ParticleâFilm Plasmons on Periodic Silver Film over Nanosphere

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Particle-film plasmons on periodic silver film over nanosphere (AgFON): A hybrid plasmonic nanoarchitecture for surface-enhanced Raman spectroscopy Jiwon Lee, Qianpeng Zhang, Seungyoung Park, Ayoung Choe, Zhiyong Fan, and Hyunhyub Ko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09753 • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 28, 2015

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ACS Applied Materials & Interfaces

Particle-film plasmons on periodic silver film over nanosphere (AgFON): A hybrid plasmonic nanoarchitecture for surface-enhanced Raman spectroscopy Jiwon Lee,†,§ Qianpeng Zhang,‡,§ Seungyoung Park,† Ayoung Choe,† Zhiyong Fan,*,‡ and Hyunhyub Ko*,†

AUTHOR ADDRESS †

School of Energy and Chemical Engineering, Ulsan National Institute Science & Technology

(UNIST), Ulsan, Republic of Korea. ‡

Department of Electronic & Computer Engineering, Hong Kong University of Science &

Technology (HKUST), Hong Kong SAR, China. KEYWORDS: Particle-film plasmon coupling, AgFON, SERS, surface plasmons, chemical sensor

Abstract

ACS Paragon Plus Environment

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Plasmonic systems based on particle-film plasmonic couplings have recently attracted great attention because of the significantly enhanced electric field at the particle-film gaps. Here, we introduce a hybrid plasmonic architecture utilizing combined plasmonic effects of particle-film gap plasmons and silver film over nanosphere (AgFON) substrates. When gold nanoparticles (AuNPs) are assembled on AgFON substrates with controllable particle-film gap distances, the AuNP-AgFON system supports multiple plasmonic couplings from interparticle, particle-film, and crevice gaps, resulting in a huge surface-enhanced Raman spectroscopy (SERS) effect. We show that the periodicity of AgFON substrates and the particle-film gaps greatly affects the surface plasmon resonances, and thus, the SERS effects due to the interplay between multiple plasmonic couplings. The optimally designed AuNP-AgFON substrate shows a SERS enhancement of 233 times compared to the bare AgFON substrate. The ultra-sensitive SERS sensing capability is also demonstrated by detecting glutathione, a neurochemical molecule that is an important antioxidant, down to the 10 pM level.

Introduction Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that utilizes the surface plasmon resonance (SPR) of metal nanostructures to enhance the Raman scattering of analytes.1-4 For SERS chemical sensors, SERS-active substrates with metallic nanoscale gaps or edges have been fabricated by several approaches such as nanoparticle self-assembly,5 e-beam lithography,6 and nanosphere lithography.7 Among the various approaches, nanosphere lithography has captivated the scientific community because it is an inexpensive, robust, and reproducible technique for the production of large-area and precisely defined periodic metal nanostructures. In nanosphere lithography, two-dimensional hexagonal arrays of colloidal crystals are used


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as nanoscale templates or masks for the subsequent formation of metal nanostructures, where the plasmonic properties can be easily tuned by controlling the size of the nanospheres and the thickness of the evaporating metals. In particular, silver film coating on colloidal crystals, the so-called Ag film over nanospheres (AgFON) substrate, has been widely used in SERS chemical and biological applications because of its reasonably large SERS enhancements.8-14 Here, the sharp crevices between adjacent metallic halfdomes have been known to provide SERS “hot spots” with the greatest local field enhancement, which mostly contribute to the generation of entire SERS signals.15-16 In addition, the periodic metallic nanostructures provide additional SERS enhancements due to light coupling to the propagating surface plasmon polaritons (SPPs).17-18 However, the existence of hot spots only at the crevices limits the number of hot spots and subsequently the SERS enhancements. The molecules adsorbed at the smooth metallic area between crevices are not fully utilized for the generation of SERS signals. Recently, metal nanoparticles on smooth metal films; the so-called particle-on-film systems, have been greatly investigated for the fundamental study of surface plasmons and for applications in chemical sensors.19-27 In this system, the nanometer scale gap between the particle and the film provides a hot spot with large field enhancements.28-29 The field enhancements at the particle-film gaps can be attributed to the hybridization of localized surface plasmons (LSPs) of metal nanoparticles with the SPPs of metal films that propagate along the metal-dielectric interfaces at the surface of metal films. In addition, the LSPs interact and couple with their own mirror charges in the metal films, contributing to the field enhancements. These particle-film plasmonic interactions enable the fabrication of tunable and versatile plasmonic systems, where the plasmonic


