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Silver nanotriangles-loaded filter paper for ultrasensitive SERS detection application benefited by interspacing of sharp edges
Accepted Manuscript Title: Silver Nanotriangles-Loaded Filter Paper for Ultrasensitive SERS Detection Application Benefited by Interspacing of Sharp Edges Author: Chong Wang Bingxin Liu Xincun Dou PII: DOI: Reference:
S0925-4005(16)30324-0 http://dx.doi.org/doi:10.1016/j.snb.2016.03.030 SNB 19835
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
14-12-2015 4-3-2016 8-3-2016
Please cite this article as: Chong Wang, Bingxin Liu, Xincun Dou, Silver Nanotriangles-Loaded Filter Paper for Ultrasensitive SERS Detection Application Benefited by Interspacing of Sharp Edges, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.03.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Silver Nanotriangles-Loaded Filter Paper for Ultrasensitive SERS Detection Application Benefited by Interspacing of Sharp Edges Chong Wang,a,b‡ Bingxin Liua‡ and Xincun Dou*a a
Laboratory of Environmental Science and Technology, Xinjiang Technical Institute of Physics & Chemistry; Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China b
University of Chinese Academy of Sciences, Beijing 100049, China Email: [email protected]
‡
Chong Wang and Bingxin Liu contribute equally to this work.
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Graphical abstract
The combination of experimental and computational results demonstrate that the sharp edges and tips boosted Ag nanotriangles provides a sensitive and good reproducibility approach for sensing pATP and PA.
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Highlights 1．A systematical study is presented on how to gain more and strong electromagnetic “hotspots” by tailoring the edges of nanotriangles. 2. A flexible SERS substrate with good activity and reproducibility was obtained and the ultratrace detection of pATP with a concentration as low as 10-8 M was achieved. 3. The present silver nanotriangles-loaded filter paper SERS substrate can be employed to sensitively and selectively detecting ultratrace military explosives PA of 10-6 M.
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Abstract The strongly enhanced electromagnetic “hotspots” from the edges and tips of nanoplates are highly preferred to significantly promote the surface enhanced Raman scattering (SERS) activity. Here, Ag nanotriangles with a sharp edge of 20 nm were prepared by the reduction of AgNO3 using N,N-Dimethylformamide (DMF) with the assistance of Polyvinyl pyrrolidone (PVP). By loading of Ag nanotriangles onto the filter paper, a flexible SERS substrate with good activity and reproducibility was obtained and the ultratrace detection of p-aminothiophenol (pATP) with a concentration as low as 10-8 M was achieved. The sharp edges and tips boosted SERS activity was proved by the finite element method modeling of the plasmonic properties of Ag nanotriangles with different edge thicknesses (10 - 80 nm), Ag nanosheets with different tip acutances (squares and hexagons) together with Ag nanorods, nanocubes and nanospheres. The experimental and theoretical results confirmed that the interspacing created by filter paper plays an important role in enhancing the overall SERS activity. Furthermore, it is shown that the present silver nanotriangles-loaded filter paper SERS substrate can be employed to sensitive and selective detecting ultratrace military explosives picric acid (PA) of 10−6 M.
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1. Introduction The surface enhanced Raman scattering based detection is a surface-sensitive technique providing the specific structural and vibrational fingerprints of the interested analytes.[1-6] The enhanced sensitivity makes SERS serve as one of the most powerful analytical tools in life sciences, environmental monitoring and homeland security.[7, 8] The SERS sensitivity depends largely on the electromagnetic “hotspots” where local electric field is extremely intense, thus, the Raman signals from molecules at these sites are particularly strong and contribute to the main fraction of the overall intensity.[9] The creation of sharp edges or tips is one of the effective way to enhance electromagnetic “hotspots” for noble metal nanoparticles. [9-15] Thus, it is desirable to pursue appropriate noble metal nanoparticles that have sharp edges and tips which can afford ultrasensitive SERS activities. Silver nanotriangles are representative noble metal nanostructures have sharp edges and tips. [16] Due to the high anisotropy in dimensions, the nanotriangles systems usually have ultrasensitive SERS activities and have been proved to be powerful SERS substrate. [16] For example, silver nanotriangles obtained by a seed-mediated growth method [16] and a photoinduced method [17] have been demonstrated to be more superior than the SERS substrates constructed by the normal nanoparticles without sharp edges and tips. However, there lacks a systematical study on how to gain more and strong electromagnetic “hotspots” by tailoring the edges of nanotriangles, which is considered to greatly favor the deep understanding and the design of nanoplate structures for specific SERS applications. In addition, it is shown that filter paper can serve as a suitable support material for noble metal nanoparticles due to the combination of hydroxyl groups on the fibers surface with silver nanoparticles. [18-20] Moreover, particle spacing, which is a key determinant of the plasmonic properties, can be achieved by using self-assembly of nanoparticles on the filter paper, and the resulting filter paper-based SERS substrate can afford more "hotspots" than planar substrates.[21] Thus, a filter paper SERS substrate loaded by silver nanotriangles with sharp edges would be of great promise to realize the ultrasensitive detection. Explosives detection is one the most popular fields demanding ultratrace detection due to the low room-temperature vapor pressure of explosives, [22-24] and it would be much more effective to verify the performance of a SERS substrate by applying it into explosives detection. Moreover, selective and sensitive detection of 5
these explosives is of great importance in countering terrorist threats and pollution of soil and groundwater.[25] However, to the best of our knowledge, there lacks reports on rapid and selective detection of PA, especially by a SERS method. [26, 27] In this work, we report a fast preparation of silver nanotriangles (Ag nanotriangles) of 200 mm edge length and 20 nm thickness in a large scale. The sensitive SERS substrate was constructed based on simple immersing of filter paper in a concentrated silver nanotriangle dispersion, and it can detect 10-8 M pATP as probe molecules. The substrate remains SERS active even after used in the open air for 42 days. Finite element method (FEM) by controlling over the thickness and degree of nanoplates confirms the strong electric field distribution created by Ag nanotriangles with appropriate thickness. Compared to the normal nanostructures, such as Ag nanorod array, round corner Ag cubes-loaded filter paper and Ag nanospheres-loaded filter paper, the remarkable SERS activity of the Ag nanotriangles-loaded filter paper substrate is proved to be boosted by the sharp edges and tips together with the interparticle spacing. Moreover, the present SERS substrate allows the sensitive and selective detection of PA with a concentration as low as 10-6 M. 2. Experimental Section 2.1. Materials All reagents were analytical grade and were used as received without further purification. AgNO3, N,N-Dimethylformamide (DMF), Na2S·9H2O and boric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinyl pyrrolidone (PVP), 4-aminothiophenol (pATP), picric acid (PA), glycol were purchased from Aladdin. TNT (2,4,6-Trinitrotoluene), DNT (2,4-Dinitrotoluene) and RDX (Hexogen) were obtained from the National Security Department of China. 2.2. Synthesis of Ag Nanotriangles In a typical synthesis of Ag nanotriangles, 15 mL DMF was refluxed at 140 oC for 30 min, then 14 mL DMF solution of AgNO3 (0.035 M) and 0.5 g PVP (MW=1300 K) was injected. After injection, the reaction mixture was stirred at 350 rpm and further refluxed at 140 oC for 3.5 h. When the reaction finished, the mixture was cooled to room temperature by rapidly using the ice water bath. The product was purified by centrifugation with ethanol repeatedly (5 mL of the prepared Ag nanotriangles solution was diluted with ethanol and centrifuged at 9500 rpm for 15 6
minutes). The precipitated Ag nanotriangles were then re-dispersed in ethanol and stored at 4 oC until use. 2.3. Fabrication of SERS substrate Ag nanotriangles-loaded filter paper substrate: the filter paper was immersed in Ag nanotriangles solution for 20 min and then allowed to drain for a few seconds on a paper towel. The color of the filter paper changed from white to wathet in the process. The filter paper was dried at 50 oC in Petri dishes. Ag nanotriangles-loaded glass substrate: 200 μL Ag nanotriangles solution dropwise add to the surface of the glass slide evenly. Then the glass substrate was dried at 50 oC in petri dishes. 2.4. Characterization Field-emission scanning electron microscopy (FESEM, ZEISS SUPRA 55VP) was used to characterize the morphology of the samples. All the SERS spectra were measured by a confocal microprobe Raman spectrometer system (RTS-B, Titan Electro-Optics Co., LTD) with a 532 nm laser line, where the effective power of the laser source was about 15 mW. The laser spot focused on the sample surface is about 10 mm in diameter. 2.5 Theoretical Simulation FEM modeling was conducted by using the RF module of Comsol Multiphysics V3.5a, parameters were based on those of the Ag-nanostructures from scanning electron microscopy (SEM) observations, together with the excitation line of 532 nm. Optical constants of Ag were acquired from the literature.
