Signature of Plasmonic Nanostructures Synthesised by Electrical Exploding Wire Technique on Surface-Enhanced Raman Scattering

This work aims to fabricate two types of plasmonic nanostructures by electrical exploding wire (EEW) technique and study the effects of the different morphologies of these nanostructures on the absorption spectra and Surface-Enhanced Raman Scattering (SERS) activities, using Rhodamine 6G as a probe molecule. The structural properties of these nanostructures were examined using X-Ray diffraction (XRD). The morphological properties were examined using field emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (STEM). The absorption spectra of the mixed R6G laser dye (concentration 1×10 -6 M) with prepared nanostructures were examined by double beam UV-Vis Spectrophotometer. The Raman spectra of the R6G mixed with the prepared nanostructures were examined using a Horiba HR Evolution 800 Raman microscope system with an objective lens (50 ×). The FESEM and STEM images indicated that the Ag nanoparticles (AgNPs) with 35 nm average particle sizes were decorated on the surface of the AgNWs and the PDA layer by EEW technique, forming AgNW@AgNPs and AgNW@PDA@AgNPs nanostructures. The results indicated that the increased intensities of the absorption spectra peaks and the SERS arise from the hot spots and the roughness of the surface of nanostructures. The SERS enhancement factor of R6G (1×10 -6 M) was reached at 2.3×10 7 and 2.5×10 7 , at the wave number of 1650 cm -1 , for the AgNW@AgNPs and AgNW@PDA@AgNPs nanostructures, respectively, after being excited by (λexc. = 532 nm) laser source. It can be concluded that the AgNW@AgNPs and AgNW@PDA@AgNPs nanostructures were fabricated with an easy and simple way without the need for additional chemical compounds. These nanostructures attained a reliable and sensitive detection and can be utilized in a variety of SERS applications, such as chemical and biological sensors.


