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Transparent and Flexible Surface Enhanced Raman Scattering (SERS) Sensors Based on Gold Nanostar Arrays Embedded in Silicon Rubber Film Seungyoung Park, Jiwon Lee, and Hyunhyub Ko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14022 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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ACS Applied Materials & Interfaces

Transparent and Flexible Surface Enhanced Raman Scattering (SERS) Sensors Based on Gold Nanostar Arrays Embedded in Silicon Rubber Film Seungyoung Park†, Jiwon Lee†, Hyunhyub Ko†,* †

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

(UNIST), Ulsan Metropolitan City, 689-798, Republic of Korea. * To whom correspondence should be addressed: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Integration of surface-enhanced Raman scattering (SERS) sensors onto transparent and flexible substrates enables lightweight and deformable SERS sensors which can be wrapped or swabbed on various non-planar surfaces for the efficient collection and detection of analytes on various surfaces. However, the development of transparent and flexible SERS substrates with high sensitivity is still challenging. Here, we demonstrate a transparent and flexible SERS substrate with high sensitivity based on a polydimethylsiloxane (PDMS) film embedded with gold nanostar (GNS) assemblies. The flexible SERS substrates enables the conformal coverage on arbitrary surfaces and the optical transparency allow the light interaction with the underlying contact surface, thereby providing highly sensitive detection of analytes adsorbed on arbitrary metallic and dielectric surfaces which otherwise do not provide any noticeable Raman signals of analytes. In particular, when the flexible SERS substrates are covered onto metallic surfaces, the SERS enhancement is greatly improved due to the additional plasmon couplings between GNS and metal film. We achieve the detection capability of a trace amount of benzenethiol (108 M) and enormous SERS enhancement factor (~1.9  108) for flexible SERS substrates on Ag film. In addition, due to the embedded structure of GNS monolayers within the PDMS film, SERS sensors maintain the high sensitivity even after mechanical deformations of stretching, bending, and torsion for 100 cycles. The transparent and flexible SERS substrates introduced in this study is applicable to various SERS sensing applications on non-planar surfaces, which are not achievable for hard SERS substrates. KEYWORDS: Gold nanostar, Surface enhanced Raman scattering, Flexible, Transparent, Polydimethylsiloxane.


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Introduction Surface enhanced Raman scattering (SERS) has been widely explored for ultrasensitive molecular detections.1,2 Most of current researches about SERS sensors have focused on the design and fabrication of highly sensitive SERS substrates based on the controlled metal nanostructures on hard substrates. To obtain highly sensitive SERS substrates, various metallic structures have been suggested including three-dimentional (3D) nanostructures such as nanocanals, porous structures, nanohole arrays, and metal film over nanosphere (MFON)3-8 on top of brittle and stiff substrates (silicon, glass, and alumina).3,9,10 Although these SERS substrates showed highly sensitive detection capabilities, the target analytes in solution or gas phases should be collected and adsorbed on the SERS substrates for the SERS measurements. In many cases, the adsorption of target molecules onto metal nanostructures of SERS substrates is not a facile work, requiring careful selections of surface bonding chemistries and coating methods. Although the hard SERS substrates have been predominantly used in various sensor applications, the traditional hard substrates are not suitable for the direct analysis of target analytes adsorbed on various surfaces, especially on curved or not easily-accessible surfaces. Recently, the demand of SERS detection of analytes adsorbed on arbitrary flat or curved surfaces for the real-world sensing applications have imposed the development of flexible SERS substrates.11-14 The flexible SERS sensors can be wrapped and swabbed onto various irregular, curved and non-planar substrates to collect the unknown molecules by simply covering the underlying substrates.15 In addition, the detection of analytes by covering the surfaces with flexible SERS substrates offer the nondestructive detection of molecules. Various flexible SERS substrates have been reported based on the processes of soaking and coating of plasmonic nanoparticles on the flexible supporting films such as filter paper, PDMS, polymer nanofiber mats, flexible polymer


