communications Biophotonic Surfaces
Biologically Inspired Biophotonic Surfaces with Self-Antireflection Young-Jae Oh, Jae-Jun Kim, and Ki-Hun Jeong* Biologically inspired nanoarchitectures exhibit intriguing functions at solid-surrounding medium interfaces such as self-cleaning,[1,2] strong adhesion,[3,4] antireflection,[5,6] or environmental adaptation.[7,8] In particular, antireflective structures (ARS) inspired from the corneal surface of moth eyes can simply modulate the effective index by controlling the fill factor (FF), i.e., the relative areal fraction of subwavelength nanostructures, thereby satisfying an antireflection condition at air-solid interface (Figure 1a).[9,10] In the last decade, ARS have been extensively utilized for assorted photonic applications such as solar cells,[11,12] photodetectors,[13] display panels,[14–16] or optical lenses[17,18] to reduce specular reflection on the interfaces. However, all the previous works still leave out an additional important function, i.e., self-antireflection. The effective index of ARS is significantly affected by the index of a surrounding medium, which entirely fills the interstitial nanogap spacing between the nanostructures. Not only the air, but also solution media can fill the interstitial space to modulate the effective index of ARS. A particular FF of ARS can offer the spontaneous antireflection for diverse surrounding media with different indices. The index mismatch at either air-substrate or solution-substrate interfaces always causes specular reflection, particularly on either or both sides of a substrate, which substantially hinders highly sensitive biophotonic sensing and imaging due to optical loss or unfavorable interferences.[19] Consequently, this new function can provide the full benefits of ARS for highly efficient optical bioassays such as colorimetric, fluorescent, or luminescent assays. In this work, we report biologically inspired biophotonic surfaces with self-antireflection for highly sensitive optical biosensing and bioimaging. Both surfaces of the substrates enclose large-scale and low-cost glass nanopillar arrays (GNA) with ∼0.5 in FF. The top-side GNA serve as either antireflective or plasmonic surfaces at a solution-substrate interface and the bottom as antireflective surfaces at air-substrate interface, respectively (Figure 1b, Supporting Information Figure S1 for the enlarged SEM images). The GNA filled
Y.-J. Oh, J.-J. Kim, Prof. K.-H. Jeong Department of Bio and Brain Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305–701, Korea E-mail: [email protected] DOI: 10.1002/smll.201303876
2558
wileyonlinelibrary.com
with interstitial medium can be considered as single-layer ARS. Based on the Maxwell Garnett model,[5] the self-antireflection can be realized by using the spontaneous index modulation of ARS, i.e., medium-filled GNA with ∼0.5 FF, at both air-substrate (nair and nsub) and solution-substrate (nsol and nsub) interfaces, where a surrounding medium with an index ranging from 1.0 to 1.8 completely fills the interstitial gaps between the nanopillars. As a result, the effective index naturally meets an antireflection condition (Supporting Information Figure S2). The biophotonic surfaces exhibit diverse examples such as antireflective substrates with single-side GNA (SS-GNA) and double-side GNA (DS-GNA) or nanoplasmonic surfaces for highly sensitive fluorescence sensing or surface enhanced Raman scattering (SERS) as well as high contrast imaging (Figure 1c). A wafer-level nanofabrication of GNA with self-antireflection was done by using thin silver film annealing and reactive ion etching (RIE) on a borosilicate glass substrate (n≈1.47). Thermal annealing transforms thin silver film into size-controllable silver nanoislands, which serve as an etching mask for nanopillar formation during the RIE process. Note that the FF of GNA was precisely controlled with the density of Ag nanoislands depending on the initial thickness of thin silver film. This method was applied on both sides of the glass wafer, where the top-side GNA can also be further functionalized as plasmonic nanostructures by evaporating thin silver or gold film (Figure 2a). The GNA were etched down by a quarter-wavelength to suppress specular reflection at air-substrate interface. The effective index for 0.5 ± 0.01 FF GNA with 130 nm in nanopillar height naturally varies from 1.22 to 1.64 for 1.0 to 1.8 in surrounding index, close to the geometric mean of substrate and surrounding indices, i.e., an ideal antireflection condition at the interface (Supporting Information Figure S2). In experiment, the maximum transmission through the SS-GNA is clearly shown at 130 nm in nanopillar height, which improves transmission by ∼3.8 percent at air-substrate interface, compared with the flat surface (Figure 2b). The DS-GNA remarkably double the transmission improvement, which shows transmittance over 99 percent. In particular, the size distribution of GNA offers broadband antireflection in a visible range from 485 nm to 655 nm. The optical images show specular reflections from flat surfaces, SS-GNA, and DS-GNA on 4-inch glass wafers, respectively (Figure 2b). The DS-GNA provides a clear image of the letters whereas those behind the flat surfaces are visually unreadable due to the substantial specular reflection.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, 10, No. 13, 2558–2563
Biologically Inspired Biophotonic Surfaces with Self-Antireflection
Figure 1. Biologically inspired surfaces with self-antireflection for highly sensitive biophotonic sensing and imaging. (a) Self-antireflection inspired from the corneal nanostructures of a Lepidoptera: Nolidae. (b) Biologically inspired biophotonic surfaces with self-antireflective structures (SARS). Both sides of glass nanopillar arrays (GNA) serve as SARS, i.e., antireflective structures with ∼0.5 fill factor (FF), where topside GNA can also be further functionalized as plasmonic nanostructures with nanogap-rich metal nanoislands. The effective indices (nbot and ntop) of SARS at either air-glass or solution-glass interface spontaneously meet antireflection conditions for diverse surrounding media with different indices when a surrounding medium completely fills the interstitial gap spacing between the nanostructures. (c) Optical images of substrates with biophotonic surfaces, i.e., antireflective and nanoplasmonic surfaces. The antireflective substrates includes a single (SS-GNA), double-side (DS-GNA) antireflective glass nanopillar arrays and the nanoplasmonic surfaces enclose plasmonic nanostructures at different plasmon resonances (scale bars : 1 cm). Light reflection from the surface is clearly suppressed by DS-GNA, compared with flat surfaces or even SS-GNA.
