3 April 2018 Received in revised form 19 June 2018 Accepted 21 June 2018 Available online 22 June 2018 Keywords: Aggregation Multi-branched nanostructure Bimetallic SERS Thiram Detection
a b s t r a c t The super-multibranched Au\\Ag bimetallic nanostructures are synthesized due to the aggregation of Au nanostars in the process of silver coating. The super-multibranched bimetallic nanostructures with different silver coating thickness are obtained by changing the concentration of silver nitrate and ascorbic acid. It has been found that the formation of these nanostructures is due to the stacking of several nanostars during the process of silver coating. By comparing the silver coating process of gold nanostars with different branch lengths, we found that the nanostars with longish branches are easy to aggregate and form the super-multibranched nanostructures in the process of silver coating. In the Au\\Ag bimetallic nanostructures, the silver layer is mainly covered on the surface of the cores and the thickness increases with the increasing of the AgNO3, which leads to the change of the surface-enhanced Raman scattering (SERS) activity. It has been found that the SERS activity is stronger when the silver layer is thin and the Au branches are still exposed to the outside of the Ag shell. The sample with the strongest SERS activity has been used to detect thiram with different concentrations. The Raman intensity increases linearly with the logarithmic concentration of thiram ranging from 10−3 to 10−7 M with a detection limit of 6.3 × 10−7 M. These experimental results show that the super-multibranched bimetallic nanostructures have a broad application prospect in molecular detection and biologic sensing based on SERS. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Surface enhanced Raman scattering (SERS) is an important analytical technique and has been widely applied in life sciences, archaeology, forensic medicine, environmental protection, etc. due to its high sensitivity, selectivity and nondestructive detection. It is generally believed that SERS includes two major contributors that are the electromagnetic enhancement (EM) and the chemical enhancement (CM). EM results from the increase in local electromagnetic field due to the excitation of localized surface plasmon resonances (LSPRs), while CM is mainly due to charge transfer [1, 2]. It is well acknowledged that in most circumstances EM is the major contributor of the total enhancement factor and can be N1010, while the chemical enhancement factor is usually no N102 and highly molecule specific [3]. The SERS enhancement factor (EF), especially the electromagnetic enhancement factor, is closely related to the composition, size, morphology and interparticle distance of the SERS substrate which usually consist of gold and silver nanostructures. For anisotropic nanostructures and nanostructure assemblies, the electromagnetic enhancement factor at the “hot spots” such as the tips, crevices and interparticle nanogaps are much larger than the rest of the ⁎ Corresponding author at: School of Life Science and Technology, Xi'an Jiaotong University, Xi'an 710049, China. E-mail address: [email protected] (J.-W. Zhao).
https://doi.org/10.1016/j.saa.2018.06.080 1386-1425/© 2018 Elsevier B.V. All rights reserved.
areas and hence larger than the average EF of the entire substrate [4, 5]. By deliberately measuring the SERS at the “hot spots” such as the nanogap of a silver nano-dumbbell, single molecule detection can be achieved [6–8]. However, the application of single-molecule surfaceenhanced Raman spectroscopy (SM-SERS) has its own difficulties such as fluctuation, unrepeatability, and uncertainty of the location [9–11]. In addition, Raman background from the photoluminescence of metals, contaminant molecules, in-band emission of metals, etc. may further reduces the signal-to-noise ratio [12, 13]. An alternative approach is to increase the density of hot spots in the SERS substrate so as to improve the overall EF and reduce its spatial variation. The multi-branched gold nanoparticles contain a large number of branches and hence high density of hot spots. Rodriguez et al. [14] found that multi-branched gold nanoparticles have very high Raman enhancement factor and can be used for molecular detection with an ultralow detection limit of 10−21 M. Zhu et al. [15] studied the Raman spectral properties of Rhodamine 6G in the presence of gold nanoparticles. The maximum Raman enhancement factor of Rhodamine 6G was EF = 1.17 × 105. Yuan et al. [16] found that the SERS on nanostars was several orders of magnitude greater than that of nanospheres. In their study, 4 unique nanostars were used as SERS probes in both in-vivo solutions and ex-vivo tissue samples under near infrared ray excitation. In order to realize quantitative multiplex detection, Liu et al. [17] created a pHsensing nanoprobe based on SERS using nanostars. Li et al. [18] used