Introduction
Recently, fabrication of nanomaterials using physical techniques, especially those created using electro-exploding wire (EEW) technique has received remarkable consideration because of its high purity (no requirement for additives as those used in the chemical means) and fast preparation. In addition, the resulting nanoparticles retain the chemical identity of the original materials and are therefore capable of producing metal nanomaterials for any required chemical [1]. Nanoparticles can be synthesized through the EEW technique, while surrounded by a non-flammable liquid medium, such as deionized water. The particles evaporated during the explosion will condense into the liquid more efficiently than into the surrounding air. The properties of nanoparticles created by EEW rely upon numerous parameters, which include wires' material and diameter, features of the electrical circuit, and the surrounding medium [2].
Noble metal nanostructures show unique optical properties because of the excitation of localized plasmons [3], which have been widely used to control a variety of optical signals [4]. The optical properties enhancement, especially Surface Enhanced Raman Scattering (SERS), using metal nanostructures is considered one of the most efficient methods for ultra-sensitive chemical sensing of molecules adsorbed on the surface of metal nanostructure substrates. The active SERS substrates provide a large enhancement to Raman signals, which can be mainly attributed to two mechanisms; the first is the chemical mechanism based on the transfer of charge between molecules and metallic substrates. The second is the electromagnetic mechanism, which is based on the enhancement of local electromagnetic fields because of the resonance of surface plasmons stimulated by incident light [5]. The enhancement of large electromagnetic fields occurs in active positions (hot spots) on the metal's surface at a nanometre gap between two nanoparticles, due to coupling the Localized Surface Plasmon Resonance (LSPR) between them [6]. Therefore, it has been necessary to fabricate nanostructures, containing rough surface and having a large number of hot spots, which serve as effective substrates to enhance Raman spectrum. Therefore, a high density of AgNPs on the surface of AgNWs and the AgNW@PDA which served as SERS hot spots was designed and fabricated for increasing SERS intensity and target molecule detection (R6G dye molecules). As it is well known, the influence of hot spots depends on the size and shape of the nanoparticles and the space between two neighboring nanoparticles. Theoretical calculations showed that the spacing of neighboring nanoparticles on any surface can provide a strongly enhanced Raman signal, with the likelihood of having greatest enhancement of SERS in the presence of nanogaps between the metal nanostructures [7]. Several pioneering works verify that strong SERS intensity should occur when the wavelength of LSPR overlaps with the excitation laser wavelength [8]. Therefore, development of reliable as well as lowcost and simple preparation techniques of the noble metal nanostructures, which contain rough surfaces and active hot spot, are our priorities in this research.
At the present time, the noble metallic nanomaterials, such as gold, silver, and copper are widely used in the manufacture of active SERS substrates, due to noble metal nanoparticles that could generate a powerful local electric field with the excitation of incident light [9]. Compared to other noble metal nanomaterials, silver nanostructures are more widely utilized as an active SERS substrate through exploiting their wide range of plasmon resonance influences in the visible region, strong signal enhancement, and reliable synthesis technique [10]. So far, silver nanostructures with varied morphologies, such as nanospheres [11], nanorods [12], nanowires [13], and nanosheets [14], have been described as active SERS substrates that attain very sensitive molecular detection. Among the nanostructures mentioned above, the metal nanostructures with rougher surfaces (e.g. AgNPs decorated AgNWs forming metal-metal nanostructures) are utilized as active SERS structures in scientific researches or highly sensitive application fields [15]. These nanostructures allow increasing the electric fields resulting from the LSPR of AgNPs and their overlap with those from the SPR of AgNWs. Thus, they achieve high intensity for SERS [16].
Dopamine (DA) has plentiful catechol and amine active groups that exist on the surface of molecules. These groups could be employed as a multipurpose rostrum due to their distinguished properties, for example, anchor ability, chemical recognition, and self-polymerization [17]. Especially, the self-polymerization of DA could supply a facilitated and effective method to surface adjustment and create a very strong adhesive layer by immersing the material into the solution. More importantly, due to the existence of plenty of groups from catechol and amine on the surface, polydopamine (PDA) molecules could have fairly robust complexing behaviours with metallic ions, and they can automatically in situ reduce them into the metallic nanoparticles via oxidizing catechol into the correspondent quinine groups under alkaline aqueous solution [18].
At the present time, regular metal nanostructures with desired size and coating are deposited on all types of substrates, even including super hydrophobic surface, which supplied a promising opportunity to create sharp edges and nanogaps as controllable SERS "hot spots" on the AgNWs surface. Therefore, from these unique properties of PDA, an effective and multipurpose strategy was developed to controllably synthesize a series of AgNW@PDA@AgNPs nanostructures based on AgNWs that are coated with PDA molecules, which achieved highly a sensitive detection to SERS. By EEW technique, the AgNPs were decorated on the surface of PDA layer for the formation of metalinsulator-metal nanostructures, which allow increasing the electric fields resulting from the LSPR of AgNPs and their overlap with those from the SPR of AgNWs. For the nanostructure of AgNW@PDA@AgNPs, there were the two types of coupling localized surface plasmon-localized surface plasmon (LSP-LSP) and control localized surface plasmon-surface plasmon polariton (LSP-SPP) by PDA layer. The AgNW@PDA@AgNPs nanostructures could give a large number of "hot spots" and showed ultra-high sensitivity of SERS by the regulation of a synergistic effect between the AgNWs and AgNPs along the longitudinal axis. It could be possible by this fabrication strategy to demonstrate a large capability to fabricate varied-active substrates and attain reliable and sensitive detection in chemical and biological fields.