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properties are controlled by the particle-film gap distances as well as the nanoparticle shapes and sizes.30-34 For high-density nanoparticle assembly on metal film, interparticle couplings between LSPs also contribute to the field enhancements in addition to the particle-film plasmon couplings. Here, the interplay between interparticle and particlefilm plasmon couplings has been known to greatly affect the maximum field enhancements. In addition, the location of hot spots varies depending on the excitation wavelength on particle-film plasmon coupling.35 Although these particle-on-film systems can provide synergistic plasmonic effects when they are combined with periodic metallic films, there have only been a few reports on the detailed study of plasmonic properties of these systems. Here, we introduce a particle-on-film plasmonic configuration based on the assembly of gold nanoparticles (AuNPs) on AgFON substrate (denoted as AuNP-AgFON) with larger SERS enhancements compared to conventional particle-on-film or AgFON systems. Such AuNP-AgFON systems support multiple field enhancements from interparticle, particle-film, and crevice plasmon couplings, which enables the fabrication of highly sensitive SERS substrates. In addition, by controlling the size of the nanosphere in the hexagonal colloidal assembly, we controlled the interplay between multiple plasmon couplings and demonstrated wavelength-customizable SERS substrates. For 785 nm excitation wavelength, the AuNP-AgFON substrate based on 750 nm nanospheres shows 233 times stronger SERS enhancement than bare AgFON substrate.

Experimental Section Synthesis of gold nanoparticles


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Gold nanoparticles (AuNPs) were prepared by the Turkevich–Frens method,36-37 using sodium citrate as a reducing agent in an aqueous solution. Briefly, an aqueous solution of HAuCl4 (Sigma-Aldrich, 150 mL, 0.25 mM) was heated to the boiling point under stirring, and subsequently, an aqueous solution of sodium citrate (Sigma-Aldrich, 1.15 mL, 0.1 M) was added to the boiling solution. The solution was kept under boiling conditions until the color changed to wine red. The solution was then cooled to room temperature. Fabrication of AgFON arrays Two-dimensional (2D) colloidal arrays consisting of polystyrene (PS) nanospheres (Polysciences Inc.) were fabricated on clean silicon substrates via a spin-coating method. Hydrophilic silicon substrates (1 cm x 1 cm) were fabricated by treating them in a mixed solution (3:1 v/v) of H2SO4 (JUNSEI, 98%) and H2O2 (DUKSAN, 28%) for 1 h and then washing thoroughly with DI water. For the self-assembly of polystyrene nanosphere arrays by a spin-coating method, a droplet (23 µL for 220 nm and 505 nm PS, 26 µL for 750 nm and 990 nm PS) of PS suspension (10% w/v) was placed on the silicon substrates. Then, the spin coating process was performed at the following spin speeds and times: 0 rpm 60 s, 200 rpm 60 s, 400 rpm 60 s, 600 rpm 60 s, 800 rpm 60 s, 1000 rpm 60 s, 1200 rpm 60 s, 1400 rpm 120 s, 1600 rpm 1200 s, 0 rpm 60 s, 2000 rpm 1200 s. Finally, in order to fabricate the AgFON, 200 nm Ag film with a 10-nm Cr adhesion layer was deposited onto the surface of the 2D PS arrays by a sputtering method (SORONA, SRN120M). Immobilization of AuNP on AgFON arrays