3. Results and Discussion 3.1. SERS performance evaluation of Ag nanotriangles-loaded filter paper substrate
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Fig. 1 (a) SEM image of Ag nanotriangles and (b) Ag nanotriangles loaded on a filter paper, (c) SERS spectra recorded from the Ag nanotriangles-loaded filter paper substrate after being dispersed with (1) 10-5 M, (2) 10−6 M, (3) 10-7 M, (4) 10−8 M pATP (Integration time 5 s, laser power about 15 mW) and (d) signal derivation detected at the same point on the substrate for 8 times with pATP at different concentrations (intensity at 1437 cm-1 as the reference). Silver nanotriangles were obtained by using DMF reduction of AgNO3 with the assistance of PVP as a polymer surfactant. It is shown from the SEM image (Fig. 1a) that Ag nanoplates hold triangle shape with an edge length of around 200 nm. Ag nanotriangles-loaded filter paper substrate can be further obtained by immersing the filter paper into Ag nanotriangles solution for 20 min and drying at 50 oC. SEM image in Fig. 1b suggests that the Ag nanotriangles are uniformly distributed on the surface of the filter paper and the majority of Ag nanotriangles in the substrate are assembled in many-particle densely packed aggregates. Furthermore, the average thickness of the Ag nanotriangles is measured to be 20 nm from the side view (Fig. S1). As depicted in Fig. 1c, the SERS spectra were recorded from the Ag nanotriangles-loaded filter paper substrate at an excitation wavelength of 532 nm with loading of varied concentrations of pATP molecules (10-5, 10-6, 10-7 and 10-8 M). The two main peaks at 8
1079 and 1577 cm-1 can be assigned to the totally symmetric modes 7a (C-S stretch) and 8a (C-C stretch) of pATP, respectively.[28] The other distinct peaks at 1144, 1189, 1390 and 1437 cm-1 can be attributed to the fundamental benzene ring vibrations of pATP (9b, 14b, 3b and 19b, respectively), as detailedly shown in Table S1.[29] It is shown that with the decrease of the concentration of pATP, the intensity of the Raman vibration signals weakens gradually. It should be noted that the vibration modes at 1079 and 1437 cm-1 can still be observed when the concentration of pATP decreased to 10-8 M, indicating the excellent SERS activity of the present Ag nanotriangles-loaded filter paper substrate. It is supposed that the sharp edges and tips on the Ag nanotriangles can result in substantial "hotspots", thus giving strong SERS signals towards extremely low concentration of pATP. In addition, the relative standard deviation (RSD) of the SERS signals detected at the same point on the substrate is less than 17% (Fig. 1d), typically, 6% towards 10-5 M pATP, indicating that the Ag nanotriangles-loaded filter paper substrate is very sensitive to the analyte's concentration variation rather than the intrinsic SERS signal evolution with time.
Fig. 2 (a) The reproducibility of the Ag nanotriangles-loaded filter paper substrates (The transverse dot line shows the average SERS intensity from 7 substrstes, and the error bar indicates the signal derivation of 8 spots within single substrate). (b) SERS intensity change over time, and the error bar indicates the signal derivation of 8 spots within single substrate. To evaluate the reproducibility of the present fabrication method, 7 Ag nanotriangles-loaded filter paper substrates were prepared with the same route, and 8 different spots were randomly selected to record the SERS signals of 10-4 M pATP for each substrate (Fig. 2a). It is shown that the average peak intensity at 1079 cm-1 is 18625, and the average RSD for the SERS signals is 17%, which is a firm evidence of 9
the good reproducibility of the present fabrication method. Besides, the error bar indicates that the RSD of the SERS signals within a single substrate ranges from 10% to 24%, indicating the good consistency of the Ag nanotriangles-loaded filter paper SERS substrates. Fig. 2b shows the SERS intensity change over time at 1079 cm-1 towards 10-4 M pATP, and it is clearly shown that although the SERS intensity of the Ag nanotriangles-loaded filter paper substrate decreases gradually from 21240 to 9187 over time, they remain SERS active after 42 days placement in open air. The above results indicate that the Ag nanotriangles-loaded filter paper substrate could be used as highly sensitive, reproducible and reliable SERS substrate for practical trace detection applications. 3.2 The role of edges and tips in enhancing SERS activity
Fig. 3 E-field intensity distributions (indicated by the color bar) of the single Ag nanotriangles with a thickness of (a) 10 nm, (b) 15 nm, (c) 20 nm, (d) 40 nm, (e) 60 nm, and (f) 80 nm by FEM simulations. The circumcircle diameter of these geometric model are fixed as 230.94 nm. To provide a deep insight into the strong SERS activities at edges and tips, near-field calculations for a series of Ag nanoplates with a diverse range of sharpness or angle was performed. Firstly, FEM modeling was used to investigate the localized electric field intensity (E-max) of the Ag nanotriangles with thicknesses ranging from 10 to 80 nm and a fixed edge length of 200 nm. The E-field intensity distributions of the single Ag nanotriangles by FEM simulations are shown in Fig. 3. The maximum values of the electric field intensity in the presented cross sections are 652, 351, 159, 10
54, 47 and 26 V/m for the Ag nanotriangles with an edge thickness of 10, 15, 20, 40, 60 and 80 nm, respectively. The E-max decreases gradually with the increase of the thickness of Ag nanotriangles, namely bluntness (Fig. S2). Considering the fourth power dependence of the enhancement factor (EF) on the electric field intensity (ISERS ≈ |E|4), [30] the Ag nanotriangles with a small thickness undoubtedly possess a higher EF. This simulation result suggests that the electric field focusing effect of the Ag nanotriangles depends highly on the sharpness of the nanoplates.
Fig. 4 E-field intensity distributions of (a) Ag nanosquare and (b) Ag nanohexagon. The circumcircle diameter of these geometric model are fixed as 230.94 nm. To simulate the effects of tips angle on the near-field enhancement, the Ag nanoplates with a fixed thickness of 20 nm and different tip acutances, namely Ag nanosquares and nanohexagons, were simulated in Fig 4a and 4b, and the E-max in the presented cross sections are 38 and 74 V/m, respectively. Thus, it is clearly shown that compared to nanosquares and nanohexagons, Ag nanotriangles, which have more sharp tips possess the largest E-max.
