Spontaneous Self-Formation of 3D Plasmonic Optical Structures Inhee Choi,†,‡,§,¶ Yonghee Shin,∥,¶ Jihwan Song,⊥,¶ SoonGweon Hong,†,‡ Younggeun Park,†,‡ Dongchoul Kim,*,†,⊥ Taewook Kang,*,†,∥ and Luke P. Lee*,†,‡ †
Department of Bioengineering and ‡Berkeley Sensor and Actuator Center, University of California, Berkeley, California 94720, United States § Department of Life Science, University of Seoul, Seoul 130-743, Republic of Korea ∥ Department of Chemical and Biomolecular Engineering and ⊥Department of Mechanical Engineering, Sogang University, Seoul 121-742, Republic of Korea., S Supporting Information *
ABSTRACT: Self-formation of colloidal oil droplets in water or water droplets in oil not only has been regarded as fascinating fundamental science but also has been utilized in an enormous number of applications in everyday life. However, the creation of three-dimensional (3D) architectures by a liquid droplet and an immiscible liquid interface has been less investigated than other applications. Here, we report interfacial energy-driven spontaneous self-formation of a 3D plasmonic optical structure at room temperature without an external force. Based on the densities and interfacial energies of two liquids, we simulated the spontaneous formation of a plasmonic optical structure when a water droplet containing metal ions meets an immiscible liquid polydimethylsiloxane (PDMS) interface. At the interface, the metal ions in the droplet are automatically reduced to form an interfacial plasmonic layer as the liquid PDMS cures. The self-formation of both an optical cavity and integrated plasmonic nanostructure significantly enhances the fluorescence by a magnitude of 1000. Our findings will have a huge impact on the development of various photonic and plasmonic materials as well as metamaterials and devices. KEYWORDS: self-formation, interfacial energy, liquid droplets, nanoparticles, plasmonic optical structures, plasmonic optical cavity
S
Here we report the spontaneous self-formation of 3D plasmonic optical structure with self-integrated plasmonic thin film layer from an ionic water droplet in liquid polydimethylsiloxane (PDMS) at room temperature. Our proposed method is illustrated in Figure 1. When the ionic water droplet is placed on the surface of the liquid PDMS, the water droplet is immediately surrounded by the liquid PDMS (shown in the middle image in Figure 1a). In this process, the surface tension of the water droplet is an important factor that determines the fast and spontaneous formation of a spherical cavity. As time proceeds, interfacial reduction of metal ions as well as solidification of the liquid PDMS automatically occurs without an additional step (shown in the right image in Figure 1a). Hollow cavity structures integrated with either Ag or Au plasmonic nanoparticles can be obtained, depending on the selection of metal ions (shown in Figure 1b and 1c). Creation
mall oil droplets in water (oil-in-water) and the reverse (water-in-oil) have been extensively utilized in the development of various functional materials for applications ranging from biomedicine1−8 to catalysis.9−16 Thanks to their easy compartmentalization and isotropic reaction conditions, droplets ranging from a few micrometers to tens of nanometers in size can serve as excellent chemical reactors for e.g. producing inorganic and organic nanoparticles.11−13,17−21 Furthermore, liquid droplets allow for the intradroplet assembly of a variety of materials including lipid molecules, polymers, and micro/nanoparticles.4−10 Lastly, the self-assembly of such droplets with a similar size can also act as a template for the construction of porous materials via a typical sol−gel process or polymerization.14−16,22 However, the creation of integrated three-dimensional (3D) solid architectures by a liquid droplet and an immiscible liquid interface has been less thoroughly investigated. Moreover, if the size of such a droplet increases over several hundreds of micrometers, where the gravitational force is no longer negligible, the droplet is no longer considered useful for this purpose. © 2016 American Chemical Society
Received: May 2, 2016 Accepted: June 27, 2016 Published: June 27, 2016 7639
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Figure 1. Spontaneous, single-step, and room-temperature formation of plasmonic optical cavities: ionic water droplet on liquid PDMS. (a) Schematic illustration showing one-step formation of a plasmonic optical cavity by a droplet of ionic water and sequential metal ion reduction along the surface. When the ionic water droplet is placed on the surface of the liquid PDMS, the water droplet is immediately surrounded by the liquid PDMS due to its higher surface tension. As time proceeds, interfacial reduction of metal ions and solidification of the liquid PDMS automatically occur. (b, c) Side views of representative Ag and Au plasmonic optical cavities formed by dropping ionic water solution with dissolved metal ion precursors (HAuCl4 and AgNO3) onto a liquid PDMS. Scale bars, 5 mm. (d) Top view of close-packed Au plasmonic optical cavities. Scale bar, 1 cm. (e) Array of Au plasmonic optical cavities. Scale bars, 1 cm.