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film, flat PET, carbon nanotube and graphene films.16-24 For example, paper substrates have attracted great attentions due to the advantages of cheap and easy fabrication processes, environmentally friendly and disposable materials.16,25-26 Free-standing polymer nanofiber films can be easily fabricated in large quantities by electrospinning methods and used to integrate nanoparticle assemblies for flexible SERS sensors.18,27 However, one of significant problems of previous flexible SERS sensors based on paper and polymer fiber films is the optically nontransparent (opaque) property, which prevent their applications in the direct analysis of molecules adsorbed on the arbitrary substances. Compared to conventional rigid and non-transparent substrates, transparent and deformable SERS substrates provide the possibility of detect analysis of underlying surface chemistries by simply covering onto the irregular and non-planar surfaces. Transparent and flexible SERS substrates have been reported by combining the plasmonic metal nanostructures and flexible PDMS films with a high optical transparency.28-31 Lu et. al. reported the decoration of PDMS films with spherical gold nanoparticles (GNPs) to fabricate transparent and flexible SERS substrates.28,29 Although the GNP-PDMS flexible SERS substrates can be used to detect the analytes on the target surfaces by covering the substrates on the underlying surfaces, the spherical nanoparticles usually provide weak SERS effects, limiting their applications in trace level detection of analytes. To overcome the low sensitivity, gold nanostar assemblies on PDMS substrates have been introduced with high SERS sensitivities.31 However, the nanostar assemblies attached on the surface of a PDMS film via the linker molecules have a limited chemical and mechanical stability due to the direct exposure to various chemical and mechanical damages. Although there is a recent report on the direct in-situ synthesis of Au nanostars on the PDMS surface without using the linker molecules,32 it is hard to control the shape of nanostars. In addition, the hard surface layers of nanostar assemblies on top of soft substrates prevent conformal contacts


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between soft substrates and target surfaces. Meanwhile, the gold nanostar assemblies embedded into the PDMS film without an external exposure can be protected from the direct exposure to chemical and mechanical damages as well as from the deterioration by the humidity unlike gold nanostar attached PDMS film via the linker molecules such as 3-Aminopropyl-triethoxysilane (APTES) which can be slightly deteriorated by the humidity causing the loss of the gold nanostar assemblies.33 In addition, gold nanostar assemblies embedded PDMS which result in the conformal contact between SERS substrates and target surface provide an enhanced SERS effects. Here, we introduce transparent and flexible SERS substrates with ultrahigh sensitivity based on the GNS assemblies embedded into transparent and flexible PDMS films. In the suggested design of SERS substrates, the strong E-field enhancements at the sharp tips of GNS enable the enormous Raman enhancements of analytes attached on non-SERS active surfaces. Since the GNS assemblies are embedded within the soft PDMS films, the SERS substrates can provide conformal contacts with the target surfaces by covering onto the underlying substrates, which lead to the closer proximity to the analytes and thus high SERS effects. We demonstrate that the Raman enhancement can be dramatically improved when the flexible SERS substrates are covered onto metal films due to the additional plasmon couplings between GNS and metal films. Furthermore, we show that the GNS assemblies can be embedded into the patterned PDMS substrates, enabling the selective chemical imaging of p-aminothiophenol (PAPT) and graphene oxide attached on metal films.

RESULTS AND DISCUSSION Fabrication of transparent and flexible SERS substrates


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Figure 1 schematically illustrate the fabrication of transparent and flexible SERS substrates based on GNS assemblies. First, high-density and uniform assemblies of GNSs are produced on the silicon substrate by a dip coating process. Here, the electrostatic interactions between the positively charged poly(diallyl-dimethylammonium) (PDDA) layers coated on the substrates and the GNS modified with negatively-charged sodium citrate enable the uniform self-assembly of GNS monolayers onto Si substrates.3,34 Next, liquid mixture of PDMS pre-polymer and curing agent is poured onto the Si substrate containing the assembled GNS monolayers and cured at the room temperature for 24 hrs. Here, the room temperature curing condition is used to prevent the shape change of GNS by the heat treatment and increase the softness of PDMS substrate. After curing, PDMS thin film is slowly detached from Si substrate, resulting in the transfer of GNS assemblies from Si surface into the flexible PDMS films. GNSs have a spherical core decorated with multi-branched sharp tips, which cause strong electromagnetic field concentration at the end of tips by lightening rod effects, thereby forming SERS ‘hot spots’.4 The synthesis of GNS is performed by surfactantless synthesis method based on the enlargement of spherical gold nanoparticles by the reduction of Au ion to Au with the aid of hydroxylamine (NH2OH).35 Here, 2-[4-(2-hydroxyethyl)-1-piperazinyl] ethane-sulfonic acid (HEPES) is used as the shape-control agent to control the tip-shaped branches on the Au surface during the reduction process. This seed mediated GNS synthesis enables the control of branching and size of GNS depending on the concentration of HEPES and volume of seed particles. Figure 2(a) show the scanning electron microscopy (SEM) and transmission electron microcopy (TEM) images of GNS. These images indicate the uniform formation of GNSs with the spherical core diameter of 57 ± 9 nm and the branched tip length of 11 ± 3 nm. Figure 2(b) shows a strong UVvis absorption peak at ~745 nm, which is attributed to the localized surface plasmon resonance


