Droplet-Guiding Superhydrophobic Arrays of Plasmonic Microposts for

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Droplet-Guiding Superhydrophobic Arrays of Plasmonic Microposts for Molecular Concentration and Detection Hyelim Kang, Yongjoon Heo, Dong Jae Kim, Ju Hyeon Kim, Tae Yoon Jeon, Soojeong Cho, Hye-Mi So, Won Seok Chang, and Shin-Hyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11506 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Applied Materials & Interfaces

Droplet-Guiding Superhydrophobic Arrays of Plasmonic Microposts for Molecular Concentration and Detection Hyelim Kang,† Yong Joon Heo,† Dong Jae Kim,† Ju Hyeon Kim,† Tae Yoon Jeon,† Soojeong Cho,† Hye-Mi So,§ Won Seok Chang,§,‡ and Shin-Hyun Kim†* †

Department of Chemical and Biomolecular Engineering (BK21+ Program), KAIST, Daejeon

34141 Korea, Email: Shin-Hyun Kim ([email protected]) §

Nano-Convergence Mechanical Systems Research Division, Korea Institute of Machinery and

Materials, Daejeon 34103 Korea ‡

Department of Nanomechatronics, Korea University of Science and Technology, Daejeon

34113 Korea

KEYWORDS: Colloidal lithography, photolithography, superhydropobic, surface-enhanced Raman scattering, concentration

ACS Paragon Plus Environment

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ABSTRACT

Droplet-guiding superhydrophobic SERS substrates are created by a combinatorial lithographic technique. Photolithography defines the pattern of a micropillar array with a radial density gradient, whereas colloidal lithography features a nanotip array on the top surface of each micropillar. The nanotip array renders the surface superhydrophobic and the pattern of micropillars endows the radial gradient of contact angle, enabling the spontaneous droplet migration toward the center of the pattern. Water droplets containing target molecules are guided to the center and the molecules dissolved in the droplets are concentrated at the surface of the central micropillar during droplet evaporation. Therefore, the molecules can be analyzed at the predefined position by Raman spectra without scanning the entire substrate. At the same time, SERS-active nanotip array provides high sensitivity of Raman measurement.


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INTRODUCTION Raman scattering has provided fingerprints for molecular identification and detection due to its high specificity on molecular composition and structure. However, high intensity of electromagnetic wave and high concentration of analyte molecules are prerequisite for acquiring sufficient Raman intensity for measurement. To overcome the first limitation, plasmonic metal nanostructures with nanogaps or sharp edges have been employed. Such structures dramatically increase the intensity of incident electromagnetic wave on their surface through surface plasmon resonance (SPR), amplifying Raman signals of molecules adsorbed on the surface; this amplification is referred to as surface-enhanced Raman scattering (SERS).1-6 The enhancement factor of Raman signal can be as high as 1015 for silver nanoparticles,1 which provides high Raman intensity for molecules adsorbed with high surface density. Nevertheless, when sample fluids are dilute, the molecules are slowly adsorbed over the wide surface in a diffusion-limited rate, resulting in a low surface density of molecules and achieving low Raman signal. To concentrate molecules in a local site of the plasmonic nanostructure, superhydrophobic surfaces have been used.7-15 Water drops deposited on the superhydrophobic surface have low contact angle (CA) hysteresis due to a low areal fraction of solid-liquid contact,16 which enables to sustain spherical shape of water drops in a course of evaporation except the last stage. The molecules dissolved in the drops can be enriched, which are finally deposited on the very small surface area; this approach, therefore, addresses the second limitation. However, drops migrate on the superhydrophobic surface during shrinkage, making a position of final destination uncertain. Therefore, entire surface should be inspected for SERS analysis unless target molecules are fluorescently labeled. To manipulate the drop migration, wettability of superhydrophobic surfaces has been spatially modulated by various techniques;


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chemical or temperature gradients and surface density gradient are used to cause spontaneous migration of drops.17-23 In addition, pinning spots with high wettability are patterned in the surfaces with low wettability to deposit accumulated molecules in a pre-defined position.23-26 To the best of our knowledge, however, only a single report has described the successful preparation of