Figure 2. Computational and experimental analyses of spontaneous and rapid formation of a spherical cavity by a water droplet on liquid PDMS. (a) Three-dimensional geometry of a liquid droplet in liquid PDMS with the forces acting on the system depicted: gravitational and buoyancy forces on the droplet, surface energies of the liquid droplet and liquid PDMS, and interfacial energy between the liquid droplet and liquid PDMS. (b) Simulation results for three cases: (1) ρ > ρliq PDMS, (2) ρ < ρliq PDMS and γ ≤ γliq PDMS, and (3) ρ < ρliq PDMS and γ ≫ γliq PDMS. Experimental results for each case: (1) ethylene glycol (ρethylene glycol > ρliq PDMS), (2) ethanol (ρethanol < ρliq PDMS and γethanol ≤ γliq PDMS), and (3) water (ρwater < ρliq PDMS and γwater ≫ γliq PDMS). Scale bars, 1 mm. (c) Simulation and experimental results for the shape evolution of a single water droplet upon contact with liquid PDMS. The volume of the water droplet is 10 μL, and blue dye was used for visualization. Scale bars, 1 mm. (d) Plot for horizontal diameter versus vertical diameter (respectively denoted as “x” and “y” in inset) of the fabricated cavity and comparison of spheres having same radial diameters (black line).
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Figure 3. Self-formation of a plasmonic cavity with an interfacial layer of nanoparticles integrated along its surface. (a) Time-lapse photographs of the formation of an Ag plasmonic cavity. Images were collected after dropping 50 μL of 5 mM AgNO3 aqueous solution onto the liquid PDMS. Scale bars, 2 mm. (b, c) Dark-field scattering images of cross-sectional surface of Ag plasmonic cavities (b) and Au plasmonic cavities (c) formed using different concentrations of AgNO3 and HAuCl4, respectively. Scale bars, 20 μm. (d, e) Dark-field scattering images of the surface of Ag plasmonic cavities (d) and Au plasmonic cavities (e) formed with varying concentrations of AgNO3 and HAuCl4, respectively. Scale bars, 20 μm. (f, g) Corresponding scattering spectra collected from the surface of Ag plasmonic cavities (f) and Au plasmonic cavities (g), respectively. (h) Size distribution of the formed nanoparticles at the cavity’s surface. 70 particles in each electron micrograph were randomly selected for statistical analysis.
Using our model, the motion of the liquid droplet after making contact with the surface of the liquid PDMS was simulated by varying the properties of the liquid droplet (i.e., ρliq droplet and γliq droplet). Three cases were examined and compared with experimental findings: (1) ρ > ρliq PDMS, (2) ρ < ρliq PDMS and γ ≤ γliq PDMS, and (3) ρ < ρliq PDMS and γ ≫ γliq PDMS (Figure 2b). The simulation results show that if the density of the liquid droplet is higher than that of the liquid PDMS, the droplet tends to sink completely into the liquid PDMS (case 1). On the other hand, if the liquid droplet is less dense than the liquid PDMS (cases 2 and 3) the droplet becomes positioned near the surface of the PDMS, and interestingly, its shape depends critically on its surface tension. If the surface tension of the liquid droplet is comparable or lower than that of the liquid PDMS [γliq PDMS is 22−25 mN/ m23−25], the droplet becomes ellipsoidal (case 2). However, if the surface tension of the liquid droplet is much higher than that of the liquid PDMS, for example, a water droplet, with γwater of 72.8 mN/m,24 the droplet becomes spherical (case 3). These computational results are consistent with the experimental results. A droplet of ethylene glycol (ρethylene glycol is 1.115 g/mL at 20 °C) sinks to the bottom of the liquid PDMS (case 1). An ethanol droplet [γethanol of 22.4 mN/m26,27] becomes positioned near the surface and takes an ellipsoidal shape (case 2), while a water droplet produces a more spherical cavity (case 3). The observed results can be summarized as the
of closely packed cavities or a periodic array of the cavities (Figure 1d and 1e) is also achievable via consecutively dropping of the metal ionic solution.
