Small and Sharp Triangular Silver Nanoplates Synthesized Utilizing

Article

Small and Sharp Triangular Silver Nanoplates Synthesized Utilizing Tiny Triangular Nuclei and Their Excellent SERS Activity for Selective Detection of Thiram Residue in Soil Chun-Hong Zhang, Jian Zhu, Jian-Jun Li, and Jun-Wu Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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ACS Applied Materials & Interfaces

Small and Sharp Triangular Silver Nanoplates Synthesized Utilizing Tiny Triangular Nuclei and Their Excellent SERS Activity for Selective Detection of Thiram Residue in Soil

Chun-Hong Zhang,

Jian Zhu,

Jian-Jun Li *,

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 * Corresponding author Telephone: 86-29-82664224 Fax numbers: 86-29-82664224 Email: [email protected] (Jun-Wu Zhao) [email protected] (Jian-Jun Li) Address: School of Life Science and Technology, Xi’an Jiaotong University, Xi’an, 710049, Peoples Republic of China KEYWORDS: triangular silver nanoplates, tiny seeds-midiated chemical reduction route, triangular silver nuclei, surface-enhanced Raman scattering, local field enhancement, thiram residue 1

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ABSTRACT: The great harm of thiram residue in soil to environment and human health is usually ignored. Due to the complexity of soil compositions, the detection of thiram residue in soil faces considerable difficulties. In this work, a highly sensitive and selective surface-enhanced Raman scattering (SERS) substrate based on the triangular silver nanoplates (TSNPs) with small size and sharp corners is developed and used for the detection of thiram residue in soil for the first time. These TSNPs are synthesized by replacing the conventional seeds in the seed-mediated chemical reduction route with the tiny and uniform triangular silver nuclei (TSN) which can provide more growing space for generating sharp corners during the growth of TSNPs. It is interesting that the TSNPs with the smaller size have the better SERS performance. The possible mechanism behind this phenomenon is explained by the electromagnetic enhancement theory. Based on the Raman activity of the smallest TSNPs, a SERS-active substrate is prepared for detecting the thiram residue in soil. The thiram solution detection shows that the limit of detection (LOD) of these smallest TSNPs is lower than other nanoparticles, such as nanospheres, nanocubes, etc. For sensing the thiram residue in soil, the addition of poly(sodium 4-styrenesulfonate) realizes the specific adsorption of thiram by TSNPs. This method exhibits a good linear response from 0.12 to 4.8 µg/g with a low LOD of 90 ng/g, which is better than conventional methods. This work shows the great potential of the small TSNPs as a novel SERS substrate and its broader applications in pesticides detection.

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INTRODUCTION

Pesticide residues have been a worldwide endangering problem in environment safety and human health. Thiram, a dithiocarbamate fungicide, is widely used as seed dressing to prevent and treat many kinds of crop diseases.1 Its overuse can pollute soil and may further seep into groundwater or mix with air dust, leading to high toxicity for human skin and mucosa.2 Generally, the detection methods of thiram in soil mainly include spectrophotometry combined with solid-phase extraction, ion mobility spectrometry combined with headspace solid-phase microextraction (HS-SPME-IMS) and high pressure liquid chromatography (HPLC).3-6 However, these methods always suffer from the shortcomings of unsatisfactory limit of detection (LOD), complex extraction procedures, long detection time, high cost and so on, which can hardly meet the needs of practical detection. Surface-enhanced Raman scattering (SERS), as an extremely sensitive detection technique with high specificity and short detection time, has been widely applied in chemical and biological analyses.7-10 In recent years, with the development of nanotechnology, noble metallic nanoparticles have been widely used to enhance Raman signals and dramatically reduced the LOD of SERS because their unique localized surface plasmon resonance (LSPR) properties can induce strong local electric field enhancement.11, 12 In the application of thiram detection, a variety of morphologies and composite structures, such as Au nanorod, Ag nanocube, Ag@mesoporous SiO2 and Au@thiram@Ag structures, have been used as SERS substrate.13-16 However, the nanoparticles either cannot achieve satisfactory LOD or the preparation procedures are too complex. More importantly, most researchers focus on determining the quantity of pure thiram in water or that on the surface of fruits and vegetables, while neglecting the damage and detection of thiram residue in soil. As we all know, the components of soil are more complex than that on the surface of fruits and vegetables, which brings great challenge toward the extraction of thiram and the specificity of thiram detection.17, 18 Therefore, the nanoparticles with simple preparation process, which can also provide high sensitivity and specificity and simplify the extract process of thiram, are the key to solve these problems. Recently, triangular nanoplates have attracted considerable attention due to their intense local electric field enhancements caused by the anisotropic morphology and sharp corners compared with other morphologies such as nanosphere and nanorod.19-21 And the silver nanoparticles show higher LSPR energy and stronger local electric field than gold nanoparticles,22 which makes the triangular 3



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silver nanoplates (TSNPs) be an ideal SERS substrate.23-25 For example, Zhang et al. synthesized high-quality TSNPs and found that the SERS enhancement factor (EF) for the disperse TSNPs could reach ~104.26 Furthermore, the disulfide bond in thiram can break spontaneously upon exposure to the Ag surface and then bind to the TSNPs through the Ag-S bond.27, 28 Thus, the thiram in soil could be easily adsorbed and enriched on the surface of TSNPs, avoiding the complex extraction process. Therefore, it is feasible to use TSNPs as SERS substrate for detecting thiram in soil. In order to obtain a higher SERS signal, the size of TSNPs is also an important factor to consider. However, the correlations between the SERS intensity and the size of TSNPs have hardly been investigated. In general, the larger the size, the stronger the SERS intensity for morphologies such as gold nanosphere, nanostar and nanooctahedra.29-32 However, triangular nanoplates have a unique anisotrpic morphology with a larger proportion of edges and corners. And these sharp corners and edges are the main causes of local electric field enhancement.19 Remarkably, unlike the principle mentioned above, our preliminary experiments indicated that the smaller the size and the sharper the corners of the TSNPs, the stronger the intensity of SERS. Inspired by this, we deduce that the key to getting more intense SERS response is to synthesize the TSNPs with small size and sharp corners. Currently,