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polydopamine-coated gold nanostars (Au NSs) for computed tomography imaging and enhanced photothermal therapy of tumors. Bimetallic nanoparticles have attracted wide attentions due to their optical, chemical and electrical properties depending on the shell thickness and composition ratio [19–24]. The formation of gold and silver bimetallic nanoparticles is mainly formed by coating a layer of silver on the surface of various morphologies of gold nanoparticles to form gold silver core-shell nanoparticles, such as core-shell structure Au\\Ag nanorods [20], Au\\Ag nanospheres [21], Au\\Ag triangle plate [22], etc. Different from the morphology of nanospheres and nanorods with gold silver core-shell structure, the branched nanoparticles such as nanostars and nanoflowers will form bimetallic nanoparticles by coating a layer of silver on the surface. The feature of this kind of particles is that the silver layer is mainly covered on the gold cores, and the branches of gold will expose different lengths out of the silver layer. Thus there are two kinds of metals on the surface of the nanoparticles at the same time. For non-branched nanoparticles, there is only silver element on the surface of the nanoparticles after the silver coating. This feature has significant advantages for SERS and other biosensors. For example, the multibranched Au\\Ag bimetallic nanostructures can also be used to detect two kinds of tumor markers at same time based on surface enhanced Raman scattering. By different biologic or chemical modifications on gold and silver surface, different kinds of biologic molecules could be specifically bound on the gold branch or silver shell. Therefore, we can use its bimetallic character and good SERS properties to detect two kinds of tumor markers based on SERS. Thiram, a pesticide widely used in vegetables, fruits and crops, is difficult to estimate from the skin and mucous membranes of the human body. Therefore, the SERS detection and determination of thiram is more urgent. At present, the main methods to detect thiram are UV– Vis spectrophotometry, chemiluminescence analysis, gas chromatography/mass spectrometry, and high-performance liquid chromatography, but the traditional method for detection of thiram is time consuming, complex and expensive [25–27]. In recent years, SERS has attracted much attention because of its excellent sensitivity. What's more, the molecules of thiram could disconnect S\\S bond to adsorb to the surface of metallic nanoparticles, and form the strongest SERS characteristic peak at 1386 cm−1 [28]. Therefore, thiram was selected in this study for determination of SERS activity of super-multibranched metallic nanoparticles. In this study, we use AgNO3 to induce the aggregation of nanostars and synthesize super-multibranched Au\\Ag bimetallic nanoparticles for the first time. What's more, the effects of the concentration of AgNO3 and AA on the morphology of the multibranching have been studied. Then we discussed the effect of different branching length on the formation of the super-multibranched bimetallic nanoparticles. In the experiment, the changes of SERS activity under different silver layer thickness were analyzed by using rhodamine 6G as Raman probe molecule and the synthesized super-multibranched bimetallic nanoparticles as the substrate. Finally, thiram with different concentrations was detected by using this kind of super-multibranched nanostructure.
2. Experimental 2.1. Materials and Chemicals Polyethylene glycol octyl phenyl ether (TX100) and silver nitrate (AgNO3, N99%) were purchased from Sigma-Aldrich. Chloroauric acid tetrahydrate (HAuCl4.4H2O) was achieved from Sinopharm Chemical Reagent Co. Ltd., China. Sodium borohydride (NaBH4, 98%) and ascorbic-acid (AA, 99.99%) were obtained from Aladdin Industrial Corporation. Ammonium hydroxide (25%) was purchased from Tianjin Fu Yu Chemical Reagent Co. Ltd., China. The ultrapure water was obtained from Millipore water purification system (Millipore, USA).