small 2014, 10, No. 13, 2558–2563
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
2559
communications
Y.-J. Oh et al.
Figure 2. Nanofabrication methods and optical properties of surfaces with SARS at air-glass interface. (a) Wafer-level nanofabrication for GNA with ∼0.5 FF on both sides of a glass substrate obtained by reactive ion etching with a silver nanoisland mask. Plasmonic nanopillar arrays can be further formed by using additional silver or gold deposition. (b) Light transmission through the antireflective substrates with GNA of different heights. The SS-GNA with 130 nm in nanopillar height show the maximum transmittance of 0.96 in visible region. The DS-GNA further increases light transmittance over 0.99. The reduced reflection from DS-GNA on 4-inch glass wafers clearly makes the letters behind the substrate readable compared with the SS-GNA or the flat surfaces.
SERS and fluorescence signal improvements were quantitatively investigated with the biophotonic surfaces with self-antireflection. Both the nanoplasmonic and antireflective surfaces were utilized for SERS and fluorescence experiments, respectively. The nanoplasmonic substrates contain the ∼0.5 FF GNA on both sides, where the top GNA was fully covered with nanogap-rich silver nanoislands by using thermal evaporation of 30 nm thick silver film.[20] The bottom GNA serve as self-antireflective structures (SARS), which substantially improve SERS signals from reference molecules (benzenethiol) on top-side silver nanoislands during the excitation and collection. The bottom GNA with 130 nm in nanopillar height increase the SERS signals by over 20 percent, compared with a flat bottom surface (Figure 3a). The asterisks shown in the inset of Figure 3a indicate the major SERS peaks for the quantitative comparisons. Fluorescence signals were also compared with DS-GNA and SS-GNA on the bottom-side. Three different fluorescent dye solutions in red, green, and blue were wet on the top surfaces. All the fluorescence intensities were measured and averaged from the areal fluorescent images of 180 µm × 180 µm (see methods for details). The red fluorescence signals from nile red solution are increased by 14 percent from SS-GNA and by 16 percent from DS-GNA, respectively, compared with the flat surfaces. The green signals from rhodamine 6G (R6G) are improved by about 16 percent from SS-GNA and by 19 percent from DS-GNA. The blue signals from 7-diethylaminocoumarin-3-carboxylic acid (succinimidyl ester) are
2560 www.small-journal.com
also increased by about 15 percent from SS-GNA and by 18 percent from DS-GNA (Figure 3b). Compared with the flat surfaces, SS-GNA provide 15 percent improvement and DS-GNA increase the fluorescent signals up to 18 percent on average. The fluorescent signals from the DS-GNA exhibit an additional increment despite the small index mismatch at the solution-substrate interfaces, compared with SS-GNA. Note that the signal improvements of both SERS and fluorescence are higher than the expected values (∼7.7%) from transmittance improvement through the GNA during both the excitation and the collection process. This substantial improvement can be explained by the following reasons; both light coupling and extraction efficiency through the ARS increase with the incident angle, compared with those from the flat surface.[21,22] This angular dependency directly contributes to both the excitation and the collection light through a microscope objective. Besides, fluorescence and SERS signals have a slightly nonlinear relationship with the low excitation power (Supporting Information Figure S3).[23,24] High contrast imaging was also demonstrated with the biophotonic surfaces with self-antireflection. Specular reflection from the top-side solution-substrate interface is relatively low due to the small index-mismatch unlike that from the bottom air-substrate interface. However, this small reflection may cause significant background noises or unfavorable interferences during optical bioimaging. Figure 4a shows transmittance at solution-substrate interfaces depending on the refractive index. Transmission through a flat glass surface,
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, 10, No. 13, 2558–2563
Biologically Inspired Biophotonic Surfaces with Self-Antireflection
Figure 3. Highly sensitive optical biosensing based on biologically inspired biophotonic surfaces. (a) SERS signal improvements from the nanoplasmonic substrates with SS-GNA depending on the nanopillar height. The signal improvement of SERS peaks shows the maximum value for GNA with 130 nm in height where the SERS signals are increased by over 20 percent, compared with a flat bottom surface. The asterisks in inset figure indicate the major SERS peaks of benzenethiol. (b) Fluorescence signal improvement from the antireflective substrate with GNA of 130 nm in nanopillar height. The SS-GNA increase the fluorescence signals from red, green and blue fluorescent dyes by 15 percent and the DS-GNA increase the signals by 18 percent, compared with the substrate with flat surfaces.
