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Concentric microring structures containing gold nanoparticles for SERS-based applications
Applied Surface Science 497 (2019) 143752
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Concentric microring structures containing gold nanoparticles for SERSbased applications
T
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Evaldas Stankevičiusa, , Elena Daugnoraitėa, Ilja Ignatjevb, Zenonas Kuodisb, Gediminas Niaurab, Gediminas Račiukaitisa a b
Department of Laser Technologies, Center for Physical Sciences and Technology, Savanoriu 231, LT-02300 Vilnius, Lithuania Department of Organic Chemistry, Center for Physical Sciences and Technology, Sauletekio av. 3, LT-10257 Vilnius, Lithuania
A R T I C LE I N FO
A B S T R A C T
Keywords: Gold nanoparticle Multi-ring structure Surface enhanced Raman spectroscopy SERS Interference lithography
Concentric microring structures containing gold nanoparticles were fabricated using a Bessel-like beam array with the 30 μm and 60 μm periods. The fabricated structures coated with 5 nm thick gold layer demonstrate enhancement of Raman signals due to their induced localized plasmon resonance. The measured Raman signal intensity of self-assembled monolayers from thiols with terminal phenylalanine ring group with using various Raman spectrometers exhibit that concentric gold nanoparticles structures enhance the Raman signal from different size of analyzing area. The demonstrated method provides a facile fabrication of microring structures containing gold nanoparticles and shows their potential to exploit them for surface-enhanced Raman scattering (SERS) based applications.
1. Introduction Since the high enhancement of Raman cross-sections has been demonstrated using a roughened noble metal surface [1], a plenty type of substrates has been discovered to enable surface-enhanced Raman spectroscopy [2] such as metal island films [3], metal film over nanospheres [4], aggregated noble metal colloids [5–7], particles grafted on silanized glasses [8], anisotropic metal nanoparticles [9], regular holes in thin noble metal films [10] regular arrays of nanoparticles [11–14], and concentric ring structures [15–18]. The SERS enhancement in the arrays of noble metal nanoparticles relies on both the properties of the constitutive elements of nanoparticle arrays such as composition, roughness, shape and size of nanoparticles, as well as the geometric characteristics of the whole array of nanoparticles, such as array size, period, and geometry [12,14,19]. When the nanoparticles are split up by small gaps (less than λ/(2π), where λ is the coupling wavelength), the strong quasi-static near-field interactions dominate the response of the array [14]. Therefore, the localized modes with highly enhanced local fields are excited. Theoretical [20,21] and experimental studies [22,23] have shown that high enhancement of the electromagnetic field appears in the intersection between individual nanoparticles, where the increase of E-field can be orders of magnitude larger than on single nanoparticles [14]. Ring-type architecture is hugely desirable because of the high
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degrees of symmetry [24,25]. Concentric multi-ring structures compared to single-ring structures have higher degrees of freedom and can create hybridized plasmon resonances [17,26,27]. Tightly packed ringtype monolayers of metallic nanoparticles with micron-scale widths and millimeter-scale lengths can trigger localized surface plasmon resonance, which leads to the enhancement of the optical field due to the concentration of the optical energy in the space among nanoparticles [28]. The important advantage of concentric microrings is a more uniform distribution of localized enhanced electric field [18]. Therefore, concentric-ring structures containing metallic nanoparticles can be useful substrates for surface-enhanced Raman scattering (SERS) [28]. A diversity of ring-shape micro−/nanostructures containing metallic nanoparticles have been created using different techniques such as solvent evaporation leading to dry-hole formation [29,30], particle deposition at the edges of a nanoparticle-rich droplet [31], self-organization [32,33], biopolymer template [34], templated self-assembly of gold-containing block copolymer micelles [35], and metal vapor deposition onto a silicone oil surface [36]. Of all these techniques, selfassembly of colloidal nanoparticles is especially attractive due to its cost-effectiveness and the credibility for mass production [28,37]. However, the fabrication of multiple concentric-ring nanostructures using self-assembly is challenging. A simple laser-based fabrication method of concentric-ring nanostructures has already been demonstrated in our previous work [38]. This method is based on the Bessel-
Corresponding author. E-mail address: [email protected] (E. Stankevičius).