Experimental Section 1-Materials and characterization instruments
Rhodamine 6G (R6G) (C 28 H 31 N 2 O 3 Cl, molecular weight 479.02 g/Mol) was purchased from Exciton Chemical CO.INC. The dye solution was prepared by gravimetric method, where 0.47902 g of the dye powder was dissolved in 10 ml of deionized water to produce a solution of the R6G dye with a concentration of 1×10 -1 M. Then, by the dilution method, we obtained dye concentrations from 1×10 −1 to 1×10 −6 M). Silver nanowire (AgNW) was purchased from XFNANO with an average diameter of 90 nm and length of 1 micron. A weight of 500 mg was dissolved in 25 ml water, reaching a concentration of ≈ 0.2 M. Dopamine hydrochloride (tris-base, ≥ 99.9 %) was purchased from Sigma-Aldrich. Silver wire with a diameter of 0.3 mm and silver plate (3cm x 2cm x 3mm) at 99.9% purity were purchased from a jeweller shop in Baghdad-Iraq. Raman spectra of the prepared samples were recorded using a Horiba HR Evolution 800 Raman microscope system, after being excited with a 532 nm laser (objective lens 50 ×). The integration time was 8 seconds and the laser power was 1 mW.
Raman spectra of the R6G dye were measured for different concentrations after mixing it with the plasmonic nanostructures. These samples were examined after placing few drops of the mixture on a glass substrate and drying them under ambient condition. The absorption spectra of the mixed R6G with the prepared nanostructures were measured in different concentrations in the wavelength range of 300-800 nm at room temperature for all samples, using Shimadzu-1800 UV-VIS double beam spectrophotometer. The concentration of AgNPs prepared by EEW technique was measured using AA-7000 Atomic Absorption Spectrophotometer (Shimadzu). The details of internal nanostructure features were examined by XRD; few drops of the aqueous colloid of AgNPs, AgNWs, AgNW@AgNPs, AgNW@PDA and AgNW@PDA@AgNPs nanostructures were dried on glass substrate and XRD data were taken in the range of 2θ (10-90) degree. The XRD was performed with XRD 6000 X-ray Diffractometer (Shimadzu) using CuKα 1 with radiation of λ= 1. 5406 Å. The morphologies of the AgNWs and AgNW@AgNPs nanostructures were characterized with a fieldemission scanning electron microscope. The analysis was performed on samples deposited onto a rinsed silicon wafer and dried under ambient conditions. The particles size and morphologies of AgNPs, AgNW@PDA, and AgNW@PDA@AgNPs nanostructures were further characterized by STEM (FESEM, STEM Nova NanoSEM 450) with 1000 KV accelerating voltage.

2-Preparation of nanostructures 2.1-Preparation of AgNPs by EEW technique
A volume of 30 ml of deionized water was placed in a shock wave resistant container with magnetic stirring for the entire time of preparation at room temperature, as shown in Figure-1. Preparation of AgNPs was achieved by exploding a highly pure Ag wire (0.3 mm in diameter) against an Ag plate that was held under a voltage of 82.2 V respect to the wire and a steady current of 100A . The electric exploding of the wire was performed by touching the wire with the plate mechanically. The concentration of the prepared AgNPs was 2×10 -4 M, which was diluted to obtain other concentrations.

2.2-Preparation of AgNW@AgNPs nanostructures
To prepare the AgNWs decorated by AgNPs, 1.25 ml of aqueous colloid of the AgNWs (0.2 M) was dispersed by 30 ml deionized water. For the purpose of obtaining a homogeneous aqueous colloid, magnetic stirrer was used for 30 min at room temperature, followed by ultra-sonication for 15 min to ensure complete dispersion. The final aqueous colloid of AgNWs was placed in shock wave resistant container. Then, the same steps were used for forming AgNW@AgNPs nanostructures (e.g. metalmetal nanostructures).

2.3-Preparation of AgNW@PDA@AgNPs nanostructures
A volume of 1.25 ml of aqueous colloid of the AgNWs (0.2 M) was dispersed in 30 ml deionized water with magnetic stirring for 30 min at room temperature, then ultra-sonicated for 15 min for the purpose of ensuring complete dispersion. Then, 0.05 g of dopamine hydrochloride powder was added into the aqueous colloid of the AgNWs. The pH of the final reaction solution was adjusted to 8.5 by adding Tris-HCl buffer solution with a gentle stirring speed at room temperature for 24 hours. When the color of the aqueous colloid was started to become brown, this was an indication that the polydopamine has coated the AgNWs. After centrifuging the solution at 4000 rpm and washing with deionized water several times, the AgNWs@PDA were obtained and stored in deionized water to be used later. Then, the PDA-coated AgNWs were dispersed in 30 ml deionized water with ultrasonicating. The final aqueous colloid of the AgNWs@PDA was decorated by AgNPs using the EEW technique for forming AgNW@PDA@AgNPs nanostructures (e.g. metal-insulator-metalnanostructures).

