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Highly ordered Au-decorated Ag nanorod arrays as an ultrasensitive and reusable substrate for surface enhanced Raman scattering
Colloids and Surfaces A 560 (2019) 360–365
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Highly ordered Au-decorated Ag nanorod arrays as an ultrasensitive and reusable substrate for surface enhanced Raman scattering Yaqian Dana, Chengquan Zhonga, Huanwen Zhua, Jun Wanga,b, a b
T
⁎
Department of Physics, Faculty of Science, Ningbo University, Ningbo, 315211, China College of Physics and Electronic Information Engineering, Minjiang University, Fuzhou, 350108, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Key words: Electroless deposition Ag nanorod arrays Micro-nano structure Reproducibility SERS
For surface-enhanced Raman spectroscopy (SERS) substrates, complexity of the fabrication process, sensitivity and stability are factors that need to be considered. We report a simple and novel method for preparing an effective substrate that greatly enhances SERS signals. The SERS substrate is fabricated by selectively electroless deposition of Ag nanoparticles in the highly ordered porous anodized aluminum oxide (AAO) template and then etching it to form Ag nanorod bundles structure, so that the Raman enhancement occurs in the gap between the nanorods and the top of the nanorod bundle, which is verified by finite element analysis. A peculiarity of this method is that this is the first time to synthesize Ag nanorod arrays structure by using it. The results indicated that this prepared substrate has special micro-nano structure and when rhodamine 6 G is selected as the detection reagent, the detection limit is as low as 10−16 M with excellent recyclability. This outstanding array structure has a promising practical application in SERS field.
1. Introduction As early as 1970s [1], in order to solve a difficult problem which Raman scattering sensitivity was not high, SERS techniques came into being [2,3]. SERS is an optical phenomenon in which the Raman scattering signal of adsorbed molecules is dramatically higher than ordinary signals due to the enhancement of the electromagnetic field on or near the surface of a certain metal good conductor and sol [4,5]. The enhancement effect mainly occurs at the surface of micro-nano structures of noble metal materials because of the fact that the micro-nano structures can amplify the SERS signals through the enhancement of the ⁎
electromagnetic field induced by the localized surface plasmon resonance (LSPR) [6–9]. It should be noted that, SERS technology not only has most of the advantages of Raman spectroscopy, but also can provide more abundant chemical molecular structure information, realizing real-time and in-situ detection, high sensitivity, and no sample pre-treatment or destructive [10,11]. It is a powerful trace-level analysis tool as well as detection of low concentrations of analytes or contaminants without damaging the samples [12,13]. As a result, SERS has been widely used in the fields of interface and surface science [14], material analysis [15], biology medicine [16,17], food safety [18] and environmental monitoring [19,20]. For practical applications, it is
Corresponding author at: Department of Physics, Faculty of Science, Ningbo University, Ningbo, 315211, China. E-mail address: [email protected] (J. Wang).
https://doi.org/10.1016/j.colsurfa.2018.10.040 Received 3 September 2018; Received in revised form 16 October 2018; Accepted 16 October 2018 Available online 22 October 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 560 (2019) 360–365
Y. Dan et al.
desired for a high-performance SERS substrate that not only has highdensity hot spots to ensure high sensitivity, but also has good stability to realize SERS signal reproducibility [21,22]. In general, the more hot spots of a certain micro-nano structured material, the more favourable to enlarge the SERS signal [23]. On the basis of these considerations, the current research effort in SERS mainly focuses on the preparation of various and high-performance micro-nano structured materials for SERS substrates [24]. These micro-nano structures can generate a great deal of hot spots, thereby effectively enhancing SERS signals. In recent years, silver nanorods as a promising substrate have been extensively studied and anodized aluminum oxide (AAO) templates are also commonly used as auxiliary material in the field of SERS. For example, Saravanan et al. prepared Ag nanorod arrays (AgNRs) in a double-pass AAO membrane by chemical reduction [25]; Yan et al. used ion-sputtering on the surface of conicalpore-AAO template which had been formed after multistep process of anodization and etching to prepare Ag nanoparticle arrays [26]; Wang et al. prepared Ag-decorated nanorod arrays by vacuum deposition of Ag on a patterned fingerlike AAO film which had been achieved by using the nanosphere lithography (NSL) and Al deposition methods [27]; etc. Although the Ag nanorods prepared by these experimental schemes were evenly distributed, their morphology was difficult to control, and the resultant double-pass AAO template was easily broken, which still haveroom for optimization. Inspired by these experiments, herein, we present a simple, less material consumption and low-cost method for fabricating an excellent substrate with high-density hot spots for SERS detection based on selectively electroless deposition. At first, AgNPs were deposited in the AAO template, and the distribution of AgNPs was very uniform because AAO had an ordered porous structure, leading to the reaction being confined into the pores. As the deposition time increased, Ag nanorod arrays were grown in highly ordered pores by electroless deposition. Then, the AAO template was etched with an aqueous solution of sodium hydroxide so that Ag nanorods were automatically assembled into bundles and many hot spots were formed between the nanorods, capturing the analytes at these locations. Finally, the previously prepared sample was used as substrate to rapidly detect rhodamine 6 G (R6 G) that exhibitedsurprisingly high sensitivity (10−16 M) and Raman enhancement.