magnesium fluoride (MgF2), and GNA was calculated by using a finite difference time domain (FDTD) method. The self-antireflection substantially varies with the FF and height of GNA for diverse surrounding media with different indices. The results obviously support the GNA with 0.5 FF and 130 nm in nanopillar height have an ideal self-antireflection for diverse surrounding media (Supporting Information Figure S4). Distinct from conventional antireflection coating such as MgF2, the GNA with spontaneous index modulation can provide exceptional advantages for cellular or biomolecular imaging within a broad range of 1.0 to 1.6 in refractive index (Figure 4a). The result clearly shows MgF2 with a constant index of n≈1.38 is not suitable for diverse solution-glass interfaces. Figure 4b shows reflection images of microspheres on the antireflective substrate with DS-GNA immersed in diverse solution media. The experimental results clearly demonstrate the spontaneous index modulation of DS-GNA completely removes the reflection at the solution-substrate interface and provides exceptionally high contrast images of microspheres, compared with those from the flat surface or SS-GNA. White-light images can also take full benefits for high contrast imaging owing to the self-antireflection (Supporting Information Figure S5). To be concluded, this work successfully demonstrates the novel biologically inspired biophotonic surfaces with selfantireflection for highly sensitive fluorescence detection and SERS as well as high contrast optical imaging. The ∼0.5 FF GNA with size distribution effectively serve as SARS in a broadband visible range. The ∼0.5 FF GNA directly contributes to the self-antireflection for diverse media with different indices and the size distribution reduces optical mismatches in a broadband visible range by spatial averaging of refractive indices as well. The GNA improve both the excitation and collection of light and result in the substantial increase of SERS and fluorescence signals by about 20 percent. In small 2014, 10, No. 13, 2558–2563
addition, the spontaneous index modulation of the GNA also enables exceptionally high contrast imaging. This selfantireflection can also be further expanded not only to glass materials for biophotonic applications but also to miscellaneous substrate materials for light gathering or light emitting (Supporting Information Figure S6). This biological inspiration provides a new direction for mining the smartness from natural photonic structures of insect larvae under water or amphibiotic insects with ultimate environmental adaptation.
Experimental Section Nanofabrication of the Glass Nanopillar Arrays and Plasmonic Nanostructures: Borosilicate glass substrates were used for constructing the biophotonic surfaces with antireflective nanostructures and plasmonic nanostructures. The silver nanoislands as etching mask were formed by using thermal evaporation of thin silver film (10 nm) in Volmer-Weber mode[25] and thermal annealing at 380 degrees for one hour. The height of GNA was controlled by RIE time. The second silver layer (30 nm in thickness) for plasmonic nanostructures was additionally deposited onto the 130 nm height GNA by thermal evaporation. Chemical Sample Preparation: The chemical samples were purchased from Sigma Aldrich. Ethanol for nile red (50 µM), distilled water for R6G (100 µM), and dimethyl sulfoxide solution for 7-diethylaminocoumarin-3-carboxylic acid (succinimidyl ester, 1 mM) were served as solvents. Benzenethiol solution (diluted with ethanol, 4 mM) was used for self-assembled monolayer onto the plasmonic nanostructures. The microspheres of 15 µm in diameter were purchased from Invitrogen (Cat No F-8842, medium: 2-ethoxyethyl acetate, n ≈ 1.406). The sample was used as received, or the medium was exchanged to water (n ≈ 1.333) or ethanol (n ≈ 1.363) by using centrifuge.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
2561
communications
Y.-J. Oh et al.
Figure 4. High contrast optical imaging based on biologically inspired biophotonic surfaces. (a) Self-antireflection from the biophotonic surface. Unlike a flat glass surface or magnesium fluoride coating with 100 nm in thickness, the effective index of GNA with 0.5 FF and 130 nm in height spontaneously meets antireflection conditions for diverse surrounding media with different indices. The inset letters and lines indicate the refractive indices of representative surrounding media. (b) High contrast optical images of microspheres in 2-ethoxyethyl acetate, ethanol and water. Unlike flat surfaces, DS-GNA provides high contrast optical imaging without fringes and background noises for surrounding media with different indices due to the self-antireflection (scale bars : 50 µm).