https://doi.org/10.1016/j.apsusc.2019.143752 Received 12 June 2019; Received in revised form 14 August 2019; Accepted 21 August 2019 Available online 22 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 497 (2019) 143752
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Fig. 1. Formation of microring structures consisting of Au NPs: a) preparation of the sample; b) treatment of the sample with 30 μm and 60 μm period polymeric structure; c–f) the fabricated microring structures manufactured using different period (30 μm (c, d) and 60 μm (e, f)) polymeric structures (shown in b). These structures were coated with 5 nm thick gold layer after treatment process; g) the transmittance spectra of different period (red curve – 30 μm, blue curve - 60 μm) structures and unstructured area coated with uniform layer of Au NPs and 5 nm gold film (black curve) when reference spectrum is a glass substrate; h) the structural chemical formula of MOPHE compound used in SERS measurements. Scale bars in all pictures except a) represent 30 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
simple method provides the possibility for realization of SERS substrates with a significant and reproducible signal enhancement for diagnostic applications.
like beam generation using polymeric microstructures, which were fabricated using laser interference lithography [39,40]. In this work, we show that concentric-ring gold nanoparticles structures coated with a thin gold layer enhance the Raman signal more than two times better than the unstructured gold nanoparticles with the same thin gold layer. The enhancement of Raman signal depends on the laser treatment parameters used in the fabrication of gold microring structures. This
2. Materials and methods The process of microring structures array containing gold 2
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Fig. 2. Microring structures of 60 μm (a) and 30 μm (b) periods with marked places (red circles) where Raman signal was measured; c) and d) Raman spectra measured at marked places in a) and b), respectively; e) and f) measured maximum Raman intensity of the MOPHE compound peak at marked places in a) and b), respectively; the control sample was an unstructured gold nanoparticles sample coated with 5 nm thick gold layer. Scale bars in a) and b) represent 10 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nanoparticles (Au NPs) formation is depicted in Fig. 1. At first, Au NPs were produced using laser ablation of the gold target in the deionized and distilled water. The average size of the prepared Au NPs was about 30 nm. The generated Au NPs were spread on the silanized glass substrate by immersing the glass sample into the solution of Au NPs for 72 h. Afterward, the silanized glass was rinsed by distilled water and dried up at the room temperature. The typical Au NPs distribution on the prepared sample is shown in Fig. 1a. The silanized glass with Au NPs was processed by laser irradiation using Bessel-like beamlets, which were created using self-made polymeric structures with 30 μm and 60 μm period (Fig. 1b) [39]. The polymeric microstructures were illuminated by a Gaussian beam of the Atlantic HE (Ekspla) laser using different average laser power (0.8 W, 0.6 W, and 0.4 W) with 1 kHz repetition rate at 532 nm wavelength. The duration of laser pulses in all experiments was ~300 ps. A silanized glass substrate with Au NPs was located at ~350 μm distance from the polymeric microstructures (shown in Fig. 1b). After laser irradiation of the prepared sample by the
Bessel-like beamlets, an arrangement of gold nanoparticles into concentric circles, which correspond to intensity minima of generated beamlets, was obtained. Afterward, Au NPs containing microring structures were coated with 5 nm thick gold layer using a commercial magnetron sputter Quorum Q150T. The thin metallic film layer increases the efficiency of SERS substrates due to higher excitation of localized plasmons [41]. The microring structures array containing Au NPs and covered with 5 nm thick gold layer are shown in Fig. 1c–f. The morphology of produced microring structures depends on the used polymeric structures (Fig. 1c–f). As can be seen from Fig. 1c–f, structures fabricated with larger period polymeric structures has a larger number of rings containing gold nanoparticles. Arrangements with a larger number of rings are more suitable for SERS applications as they have more rough metal surfaces which are very important in the enhancement of Raman scattering by the adsorbed molecules on rough metallic nanostructures. The fabricated structures exhibit the plasmonic properties, which 3