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Fig. 5 SEM image and E-field intensity distributions (indicated by the color bar) of the different silver nanostructures by FEM simulations of (a, b) Ag nanorods array, (c, d) Ag nanocubes on a filter paper, (e, f) Ag nanospheres on a filter paper. In order to further understand the function of sharp edges and tips on the overall SERS activities, computational and experimental SERS evaluation on three kinds of popular silver nanostructures without sharp edges and tips, namely Ag nanorods array, Ag nanocubes and Ag nanospheres, were conducted simultaneously. The large-area Ag nanorods array were grown via an electrochemical deposition method using porous anodic alumina oxide (AAO) template. It is shown from the top-view SEM image that the Ag nanorods distribute uniformly and have a height of around 200 nm 12
and a diameter of 40 nm (Fig. 5a). According to this geometric parameter, the E-max modeled by FEM in the presented cross sections is 5 V/m (Fig. 5b). The E-max for Ag nanocube with an edge length of 70 nm (Fig. 5c) is modeled to be 17 V/m (Fig. 5d). While the Ag nanosphere with a diameter of 100 nm (Fig. 5e) possesses the lowest E-max of 1.5 V/m (Fig. 5f).
Fig. 6 (a) SERS spectra of pATP collected from (1) Ag nanotriangles-loaded filter paper substrate for 10-5 M pATP, (2) Ag nanospheres and (3) Ag nanocubes-loaded filter paper substrate for 10-4 M pATP and (4) Ag nanorods array for 10-4 M pATP. (b) The reproducibility of the four kinds of SERS substrates, error bar indicates the signal derivation of 8 spots within single substrate. From the SERS spectra of the Ag nanorods array, the Ag nanocubes and the Ag nanospheres-loaded filter paper substrates towards 10-4 M pATP (Fig. 6a), it is clearly shown that the intensities of SERS signals are around 3220, 4446 and 12398 at 1079 cm-1. It is surprised that the Ag nanospheres, which possess the weakest E-max in the three Ag nanostructures as simulated, show the strongest SERS activity after loading on a filter paper. From a comparison of the morphology of the Ag nanocubes and the Ag nanospheres-loaded filter paper substrates, one can find that Ag nanospheres are more condensed and thus much more and stronger SERS "hotspots" were boosted by interspacing, which will be discussed in section 3.3. Generally, these three much smaller nanostructures should present more excellent SERS performance than the Ag nanotriangles due to the size effect (40-100 nm vs. 200 nm).[30] However, all these three SERS substrates show much weaker SERS signals compared to the Ag nanotriangles-loaded filter paper since it shows an intensity of 16374 even towards 10-5 M pATP. It should be noted that this result is based on a statistical analysis and 8 spots were selected for each substrate (Fig. 6b). Thus, it is further confirmed that the silver nanostructures with sharp edges and tips exhibit much larger field enhancement. 13
To understand the influence of the excitation source to the optical properties of these different nanostructures, FEM modeling was conducted to investigate the local electric field intensity of the Ag nanotriangles with different excitation wavelength. The E-max in the presented cross sectionsare 169, 159, 147 and 156 V/m for the Ag nanotriangles with excitation wavelengths of 514 nm, 532 nm, 633 nm and 785 nm, respectively (Fig. S3). This simulation result suggests that the excitation wavelength of the laser source has very little influence on the electric field focusing effect of the Ag nanotriangles. This conclusion is further evidenced by the similar FEM simulations of Ag nanorod, Ag nanocube and Ag nanosphere with different excitation wavelengths (Fig. S4-S6). 3.3 The role of Interspacing in enhancing SERS activity
Fig. 7 (a) SERS spectra recorded from the Ag nanotriangles-loaded filter paper substrate and Ag nanotriangles-loaded glass substrate after being dispersed with 10-5 M pATP. (b) E-field intensity distributions (indicated by the color bar) of the double Ag nanotriangles with 20 nm gaps. To further understand the advantages of the Ag nanotriangles-loaded filter paper substrate, the SERS properties of the Ag nanotriangles-loaded on a glass substrate was also evaluated. The SERS sprctra of both substrates towards pATP with a concentration of 10-5 M obtained by maintaining the same measurement condition shown that all the characteristic scattering peaks of pATP can be clearly observed (Fig. 7a and S7). However, it is obvious that the SERS intensity of the Ag nanotriangles-loaded filter paper substrate is much stronger than that of the Ag nanotriangles-loaded glass substrate, typically, 3.1 times at a wavelength of 1079 cm-1. The filter paper substrate induced SERS efficiency enhancement may be attributed to the more "hotspots" which emerge from the interparticle spacing or gaps between the 14
adjacent Ag nanotriangles. The three-dimensional and porous structure of filter paper is able to provide a greater surface area to localize the adsorption of Ag nanotriangles via abundant hydroxyl groups that lead to the Ag nanotriangles aggregation. Therefore, additional simulation on the interparticle spacing between Ag nanotriangles was performed in Fig. 7b and the result shows that the E-max reaches 211 V/m, indicating that the electromagnetic enhancement is highly localized in the gap region of Ag nanotriangles, which is preferentially formed in the nanotriangles closely-packed filter paper substrate. As the Ag nanotriangles are contacted to form aggregates with interparticle spacing or gaps, their transition dipoles couple to each other and the enhanced fields of each nanotriangle start to coherently interfere at their „„hotspots‟‟.[31] Thus, the intricate network of fibers enables potentially high density of electromagnetic "hotspots" with the decoration of plasmonic Ag nanotriangles.
Fig. 8 (a) SERS spectrum evolution of pATP (10–5 M) with evaporation of the ethanol solution loaded on Ag nanotriangles - filter paper substrate at intervals of 5 s and (b) the maximum SERS intensity recorded at 1079 cm-1. The function of the interparticle spacing distance can be further studied by solvent evaporation induced spacing change after dispersing of pATP solution onto the filter paper. As shown in Fig. 8, the initially weak signals become stronger and reach the maximum value at 40 s. It is considered that the filter paper cellulose deforms with the change of the surface tension caused by the solvent evaporation, resulting in the change of the gaps between Ag nanotriangles in the evaporation process. Thus, the intricate network of fibers provides an efficient platform for the formation of electromagnetic "hotspots" induced by interparticle spacing between Ag nanotriangles.