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For the assembly of AuNPs on the AgFON substrates, we exploited the electrostatic interactions between negatively-charged sodium citrate on the AuNP surface and the positively-charged poly-(diallyldimethylammonium chloride) (PDDA, 100 000 Mw, Sigma-Aldrich) layer on the AgFON substrates.20, 31, 38-42 The PDDA layer was prepared by spin-coating 0.2% PDDA solution followed by DI water rinsing, which resulted in a ∼1 nm thick layer on the substrates.26 PDDA/AgFON substrates were immersed in the AuNP solution for 2 h followed by washing with DI water and nitrogen blowing. The gap distance between the AuNP and the AgFON substrate was controlled by using layer-by-layer (LBL) polymer multilayer, where positively-charged PDDA and negatively-charged poly(sodium 4-styrenesulfonate) (PSS, Sigma-Aldrich) polymers are alternatively deposited on the AgFON surface. In a typical process, the substrate was coated with polymers via spin-coating (4000rpm, 60s) the polymer solution (0.2 wt% in water) and rinsing with ultra-pure water twice. The above process was repeated to fabricate polymer layers of PDDA, (PDDA/PSS)PDDA, (PDDA/PSS)2PDDA, and (PDDA/PSS)3PDDA, resulting in different gap distances. Sample characterization The morphologies of the samples were observed by field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4800). The absorption spectra were measured using an ultraviolet-visible (UV-vis) microspectrophotometer (CRAIC,

20/20 PV). SERS

measurements were performed using a confocal Raman microscope (WITec, Alpha 300) with excitation wavelengths of 785 nm, 633 nm, and 532 nm. Benzenethiol (SigmaAldrich) was used as the SERS probe molecule to evaluate the SERS performance of the nanostructures. The plasmonic nanostructures (AuNP-AgFON and bare-AgFON


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substrates) were incubated in an ethanol solution of benzenethiol (100 mM) for 15 min, rinsed with ethanol, and then dried using nitrogen gas. All the Raman measurements were conducted under 1 mW laser power, 100× objective lens (numerical aperture = 0.9), 0.5 s integration time, and 5 counts of accumulation. Raman spectral images (10 × 10 µm2) were obtained under 1 mW laser power and 0.2 s integration time.

Results and discussion Figure 1 presents the fabrication of AuNP-AgFON plasmonic systems, where regular AgFON arrays are prepared on silicon substrates and AuNPs (18 nm) are subsequently assembled on the periodic AgFON surfaces. For the fabrication of 2D hexagonal arrays of colloidal crystals with controlled periodicity, polystyrene (PS) nanospheres with different diameters (220, 505, 750, 990 nm) were self-assembled on the silicon substrates by a spin coating technique (Figure 1a).43 In order to fabricate AgFON arrays, Ag (thickness of ∼200 nm) was sputtered onto the surface of the 2D PS nanosphere arrays with four different nanosphere diameters, 220, 505, 750, and 990 nm, denoted as AgFON-220, AgFON-505, AgFON-750, and AgFON-990, respectively (Figures 1b, S1). For the uniform assembly of AuNPs over AgFON surface (AuNP-AgFON), we employed electrostatic assembly of negatively charged nanoparticles on positively charged surfaces.26 In this process, the negatively charged AuNPs with sodium citrate ligands were assembled on the positively charged AgFON surfaces modified with poly (diallyl dimethylammonium) chloride (PDDA) (Figure 1c). Figure 1d shows a representative scanning electron microscope (SEM) image of the AuNP-AgFON system, where AuNPs are uniformly assembled on top of AgFON substrates with periodic crevice gaps.


