Surface-Nanostructured Single Silver Nanowire: A New One

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Surface-nanostructured single silver nanowire: a new one-dimensional microscale SERS interface Mengmeng Chen, Huanhuan Zhang, Yue Ge, Shuo Yang, Peijie Wang, and Yan Fang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02854 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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Surface-nanostructured single silver nanowire: a new one-dimensional microscale SERS interface Mengmeng Chen,† ,‡Huanhuan Zhang,† ,‡ Yue Ge,† Shuo Yang,† Peijie Wang,† Yan Fang*,† † The Beijing Key Laboratory for Nano-Photonics and Nano-Structures, Department of Physics, Capital Normal University, Beijing 100048, China.

KEYWORDS: Silver nanowire, Raman, Micro SERS interfaces, Hot spots, Charge transfer

ABSTRACT

One-dimensional microscale SERS-active interfaces have been intriguing as a newly emerging class of SERS interfaces compared to conventional macroscale SERS substrates. In this work, a stable surface-nanostructured single silver nanowire was fabricated. The nanostructures on the nanowire are formed by nanoscale silver crystal dots with diameters of 20-50 nm. The SERS signals of the crystal violet probe molecules adsorbed on the nanostructures are dramatically enhanced by both electromagnetic and chemical effects. The hot spots generated at the junctions of adjacent nanoscale dots yield highly efficient surface plasmon resonance. Simultaneously, the charge transfer on the atomic-scale silver cluster located at the nanostructured interface causes an enhancement similar to a Raman resonance. The intensity distributions of the SERS peaks on the

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surface-nanostructured single nanowire are characterized by SERS mapping. It is found that, although the intensities of the SERS peaks are different, their SERS mapping images show uniform SERS enhancement distributions, whereas the noticeable SERS intensity distributions on the single smooth silver nanowire are mainly located on the two ends of the nanowire. The large number of nanoscale crystal dots along with the atomic-scale silver clusters are uniformly and densely distributed on the surface of the single roughened nanowire; these structural attributes induce a uniform and large surface plasmon resonance and charge transfer enhancements on the entire surface of the nanowire. This work indicates that the surfacenanostructured single silver nanowire, synthesized using a quite simple preparation method, performs as an excellent one-dimensional microscale SERS substrate with uniform and high enhancement characteristics, which shows high potential for applications as a new class of SERS-active substrates. Furthermore, the higher enhancement factor of the microscale SERS interfaces can be achieved by introducing other roughened nanowires to assemble a dimer and a trimer as micro SERS substrates, which is consistent with the dark field (DF) measurements.

INTRODUCTION Surface-enhanced Raman scattering (SERS) has been intensively studied as the most sensitive in situ detection technique of molecular structures and interface effects due to the high enhancement factors up to 1015.1,2 This technique shows great promise for use in diverse applications in chemical and biological sensing,3,4 biomarkers,5,6 cancer therapy,7,8 trace analysis,9,10 optical data storage11 and even single molecule detection.12 Moreover, SERS interfaces with large enhancement factors can be applied to the detection of biomolecular structures and the interaction between drugs and biomolecules in a close to real-life environment


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because the SERS signals of water molecules are usually very weak and can be ignored.13 These enhancement characteristics seem to be superior to those of the conventional biomolecule detection methods. It is crucial to fabricate SERS-active metallic substrates with highly efficient enhancement for the effective applications of the SERS technique. A rather large number of novel SERS substrates have been synthesized with a variety of methods, such as metallic nanoparticles,14,15 nanowires,16,17 nanosheets,18 nanobelts,19 nanorods,20 nanocubes,21,22 nanodisk,23 aligned nanowires,24 core-shell nanoparticles,25,26 nanoraspberries,27 nanoroughened metal surfaces and electrodes,28 which have greatly and effectively expanded the applications of the SERS effect as well as promoted the development of surface plasmonics.29 However, these substrates are macroscale SERS substrates that were fabricated with the accumulation and aggregation of a large number of nanoparticles, nanowires and other nanomaterials forming macroscale SERS active interfaces. More recently, microscale SERS substrates that are micro sized with surfaces modified with nanostructures, have attracted attention as a newly emerging class of SERS substrates due to the unique microzone features and the great potential for optical data storage,30 micro catalysis,31 micro detection and analysis devices,32 micro-optical switching of micro electro mechanical systems (MEMS) and light coupling microdevices of integrated plasmonic circuits, etc. Research regarding SERS substrates has entered into a new age of fabricating and characterizing more complicated but highly significant microscale SERS substrates. Among these substrates, a one-dimensional microscale SERS substrate that is a single nanowire with a microscale length, a nanoscale diameter and a nanostructured interface, shows unique properties and distinct advantages over conventional SERS substrates. The surface plasmon resonance frequency and bandwidth, SERS enhancement factor, and the coupling of nanostructures on the entire nanowire surface can be modulated by