SERS-active

substrates

with

drop-guiding

and

deposition

functions,

which

is

superhydrophobic bull’s eye.27 The bull’s eye composed of radially-aligned microfins with constant thickness is designed to have relatively low CA at the center;27 the radial microfins make gradual increase of solid fraction toward the center. To form plasmonic nanostructure on the bull’s eye, an aqueous suspension of metal nanoparticles is first deposited, which makes random aggregates of nanoparticles at the center. An aqueous solution of analyte molecules is then deposited on the substrate, which is concentrated on the surface of the random aggregates, providing Raman spectrum of the analyte enhanced by the aggregates. However, the formation of random aggregates is not highly reproducible. More importantly, the aggregates are fragile and easily disintegrated from the bull’s eye, permitting only single use. Therefore, designing of welldefined plasmonic structures integrated into superhydrophobic drop-guiding surface still remains an important challenge. Here, we report a hybrid approach of photolithography and colloidal lithography to make superhydrophobic plasmonic nanostructures for molecular concentration and SERS analysis at a pre-defined location. Colloidal monolayer partially embedded in the negative photoresist is used as a template to make plasmonic nanotip arrays at a period of 540 nm, which is further patterned into cylindrical microposts with a diameter of 30 µm by photolithography. The number density of the posts is spatially modulated to be highest at the center and gradually reduced along the radial direction. This leads to wettability gradient on the resulting


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superhydrophobic micropattern. Therefore, aqueous drops deposited in the pattern experience Laplace pressure gradient, leading to the migration toward central posts.17, 18 In addition, the drops maintain spherical shape in a course of evaporation until their radii become comparable to the inter-post separation. This provides a considerable enrichment of molecules in the drop and leads to highly focused deposition of the molecules only on the surfaces of several central posts, enabling simple Raman characterization of molecules dissolved in water. In addition, high stability of the plasmonic post patterns enables the reuse after washing with high reproducibility of Raman intensity. Moreover, an array of the patterns can be used for analysis of multiple samples using single substrate. RESULTS AND DISCUSSION Designing SERS-active substrates. To make periodic metal nanostructures for SERS, a monolayer of the colloidal array is used as a template. Monodisperse silica particles with a diameter of 540 nm are spin-casted on the surface of the negative photoresist, SU-8, to form a hexagonal array, which is then partially embedded by thermal annealing at 80°C for 6 min.6, 28, 29 The monolayer formed on the SU-8 is shown in Figure S1 of the Supporting Information. The diameter of 540 nm is selected to make a well-defined nanotip array at high density; a larger diameter reduces the density of nanotips and smaller diameter loses structural features of nanotips due to the fragility of a thin structure. As the partial embedding of silica particle is driven by capillary force, the embedding ceases when the silica particles form an equilibrium contact angle at the interface between air and molten SU-8. Therefore, silica particles have the comparable level of the partial exposure with a negligible variation in a wide area, which makes the production of the nanostructure highly reproducible. After photopolymerization of the resist, silica particles are etched out and resulting hexagonal array of cavities is subjected to reactive


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ion etching (RIE) with SF6 gas to make sharp tips at the positions of cavity interstices.29, 30 Silver deposition over the sharp tips renders the structure SERS-active. When the silver-coated substrate is treated by perfluorothiol by vapor deposition, the CA of the SERS-active surface increases to 150°, as shown in Figure S2a. However, the CA hysteresis remains as high as 30° as shown in Figure S2b and c, achieving only limited enrichment; deposition of 10 µL water drop leaves behind a stain with a diameter of 1.40 mm, as shown in Figure S3a of the Supporting Information. To render the SERS-active substrate to have small CA hysteresis, an areal fraction of the solid-liquid interface should be further reduced. At the same time, the fraction is required to be spatially modulated to manipulate migration of water drop. To accomplish these, silica particleembedded photoresist is micropatterned by photolithography to create an array of cylindrical posts with a diameter of 30 µm and height of 70 µm whose separation is increased along a radial direction from the center to the border within 1 cm circle area. The spacing between two neighboring posts near the border is 320 µm, which supports water droplets with a diameter of 13 mm, while avoiding the infiltration into the interstitial area among the posts. The micropattern is subjected to the same procedure of silica particle removal, RIE treatment, silver deposition, and surface fluorination. The overall procedure of the fabrication is schematically summarized in Figure 1a. The resulting micropattern of plasmonic posts at which water drop is deposited on the center is shown in Figure 1b. The posts are positioned in a series of concentric circles; the innermost circle has a radius of 60 µm, and the outer circles are placed to make gradual increase of inter-post distance along the radial direction, as shown in Figure 1c. Each post has silver nanotip array on the top surface for SERS, as shown in Figure 1d. As the nanotips are formed by etching of polymers in the interstitial regions among cavities, they take a shape of inverse