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(LSPR) of branched tips attached on GNS. We also observe a weak shoulder peak around 530 nm due to the LSPR of spherical core of GNS.36,37 Figure 2(c) shows the resulting GNS assemblies on silicon substrate after the coating of GNS monolayer on the Si substrate in comparison with the bare silicon showing the color change of the Si substrate after GNS coatings. Figure S1 shows the uniformity of coated GNS assemblies on the Si substrate, which results in the reproducible fabrication of SERS sensors (Figure S2). In addition, PDMS film embedded with GNS monolayers shows a bluish color which can be clearly compared with the bare PDMS film. Figure S3 shows the AFM images of GNS embedded PDMS SERS films, indicating that the morphology of the GNSs is slightly exposed to the outside of PDMS surface. The flexibility of flexible SERS sensor can be evaluated with the mechanical deformations including the bending, stretching and torsion of the flexible SERS sensor (Fig. 2c). Figure 2(d) shows the high-density, uniform assemblies of GNS monolayer self-assembled on the Si substrate via dip-coating method. As shown in Figure 2(e), the high-density GNS monolayers on Si substrates were nicely transferred and embedded within the PDMS films without any residual GNS nanoparticles on the original Si substrates. Figure S4 demonstrates that assembled GNS arrays can be completely transferred from the silicon substrate to the PDMS substrate. And flexible SERS sensor shows the enhanced absorbance on Si substrate in comparison with bare Si substrate and bare PDMS on Si substrate (Figure S5).

Raman enhancement effects of transparent and flexible SERS films To investigate the Raman enhancement effect of flexible SERS active films, we prepared silicon substrates coated with the target analytes. Then, the silicon substrate was covered with the flexible


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SERS active film for the analysis of Raman spectroscopy. Figure 3a shows the Raman spectra of 0.1 % benzenethiol molecules adsorbed onto the silicon substrate with and without covering of the SERS active film. In the absence of SERS active film, we did not observe any noticeable Raman signal from the adsorbed benzenethiol molecules. On the other hand, when the SERS active film is covered on top of Si substrate with the benzenethiol molecules, we observed the distinct Raman spectra of benzenethiol for the C–H out-of-plane bending (1006 cm-1) and C–C symmetric stretching (1029, 1079 cm-1) vibrations (Figure 3b). This result clearly indicates that the simple covering of flexible SERS film on the target surface can easily induce the Raman signal enhancement of the target analyte on the SERS-inactive surface. The Raman enhancement of our SERS active film is attributed to the strong interparticle surface plasmon coupling between the GNSs embedded into the PDMS film, which induces the strong electromagnetic field in and near the gaps between assembled GNSs.4 In addition to the interparticle plasmon couplings, the Raman enhancement effect is also caused by the strong electromagnetic field concentrated in the sharp multi-branch of GNSs due to the so-called lightning rod effect.38 With all these Raman enhancement effects, the SERS active film can significantly enhance the Raman spectra of target analytes adsorbed on the Raman-inactive surfaces. This approach based on free-standing SERS film covering on the target surface is clearly different from the conventional SERS substrates, where the target molecules need to be directly adsorbed onto the SERS substrate. For the conventional SERS substrates, the interaction between the target molecules and SERS substrate is a critical factor affecting the amounts of adsorbed target molecules and thus the final sensor performances. On the other hand, our approach based on freestanding SERS film covering on the target surface does not require the tedious sample preparation and surface treatment processes for the efficient adsorption of target molecules on the SERS


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substrates. Only the covering of free-standing SERS active film onto the target surface enables the Raman enhancement and the detection of target molecules. To further understand the effects of substrates underneath the SERS active film, we investigated and compared Raman enhancement effects on various underneath substrates including Ag and Au film coated onto silicon substrates, glass, silicon substrates. Figure 3c shows the Raman spectra of benzenethiol molecules adsorbed onto various substrates covered with flexible SERS films. As can be seen in Figure 3d, the silicon substrates coated with metal films (Ag, Au) exhibited significantly larger Raman intensities than those substrates (silicon, glass) without metal films. In particular, we noted that the silver-coated silicon substrate exhibits 591 times higher Raman intensity in the Raman fingerprint peak of 1079 cm-1 than that of silicon substrate. SERS enhancement factor (EF) of 1.9  108 was achieved on the Ag surface covered with the flexible SERS film embedding the uniform GNS assemblies (detailed description in Supporting Information S1). This significant increase in the SERS EF indicates the critical role of particlefilm plasmon couplings between GNSs and metal films in addition to the particle-particle plasmon couplings in high-density GNS assemblies of SERS active film on metal film-coated substrates. The different Raman enhancement on different metal surface (silver, gold) can be explained by the difference of optical loss between the Ag and Au films. It has been reported that the interaction of the LSPs of metal nanoparticles on the metal surface can be enhanced by the propagating SPP mode excited in the metal film surface.39 Ag surface has a low optical loss and the longer propagating SPPs than the Au surface, thus Ag films can contribute the stronger plasmon couplings in the GNS assemblies and thus the larger SERS EFs.40 To understand the Raman enhancement effect which is dependent on the GNS couplings within the PDMS film, we investigated and