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Fig. 9 E-field intensity distributions (indicated by the color bar) of double (a) Ag nanocubes and (b) Ag nanospheres with an interspacing distance of 20 nm by FEM simulations. The role of inter-particle spacing in Ag nanocubes and Ag nanospheres on the formation of "hotspots" was further investigated (Fig 9). It is observed that the E-max of Ag nanocubes and Ag nanospheres are 23.8 and 2.7 V/m respectively when the spacing between these Ag nanostructures edges is fixed to 20 nm. Compared with the simulation results of individual nanostructure shown in Figure 5, this result suggests that inter-particle spacing, which is generally formed via hydroxyl group on filter paper induced self-assembly of these nanostructures, can enhance the electric field intensity to a certain extent. Hence, inter-particle spacing of nanoparticles is more efficient than individual nanoparticles. However, compared with nanotriangles, these nanostructures without sharp edges and tips do not show distinct advantages in "hotspots" generation, even with a short inter-particle spacing distance. Thus, Ag nanotriangles with sharp edges, as well as filter paper, play a critical role in SERS signal enhancement. Based on the above analysis, the nanotriangle shape, a relatively small thickness and the porous filter paper substrate coordinately composed the characteristics of a highly active SERS substrate. 3.3 Ultrasensitive detection of typical military explosives-a tentative study
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Fig. 10 (a) SERS spectra of PA at different concentrations and (b) statistical analysis of 8 spots within a single substrate at 825 cm-1 towards different concentrations of PA. SERS has been demonstrated for the detection of trace levels of explosives due to its high sensitivity, speed of detection and fingerprint feature.[22] Picric acid is a common explosive with a room-temperature vapor pressure of 0.97 ppb. Besides, it is used in metal etching, dye, leather and glass industries, thus, PA exists as a trace pollutant in water and soil.[27] Thus, to effectively verify the performance of the Ag nanotriangles-loaded filter paper SERS substrate, picric acid was selected as the target analyte. As shown in Fig. 10a, well-resolved SERS spectra could be readily obtained from both the solid and diluted PA with concentrations varied from 10-4 to 10-6 M. The characteristic peaks (Table S2) of PA at 935 cm-1 (ring breathing), 1085 cm-1 (phenplic C-O stretching), 1310 cm-1 (C-C stretching) and 1557 cm-1 (C-NO2 stretching) are observed at different concentrations.[32] It is noteworthy that the Raman peaks located at 825 and 1336 cm-1, which are assigned to be the out-of-plane bending modes and the asymmetric stretching of nitro group of PA, [33] become weaker and weaker. The phenplic C-O stretching of PA at 1085 cm-1 still can be clearly identified at a concentration of 10-6 M (Fig. S8). From the statistical analysis of 8 spots within a single substrate, one can find that the intensity of the peak at 825 cm-1 decreases linearly with the decrease of the concentration of PA (Fig. 10b). The detection ability of the present Ag nanotriangles-loaded filter paper substrate is obviously improved compared to the recently reported detection limit which are summarized in Table 1. Moreover, we cannot detect any Raman signal when Ag nanorod array, Ag nanocubes and Ag nanospheres-loaded filter papers were employed as the SERS substrates even at a concentration as high as 10-4 M. Thus, it can be concluded that sharp edges and tips help to boost the SERS activity of the Ag nanotriangles-loaded filter paper 17
substrate and they play a vital role in ultratrace detection towards practical analytes. Table 1 Summary of literature reports of SERS protocol to PA sensing. Substrate Positively charged silver nanoparticles Unmodified gold nanoparticles Ag nanotriangles-loaded filter paper a
Detection Limit (M) 2.5×10-5 2×10-5 a 10-6
Ref 24
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This work
The characteristic symmetric nitro stretching Raman signal of nitroaromatic
compounds at 1335 cm-1 is absent at a concentration of 2×10-5 M. Surprisingly, no significant variation in the SERS spectra of nitro explosives TNT (2,4,6-Trinitrotoluene), DNT (2,4-Dinitrotoluene) and RDX (Hexogen) were observed by employing the same Ag nanotriangles-loaded filter paper substrate (Fig. S9). The reason can be attributed to the coordination ability of PA to the surface of Ag nanotriangles via the hydroxy group[34]. In addition, the thiol group of pATP is apt to adsorb onto the surface of Ag nanotriangles due to the stronger aurophilic interaction.[35] Thus, the Ag nanotriangles-loaded filter paper SERS substrate shows a better detection performance towards pATP than PA. However, it is difficult to light on the SERS signals for these structurally similar explosives without hydroxy group or thiol group. The result demonstrates that the Ag nanotriangles-loaded filter paper SERS substrate can be employed to selectively detect nitro explosives PA. 4.
Conclusion In summary, a flexible SERS substrate fabricated by incorporating Ag
nanotriangles with a sharp edge of 20 nm into a filter paper was demonstrated and good SERS activity and reproducibility were achieved. Typically, an ultratrace detection of pATP with a concentration as low as 10-8 M was obtained. Through the FEM modeling of the plasmonic properties of Ag nanotriangles with different edge thicknesses together with Ag nanosheets with different tip acutances, it is proved that the sharp edges and tips of the Ag nanotriangles boosted the SERS activity. The Ag nanotriangles also show a much better SERS activity compared to Ag nanorods, nanocubes and nanospheres no matter by experimental results or by computational 18
simulations, which further demonstrates the importance of creation of sharp edges and tips in enhancing the overall SERS activity. Besides, the intricate network of fibers provides an efficient platform for the formation of electromagnetic "hotspots" induced by interparticle spacing between Ag nanotriangles. Furthermore, the present Ag nanotriangles-loaded filter paper SERS substrate also can be employed to selectively detect ultratrace military explosive compound PA with a detection concentration of 10−6 M. We expect this study to open up a new perspective in the design of SERS sensors for various analytes based on sharp edges and tips boosted noble metal nanotriangles. Acknowledgements We thank the financial supports from the National Natural Science Foundation of China (51201180) and "Hundred Talents Program" of CAS. We also acknowledge the helpful discussions with Dr. Zhulin Huang in Institute of Solid State Physics, Chinese Academy of Sciences. References [1] S. Cong, Y. Yuan, Z. Chen, J. Hou, M. Yang, Y. Su, Y. Zhang, L. Li, Q. Li, F. Geng, Z. Zhao, Noble metal-comparable SERS enhancement from semiconducting metal oxides by making oxygen vacancies, Nat. Commun. 6 (2015) 7800. [2] I. Alessandri, Enhancing Raman scattering without plasmons: unprecedented sensitivity achieved by TiO2 shell-based resonators, J. Am. Chem. Soc. 135 (2013) 5541-5544. [3] W. Xu, J. Xiao, Y. Chen, Y. Chen, X. Ling, J. Zhang, Graphene-veiled gold substrate for surface-enhanced Raman spectroscopy, Adv. Mater. 25 (2013) 928-933. [4] Z. Huang, G. Meng, Q. Huang, B. Chen, F. Zhou, X. Hu, Y. Qian, H. Tang, F. Han, Z. Chu, Polyacrylic acid sodium salt film entrapped Ag-nanocubes as molecule traps for SERS detection, Nano Res. 7 (2014) 1177-1187. [5] J. Lu, H. Liu, E.S. Tok, C.H. Sow, Interactions between lasers and two-dimensional transition metal dichalcogenides, Chem. Soc. Rev. (2016) DOI: 10.1039/C5CS00553A. [6] J. Lu, J.H. Lu, H. Liu, B. Liu, L. Gong, E.S. Tok, K.P. Loh, C.H. Sow, Microlandscaping of Au Nanoparticles on Few-Layer MoS2 Films for Chemical Sensing, Small 11 (2015) 1792-1800.
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[18] P. Rajapandiyan, J. Yang, Photochemical method for decoration of silver nanoparticles on filter paper substrate for SERS application, J. Raman Spectrosc. 45 (2014) 574-580. [19] K. Zhang, J. Zhao, H. Xu, Y. Li, J. Ji, B. Liu, Multifunctional paper strip based on self-assembled interfacial plasmonic nanoparticle arrays for sensitive SERS detection, ACS Appl. Mater. Inter. 7 (2015) 16767-16774. [20] C.H. Lee, M.E. Hankus, L. Tian, P.M. Pellegrino, S. Singamaneni, Highly sensitive surface enhanced Raman scattering substrates based on filter paper loaded with plasmonic nanostructures, Anal. Chem. 83 (2011) 8953-8958. [21] Y.H. Lai, S.W. Chen, M. Hayashi, Y.J. Shiu, C.C. Huang, W.T. Chuang, C.J. Su, H.C. Jeng, J.W. Chang, Y.C. Lee, Mesostructured arrays of nanometer‐spaced gold nanoparticles for ultrahigh number density of SERS hot spots, Adv. Funct. Mater. 24 (2014) 2544-2552. [22] A. Hakonen, P.O. Andersson, M. Stenbæk Schmidt, T. Rindzevicius, M. Käll, Explosive and chemical threat detection by surface-enhanced Raman scattering: A review, Anal. Chim. Acta 893 (2015) 1-13. [23] M. López-López, C. García-Ruiz, Infrared and Raman spectroscopy techniques applied to identification of explosives, TrAC-Trend. Anal. Chem. 54 (2014) 36-44. [24] A. Chou, E. Jaatinen, R. Buividas, G. Seniutinas, S. Juodkazis, E.L. Izake, P.M. Fredericks, SERS substrate for detection of explosives, Nanoscale 4 (2012) 7419-7424. [25] B. Liu, C. Tong, L. Feng, C. Wang, Y. He, C. Lü, Water‐soluble polymer functionalized CdTe/ZnS quantum dots: a facile ratiometric fluorescent probe for sensitive and selective detection of nitroaromatic explosives, Chem-Eur J. 20 (2014) 2132-2137. [26] A. Stewart, S. Murray, S.E.J. Bell, Simple preparation of positively charged silver nanoparticles for detection of anions by surface-enhanced Raman spectroscopy, Analyst 140 (2015) 2988-2994. [27] A.K.M. Jamil, E.L. Izake, A. Sivanesan, P.M. Fredericks, Rapid detection of TNT in aqueous media by selective label free surface enhanced Raman spectroscopy, Talanta 134 (2015) 732-738.