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triangular pyramid whose top triangle has a dimension of approximately 100 nm, as shown in Figure 1e, f. The nanotip arrays formed on the surfaces of all the microposts helps render the surface superhydrophobic; in the absence of the arrays, the micropattern is not superhydrophobic and exhibits high contact angle hysteresis. Droplet-guiding and molecular concentration. The hierarchical micropattern with radial density gradient of plasmonic posts can guide water drops to the center as shown in Figure 2a and Movie S1 of the Supporting Information. Regardless of deposition position, water drops much larger than post separation have different contact angles at two edges along the radial direction. This results in different curvatures of free interface and therefore different Laplace pressures. The Laplace pressure gradient within the drops leads to internal flow from the edge with high CA to that with low CA, as illustrated in Figure S4.18 Therefore, the center of mass of the water drop moves toward the edge with low CA and the drop migrates toward the center of the post array.17-23 To confirm the gradient of CA, we measure both advancing and receding CAs on the central region and near the border of the circle. The advancing and receding CAs on the center are measured as 153° and 142° as shown in Figure 2b, whereas those near the border are 152° and 149° as shown in Figure 2c. The average CA on the center is lower than that near the border as expected from the larger areal fraction of the solid-liquid interface on the center; an areal fraction of solid-liquid interface near the border is 34 times smaller than that on the center (see the Supporting Information for detailed calculation). The CA gradient along the radial direction provides the driving force for the drop motion. In addition, CA hysteresis, defined as the difference between the advancing and receding CAs, is as small as 3° near the border owing to a small solid fraction. This yields low resistance against drop motion and enables the migration even with small Laplace pressure gradient. It is well-known that the migration velocity


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of droplets is linearly proportional to the droplet thickness.31 Therefore, a droplet with larger volume shows faster migration toward the center. The small hysteresis is beneficial for localization of analyte during evaporation of water drop. To confirm that molecules dissolved in water drop are concentrated near the central posts, we deposit a 10 µL water drop containing 10-7 M fluorescent dye molecules of rhodamine 6G (R6G) and evaporate the water at ambient condition. The water drop smoothly shrinks with a constant number of wetted posts at the initial stage of evaporation, of which center is then relocated through stick-slip motion as the number is discretely reduced as shown in Movie S2 of the Supporting Information. This relocation during the shrinkage helps the spherical drop find a central post as shown in Figure 3a; the drop guided by Laplace pressure gradient is slightly offcentered, which is then further guided during evaporation. At the last stage, droplet size becomes comparable to the separation of posts and therefore, CA is not determined by surface property averaged from the air and solid; Cassie-Baxter model is invalid at this stage. Therefore, small CA hysteresis is not retained, resulting in pinning of the drop on a few central posts as shown in the last image of Figure 3a. At this moment of transition, the concentration of molecules in drops is approximately 104 times enriched, as estimated from drop diameters at the initial and transition moment; the droplet diameter is reduced from 2.8 mm to 130 µm until the transition. The CA of the drop is rapidly reduced after the triple line is pinned while maintaining underlying air mat. This selectively deposits concentrated R6G on the top surfaces of 9 central posts including the centermost and their interstices. We further confirm this by imaging R6G with confocal laser scanning microscope. The micropattern is imaged without deposition of drop for a comparison as shown in Figure 3b, which shows auto-fluorescence from posts.32, 33 When we deposit a water drop, only several central posts, and their interstices are selectively stained by red dye molecules,