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2.2. The Preparation of Gold Nanostars Gold nanostars were synthesized as previously reported [29]. The gold seed was prepared by adding 5 mL 0.5 mM HAuCl4 to 5 mL 0.2 M TX100 solution. Then 0.6 mL ice NaBH4 was added to the mixture solution with gently string. At this point, the color of the solution changed from light yellow to reddish brown. After the color of the solution was stabilized, the solution was placed at 4 °C for several hours until use. To prepare the growth solution, 1.25 mL 4 mM AgNO3 was added to 2.5 mL 0.2 M TX100 solution, followed by the addition of 2.2 mL 1 mM HAuCl4. At present, the solution appears yellowish. Then adding 85 μL 0.078 M of ascorbic acid solution into the mixed solution, the color of the mixture gradually changed from yellow to colorless transparent liquid. Finally, adding 5 μL of seed solution to the mixed liquid and slightly shake, wherein the mixed solution was rendered pink immediately and gradually changed into blue-green, the mixed solution was taken as dark green solution. The gold nanostars can be obtained after being placed in a water bath environment at 27 °C for 1 h. 2.3. Replacement of Surfactants Because TX-100 is a nonionic surfactant, its combination with gold nanoparticle is a physical adsorption, and the binding is so weak that it cannot be centrifuged. Therefore, it is necessary to replace TX-100 with a more stable surfactant. CTAB is a relatively stable surfactant, and it is easy to replace the TX100 on the surface of gold nanoparticles. Therefore, 0.1 M CTAB is chosen to mix with gold nanoparticles in the ratio of 2:1. 2.4. Silver Coating and Aggregation of Gold Nanostars Silver was coated onto the nanostars using an established protocol [30]. Added 1 μL 0.1 M AgNO3 solution and equivalent ascorbic acid into 1 mL prepared gold nanostars solution, and then adding 3 μL NH4OH. The color of the solution changed from turquoise to light purple, then deepened gradually, and finally changed to purplish red. When the color of the solution no longer changed, it indicated that silver had been coated on the gold nanostars. 2.5. The Preparation of SERS Samples To study the SERS characteristics of Ag coated gold nanostars (Au@Ag nanostars), we used rhodamine 6G as Raman probe molecule. 250 μL 10 −6 M rhodamine 6G solution was added to 1 mL Au@Ag nanostars solution. After mixing for 1 h, the solution were centrifuged down at 6000 rpm for 10 min and then resuspended in 50 μL water. Took 20 μL of the solution, dropped it on the clean glass, and measured the Raman spectrum after drying. 2.6. Equipment and Characterization The measurement of absorption spectrum was carried out by a Shimadzu UV-3600 UV/Vis/near-IR spectrometers. H-600 (Hitachi, Japan) transmission electron microscope (TEM) under an acceleration voltage of 75 kV was used to obtain TEM images. The Raman spectra were recorded using HORIBA JOBIN YVON Raman spectrometer (HORIBA, France) equipped with 633 nm laser and 10× objective. HRTEM images and energy dispersive X-ray spectroscopy (EDS) were obtained by a JEM-2100Plus transmission electron microscope (JEOL, Japan). 3. Results and Discussions 3.1. Plasmonic Absorption Spectrum Properties of Au Nanostars and Agcoated Au Nanostars The absorption spectra and TEM of gold nanostars are shown in Fig. 1. From the TEM, one can find the number of gold branches is 4–6
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Fig. 1. (a) Absorption spectra and (b) TEM image of gold nanostars.
and the gold core size is about 10 nm. From the core to the tip of the branch, the branches get thin gradually. What is more, there are two branches on the same line. From the absorption spectrum in Fig. 1a, we can see that there are three plasmon resonance absorption peaks. For instance, when the amount of AA is 60 μL, the first absorption peak at 510 nm is own to the LSPR of gold core, the second absorption peak at 700 nm is due to the LSPR of the gold branches, and the third absorption peak at 1250 nm (in the near infrared region) is the result of the aligned branches. Thus, the transverse oscillations of valence electrons form the shorter LSPR at 510 nm. The longitudinal oscillation of electrons along the aligned branches is the reason for the long LSPR in the near infrared region. On the other hand, at least one branch forms
an angle with all branches. So the rest of the branches are not coupled and the intermediate LSPR are generated [29]. It can also be seen from the absorption spectrum that the second absorption peak gradually red shifts by increasing the amount of AA, which indicates that the length of branch become longer with the increase of AA. The absorption spectra and TEM of Ag-coated Au nanostars (Au@Ag nanostars) are shown in Fig. 2. The gold nanostars with 80 μL AA (the corresponding second LSPR peak is located at 940 nm) was used in Ag coating. Fig. 2a is the absorption spectrum of Au@Ag nanostars with different silver layer thickness. It can be seen that the second absorption peak blue shifts to 510 nm, the third absorption peak gradually blue shifts and the absorption peak of 510 nm was gradually increased
Fig. 2. (a) Absorption spectra of Ag coated Au nanostars (80 μL AA) with different Ag coating thicknesses; TEM images of (b) dispersion and (c) aggregation of Au@Ag nanostars.