many

procedures have

been

reported

to

synthesize

TSNPs, including

photo-induced,33-35 thermal synthetic,36 seed-mediated methods,37 and so on. The TSNPs synthesized by photo-induced methods have high quality, but the reaction time is usually too long (~ 50 h) and large-scale synthesis is limited by the light source power. The thermal synthetic and seed-mediated methods are easier to perform on a large scale, but the corners of the obtained TSNPs are often truncated and the size is large and nonuniform. Therefore, these methods cannot be used to synthesize TSNPs with small size, sharp corners and good uniformity on a large scale. Further researches discovered the TSNPs grown on the preformed triangular silver nuclei (TSN) under low temperature and coordinating ligand conditions exhibit well-defined morphology.38-40 However, the edge length of the TSNPs is usually larger than 100 nm because the seeds are too big. Thus, if we can prepare tiny TSN and use them as seeds for the growth of TSNPs, the TSNPs with small size and sharp corners may be obtained. However, there is no report on the preparation of tiny TSN. And it is very difficult to synthesize tiny TSN, for the morphological features are hard to be sustained due to the larger specific surface area. Many studies have shown that the block copolymers are very 4


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efficient on improving the stability of nanoparticles.41-43 For example, the Pluronic F127 can form micelles on the surface of nanoparticles, which can efficiently improve the nanoparticle stabilization and uniformity. Therefore, we supposed that we can synthesize tinier and more uniform TSN by adding Pluronic F127 to the system of seed preparation. And the TSNPs with small size, sharp corners and good uniformity could be achieved by further growth on the basis of these seeds. Herein, we used an improved thermal synthetic route, in which the Pluronic F127 was added to the reaction system, to synthesize tiny and uniform TSN. And then, a series of small TSNPs with sharp corners and good uniformity were synthesized by chemical reduction on the basis of the above seeds. The spectra, morphologies and size distributions of the TSNPs were also characterized and statistical analyzed. Using these TSNPs as substrates, the relationship between the SERS intensity and the size of the TSNPs has been demonstrated for the first time and the origin has also been illustrated. According to the relationship of the SERS intensity and the size and the sharpness, the TSNPs with an excellent SERS performance could be achieved. And using these TSNPs, a simple, highly sensitive and selective detection of thiram residue in soil was realized (Scheme 1).

Scheme 1. The schematic diagram of the growth process of TSNPs with small size, sharp corners and good uniformity and the SERS-based scheme for the detection of thiram residue in soil.

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EXPERIMENTAL SECTION Materials. Silver nitrate (AgNO3), sodium citrate (C6H5Na3O7·2H2O), poly(vinylpyrrolidone)

(PVP, weight-average molecular weight MW~58000 g/mol), acetonitrile, ascorbic acid (AA), 4-mercaptobenzoic acid (4-MBA), ethanol, thiram, triadimefon, diniconazole, carbendazim, imidacloprid, chlorothalonil and parathion-methyl were purchased from Aladdin. Sodium borohydride (NaBH4), Pluronic F127 and poly(sodium 4-styrenesulfonate) (PSS, weight-average molecular weight MW~70000 g/mol) were purchased from Sigma-Aldrich. Hydrogen peroxide (H2O2, 30 wt%) was purchased from Tianli Chemical Reagent Co. Ltd (Tianjin, China). The ultrapure water used in all experiments was obtained from Millipore water purification system (Milli-Q, Millipore, USA). The resistivity of the ultrapure water was 18.2 MΩ.

Synthesis of tiny and uniform TSN seeds. The tiny and uniform TSN were prepared by a modified thermal synthetic route. In a typical synthesis, an aqueous solution of Pluronic F127 (2 g/L, 195 mL) was mixed with AgNO3 (50 mM, 400 µL), C6H5Na3O7·2H2O (75 mM, 4 mL) and H2O2 (30 wt%, 480 µL) at room temperature. Under vigorous stirring, NaBH4 (100 mM, 400 µL) was quickly injected into the mixture, producing a pale yellow solution. After 15 min, the solution became dark yellow with a great deal of foam. In the synthesis that PVP was used as a stabilizer, Pluronic F127 was replaced by PVP, leaving other steps the same. 50 mL solution of TSN was centrifuged at 12000 rpm for 30 min. The concentrated TSN were redispersed in aqueous solution of sodium citrate (1.5 mM, 10 mL) as the seed solution.

Synthesis of TSNPs with small size and sharp corners. In a typical synthesis of the smallest TSNPs with sharp corners, AA (100 mM, 100 µL) and 10 mL of acetonitrile were added in 20 mL of ultrapure water, followed by addition of 10 mL of the seed solution. Into this mixture, AgNO3 (50 mM, 50 µL) was injected dropwise under vigorous stirring to start the seeded growth. After reaction for 30 min at 0 °C, the TSNPs were collected by centrifugation. In order to synthesize TSNPs of different sizes, two methods of the seeded growth were proposed. One was to adjust the volume of AgNO3, which applied to synthesize small TSNPs. This method is convenient for controlling the size of TSNPs exactly. Specifically, the volume of AgNO3 was 50 µL, 75 µL, 100 µL and 150 µL for the synthesis of TSNPs with LSPR band of 560 nm, 575 nm, 590 nm and 630 nm, respectively, and the ratio of the volume between AA and AgNO3 was 2:1. However, the self-nucleation of Ag 6


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would happen due to high concentration of AgNO3 if one wanted to produce large TSNPs just by adding the volume of AgNO3. Therefore, we could synthesize large TSNPs by reducing the volume of seed solution. Specifically, the volumes of the seed solution were 6.5 mL, 5.5 mL, 4 mL and 3 mL for the synthesis of TSNPs with LSPR band of 670 nm, 700 nm, 770 nm and 795 nm, respectively. The volume of AgNO3 was 170 µL and the ratio of the volume between AA and AgNO3 was 2:1.