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as shown in Figure 3c. Deposition of molecules in the post interstices is inevitable due to the pinning of the triple line along the outermost contour of the droplet at the moment of the transition; this reduces final concentration on the posts to be approximately half. The guided evaporation of water drops results in significant concentration. In comparison with a nonpatterned substrate composed of fluorinated silver nanotip array, the micropattern provides 75 times higher concentration for the same volume of a water drop, as shown in Figure S3b of the Supporting Information. Molecular concentration and Raman analysis at predefined position. The silver nanotip arrays strongly absorb a light with wavelength of 350 nm and the absorbance decreases with wavelength, as shown in Figure S5a. Nevertheless, Raman scattering can be significantly enhanced with a laser with wavelength of 514 nm as the absorbance is still significant, as shown in Figure S5b; Raman enhancement is insignificant at 633 nm and almost negligible at 785 nm. The strong localization of electric field around the silver nanotip under the irradiation with a laser of 514 nm wavelength is also confirmed by finite-difference time-domain (FDTD) calculation, as shown Figure 4a; the model structure is constructed with 20-nm-thick silver layer on a triangular pillar made of SU-8 with a dimension of 100 nm. The localization is the maximum at the edges of triangle at which intensity of electromagnetic field is enhanced in a factor of |E|2/|E0|2 = 53.7; SERS enhancement of |E|4/|E0|4 = 2880 is expected. The destination of the concentrated molecules always includes the central post. Therefore, we measure Raman spectra of the molecules from the central post using a laser of 514 nm wavelength. We confirm negligible Raman intensity of perfluorothiol before drop deposition which is coated on the silver surface for surface fluorination. When 10 µL water drop containing 10-3 M R6G is deposited and evaporated at ambient condition, we obtain typical Raman


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spectrum of R6G as shown in Figure 4b; characteristic peaks are located at 610, 775, 1127, 1183, 1310, 1360, 1507, 1573, and 1648 cm-1.34 To study the influence of initial R6G concentration on Raman intensity, we reduce the concentration from 10-6 M by a factor of 10 and measure corresponding Raman spectrum as shown in Figure 4c; we include the peaks at 610 cm-1 and 775 cm-1 in the measurement range. Raman intensity is linearly proportional to logarithmic concentration, and the intensity remains sufficiently high for the concentration of 10-9 M, as shown in Figure 4d. This indicates that our pattern can be used for quantitative detection of analyte molecules.35 For a comparison, the patterns are immersed in ethanolic R6G solutions with the same set of concentrations for 1 hour respectively, which are then washed with ethanol several times to measure Raman intensity without concentrating effect; a soaking method is typically used in sample preparation for SERS measurement.36 Ethanolic solutions are used, instead of aqueous solutions, to fully infiltrate the pores of the superhydrophobic surface with the solution. The pattern exhibits a very low intensity of Raman spectrum even for the concentration of 10-6 M and no signal for lower concentrations, as shown in Figure 4d. This manifests that dilute analyte molecules can be effectively enriched and localized on the central posts of the superhydrophobic micropatterns. The intensity from 10-9 M aqueous solution is much higher than that from 10-6 M ethanolic solution, indicating that the micropattern provides an enhancement effect corresponding to an at least 1000-fold increase in the analyte concentration; this factor can be increased if a larger volume of the water drop is deposited as more molecules are concentrated on the central part of the micropattern. On the other hand, we further confirm with Raman measurement that molecules are selectively deposited in the several centermost posts, as shown in Figure S6. The post of the second centermost with R6G deposition shows Raman intensity comparable to that from the centermost, whereas the post of the third