2.3. Electroless deposition of Ag nanorod arrays on AAO (Ag@AAO) The Ag nanorod arrays were realized by immersing the AAO template which obtained above in 1 M AgNO3 aqueous solution based on an electroless deposition mechanism. The reaction vessel is 25 mL Teflon autoclave. The AAO template was embedded in a Teflon mold to ensure that the reaction zone was not changed, and then the reaction temperature was controlled at T = 333 K for 11 h. After the reaction was completed, the Teflon autoclave was naturally cooled to room temperature. The reaction time and the amount of reaction solution could affect the growth of silver nanorod array. Finally, taking out the sample and washing it several times with ethanol and deionized water. The sample was soaked in a 1 M NaOH for a certain period of time using the reaction of sodium hydroxide and aluminum oxide, and then rinsed with deionized water, so that highly uniform Ag nanorod arrays were obtained. We placed it in a vacuum drying oven to dry. 2.4. Gold nanoparticles (AuNPs) decorated Ag@AAO nanorod arrays AuNPs were sputtered onto the surface of the prepared sample by using an electron beam vacuum coater (Auto 306, England HHV). During the entire evaporation process, the coating rate was 0.2 A/S and the film thickness was 50 nm. 2.5. Characterization Morphology and structure of the as-fabricated Ag nanorod arrays were characterized by field emission scanning electron microscopy (FESEM, Hitachi SU-70), energy dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM, NanoScope IIIa, Veeco). 2.6. SERS measurements SERS measurements were performed at room temperature with Renishaw inVia. A laser with a wavelength of 785 nm is used as the excitation source. The incident power is 0.5 mW, and the SERS signal integration time was 10 s, totalling 5 scans. Before testing, the samples were prepared by immersing in R6 G solution overnight, followed by drying with nitrogen and finally subjected to SERS testing. It should be noted that the Raman spectra of all samples had the same accumulation time and laser power.
2. Experimental section
3. Results and discussion
2.1. Materials
3.1. Characterization of AAO
Aluminum sheet (99.999%), perchloric acid (HClO4), ethanol (C2H5OH), oxalic acid (H2C2O4), phosphoric acid (H3PO4), chromic acid (H2CrO4), silver nitrate (AgNO3), sodium hydroxide (NaOH), and R6 G were purchased from Aladdin Chemistry Co. Ltd. All of the materials were not further purified. In addition, deionized water was used as a solvent throughout the experiment.
As can be seen from Fig. 1a, the nanopores on the AAO template prepared by two-step anodization method are uniform in size, ordered in arrangement and distributed independently. Furthermore, it can be seen from the Fig. 1b that the hexagonal pore size is 50 ± 10 nm. 3.2. Characterization of Ag@AAO The fabrication procedure for Ag@AAO was described schematically in Fig. 2. Ag nanorods were electrolessly deposited in the pores, in this process, the detailed growth mechanism of Ag nanorods is not only attributed to the transfer of electrons of Al (reducing agent) in the AAO template to Ag ions (oxidant) in the reaction solution, but also the redox potential of Al is generally higher than that of Ag, so Ag ions are deoxidized to form an Ag atom which attached to the Al layer. In the high temperature and pressure environment, metal ions are continuously reduced and grow along the template pores to form nanorods. It follows the reaction route as described below. To obtain specific structural characteristics and elements distributions of the as-synthesized Ag@AAO, FE-SEM, EDS and AFM analyses were carried out. As can be seen from Fig. 3a, Ag nanorods were
2.2. Preparation of AAO template Preparation of highly ordered porous AAO template was divided into three parts: First, high-purity aluminum sheet was subjected to high-temperature vacuum calcining at 500℃ for 4 h, and then washed with acetone, ethanol and deionized water in turn for 10 min; second, the above-mentioned processed aluminum sheet was cut into small round wafers and polished in perchloric acid and ethanol solutions with a volume ratio of 1: 9; and finally, using a secondary anodic oxidation process in 0.4 M oxalic acid under 40 V at 2–8℃ to obtain single-pass AAO products (only the bottom of the nanopore has an Al layer) with uniform pore arrangement. It should be noted that the working voltage must be kept constant throughout the anodizing process. 361
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Fig. 1. SEM images of the highly ordered porous AAO template.