Measurements: The visible light transmission was measured by using a spectrometer (SM642, Spectral Products) coupled with a collimated white-light LED source. Due to time variation of the LED source, optical power of the LED source was measured every time after the optical power measurement through the sample. All the measured values of transmission were averaged after ten point measurements. The FF of the GNA was measured by SEM images of three individually fabricated devices. The inverted confocal laser scanning microscope (CLSM, Carl Zeiss Axiovert 200M) was used for fluorescence measurement. The samples were excited with the built-in laser sources. The excitation wavelengths were 543 nm for nile red, 488 nm for R6G, and 458 nm for 7-diethylaminocoumarin3-carboxylic acid (succinimidyl ester) sample. The fluorescence intensities were measured from target solutions in PDMS reservoir. The reflection images of Figure 4 were also measured by using an inverted confocal microscope where the scanning wavelength was 543 nm. For the SERS measurement, a laser (632.8 nm, 5 mW) and a spectrometer were coupled to an inverted microscope. The excitation power was controlled by neutral density filter and the SERS spectra were recorded using a MicroSpec 2300i spectrometer
2562 www.small-journal.com
equipped with a charge-coupled device (CCD) camera (Princeton Instruments, Model PIXIS: 400BR). The SERS signal improvement plotted in Figure 3a is the average value from three individually fabricated nanoplasmonic surfaces. The laser excitation and signal collection for SERS and fluorescence measurements were done by a 50X objective lens (NA 0.5).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant (No. 2013035236, No. 2013050154),
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, 10, No. 13, 2558–2563
Biologically Inspired Biophotonic Surfaces with Self-Antireflection
Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project / (CISS2012M3A6A6054199), and Ministry of Trade, Industry, and Energy (MOTIE 10041120).
[1] F. Xia, L. Jiang, Adv. Mater. 2008, 20, 2842. [2] K. Liu, X. Yao, L. Jiang, Chem. Soc. Rev. 2010, 39, 3240. [3] Y. Tian, N. Pesika, H. Zeng, K. Rosenberg, B. Zhao, P. McGuiggan, K. Autumn, J. Israelachvili, Proc Natl. Acad. Sci. USA 2006, 103, 19320. [4] B. Aksak, M. P. Murphy, M. Sitti, Langmuir 2007, 23, 3322. [5] S. Chattopadhyay, Y. F. Huang, Y. J. Jen, A. Ganguly, K. H. Chen, L. C. Chen, Mater. Sci. Eng. R. 2010, 69, 1. [6] Y. Li, J. Zhang, B. Yang, Nano Today 2010, 5, 117. [7] M. Rassart, J.-F. Colomer, T. Tabarrant, J. P. Vigneron, New J. Phys. 2008, 10, 033014. [8] J. H. Kim, J. H. Moon, S.-Y. Lee, J. Park, Appl. Phys. Lett. 2010, 97, 103701. [9] P. Lalanne, G. M. Morris, Nanotechnology 1997, 8, 53. [10] J.-W. Ha, I. J. Park, S.-B. Lee, Macromolecules 2008, 41, 8800. [11] Y.-J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, J. W. P. Hsu, Nano Lett. 2008, 8, 1501. [12] S. A. Boden, D. M. Bagnall, Prog. Photovolt: Res. Appl. 2010, 18, 195.
small 2014, 10, No. 13, 2558–2563
[13] D.-S. Tsai, C.-A. Lin, W.-C. Lien, H.-C. Chang, Y.-L. Wang, J.-H. He, ACS Nano 2011, 5, 7748. [14] M. Ibn-Elhaj, M. Schadt, Nature 2001, 410, 796. [15] S. L. Diedenhofen, G. Vecchi, R. E. Algra, A. Hartsuiker, O. L. Muskens, G. Immink, E. P. A. M. Bakkers, W. L. Vos, J. G. Rivas, Adv. Mater. 2009, 21, 973. [16] G. C. Park, Y. M. Song, J.-H. Ha, Y. T. Lee, J. Nanosci. Nanotechnol. 2011, 11, 6152. [17] J.-J. Kim, Y. Lee, H. G. Kim, K.-J. Choi, H.-S. Kweon, S. Park, K.-H. Jeong, Proc Natl. Acad. Sci. USA 2012, 109, 18674. [18] H. Jung, K.-H. Jeong, Appl. Phys. Lett. 2012, 101, 203102. [19] A. L. Mattheyses, K. Shaw, D. Axelrod, Microsc. Res. Techniq. 2006, 69, 642. [20] Y.-J. Oh, K.-H. Jeong, Adv. Mater. 2012, 24, 2234. [21] Y.-H. Ho, C.-C. Liu, S.-W. Liu, H. Liang, C.-W. Chu, P.-K. Wei, Opt. Express 2011, 19, A295. [22] B. D. Ryu, P. Uthirakumar, J. H. Kang, B. J. Kwon, S. Chandramohan, H. K. Kim, H. Y. Kim, J. H. Ryu, H. G. Kim, C.-H. Hong, J. Appl. Phys. 2011, 109, 093116. [23] C. V. Bindhu, S. S. Harilal, V. P. N. Nampoori, C. P. G. Vallabhan, Mod. Phys. Lett. B 1999, 13, 563. [24] C. Viets, W. Hill, J. Phys. Chem. B 2001, 105, 6330. [25] C. T. Campbell, Surf. Sci. Rep. 1997, 27, 1.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: December 19, 2013 Revised: January 22, 2014 Published online: March 13, 2014
www.small-journal.com
2563
Biologically Inspired Biophotonic Surfaces with Self-Antireflection Young-Jae Oh, Jae-Jun Kim, and Ki-Hun Jeong* Biologically inspired nanoarchitectures exhibit intriguing functions at solid-surrounding medium interfaces such as self-cleaning,[1,2] strong adhesion,[3,4] antireflection,[5,6] or environmental adaptation.[7,8] In particular, antireflective structures (ARS) inspired from the corneal surface of moth eyes can simply modulate the effective index by controlling the fill factor (FF), i.e., the relative areal fraction of subwavelength nanostructures, thereby satisfying an antireflection condition at air-solid interface (Figure 1a).