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compared the SERS activity of the SERS sensors with different GNS densities. Depending on the GNS density, SERS sensors exhibit different SERS activities (Figure S6). The Raman spectra of benzenethiol molecules are not observed for the glass and silicon substrates without covering the SERS active film. However, when the SERS active film is covered onto the glass and silicon substrates, the specific Raman peaks of benzenethiol molecules can be clearly observed. Different dielectric substrates (silicon, glass) without metal film coatings exhibited different Raman enhancement that can be attributed to the difference of permittivity. The substrates with a higher permittivity (ε) can provide larger plasmon couplings with the LSPs of metal nanoparticles due to the stronger image charges.41 Therefore, Si substrates (ε ~ 11.76 at the frequency of 0.5 MHz)42 provided a higher SERS EF than that of glass substrates (ε ~ 4.6 at the frequency of 94 GHz).43

Sensing capabilities of transparent and flexible SERS films To investigate the detection limit of our SERS chemical sensors, the Raman spectra have been analyzed for the different concentration of benzenethiol molecules absorbed onto the Ag filmcoated silicon substrate covered with the SERS active film (Figure 4). For various concentrations (103 to 108 M) of benzenethiol, the Raman spectra of benzenethiol were observed for C–H outof-plane bending at 1006 cm-1 and C–C symmetric stretching at 1029 and 1079 cm-1 modes (Figure 4a). The Raman intensity gradually decreased with the decrease of benzenethiol concentration (Figure 4b). In particular, the Raman peak of benzenethiol molecules at the 1079 cm-1 can be clearly observed down to 10-8 M demonstrating the highly sensitive detection capability of our SERS chemical sensors. Our SERS sensors provide a sensitive detection of analytes (down to 10-


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M) with a high enhancement factor (1.9  108), which can be favorably compared to the previous

GNS-based SERS sensors (Table S1). Besides the sensing performance of the flexible SERS chemical sensors, the mechanical stability of flexible SERS sensors is important for practical applications. The mechanical durability of flexible SERS sensor was investigated by applying various mechanical deformation stimuli including the stretching, bending and torsion for 100 cycles. For the stretching test, the flexible SERS sensor was stretched to ~150 % (L/L0) (Figure 5a). For bending and torsion tests, the flexible SERS sensors were bent in half and twisted to ~180° (Figure 5b and 5c). The SERS signals were analyzed for benzenethiol (10-3 M) molecules absorbed onto the Ag substrate by covering flexible SERS active films after each mechanical stimuli (Figure 5a-c). For the 100 cycles of stretching, bending and torsion mechanical deformations, flexible SERS sensors showed no detectable change in the SERS signal maintaining the similar SERS intensity in the Raman fingerprint peak of 1079 cm-1 as that of the original state before the mechanical stimuli. The flexible SERS sensors are advantageous in the detection of analytes on the non-planar surfaces. We have performed SERS measurements of thiram which is one of the pesticide on the apple skin. Figure S7 exhibits that the flexible SERS sensors enable the detection of thiram on the non-planar apple skin. These results

demonstrate the high sensing performance and excellent mechanical durability of our flexible SERS sensors, which can be attributed to the embedded structure of GNS assemblies into the soft and thin PDMS film. Our free-standing SERS film-on-metal surface approach shows a highly sensitive detection capability of analyte molecules. To further investigate the site-specific Raman enhancement of flexible SERS film on metal surface, we fabricated flexible SERS sensors with micro-patterned GNS assemblies. GNS coated line-patterned Si substrate are shown in Figure S8. In this sensor


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design, GNS assemblies embedded in the protruding region of flexible SERS sensors can be selectively activated when the line-patterned flexible SERS sensors are in contact with the metal surface (Figure 6a). For the visual observation of site-specific selective Raman enhancement, we performed Raman mappings with p-aminothiophenol (PATP) molecules and graphene oxide adsorbed onto Ag films after covering the flexible SERS substrates patterned with GNS assemblies. The strong plasmon coupling effects between Ag film and patterned GNS assemblies within flexible SERS film selectively enhance the Raman spectra of adsorbed molecules (Figure 6b). Figure 6c shows the Raman mapping images of PATP with Raman bands at 1071 cm-1 and graphene oxide with Raman bands at 1335 cm-1 (D peak), which clearly visualize the micropatterned lines in Raman mapping images. The Raman mapping images indicate that the covering of flexible SERS sensors patterned with GNS assemblies onto the metal surface leads to the sitespecific selective enhancement of Raman signals for the surface adsorbed molecules.