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Biographies Chong Wang received his bachelor's degree in Chemistry from Shihezi University in 2014. Now, he is a graduate student in Prof. Dou‟s group. His research interest centers around sensors and analytical applications by means of nanomaterials and surface-enhanced Raman spectroscopy. Bingxin Liu completed his Ph.D. in 2015 at Northeast Normal University, China. Currently he is an assistant professor at the Laboratory of Environmental Science and Technology at The Xinjiang Technical Institute of Physics & Chemistry of the Chinese Academy of Sciences. His research interests are constructing nanosensors for rapid and non-contact sensing of military and improvised explosives. Xincun Dou obtained his Ph.D. in Materials Physics and Chemistry from Institute of Solid State Physics in Chinese Academy of Sciences in 2009. Following postdoctoral work at Nanyang Technological University in Singapore in Materials Science, he joined the Laboratory of Environmental Science and Technology at Xinjiang Technical Institute of Physics & Chemistry in 2011 as a "Hundred Talents Program" professor. His research interests are focused on the exploratory design of nanomaterials and nanodevices, with an emphasis on explosives sensing applications.
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S0925-4005(16)30324-0 http://dx.doi.org/doi:10.1016/j.snb.2016.03.030 SNB 19835
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
14-12-2015 4-3-2016 8-3-2016
Please cite this article as: Chong Wang, Bingxin Liu, Xincun Dou, Silver Nanotriangles-Loaded Filter Paper for Ultrasensitive SERS Detection Application Benefited by Interspacing of Sharp Edges, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.03.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Silver Nanotriangles-Loaded Filter Paper for Ultrasensitive SERS Detection Application Benefited by Interspacing of Sharp Edges Chong Wang,a,b‡ Bingxin Liua‡ and Xincun Dou*a a
Laboratory of Environmental Science and Technology, Xinjiang Technical Institute of Physics & Chemistry; Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China b
University of Chinese Academy of Sciences, Beijing 100049, China Email: [email protected]
‡
Chong Wang and Bingxin Liu contribute equally to this work.
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Graphical abstract
The combination of experimental and computational results demonstrate that the sharp edges and tips boosted Ag nanotriangles provides a sensitive and good reproducibility approach for sensing pATP and PA.
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Highlights 1．A systematical study is presented on how to gain more and strong electromagnetic “hotspots” by tailoring the edges of nanotriangles. 2. A flexible SERS substrate with good activity and reproducibility was obtained and the ultratrace detection of pATP with a concentration as low as 10-8 M was achieved. 3. The present silver nanotriangles-loaded filter paper SERS substrate can be employed to sensitively and selectively detecting ultratrace military explosives PA of 10-6 M.
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Abstract The strongly enhanced electromagnetic “hotspots” from the edges and tips of nanoplates are highly preferred to significantly promote the surface enhanced Raman scattering (SERS) activity. Here, Ag nanotriangles with a sharp edge of 20 nm were prepared by the reduction of AgNO3 using N,N-Dimethylformamide (DMF) with the assistance of Polyvinyl pyrrolidone (PVP). By loading of Ag nanotriangles onto the filter paper, a flexible SERS substrate with good activity and reproducibility was obtained and the ultratrace detection of p-aminothiophenol (pATP) with a concentration as low as 10-8 M was achieved. The sharp edges and tips boosted SERS activity was proved by the finite element method modeling of the plasmonic properties of Ag nanotriangles with different edge thicknesses (10 - 80 nm), Ag nanosheets with different tip acutances (squares and hexagons) together with Ag nanorods, nanocubes and nanospheres. The experimental and theoretical results confirmed that the interspacing created by filter paper plays an important role in enhancing the overall SERS activity. Furthermore, it is shown that the present silver nanotriangles-loaded filter paper SERS substrate can be employed to sensitive and selective detecting ultratrace military explosives picric acid (PA) of 10−6 M.
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1. Introduction The surface enhanced Raman scattering based detection is a surface-sensitive technique providing the specific structural and vibrational fingerprints of the interested analytes.[1-6] The enhanced sensitivity makes SERS serve as one of the most powerful analytical tools in life sciences, environmental monitoring and homeland security.[7, 8] The SERS sensitivity depends largely on the electromagnetic “hotspots” where local electric field is extremely intense, thus, the Raman signals from molecules at these sites are particularly strong and contribute to the main fraction of the overall intensity.[9] The creation of sharp edges or tips is one of the effective way to enhance electromagnetic “hotspots” for noble metal nanoparticles. [9-15] Thus, it is desirable to pursue appropriate noble metal nanoparticles that have sharp edges and tips which can afford ultrasensitive SERS activities. Silver nanotriangles are representative noble metal nanostructures have sharp edges and tips. [16] Due to the high anisotropy in dimensions, the nanotriangles systems usually have ultrasensitive SERS activities and have been proved to be powerful SERS substrate. [16] For example, silver nanotriangles obtained by a seed-mediated growth method [16] and a photoinduced method [17] have been demonstrated to be more superior than the SERS substrates constructed by the normal nanoparticles without sharp edges and tips. However, there lacks a systematical study on how to gain more and strong electromagnetic “hotspots” by tailoring the edges of nanotriangles, which is considered to greatly favor the deep understanding and the design of nanoplate structures for specific SERS applications. In addition, it is shown that filter paper can serve as a suitable support material for noble metal nanoparticles due to the combination of hydroxyl groups on the fibers surface with silver nanoparticles. [18-20] Moreover, particle spacing, which is a key determinant of the plasmonic properties, can be achieved by using self-assembly of nanoparticles on the filter paper, and the resulting filter paper-based SERS substrate can afford more "hotspots" than planar substrates.[21] Thus, a filter paper SERS substrate loaded by silver nanotriangles with sharp edges would be of great promise to realize the ultrasensitive detection. Explosives detection is one the most popular fields demanding ultratrace detection due to the low room-temperature vapor pressure of explosives, [22-24] and it would be much more effective to verify the performance of a SERS substrate by applying it into explosives detection. Moreover, selective and sensitive detection of 5
these explosives is of great importance in countering terrorist threats and pollution of soil and groundwater.[25] However, to the best of our knowledge, there lacks reports on rapid and selective detection of PA, especially by a SERS method. [26, 27] In this work, we report a fast preparation of silver nanotriangles (Ag nanotriangles) of 200 mm edge length and 20 nm thickness in a large scale. The sensitive SERS substrate was constructed based on simple immersing of filter paper in a concentrated silver nanotriangle dispersion, and it can detect 10-8 M pATP as probe molecules. The substrate remains SERS active even after used in the open air for 42 days. Finite element method (FEM) by controlling over the thickness and degree of nanoplates confirms the strong electric field distribution created by Ag nanotriangles with appropriate thickness. Compared to the normal nanostructures, such as Ag nanorod array, round corner Ag cubes-loaded filter paper and Ag nanospheres-loaded filter paper, the remarkable SERS activity of the Ag nanotriangles-loaded filter paper substrate is proved to be boosted by the sharp edges and tips together with the interparticle spacing. Moreover, the present SERS substrate allows the sensitive and selective detection of PA with a concentration as low as 10-6 M. 2. Experimental Section 2.1. Materials All reagents were analytical grade and were used as received without further purification. AgNO3, N,N-Dimethylformamide (DMF), Na2S·9H2O and boric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinyl pyrrolidone (PVP), 4-aminothiophenol (pATP), picric acid (PA), glycol were purchased from Aladdin. TNT (2,4,6-Trinitrotoluene), DNT (2,4-Dinitrotoluene) and RDX (Hexogen) were obtained from the National Security Department of China. 2.2. Synthesis of Ag Nanotriangles In a typical synthesis of Ag nanotriangles, 15 mL DMF was refluxed at 140 oC for 30 min, then 14 mL DMF solution of AgNO3 (0.035 M) and 0.5 g PVP (MW=1300 K) was injected. After injection, the reaction mixture was stirred at 350 rpm and further refluxed at 140 oC for 3.5 h. When the reaction finished, the mixture was cooled to room temperature by rapidly using the ice water bath. The product was purified by centrifugation with ethanol repeatedly (5 mL of the prepared Ag nanotriangles solution was diluted with ethanol and centrifuged at 9500 rpm for 15 6
minutes). The precipitated Ag nanotriangles were then re-dispersed in ethanol and stored at 4 oC until use. 2.3. Fabrication of SERS substrate Ag nanotriangles-loaded filter paper substrate: the filter paper was immersed in Ag nanotriangles solution for 20 min and then allowed to drain for a few seconds on a paper towel. The color of the filter paper changed from white to wathet in the process. The filter paper was dried at 50 oC in Petri dishes. Ag nanotriangles-loaded glass substrate: 200 μL Ag nanotriangles solution dropwise add to the surface of the glass slide evenly. Then the glass substrate was dried at 50 oC in petri dishes. 2.4. Characterization Field-emission scanning electron microscopy (FESEM, ZEISS SUPRA 55VP) was used to characterize the morphology of the samples. All the SERS spectra were measured by a confocal microprobe Raman spectrometer system (RTS-B, Titan Electro-Optics Co., LTD) with a 532 nm laser line, where the effective power of the laser source was about 15 mW. The laser spot focused on the sample surface is about 10 mm in diameter. 2.5 Theoretical Simulation FEM modeling was conducted by using the RF module of Comsol Multiphysics V3.5a, parameters were based on those of the Ag-nanostructures from scanning electron microscopy (SEM) observations, together with the excitation line of 532 nm. Optical constants of Ag were acquired from the literature.