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centermost without R6G only shows noise-level signal. To further study the influence of dropletguiding micropatterns on the Raman intensity, 10 µL water drop of 10-7 M R6G is deposited on the micropattern-free superhydrophobic nanotip arrays. The peak intensity at 610 cm-1 is 1.8 times and 1.3 times smaller than that obtained at micropatterns with 10-7 M and 10-8 M R6G, respectively as shown in Figure S7; enhancement factor of 40 is obtained from a linear relation between Raman intensity and logarithmic concentration. This result is roughly consistent with the enrichment factor of 75 achieved by the micropattern, as we confirm in Figure S3. High stability of metal nanostructures. The silver nanotip array integrated on the top surface of each micropost has high mechanical stability, enabling the repeated use without deterioration of SERS activity. To confirm this, we measure Raman spectra during 3 cycles of water drop deposition and washing, as shown in Figure 4e, where water drops contain 10-6 M R6G. During washing the surface with water, deposited molecules are removed, yielding no meaningful Raman signal. The superhydrophobic property remains unchanged after the washing. Therefore, Raman intensity is retained for all three measurements with only small variation, proving high reproducibility as well as recyclability. In addition, the underlying polymer structure is made of highly crosslinked SU-8, providing high chemical resistance. Therefore, the composite structure is compatible with the use of organic solvents such as ethanol and isopropanol. Moreover, the high mechanical stability of the integrated metal nanostructure allows the mechanical agitation and sonication. We prove the stability by repeatedly depositing and washing highly adhesive protein of bovine serum albumin (BSA) tagged with fluorescein isothiocyanate (FITC). The BSA deposited on the central part of pattern for the first drop is completely washed out with ethanol jet and subsequent sonication, as shown in Figure S8. For the second drop, the BSA is deposited on the central part in the same manner to the first drop, proving that the superhydrophobicity and


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droplet-guiding performance remain unchanged. The surface morphology of nanotip arrays is also conserved even after sonication and washing as shown in Figure S9, indicating that the structure has sufficient mechanical stability for reuses. Therefore, molecules with hydrophobic moieties can also be repeatedly deposited and washed out, potentially enabling the analysis of molecules with a wide spectrum of polarity. The post patterns with the density gradient can be further arrayed for analysis of multiple samples on a single substrate; because the micropattern is featured by photolithography, any shape and geometry can be prepared. For example, 2 × 2 array of patterns can be prepared, each of which can accommodate own aqueous drop as shown in top panel of Figure 4f. Therefore, four different samples can be analyzed with a single substrate. As a demonstrative purpose, we place four different water drops containing 10-7 M malachite green (MG), 10-7 M R6G, 10-6 M crystal violet (CV), and 10-5 M benzenethiol (BT) on the four distinct spots, respectively. Raman spectrum obtained from each spot has the characteristic peaks of its own molecule without crosscontamination, as shown in the bottom panel of Figure 4f.

CONCLUSIONS In this work, we report a lithographic method to create SERS-active hierarchical patterns with functions of molecular concentration and localization. The pattern is photolithographically designed to have cylindrical micropost array with radial density gradient which yields CA gradient on the superhydrophobic surface. Plasmonic nanotip array is formed on the top surface of each post by colloidal lithography which provides SERS activity. An aqueous drop containing analyte molecules is guided to migrate to the center of the pattern by Laplace pressure gradient in


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the drop. Subsequent evaporation of the drop concentrates the molecules and deposits them selectively on the central microposts. Therefore, Raman spectra of the molecules are simply measured from the central posts with dramatically enhanced sensitivity in comparison with free adsorption of analytes in bulk samples; Raman intensity is enhanced by a factor of at least 103 without change of metal nanostructures. In addition, the pattern can be further arrayed by photolithography to have discrete sites for independent SERS characterization of many aqueous samples. This combinatorial lithographic method is highly reproducible. This is because most delicate procedure in the fabrication, which is the partial embedding of silica particles into SU-8, is precisely controlled by a capillary wetting. Moreover, SERS-active metal nanostructures are fully integrated into micropatterns, providing high structural stability and therefore allowing repeated uses. More importantly, a small amount of sample is easily deposited on the pattern, and the analyte molecules are concentrated and localized, thereby enabling simple and fast analysis with high precision; it typically takes 40 min to evaporate water fully at the normal ambient condition. We summarize the characteristics of our substrates in a comparison with previous reports on superhydrophobic SERS-active substrates in Table S1 of the Supporting Information. There have been only few reports which simultaneously achieve molecular enrichment with high reproducibility, reusability, and cost-effective production. Moreover, there is only one report which provides the concentration of molecules at a predefined position. Therefore, we believe that our hierarchical patterns with wettability gradient are promising for Raman analysis of water-soluble molecules. One remaining concern on the superhydrophobic SERS-active substrates is a lack of selectivity on target molecules. All the molecules including targets and impurities are concentrated, which increases Raman intensities of impurities as well as targets. If the hydrogel layer is introduced on the top of the central post in our patterns, the hydrogel