uniformly distributed, highly ordered and had high filling rates. The diameter of the nanorods was 50 ± 10 nm, which was consistent with the pore size of the AAO template. Fig. 3b shows the picture of the Ag@AAO being treated with sodium hydroxide, and it was clear that the exposed length of Ag nanorods increased and the gap between adjacent nanorods was also small, so the surface area of Ag nanorods increases, which could be used to attach more AuNPs. It was evident in Fig. 4, the nanorod bundles were surrounded by many AuNPs, and on the nanorod bundles, a large number of AuNPs were continuously deposited, followed by aggregating to form many big Au spheres. And this special nanostructure made the sample surface rough enough and the surface area was significantly increased, when the light was irradiated on the sample, free electrons on the surface of the Ag nanorod interact with photons to produce collective oscillations (surface plasmon resonance), thereby enhancing the electromagnetic (EM) field [28,29]. At the same time, the SERS effect was closely related to the enhancement of the EM field of nanostructures [30], so the SERS enhanced effect was remarkable. For the enhancement effect, a finite difference time domain (FDTD) simulation was used to perform a preliminary calculation. The simulation model created by FDTD was based on SEM images (Fig. 4), where it was assumed that the AuNPs were uniformly distributed and were ideal spheres, and the diameters of nanorods and AuNPs were 60 and 10 nm, respectively. Then, FDTD simulation was performed with a monochromatic incident wavelength of 785 nm on the side and above of the nanorod bundles. All the simulations were described schematically in Fig. 5. The blue balls represented AuNPs, while the yellow and red area represented large EM fields.It was obvious that regardless of whether it was on the side or on the top, there were a certain EM field enhancement which mainly came from the gap, resulting in synergistic plasmonic enhancements. Moreover, the gaps between adjacent nanorod bundles provided SERS hot spots with local EM field enhancement, which contribute to the generation of the SERS signals, so the whole substrate would produce a huge SERS effect [31]. The EDS mapping analysis in Fig. 6(a) displays a homogeneous distribution of the three elements Ag, O, and Al on the two-dimensional projection of nanostructure chemical maps. The compositional mass
fractions of Ag, O, and Al in the Ag@AAO nanostructure arrays were 5.98%, 46.98%, and 47.04%, respectively. Besides, Fig. 6(b) shows XRD analysis of Ag nanorods, which demonstrates that the diffraction peaks were observed. All peaks located at ∼38.12°, ∼44.31°, ∼64.45°, ∼77.41° and ∼81.55°corresponded to the cubic Ag crystal planes of (111), (200), (220), (311) and (222), respectively. These two characterization demonstrated the presence of Ag elements in addition to Al and O, indicating that the nanowire composition was Ag. In addition, the AFM was further used to confirm the nanostructures of the uniformly distributed Ag@AAO. As Fig. 7(a, b) depicts, silver nanorods were evenly distributed and highly ordered. These results indicated that large-scale uniform Ag nanorod arrays can be synthesized by electroless deposition. 3.3. SERS detection of R6 G dye In order to test whether our substrates of AuNPs decorated Ag@AAO were effective or not for trace detection, the SERS performance of the synthesized nanosubstrate was evaluated by using R6 G as a probe molecule. And it is well known that major characteristic peaks of the R6 G molecule are observed at 611, 778, 1181, 1313, 1361, 1511, and 1651 cm−1, respectively. The vibrational modes corresponding to these main characteristic peaks were illustrated in Table 1 [32–34]. As is shown in Fig. 8a, comparing the SERS enhancement effects of blank AAO and AuNPs decorated Ag@AAO, it was apparent that the SERS intensity of the latter was stronger, about 5 times higher than that of blank AAO. According to previous research, sensitivity of SERS substrates was related to the microstructure of sample [21], the reason why the SERS of our sample exhibit higher enhancement was the formation of coarse micro-nano structures on the sample surface and the exposed length of clustered Ag nanorods with gaps were up to 300 nm, resulting in a huge surface area increase that R6 G molecules achieved adequate adsorption sites. On the one hand, the chemical interactions of R6 G molecules with nanorods surfaces resulted in chemical mechanisms (CM) enhancement; on the other hand, Ag nanorods with gaps generated numerous SERS “hot spots” since their surface plasmon resonance
Fig. 2. Schematic illustration of the synthesis process of Ag@AAO SERS substrates: (I) fabrication of AAO template; (II) formation of Ag@AAO; (III) Ordered arrays of Ag nanorods bundles achieved by sodium hydroxide etching. 362
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Fig. 3. SEM images: (a) the top view of Ag@AAO; (b) 30°-tilted side-view of the Ag@AAO being treated with sodium hydroxide.