3. Results and Discussion 3.1. SERS performance evaluation of Ag nanotriangles-loaded filter paper substrate
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Fig. 1 (a) SEM image of Ag nanotriangles and (b) Ag nanotriangles loaded on a filter paper, (c) SERS spectra recorded from the Ag nanotriangles-loaded filter paper substrate after being dispersed with (1) 10-5 M, (2) 10−6 M, (3) 10-7 M, (4) 10−8 M pATP (Integration time 5 s, laser power about 15 mW) and (d) signal derivation detected at the same point on the substrate for 8 times with pATP at different concentrations (intensity at 1437 cm-1 as the reference). Silver nanotriangles were obtained by using DMF reduction of AgNO3 with the assistance of PVP as a polymer surfactant. It is shown from the SEM image (Fig. 1a) that Ag nanoplates hold triangle shape with an edge length of around 200 nm. Ag nanotriangles-loaded filter paper substrate can be further obtained by immersing the filter paper into Ag nanotriangles solution for 20 min and drying at 50 oC. SEM image in Fig. 1b suggests that the Ag nanotriangles are uniformly distributed on the surface of the filter paper and the majority of Ag nanotriangles in the substrate are assembled in many-particle densely packed aggregates. Furthermore, the average thickness of the Ag nanotriangles is measured to be 20 nm from the side view (Fig. S1). As depicted in Fig. 1c, the SERS spectra were recorded from the Ag nanotriangles-loaded filter paper substrate at an excitation wavelength of 532 nm with loading of varied concentrations of pATP molecules (10-5, 10-6, 10-7 and 10-8 M). The two main peaks at 8
1079 and 1577 cm-1 can be assigned to the totally symmetric modes 7a (C-S stretch) and 8a (C-C stretch) of pATP, respectively.[28] The other distinct peaks at 1144, 1189, 1390 and 1437 cm-1 can be attributed to the fundamental benzene ring vibrations of pATP (9b, 14b, 3b and 19b, respectively), as detailedly shown in Table S1.[29] It is shown that with the decrease of the concentration of pATP, the intensity of the Raman vibration signals weakens gradually. It should be noted that the vibration modes at 1079 and 1437 cm-1 can still be observed when the concentration of pATP decreased to 10-8 M, indicating the excellent SERS activity of the present Ag nanotriangles-loaded filter paper substrate. It is supposed that the sharp edges and tips on the Ag nanotriangles can result in substantial "hotspots", thus giving strong SERS signals towards extremely low concentration of pATP. In addition, the relative standard deviation (RSD) of the SERS signals detected at the same point on the substrate is less than 17% (Fig. 1d), typically, 6% towards 10-5 M pATP, indicating that the Ag nanotriangles-loaded filter paper substrate is very sensitive to the analyte's concentration variation rather than the intrinsic SERS signal evolution with time.
Fig. 2 (a) The reproducibility of the Ag nanotriangles-loaded filter paper substrates (The transverse dot line shows the average SERS intensity from 7 substrstes, and the error bar indicates the signal derivation of 8 spots within single substrate). (b) SERS intensity change over time, and the error bar indicates the signal derivation of 8 spots within single substrate. To evaluate the reproducibility of the present fabrication method, 7 Ag nanotriangles-loaded filter paper substrates were prepared with the same route, and 8 different spots were randomly selected to record the SERS signals of 10-4 M pATP for each substrate (Fig. 2a). It is shown that the average peak intensity at 1079 cm-1 is 18625, and the average RSD for the SERS signals is 17%, which is a firm evidence of 9
the good reproducibility of the present fabrication method. Besides, the error bar indicates that the RSD of the SERS signals within a single substrate ranges from 10% to 24%, indicating the good consistency of the Ag nanotriangles-loaded filter paper SERS substrates. Fig. 2b shows the SERS intensity change over time at 1079 cm-1 towards 10-4 M pATP, and it is clearly shown that although the SERS intensity of the Ag nanotriangles-loaded filter paper substrate decreases gradually from 21240 to 9187 over time, they remain SERS active after 42 days placement in open air. The above results indicate that the Ag nanotriangles-loaded filter paper substrate could be used as highly sensitive, reproducible and reliable SERS substrate for practical trace detection applications. 3.2 The role of edges and tips in enhancing SERS activity
Fig. 3 E-field intensity distributions (indicated by the color bar) of the single Ag nanotriangles with a thickness of (a) 10 nm, (b) 15 nm, (c) 20 nm, (d) 40 nm, (e) 60 nm, and (f) 80 nm by FEM simulations. The circumcircle diameter of these geometric model are fixed as 230.94 nm. To provide a deep insight into the strong SERS activities at edges and tips, near-field calculations for a series of Ag nanoplates with a diverse range of sharpness or angle was performed. Firstly, FEM modeling was used to investigate the localized electric field intensity (E-max) of the Ag nanotriangles with thicknesses ranging from 10 to 80 nm and a fixed edge length of 200 nm. The E-field intensity distributions of the single Ag nanotriangles by FEM simulations are shown in Fig. 3. The maximum values of the electric field intensity in the presented cross sections are 652, 351, 159, 10
54, 47 and 26 V/m for the Ag nanotriangles with an edge thickness of 10, 15, 20, 40, 60 and 80 nm, respectively. The E-max decreases gradually with the increase of the thickness of Ag nanotriangles, namely bluntness (Fig. S2). Considering the fourth power dependence of the enhancement factor (EF) on the electric field intensity (ISERS ≈ |E|4), [30] the Ag nanotriangles with a small thickness undoubtedly possess a higher EF. This simulation result suggests that the electric field focusing effect of the Ag nanotriangles depends highly on the sharpness of the nanoplates.