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with the increasing of AgNO3. It indicates that the length of exposed gold branches is shorter with the increasing of silver layer thickness. The TEM image of Fig. 2b shows that the silver layer is mainly coated on the gold core and the total size of the core is gradually increasing, which leads to the exposed branches of gold become shorter gradually. When the volume of AgNO3 reaches 12 μL (the corresponding concentration of AgNO3 in the sample is 1.1 mM), the absorption peak of 510 nm red shifts slightly, while the third absorption peak still blue shifts. This spectral change indicates the formation of supermultibranched bimetallic nanostars. Fig. 2c shows the TEM of aggregated Au@Ag nanostars. Compared with the dispersed ones, the aggregation state has the following characteristics: 1. Branches are more numerous and dense; 2. The core size is larger; 3. The length of branches is shorter. The analysis of elements is shown in Fig. 3. It is found that the main elements of the core are gold and silver, and the main elements of branches are gold, which further verifies that the silver layer is actually coated on the cores, which also indicates that the formation of this kind of multi-branching is due to aggregation instead of the re-growth. In order to further understand the property of super-multibranched Au@Ag nanostars, the amount of AgNO3 is increased continuously and
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the TEM images are shown in Fig. S1. It can be seen from Fig. S1a, when the amount of AgNO3 is small, branches can be longer and the number of branches increases. One can also find the aggregation is due to the accumulation of several gold nanostars after the addition of AgNO3. When the amount of AgNO3 increases, the exposed branches of the dispersed Au@Ag nanostars are relatively short. When the amount of AgNO3 is increased to 18 μL, the majority of branches have been completely coated with silver, and form a variety of irregular polygons. 3.2. The Effect of Different Branch Length on the Formation of Bimetallic Nanoparticles In order to investigate the effect of different branch length of gold nanostars on the formation of super-multibranched bimetallic nanoparticles in the process of silver coating, we selected gold nanostars synthesized with different concentrations of AA for silver coating. Fig. S2a shows the absorption spectra of Au@Ag nanostars with different silver layer thickness, the initial gold nanostars have the second absorption peak at 690 nm. From the absorption spectra, it can be seen that the
Fig. 3. EDS elemental mapping of aggregated Au@Ag nanostars. (a) the SEM of aggregated Au@Ag nanostars; (b) the mapping of Ag element; (c) the mapping of Au element and (d) comprehensive distribution map of Ag and Au element.
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third absorption peak continuously blue shifts with the increase of silver nitrate concentration. When the amount of AgNO3 reaches 4 μL (the corresponding concentration of AgNO3 in the sample is 0.39 mM), the second absorption peak blue shifts to 510 nm, and no longer shifts when the AgNO3 amount is further increased. When the amount of silver nitrate is 7 μL, as shown in Fig. S2b, most of the Au@Ag nanostars still have exposed branches. When the amount of AgNO3 reaches 10 μL (the corresponding concentration of AgNO3 in the sample is 0.99 mM), as shown in Fig. S2c, the gold nanostars are completely covered by silver layer, and no aggregation and super-multibranched bimetallic nanoparticles are produced. In Fig. S3, the absorption spectra and TEM images of Ag coated gold nanostars (70 μL AA) with different Ag coating thicknesses are studied. In this case, the second LSPR peak is located at 880 nm. Similar to the results in Fig. S2, with the increase of AgNO3 content, the second absorption peak blue shifts to 510 nm firstly and then fixed at the position, whereas the third absorption peak blue shifts continuously. When the amount of AgNO3 is increased to 12 μL, as shown in Fig. S3b, some of the tips of branches are not completely covered by the silver layer. When the amount of AgNO3 increases to 18 μL, the corresponding TEM is shown in Fig. S3c, it can be seen that the branches of gold are completely coated by the silver layer, forming irregular silver nanoparticles. From the above results, we can see that when the branches