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Compared to traditional AgFON substrates with only gap plasmons and grating coupling

effects,

AuNP-AgFON

plasmonic

systems

provide

multiple

E-field

enhancements from the localized SPRs by the plasmon couplings between AuNPs (interparticle plasmons), couplings between the LSPs of AuNPs and the propagating SPPs in the metal film (particle-film plasmons), and the gap plasmons at the crevice gap regions (gap plasmons), all of which can be utilized as highly-sensitive SERS substrates (Figure 1e). In addition, the regular metal nanostructures from 2D hexagonal arrays can provide grating coupling effects, where the incident light is efficiently coupled into propagating SPPs.17 The propagating SPPs are known to further enhance the plasmon couplings between LSPs of metal nanoparticles.44-45 The SEM top-view image of the AgFON-505 substrate (Figure 2a) shows a periodic metallic nanostructure with hexagonally arranged, close-packed spherical arrays, where sharp crevice gaps are formed between adjacent metallic half-domes on the regular AgFON surface. The subsequent electrostatic assembly of AuNPs resulted in uniform coverage of AuNPs on the AgFON surface (Figure 2b,c). The high-resolution tilt-view SEM image of AuNP-AgFON-505 substrate (Figure 2c) indicates that AuNPs are assembled mostly on the upper hemisphere without disturbing the sharp crevices between the metallic half-domes. To evaluate the SERS performance of AuNP-AgFON-505 substrates as compared to the AgFON-505 substrates, the SERS spectra of benzenethiol (BT) molecules were measured using 785 nm laser excitation. BT molecules were adsorbed on the SERS substrates by incubating SERS substrates in 100 mM BT solution for 15 min. We chose benzenethiol molecule as the SERS marker since it can be easily adsorbed on the Ag and


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Au surfaces forming a self-assembled monolayer and provide strong SERS signals.26, 46 Figure 2d shows the SERS spectra of AuNP-AgFON-505, AgFON-505, and AuNP-Ag film substrates. The Raman spectra of benzenethiol shows three prominent Raman bands including the in-plain ring-breathing mode (997 cm-1), in-plain C-H bending mode (1021 cm-1), and in-plane ring-breathing mode coupled with the C-S stretching mode (1071 cm1 47-48

).

Notably, the Raman intensity of AuNP-AgFON-505 at 1071 cm-1 is ∼30 times

larger than that of AgFON-505, indicating the significant role of AuNP assemblies on AgFON-505 substrate in the improvement of the SERS effect. This huge improvement of SERS signal for the AuNP-AgFON system can be attributed to the additional E-field enhancements from the particle-film and interparticle plasmon couplings when AuNPs are assembled on AgFON substrate. The increase in field enhancements and the number of hot spots after the AuNP assembly on AgFON substrates can be clearly observed in the confocal Raman spectral mapping images (Figure 2e, f). When AuNPs are assembled on a plane Ag film without periodic crevice gaps (AuNP-Ag film), the Raman intensity is ∼6000 times lower than that of AuNP-AgFON (Figure 2d), indicating that the particlefilm and interparticle plasmon couplings alone are not the main factors contributing to the huge SERS signal improvements. Therefore, the huge SERS effects for the AuNPAgFON system can be attributed to the multiple field enhancements from crevice gaps, particle-film, interparticle, periodic grating coupling, and the mutual interactions between these plasmon modes. To understand the SERS enhancements on different plasmonic surfaces, we examined the finite-difference time-domain (FDTD) calculation of E-fields on AuNP-AgFON-505 substrates under the 785 nm excitation condition (Figure 2g). The enlarged E-field


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profiles indicate that the E-fields (60–70 V/m) of interparticle and particle-film gaps (g1 in Figure 2g) are 21–25 times higher than the E-field (2.8 V/m) of the crevice gap (g2 in Figure 2g). For AgFON-505 substrates without AuNPs, only crevice gaps provide E-field enhancement (2.5 V/m, Figure S2), but the intensity is much lower than the E-fields of the AuNP-AgFON-505 substrates. This FDTD simulation result also corresponds to the experimental result, where the SERS intensity of AuNP-AgFON-505 was ∼30 times higher than AgFON-505 at 785 nm excitation wavelength (Figure 2d). The E-field distribution of AuNP-AgFON and AgFON substrates can be also observed by scanning near-field optical microscopy (SNOM) images (Figure S3). As compared to the bare AgFON-505 substrates, which show E-field enhancements only at the crevice gap between nanospheres, AuNP-AgFON-505 substrates show multiple E-field enhancements at crevice gaps as well as on the nanosphere surfaces because of the plasmon couplings between interparticle and particle-film gaps. All these results indicate that AuNPAgFON-505 substrates can induce multiple hotspots at the interparticle, particle-film, and crevice gaps, which result in large E-field enhancements. For the AuNP-AgFON system, the periodicity of AgFON substrates affects the SERS enhancements. Figures 3a–f show the SEM images of AuNP-AgFON systems with nanosphere diameters of 220, 750, and 990 nm. The confocal Raman images indicate that the AuNP-AgFON-220 substrate exhibits mostly dark areas with a small number of bright spots, indicating a lower overall SERS intensity and smaller number of hot spots compared to those of the AuNP-AgFON system with 750 and 990 nm nanospheres (Figure 3g-i). The SERS spectra of AuNP-AgFON with different periodicities (220, 505, 750, and 990 nm) are compared in Figure 3j, where AuNP-AgFON-505 system shows the