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obtained from the area larger than the size of fabricated microring structures (shown in Fig. 3a). Raman spectra obtained with RamanFlex 400 spectrometer are shown in Fig. 3c. In this case, Raman spectra were measured for the 60 μm period microring structures which were fabricated using different average laser power settings: 0.4 W, 0.6 W, and 0.8 W. Other laser processing parameters were the same for all fabricated microring structures (wavelength 532 nm; repetition rate 1 kHz; pulse duration ~300 ps). All structures after the treatment were coated with 5 nm thick gold layer. The results show that the microring structures fabricated with the 0.4 W laser power have the highest enhancement of Raman signal intensity (Fig. 3c,d) (more than two times higher compared with structures manufactured with higher laser power). Structures fabricated with higher laser power have similar enhancement of Raman signal intensity to control sample (unstructured Au NPs with 5 nm thick gold layer). According to Kuchmizhak FDTD calculations performed for the porous and solid microring structures, the smooth microring structure containing from solid gold has a lower enhancement of SERS signal compared to the porous microring structures [44]. Furthermore, nanosized surface features produce multiple strongly enhanced E-fields, homogeneously distributed along the microring surface independently of the polarization direction, while the Efield enhancement in the solid gold microring structure follows the polarization direction and is distributed non-uniformly along the ring circumference, yielding similar distribution of the SERS signal. These theoretical simulations explain the results given above. The structures fabricated with 0.8 W laser power have higher smoothness compared with structures manufactured with lower laser power. By using the higher laser power, the intensity in the minimum intensity area (Fig. 3b) is enough to melt Au NPs in this area. The melting/solidification process leads to the fusion of Au NPs in the Bessel-like beam minimum intensity area [38]. Therefore, the microring structures fabricated with higher laser power consist of larger Au NPs and have higher smoothness, which leads to the lower enhancement of Raman signal. The average SERS enhancement factor (EF) values of multiring structures were calculated similarly as previously reported [45–47] assuming the formation of the complete self-assembled monolayer at Au NPs. The EF values were estimated by comparing the intensity of ν12 mode of MOPHE compound in SERS (ISERS) and ordinary Raman solution (IRS) spectra: EF = (ISERS/Nsurf)/(IRS/Nsol), where Nsurf and Nsol are the number of molecules probed in surface and solution spectra, respectively. The concentration of MOPHE compound in ethanolic solution was 0.1 M. The Nsurf and Nsol values were estimated correspondingly as reported previously [44,46] assuming similar surface density for benzethiol and MOPHE molecules. It should be noted that Nsurf = N'surf × n, where N'surf and n are the number of molecules on flat surface and surface roughness factor, respectively. We have not estimated experimentally the n value; however, assuming that the nanostructured surface consists of closely spaced hemispheres, the n value would be 2 [48]. This n was adopted for EF calculation. The highest EF value for our studied multiring structures was found to be 0.8 × 106. The obtained EF is lower comparing with previously reported value for coupled-concentric ring structures (1.67 × 107) [48] or concentric necklace nanolenses (1.3 × 107) [49] and similar with average Raman enhancement value of isolated dimer in a necklace (0.7 × 106) [49] and conventional SERS EF (106) [47]. It should be noted that dimensions of initial nanoparticles used in our SERS experiments (40 nm) are not optimized for 785 nm excitation wavelength; higher enhancement factor is expected for larger nanoparticles [50]. The important novelty of our work comparing with published concentric microring SERS studies is easiness in the fabrication of microring nanostructures.
can be observed in transmittance spectra of different period structure (Fig. 1g). The transmittance spectra were measured by using a microscope Nikon Eclipse LV100 and spectrometer Avant-AvaSpec ULS2048 when reference spectrum was a clean glass substrate. It means that 100% corresponds to a transmittance of the clean glass substrate. The area from which the transmittance spectra were taken was ~0.45 mm2. In Fig. 1g is shown the transmittance spectrum of unstructured area coated with a uniform layer of Au NPs and 5 nm gold film (black curve), and the transmittance spectra of different period structure (30 μm – red curve and 60 μm – blue curve). All given transmittance spectra have a minimum in the range of 540–570 nm. This part is responsible for a surface plasmon resonance caused by collective oscillation of electrons on the gold nanoparticle surface and resulting in the strong extinction of light (absorption and scattering). In the range of 600–800 nm, the light transmission is higher for the structured surfaces comparing to the random distributed Au NPs (Fig. 1g). This result is related to the ablation of nanoparticles as gold has high reflectivity for wavelengths longer than 600 nm. The decrease of Au NPs covered area increases the light transmission in this range. The increase of light transmission in the range of 600–800 nm is higher for the 60 μm period structures compared to the 30 μm period structures due to a lower density of Au NPs in 60 μm period structures (Fig. 1c–f). For SERS measurements, 8-mercapto-N-phenethyloctanamide (MOPHE) compound (synthesized