of the nanoparticles are relatively short, the particle aggregation is not easy to take place, and the supermultibranched bimetallic nanoparticles cannot be obtained in the process of silver coating. Fig. S4 shows the absorption spectra and TEM images of Ag coated gold nanostars (90 μL AA) with different Ag coating thicknesses. In this case, the second LSPR peak is located at 1030 nm. With the increase of the silver layer thickness, the third absorption peak is continuously blue shifted. The second absorption peak firstly blue shifts to 485 nm, and then red shifts when the amount of silver nitrate reaches 12 μL (the corresponding concentration of AgNO 3 in the sample is 1.1 mM). Fig. S4b shows TEM image when the amount of silver nitrate is 12 μL. As can be seen from the figure, the thickness of the silver layer is not uniform, and a small number of nanoparticles aggregate to form multi-branched nanoparticle, but most of the nanoparticles are still in the dispersed state. When the amount of silver nitrate increases to 20 μL, as shown in Fig. S4c, the silver layer thickness increases and the thickness is relatively uniform. Only a few nanoparticles aggregate to form multi-branched bimetallic nanoparticles. Compared with the results in Fig. S1, the AA amount is 80 μL, the effect of agglomeration is not obvious, and the thickness of silver layer is not uniform.
From the above results, it can be seen that the length of branches does have an effect on the formation of super-multibranched bimetallic nanoparticles. When the length of branches is short, there is no aggregation. Whereas when the length of branches is longer, the aggregation effect is also not obvious. When the AA content is 80 μL and the second absorption peak is about 900 nm, it is easy to aggregate in the process of silver coating and form super-multibranched bimetallic nanoparticles. Based on the above experimental results, we speculate that the formation of the multi-branched bimetallic nanoparticles is due to the following reasons. In the longer branched gold nanostars, the AA is excessive, which increases the negative charge in the solution. After the replacing of surfactant on the gold nanostars with CTAB, the interaction of positive and negative charge reduces the effect of CTAB. When silver nitrate and AA are added, a part of AA is served as reducing agent to reduce silver ion to silver. The presence of excess AA causes the solution to be negatively charged. The combination of CTAB and nanoparticles is weakened, which results in the aggregation of nanoparticles to form super-branched bimetallic nanoparticles. In the process of silver coating, the second and third absorption peaks are constantly shifting blue and the intensity of the absorption peaks at about 510 nm is increased continuously. However, the absorption peaks of the gold branches have not completely disappeared, but constantly blue shift. So there are still bare branches of gold, which could be demonstrated by the absorption spectrum in Figs. S3 and S4. EDS technology is used to analyze the distribution of elements. In this paper, the main purpose is to analyze the distribution of silver elements, mainly to explain whether the branches of nanostructure are gold or silver. From the analysis of EDS, the elements on the branches are mainly gold, and the silver is distributed on the nucleus. The results indicate that the formation of the nanoparticles is due to aggregation instead of growing silver branches again. 3.3. Improve the SERS Activity of the Super-multibranched Nanoparticles by Tuning the Ag Coating In order to study the SERS activity of the super-multibranched nanostars, rhodamine 6G was used as Raman active molecule and 633 nm laser as excitation light to measure the Raman spectra. Fig. 4a shows the absorption spectra of Ag coated gold nanostars (80 μL AA) with different Ag coating thicknesses. The blue shift of the longer wavelength peak and the increase of the peaks in the visible region indicate the silver is coated on the gold nanostars. The Raman spectra are shown in Fig. 4b. The characteristic peak of rhodamine 6G is represented by SERS spectrum, and the characteristic peak at 613 cm−1 exhibits the vibrational mode of C\\C\\C ring. The characteristic peak at 1312 cm−1
Fig. 4. (a) Absorption spectra and (b) SERS spectra of Ag coated gold nanostars (80 μL AA) with different Ag coating thicknesses and aggregated Au@Ag nanostars.