CONCLUSIONS We demonstrated the fabrication of the transparent and flexible SERS sensor as simple and convenient SERS technique for highly sensitive Raman detection by embedding the GNSs monolayer into the thin PDMS film. The flexible SERS substrates enables the conformal coverage on arbitrary surfaces and the optical transparency allow the light interaction with the contact surface, thereby providing highly sensitive detection of analytes adsorbed on arbitrary metallic and enormous enhancement factor of ~1.9  108. Also, the flexible SERS sensor facilitated the observation of the distinct SERS signal even on the dielectric surfaces which do not provide any


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noticeable Raman signals of analytes. The interparticle couplings between the GNSs and lightening rod effects of sharp tips of GNSs as well as multiple field enhancement between GNSmetal film contribute to the enormous Raman enhancements. In addition, the flexible SERS sensor with GNS monolayer-embedded structure into PDMS maintain the stable SERS signal without any deterioration under various physical stimuli such as stretching, bending, torsion for each 100 cycle. And the reproducible and clear Raman signal is observed for the enhanced Raman factor. Flexible SERS substrate with large hot-surface is useful for chemical imaging. The use of a transparent, freestanding and flexible SERS active film also is useful for the application that is direct and nondestructive for observation of samples in the various state.

EXPERIMENTAL SECTION Synthesis of goldnanostar First, we employ the gold nanoparticle that has the multi branches called gold nanostar(GNS), made from round shaped gold nanoparticle as the seed. This gold nanoparticle of 18 nm diameter is prepared by Turkevich-Frens method44. 28 ul of HAuCl4 solution is soluble in the D.I water and heated to 100 ℃ and then 1.15 ml of 0.1 mol the trisodium citrate is added into the boiled HAuCl4 solution with stirring. Within the number of minutes, the color of solution is changed to red wine color and the solution is cooled to the room temperature. The synthesis of GNSs from spherical AuNPs was carried out in 75 ml of aqueous HEPES solution (25 mM, pH 7, Sigma-Aldrich). And the proper amount of the AuNP seed and 360 ul of 100 mM NH2OH is added into as-prepared mixture. And the enlargement of the Au nanoparticle seeds was achieved by dropwise adding 5


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mL of an aqueous solution of HAuCl4 (0.8 mM).The mixture is kept to the 4 ℃ for 5 h by using the cool plate.

Fabrication of transparent and flexible SERS film The flexible SERS film was made from high-density assembled GNSs onto silicon substrate. For the assembly of GNSs, the electrostatic interaction between the positively-charged poly(diallyl dimethylammonium) (PDDA) and negatively charged trisodium citrate surrounded GNS solution is introduced.3,10,45 The PDDA layer is formed onto the Si substrate by spin coating of the 0.2 % PDDA solution, resulting in about ~1 nm layer. And this PDDA coated Si substrate is deep coated into the GNS solution for 18 h. And GNS particles were uniformly assembled onto the silicon substrate in a monolayer followed by D.I washing and N2 blowing. Then, the appropriate amount of the PDMS is poured on the GNS assembled Si substrate and cured at the room temperature overnight. And the cured PDMS was carefully detached from the Si substrate. Finally, we fabricated the flexible SERS film achieving the transfer of assembled GNS into inner surface of PDMS thin film. The thickness of the film is measured to be ~200 μm (Figure S9).

Material characterizations The synthesized gold nanostar and SERS film were characterized by UV-vis-spectrophotometry (V-670, JASCO) and UV-vis-microspectrometer (20/20/PV, CRAIC), Field-emission scanning electron microscopy (FE-SEM, Nano 230, FEI), Transmission electron microscopy (JEM-2100, JEOL). For the observation of the SERS effect with the SERS film, we prepared the various substrates such as glass, Si, Au film, Ag film. We treated the benzenethiol on the desired substrate


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according to the following process. The desired substrates are submerged in the benzenethiol solution with the desired concentration dissolved in the ethanol solvent for 30 min. Then, the desired substrates are rinsed with the ethanol solvent for several times and the substrates are gently dried by N2 gas blowing. And the SERS film cover the substrates and it is possible to analyze the analyte between substrate and SERS film for the Raman spectroscopy. And the Raman spectra of analyte is excited with 785 nm laser using a confocal Raman spectroscope (alpha 300, WITec) and objective lens (50x). The integration time was 0.5 s; the laser power was ~15 mW.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: OM and SEM image of patterned Si, UV-vis resistivity (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. ORCID Hyunhyub Ko: Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This

work

was

supported

by

the

National

Research

Foundation

of

Korea

(2015R1A2A1A10054152).