Fig. 4. SEM images of AuNPs decorated Ag@AAO being treated with sodium hydroxide.
Fig. 5. FDTD simulated EM field distribution around a bundle of AuNPs decorated Ag@AAO under normal incidence with a wavelength of 785 nm: (a) Cross-section; (b) vertical section.
Fig. 6. (a) EDS mapping and EDS results of the Ag@AAO nanostructure arrays before sputtering. (b) XRD pattern of the Ag nanorods. 363
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Fig. 7. AFM images of Ag@AAO without sodium hydroxide treatment: (a) 2D picture; (b) 3D picture. Table 1 Major SERS bands and corresponding vibrational modes of R6 G. Peak position(cm−1)
Vibrational mode
611 778 1181 1313 1361、1511、1575、1651
CeCeC ring in-plane bending vibrations CeH out-of-plane bending vibrations CeH in-plane bending mode NeH in plane bending mode CeC stretching of the aromatic ring
contributes to the EM field enhancement, so CM and EM could synergistically enhance SERS signals [35]. Furthermore, we prepared different concentrations R6 G aqueous solutions from 10−4 to 10−16 M and collected the corresponding SERS spectra of them on the sample. As presented in Fig. 8b, it is clear that SERS spectra could be detected when the concentration of R6 G down to 10−16 M. This result revealed that as-prepared substrate had high sensitivity. Last but not least, we investigated the stability of our substrate via examining the Raman activity of it after three months of storage. The testing procedure included ultrasonic treatment of 60 s with acetone; then cleaning in alcohol and DI water followed by drying in ambient air; furthermore, we could repeat the above steps many times as necessary for cleaning well; at last, immersion in the same R6 G concentration to detect the SERS spectrum. Fig. 9 displays the substrate could still detect the SERS signal of R6 G after being placed for 3 months (curve 2b). Curve 2a was almost straight and similar to curve 1a, indicating that the sample was washed well. Therefore, the as-prepared Ag nanorod arrays substrate displays high SERS stability for detecting R6 G molecules.
Fig. 9. SERS signal for R6 G molecules (10−4 M): (1a) blank AAO as a reference; (1b) Au-decorated Ag@AAO; (2a) washed; (2b) Au-decorated Ag@AAO after R6 G re-deposition.
4. Conclusions In summary, we proposed a simple and novel method to prepare high-performance SERS substrates. In other words, the hexagonal AAO nanohole arrays served as a template for selective electroless deposition of Ag nanorods. Due to the spatial confinement effect of nanopores, the resultant Ag nanorods were uniformly distributed and arranged over a large area. Additionally, by etching Ag nanorod with aqueous sodium hydroxide solution, we can obtain Ag nanorod bundles which have
Fig. 8. (a) The Raman spectra of R6 G (10−4 M): curve i, blank AAO + Au; curve ii, Ag@AAO + Au; (b) SERS signals of R6 G with different concentrations. 364
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large hot spot adsorption areas, so Ag@AAO showed the highest SERS intensity, and when the concentration of the R6 G was as low as 10−16 M, the SERS signal could still be detected. We believe this unique synthesis method can be used to prepare other metallic materials with outstanding structure and used for many fields.