in our laboratory) was used to form self-assembled monolayers (SAMs) on our nanostructured surfaces. Because of the presence of the intrachain amide group, SAMs of MOPHE on the gold surface are incredibly stable and well-organized [42]. Also, these monolayers are useful for probing the Raman enhancement effect because the terminal phenylalanine ring possesses characteristic intense ν12 mode at 1002–1004 cm−1 [43]. The sample with microring structure was immersed in an ethanol solution containing 1 mM MOPHE for about 16 h and afterward was washed with ethanol solution and was dried at room temperature under N2 slow flow. The structural formula of MOPHE compound is shown in Fig. 1h. Molecules of this compound form a monolayer on the surface of the sample. In this case, the hydrogen atom is replaced by a gold atom to form a bond with sulfur. 3. Results and discussion SERS measurements were performed using Renishaw InVia and RamanFlex 400 spectrometers with an excitation wavelength of 785 nm by using 0.9 and 50 mW laser power, respectively. The Renishaw InVia spectrometer collected Raman spectra from the area about 2.4 μm in diameter (red circle in Fig. 2a,b) and RamanFlex 400 from the area about 200 μm in diameter (pink circle in Fig. 3b). By using Renishaw InVia spectrometer, the microring structures with 30 μm and 60 μm period were tested. It was measured in various areas of structures by moving the sample with a 5 μm step. Altogether, five different places were measured for 60 μm period microring structures and four different locations for 30 μm period microring structures. The measured areas are marked in Fig. 2a,b. The results show that the maximum Raman intensity is the highest at the edge of the rings, but the enhancement of the Raman signal in the center part of microring structures due to the concentration of the surface plasmons energy in the presence of concentric rings is also observed [28]. The increase of Raman signal intensity in various parts of microring structures fluctuates between 2 and 4 times compared with the control sample (Fig. 2c–f). The control sample was an unstructured gold nanoparticles sample coated with a 5 nm thick gold layer. The results show that the assembled concentric gold microring structures can significantly enhance SERS signals due to their induced localized plasmon resonance. Local enhancement of Raman signal from different part of microring structures was confirmed by the previous experiments using the Renishaw InVia spectrometer. In this case, the Raman signal was obtained from the area smaller than the size of fabricated microring structures. By using RamanFlex 400 spectrometer, Raman signal was
4. Conclusions Concentric microring structures containing gold nanoparticles were fabricated using a Bessel-like beam array with 30 μm and 60 μm period. 4
Applied Surface Science 497 (2019) 143752
E. Stankevičius, et al.
Fig. 3. a) Microring structure containing Au NPs coated with 5 nm thick gold layer. Marked area (pink circle) show analyzing area using RamanFlex 400 spectrometer; b) Bessel-like beam intensity distribution behind the self-made polymeric structure; c) Raman spectra measured for Au NPs microring structure with period 60 μm which were fabricated using 0.4 W, 0.6 W, 0.8 W average laser power; d) measured maximum Raman intensity at MOPHE compound peak for microring structures fabricated with 0.4 W, 0.6 W, 0.8 W average laser power. The control sample was an unstructured gold nanoparticles sample coated with a 5 nm thick gold layer. Scale bar in a) represents 30 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The fabricated structures coated with 5 nm thick gold layer demonstrate enhancement of SERS signals due to their induced localized plasmon resonance. The measured Raman signal intensity of MOPHE compound using different Raman spectrometers exhibit that concentric microring structures containing gold nanoparticles enhance the Raman signal. The enhancement of Raman signal depends on the laser treatment parameters used in the fabrication of gold microring structures. Microring structures manufactured with 0.4 W laser power exhibit more than two times higher enhancement of Raman signal compared with microring structures fabricated with higher laser power due to higher roughness. The higher smoothness of structures manufactured with higher laser pulse energy is determined by the fusion of gold nanoparticles in the Bessel-like beam minimum intensity area due to sufficient energy to melt gold nanoparticles in this area. Finally, not only gold nanoparticles can be used in our demonstrated fabrication approach to assemble nanoparticles in concentric microring structures, but it is also applicable to other metals or even nonmetals like polymers or semiconductors nanoparticles.
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Acknowledgments [11]
E. Stankevičius thanks Lithuanian Academy of Sciences for the financial support under the Young Scientist Scholarship. G.N. gratefully acknowledges the Center of Spectroscopic Characterization of Materials and Electronic/Molecular Processes (SPECTROVERSUM Infrastructure) for the use of Raman spectrometers.