RESULTS AND DISCUSSION First, to investigate the mechanism for spontaneous cavity formation by a water droplet, the forces acting on the liquid droplet and liquid PDMS are considered (Figure 2a). The liquid droplet and the liquid PDMS have their own energies along the free surface area (i.e., surface energy) and at the interface between them (i.e., interface energy), which depend on the nature of the molecular contact between them. They are also affected by gravity according to their density: in addition, a buoyancy force acts on the liquid droplet when it sinks into the liquid PDMS. Thus, the system energy that includes all of the energetic contributions can be expressed as G = G bulk + Gsurface + Ginterface + Ggravity + G buoyancy
(1)
where Gbulk, Gsurface, Ginterface, Ggravity, and Gbuoyancy are the energies corresponding to bulk, surface, interface, gravitational potential, and buoyancy, respectively. Given this description of system energy, a 3D dynamic model based on the phase field approach was developed to describe the motion of the liquids. Further details regarding its computation and scale are described in the Supporting Information. 7641
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Figure 4. Optical properties of the spherical plasmonic cavity. (a) Side views of the simulated ray traces in the Ag plasmonic optical cavity at different focal points (top) and representative intensity profiles of the accumulated rays for different focal planes (bottom), and their comparison with those of flat PDMS case. Scale bars, 200 μm. (b) Enhanced electric field for an oligomeric assembly of Ag nanoparticles for the signal amplifications of biological or chemical assays. As an example, simulation was carried out for Ag nanoparticles 154 nm in diameter (from Figure 3h) and having an interparticle distance of 1 nm. (c) Confocal imaging configuration used to measure fluorescence intensity at different focal planes of the cavity. (d) Representative confocal laser scanning images of a fluorophore (PI) loaded Ag plasmonic optical cavity obtained from the different focal planes. Scanning direction is from bottom to top of the cavity. Scale bars, 200 μm. (e, f) Fluorescence images of the cavities collected at the focal plane i (e) and ii (f) denoted in c. Ag plasmonic optical cavities prepared under varying Ag concentrations from 0.5 mM to 5 mM were tested. A flat PDMS film and a PDMS cavity without the integrated nanoparticles (i.e., 0 mM) were also tested for comparison. All images were taken under the same light exposure with a 10× objective lens. Scale bars, 200 μm. (g) Plot depicting the differences in fluorescence intensities measured in different focal planes (in e and f) for all Ag plasmonic optical cavities. An inset shows a plot for fluorescence enhancement factor (EF) of each cavity compared with that of the flat PDMS. The EF increases with increasing concentration of Ag ion used in forming the Ag plasmonic optical cavity.