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REFERENCES (1) Nie, S. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102-1106. (2) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single molecule detection using surface-enhanced Raman scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667. (3) Ko, H.; Chang, S.; Tsukruk, V. V. Porous substrates for label-free molecular level detection of nonresonant organic molecules. Acs Nano 2008, 3, 181-188. (4) Lee, J.; Hua, B.; Park, S.; Ha, M.; Lee, Y.; Fan, Z.; Ko, H. Tailoring surface plasmons of high-density gold nanostar assemblies on metal films for surface-enhanced Raman spectroscopy. Nanoscale 2014, 6, 616-623. (5) Liu, G.; Li, Y.; Duan, G.; Wang, J.; Changhao; Liang; Cai, W. Tunable surface plasmon resonance and strong SERS performances of Au opening-nanoshell ordered arrays. ACS Appl. Mater. Interfaces 2012, 4, 1-5. (6) Velev, O. D.; Kaler, E. W. Structured porous materials via colloidal crystal templating: from inorganic oxides to metals. Adv. Mater. 2000, 12, 531-534. (7) Brolo, A. G.; Arctander, E.; Gordon, R.; Leathem, B.; Kavanagh, K. L. Nanohole-enhanced Raman scattering. Nano Lett. 2004, 4, 2015-2018. (8) Lee, J.; Zhang, Q.; Park, S.; Choe, A.; Fan, Z.; Ko, H. Particle-Film Plasmons on Periodic Silver Film over Nanosphere (AgFON): A Hybrid Plasmonic Nanoarchitecture for SurfaceEnhanced Raman Spectroscopy. ACS Appl. Mater. Interfaces 2016, 8, 634-642. (9) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G. Self-assembled metal colloid monolayers: an approach to SERS substrates. Science 1995, 267, 1629-1632. (10) Ko, H.; Singamaneni, S.; Tsukruk, V. V. Nanostructured surfaces and assemblies as SERS media. Small 2008, 4, 1576-1599. (11) Aksu, S.; Huang, M.; Artar, A.; Yanik, A. A.; Selvarasah, S.; Dokmeci, M. R.; Altug, H. Flexible plasmonics on unconventional and nonplanar substrates. Adv. Mater. 2011, 23, 4422-4430. (12) Osberg, K. D.; Rycenga, M.; Bourret, G. R.; Brown, K. A.; Mirkin, C. A. Dispersible surface-enhanced Raman scattering nanosheets. Adv. Mater. 2012, 24, 6065-6070. (13) Singh, J. P.; Chu, H.; Abell, J.; Tripp, R. A.; Zhao, Y. Flexible and mechanical strain resistant large area SERS active substrates. Nanoscale 2012, 4, 3410-3414. (14) Zhang, R.; Xu, B. B.; Liu, X. Q.; Zhang, Y. L.; Xu, Y.; Chen, Q. D.; Sun, H. B. Highly efficient SERS test strips. Chem. Commun. 2012, 48, 5913-5915. (15)Polavarapu, L.; Liz-Marzan, L. M. Towards low-cost flexible substrates for nanoplasmonic sensing. Phys. Chem. Chem. Phys. 2013, 15, 5288-5300. (16) Lee, C. H.; Tian, L.; Singamaneni, S. Paper-based SERS swab for rapid trace detection on real-world surfaces. ACS Appl. Mater. Interfaces 2010, 2, 3429-3435. (17) Zhang, W.; Li, B.; Chen, L.; Wang, Y.; Gao, D.; Ma, X.; Wu, A. Brushing, a simple way to fabricate SERS active paper substrates. Anal. Methods 2014, 6, 2066. (18) He, D.; Hu, B.; Yao, Q.-F.; Wang, K.; Yu, S.-H. Large-scale synthesis of flexible freestanding SERS substrates with high sensitivity: electrospun PVA nanofibers embedded with controlled alignment of silver nanoparticles. ACS nano 2009, 3, 3993-4002.