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Full length article
Concentric microring structures containing gold nanoparticles for SERSbased applications
T
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Evaldas Stankevičiusa, , Elena Daugnoraitėa, Ilja Ignatjevb, Zenonas Kuodisb, Gediminas Niaurab, Gediminas Račiukaitisa a b
Department of Laser Technologies, Center for Physical Sciences and Technology, Savanoriu 231, LT-02300 Vilnius, Lithuania Department of Organic Chemistry, Center for Physical Sciences and Technology, Sauletekio av. 3, LT-10257 Vilnius, Lithuania
A R T I C LE I N FO
A B S T R A C T
Keywords: Gold nanoparticle Multi-ring structure Surface enhanced Raman spectroscopy SERS Interference lithography
Concentric microring structures containing gold nanoparticles were fabricated using a Bessel-like beam array with the 30 μm and 60 μm periods. The fabricated structures coated with 5 nm thick gold layer demonstrate enhancement of Raman signals due to their induced localized plasmon resonance. The measured Raman signal intensity of self-assembled monolayers from thiols with terminal phenylalanine ring group with using various Raman spectrometers exhibit that concentric gold nanoparticles structures enhance the Raman signal from different size of analyzing area. The demonstrated method provides a facile fabrication of microring structures containing gold nanoparticles and shows their potential to exploit them for surface-enhanced Raman scattering (SERS) based applications.
1. Introduction Since the high enhancement of Raman cross-sections has been demonstrated using a roughened noble metal surface [1], a plenty type of substrates has been discovered to enable surface-enhanced Raman spectroscopy [2] such as metal island films [3], metal film over nanospheres [4], aggregated noble metal colloids [5–7], particles grafted on silanized glasses [8], anisotropic metal nanoparticles [9], regular holes in thin noble metal films [10] regular arrays of nanoparticles [11–14], and concentric ring structures [15–18]. The SERS enhancement in the arrays of noble metal nanoparticles relies on both the properties of the constitutive elements of nanoparticle arrays such as composition, roughness, shape and size of nanoparticles, as well as the geometric characteristics of the whole array of nanoparticles, such as array size, period, and geometry [12,14,19]. When the nanoparticles are split up by small gaps (less than λ/(2π), where λ is the coupling wavelength), the strong quasi-static near-field interactions dominate the response of the array [14]. Therefore, the localized modes with highly enhanced local fields are excited. Theoretical [20,21] and experimental studies [22,23] have shown that high enhancement of the electromagnetic field appears in the intersection between individual nanoparticles, where the increase of E-field can be orders of magnitude larger than on single nanoparticles [14]. Ring-type architecture is hugely desirable because of the high
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degrees of symmetry [24,25]. Concentric multi-ring structures compared to single-ring structures have higher degrees of freedom and can create hybridized plasmon resonances [17,26,27]. Tightly packed ringtype monolayers of metallic nanoparticles with micron-scale widths and millimeter-scale lengths can trigger localized surface plasmon resonance, which leads to the enhancement of the optical field due to the concentration of the optical energy in the space among nanoparticles [28]. The important advantage of concentric microrings is a more uniform distribution of localized enhanced electric field [18]. Therefore, concentric-ring structures containing metallic nanoparticles can be useful substrates for surface-enhanced Raman scattering (SERS) [28]. A diversity of ring-shape micro−/nanostructures containing metallic nanoparticles have been created using different techniques such as solvent evaporation leading to dry-hole formation [29,30], particle deposition at the edges of a nanoparticle-rich droplet [31], self-organization [32,33], biopolymer template [34], templated self-assembly of gold-containing block copolymer micelles [35], and metal vapor deposition onto a silicone oil surface [36]. Of all these techniques, selfassembly of colloidal nanoparticles is especially attractive due to its cost-effectiveness and the credibility for mass production [28,37]. However, the fabrication of multiple concentric-ring nanostructures using self-assembly is challenging. A simple laser-based fabrication method of concentric-ring nanostructures has already been demonstrated in our previous work [38]. This method is based on the Bessel-
Corresponding author. E-mail address: [email protected] (E. Stankevičius).