Preparation of SERS substrates. In order to examine the dependence of SERS activity on the size of the TSNPs, we need to prepare the SERS substrates under the same particle concentration of each sample. Thus, 8 mL of TSNPs solutions with different LSPR bands (560 nm, 630 nm, 700 nm, 760 nm) were centrifuged down at 8000 rpm for 20 min and then resuspended in 400 µL, 400 µL, 220 µL and 160 µL of water, respectively. These resulting samples of concentrated TSNPs would be sure of the same particle concentration. 4-MBA was adsorbed on the TSNPs by placing 20 µL of 10-6 M solution and letting it dry in air.

Characterization. A UV-3600 UV-VIS-NIR spectrophotometer (Shimadzu, Japan) was used to record the absorption spectra. Transmission electron microscopy (TEM) images were performed using a JEM-200CX (JEOL Ltd., Japan) transmission electron microscopy. For the preparation of the TEM samples, 20 µL of concentrated TSN and TSNPs were placed on copper grids and the excess solution was removed using filter paper. Then, these grids were dried at room temperature. The size distributions were measured by counting a minimum of 100 particles per sample. Scanning electron microscopy (SEM) images were acquired from JSM-7000F (JEOL Ltd., Japan). The preparation of the SEM sample was the same as that of the SERS substrate. The Raman spectra were recorded using HORIBA JOBIN YVON Raman spectrometer (HORIBA, France) equipped with 633 nm laser and 10× objective. The laser power reaching the samples was 3.2 mW and the integration time was 20 s. The numerical aperture (NA) was 0.25 and the focused spot size was ~3 µm.

SERS detection of thiram. First, a series of ethanolic solutions containing 10-4-10-8 M thiram were prepared. Then 20 µL of concentrated smallest TSNPs solution was mixed respectively with 20 µL different concentrations of thiram solution. The mixtures were incubated at room temperature for 10 min. The final concentration of thiram ranged from 5 × 10-5-5 × 10-9 M. 7



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Soil samples pretreatment. Soil samples were collected from farmland and nursery garden located near Xi’an and Beijing, China. First, the soil samples were sifted to remove block masses and sundries and grinded into powder after drying at 60 °C for 2 h. Then 0.024-2.16 µg of thiram was added into 0.2 g of soil sample. In order to extract the thiram in soil samples, 1 mL of ethanol was mixed with the soil sample. And we could get the thiram-dissolved supernatant by centrifugation. To remove the interfering substances in soil, 10 µL of PSS solution (10 mM) was added into 100 µL of above supernatant. Finally, 20 µL of the soil extract containing different concentrations of thiram was mixed respectively with 20 µL of concentrated smallest TSNPs solution. The final concentration of thiram ranged from 0.12-10.8 µg/g.

RESULTS AND DISCUSSION Spectra properties and morphology characterization of the tiny and uniform TSN

synthesized with Pluronic F127 as a stabilizer. To synthesize small, sharp and uniform TSNPs, the tiny and uniform TSN should be prepared as seeds to provide more space for the corners to grow. In this paper, we improved the preparation of TSN by adjusting the amount of NaBH4 and replacing the PVP with Pluronic F127. According to the literature,36 the size of the silver nanoplates with a triangular nuclei crystal structure decreases as the amount of NaBH4 reduces. However, the LSPR bands were usually greater than 600 nm, because the amount of NaBH4 was only tuned in a limited range. Hence, in this study, the amount of NaBH4 was drastically reduced to obtain tiny seeds. As shown in Figure 1a, the LSPR bands of silver nanoplates blue-shift from 620 nm to 490 nm with reducing the amount of NaBH4 from 1500 µL to 500 µL, indicating the decrease in the silver nanoplates size. Among them, the smallest silver nanoplates still have the feature of triangular nuclei and could grow into well-defined TSNPs (Figure S1). However, when the amount of NaBH4 is below 500 µL, the spectrum shows only one peak at about 400 nm caused by the spherical nanoparticles, as shown in Figure 1b. And the well-defined TSNPs could not be achieved by further growth used these nanoparticles as seeds (the inset of Figure 1b). This phenomenon could be attributed to the disability of PVP for protecting the TSN from the etching of H2O2, making the prepared nanoparticles close to sphere and lose the feature of triangular nuclei. So it becomes difficult to obtain tinier seeds with the feature of triangular nuclei with a small amount of NaBH4. In order to further reduce the size and improve the uniformity of the TSN, we replaced the PVP 8