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controlling the length of the nanowire, the roughness and the nanostructure distribution. These attributes enable a typical microscale SERS substrate with better adaptability, more flexibility and more widespread applications. However, few studies regarding one-dimensional microscale SERS substrates have been reported to date. Even though there was a study on transferring nanoparticles to the surfaces of nanowires as the microscale SERS substrate reported,33 transferring nanoparticles to the surfaces of nanowires suffers from complex processes and severe conditions, and the nanoparticles that were only attached to the nanowires were unstable and easily shed, which considerably affected the stability of SERS enhancement effect. It is significant to fabricate one-dimensional microscale SERS substrate with a high enhancement factor,

stable

and

uniformly

distributed

nanostructured

interfaces,

good

reproducibility/reproducibility, and prepared using a quite simple preparation method, not only for a clear understanding and a theoretical simulation of the SERS mechanism but also for effective applications of the SERS effect. In this study, a stable surface-nanostructured single silver nanowire was fabricated. The TEM and HRTEM images reveal that the nanostructures on the nanowire are formed by nanoscale silver crystal dots. The SERS signals of the crystal violet (CV) probe molecules adsorbed on the silver nanostructured interface are dramatically enhanced. The SERS mapping images show uniform SERS enhancement distributions, which is in contrast to the case on a single smooth silver nanowire in which the noticeable SERS intensities are mainly localized on the two ends of the nanowire. The large number of nanoscale silver crystal dots accompanied with the atomic-scale silver clusters, are uniformly and densely distributed on the surface of the single roughened silver nanowire, which induce both the uniform and large surface plasmon resonance enhancement on the junctions of adjacent nanoscale crystal dots and the charge


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transfer enhancement at the atomic-scale cluster active-sites on the entire surface of the nanowire. This work indicates that the surface-nanostructured single silver nanowire, produced using a quite simple preparation method, performs as an excellent one-dimensional microscale SERS substrate with uniform and shows potential as a newly emerging class of SERS-active substrates.

EXPERIMENTAL SECTION Silver nanowires were synthesized by polyol reduction using polyvinylpyrrolidone (PVP) as the protecting agent.34 First, 5 ml ethylene glycol (EG) was refluxed in a three-necked roundbottomed flask at 160 ℃ for 20 min. Second, 5 ml of EG solution of 0.2 M AgNO3 and 5 ml of EG solution of 3 M PVP (MV=58000, Alfa Aesar) were mixed uniformly with continuous magnetic stirring, and a small amount of NaCl was added. The mixture was added dropwise to the refluxing EG solution over a period of 10 min, and then constantly heated at 160 ℃ for 90 min. Third, the silver nanowires in this solution were washed for further purification via centrifugation in acetone once to remove the EG and three times with ethanol to remove the surfactant PVP. Then, the precipitates and silver nanowires, were collected and dispersed into ethanol for further use. Silver nanowires were placed onto a silicon substrate; then, this silver nanowire substrate was immersed into aqua regia (hydrochloric acid: nitric acid = 3:1) for 10 s to form rough nanoscale interfaces, and was subsequently rinsed and washed with deionized water and then immersed in deionized water for 12 h to thoroughly rinse the remaining aqua regia on the silver nanowire substrate and dried. Finally, the cleaned and surface-nanostructured single silver nanowire and coupled nanowires were obtained. The solution of CV molecules was dropped onto


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the clean silicon surface, the smooth silver nanowire, the surface-nanostructured silver nanowire for Raman and SERS measurements. The morphology of the silver nanowires and coupled nanowires were observed by scanning electron microscopy (SEM: Hitachi S-4800), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) (Model JEOL, JEM2100). The micro Raman and SERS spectra were measured with a Renishaw H13325 spectrophotometer with an excitation wavelength at 532 nm. The power on the samples is 0.5 mW. The Raman mapping was recorded with excitation at 514 nm. The dark field (DF) scattering spectra were obtained with an inverted optical microscope equipped with a DF condenser.

RESULTS AND DISCUSSION Figure 1(a) shows a typical SEM image of a single smooth silver nanowire with a diameter of approximately 310 nm and a length of approximately 7 μm. The HRTEM images of the ends and the side of the silver nanowire (insets in Figure 1(a)) show lattice constants of approximately 0.24 nm and 0.20 nm that are assigned to the Ag{111} and Ag{200} facets,34,35 respectively, which demonstrate the high crystallinity of the single smooth silver nanowire. Figure 1(b) shows the SEM image of a surface-nanostructured single silver nanowire, revealing that the nanostructures are composed of raised granules like dots with diameters of 20-50 nm (top right inset in Figure 1(b)). These nanostructures are uniformly and densely distributed on the entire surface of the silver nanowire. Statistically the diameter of the roughened nanowire is slightly larger than that of the smooth nanowire due to a decrease in the surface tension after roughness. The corresponding HRTEM image of the silver nanostructures (top left inset in Figure 1(b))


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shows a lattice fringe distance of approximately 0.24 nm that is attributed to the Ag{111} facet, indicating that the silver nanocrystal dots on the nanowire retain the high crystallinity.