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meshes with consistent size will allow the selective diffusion of small molecules onto the surface of metal nanostructures.37 Therefore, the limitation on the molecular selectivity can be partially overcome. METHODS Preparation of substrates. Thin adhesion layer of SU-8 2 (MicroChem) with a thickness of 1.5 µm is spin-casted on the wafer, which is then fully polymerized by ultraviolet (UV) irradiation with an intensity of 14.5 mW/cm2 for 30 s and post-exposure baking at 95 °C for 1 min. Over the adhesion layer, SU-8 50 (MicroChem) is spin-coated to be 70 µm thick and then baked at 65°C for 15 min and 95°C for 50 min. The film is treated with oxygen plasma for 1 min to render the surface hydrophilic. Monodisperse silica particles with a diameter of 540 nm are dispersed in ethanol at the concentration of 7 w/w%, which is spin-coated on the surface of the SU-8 film at 2900 rpm to form a monolayer of a hexagonal array. The substrate is then thermally annealed at 80°C for 6 min to partially embed the monolayer into the molten SU-8 film. To make an array of microposts with a radial gradient, the silica particle-embedded SU-8 film is exposed to UV light with an intensity of 14.5 mW/cm2 for 80 s through a photomask with design same to the target array. The film is baked at 65°C for 3 min and at 95°C for 12 min, which is then developed with propylene glycol monomethyl ether acetate (PGMEA, Sigma-Aldrich). Silica particles are removed from microposts by immersing the substrate in 2 w/w% hydrofluoric acid for 2 min and washed with isopropyl alcohol. To create nanotip array on the top surfaces of microposts, the substrate is subjected to RIE (Vacuum Science Inc.) with SF6 gas for 100 s with the power of 100 W and flow rate of 100 sccm. Silver is deposited over the microposts with nanotip arrays using thermal evaporator (Korea Vacuum Tech., KVE-T2004L) to be 20 nm thick. To render the


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surface superhydrophobic, the silver is coated with 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10heptadecafluoro-1-decanethiol (Sigma-Aldrich) through vapor deposition method. Characterization. The arrays of SERS-active microposts are observed with an optical microscope (Nikon, L150) and scanning electron microscope (SEM, Hitachi, S-4800); for SEM observation, the samples are coated with gold to render them conductive. The arrays with and without deposition of R6G are also observed with confocal laser scanning microscope (Zeiss, LSM 5 PASCAL). Contact angles of water drops are measured using drop shape analyzer (Kruss, DSA10-Mk2). Raman spectra are obtained using dispersive Raman (Horiba Jobin Yvon) equipped with a laser beam with 514 nm wavelength and 1.24 mW power. The spectra are acquired for 5 s from measurement spot with a diameter of 1 µm. For the measurement, 10 µL drops of water containing R6G (Sigma-Aldrich) are deposited on the arrays and vaporized in ambient condition. To evaluate reusability, the array with deposited molecules is washed with a water jet from a squeeze bottle. FDTD calculation. A model geometry of single nanotip is constructed from SEM image in Figure 1d. The linearly polarized 514 nm of the laser is vertically irradiated to the nanotip. Dispersive and lossy nature of silver is reflected in Drude model, and the refractive index of SU8 is set to 1.65.