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with the Pluronic F127. The Pluronic F127 has the ability to form tiny micelles on the surface of nanoparticles, which could stabilize the nanoparticles, thus improving the uniformity and stability of tiny TSN. Compared with the PVP, the synthesized seeds with the Pluronic F127 as a stabilizer still have the feature of triangular nuclei even when the volume of NaBH4 is reduced to 400 µL. The LSPR band could blue-shift to 440 nm and the edge length could reduce to 12.5 ± 2 nm, while the minimum edge length of the TSN prepared with PVP is 19 ± 5 nm with the LSPR band at 490 nm. Moreover, the Pluronic F127 stabilized TSN are more uniform and the repeatability of this procedure is better. The TEM images of the tiniest TSN prepared by the above two approaches are shown in Figure 1c and d, respectively. The tiniest TSN synthesized with the Pluronic F127 as a stabilizer could grow into TSNPs successfully, as shown in the inset of Figure 1b. This illustrates that the addition of the Pluronic F127 could enlarge the adjustment range of the NaBH4 amount, by this, the tinier and more uniform TSN could be synthesized. Although both PVP and Pluronic F127 are amphiphilic polymers that can improve the stability of the TSN, the Pluronic F127 appears to be more efficient than the PVP based on our experimental results. The PVP usually has a hydrophobic group formed by the backbone of the vinyl polymer, which surrounds the TSN, whereas the hydrophilic pendant groups of the polymer interact with water.44 However, unlike the PVP, the Pluronic F127 is an amphiphilic triblock copolymers containing the hydrophobic central block (poly(propylene oxide), PPO) and the hydrophilic block (poly(ethylene oxide), PEO) at both ends. In aqueous solution, Pluronic F127 copolymers self-assemble into micelles, the core of the micelle is composed of the hydrophobic central PPO chains, while the hydrophilic PEO chains form the corona of the micelle.45 These micelles coating the TSN might provide a more stretched conformation because of self-exclusion between the hydrophilic and hydrophobic parts of the copolymers.43 This self-exclusion may increase the average copolymer layer thickness around the TSN, improving the stability of the TSN and protecting the TSN from the etching of H2O2. From the comparison of PVP and Pluronic F127, we find that the latter could provide effective protection in the synthesis of TSN, making the nuclei obtain tinier size, higher uniformity and better repeatability. These tiny TSN provide the synthesis of TSNPs with more uniform seeds and more growing space for generating sharp corners. Hence, the small, sharp and uniform TSNPs are expected to be synthesized by using those tiniest TSN as seeds. 9



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Figure 1. (a) The spectra of TSN synthesized with PVP by adjusting the amount of NaBH4. (b) The spectra of the tiniest TSN synthesized with PVP and Pluronic F127 by adding 400 µL of NaBH4, the inset is the spectra after growth using the seeds synthesized by the above two approaches. (c) and (d) The TEM images of the tiniest TSN prepared with PVP and Pluronic F127, respectively.

Spectra properties and morphology characterization of TSNPs with sharp corners and different sizes. In this work, we synthesized the small and large TSNPs by changing the amount of seeds and AgNO3, respectively. Although both the amount of seeds and that of AgNO3 solution can be used to adjust the TSNPs size, these two methods have their own advantages. For small-size TSNPs, we can control the TSNPs size more accurately and obtain the TSNPs with an appropriate concentration by adjusting the amount of AgNO3 solution and fixing the amount of seeds. For the large-size ones, the amount of seeds can be adjusted to rapidly increase the size of TSNPs and avoid the self-nucleation caused by the high concentration of AgNO3. As shown in Figure 2, for the large TSNPs, the LSPR band blue-shifts from 795 nm to 670 nm as the seeds solution volume increases from 3 mL to 6.5 mL. For the small TSNPs, the LSPR band blue-shifts from 630 nm to 560 nm as the amount of the AgNO3 in the growth system reduces from 150 µL to 50 µL. But when the 10


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amount of the AgNO3 is less than 50 µL, double peaks appear in the longitudinal LSPR band (Figure S2), indicating the truncated morphology of the TSNPs. This may be attributed to the fact that the amount of AgNO3 is so little that no enough Ag atoms attach to the TSN to generate sharp corners. Thus, it can be concluded that the smallest and sharp TSNPs could be obtained using 50 µL of AgNO3. As shown in the inset of Figure 2, the LSPR bands of the TSNPs synthesized by this approach could cover most of the visible light region, and the color of the colloid could change from purple to blue, green and light grey. The morphologies of the TSNPs, of which LSPR bands were 630 nm, 590 nm and 560 nm, were characterized by TEM (Figure 3a-c). From that, we can see the TSNPs have very sharp corners, high productivity and good uniformity. And the corresponding size distributions are very narrow (Figure 3d-f), revealing the TSNPs are uniform. The edge lengths of the TSNPs are 46 ± 5 nm, 35 ± 4 nm and 29 ± 3 nm corresponding to the LSPR bands at 630 nm, 590 nm and 560 nm, respectively. The thicknesses of the TSNPs were measured by coating the TSNPs with thick silica shells (Figure S3), which could make the TSNPs stand vertically on their edges and deposit onto the copper grid. The thickness of the TSNPs is about 10 ± 1 nm. In the current studies, few researchers pay attentions to the synthesis of small TSNPs. Especially in the chemical reduction method, the synthesis of the TSNPs with small size and sharp corners is more difficult than the large ones. However, the TSNPs synthesized by the method proposed in this paper are smaller, sharper and more uniform compared with other methods, such as photo-induced route, seed-mediated route and thermal synthesis (Table 1). This method also efficiently overcomes the defects of discontinuity of the LSPR bands in the photo-induced approach. Thus the adjustment of the LSPR bands and the study about the effect of size on SERS activity are allowed in a larger range. In addition, this method has a simple synthesis route and short reaction time and can be used for large scale preparation, providing probability for the applications in various fields.

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Figure 2. The spectra of TSNPs with different sizes by further growth used the tiniest TSN prepared with Pluronic F127 as the seeds. The inset is the color of the colloid corresponding to the TSNPs with different sizes.