droplet, water droplets, which have high surface energy, take a spherical shape in order to minimize the system energy since the spherical shape has the smallest surface-area-to-volume
position of a droplet depends on the relative densities of the liquids, while the morphology of a droplet is determined by GSurface. Specifically with regard to the morphology of the 7642
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To investigate the optical properties of the spherical plasmonic cavities, ray optics simulation was conducted using commercial software (COMSOL Multiphysics 5.0; Comsol, Inc.). Based on Figure 3, we assumed that the Ag and Au nanoparticles formed at the interface generate continuous plasmonic films on the cavity. The simulated ray trace in the Ag plasmonic cavity is shown in Figure 4a and supplementary movie S1. The results show that the accumulation of light is maximized near the bottom of the cavity, and considerable increases in the accumulated ray intensity are also observed around the center and the sidewall of the cavity (Figure 4a). The accumulated ray intensity of the Ag plasmonic cavity is higher than that of the Au plasmonic cavity (Figure S5) due to the wavelength of the incident light (543 nm). To estimate the near-field enhancement between the integrated nanoparticles, electromagnetic simulation was also conducted for an oligomeric assembly of nanoparticles and with interparticle distances of 1, 5, and 10 nm (Figure S6). The interparticle distance was determined from the appearance of a longer plasmon band around 500−600 nm (Figure 3f) induced by plasmonic coupling between Ag nanoparticles.31−33 The PDMS layer integrated with Ag nanoparticles shows a 400-fold higher maximum electric field intensity than that of the solid PDMS (Figure 4b and Figure S6). To confirm the enhanced optical performance of the plasmonic cavities we developed, the fluorescence signal from the propidium iodide (PI) in the Ag plasmonic cavity was measured by a confocal laser scanning microscopy setup with a 543 nm laser as the light source (Figure 4c). The Ag plasmonic optical cavities were prepared under varying Ag concentrations. The fluorescence excitation maximum and emission maximum of PI are around 535 and 617 nm, respectively. Figures 4d presents representative confocal laser scanning images obtained from the different focal planes in the Ag plasmonic optical cavity. The maximum fluorescence intensity is measured near the bottom surface of the cavities (Figure 4d,e). Along the sidewall surface of the cavity, the fluorescence intensity is also considerably enhanced (Figure 4d,f and Figure S7). Enhanced fluorescence signals of PI are observed from all tested Ag plasmonic optical cavities (Figure 4e,f). In contrast, no significant fluorescence signals are observed from the flat PDMS and the PDMS cavity without the integrated nanoparticles (i.e., 0 mM cavity). From the intensity analysis, the fluorescence enhancement increases with increasing concentration of Ag ion used in forming the Ag plasmonic optical cavity (Figure 4g). Fluorescence intensity can be maximized to 1000 times that of the flat PDMS (an inset in Figure 4g). This significant fluorescence enhancement can be attributed to multiple effects derived from the spherical cavity structure (i.e., light confining and focusing) and the integrated Ag plasmonic nanoparticles (i.e., efficient light reflection and near-field enhancement).
ratio. By the same token, ethanol droplets also change to minimize the system energy, but because they have low surface energy, they become ellipsoidal and occupy a larger surface area to achieve the same end. The formation kinetics of the spherical cavity created by a droplet of water after it contacts the liquid PDMS are shown in Figure 2c. The simulation results show that a water cavity is formed near the surface of liquid PDMS within a few seconds, which agrees well with experimental results (Figure 2c). It is also observed experimentally that the spherical shape of the water cavity is maintained irrespective of its diameter, from tens of micrometers to a few millimeters (Figure 2d and Figure S1). An experimental demonstration of the interfacial formation of metallic nanoparticles along the surface of the water cavity is presented in Figure 3a. On the basis of the previous reports,28−30 Si−H groups in the curing agent (Figure S2) act as the direct reduction reagent to form metallic nanoparticles; that is, the metal ions are diffused into the liquid PDMS and react with the residual Si−H groups, resulting in the formation of metallic