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Figure 1. (a) Schematic diagram illustrating the preparation procedure of droplet-guiding superhydrophobic surface composed of plasmonic micropost array. A monolayer of silica particles is deposited on the photoresist film, which is then spontaneously embedded by thermal annealing. The photoresist film is micropatterned to have micropost arrays with density gradient through photolithography, from which silica particles are etched out to make void arrays on top surfaces of microposts. The top surface is further subjected to reactive ion etching (RIE) with SF6 gas to make sharp tip arrays, over which Ag is deposited to form plasmonic nanotip arrays. (b) Photograph showing aqueous droplet placed on the droplet-guiding superhydrophobic substrate. (c) Optical microscope (OM) image showing the central part of the superhydrophobic post arrays with a radial density gradient. (d) Scanning electron microscope (SEM) image showing the top surface of single micropost. (e, f) SEM images showing the nanotip arrays: top view (e) and side view (f). The nanotips take a shape of inverse triangular pyramid. Triangular surfaces are denoted in (e).


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Figure 2. (a) Series of still shot images showing the migration of droplet toward the center of the superhydrophobic substrate, where 20 µL of the water droplet is deposited. (b, c) Advancing and receding contact angles (CAs) of a water droplet on the (b) central and (c) outer parts of the superhydrophobic substrate.


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Figure 3. (a) Series of still-shot images showing evaporation-induced shrinkage of a droplet on the superhydrophobic substrate. The centers of drop and pattern in each image are denoted with white and red dots, respectively. Each image is taken at the denoted time after the droplet deposition. (b, c) Confocal laser scanning microscope images of micropost array in the center of the superhydrophobic substrate without (b) and with deposition of rhodamine 6G (R6G). 10 µL water drop containing 10-7 M R6G is deposited and evaporated in (c). The microposts show weak autofluorescence, whereas concentrated R6G shows strong fluorescence. The 7 centermost dots are denoted with white circles.


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Figure 4. (a) Spatial distribution of the electric field intensity normalized by the electric field of the 514-nm incident light, |E|2/|E0|2, in silver-coated nanotip. (b) Raman spectrum of R6G molecules measured from the centermost micropost. The molecular structure of R6G is shown in the inset. (c) Raman spectra of R6G measured from the centermost micropost, where water droplets containing four different concentrations of R6G, 10-6, 10-7, 10-8, and 10-9 M, are deposited on the superhydrophobic substrate and evaporated (red curves) or the substrate is immersed in ethanolic solution of R6G at concentration of 10-6 M for 1 h and washed (a blue curve) prior to the measurement. Raman spectrum measured from the centermost micropost without R6G deposition is included for comparison (a gray curve). The asterisks indicate characteristic peak positions of R6G molecules. (d) Concentration dependence of Raman intensity of R6G at 610 cm-1 measured from the superhydrophobic substrate, where water droplet containing R6G is deposited on the substrate and evaporated (red squares) or the substrate is immersed in an ethanolic solution of R6G and washed (blue squares). (e) Raman spectra measured from the centermost micropost in the superhydrophobic substrate during three cycles of R6G deposition and washing. (f) Photograph showing an array of four superhydrophobic patterns, each of which has own aqueous droplet (top panel) and Raman spectra measured from four different sites at which malachite green (MG), R6G, crystal violet (CV), and benzenethiol (BT) are deposited from four different droplets respectively (bottom panel).

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge on the ACS Publication website. Confocal microscope images and absorption spectrum of the substrates, Raman spectra of target molecules acquired with various laser sources, locations of measurement spot, concentrations of the molecules, and table comparing previously reported SERS substrates. (PDF) Movie S1 and S2 showing the droplet guiding toward the center of micropattern and evaporation-induced deposition of molecules at the center. (AVI)

AUTHOR INFORMATION


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Corresponding Author * Email:[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This

work

was

supported

by

the

Midcareer

Researcher

Program

(NRF-

2017R1A2A2A05001156) and Global Research Laboratory (NRF-2015K1A1A2033054) through the National Research Foundation (NRF), funded by the Ministry of Science, ICT and Future Planning (MSIP). Dr. W. S. Chang appreciates the Global Frontier R&D Program by the Center for Advanced Meta-materials (CAMM) (2014M3A6B3063707) and the Nano Material Technology Development Program (2014M3A7B6020163) by MSIP/NRF and the Industrial Core Technology Program (10062837) by MOTIE/ KEIT.

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Droplet-Guiding Superhydrophobic Arrays of Plasmonic Microposts for

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