Figure 3. The TEM images and size distributions of the sharp TSNPs with different sizes. (a)-(c) The TEM images of the TSNPs with different sizes: 46 ± 5 nm, 35 ± 4 nm and 29 ± 3 nm. (d)-(f) The size distributions of the TSNPs with different sizes: 46 ± 5 nm, 35 ± 4 nm and 29 ± 3 nm. Table 1. The comparison of the TSNPs between the method in this paper and those in literatures. (The sharpness and uniformity of the TSNPs are estimated by the TEM images in the literatures.) Minimal edge length/nm

Thickness/nm

Sharpness

Uniformity

Reaction time/h

38 ± 7

9.8 ± 1

excellent

good

~50

88

24

good

excellent

~2

31 ± 7

7 ± 1.5

good

normal

~0.5

Seed-mediated route

20 ± 3

5.1 ± 1.1

normal

excellent

~0.05

Triangular nanoplate seed-mediated route40

150

6.9

excellent

excellent

~0.5

This work (tiny TSN seed-mediated route)

29 ± 3

10 ± 1

excellent

excellent

~0.5

Method Photo-induced route34 26

Photo-induced route 36

Thermal synthesis

37

SERS of the TSNPs with different sizes. To investigate the SERS changes against the TSNPs sizes, the 4-MBA is used as a Raman probe molecule because of its broad use in the studies of SERS.46, 47 In this work, the particle concentration of all Raman samples remains consistent. Figure 12


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4a shows the SERS spectra of the sharp TSNPs with sizes of 70 ± 9 nm, 55 ± 7 nm, 46 ± 5 nm and 29 ± 3 nm (corresponding to the LSPR bands of 770 nm, 700 nm, 630 nm and 560 nm). The SEM image of the smallest and sharp TSNPs on the substrate surface is shown in Figure 4b. One can find the TSNPs were almost arranged in a dense monolayer on the substrate and stacking effects were not obvious. Here we denote them as TSNPs770, TSNPs700, TSNPs630 and TSNPs560, respectively. The intensities of the peak at ~1080 cm-1, which belongs to the benzene ring-breathing modes, are selected as a basis for analysis. The SERS EFs of the TSNPs with different sizes are shown in Figure 4c. The EF is calculated by the formula: EF = (ISERS/IRaman) × (CRaman/CSERS).26 In this formula, ISERS and IRaman are the intensities of SERS and the normal Raman spectra, respectively, CSERS and CRaman represent the concentrations of the 4-MBA in SERS and the normal Raman measurements, respectively. The SERS EFs are 2.7 × 104, 5.5 × 104, 2.72 × 105 and 3.94 × 105 for the sharp TSNPs with edge lengths of 70 ± 9 nm, 55 ± 7 nm, 46 ± 5 nm and 29 ± 3 nm, respectively. We can find that the SERS EF increases as the size of the TSNPs decreases with 633 nm laser. To confirm the universality of the above principle in the TSNPs, the truncated TSNPs with different sizes (corresponding to the LSPR bands of 565 nm, 510 nm and 440 nm) were synthesized by thermal synthetic route. And the SERS spectra were also measured. The results reveal that the SERS intensity still increases as the size decreases for the truncated TSNPs, indicating that the size of the TSNPs plays a significant role in SERS, as shown in Figure 4d. Furthermore, in order to explore whether the SERS is influenced by the sharpness of the TSNPs, the SERS spectrum of the TSNPs560 was compared with that of the truncated TSNPs with the same LSPR band (560 nm). As the TEM images shown in Figure 5, the size of the truncated TSNPs and the sharp ones are similar, but the latter have the sharper corners and more uniform size. From the SERS spectra shown in Figure 5a, it’s easy to find that the SERS intensity of the TSNPs560 is higher than that of the truncated ones. Therefore, we can draw a conclusion that the SERS intensity of the TSNPs is determined by both the size and the sharpness. That is, the smaller and sharper the TSNPs, the higher the SERS intensity. Further, we compared the SERS intensity of the TSNPs560 (Figure 4a) with the smallest truncated ones (Figure 4d) (corresponding to the LSPR band of 440 nm). As shown in Figure 4e, we can find that although the size of TSNPs560 is not as small as the truncated ones, the SERS intensity of the TSNPs560 is much stronger. This indicates that the SERS activity of the TSNPs560 is stronger than that of the truncated ones with any size prepared by thermal synthetic 13



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route. Given the above, the smallest TSNPs with sharp corners (TSNPs560) synthesized by our approach have the strongest SERS activity. It is also worth noting that the biggest SERS EF is an order of magnitude higher than that of the disperse TSNPs prepared by the photo-induced route.26

Figure 4. (a) The SERS spectra and (c) the EFs of 4-MBA with the sharp TSNPs with different sizes of 70 ± 9 nm, 55 ± 7 nm, 46 ± 5 nm and 29 ± 3 nm (corresponding to the LSPR band of 770 nm, 700 nm, 630 nm and 560 nm). (b) The SEM image of the smallest and sharp TSNPs on the substrate surface. (d) The SERS spectra of 4-MBA with different-sized truncated TSNPs. (e) The comparison of the SERS intensities between the TSNPs560 and the smallest truncated TSNPs.