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365
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Highly ordered Au-decorated Ag nanorod arrays as an ultrasensitive and reusable substrate for surface enhanced Raman scattering Yaqian Dana, Chengquan Zhonga, Huanwen Zhua, Jun Wanga,b, a b
T
⁎
Department of Physics, Faculty of Science, Ningbo University, Ningbo, 315211, China College of Physics and Electronic Information Engineering, Minjiang University, Fuzhou, 350108, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Key words: Electroless deposition Ag nanorod arrays Micro-nano structure Reproducibility SERS
For surface-enhanced Raman spectroscopy (SERS) substrates, complexity of the fabrication process, sensitivity and stability are factors that need to be considered. We report a simple and novel method for preparing an effective substrate that greatly enhances SERS signals. The SERS substrate is fabricated by selectively electroless deposition of Ag nanoparticles in the highly ordered porous anodized aluminum oxide (AAO) template and then etching it to form Ag nanorod bundles structure, so that the Raman enhancement occurs in the gap between the nanorods and the top of the nanorod bundle, which is verified by finite element analysis. A peculiarity of this method is that this is the first time to synthesize Ag nanorod arrays structure by using it. The results indicated that this prepared substrate has special micro-nano structure and when rhodamine 6 G is selected as the detection reagent, the detection limit is as low as 10−16 M with excellent recyclability. This outstanding array structure has a promising practical application in SERS field.
1. Introduction As early as 1970s [1], in order to solve a difficult problem which Raman scattering sensitivity was not high, SERS techniques came into being [2,3]. SERS is an optical phenomenon in which the Raman scattering signal of adsorbed molecules is dramatically higher than ordinary signals due to the enhancement of the electromagnetic field on or near the surface of a certain metal good conductor and sol [4,5]. The enhancement effect mainly occurs at the surface of micro-nano structures of noble metal materials because of the fact that the micro-nano structures can amplify the SERS signals through the enhancement of the ⁎
electromagnetic field induced by the localized surface plasmon resonance (LSPR) [6–9]. It should be noted that, SERS technology not only has most of the advantages of Raman spectroscopy, but also can provide more abundant chemical molecular structure information, realizing real-time and in-situ detection, high sensitivity, and no sample pre-treatment or destructive [10,11]. It is a powerful trace-level analysis tool as well as detection of low concentrations of analytes or contaminants without damaging the samples [12,13]. As a result, SERS has been widely used in the fields of interface and surface science [14], material analysis [15], biology medicine [16,17], food safety [18] and environmental monitoring [19,20]. For practical applications, it is
Corresponding author at: Department of Physics, Faculty of Science, Ningbo University, Ningbo, 315211, China. E-mail address: [email protected] (J. Wang).
https://doi.org/10.1016/j.colsurfa.2018.10.040 Received 3 September 2018; Received in revised form 16 October 2018; Accepted 16 October 2018 Available online 22 October 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
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desired for a high-performance SERS substrate that not only has highdensity hot spots to ensure high sensitivity, but also has good stability to realize SERS signal reproducibility [21,22]. In general, the more hot spots of a certain micro-nano structured material, the more favourable to enlarge the SERS signal [23]. On the basis of these considerations, the current research effort in SERS mainly focuses on the preparation of various and high-performance micro-nano structured materials for SERS substrates [24]. These micro-nano structures can generate a great deal of hot spots, thereby effectively enhancing SERS signals. In recent years, silver nanorods as a promising substrate have been extensively studied and anodized aluminum oxide (AAO) templates are also commonly used as auxiliary material in the field of SERS. For example, Saravanan et al. prepared Ag nanorod arrays (AgNRs) in a double-pass AAO membrane by chemical reduction [25]; Yan et al. used ion-sputtering on the surface of conicalpore-AAO template which had been formed after multistep process of anodization and etching to prepare Ag nanoparticle arrays [26]; Wang et al. prepared Ag-decorated nanorod arrays by vacuum deposition of Ag on a patterned fingerlike AAO film which had been achieved by using the nanosphere lithography (NSL) and Al deposition methods [27]; etc. Although the Ag nanorods prepared by these experimental schemes were evenly distributed, their morphology was difficult to control, and the resultant double-pass AAO template was easily broken, which still haveroom for optimization. Inspired by these experiments, herein, we present a simple, less material consumption and low-cost method for fabricating an excellent substrate with high-density hot spots for SERS detection based on selectively electroless deposition. At first, AgNPs were deposited in the AAO template, and the distribution of AgNPs was very uniform because AAO had an ordered porous structure, leading to the reaction being confined into the pores. As the deposition time increased, Ag nanorod arrays were grown in highly ordered pores by electroless deposition. Then, the AAO template was etched with an aqueous solution of sodium hydroxide so that Ag nanorods were automatically assembled into bundles and many hot spots were formed between the nanorods, capturing the analytes at these locations. Finally, the previously prepared sample was used as substrate to rapidly detect rhodamine 6 G (R6 G) that exhibitedsurprisingly high sensitivity (10−16 M) and Raman enhancement.