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(19) Xu, W.; Ling, X.; Xiao, J.; Dresselhaus, M. S.; Kong, J.; Xu, H.; Liu, Z.; Zhang, J. Surface enhanced Raman spectroscopy on a flat graphene surface. Proc. Natl. Acad. Sci. U.S.A 2012, 109, 9281-9286. (20) Hasell, T.; Lagonigro, L.; Peacock, A. C.; Yoda, S.; Brown, P. D.; Sazio, P. J. A.; Howdle, S. M. Silver Nanoparticle Impregnated Polycarbonate Substrates for Surface Enhanced Raman Spectroscopy. Adv. Func. Mater. 2008, 18, 1265-1271. (21) Sun, Y.; Liu, K.; Miao, J.; Wang, Z.; Tian, B.; Zhang, L.; Li, Q.; Fan, S.; Jiang, K. Highly sensitive surface-enhanced Raman scattering substrate made from superaligned carbon nanotubes. Nano Lett. 2010, 10, 1747-1753. (22) Zhou, Y.; Cheng, X.; Yang, J.; Zhao, N.; Ma, S.; Li, D.; Zhong, T. Fast and green synthesis of flexible free-standing silver nanoparticles–graphene substrates and their surface-enhanced Raman scattering activity. RSC Adv. 2013, 3, 23236. (23) Zuo, Z.; Zhu, K.; Gu, C.; Wen, Y.; Cui, G.; Qu, J. Transparent, flexible surface enhanced Raman scattering substrates based on Ag-coated structured PET (polyethylene terephthalate) for in-situ detection. Appl. Surf. Sci. 2016, 379, 66-72. (24) Xu, K.; Wang, Z.; Tan, C. F.; Kang, N.; Chen, L.; Ren, L.; Thian, E. S.; Ho, G. W.; Ji, R.; Hong, M. Uniaxially Stretched Flexible Surface Plasmon Resonance Film for Versatile Surface Enhanced Raman Scattering Diagnostics. ACS Appl. Mater. Interfaces 2017, 9, 26341-26349. (25) Qu, L.-L.; Li, D.-W.; Xue, J.-Q.; Zhai, W.-L.; Fossey, J. S.; Long, Y.-T., Batch fabrication of disposable screen printed SERS arrays. Lab Chip 2012, 12, 876-881. (26) Tseng, S.-C.; Yu, C.-C.; Wan, D.; Chen, H.-L.; Wang, L. A.; Wu, M.-C.; Su, W.-F.; Han, H.-C.; Chen, L.-C. Eco-friendly plasmonic sensors: using the photothermal effect to prepare metal nanoparticle-containing test papers for highly sensitive colorimetric detection. Anal. Chem. 2012, 84, 5140-5145. (27) Zhang, C. L.; Lv, K. P.; Cong, H. P.; Yu, S. H. Controlled Assemblies of Gold Nanorods in PVA Nanofiber Matrix as Flexible Free‐Standing SERS Substrates by Electrospinning. Small 2012, 8, 648-653. (28) Lu, G.; Li, H.; Zhang, H. Nanoparticle-coated PDMS elastomers for enhancement of Raman scattering. Chem. Commun. 2011, 47, 8560-8562. (29) Lu, G.; Li, H.; Zhang, H. Gold-nanoparticle-embedded polydimethylsiloxane elastomers for highly sensitive Raman detection. Small 2012, 8, 1336-1340. (30) Wen, X. L.; Li, G. Y.; Zhang, J.; Zhang, Q.; Peng, B.; Wong, L. M.; Wang, S. J.; Xiong, Q. H. Transparent free-standing metamaterials and their applications in surface-enhanced Raman scattering. Nanoscale 2014, 6, 132-139. (31) Shiohara, A.; Langer, J.; Polavarapu, L.; Liz-Marzan, L. M. Solution processed polydimethylsiloxane/gold nanostar flexible substrates for plasmonic sensing. Nanoscale 2014, 6, 9817-9823. (32) Fortuni, B.; Fujita, Y.; Ricci, M.; Inose, T.; Aubert, R.; Lu, G.; Hutchison, J. A.; Hofkens, J.; Latterini, L.; Uji-i, H. A novel method for in situ synthesis of SERS-active gold nanostars on polydimethylsiloxane film. Chem. Commun. 2017, 53, 5121-5124. (33) Smith, E. A.; Chen, W. How to prevent the loss of surface functionality derived from aminosilanes. Langmuir 2008, 24, 12405-12409. (34) Ko, H.; Jiang, C.; Tsukruk, V. V. Encapsulating nanoparticle arrays into layer-by-layer multilayers by capillary transfer lithography. Chem. Mater. 2005, 17, 5489-5497.