Fig. 4 E-field intensity distributions of (a) Ag nanosquare and (b) Ag nanohexagon. The circumcircle diameter of these geometric model are fixed as 230.94 nm. To simulate the effects of tips angle on the near-field enhancement, the Ag nanoplates with a fixed thickness of 20 nm and different tip acutances, namely Ag nanosquares and nanohexagons, were simulated in Fig 4a and 4b, and the E-max in the presented cross sections are 38 and 74 V/m, respectively. Thus, it is clearly shown that compared to nanosquares and nanohexagons, Ag nanotriangles, which have more sharp tips possess the largest E-max.
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Fig. 5 SEM image and E-field intensity distributions (indicated by the color bar) of the different silver nanostructures by FEM simulations of (a, b) Ag nanorods array, (c, d) Ag nanocubes on a filter paper, (e, f) Ag nanospheres on a filter paper. In order to further understand the function of sharp edges and tips on the overall SERS activities, computational and experimental SERS evaluation on three kinds of popular silver nanostructures without sharp edges and tips, namely Ag nanorods array, Ag nanocubes and Ag nanospheres, were conducted simultaneously. The large-area Ag nanorods array were grown via an electrochemical deposition method using porous anodic alumina oxide (AAO) template. It is shown from the top-view SEM image that the Ag nanorods distribute uniformly and have a height of around 200 nm 12
and a diameter of 40 nm (Fig. 5a). According to this geometric parameter, the E-max modeled by FEM in the presented cross sections is 5 V/m (Fig. 5b). The E-max for Ag nanocube with an edge length of 70 nm (Fig. 5c) is modeled to be 17 V/m (Fig. 5d). While the Ag nanosphere with a diameter of 100 nm (Fig. 5e) possesses the lowest E-max of 1.5 V/m (Fig. 5f).
Fig. 6 (a) SERS spectra of pATP collected from (1) Ag nanotriangles-loaded filter paper substrate for 10-5 M pATP, (2) Ag nanospheres and (3) Ag nanocubes-loaded filter paper substrate for 10-4 M pATP and (4) Ag nanorods array for 10-4 M pATP. (b) The reproducibility of the four kinds of SERS substrates, error bar indicates the signal derivation of 8 spots within single substrate. From the SERS spectra of the Ag nanorods array, the Ag nanocubes and the Ag nanospheres-loaded filter paper substrates towards 10-4 M pATP (Fig. 6a), it is clearly shown that the intensities of SERS signals are around 3220, 4446 and 12398 at 1079 cm-1. It is surprised that the Ag nanospheres, which possess the weakest E-max in the three Ag nanostructures as simulated, show the strongest SERS activity after loading on a filter paper. From a comparison of the morphology of the Ag nanocubes and the Ag nanospheres-loaded filter paper substrates, one can find that Ag nanospheres are more condensed and thus much more and stronger SERS "hotspots" were boosted by interspacing, which will be discussed in section 3.3. Generally, these three much smaller nanostructures should present more excellent SERS performance than the Ag nanotriangles due to the size effect (40-100 nm vs. 200 nm).[30] However, all these three SERS substrates show much weaker SERS signals compared to the Ag nanotriangles-loaded filter paper since it shows an intensity of 16374 even towards 10-5 M pATP. It should be noted that this result is based on a statistical analysis and 8 spots were selected for each substrate (Fig. 6b). Thus, it is further confirmed that the silver nanostructures with sharp edges and tips exhibit much larger field enhancement. 13
To understand the influence of the excitation source to the optical properties of these different nanostructures, FEM modeling was conducted to investigate the local electric field intensity of the Ag nanotriangles with different excitation wavelength. The E-max in the presented cross sectionsare 169, 159, 147 and 156 V/m for the Ag nanotriangles with excitation wavelengths of 514 nm, 532 nm, 633 nm and 785 nm, respectively (Fig. S3). This simulation result suggests that the excitation wavelength of the laser source has very little influence on the electric field focusing effect of the Ag nanotriangles. This conclusion is further evidenced by the similar FEM simulations of Ag nanorod, Ag nanocube and Ag nanosphere with different excitation wavelengths (Fig. S4-S6). 3.3 The role of Interspacing in enhancing SERS activity
Fig. 7 (a) SERS spectra recorded from the Ag nanotriangles-loaded filter paper substrate and Ag nanotriangles-loaded glass substrate after being dispersed with 10-5 M pATP. (b) E-field intensity distributions (indicated by the color bar) of the double Ag nanotriangles with 20 nm gaps. To further understand the advantages of the Ag nanotriangles-loaded filter paper substrate, the SERS properties of the Ag nanotriangles-loaded on a glass substrate was also evaluated. The SERS sprctra of both substrates towards pATP with a concentration of 10-5 M obtained by maintaining the same measurement condition shown that all the characteristic scattering peaks of pATP can be clearly observed (Fig. 7a and S7). However, it is obvious that the SERS intensity of the Ag nanotriangles-loaded filter paper substrate is much stronger than that of the Ag nanotriangles-loaded glass substrate, typically, 3.1 times at a wavelength of 1079 cm-1. The filter paper substrate induced SERS efficiency enhancement may be attributed to the more "hotspots" which emerge from the interparticle spacing or gaps between the 14
adjacent Ag nanotriangles. The three-dimensional and porous structure of filter paper is able to provide a greater surface area to localize the adsorption of Ag nanotriangles via abundant hydroxyl groups that lead to the Ag nanotriangles aggregation. Therefore, additional simulation on the interparticle spacing between Ag nanotriangles was performed in Fig. 7b and the result shows that the E-max reaches 211 V/m, indicating that the electromagnetic enhancement is highly localized in the gap region of Ag nanotriangles, which is preferentially formed in the nanotriangles closely-packed filter paper substrate. As the Ag nanotriangles are contacted to form aggregates with interparticle spacing or gaps, their transition dipoles couple to each other and the enhanced fields of each nanotriangle start to coherently interfere at their „„hotspots‟‟.[31] Thus, the intricate network of fibers enables potentially high density of electromagnetic "hotspots" with the decoration of plasmonic Ag nanotriangles.
Fig. 8 (a) SERS spectrum evolution of pATP (10–5 M) with evaporation of the ethanol solution loaded on Ag nanotriangles - filter paper substrate at intervals of 5 s and (b) the maximum SERS intensity recorded at 1079 cm-1. The function of the interparticle spacing distance can be further studied by solvent evaporation induced spacing change after dispersing of pATP solution onto the filter paper. As shown in Fig. 8, the initially weak signals become stronger and reach the maximum value at 40 s. It is considered that the filter paper cellulose deforms with the change of the surface tension caused by the solvent evaporation, resulting in the change of the gaps between Ag nanotriangles in the evaporation process. Thus, the intricate network of fibers provides an efficient platform for the formation of electromagnetic "hotspots" induced by interparticle spacing between Ag nanotriangles.