2.3. Electroless deposition of Ag nanorod arrays on AAO (Ag@AAO) The Ag nanorod arrays were realized by immersing the AAO template which obtained above in 1 M AgNO3 aqueous solution based on an electroless deposition mechanism. The reaction vessel is 25 mL Teflon autoclave. The AAO template was embedded in a Teflon mold to ensure that the reaction zone was not changed, and then the reaction temperature was controlled at T = 333 K for 11 h. After the reaction was completed, the Teflon autoclave was naturally cooled to room temperature. The reaction time and the amount of reaction solution could affect the growth of silver nanorod array. Finally, taking out the sample and washing it several times with ethanol and deionized water. The sample was soaked in a 1 M NaOH for a certain period of time using the reaction of sodium hydroxide and aluminum oxide, and then rinsed with deionized water, so that highly uniform Ag nanorod arrays were obtained. We placed it in a vacuum drying oven to dry. 2.4. Gold nanoparticles (AuNPs) decorated Ag@AAO nanorod arrays AuNPs were sputtered onto the surface of the prepared sample by using an electron beam vacuum coater (Auto 306, England HHV). During the entire evaporation process, the coating rate was 0.2 A/S and the film thickness was 50 nm. 2.5. Characterization Morphology and structure of the as-fabricated Ag nanorod arrays were characterized by field emission scanning electron microscopy (FESEM, Hitachi SU-70), energy dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM, NanoScope IIIa, Veeco). 2.6. SERS measurements SERS measurements were performed at room temperature with Renishaw inVia. A laser with a wavelength of 785 nm is used as the excitation source. The incident power is 0.5 mW, and the SERS signal integration time was 10 s, totalling 5 scans. Before testing, the samples were prepared by immersing in R6 G solution overnight, followed by drying with nitrogen and finally subjected to SERS testing. It should be noted that the Raman spectra of all samples had the same accumulation time and laser power.
2. Experimental section
3. Results and discussion
2.1. Materials
3.1. Characterization of AAO
Aluminum sheet (99.999%), perchloric acid (HClO4), ethanol (C2H5OH), oxalic acid (H2C2O4), phosphoric acid (H3PO4), chromic acid (H2CrO4), silver nitrate (AgNO3), sodium hydroxide (NaOH), and R6 G were purchased from Aladdin Chemistry Co. Ltd. All of the materials were not further purified. In addition, deionized water was used as a solvent throughout the experiment.
As can be seen from Fig. 1a, the nanopores on the AAO template prepared by two-step anodization method are uniform in size, ordered in arrangement and distributed independently. Furthermore, it can be seen from the Fig. 1b that the hexagonal pore size is 50 ± 10 nm. 3.2. Characterization of Ag@AAO The fabrication procedure for Ag@AAO was described schematically in Fig. 2. Ag nanorods were electrolessly deposited in the pores, in this process, the detailed growth mechanism of Ag nanorods is not only attributed to the transfer of electrons of Al (reducing agent) in the AAO template to Ag ions (oxidant) in the reaction solution, but also the redox potential of Al is generally higher than that of Ag, so Ag ions are deoxidized to form an Ag atom which attached to the Al layer. In the high temperature and pressure environment, metal ions are continuously reduced and grow along the template pores to form nanorods. It follows the reaction route as described below. To obtain specific structural characteristics and elements distributions of the as-synthesized Ag@AAO, FE-SEM, EDS and AFM analyses were carried out. As can be seen from Fig. 3a, Ag nanorods were
2.2. Preparation of AAO template Preparation of highly ordered porous AAO template was divided into three parts: First, high-purity aluminum sheet was subjected to high-temperature vacuum calcining at 500℃ for 4 h, and then washed with acetone, ethanol and deionized water in turn for 10 min; second, the above-mentioned processed aluminum sheet was cut into small round wafers and polished in perchloric acid and ethanol solutions with a volume ratio of 1: 9; and finally, using a secondary anodic oxidation process in 0.4 M oxalic acid under 40 V at 2–8℃ to obtain single-pass AAO products (only the bottom of the nanopore has an Al layer) with uniform pore arrangement. It should be noted that the working voltage must be kept constant throughout the anodizing process. 361
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Fig. 1. SEM images of the highly ordered porous AAO template.