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highest SERS intensity and AuNP-AgFON-220 system exhibits the lowest SERS signal. The comparison of the average SERS intensities as a function of periodicity (Figure 3k) indicates that the AuNP-AgFON-505 system exhibits 6–20 times larger SERS intensities than those of AuNP-AgFON systems with other periodicities. We also observe that the SERS intensities of AuNP-AgFON systems are 30–230 times larger than those of the AgFON systems for all periodicities, indicating the significant effect of AuNP assemblies deposited on AgFON substrates for the improvement of SERS signals. Without the addition of AuNPs on top of AgFON systems, we observe 19–43 times improvements of SERS intensities for AgFON-505 systems as compared to other periodicities (220, 750, and 990 nm). These results demonstrate that the effects of particle-film plasmon couplings are larger than the grating coupling effects for the enhancement of SERS effects. The different SERS intensities depending on the periodicity can be correlated with the spectral positions of SPRs. As can be seen in Figure 4a, AuNP-AgFON substrates with larger nanosphere diameters (505, 750, 990 nm) exhibit several optical absorption maxima, which can be attributed to the absorption of incident light via resonant couplings with surface plasmon resonances of AuNP-AgFON plasmonic systems. On the other hand, the AuNP-AgFON-220 substrate does not exhibit any noticeable absorption maxima because of the weak light coupling effect. For AuNP-AgFON-220 substrates, the periodicity is too small for efficient grating couplings of light and the small nanosphere size (220 nm) does not provide sharp enough crevice gaps for efficient light coupling into gap plasmons. While the AuNP-AgFON-505 substrate shows the maximum absorption peak at 650 nm, which broadly covers the wavelength range between 500–900 nm, for


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AuNP-AgFON-750 and 990 substrates, the absorption maxima over the range of 450–950 nm show weak spectral intensities as compared to that of AuNP-AgFON-505. The absorption spectra of AuNP-AgFON substrates differ from those of AgFON substrates. When compared to the absorption spectra of the AgFON-505 substrate, it is clear that the additional interparticle and particle-film plasmon couplings for the AuNPAgFON-505 substrate results in the blue-shift of the absorption maxima (Figure 4b). To further investigate the effects of particle-film gaps on the absorption spectra, we increased the particle-film gaps by increasing the number of layer-by-layer assembled polyelectrolyte multilayers (PDDA/PSS layers), in which the gap distance can be controlled on the nanometer scale.26 As compared to the AuNP-PDDA-AgFON-505 substrate, we observed that further increase in the particle-film gaps with the increase in PDDA/PSS multilayer thickness resulted in a slight red-shift of the absorption maxima as well as decreased spectral intensity (Figure 4b). This behavior is attributed to the reduced particle-film plasmon couplings with increase in the particle-film gap distances.46, 49-50 We also observed that the absorption spectra of AuNP-AgFON substrates with larger nanosphere diameters (505, 750, 990 nm) exhibit either blue-shifted spectra or additional absorption spectra in the lower wavelength range compared to the spectra of AgFON substrates (Figure S4), suggesting the existence of additional particle-film plasmon couplings.51-52 The field enhancements from particle-film plasmon couplings are known to be much stronger than those from interparticle couplings, resulting in large SERS enhancements.20, 23-24 Consequently, the different light absorption properties of AgFON, AuNP-AgFON, and AuNP-(PDDA/PSS)nPDDA-AgFON substrates indicate that various