Figure 1. (a) The SEM image of a single smooth silver nanowire. The top left and right insets are the HRTEM images of the ends and the side surfaces of the nanowire, respectively. (b) The SEM image of a single roughened silver nanowire. The top right inset is the enlarged SEM image of a single roughened silver nanowire. The top left inset is the HRTEM image of the silver nanocrystal dots as the nanostructures on the nanowire. Figure 2(b) exhibits the SERS spectrum of the CV probe molecules adsorbed on the single smooth silver nanowire. The SERS enhancement factor (EF) is 2.6×102. The profile of this SERS spectrum is almost same as that of the normal Raman spectrum of the CV molecules (Figure 2(a)), i.e., there are no obvious changes in the SERS spectrum, such as the appearance of new peak and peak shift that might be related to a chemical effect are observed, except for the enhancement of SERS intensity. This behavior indicates that SERS enhancement is mainly derived from the surface plasmon resonance effect on the single smooth silver nanowire.


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Intensity (a.u.)

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-1 Wave number (cm )

Figure 2. (a) Raman spectrum of CV coated on the silicon surface. (b) SERS spectrum of CV adsorbed on the single smooth Ag nanowire (EF≈2.6×102). (c) SERS spectrum of CV on the surface-nanostructured single silver nanowire (EF≈3.7×104). As CV probe molecules are adsorbed on the surface-nanostructured single silver nanowire, the SERS signals are further enhanced (Figure 2(c)) compared to those on the single smooth silver nanowire. According to the hot spots theory,36-38 the densely distributed silver nanocrystal dots on the entire surface of the single roughened silver nanowire could induce a large number of hot spots on the junctions of adjacent dots, which yield highly efficient surface plasmon resonance enhancement. On the other hand, notably, there is a new strong peak at 237 cm-1 that overlaps with the original Raman peak of CV at 211 cm-1. This vibration is attributed to the N-Ag symmetric stretching mode, in which the lone pair electrons on the N atom of CV occupies the vacant sp hybrid orbitals of the silver atom forming the chemical N-Ag bond. These


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CV molecules are adsorbed on certain surface sites at an atomic-scale like the silver clusters acting as SERS active-sites39-43 on the surface-nanostructured single silver nanowire through NAg bonding, and electronic coupling of the adsorbed molecules with the silver surface takes place, i.e., the charge transfer occurs. This result is clear evidence of charge transfer between the adsorbed CV and the silver surface, which is associated with the chemical enhancement effect of SERS. Therefore, the SERS enhancement on the single roughened silver nanowire is not only from the surface plasmon resonance effect but also is from the charge transfer effect. Consequently, the surface-nanostructured single silver nanowire as a one-dimensional microscale SERS substrate exhibits a highly efficient enhancement that far exceeds that of the single smooth silver nanowire. The distribution uniformity of the SERS enhancements, including the surface plasmon resonance and charge transfer enhancements on the nanostructured interfaces, is one of the most important indicators of a SERS substrate, except for the highly efficient increase in SERS intensity. To characterize the SERS enhancement distribution on the surface-nanostructured single silver nanowire, the SERS mapping image which exhibits the intensity distribution of the Raman peak of CV at 913 cm-1 is measured by a color decoding method (Figure 3 (a)). The SERS intensity distribution on the single roughened silver nanowire is almost uniform and the images from the other Raman peaks intensities show the same distribution (see the supporting information), which is in contrast to the case on the single smooth silver nanowire in which the noticeable SERS intensity distributions are mainly located at the two ends (Figure 3 (b)). The densely and uniformly distributed nanostructures on the silver nanowire are composed of silver nanocrystal dots with diameters of 20-50 nm, which induce a large number of uniformly distributed hot spots at the junctions of adjacent dots, leading to the uniform and large


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enhancement of the local surface plasmon resonance on the entire surface of the single silver nanowire. Simultaneously, the atomic-scale silver clusters coexist with the silver nanocrystal dots as the SERS active-sites on which the CV probe molecules are chemadsorbed through N-Ag bonding and a charge transfer from the CV molecules to the silver surface occurs. Consequently, a large number of atomic-scale silver clusters are also uniformly and densely distributed on the surface of the single silver nanowire, leading to uniform chemical enhancement. Consequently, the surface-nanostructured single silver nanowire, prepared using a quite simple preparation method, performs as an excellent one-dimensional microscale SERS interface with uniform and large enhancements of the surface plasmon resonance at the hot spots and charge transfer at the active-sites. This nanowire shows high potential for applications as a new class of SERS-active substrates.