[9,10] In the last decade, ARS have been extensively utilized for assorted photonic applications such as solar cells,[11,12] photodetectors,[13] display panels,[14–16] or optical lenses[17,18] to reduce specular reflection on the interfaces. However, all the previous works still leave out an additional important function, i.e., self-antireflection. The effective index of ARS is significantly affected by the index of a surrounding medium, which entirely fills the interstitial nanogap spacing between the nanostructures. Not only the air, but also solution media can fill the interstitial space to modulate the effective index of ARS. A particular FF of ARS can offer the spontaneous antireflection for diverse surrounding media with different indices. The index mismatch at either air-substrate or solution-substrate interfaces always causes specular reflection, particularly on either or both sides of a substrate, which substantially hinders highly sensitive biophotonic sensing and imaging due to optical loss or unfavorable interferences.[19] Consequently, this new function can provide the full benefits of ARS for highly efficient optical bioassays such as colorimetric, fluorescent, or luminescent assays. In this work, we report biologically inspired biophotonic surfaces with self-antireflection for highly sensitive optical biosensing and bioimaging. Both surfaces of the substrates enclose large-scale and low-cost glass nanopillar arrays (GNA) with ∼0.5 in FF. The top-side GNA serve as either antireflective or plasmonic surfaces at a solution-substrate interface and the bottom as antireflective surfaces at air-substrate interface, respectively (Figure 1b, Supporting Information Figure S1 for the enlarged SEM images). The GNA filled
Y.-J. Oh, J.-J. Kim, Prof. K.-H. Jeong Department of Bio and Brain Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305–701, Korea E-mail: [email protected] DOI: 10.1002/smll.201303876
2558
wileyonlinelibrary.com
with interstitial medium can be considered as single-layer ARS. Based on the Maxwell Garnett model,[5] the self-antireflection can be realized by using the spontaneous index modulation of ARS, i.e., medium-filled GNA with ∼0.5 FF, at both air-substrate (nair and nsub) and solution-substrate (nsol and nsub) interfaces, where a surrounding medium with an index ranging from 1.0 to 1.8 completely fills the interstitial gaps between the nanopillars. As a result, the effective index naturally meets an antireflection condition (Supporting Information Figure S2). The biophotonic surfaces exhibit diverse examples such as antireflective substrates with single-side GNA (SS-GNA) and double-side GNA (DS-GNA) or nanoplasmonic surfaces for highly sensitive fluorescence sensing or surface enhanced Raman scattering (SERS) as well as high contrast imaging (Figure 1c). A wafer-level nanofabrication of GNA with self-antireflection was done by using thin silver film annealing and reactive ion etching (RIE) on a borosilicate glass substrate (n≈1.47). Thermal annealing transforms thin silver film into size-controllable silver nanoislands, which serve as an etching mask for nanopillar formation during the RIE process. Note that the FF of GNA was precisely controlled with the density of Ag nanoislands depending on the initial thickness of thin silver film. This method was applied on both sides of the glass wafer, where the top-side GNA can also be further functionalized as plasmonic nanostructures by evaporating thin silver or gold film (Figure 2a). The GNA were etched down by a quarter-wavelength to suppress specular reflection at air-substrate interface. The effective index for 0.5 ± 0.01 FF GNA with 130 nm in nanopillar height naturally varies from 1.22 to 1.64 for 1.0 to 1.8 in surrounding index, close to the geometric mean of substrate and surrounding indices, i.e., an ideal antireflection condition at the interface (Supporting Information Figure S2). In experiment, the maximum transmission through the SS-GNA is clearly shown at 130 nm in nanopillar height, which improves transmission by ∼3.8 percent at air-substrate interface, compared with the flat surface (Figure 2b). The DS-GNA remarkably double the transmission improvement, which shows transmittance over 99 percent. In particular, the size distribution of GNA offers broadband antireflection in a visible range from 485 nm to 655 nm. The optical images show specular reflections from flat surfaces, SS-GNA, and DS-GNA on 4-inch glass wafers, respectively (Figure 2b). The DS-GNA provides a clear image of the letters whereas those behind the flat surfaces are visually unreadable due to the substantial specular reflection.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, 10, No. 13, 2558–2563
Biologically Inspired Biophotonic Surfaces with Self-Antireflection
Figure 1. Biologically inspired surfaces with self-antireflection for highly sensitive biophotonic sensing and imaging. (a) Self-antireflection inspired from the corneal nanostructures of a Lepidoptera: Nolidae. (b) Biologically inspired biophotonic surfaces with self-antireflective structures (SARS). Both sides of glass nanopillar arrays (GNA) serve as SARS, i.e., antireflective structures with ∼0.5 fill factor (FF), where topside GNA can also be further functionalized as plasmonic nanostructures with nanogap-rich metal nanoislands. The effective indices (nbot and ntop) of SARS at either air-glass or solution-glass interface spontaneously meet antireflection conditions for diverse surrounding media with different indices when a surrounding medium completely fills the interstitial gap spacing between the nanostructures. (c) Optical images of substrates with biophotonic surfaces, i.e., antireflective and nanoplasmonic surfaces. The antireflective substrates includes a single (SS-GNA), double-side (DS-GNA) antireflective glass nanopillar arrays and the nanoplasmonic surfaces enclose plasmonic nanostructures at different plasmon resonances (scale bars : 1 cm). Light reflection from the surface is clearly suppressed by DS-GNA, compared with flat surfaces or even SS-GNA.