https://doi.org/10.1016/j.apsusc.2019.143752 Received 12 June 2019; Received in revised form 14 August 2019; Accepted 21 August 2019 Available online 22 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Formation of microring structures consisting of Au NPs: a) preparation of the sample; b) treatment of the sample with 30 μm and 60 μm period polymeric structure; c–f) the fabricated microring structures manufactured using different period (30 μm (c, d) and 60 μm (e, f)) polymeric structures (shown in b). These structures were coated with 5 nm thick gold layer after treatment process; g) the transmittance spectra of different period (red curve – 30 μm, blue curve - 60 μm) structures and unstructured area coated with uniform layer of Au NPs and 5 nm gold film (black curve) when reference spectrum is a glass substrate; h) the structural chemical formula of MOPHE compound used in SERS measurements. Scale bars in all pictures except a) represent 30 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
simple method provides the possibility for realization of SERS substrates with a significant and reproducible signal enhancement for diagnostic applications.
like beam generation using polymeric microstructures, which were fabricated using laser interference lithography [39,40]. In this work, we show that concentric-ring gold nanoparticles structures coated with a thin gold layer enhance the Raman signal more than two times better than the unstructured gold nanoparticles with the same thin gold layer. The enhancement of Raman signal depends on the laser treatment parameters used in the fabrication of gold microring structures. This
2. Materials and methods The process of microring structures array containing gold 2
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Fig. 2. Microring structures of 60 μm (a) and 30 μm (b) periods with marked places (red circles) where Raman signal was measured; c) and d) Raman spectra measured at marked places in a) and b), respectively; e) and f) measured maximum Raman intensity of the MOPHE compound peak at marked places in a) and b), respectively; the control sample was an unstructured gold nanoparticles sample coated with 5 nm thick gold layer. Scale bars in a) and b) represent 10 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nanoparticles (Au NPs) formation is depicted in Fig. 1. At first, Au NPs were produced using laser ablation of the gold target in the deionized and distilled water. The average size of the prepared Au NPs was about 30 nm. The generated Au NPs were spread on the silanized glass substrate by immersing the glass sample into the solution of Au NPs for 72 h. Afterward, the silanized glass was rinsed by distilled water and dried up at the room temperature. The typical Au NPs distribution on the prepared sample is shown in Fig. 1a. The silanized glass with Au NPs was processed by laser irradiation using Bessel-like beamlets, which were created using self-made polymeric structures with 30 μm and 60 μm period (Fig. 1b) [39]. The polymeric microstructures were illuminated by a Gaussian beam of the Atlantic HE (Ekspla) laser using different average laser power (0.8 W, 0.6 W, and 0.4 W) with 1 kHz repetition rate at 532 nm wavelength. The duration of laser pulses in all experiments was ~300 ps. A silanized glass substrate with Au NPs was located at ~350 μm distance from the polymeric microstructures (shown in Fig. 1b). After laser irradiation of the prepared sample by the
Bessel-like beamlets, an arrangement of gold nanoparticles into concentric circles, which correspond to intensity minima of generated beamlets, was obtained. Afterward, Au NPs containing microring structures were coated with 5 nm thick gold layer using a commercial magnetron sputter Quorum Q150T. The thin metallic film layer increases the efficiency of SERS substrates due to higher excitation of localized plasmons [41]. The microring structures array containing Au NPs and covered with 5 nm thick gold layer are shown in Fig. 1c–f. The morphology of produced microring structures depends on the used polymeric structures (Fig. 1c–f). As can be seen from Fig. 1c–f, structures fabricated with larger period polymeric structures has a larger number of rings containing gold nanoparticles. Arrangements with a larger number of rings are more suitable for SERS applications as they have more rough metal surfaces which are very important in the enhancement of Raman scattering by the adsorbed molecules on rough metallic nanostructures. The fabricated structures exhibit the plasmonic properties, which 3