Figure 5. (a) The comparison of the SERS intensities between the TSNPs560 and the truncated TSNPs with the same LSPR band (560 nm). (b)-(c) The TEM images of the truncated TSNPs and the sharp TSNPs with the LSPR band of 560 nm. 14


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Unlike the principles in other morphological nanoparticles, we found that the SERS intensity became stronger as the size of TSNPs decreased. The enhancement effects for SERS, including the long-range electromagnetic enhancement and the short-range chemical enhancement, have been widely admitted.48 Among them, the local field enhancement plays a major role in the enhancement of SERS. When the incident light resonantly couples with the surface plasmon of nanoparticles, the surface conduction electrons are localized in a confined volume. As a result, the local field could be remarkably enhanced. The theoretic calculations and experiments have demonstrated that the electric field enhancement of TSNPs was mainly localized on the sharp corners and edges,49-51 which plays an important role in improving the SERS effect, as shown in Figure 5. However, the electric field enhancement of the surfaces of TSNPs is very weak. In this study, the ratios of the side faces to the top and bottom surfaces (Sside:Stop

and bottom)

are about 0.49, 0.63, 0.75 and 1.2

corresponding to the TSNPs with sizes of 70 ± 9 nm, 55 ± 7 nm, 46 ± 5 nm and 29 ± 3 nm, respectively. The schematic diagram of unfolded TSNPs and the distributions of the probe molecules are shown in Scheme 2. One can find that the Sside:Stop and bottom of the small TSNPs (TSNPs560, ~1.2) is remarkably larger than that of the large ones (TSNPs770, ~0.49). This indicates the small TSNPs can adsorb more probe molecules localized on the sharp corners and edges under the condition of the same number of particles and probe molecules used in all Raman samples. Moreover, due to the smaller volume, the average distance between the hotspots produced by the corners and the probe molecules of small TSNPs is closer than that of large ones. Given the above mechanism, the small TSNPs have higher SERS activity than the large ones.

Scheme 2. (a) The schematic diagram of the unfolded TSNPs. Sside and Stop and bottom are the areas of the side faces and the top and bottom surfaces, respectively. The small and large TSNPs correspond to TSNPs560 and TSNPs770, respectively. (b) The distributions of the 4-MBA on the small and large TSNPs. 15



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Detection of thiram using the TSNPs with smallest size and sharp corners as substrate. The pure thiram with different concentrations were detected first to verify the feasibility and the superiority of the SERS-based thiram detection on TSNPs substrate and facilitate the comparison with other morphologies. Figure 6 shows the SERS spectra of thiram from the TSNPs substrates. All the characteristic peaks for thiram from 400 to 1800 cm-1 could be clearly identified. According to the previous reports,52 the strongest peak at 1380 cm-1 is attributed to the CN stretching mode and symmetric CH3 deformation mode. Firstly, the influence of the TSNPs size on the SERS activity for thiram was investigated. As shown in Figure 6a, when the concentration of thiram is 5 µM, the SERS activity of the small TSNPs (TSNPs560) was significantly stronger than that of the large ones (TSNPs630). And even when the concentration decreased to 0.5 µM, the TSNPs560 still showed strong enhancement ability but the TSNPs630-based SERS spectrum didn’t show any peaks. These results indicate the principle that the small TSNPs could produce stronger SERS activity is also true for the detection of thiram. And using small TSNPs can obtain higher sensitivity for detecting thiram with low concentrations. Therefore, we choose the smallest TSNPs with sharp corners (TSNPs560) to enhance the Raman signals of thiram and then achieve lower LOD. By using the smallest TSNPs proposed in this paper as the SERS substrate, we studied the SERS properties for different final thiram concentrations, as shown in Figure 6b and c. We can find that the SERS signal intensities increase with the concentration of thiram. As shown in the inset of Figure 6c, the peak intensities at 1380 cm-1 have a linear positive correlation with the concentrations of thiram in the range of 5 × 10-9-1 × 10-6 M. And the corresponding LOD (S/N = 3) can reach 3.3 nM. These above results indicate the SERS detection of thiram using the TSNPs with small size and sharp corners as substrate is feasible. A performance comparison of the small and sharp TSNPs with other commonly used SERS substrates in literatures for the detection of pure thiram solution is summarized in Table 2. Compared with these substrates, the quantification range and LOD of thiram obtained by using the small and sharp TSNPs as substrate are superior to all of single-component nanoparticles and most composite ones. Only a few composite nanoparticles have a lower LOD. For example, the LOD of thiram is 1 nM when Au@thiram@Ag nanorods is used as substrate,16 and the LOD is as low as 80 pM when Au@Ag nanocuboids is used as SERS substrate.52 However, the composite nanoparticles have the disadvantage of complicated preparation process, making them inconvenient to use. 16


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Conversely, the small and sharp TSNPs have the advantages of simple preparation process and no need of complex modifications. Therefore, the TSNPs with small size and sharp corners proposed in this paper can make the detection of thiram with low concentration easier.

Figure 6. (a) SERS properties of small (TSNPs560) and large TSNPs (TSNPs630) for the different final concentrations of pure thiram solution. (b) SERS properties of the smallest TSNPs with sharp corners (TSNPs560) for the different final concentrations of pure thiram solution and (c) the relationship between peak intensities and thiram concentrations. Error bars represent relative standard deviations (RSD) from three replicate samples. Table 2. Comparison of different morphologies of nanoparticles for the SERS detection of pure thiram solution. Morphology 53

Dogbone shaped Au nanoparticles Gold nanorods13 Silver nanocubes14 Silver nanowires28 Single clusters of self-assembled silver nanoparticles54 Cicada wing decorated by silver nanoparticles55 Au nanoparticle-decorated cicada wing56 Gold nanoplate-in-shell57 Au@Ag nanocuboids52 Fe3O4/Ag composites58 Ag@mesoporous SiO215 Au@thiram@Ag nanorods16 This work (small TSNPs)

LOD

Detection Linear Range

11.8 ± 3.2 nM 11.00 ± 0.95 nM 50 nM 100 nM 100 nM

25-225 nM 100-900 nM NG NG NG

100 nM

100 nM to 1 mM

100 nM

100 nM-1 mM

12.29 nM 80 pM 500 nM 10 nM 1 nM 3.3 nM

200-600 nM NG NG 10 nM-10 mM. NG 5 nM-1 µM

Detection of thiram in soil samples. In agriculture, thiram is usually used as seed dressing to prevent and treat many kinds of crop diseases which may cause thiram residue in soil.1 To verify the detection performance of this method for thiram in soil, different amounts of thiram were added into soil samples and detected after being leached with ethanol. Unlike the pure thiram solution, the 17