nanoparticles. Therefore, as the liquid PDMS is cured, the metal ions are naturally reduced to nanoparticles, resulting in a solid spherical cavity with a selfintegrated nanoparticle layer. For example, 10 min after placing an AgNO3 aqueous droplet on the liquid PDMS, the transparent cavity gradually turns into yellow, indicating the formation of plasmonic Ag nanoparticles (Figure 3a). For the case of an HAuCl4 aqueous droplet, a change in the interfacial color from transparent to purple is observed with time, indicating the formation of plasmonic Au nanoparticles (Figure S3). In order to estimate the thickness of the interfacial nanoparticle layer along the surface of the cavity, cross-sectional scattering images of the interface were obtained by varying the concentration of Ag and Au ions using a dark-field microscope (Figure 3b,c). For both ions, as their concentrations increase (from 0.1 to 5 mM), the thicknesses of the resulting plasmonic nanoparticle layers gradually increase. In order to estimate the size and density of the nanoparticles, scattering images, spectra, and transmission electron microscopy (TEM) images were collected. Scattering images and corresponding spectra from the nanoparticles in the layer also show distinct color transitions and concomitant plasmon band shifts (wavelength and intensity) in accordance with the increase in concentration (Figure 3d−g). Note that in the case of the Au plasmonic cavity a red shift of the plasmon band (to the near-infrared range around 700 nm) is more prominent than in that of the Ag plasmonic cavity. In the scattering images (Figure 3d,e), increase in the density of scattering spots implies the increase in the number of nanoparticles in the layer. In order to further characterize the size of the nanoparticles formed in the plasmonic layer, nanoparticles formed at the water−PDMS interface were sampled onto TEM grids. In each electron micrograph, 70 particles were randomly selected, and their sizes were statistically analyzed. As shown in Figure 3h and Figure S4, average sizes of Ag and Au nanoparticles tend to increase as the concentration of the precursor solution increases (Ag: from 20.0 ± 8.1 nm to 154.0 ± 63.5 nm, Au: from 26.0 ± 8.4 nm to 90.0 ± 52.9 nm). Both the size increase and broader size distribution in accordance with the increase of the salt concentration are responsible for the red-shift and broadening of the scattering peaks (Figure 3f,g). Collectively, observed color transitions, plasmon band shifts, and TEM results can be interpreted as increases in both the density and the size of nanoparticles on the surface of the cavity.
CONCLUSIONS In conclusion, we have demonstrated the self-transformation of photonic cavity with a self-integrated plasmonic thin film layer from a small water droplet containing metal ions placed on an immiscible liquid at room temperature without an external force. Rapid and spontaneous formation of the plasmonic optical cavity on liquid PDMS is found to be driven mainly by the interfacial energy of water. We observed the concomitant reduction of metal ions to an interfacial metallic thin film as the liquid PDMS cures. The physical properties of these interfacial 7643
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which plasmon bands are well overlapped with the excitation band of PI. We note that the fluorescence excitation maximum and emission maximum of PI is around 535 and 617 nm, respectively. In order to maximize the fluorescence signal, imaging was carried out under a matched excitation condition with the plasmon band of the Ag cavity and the absorption (excitation) band of the analytes by utilizing a 543 nm laser as a light source. Ag cavities were prepared with varying AgNO3 concentrations, and then a 10 μM PI solution was loaded in them to measure the enhanced fluorescent signals. Serial scans in the z-direction were started from the bottom of the cavity. Transmission Electron Microscope Analysis. In order to characterize the metal nanoparticles formed at the surface of plasmonic cavity, TEM analysis was carried out. Metal ion precursor (AgNO3 and HAuCl4) solutions were prepared with varying concentrations: 0.1, 0.5, 1, 2, and 5 mM. Each metal ion precursor solution was dropped to liquid PDMS for formation of plasmonic cavity at room temperature. After 12 h, the solution at the interface was collected for the TEM analysis. A 10 μL portion of each solution was dropped onto a carboncoated 300 mesh TEM grid (Ted Pella, Inc.). TEM images were obtained with a JEOL JEM 1010 electron microscope operating at an acceleration voltage of 80 kV.