Figure 3. (a) SERS mapping image on the surface-nanostructured single silver nanowire using the intensity of the Raman peak at 913 cm-1. (b) SERS mapping image on the single smooth silver nanowire corresponding to the intensity of the Raman peak at 913 cm-1. The scale bar on the right of each image displays the color decoding scheme for different SERS intensities. The brighter the color of the bar, the higher the SERS enhancement.


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To further increase the SERS enhancement efficiency of the microscale SERS substrate, another silver nanowire is introduced to assemble a coupled nanowire interface. Figure 4(a) shows the side-by-side assembly of two roughened nanowires. To facilitate a direct comparison of the SERS intensity distributions between the coupled and uncoupled areas, a long nanowire and a short nanowire were assembled together. The corresponding SERS mapping image of CV shows the uniform and large enhancement in the coupled area, which obviously exceeds the intensity in the uncoupled area (Figure 4(c)), whereas the noticeable SERS intensity distribution areas of the coupled smooth nanowires (Figure 4(b)) are mainly focused at the ends of the short nanowire (Figure 4(d)). The profile of the SERS spectrum in the coupled area are similar to that of the SERS spectrum on the surface-nanostructured single silver nanowire (Figure 4(e)). Therefore, the coupled roughened silver nanowires system, as another microscale SERS substrate, can considerably improves the enhancement efficiency compared to the single roughened silver nanowire, indicating that a higher enhancement factor of the microscale SERS substrates can be achieved by introducing other roughened nanowires to assemble dimers and even trimers.


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Figure 4. (a) The SEM image of the coupled roughened silver nanowires. (b) The SEM image of the coupled smooth silver nanowires. (c) The SERS mapping image of the coupled roughened silver nanowires. (d) The SERS mapping image of the coupled smooth silver nanowires. (e) The corresponding SERS spectra, the upper line (red): the single roughened silver nanowire, and the lower line (black): the coupled roughened silver nanowires. (f) The DF scattering spectra, the upper line (red): the single roughened silver nanowire, and the lower line (black): the coupled roughened silver nanowires. Figure 4(f) is the dark field (DF) scattering spectra of the single roughened silver nanowire, which shows a broad surface plasmon resonance band with a full width at half


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maximum (FWHM) between 645 nm and 957 nm. The width of the FWHM can be modulated by controlling the length of nanowire. As another roughened silver nanowire is introduced to assemble the coupled nanowires interfaces, the surface plasmon resonance band is further strongly enhanced, which is consistent with the SERS mapping image in Figure 4. CONCLUSION A surface-nanostructured single silver nanowire, as a typical one-dimensional microscale SERS substrate, was fabricated. The silver nanowire still retains a high crystallinity after roughness. The nanostructures on the nanowire are composed of silver nanocrystal dots with diameters of 20-50 nm and atomic-scale silver clusters. The SERS signals of the CV probe molecules adsorbed on the silver nanostructures are dramatically enhanced by both electromagnetic and chemical effects, which far exceeds the SERS intensity on the single smooth silver nanowire. The hot spots generated at the junctions of adjacent nanocrystal dots yield highly efficient surface plasmon resonance enhancement. Simultaneously, the charge transfer at the atomic-scale silver clusters at the nanostructured interface create an enhancement similar to a Raman resonance. Furthermore, the intensity distributions of the SERS peaks at 237 cm-1, 913 cm-1, 1176 cm-1 and 1368 cm-1 on the surface-nanostructured single silver nanowire are characterized by SERS mapping. The SERS mapping images of these peaks show uniform SERS enhancement distributions although their SERS intensities are different, which contrasts to the case on the single smooth silver nanowire in which the noticeable SERS intensity distributions are mainly located at the two ends of the nanowire. The large number of nanoscale crystal dots along with the atomic-scale silver clusters are uniformly and densely distributed on the surface of the single


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roughened silver nanowire, which leads to the uniform and large surface plasmon resonance enhancement and charge transfer enhancement on the entire surface of the nanowire. This result indicates that the surface-nanostructured single silver nanowire, prepared by a quite simple preparation method, performs as an excellent one-dimensional microscale SERS interface with uniform and large enhancement. This nanowire shows great potential of being a newly emerging class of SERS-active substrates. It is further found that the higher enhancement factor can be achieved by introducing other roughened nanowires to assemble a dimer or a trimer as microscale SERS substrates, which is also demonstrated by the DF measurements.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. Author Contributions ‡ M. Chen and H. Zhang contributed equally to this work. ACKNOWLEDGMENT This work is supported by the Natural Science Foundation of China (Grant No. 21473115).

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Surface-Nanostructured Single Silver Nanowire: A New One

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