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plasmon couplings at interparticle, particle-film, and crevice gaps can be synergistically excited to significantly affect the light absorption properties. To further investigate the effects of excitation wavelength on SERS enhancements as a function of nanosphere size, we measured SERS signals on bare AgFON and AuNPAgFON substrates for excitation wavelengths of 633 and 532 nm (Figure 5). Both AgFON and AuNP-AgFON substrates show the strongest SERS intensity for the nanosphere size of 505 nm (Figure 5a,c). The AuNP-AgFON-505 systems exhibited 9 to 26 times larger average SERS intensity than those of the AuNP-AgFON substrates with other nanosphere diameters for 633 nm excitation (Figure 5b), and 4 to 6 times larger intensities for 532 nm excitation (Figure 5d). The larger SERS enhancement of AuNPAgFON-505 at 633 nm laser excitation than that at 532 nm laser excitation can be attributed to the closer location of the maximum absorption peak (650 nm) to the wavelength of the 633-nm laser compared to the wavelength of the 532-nm laser. It is known that the maximum SERS enhancements can be observed when the wavelength of SPR is located between the excitation wavelength and the wavelength of scattered Raman spectra of the analytes.53 The enhanced SERS signal for the 633-nm laser excitation can also be confirmed by the confocal Raman images of AgFON-505 substrates (Figures 5c and d). The confocal Raman images also indicate that AuNP-AgFON-505 substrates provide larger SERS enhancements than those of AgFON-505 substrates. Although we observed the strongest SERS intensity at the nanosphere size of 505 nm for all the excitation laser wavelengths (532, 633, 785 nm), the effects of nanosphere size on wavelength-dependent SERS intensity can be different depending on other sample fabrication conditions such as the substrate type (silicon, glass), metal film thickness, and


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metal deposition method (sputtering, evaporation). These factors affect the final surface plasmon resonances and thus the wavelength-dependent SERS intensity variations.54-55 The size and shape of gold nanoparticles also affect the SERS activity of the substrates. The different size of gold nanoparticles (12, 18, and 50 nm) on AgFON-505 substrates (Figure S5) resulted in different positions of absorption maximum peak (Figure S6). The AgFON-505 substrate with 18 nm AuNPs has a broad peak at 600-800 nm wavelength range, which coincide with the excitation laser wavelength (785 nm) and thus provide the highest SERS intensity (Figure S7). The SERS system with 12 nm AuNPs has the maximum peak at ~600 nm wavelength with a lower intensity and the SERS substrates with 50 nm AuNPs and gold nanostars (AuNSs) have the maximum peaks far away from the 785 nm wavelength. These UV-Vis absorption spectra result in the corresponding SERS intensity (Figure S7). It is worth to note that the effect of size and shape of AuNPs still depends on the surface density of AuNPs. In our previous work,26 it was demonstrated that the optimal density of gold nanoparticles is required to obtain the highest SERS enhancement due to the competition between the interparticle and particlefilm plasmon couplings. If the interparticle plasmon couplings are too strong at a high surface density of nanoparticles, then it screens the particle-film plasmon couplings. To demonstrate the potential use of our AuNP-AgFON plasmonic systems as an ultrasensitive SERS chemical sensor, we investigated the neurochemical detection capability of our SERS substrates. The neurochemical detection is attractive for healthcare monitoring systems. The amount of glutathione (GSH), a neurochemical that is an important antioxidant, in tissues is related to various diseases such as Alzheimer’s and Parkinson’s disease, cancers, and various aging problems.56-58 In particular, GSH is