2.4-Preparation of SERS substrates
The R6G (1×10 -6 M) dye was mixed with AgNW@AgNPs and AgNW@PDA@AgNPs nanostructures, respectively, then deposited on glass substrates and allowed to dry under the ambient conditions before examining the Raman spectrum, as illustrated in Figure- Figure-3 shows typical XRD patterns of AgNPs, AgNWs, AgNW@AgNPs, AgNW@PDA and AgNW@PDA@AgNPs nanostructures. The results demonstrated that the prepared AgNPs, AgNW@AgNPs and AgNWs@PDA@AgNPs nanostructures are having face-centred cubic phase for Ag metal. All patterns have five peaks with a slight difference in the peak location of each sample, which were observed at (2θ) degrees of 38.116 ο , 44.227 ο , 64.425 ο , 77.472 ο and 81.536 ο and have been indexed to hkl values of (111), (200), (220), (311) and (222), respectively. This indicates that no oxidation occurred during and after the preparation of AgNPs, AgNW@AgNPs and AgNW@PDA@AgNPs nanostructures by EEW technique and that the high purity and good crystalline nature were preserved. The peak position of the XRD pattern is similar to that of the standard bulk silver, which corresponded with the face-centred cubic structure of silver metal (identical with JCPDS Card No. 4-0783). Figure-4 demonstrates the morphology, structural information, and particle size of the AgNPs, AgNW@PDA and AgNW@PDA@AgNPs. As shows in Figure-4a, the average size of AgNPs was 35nm. The AgNPs were nearly spherical and slightly agglomerated. As clearly observed in Figure-4b, the PDA layer could be fixed onto AgNWs surface and the AgNWs was coated completely to form the AgNW@PDA nanostructure, where the AgNWs acted as the core and PDA formed the amorphous shell. Figure-4b demonstrates that the DA molecules coated the surface of AgNWs, with the creation of assembled functional PDA layer by self-polymerization. Figure-4c shows that the density and the distribution of AgNPs decorated on AgNW@PDA surface were random. The AgNPs were adhered to the AgNW@PDA surface very tightly to form the AgNW@PDA@AgNPs nanostructures. The PDA layer served as a spacer molecule (nanogap) to separate the two metals (AgNWs and AgNPs). The density of the AgNPs on the surface of the AgNW@PDA nanostructures depended on the number of explosions. The distance between the two the nanostructures was in the magnitude of several nanometers, which is the ideal distance for the existence of effective hot spots due to electromagnetic coupling, which depends on PDA thickness [11,19].

3-FESEM image analyses
Figure-5a illustrates that the AgNWs have a uniform straight morphology and smooth surface. In Figure-5b, the AgNW@AgNPs nanostructures displayed a relatively irregular morphology, which indicates that AgNWs surface was decorated by AgNPs. After performing the EEW technique, many silver nanoparticles appeared on the surface of AgNWs along the long axis, and all of the AgNWs were successfully decorated with AgNPs and formed a series of AgNW@AgNPs nanostructures. Also, it can be seen that the scattered AgNPs were adhered tightly and randomly to the AgNWs surface, forming an irregular rough surface that contains plentiful hot spots [20].  Figure-6 illustrates the UV-visible absorption spectra of AgNPs with different concentrations (2×10 -5 , 4×10 -5 , 6×10 -5 , 8×10 -5 , 1×10 -4 M); with and without R6G dye (1×10 -6 M). Surface plasmon resonance peaks were distinctive and their locations were almost constant. In Figure-6a, the SPR of the AgNPs appeared at 404 nm and the SPR peak intensity was increased with increasing AgNPs concentration. The increase in the intensity of SPR peaks is due to the increase in the resonance between the collective oscillations of the conduction electrons with incident electromagnetic field [21].