small 2014, 10, No. 13, 2558–2563
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
2559
communications
Y.-J. Oh et al.
Figure 2. Nanofabrication methods and optical properties of surfaces with SARS at air-glass interface. (a) Wafer-level nanofabrication for GNA with ∼0.5 FF on both sides of a glass substrate obtained by reactive ion etching with a silver nanoisland mask. Plasmonic nanopillar arrays can be further formed by using additional silver or gold deposition. (b) Light transmission through the antireflective substrates with GNA of different heights. The SS-GNA with 130 nm in nanopillar height show the maximum transmittance of 0.96 in visible region. The DS-GNA further increases light transmittance over 0.99. The reduced reflection from DS-GNA on 4-inch glass wafers clearly makes the letters behind the substrate readable compared with the SS-GNA or the flat surfaces.
SERS and fluorescence signal improvements were quantitatively investigated with the biophotonic surfaces with self-antireflection. Both the nanoplasmonic and antireflective surfaces were utilized for SERS and fluorescence experiments, respectively. The nanoplasmonic substrates contain the ∼0.5 FF GNA on both sides, where the top GNA was fully covered with nanogap-rich silver nanoislands by using thermal evaporation of 30 nm thick silver film.[20] The bottom GNA serve as self-antireflective structures (SARS), which substantially improve SERS signals from reference molecules (benzenethiol) on top-side silver nanoislands during the excitation and collection. The bottom GNA with 130 nm in nanopillar height increase the SERS signals by over 20 percent, compared with a flat bottom surface (Figure 3a). The asterisks shown in the inset of Figure 3a indicate the major SERS peaks for the quantitative comparisons. Fluorescence signals were also compared with DS-GNA and SS-GNA on the bottom-side. Three different fluorescent dye solutions in red, green, and blue were wet on the top surfaces. All the fluorescence intensities were measured and averaged from the areal fluorescent images of 180 µm × 180 µm (see methods for details). The red fluorescence signals from nile red solution are increased by 14 percent from SS-GNA and by 16 percent from DS-GNA, respectively, compared with the flat surfaces. The green signals from rhodamine 6G (R6G) are improved by about 16 percent from SS-GNA and by 19 percent from DS-GNA. The blue signals from 7-diethylaminocoumarin-3-carboxylic acid (succinimidyl ester) are
2560 www.small-journal.com
also increased by about 15 percent from SS-GNA and by 18 percent from DS-GNA (Figure 3b). Compared with the flat surfaces, SS-GNA provide 15 percent improvement and DS-GNA increase the fluorescent signals up to 18 percent on average. The fluorescent signals from the DS-GNA exhibit an additional increment despite the small index mismatch at the solution-substrate interfaces, compared with SS-GNA. Note that the signal improvements of both SERS and fluorescence are higher than the expected values (∼7.7%) from transmittance improvement through the GNA during both the excitation and the collection process. This substantial improvement can be explained by the following reasons; both light coupling and extraction efficiency through the ARS increase with the incident angle, compared with those from the flat surface.[21,22] This angular dependency directly contributes to both the excitation and the collection light through a microscope objective. Besides, fluorescence and SERS signals have a slightly nonlinear relationship with the low excitation power (Supporting Information Figure S3).[23,24] High contrast imaging was also demonstrated with the biophotonic surfaces with self-antireflection. Specular reflection from the top-side solution-substrate interface is relatively low due to the small index-mismatch unlike that from the bottom air-substrate interface. However, this small reflection may cause significant background noises or unfavorable interferences during optical bioimaging. Figure 4a shows transmittance at solution-substrate interfaces depending on the refractive index. Transmission through a flat glass surface,
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, 10, No. 13, 2558–2563
Biologically Inspired Biophotonic Surfaces with Self-Antireflection
Figure 3. Highly sensitive optical biosensing based on biologically inspired biophotonic surfaces. (a) SERS signal improvements from the nanoplasmonic substrates with SS-GNA depending on the nanopillar height. The signal improvement of SERS peaks shows the maximum value for GNA with 130 nm in height where the SERS signals are increased by over 20 percent, compared with a flat bottom surface. The asterisks in inset figure indicate the major SERS peaks of benzenethiol. (b) Fluorescence signal improvement from the antireflective substrate with GNA of 130 nm in nanopillar height. The SS-GNA increase the fluorescence signals from red, green and blue fluorescent dyes by 15 percent and the DS-GNA increase the signals by 18 percent, compared with the substrate with flat surfaces.