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known to be an oxidative stress indicator for Alzheimer’s disease.59 To evaluate the capability of detection of neurochemical molecules, GSH molecules (Figure 6a) were adsorbed on AgFON-505 substrates that were coated with agarose layers to trap analytes inside the polymer network by shrink-swell cycles.60 Then, AuNPs were attached on the AgFON surface in order to form particle-on-film plasmonic structures. As shown in Figure 6b, GSH molecules exhibit characteristic vibration modes of C-S stretching at 649 cm-1, –COO– bending modes at 775 cm-1, and CH2 bending at 1456 cm-1.61-62 Our optimized SERS substrates (AuNP-AgFON-505) can detect 10 pM of GSH. The signal intensity at 775 cm-1 gradually decreased from 10 µM to 10 pM (Figure 6c). The detection of GSH down to 10 pM is well below the best know detection limit of GSH based on SERS substrates.63

Conclusions In summary, we have shown a hybrid plasmonic nano-architecture with particle-onfilm plasmonic configuration on AgFON substrate based on the assembly of AuNPs on AgFON substrate with larger SERS enhancements compared to conventional particle-onfilm or AgFON systems. We have demonstrated that the AuNP-AgFON system showed a huge surface-enhanced Raman spectroscopy (SERS) effect because of the multiple plasmonic couplings from interparticle, particle-film, and crevice gaps. The periodicity of AgFON substrates and the particle-film gaps greatly affected the surface plasmon resonances, and thus, the SERS effects due to the interplay between multiple plasmonic couplings. The optimal AuNP-AgFON system showed an ultra-sensitive SERS sensing capability with the detection of neurochemical molecules down to 10 pM level. Our


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AuNP-AgFON plasmonic systems will provide a platform for the development of ultrasensitive plasmonic sensors.

Figure 1. Fabrication procedures of AuNP-AgFON plasmonic systems: (a) self-assembled 2D hexagonal arrays of polystyrene nanospheres on Si substrate, (b) deposition of Ag film by sputtering, and (c) electrostatic assembly of negatively-charged Au nanoparticles on positivelycharged PDDA/AgFON surface. (d) SEM cross section image of AuNP-AgFON-505, (e) multiple field enhancements are generated from particle-film, interparticle, and crevice gap plasmon couplings.


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Figure 2. SEM images of AgFON-505 (a), AuNP-AgFON-505 (b) with cross section image of AuNP-AgFON-505 (c). Raman spectra of AuNP-AgFON-505, AgFON-505, and AuNP-Ag film substrates (d). Raman mapping images of AuNP-AgFON-505 versus AgFON-505 at 785 nm excitation laser (e,f). (g) FDTD simulations of E-fields on AuNP assemblies on AgFON-505 substrate at 785 nm laser wavelength. Partially enlarged image for dimer AuNPs (g1) and crevice gap (g2) in AuNP-AgFON-505 system.


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Figure 3. SEM images of top view (a,b,c) and cross-section view (d,e,f) of AuNP-AgFON-220 (a,d), AuNP-AgFON-750 (b,e), and AuNP-AgFON-990 (c,f). Raman mapping images of AuNPAgFON-220 (g), AuNP-AgFON-750 (h), and AuNP-AgFON-990 (i) at 785 nm excitation laser. (j) Raman spectra of 100 mM BT for AuNP-AgFON-220, AuNP-AgFON-505, AuNP-AgFON750, and AuNP-AgFON-990 substrates. (k) Nanosphere size-dependent Raman intensity plots for before and after attaching AuNPs on AgFON substrates.


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Absorption (A.U.)

(a) 0.8

AuNP-AgFON-220 AuNP-AgFON-505 AuNP-AgFON-750 AuNP-AgFON-990

0.6 0.4 0.2 0.0

(b) Absorption (A.U.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

600 700 800 Wavelength (nm)

900

0.8

AgFON-505 AuNP-PDDA-AgFON-505 AuNP-(PDDA/PSS)PDDA-AgFON-505 AuNP-(PDDA/PSS)2PDDA-AgFON-505

0.6

AuNP-(PDDA/PSS)3PDDA-AgFON-505

0.4 0.2 0.0

500

600 700 800 Wavelength (nm)

900

Figure 4. (a) UV-Vis absorption spectra of AuNP-AgFON substrates with various polystyrene nanospheres; 220 nm, 505 nm, 750 nm, and 990 nm, (b) UV-Vis absorption spectra of AuNP assemblies on AgFON-505 substrate with different thickness of polymer layers.