nanoparticles, including size, thickness, and plasmon resonance, can be tuned by varying the concentration of the metal ion in the water droplet. The fluorescence signal in the cavities with integrated Ag nanoscale thin film was significantly increased, by a magnitude of 1000, which can be attributed to the spherical optical cavity structure and the surface Ag plasmonic thin film. Since 3D spherical plasmonic cavities produced using our method can play two optical functions, as angular reflectors to confine incident light in the cavity, and as nanoscale photon sources to scatter absorbed light into the cavity, they could greatly improve the signal sensitivity of most surface plasmon based detection methods, such as plasmon-enhanced fluorescence and surface-enhanced Raman scattering. For example, they could apply to amplify fluorescent signals for enzymelinked immunosorbent assay, recombinase polymerase amplification, polymerase chain reaction, etc. Finally, since PDMS is moldable, flexible, inexpensive, and optically transparent, various formats of plasmonic cavity arrays can be fabricated on 2D and 3D structures for further amplifications of biological and chemical assay signals by developing various innovative 3D photonic structures, which can be also applied in 3D metalens. We believe that our findings will open exciting avenues in fields ranging from photonics and metamaterials to biomedical applications.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02903. Computational details and supplementary figures (PDF) Movie of the representative ray tracing in the Ag plasmonic optical cavity (AVI)
METHODS Materials. Gold(III) chloride trihydrate (HAuCl4·3H2O, ≥99.9%), silver nitrate (≥99.8%), and propidium iodide (PI, 95%) were purchased from Sigma-Aldrich and used without any further pretreatment. The Sylgard 184 (including PDMS monomer and curing agent) was purchased from Dow Corning (Midland, MI). Formation of Plasmonic Optical Cavities. Plasmonic optical cavities were prepared by dropping ionic water droplets onto the liquid PDMS. Ionic water solutions were prepared by varying the concentration (from 0.1 to 5 mM) and the composition of metal ion precursors (i.e., AgNO3 and HAuCl4). The liquid PDMS layer was prepared by mixing liquid PDMS base and curing agent (10−1 ratio) for all subsequent experiments for forming cavities. After degassing of the PDMS mixture for 1 h, the prepared ionic water solution was dropped on to the liquid PDMS layer, which was cured at room temperature. Darkfield Imaging and Spectral analyses. The fabricated plasmonic cavities were characterized by using an enhanced darkfield transmission optical microscope (Olympus BX43, Tokyo, Japan) equipped with a hyperspectral imaging spectrophotometer (CytoViva Hyperspectral Imaging System (HSI), Auburn, AL). This system employs a darkfield-based illuminator that focuses a highly collimated light at oblique angles on the sample to obtain images with improved contrast and signal-to-noise ratio. This imaging technique utilizes the intrinsic scattering properties of nanoparticles, and therefore, neither staining nor a contrast agent is required to visualize the nanoparticles. A concentric imaging spectrophotometer that was capable of recording the high-quality spectrum (high signal-to-noise ratio) at visible and near-infrared (VNIR: 400−1000 nm) wavelengths at a high spectral resolution of 1.5 nm with 10 nm scan size and pixel size 25 nm was used. A motorized stage was guided by the HSI system for synchronizing sample movement with a hyperspectral image scanner. Using this system, scattering images of nanoparticles from the cavity surface were taken using an immersion darkfield condenser and a truecolor camera with a white light illumination. Scattering spectra from the surface of the plasmonic cavity were collected at more than 10 points and averaged. Confocal Laser Scanning Imaging. Fluorescence signals enhanced from our spherical plasmonic cavities were collected by using a confocal laser scanning microscopy setup (LSM510, Carl Zeiss, Jena, Germany). We measured the plasmon-enhanced fluorescent intensity of PI fluorescent dye in our Ag plasmonic optical cavities, of
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions ¶
I.C., Y.S., and J.S. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the Air Force Office of Scientific Research Grants AFOSR FA2386-13-1-4120 to L.P.L. and the International Research & Development Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2013K1A3A1A32035444) to Y.S. and T.K. This work was also supported by a Korea CCS R&D Center (KCRC) grant funded by the Korean government (Ministry of Science, ICT & Future Planning) (Grant No. 2015M1A8A1053539) to L.P.L., the National Science Foundation (EFRI-SEED grant award No. 1038279) to L.P.L. and Y.P., and a National Research Foundation of Korea (NRF) grant funded by Korea government (MSIP) (No. 2014R1A2A2A09052374, 2013R1A1A2011263) to J.S. and D.K. This research was also supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (2013K1A4A3055268) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03012845) to T.K. This work was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by 7644
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