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composition of soil extract is very complex, which usually contains some positively charged substances. These substances are easy to combine with the citrate-modified TSNPs to cause serious aggregation, so the untreated soil extract cannot be detected. PSS, a negatively charged polymer, could combine with the positively charged substances in the soil and avoid the aggregation of TSNPs. Thus, a certain amount of PSS aqueous solution (10 mM, 10 µL) was added into the soil extract containing thiram to neutralize the charges of these positively charged substances. As shown in Figure S4, the spectrum and color of the mixture of soil extract and TSNPs with the PSS have no obvious change, while that of the mixture without the PSS exhibit drastic changes. That is, the PSS effectively prevents the aggregation of TSNPs caused by the positively charged substances in soil. To test the effect of other soil components on the detection of thiram, the SERS spectrum of the soil extract containing thiram was compared to that of the soil extract adding an equal amount of ethanol. As shown in Figure S5, the characteristic peaks of thiram could be observed in the soil extract with thiram, but there is no peak in the control sample. So other components in soil have no interference on the detection of thiram. However, the linearity range and LOD of the thiram detection in soil samples are different from that in pure thiram solution because of the addition of PSS solution. Therefore, the SERS spectra for different final thiram concentrations and the relationship between peak intensities and thiram concentrations in soil samples were investigated (Figure 7). It reveals that the peak at 1380 cm-1 increases with the concentration of thiram. When the amount of thiram is more than 6.0 µg/g, the rising trend of the peak intensity slows down. The peak intensities at 1380 cm-1 are in good linearity with the concentrations of thiram in the range of 0.12-4.8 µg/g as shown in the inset of Figure 7b. And the corresponding LOD (S/N = 3) is 90 ng/g.

Figure 7. (a) SERS spectra for the different final thiram concentrations and (b) the relationship between peak intensities and thiram concentrations in soil samples. Error bars represent RSD from three replicate samples. 18


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In order to evaluate the selectivity of our method, we selected some pesticides used as seed dressing or contained CH3 as interfering substances (Figure S6). In this test, the thiram has a concentration of 0.24 µg/g, the concentration of other pesticides is 24 µg/g, which is 100-fold concentrated than thiram. From Figure 8, we can find that only thiram gives rise to a SERS signal, but other samples with 100-fold concentration do not provide a SERS response. This may be attributed to the fact that the disulfide bond in thiram breaks spontaneously upon exposure to the Ag surface and then binds to the TSNPs through the Ag-S bond.27, 28 But other pesticides cannot be bonded to the surface of TSNPs because they have no active groups such as disulfide bond. This leads to too far distance between the molecules and the TSNPs to give rise to a SERS response. Therefore, this method exhibits excellent selectivity for thriam detection in soil.

Figure 8. SERS response from the TSNPs to thiram (0.24 µg/g ) and other interfering substances (24 µg/g). Error bars represent RSD from three replicate samples.

In order to evaluate the practicality of this method, we collected some nursery garden soil located in Xi’an and Beijing to perform the spike-and-recovery experiments. The pretreatment and detection procedures are the same as the farmland soil. Three different amounts of thiram were added into the soil samples and the results are listed in Table 3. For the nursery garden soil, the recoveries are in the range of 94-100.8 % and 93-111.75 %, respectively. The variation coefficients are in the range of 2.14-4.04 % and 0.78-5.80 %, respectively. These results reveal that this detection method is reliable and could be applied to the detection of thiram residue in soil. Table 4 shows the performance comparison of this method with other thiram detection methods in soil samples. The reported methods in these literatures mainly include spectrophotometry, HS-SPME-IMS and HPLC.3-6 Especially, the spectrophotometry has a low LOD of 0.34 µg/g. However, all of the above methods have some problems in common: the LOD is unsatisfactory and 19



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the detection procedures are too complicated and expensive. In this paper, the SERS based on noble metal nanoparticles is applied to the thiram detection in soil for the first time. The LOD of this method is lower than that of all other methods. What’s more, this method also has simple procedures, strong anti-interference ability, stable signals and a comparable quantification range. Table 3. Detection of thiram in nursery garden soil samples. Sample

Added (µg/g)

Found (µg/g)

Recovery (%)

RSD (%, n = 3)

Soil collected from Xi’an

1.0 2.5 4.0 1.0 2.5 4.0

0.94 2.52 3.96 0.93 2.51 4.47

94 100.8 99 93 100.4 111.75

3.87 4.04 2.14 5.04 0.78 5.80

Soil collected from Beijing

Table 4. Comparison of different methods for the detection of thiram in soil samples. (The LOD and linear range are obtained by converting the datas in literatures.) Method 3

Spectrophotometry Spectrophotometry4 HS-SPME-IMS5 HPLC method6 This work (SERS based on small TSNPs)

LOD

Detection Linear Range

1.65 µg/g 0.34 µg/g 210 µg/g 0.6 µg/g 90 ng/g

2.5–125 µg/g 2 -20 µg/g 1-7 mg/g NG 0.12-4.8 µg/g

CONCLUSIONS

In summary, the TSNPs with small size, sharp corners and good uniformity are successfully synthesized by a tiny TSN-mediated chemical reduction route, and a novel SERS-based detection strategy for the thiram residues in soil is developed for the first time. We find that the size and uniformity of the seeds could strongly affect the size and sharpness of the TSNPs. Compared with conventional seeds, the tiny and uniform TSN synthesized by using the Pluronic F127 as an effective stabilizer provide uniform seeds and more growing space to generate sharp corners for the synthesis of TSNPs. The SERS performance studies exhibit a distinctive result that the SERS intensity increases as the size of TSNPs decreases. This new observation may be ascribed to the more proportion of the probe molecules localized on the sharp corners and edges and the closer distance between the hotspots and probe molecules for the small TSNPs. Besides, the SERS intensity of the TSNPs with sharp corners is stronger than that of the truncated ones. As a result, the smallest TSNPs (29 ± 3 nm) with sharp corners synthesized by our approach exhibit the strongest 20