4-UV-visible absorption spectra
On the other hand, as shown in Figure-6b, the absorption peak position of R6G (1×10 -6 M) was at 525 nm. The absorption peak of the R6G dye was increased with increasing AgNPs concentration. The SPR peaks of the AgNPs were decreased after AgNPs had been mixed with the dye, because the energy is transferred from the nanoparticles to the dye [22,23].   Figure-7a, the SPR of the AgNWs has two peaks; the first appeared in 351 nm, due to the longitudinal plasmon resonance absorption of the AgNWs, and the second at 374 nm, due to the transverse plasmon resonance of AgNWs [24]. The intensity of these peaks was increased with increasing AgNWs concentration. On the other hand, as shown in Figure-6b, the R6G dye molecules have an absorption peak at 524 nm and the intensity of this peak was increased with increasing AgNWs concentration. This behaviour can be illustrated as follows; Plasmonic metal nanostructures were shown to be able to enhance optical processes. The enhanced effect of surface absorption is enhanced based on the excitation of localized surface plasmons giving the enhanced electromagnetic fields. Thus, the molecules placed inside this enhanced field (which is adsorbed on the nanostructures) will be excited more often due to the electromagnetic field enhancement of the incident light on the active substrate, which leads to improve the absorption [22]. with R6G dye (1×10 -6 M). Figure-8 illustrates that two observed distinctive SPR peaks appeared at about 351 nm and 373 nm in the absorption spectrum of AgNWs. The peaks nearly vanished when the AgNWs were coated with PDA layer by self-polymerization, as seen clearly in the absorption spectrum of AgNW@PDA in Figure-8a. The absorption peaks of AgNW@AgNPs and AgNW@PDA@AgNPs were clearly observed at 391 nm and 400 nm, respectively. They have wider and higher absorption peaks, indicating the formation of plentiful AgNPs on the surfaces of the AgNWs and the PDA layer. The enhancement of the electromagnetic field arises due to the coupling between the LSP of AgNWs and the dense hot spots of the AgNW@AgNPs that were formed by the decoration of AgNPs on the surface of AgNWs. While the nanostructures were constructed by AgNPs-decorated AgNW@PDA to form AgNW@PDA@AgNPs nanostructures that have nanogaps between AgNWs and AgNPs that have surface plasmon polariton (SPP). The electromagnetic field was enhanced remarkably due to the coupling of LSP-LSP and LSP-SPP [25]. Moreover, the closely adjacent AgNPs usually exhibited a collective LSP mode, whose electromagnetic field was much stronger than that of the non-adjacent AgNPs. Therefore, AgNW@AgNPs and AgNW@PDA@AgNPs nanostructures can absorb more energy in the UV-visible range. The intensity of the electromagnetic field was increased with decreasing the distance between nanoparticles. This phenomenon needs more research.
Two peaks of the AgNWs (at 351 and 373) became less obvious due to the overlap of the two absorption peaks with the absorption peak of the AgNPs after forming the nanostructures. An absorption peak of the nanostructures became wider when they were mixed with the R6G dye. As clearly seen in Figure-8b, the R6G dye molecules have absorption peaks at 524 nm and the intensity of this peak increased when R6G dye was mixed with AgNPs, AgNWs, AgNW@AgNPs and AgNW@PDA@AgNPs, with intensities of 0.237, 0.298, 1.1 and 1.128, respectively.
The SPR peaks of the AgNW@PDA@AgNPs nanostructures were enhanced more than those of the other nanostructures due to the notion that the SPR of metal nanostructures was largely sensitive to the dielectric environment around the surface morphology [26].  Figure-9 shows the SERS spectra of R6G molecules for five nanostructures. The intensities of R6G peaks at 612, 775, 1186, 1312, 1361, 1510, 1577 and 1650 cm -1 were clearly observed. It was observed that the AgNW@AgNPs and AgNW@PDA@AgNPs nanostructures showed more remarkable Raman signal enhancement compared with AgNW@PDA, AgNPs, and AgNWs. For the AgNW@PDA nanostructures, the PDA layer weakened the SERS signal of R6G in a certain limit. The AgNW@PDA@AgNPs nanostructures have plentiful hot spots, interstices, and nanogaps (among AgNPs and AgNWs), which were synergistically subscribed to the strong SERS activity. Moreover, such small interstices among neighboring AgNPs and nanogaps between AgNWs and AgNPs in AgNW@PDA@AgNPs nanostructures were useful to achieve a very strong SERS signal [25].