magnesium fluoride (MgF2), and GNA was calculated by using a finite difference time domain (FDTD) method. The self-antireflection substantially varies with the FF and height of GNA for diverse surrounding media with different indices. The results obviously support the GNA with 0.5 FF and 130 nm in nanopillar height have an ideal self-antireflection for diverse surrounding media (Supporting Information Figure S4). Distinct from conventional antireflection coating such as MgF2, the GNA with spontaneous index modulation can provide exceptional advantages for cellular or biomolecular imaging within a broad range of 1.0 to 1.6 in refractive index (Figure 4a). The result clearly shows MgF2 with a constant index of n≈1.38 is not suitable for diverse solution-glass interfaces. Figure 4b shows reflection images of microspheres on the antireflective substrate with DS-GNA immersed in diverse solution media. The experimental results clearly demonstrate the spontaneous index modulation of DS-GNA completely removes the reflection at the solution-substrate interface and provides exceptionally high contrast images of microspheres, compared with those from the flat surface or SS-GNA. White-light images can also take full benefits for high contrast imaging owing to the self-antireflection (Supporting Information Figure S5). To be concluded, this work successfully demonstrates the novel biologically inspired biophotonic surfaces with selfantireflection for highly sensitive fluorescence detection and SERS as well as high contrast optical imaging. The ∼0.5 FF GNA with size distribution effectively serve as SARS in a broadband visible range. The ∼0.5 FF GNA directly contributes to the self-antireflection for diverse media with different indices and the size distribution reduces optical mismatches in a broadband visible range by spatial averaging of refractive indices as well. The GNA improve both the excitation and collection of light and result in the substantial increase of SERS and fluorescence signals by about 20 percent. In small 2014, 10, No. 13, 2558–2563
addition, the spontaneous index modulation of the GNA also enables exceptionally high contrast imaging. This selfantireflection can also be further expanded not only to glass materials for biophotonic applications but also to miscellaneous substrate materials for light gathering or light emitting (Supporting Information Figure S6). This biological inspiration provides a new direction for mining the smartness from natural photonic structures of insect larvae under water or amphibiotic insects with ultimate environmental adaptation.
Experimental Section Nanofabrication of the Glass Nanopillar Arrays and Plasmonic Nanostructures: Borosilicate glass substrates were used for constructing the biophotonic surfaces with antireflective nanostructures and plasmonic nanostructures. The silver nanoislands as etching mask were formed by using thermal evaporation of thin silver film (10 nm) in Volmer-Weber mode[25] and thermal annealing at 380 degrees for one hour. The height of GNA was controlled by RIE time. The second silver layer (30 nm in thickness) for plasmonic nanostructures was additionally deposited onto the 130 nm height GNA by thermal evaporation. Chemical Sample Preparation: The chemical samples were purchased from Sigma Aldrich. Ethanol for nile red (50 µM), distilled water for R6G (100 µM), and dimethyl sulfoxide solution for 7-diethylaminocoumarin-3-carboxylic acid (succinimidyl ester, 1 mM) were served as solvents. Benzenethiol solution (diluted with ethanol, 4 mM) was used for self-assembled monolayer onto the plasmonic nanostructures. The microspheres of 15 µm in diameter were purchased from Invitrogen (Cat No F-8842, medium: 2-ethoxyethyl acetate, n ≈ 1.406). The sample was used as received, or the medium was exchanged to water (n ≈ 1.333) or ethanol (n ≈ 1.363) by using centrifuge.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
2561
communications
Y.-J. Oh et al.
Figure 4. High contrast optical imaging based on biologically inspired biophotonic surfaces. (a) Self-antireflection from the biophotonic surface. Unlike a flat glass surface or magnesium fluoride coating with 100 nm in thickness, the effective index of GNA with 0.5 FF and 130 nm in height spontaneously meets antireflection conditions for diverse surrounding media with different indices. The inset letters and lines indicate the refractive indices of representative surrounding media. (b) High contrast optical images of microspheres in 2-ethoxyethyl acetate, ethanol and water. Unlike flat surfaces, DS-GNA provides high contrast optical imaging without fringes and background noises for surrounding media with different indices due to the self-antireflection (scale bars : 50 µm).