uniformly distributed, highly ordered and had high filling rates. The diameter of the nanorods was 50 ± 10 nm, which was consistent with the pore size of the AAO template. Fig. 3b shows the picture of the Ag@AAO being treated with sodium hydroxide, and it was clear that the exposed length of Ag nanorods increased and the gap between adjacent nanorods was also small, so the surface area of Ag nanorods increases, which could be used to attach more AuNPs. It was evident in Fig. 4, the nanorod bundles were surrounded by many AuNPs, and on the nanorod bundles, a large number of AuNPs were continuously deposited, followed by aggregating to form many big Au spheres. And this special nanostructure made the sample surface rough enough and the surface area was significantly increased, when the light was irradiated on the sample, free electrons on the surface of the Ag nanorod interact with photons to produce collective oscillations (surface plasmon resonance), thereby enhancing the electromagnetic (EM) field [28,29]. At the same time, the SERS effect was closely related to the enhancement of the EM field of nanostructures [30], so the SERS enhanced effect was remarkable. For the enhancement effect, a finite difference time domain (FDTD) simulation was used to perform a preliminary calculation. The simulation model created by FDTD was based on SEM images (Fig. 4), where it was assumed that the AuNPs were uniformly distributed and were ideal spheres, and the diameters of nanorods and AuNPs were 60 and 10 nm, respectively. Then, FDTD simulation was performed with a monochromatic incident wavelength of 785 nm on the side and above of the nanorod bundles. All the simulations were described schematically in Fig. 5. The blue balls represented AuNPs, while the yellow and red area represented large EM fields.It was obvious that regardless of whether it was on the side or on the top, there were a certain EM field enhancement which mainly came from the gap, resulting in synergistic plasmonic enhancements. Moreover, the gaps between adjacent nanorod bundles provided SERS hot spots with local EM field enhancement, which contribute to the generation of the SERS signals, so the whole substrate would produce a huge SERS effect [31]. The EDS mapping analysis in Fig. 6(a) displays a homogeneous distribution of the three elements Ag, O, and Al on the two-dimensional projection of nanostructure chemical maps. The compositional mass
fractions of Ag, O, and Al in the Ag@AAO nanostructure arrays were 5.98%, 46.98%, and 47.04%, respectively. Besides, Fig. 6(b) shows XRD analysis of Ag nanorods, which demonstrates that the diffraction peaks were observed. All peaks located at ∼38.12°, ∼44.31°, ∼64.45°, ∼77.41° and ∼81.55°corresponded to the cubic Ag crystal planes of (111), (200), (220), (311) and (222), respectively. These two characterization demonstrated the presence of Ag elements in addition to Al and O, indicating that the nanowire composition was Ag. In addition, the AFM was further used to confirm the nanostructures of the uniformly distributed Ag@AAO. As Fig. 7(a, b) depicts, silver nanorods were evenly distributed and highly ordered. These results indicated that large-scale uniform Ag nanorod arrays can be synthesized by electroless deposition. 3.3. SERS detection of R6 G dye In order to test whether our substrates of AuNPs decorated Ag@AAO were effective or not for trace detection, the SERS performance of the synthesized nanosubstrate was evaluated by using R6 G as a probe molecule. And it is well known that major characteristic peaks of the R6 G molecule are observed at 611, 778, 1181, 1313, 1361, 1511, and 1651 cm−1, respectively. The vibrational modes corresponding to these main characteristic peaks were illustrated in Table 1 [32–34]. As is shown in Fig. 8a, comparing the SERS enhancement effects of blank AAO and AuNPs decorated Ag@AAO, it was apparent that the SERS intensity of the latter was stronger, about 5 times higher than that of blank AAO. According to previous research, sensitivity of SERS substrates was related to the microstructure of sample [21], the reason why the SERS of our sample exhibit higher enhancement was the formation of coarse micro-nano structures on the sample surface and the exposed length of clustered Ag nanorods with gaps were up to 300 nm, resulting in a huge surface area increase that R6 G molecules achieved adequate adsorption sites. On the one hand, the chemical interactions of R6 G molecules with nanorods surfaces resulted in chemical mechanisms (CM) enhancement; on the other hand, Ag nanorods with gaps generated numerous SERS “hot spots” since their surface plasmon resonance
Fig. 2. Schematic illustration of the synthesis process of Ag@AAO SERS substrates: (I) fabrication of AAO template; (II) formation of Ag@AAO; (III) Ordered arrays of Ag nanorods bundles achieved by sodium hydroxide etching. 362
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Fig. 3. SEM images: (a) the top view of Ag@AAO; (b) 30°-tilted side-view of the Ag@AAO being treated with sodium hydroxide.