4-R6G dye Raman spectra
To quantify the SERS enhancement factors (EFs) of R6G dye on the AgNW@AgNPs and AgNW@PDA@AgNPs substrates, the peak at 1650 cm -1 was used to calculate the SERS EFs using the following equation [27]: where I SERS and I bulk represent the intensities of the R6G dye, at a wave number of 1650 cm -1 , adsorbed to the nanostructures and those dissolved as bulk in solution, respectively. The C SERS and C bulk represent the concentrations of R6G dye utilized for the SERS experiments and the Raman, respectively. To calculate the enhancement factor (EF), the highest peak of the Raman signal was taken from the data of a directly practical drawing in Figure-9. As a result, the ISERS values at the wave number of 1650 cm -1 , that is a vibration mode (C-C stretching) for the AgNW@AgNPs and AsNW@PDA@AgNPs nanostructures exhibited by the SERS spectra in Figure-9, were equal to 46497.4 and 50508, respectively. Because the fluorescence is the dominant feature of the luminescence of aqueous drops of R6G dye, it is not possible to assign an appropriate value to the Raman signal (I bulk ). Thus, we suppose a maximum value of (I bulk ) in the range of the detector noise of 200 counts (A Raman spectrum of the R6G dye in deionized water with concentration of 1×10 -1 M) [28,29]. The concentrations of R6G dye solution of 1×10 -1 and 1×10 -6 M were used for the (C bulk ) Raman without nanostructures and (C SERS ) experiments with nanostructures, respectively. As a result, the calculated EFs for the AgNW@AgNPs and AsNW@PDA@AgNPs nanostructures were 2.3×10 7 and 2.5×10 7 , respectively. The strong SERS signal enhancement of the electromagnetic field arises due to the coupling between the LSP of the AgNWs and the dense hot spots of the AgNW@AgNPs and AgNW@PDA@AgNPs nanostructures formed by the decoration of AgNPs on the surfaces of the AgNWs and AgNW@PDA. The AgNW@PDA@AgNPs nanostructure substrates have two types of coupling effect (LSP-LSP and control LSP-SPP) by PDA layer. For AgNW@AgNPs nanostructure, there exist the LSP-LSP coupling type and the non-controlled LSP-SPP coupling type [22,25]. Table  1 illustrates the assignment vibration modes corresponding to the wave number and the enhancement factor (EF) of R6G (1×10 -6 M) after being mixed with AgNW@PDA, AgNPs, AgNWs, AgNW@AgNPs and AgNW@PDA@AgNPs.

Conclusions
It can be concluded that EEW technique is a simple and efficient technique to fabricate nanostructures. The fabricated nanostructures can be used in the plasmonic substrates for Raman signal enhancement. The XRD patterns and the analysis of the images of FESTM and SETEM illustrated that the fabricated nanostructures have a rough surface and plentiful hot spots. The results indicated that the fabricated nanostructures have well absorption bands and excellent enhancement factor of R6G dye as probe molecules on the AgNW@AgNPs and AgNW@PDA@AgNPs nanostructures. It was verified that the AgNW@AgNPs and AgNW@PDA@AgNPs nanostructure substrates have the coupling effect of LSP-LSP and LSP-SPP, which was markedly related to the SERS enhancement of the R6G dye. Coupling of R6G dye molecules (as probe molecules) with fabricated nanostructures leads to the enhancement of many attractive optical and electronic properties utilized in a variety of applications such as chemical and biological sensors.