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Figure 5. Raman intensities depending on laser wavelengths: (a,c) 633 nm and (b,d) 532 nm, (a,b) Nanosphere size-dependent Raman intensity plots for before and after attaching AuNPs on AgFON substrates. (c,d) Raman mapping images of AuNP-AgFON-505 versus AgFON-505.


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(a)

Raman Intensity (a.u.)

(b)

1200 1000

649 775

(c)

1456 10 µM

800

10 nM

600 1 nM

400

100 pM

200 10 pM

0 500

Raman Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

1000 1500 Raman Shift (cm-1) 775 cm-1

250 200 150 100 50 0

10uM

10nM

1nM

100pM 10pM

Raman Shift (cm-1) Figure 6. (a) Molecular structure of glutathione (GSH). (b) Raman spectra of glutathione showing C-S stretching at 649 cm-1, -COO- bending at 775 cm-1, and CH2 bending at 1456 cm-1. (c) Raman intensity plots of AuNP-AgFON-505 with different concentration of GSH molecule at 775 cm-1.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: . SEM images of AgFON and AuNP-AgFON substrates, FDTD simulations of AgFON-505 substrates, SNOM images of AgFON and AuNP-AgFON substrates, UV-Vis absorption spectra of AgFON and AuNP-AgFON substrates with various polystyrene nanosphere sizes, and Raman intensity of various gold nanoparticles on AgFON-505 substrates. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] Author Contributions §

These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (2011-0014965, 2014M3C1B2048175) and by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (2015M3A6A5065314).


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ABBREVIATIONS AgFON, silver film over nanosphere; AuNP, gold nanoparticle; SERS, surface-enhanced Raman spectroscopy; SPR, surface plasmon resonance; SPP, surface plasmon polaritons; LSP, localized surface plasmon; AuNP-AgFON, the assembly of gold nanoparticles on AgFON substrate; AuNP-AgFON-220, AuNP-AgFON with 220 nm diameter; AuNP-AgFON-505, AuNP-AgFON with 505 nm diameter; AuNP-AgFON-750, AuNP-AgFON with 750 nm diameter; AuNPAgFON-990, AuNP-AgFON with 990 nm diameter;

AgFON-220, polystyrene nanosphere

arrays with 220 nm diameter; AgFON-505, polystyrene nanosphere arrays with 505 nm diameter; AgFON 750, polystyrene nanosphere arrays with 750 nm diameter; AgFON-990, polystyrene nanosphere arrays with 990 nm diameter; AuNP-Ag film, assembly of gold nanoparticles on silver film; PDDA, poly (diallyl dimethylammonium) chloride; PSS, poly(sodium 4styrenesulfonate); BT, benzenethiol; GSH, glutathione; SEM, scanning electron microscopy; SNOM, scanning near-field optical microscopy; UV-Vis-NIR, ultraviolet-visible-near-infrared spectrophotometer. REFERENCES (1) Chan, G. H.; Zhao, J.; Hicks, E. M.; Schatz, G. C.; Van Duyne, R. P., Plasmonic Properties of Copper Nanoparticles Fabricated by Nanosphere Lithography. Nano Lett. 2007, 7, 1947-1952. (2) Ji, X.; Song, X.; Li, J.; Bai, Y.; Yang, W.; Peng, X., Size Control of Gold Nanocrystals in Citrate Reduction: The Third Role of Citrate. J. Am. Chem. Soc. 2007, 129, 1393913948. (3) Ko, H.; Singamaneni, S.; Tsukruk, V. V., Nanostructured Surfaces and Assemblies as SERS Media. Small 2008, 4, 1576-1599.


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Table of Contents Graphic


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ParticleâFilm Plasmons on Periodic Silver Film over Nanosphere

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