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is the bending mode in N\\H bond plane. The bands at about 1362 and 1510 cm−1 are assigned to the C\\C stretching modes [31]. The SERS activity of bimetallic gold and silver nanoparticles was obviously enhanced compared with the bare gold nanostars without Ag coating. In the multibranched Ag coated gold nanostars, the Raman enhancement is mainly resulted from the physical origin based on local field effect. The location of the hot-spots induced from the local electric field enhancement may appear at the tips of the Au branches and the surfaces of the Ag shell. In the study of Atta et al., local electric field distributions for multibranched Au nanostars have been calculated using finite element simulations. One can find the hot-spots of the electric field usually take place at the tips of the branches [32]. Similar results have also been reported by Kumar et al. [33]. In the report of Pande et al., the three-dimensional finite difference time domain (3D-FDTD) method has been applied to calculate the local electric field on the spherical Au@Ag nanoparticles for various core-shell ratios [34]. One can find the hot-spots of the electric field appear at the outer surfaces of the Ag shell of the bimetallic nanoparticles. Similar results have also been observed from the Ag-coated Au nanorods [35, 36], and nanodumbbells [37]. Based on these previous reports, we believe the hot-spots may appear at the tips of the Au branches and the outer surfaces of the Ag shell. And the hot spot size and number could be tuned by the Ag coating thickness and exposed Au branches length. However, with the increase of silver layer thickness, the Raman activity decreased gradually. When the silver layer thickness is thin, the SERS activity is the highest. The reason is that when the silver layer is thin, the exposed gold branches are longer which leads to more hot spots at the tip of the branches and the stronger electromagnetic field. When most branches were covered with silver layer, the tip effect was weakened which results in the decrease of SERS activity. After the branches were completely coated by Ag shell, the SERS activity would not change again. It can also be seen from the SERS spectra that the SERS activity of bimetallic nanostructures is stronger than that of pure silver spheres. The SERS enhancement factor used in this study is defined as G ¼ I SERS =cSERS I Raman =cRaman .
[38]. The ISERS and IRaman are the intensity of Raman and
SERS signal, the cSERS and cRaman are the concentration of R6G used for the Raman and SERS measurements, respectively. According to this formula, the SERS enhancement factor of the super-multibranched nanoparticles for rhodamine 6G is 1.2 × 107. The comparison of SERS enhancement factor of rhodamine 6G with different morphological nanoparticles is shown in Table S1. The results show that the SERS activity of the synthesized super-multibranched nanoparticles is better than others. Although some nanoparticles have higher SERS enhancement factors, the preparation process of the nanostructure is too complicated.
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While, the super-multibranched nanoparticles synthesized in this paper is simple and does not require modification.
3.4. The Detection of Thiram Based on the Substrate of Supermultibranched Nanoparticles From the SERS activity of the multi-branched Au@Ag nanostars shown in Fig. 4b, it is found when the amount of silver nitrate reaches 10 μL (the corresponding concentration of AgNO3 in the sample is 0.99 mM), the multi-branched Au@Ag nanostars have the strongest SERS enhancement. Therefore, this sample was used as a Raman substrate to detect thiram with different concentrations. As shown in Fig. 5a, the Raman spectra of thiram with different concentrations were detected. The strongest Raman characteristic peak of thiram is located at 1376 cm−1, which is related to the symmetric CH3 deformation model and the CN stretching mode. While the characteristic peak at 558 cm−1 is caused by the S\\S stretching mode. The characteristic peak at 926 cm−1 is related to the CH3-N stretching pattern. The stretching mode of CN and the swaying mode of CH3 lead to the formation of characteristic peaks at 1142 cm−1 and 1498 cm−1 [39]. The characteristic peak at 1376 cm−1 is usually regarded as the characteristic peak of thiram with the largest intensity and the most obvious change. It can be seen from the Fig. 5a that the activity of SERS is enhanced with the increase of the concentration of thiram. The detectable range is 10−3–10−7 M. Fig. 5b is the linear fitting of the characteristic peak at 1376 cm−1 with the change of the logarithm of the thiram concentration. The regression equation is Y = 2055.49 + 281.82 X, where the Y is the Raman intensity of the Raman peak, and X is the logarithm of the thiram concentration. The correlation coefficient is 0.9972 and the limit of detection (LOD) of thiram is 6.3 × 10−7 M. The result shows that this method has a wide linear range and high sensitivity to detect thiram. In order to test the selectivity of thiram by this sensing method, we selected several different pesticides such as chlorothalonil, parathionmethyl, chlorpyrifos, phosmet and imidacloprid as interference factors to measure the Raman spectrum. The concentration of the interference factors is 100 μg/mL, whereas the concentration of thiram is 24 μg/mL. The result of interference experiment is shown in Fig. 6. The Raman intensity of thiram is N10 times higher than other interference factors. The result shows that the detection of thiram by this substrate based on super-multibranched Au\\Ag bimetallic nanostructures has good selectivity and specificity. The mechanism could be illuminated as that thiram molecules will disconnect S\\S bond to adsorb to the surface of super-multibranched Au\\Ag nanostructures. But other pesticides
Fig. 5. (a) SERS spectra of thiram with different concentration dropped on aggregated Au@Ag nanostars with 10 μL AgNO3; (b) The relationship between Raman peak intensity and thiram concentration.