Fig. 4. SEM images of AuNPs decorated Ag@AAO being treated with sodium hydroxide.
Fig. 5. FDTD simulated EM field distribution around a bundle of AuNPs decorated Ag@AAO under normal incidence with a wavelength of 785 nm: (a) Cross-section; (b) vertical section.
Fig. 6. (a) EDS mapping and EDS results of the Ag@AAO nanostructure arrays before sputtering. (b) XRD pattern of the Ag nanorods. 363
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Fig. 7. AFM images of Ag@AAO without sodium hydroxide treatment: (a) 2D picture; (b) 3D picture. Table 1 Major SERS bands and corresponding vibrational modes of R6 G. Peak position(cm−1)
Vibrational mode
611 778 1181 1313 1361、1511、1575、1651
CeCeC ring in-plane bending vibrations CeH out-of-plane bending vibrations CeH in-plane bending mode NeH in plane bending mode CeC stretching of the aromatic ring
contributes to the EM field enhancement, so CM and EM could synergistically enhance SERS signals [35]. Furthermore, we prepared different concentrations R6 G aqueous solutions from 10−4 to 10−16 M and collected the corresponding SERS spectra of them on the sample. As presented in Fig. 8b, it is clear that SERS spectra could be detected when the concentration of R6 G down to 10−16 M. This result revealed that as-prepared substrate had high sensitivity. Last but not least, we investigated the stability of our substrate via examining the Raman activity of it after three months of storage. The testing procedure included ultrasonic treatment of 60 s with acetone; then cleaning in alcohol and DI water followed by drying in ambient air; furthermore, we could repeat the above steps many times as necessary for cleaning well; at last, immersion in the same R6 G concentration to detect the SERS spectrum. Fig. 9 displays the substrate could still detect the SERS signal of R6 G after being placed for 3 months (curve 2b). Curve 2a was almost straight and similar to curve 1a, indicating that the sample was washed well. Therefore, the as-prepared Ag nanorod arrays substrate displays high SERS stability for detecting R6 G molecules.
Fig. 9. SERS signal for R6 G molecules (10−4 M): (1a) blank AAO as a reference; (1b) Au-decorated Ag@AAO; (2a) washed; (2b) Au-decorated Ag@AAO after R6 G re-deposition.
4. Conclusions In summary, we proposed a simple and novel method to prepare high-performance SERS substrates. In other words, the hexagonal AAO nanohole arrays served as a template for selective electroless deposition of Ag nanorods. Due to the spatial confinement effect of nanopores, the resultant Ag nanorods were uniformly distributed and arranged over a large area. Additionally, by etching Ag nanorod with aqueous sodium hydroxide solution, we can obtain Ag nanorod bundles which have
Fig. 8. (a) The Raman spectra of R6 G (10−4 M): curve i, blank AAO + Au; curve ii, Ag@AAO + Au; (b) SERS signals of R6 G with different concentrations. 364
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large hot spot adsorption areas, so Ag@AAO showed the highest SERS intensity, and when the concentration of the R6 G was as low as 10−16 M, the SERS signal could still be detected. We believe this unique synthesis method can be used to prepare other metallic materials with outstanding structure and used for many fields.
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