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obtained from the area larger than the size of fabricated microring structures (shown in Fig. 3a). Raman spectra obtained with RamanFlex 400 spectrometer are shown in Fig. 3c. In this case, Raman spectra were measured for the 60 μm period microring structures which were fabricated using different average laser power settings: 0.4 W, 0.6 W, and 0.8 W. Other laser processing parameters were the same for all fabricated microring structures (wavelength 532 nm; repetition rate 1 kHz; pulse duration ~300 ps). All structures after the treatment were coated with 5 nm thick gold layer. The results show that the microring structures fabricated with the 0.4 W laser power have the highest enhancement of Raman signal intensity (Fig. 3c,d) (more than two times higher compared with structures manufactured with higher laser power). Structures fabricated with higher laser power have similar enhancement of Raman signal intensity to control sample (unstructured Au NPs with 5 nm thick gold layer). According to Kuchmizhak FDTD calculations performed for the porous and solid microring structures, the smooth microring structure containing from solid gold has a lower enhancement of SERS signal compared to the porous microring structures [44]. Furthermore, nanosized surface features produce multiple strongly enhanced E-fields, homogeneously distributed along the microring surface independently of the polarization direction, while the Efield enhancement in the solid gold microring structure follows the polarization direction and is distributed non-uniformly along the ring circumference, yielding similar distribution of the SERS signal. These theoretical simulations explain the results given above. The structures fabricated with 0.8 W laser power have higher smoothness compared with structures manufactured with lower laser power. By using the higher laser power, the intensity in the minimum intensity area (Fig. 3b) is enough to melt Au NPs in this area. The melting/solidification process leads to the fusion of Au NPs in the Bessel-like beam minimum intensity area [38]. Therefore, the microring structures fabricated with higher laser power consist of larger Au NPs and have higher smoothness, which leads to the lower enhancement of Raman signal. The average SERS enhancement factor (EF) values of multiring structures were calculated similarly as previously reported [45–47] assuming the formation of the complete self-assembled monolayer at Au NPs. The EF values were estimated by comparing the intensity of ν12 mode of MOPHE compound in SERS (ISERS) and ordinary Raman solution (IRS) spectra: EF = (ISERS/Nsurf)/(IRS/Nsol), where Nsurf and Nsol are the number of molecules probed in surface and solution spectra, respectively. The concentration of MOPHE compound in ethanolic solution was 0.1 M. The Nsurf and Nsol values were estimated correspondingly as reported previously [44,46] assuming similar surface density for benzethiol and MOPHE molecules. It should be noted that Nsurf = N'surf × n, where N'surf and n are the number of molecules on flat surface and surface roughness factor, respectively. We have not estimated experimentally the n value; however, assuming that the nanostructured surface consists of closely spaced hemispheres, the n value would be 2 [48]. This n was adopted for EF calculation. The highest EF value for our studied multiring structures was found to be 0.8 × 106. The obtained EF is lower comparing with previously reported value for coupled-concentric ring structures (1.67 × 107) [48] or concentric necklace nanolenses (1.3 × 107) [49] and similar with average Raman enhancement value of isolated dimer in a necklace (0.7 × 106) [49] and conventional SERS EF (106) [47]. It should be noted that dimensions of initial nanoparticles used in our SERS experiments (40 nm) are not optimized for 785 nm excitation wavelength; higher enhancement factor is expected for larger nanoparticles [50]. The important novelty of our work comparing with published concentric microring SERS studies is easiness in the fabrication of microring nanostructures.
can be observed in transmittance spectra of different period structure (Fig. 1g). The transmittance spectra were measured by using a microscope Nikon Eclipse LV100 and spectrometer Avant-AvaSpec ULS2048 when reference spectrum was a clean glass substrate. It means that 100% corresponds to a transmittance of the clean glass substrate. The area from which the transmittance spectra were taken was ~0.45 mm2. In Fig. 1g is shown the transmittance spectrum of unstructured area coated with a uniform layer of Au NPs and 5 nm gold film (black curve), and the transmittance spectra of different period structure (30 μm – red curve and 60 μm – blue curve). All given transmittance spectra have a minimum in the range of 540–570 nm. This part is responsible for a surface plasmon resonance caused by collective oscillation of electrons on the gold nanoparticle surface and resulting in the strong extinction of light (absorption and scattering). In the range of 600–800 nm, the light transmission is higher for the structured surfaces comparing to the random distributed Au NPs (Fig. 1g). This result is related to the ablation of nanoparticles as gold has high reflectivity for wavelengths longer than 600 nm. The decrease of Au NPs covered area increases the light transmission in this range. The increase of light transmission in the range of 600–800 nm is higher for the 60 μm period structures compared to the 30 μm period structures due to a lower density of Au NPs in 60 μm period structures (Fig. 1c–f). For SERS measurements, 8-mercapto-N-phenethyloctanamide (MOPHE) compound (synthesized in our laboratory) was used to form self-assembled