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Fig. 9 E-field intensity distributions (indicated by the color bar) of double (a) Ag nanocubes and (b) Ag nanospheres with an interspacing distance of 20 nm by FEM simulations. The role of inter-particle spacing in Ag nanocubes and Ag nanospheres on the formation of "hotspots" was further investigated (Fig 9). It is observed that the E-max of Ag nanocubes and Ag nanospheres are 23.8 and 2.7 V/m respectively when the spacing between these Ag nanostructures edges is fixed to 20 nm. Compared with the simulation results of individual nanostructure shown in Figure 5, this result suggests that inter-particle spacing, which is generally formed via hydroxyl group on filter paper induced self-assembly of these nanostructures, can enhance the electric field intensity to a certain extent. Hence, inter-particle spacing of nanoparticles is more efficient than individual nanoparticles. However, compared with nanotriangles, these nanostructures without sharp edges and tips do not show distinct advantages in "hotspots" generation, even with a short inter-particle spacing distance. Thus, Ag nanotriangles with sharp edges, as well as filter paper, play a critical role in SERS signal enhancement. Based on the above analysis, the nanotriangle shape, a relatively small thickness and the porous filter paper substrate coordinately composed the characteristics of a highly active SERS substrate. 3.3 Ultrasensitive detection of typical military explosives-a tentative study
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Fig. 10 (a) SERS spectra of PA at different concentrations and (b) statistical analysis of 8 spots within a single substrate at 825 cm-1 towards different concentrations of PA. SERS has been demonstrated for the detection of trace levels of explosives due to its high sensitivity, speed of detection and fingerprint feature.[22] Picric acid is a common explosive with a room-temperature vapor pressure of 0.97 ppb. Besides, it is used in metal etching, dye, leather and glass industries, thus, PA exists as a trace pollutant in water and soil.[27] Thus, to effectively verify the performance of the Ag nanotriangles-loaded filter paper SERS substrate, picric acid was selected as the target analyte. As shown in Fig. 10a, well-resolved SERS spectra could be readily obtained from both the solid and diluted PA with concentrations varied from 10-4 to 10-6 M. The characteristic peaks (Table S2) of PA at 935 cm-1 (ring breathing), 1085 cm-1 (phenplic C-O stretching), 1310 cm-1 (C-C stretching) and 1557 cm-1 (C-NO2 stretching) are observed at different concentrations.[32] It is noteworthy that the Raman peaks located at 825 and 1336 cm-1, which are assigned to be the out-of-plane bending modes and the asymmetric stretching of nitro group of PA, [33] become weaker and weaker. The phenplic C-O stretching of PA at 1085 cm-1 still can be clearly identified at a concentration of 10-6 M (Fig. S8). From the statistical analysis of 8 spots within a single substrate, one can find that the intensity of the peak at 825 cm-1 decreases linearly with the decrease of the concentration of PA (Fig. 10b). The detection ability of the present Ag nanotriangles-loaded filter paper substrate is obviously improved compared to the recently reported detection limit which are summarized in Table 1. Moreover, we cannot detect any Raman signal when Ag nanorod array, Ag nanocubes and Ag nanospheres-loaded filter papers were employed as the SERS substrates even at a concentration as high as 10-4 M. Thus, it can be concluded that sharp edges and tips help to boost the SERS activity of the Ag nanotriangles-loaded filter paper 17
substrate and they play a vital role in ultratrace detection towards practical analytes. Table 1 Summary of literature reports of SERS protocol to PA sensing. Substrate Positively charged silver nanoparticles Unmodified gold nanoparticles Ag nanotriangles-loaded filter paper a
Detection Limit (M) 2.5×10-5 2×10-5 a 10-6
Ref 24
25
This work
The characteristic symmetric nitro stretching Raman signal of nitroaromatic
compounds at 1335 cm-1 is absent at a concentration of 2×10-5 M. Surprisingly, no significant variation in the SERS spectra of nitro explosives TNT (2,4,6-Trinitrotoluene), DNT (2,4-Dinitrotoluene) and RDX (Hexogen) were observed by employing the same Ag nanotriangles-loaded filter paper substrate (Fig. S9). The reason can be attributed to the coordination ability of PA to the surface of Ag nanotriangles via the hydroxy group[34]. In addition, the thiol group of pATP is apt to adsorb onto the surface of Ag nanotriangles due to the stronger aurophilic interaction.[35] Thus, the Ag nanotriangles-loaded filter paper SERS substrate shows a better detection performance towards pATP than PA. However, it is difficult to light on the SERS signals for these structurally similar explosives without hydroxy group or thiol group. The result demonstrates that the Ag nanotriangles-loaded filter paper SERS substrate can be employed to selectively detect nitro explosives PA. 4.
Conclusion In summary, a flexible SERS substrate fabricated by incorporating Ag
nanotriangles with a sharp edge of 20 nm into a filter paper was demonstrated and good SERS activity and reproducibility were achieved. Typically, an ultratrace detection of pATP with a concentration as low as 10-8 M was obtained. Through the FEM modeling of the plasmonic properties of Ag nanotriangles with different edge thicknesses together with Ag nanosheets with different tip acutances, it is proved that the sharp edges and tips of the Ag nanotriangles boosted the SERS activity. The Ag nanotriangles also show a much better SERS activity compared to Ag nanorods, nanocubes and nanospheres no matter by experimental results or by computational 18
simulations, which further demonstrates the importance of creation of sharp edges and tips in enhancing the overall SERS activity. Besides, the intricate network of fibers provides an efficient platform for the formation of electromagnetic "hotspots" induced by interparticle spacing between Ag nanotriangles. Furthermore, the present Ag nanotriangles-loaded filter paper SERS substrate also can be employed to selectively detect ultratrace military explosive compound PA with a detection concentration of 10−6 M. We expect this study to open up a new perspective in the design of SERS sensors for various analytes based on sharp edges and tips boosted noble metal nanotriangles. Acknowledgements We thank the financial supports from the National Natural Science Foundation of China (51201180) and "Hundred Talents Program" of CAS. We also acknowledge the helpful discussions with Dr. Zhulin Huang in Institute of Solid State Physics, Chinese Academy of Sciences. References [1] S. Cong, Y. Yuan, Z. Chen, J. Hou, M. Yang, Y. Su, Y. Zhang, L. Li, Q. Li, F. Geng, Z. Zhao, Noble metal-comparable SERS enhancement from semiconducting metal oxides by making oxygen vacancies, Nat. Commun. 6 (2015) 7800. [2] I. Alessandri, Enhancing Raman scattering without plasmons: unprecedented sensitivity achieved by TiO2 shell-based resonators, J. Am. Chem. Soc. 135 (2013) 5541-5544. [3] W. Xu, J. Xiao, Y. Chen, Y. Chen, X. Ling, J. Zhang, Graphene-veiled gold substrate for surface-enhanced Raman spectroscopy, Adv. Mater. 25 (2013) 928-933. [4] Z. Huang, G. Meng, Q. Huang, B. Chen, F. Zhou, X. Hu, Y. Qian, H. Tang, F. Han, Z. Chu, Polyacrylic acid sodium salt film entrapped Ag-nanocubes as molecule traps for SERS detection, Nano Res. 7 (2014) 1177-1187. [5] J. Lu, H. Liu, E.S. Tok, C.H. Sow, Interactions between lasers and two-dimensional transition metal dichalcogenides, Chem. Soc. Rev. (2016) DOI: 10.1039/C5CS00553A. [6] J. Lu, J.H. Lu, H. Liu, B. Liu, L. Gong, E.S. Tok, K.P. Loh, C.H. Sow, Microlandscaping of Au Nanoparticles on Few-Layer MoS2 Films for Chemical Sensing, Small 11 (2015) 1792-1800.
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Biographies Chong Wang received his bachelor's degree in Chemistry from Shihezi University in 2014. Now, he is a graduate student in Prof. Dou‟s group. His research interest centers around sensors and analytical applications by means of nanomaterials and surface-enhanced Raman spectroscopy. Bingxin Liu completed his Ph.D. in 2015 at Northeast Normal University, China. Currently he is an assistant professor at the Laboratory of Environmental Science and Technology at The Xinjiang Technical Institute of Physics & Chemistry of the Chinese Academy of Sciences. His research interests are constructing nanosensors for rapid and non-contact sensing of military and improvised explosives. Xincun Dou obtained his Ph.D. in Materials Physics and Chemistry from Institute of Solid State Physics in Chinese Academy of Sciences in 2009. Following postdoctoral work at Nanyang Technological University in Singapore in Materials Science, he joined the Laboratory of Environmental Science and Technology at Xinjiang Technical Institute of Physics & Chemistry in 2011 as a "Hundred Talents Program" professor. His research interests are focused on the exploratory design of nanomaterials and nanodevices, with an emphasis on explosives sensing applications.
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