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SERS intensity, e.g. the EF of 4-MBA could reach ~3.94 × 105, which is an order of magnitude higher than that of the disperse TSNPs prepared by the photo-induced route. Based on the Raman activity of the smallest TSNPs with sharp corners, a SERS-active substrate is successfully prepared to detect the pure thiram solution and further detect the thiram residue in soil. The results show the SERS intensity increases with the thiram concentration and the sensitivity of thiram detection used this substrate is better than other nanoparticles, such as silver nanospheres, nanocubes, nanowires and TSNPs with large size. For the detection of thiram residue in soil, the addition of PSS effectively improves the specific adsorption ability of TSNPs for thiram, simplifying the complex extraction procedures. A good linear response in the range of 0.12-4.8 µg/g with a low LOD of 90 ng/g is achieved, which is better than conventional methods. Additionally, this approach has good selectivity for thiram detection in soil. This work shows the great potential of the small TSNPs as a novel SERS substrate and its broader applications in pesticides detection.

ASSOCIATED CONTENT

Supporting Information The absorption spectrum of the TSNPs after growth using the seeds synthesized with PVP and 500 µL of NaBH4, the absorption spectrum and the TEM image of the TSNPs when the amount of the AgNO3 is less than 50 µL, the TEM image of silica-coated TSNPs for thickness measurement, the absorption spectra of the TSNPs and the mixture of TSNPs and soil extract with and without the PSS solution, the SERS spectra of the soil extract with and without thiram, and the molecular formulas of pesticides used in this paper. This material is available free of charge via the Internet.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Jun-Wu Zhao) [email protected] (Jian-Jun Li) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT 21



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We acknowledge the financial supports of the National Natural Science Foundation of China under grant No. 61675162.

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TOC graphic

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TOC graphic 34x14mm (300 x 300 DPI)


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Scheme 1. The schematic diagram of the growth process of TSNPs with small size, sharp corners and good uniformity and the SERS-based scheme for the detection of thiram residue in soil. 94x52mm (300 x 300 DPI)



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Figure 1. (a) The spectra of TSN synthesized with PVP by adjusting the amount of NaBH4. (b) The spectra of the tiniest TSN synthesized with PVP and Pluronic F127 by adding 400 µL of NaBH4, the inset is the spectra after growth using the seeds synthesized by the above two approaches. (c) and (d) The TEM images of the tiniest TSN prepared with PVP and Pluronic F127, respectively. 123x89mm (300 x 300 DPI)


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Figure 2. The spectra of TSNPs with different sizes by further growth used the tiniest TSN prepared with Pluronic F127 as the seeds. The inset is the color of the colloid corresponding to the TSNPs with different sizes. 56x39mm (300 x 300 DPI)



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Figure 3. The TEM images and size distributions of the sharp TSNPs with different sizes. (a)-(c) The TEM images of the TSNPs with different sizes: 46±5 nm, 35±4 nm and 29±3 nm. (d)-(f) The size distributions of the TSNPs with different sizes: 46±5 nm, 35±4 nm and 29±3 nm. 94x52mm (300 x 300 DPI)


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Figure 4. (a) The SERS spectra and (c) the EFs of 4-MBA with the sharp TSNPs with different sizes of 70 ± 9 nm, 55 ± 7 nm, 46 ± 5 nm and 29 ± 3 nm (corresponding to the LSPR band of 770 nm, 700 nm, 630 nm and 560 nm). (b) The SEM image of the smallest and sharp TSNPs on the substrate surface. (d) The SERS spectra of 4-MBA with different-sized truncated TSNPs. (e) The comparison of the SERS intensities between the TSNPs560 and the smallest truncated TSNPs. 108x69mm (300 x 300 DPI)



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Figure 5. (a) The comparison of the SERS intensities between the TSNPs560 and the truncated TSNPs with the same LSPR band (560 nm). (b)-(c) The TEM images of the truncated TSNPs and the sharp TSNPs with the LSPR band of 560 nm. 40x9mm (300 x 300 DPI)


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Scheme 2. (a) The schematic diagram of the unfolded TSNPs. Sside and Stop and bottom are the areas of the side faces and the top and bottom surfaces, respectively. The small and large TSNPs correspond to TSNPs560 and TSNPs770, respectively. (b) The distributions of the 4-MBA on the small and large TSNPs. 59x21mm (300 x 300 DPI)



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Figure 6. (a) SERS properties of small (TSNPs560) and large TSNPs (TSNPs630) for the different final concentrations of pure thiram solution. (b) SERS properties of the smallest TSNPs with sharp corners (TSNPs560) for the different final concentrations of pure thiram solution and (c) the relationship between peak intensities and thiram concentrations. Error bars represent relative standard deviations (RSD) from three replicate samples. 43x11mm (300 x 300 DPI)


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Figure 7. (a) SERS spectra for the different final thiram concentrations and (b) the relationship between peak intensities and thiram concentrations in soil samples. Error bars represent RSD from three replicate samples. 60x21mm (300 x 300 DPI)



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Figure 8. SERS response from the TSNPs to thiram (0.24 µg/g ) and other interfering substances (24 µg/g). Error bars represent RSD from three replicate samples. 56x39mm (300 x 300 DPI)


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Small and Sharp Triangular Silver Nanoplates Synthesized Utilizing

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