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Page 18 of 25

(35)Maiorano, G.; Rizzello, L.; Malvindi, M. A.; Shankar, S. S.; Martiradonna, L.; Falqui, A.; Cingolani, R.; Pompa, P. P. Monodispersed and size-controlled multibranched gold nanoparticles with nanoscale tuning of surface morphology. Nanoscale 2011, 3, 2227-2232. (36) Trigari, S.; Rindi, A.; Margheri, G.; Sottini, S.; Dellepiane, G.; Giorgetti, E. Synthesis and modelling of gold nanostars with tunable morphology and extinction spectrum J. Mater. Chem. 2011, 21, 6531–6540 (37) Yuan, H.; Khoury, C. G.; Hwang, H.; Wilson, C. M.; Grant, G. a; Vo-Dinh, T. Gold Nanostars: Surfactant-Free Synthesis, 3D Modelling, and Two-Photon Photoluminescence Imaging Nanotechnology 2012, 23, 075102. (38) Liao, P. F.; Wokaun, A. Lightning rod effect in surface enhanced Raman scattering. J. Chem. Phys. 1982, 76, 751-752. (39) Huang, F. M.; Wilding, D.; Speed, J. D.; Russell, A. E.; Bartlett, P. N.; Baumberg, J. J. Dressing plasmons in particle-in-cavity architectures. Nano Lett. 2011, 11, 1221-1226. (40) Lamprecht, B.; Krenn, J.; Schider, G.; Ditlbacher, H.; Salerno, M.; Felidj, N.; Leitner, A.; Aussenegg, F.; Weeber, J. Surface plasmon propagation in microscale metal stripes. Appl. Phys. Lett. 2001, 79, 51-53. (41) Knight, M. W.; Wu, Y.; Lassiter, J. B.; Nordlander, P.; Halas, N. J. Substrates Matter Influence of an Adjacent Dielectric on an Individual Plasmonic Nanoparticle. Nano Lett. 2009, 9, 2188-2192. (42) Dunlap Jr, W.; Watters, R. Direct measurement of the dielectric constants of silicon and germanium. Phys. Rev. 1953, 92, 1396. (43) Lide, D. R. CRC handbook of chemistry and physics. CRC press: 2004. (44) Frens, G. Particle size and sol stability in metal colloids. Kolloid-Z. u. Z. Polymere 1972, 250, 736-741. (45) Gole, A.; Orendorff, C. J.; Murphy, C. J. Immobilization of gold nanorods onto acidterminated self-assembled monolayers via electrostatic interactions. Langmuir 2004, 20, 71177122.


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Figure 1. Schematic illumination showing the fabrication process of flexible SERS sensor with gold nanostar arrays. Self-assembled gold nanostar arrays are transferred from silicon substrate into PDMS.


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Figure 2. Characteristics of as-prepared gold nanostar and Photo image and SEM images of selfassembled gold nanostar arrays embedded into PDMS for flexible SERS sensor. (a) Representative SEM image of gold nanostar. The inset show the TEM image of GNSs with sharp tips. (b) UV-vis absorption spectra showing the SPR peak of gold nanostar at ~640 nm peak. (c) Photo image of bare silicon, gold nanostar on silicon, bare PDMS and gold nanostar imbedded PDMS. And the flexible characteristics of flexible SERS sensor including the bending, stretching and torsion of gold nanostar imbedded PDMS. (d) The SEM images of self-assembled gold nanostar arrays onto silicon substrate. (e) The SEM images of self-assembled gold nanostar arrays embedded into PDMS.


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Figure 3. SERS effect with flexible SERS sensor. (a) Schematic illumination showing the detection process to demonstrate SERS effect with flexible SERS sensor. (b) Raman spectra of benzenethiol molecules before and after covering the flexible SERS sensor over the silicon substrate to confirm the SERS effect. (c,d) The comparison of Raman spectroscopy intensities of 0.1 % benzenethiol onto metal (Ag, Au film) and dielectric (Si, glass) substrate to show GNSmetal film plasmon coupling between the flexible SERS sensor and underlying substrates.


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(b) Raman intensity (a.u.)

(a) Raman intensity (a.u.)

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10000 8000 6000 4000 2000 0

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Figure 4. The comparison of Raman intensities as the concentration of benzenethiol molecules covering with flexible SERS sensor onto Ag film. (a) Raman spectra of benzenethiol molecules from 1 mM to 10 nM. (b) Raman peak of benzenethiol molecules at a 1079 cm-1 is clearly measured up to 10 nM to show the highly sensitive detection capability of flexible SERS sensor.


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Figure 5. Durability test as the mechanical stimuli of flexible SERS sensor. Schematic illustration, the comparative SERS intensity and Raman spectra (a) after the stretching to ~50 % of flexible SERS sensor, (b) after the bending in half of flexible SERS sensor, (c) after the torsion to ~180 ° of flexible SERS sensor for 100 cycles.


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Figure 6. Raman mapping images with patterned flexible SERS sensor. (a) Schematic illustration of covering the underlying substances with patterned flexible SERS sensor. (b) Raman spectra before and after covering with the patterned flexible SERS sensor onto PATP and Graphene oxide (GO), respectively. (c) Raman mapping image of PATP (bands: at 1079 cm-1) and Graphene oxide (bands: D peak at 1335 cm-1) covering with the patterned flexible SERS sensor, respectively.


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Transparent and Flexible Surface-Enhanced ... - ACS Publications

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