Measurements: The visible light transmission was measured by using a spectrometer (SM642, Spectral Products) coupled with a collimated white-light LED source. Due to time variation of the LED source, optical power of the LED source was measured every time after the optical power measurement through the sample. All the measured values of transmission were averaged after ten point measurements. The FF of the GNA was measured by SEM images of three individually fabricated devices. The inverted confocal laser scanning microscope (CLSM, Carl Zeiss Axiovert 200M) was used for fluorescence measurement. The samples were excited with the built-in laser sources. The excitation wavelengths were 543 nm for nile red, 488 nm for R6G, and 458 nm for 7-diethylaminocoumarin3-carboxylic acid (succinimidyl ester) sample. The fluorescence intensities were measured from target solutions in PDMS reservoir. The reflection images of Figure 4 were also measured by using an inverted confocal microscope where the scanning wavelength was 543 nm. For the SERS measurement, a laser (632.8 nm, 5 mW) and a spectrometer were coupled to an inverted microscope. The excitation power was controlled by neutral density filter and the SERS spectra were recorded using a MicroSpec 2300i spectrometer
2562 www.small-journal.com
equipped with a charge-coupled device (CCD) camera (Princeton Instruments, Model PIXIS: 400BR). The SERS signal improvement plotted in Figure 3a is the average value from three individually fabricated nanoplasmonic surfaces. The laser excitation and signal collection for SERS and fluorescence measurements were done by a 50X objective lens (NA 0.5).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant (No. 2013035236, No. 2013050154),
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, 10, No. 13, 2558–2563
Biologically Inspired Biophotonic Surfaces with Self-Antireflection
Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project / (CISS2012M3A6A6054199), and Ministry of Trade, Industry, and Energy (MOTIE 10041120).
[1] F. Xia, L. Jiang, Adv. Mater. 2008, 20, 2842. [2] K. Liu, X. Yao, L. Jiang, Chem. Soc. Rev. 2010, 39, 3240. [3] Y. Tian, N. Pesika, H. Zeng, K. Rosenberg, B. Zhao, P. McGuiggan, K. Autumn, J. Israelachvili, Proc Natl. Acad. Sci. USA 2006, 103, 19320. [4] B. Aksak, M. P. Murphy, M. Sitti, Langmuir 2007, 23, 3322. [5] S. Chattopadhyay, Y. F. Huang, Y. J. Jen, A. Ganguly, K. H. Chen, L. C. Chen, Mater. Sci. Eng. R. 2010, 69, 1. [6] Y. Li, J. Zhang, B. Yang, Nano Today 2010, 5, 117. [7] M. Rassart, J.-F. Colomer, T. Tabarrant, J. P. Vigneron, New J. Phys. 2008, 10, 033014. [8] J. H. Kim, J. H. Moon, S.-Y. Lee, J. Park, Appl. Phys. Lett. 2010, 97, 103701. [9] P. Lalanne, G. M. Morris, Nanotechnology 1997, 8, 53. [10] J.-W. Ha, I. J. Park, S.-B. Lee, Macromolecules 2008, 41, 8800. [11] Y.-J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, J. W. P. Hsu, Nano Lett. 2008, 8, 1501. [12] S. A. Boden, D. M. Bagnall, Prog. Photovolt: Res. Appl. 2010, 18, 195.
small 2014, 10, No. 13, 2558–2563
[13] D.-S. Tsai, C.-A. Lin, W.-C. Lien, H.-C. Chang, Y.-L. Wang, J.-H. He, ACS Nano 2011, 5, 7748. [14] M. Ibn-Elhaj, M. Schadt, Nature 2001, 410, 796. [15] S. L. Diedenhofen, G. Vecchi, R. E. Algra, A. Hartsuiker, O. L. Muskens, G. Immink, E. P. A. M. Bakkers, W. L. Vos, J. G. Rivas, Adv. Mater. 2009, 21, 973. [16] G. C. Park, Y. M. Song, J.-H. Ha, Y. T. Lee, J. Nanosci. Nanotechnol. 2011, 11, 6152. [17] J.-J. Kim, Y. Lee, H. G. Kim, K.-J. Choi, H.-S. Kweon, S. Park, K.-H. Jeong, Proc Natl. Acad. Sci. USA 2012, 109, 18674. [18] H. Jung, K.-H. Jeong, Appl. Phys. Lett. 2012, 101, 203102. [19] A. L. Mattheyses, K. Shaw, D. Axelrod, Microsc. Res. Techniq. 2006, 69, 642. [20] Y.-J. Oh, K.-H. Jeong, Adv. Mater. 2012, 24, 2234. [21] Y.-H. Ho, C.-C. Liu, S.-W. Liu, H. Liang, C.-W. Chu, P.-K. Wei, Opt. Express 2011, 19, A295. [22] B. D. Ryu, P. Uthirakumar, J. H. Kang, B. J. Kwon, S. Chandramohan, H. K. Kim, H. Y. Kim, J. H. Ryu, H. G. Kim, C.-H. Hong, J. Appl. Phys. 2011, 109, 093116. [23] C. V. Bindhu, S. S. Harilal, V. P. N. Nampoori, C. P. G. Vallabhan, Mod. Phys. Lett. B 1999, 13, 563. [24] C. Viets, W. Hill, J. Phys. Chem. B 2001, 105, 6330. [25] C. T. Campbell, Surf. Sci. Rep. 1997, 27, 1.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: December 19, 2013 Revised: January 22, 2014 Published online: March 13, 2014
www.small-journal.com
2563