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4. Conclusions
Fig. 6. SERS intensity of thiram and other interference factors (100 μg/mL), including chlorothalonil, parathion-methyl, chlorpyrifos, imidacloprid and phosmet.
This paper mainly introduces a kind of multi-branched bimetallic nanoparticles produced by silver ion induced aggregation. At the same time, the thickness of silver layer is adjusted by changing the amount of AgNO3 and AA. The properties and applications of the bimetallic multi-branched nanoparticles in SERS detection are described. The effect of branch length on the aggregation and formation of supermultibranched Au\\Ag bimetallic nanostructures is studied, and the result indicates the aggregation effect is the best when the nanostars with 80 μL AA was used for silver coating. In the SERS activity test, we used the bimetallic nanostructures as the substrate and rhodamine 6G as the Raman signal molecule. It has been found that the nanostructures have the strongest SERS activity when the silver layer was thin. Finally, for the first time, we used this new kind of super-multibranched nanoparticles to detect thiram with different concentrations. The detection range is 10−3–10−7 M, and the detection limit is 6.3×10−7 M. These results indicate that the bimetallic super-multibranched nanoparticles have a broad prospect for the application in molecular detection and biosensor. Acknowledgement
cannot absorb to the surface without the S\\S bond. Therefore, the detection of thiram has good specificity and selectivity. The comparison of the super-multibranched Au\\Ag nanostructures with other different morphological nanoparticles for the SERS thiram detection is shown in Table S2. Compared with these nanoparticles, one can find that the detection of thiram by using the super-multibranched Au\\Ag nanostructures has wider detection range. Compared with other nanoparticles, the super-multibranched nanoparticles have many sharp branches which make it produce more hot spots, thus making SERS characteristics superior to others. Thus more thiram molecules are needed to cover the whole particles. Therefore, the multi-branched bimetallic nanoparticles used as substrate for the detection of thiram have a wide range of detection. Other nanoparticles, such as Ag dendritic nanostructures [40], have a lower detection limit, but their detection range is small and the preparation process is complex. However, the preparation of the super-multibranched Au\\Ag nanostructures is simple and does not require any modification. Therefore, it is a good choice to use this super-multibranched Au\\Ag nanostructure as substrate to detect thiram.
3.5. Detection of Thiram on Tomato Peels In practical application, thiram is mainly used to prevent diseases and pests of vegetables and fruits. Therefore, we choose the peels of tomato as the sample for the detection of thiram. First, the tomato was washed by ultra-pure water and fixed on the glass plate. Then 10 μL thiram with different concentrations were dropped on the peels. After the solution was evaporated and dried, 10 μL ethanol solution was added, and then dropped it with 10 μL colloidal nanoparticles. The experimental results are shown in Table 1. For the detection of thiram on the tomato peels, the recovery rate is in the range of 94–99%. The experimental result shows that this method can be used to detect the thiram on pericarp.
Table 1 Detection of thiram on tomato peels. Sample
Added
Found
Recovery
RSD (%, n = 3)
Tomato peels
240 μg/mL 2.4 μg/mL 24 μg/mL
238.35 μg/mL 2.29 μg/mL 22.50 μg/mL
99% 96% 94%
4.04 6.03 7.64
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