monolayers (SAMs) on our nanostructured surfaces. Because of the presence of the intrachain amide group, SAMs of MOPHE on the gold surface are incredibly stable and well-organized [42]. Also, these monolayers are useful for probing the Raman enhancement effect because the terminal phenylalanine ring possesses characteristic intense ν12 mode at 1002–1004 cm−1 [43]. The sample with microring structure was immersed in an ethanol solution containing 1 mM MOPHE for about 16 h and afterward was washed with ethanol solution and was dried at room temperature under N2 slow flow. The structural formula of MOPHE compound is shown in Fig. 1h. Molecules of this compound form a monolayer on the surface of the sample. In this case, the hydrogen atom is replaced by a gold atom to form a bond with sulfur. 3. Results and discussion SERS measurements were performed using Renishaw InVia and RamanFlex 400 spectrometers with an excitation wavelength of 785 nm by using 0.9 and 50 mW laser power, respectively. The Renishaw InVia spectrometer collected Raman spectra from the area about 2.4 μm in diameter (red circle in Fig. 2a,b) and RamanFlex 400 from the area about 200 μm in diameter (pink circle in Fig. 3b). By using Renishaw InVia spectrometer, the microring structures with 30 μm and 60 μm period were tested. It was measured in various areas of structures by moving the sample with a 5 μm step. Altogether, five different places were measured for 60 μm period microring structures and four different locations for 30 μm period microring structures. The measured areas are marked in Fig. 2a,b. The results show that the maximum Raman intensity is the highest at the edge of the rings, but the enhancement of the Raman signal in the center part of microring structures due to the concentration of the surface plasmons energy in the presence of concentric rings is also observed [28]. The increase of Raman signal intensity in various parts of microring structures fluctuates between 2 and 4 times compared with the control sample (Fig. 2c–f). The control sample was an unstructured gold nanoparticles sample coated with a 5 nm thick gold layer. The results show that the assembled concentric gold microring structures can significantly enhance SERS signals due to their induced localized plasmon resonance. Local enhancement of Raman signal from different part of microring structures was confirmed by the previous experiments using the Renishaw InVia spectrometer. In this case, the Raman signal was obtained from the area smaller than the size of fabricated microring structures. By using RamanFlex 400 spectrometer, Raman signal was
4. Conclusions Concentric microring structures containing gold nanoparticles were fabricated using a Bessel-like beam array with 30 μm and 60 μm period. 4
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Fig. 3. a) Microring structure containing Au NPs coated with 5 nm thick gold layer. Marked area (pink circle) show analyzing area using RamanFlex 400 spectrometer; b) Bessel-like beam intensity distribution behind the self-made polymeric structure; c) Raman spectra measured for Au NPs microring structure with period 60 μm which were fabricated using 0.4 W, 0.6 W, 0.8 W average laser power; d) measured maximum Raman intensity at MOPHE compound peak for microring structures fabricated with 0.4 W, 0.6 W, 0.8 W average laser power. The control sample was an unstructured gold nanoparticles sample coated with a 5 nm thick gold layer. Scale bar in a) represents 30 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The fabricated structures coated with 5 nm thick gold layer demonstrate enhancement of SERS signals due to their induced localized plasmon resonance. The measured Raman signal intensity of MOPHE compound using different Raman spectrometers exhibit that concentric microring structures containing gold nanoparticles enhance the Raman signal. The enhancement of Raman signal depends on the laser treatment parameters used in the fabrication of gold microring structures. Microring structures manufactured with 0.4 W laser power exhibit more than two times higher enhancement of Raman signal compared with microring structures fabricated with higher laser power due to higher roughness. The higher smoothness of structures manufactured with higher laser pulse energy is determined by the fusion of gold nanoparticles in the Bessel-like beam minimum intensity area due to sufficient energy to melt gold nanoparticles in this area. Finally, not only gold nanoparticles can be used in our demonstrated fabrication approach to assemble nanoparticles in concentric microring structures, but it is also applicable to other metals or even nonmetals like polymers or semiconductors nanoparticles.
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Acknowledgments [11]
E. Stankevičius thanks Lithuanian Academy of Sciences for the financial support under the Young Scientist Scholarship. G.N. gratefully acknowledges the Center of Spectroscopic Characterization of Materials and Electronic/Molecular Processes (SPECTROVERSUM Infrastructure) for the use of Raman spectrometers.
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