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A Bull’s eye-plasmonic chip composed of concentric circles was applied to enhanced fluorescence microscopy. Among one dimensional (1-D), 2-D, and Bull’s eye periodic structures, the Bull’s eye-plasmonic chip provided the most enhanced fluorescence intensity under the epi-fluorescence microscope, because incident light through the objective lens with all azimuthal angles can be effectively applied to the surface plasmon resonance- field (excitation field) and the plasmon-enhanced emission was also effectively collected. In the fluorescence observation of a single nanoparticle, the enhanced fluorescence images for a microsphere with ϕ 2 μm and a nanosphere with ϕ 200 nm were observed. For the nanospheres with ϕ 40 and 20 nm, the fluorescence image, which was undetectable on a glass slide, was observed in a spatial resolution of roughly diffraction limit on the Bull’s eye-plasmonic chip. Furthermore, the use of an appropriate pinhole at the aperture stop in the incident optical system improved the fluorescence enhancement. The applicability of a Bull’s eye-plasmonic chip to fluorescence imaging was demonstrated.
© 2017 Optical Society of America
1. Introduction
Fluorescence imaging is an essential technique for bioimaging. To study a cell membrane surface and inside including various kinds of proteins, sensitive fluorescence microscopy is one of the most powerful and useful tools. Recently, single-molecule fluorescence microscopy has been developed [1–4]. Fluorescently labeled antibody plays the role of the probe to detect the distribution of a protein marker and to follow the kinetics of one.
In our laboratory, the grating substrate covered with thin metal films called a plasmonic chip has been studied for fluorescence enhancement [5–9]. The plasmonic chip with a wavelength-scale pitch can provide an enhanced electric field by direct coupling between a surface plasmon and incident light [10, 11]. The plasmonic chip has been applied to biosensing [6, 7, 12–14] and bioimaging [8, 9, 14]. In the immunosensing system, various target proteins, including epidermal growth factor receptor (EGFR) [6], C-reactive protein (CRP) [12], interleukin-6 (IL-6) [7, 13], and α-fetoprotein (AFP) [14] were sensitively detected with a plasmonic chip of one dimensional (1-D) or 2-D periodic structure and the detection sensitivity was improved 2 − 4 orders compared with that of the reference substrate such as a conventional glass slide. In cell imaging, neurons cultured on the plasmonic chip [9] and breast cancer cells [15] were sensitively observed with a fluorescence microscope. The fluorescence images were more than 10 times brighter than those taken on a glass-bottomed dish or a glass slide. These advantages of brighter fluorescence in the plasmonic chip were shown and the plasmonic chip was introduced as a useful tool for biodetection. However, for fluorescence observation under a microscope, 1-D and 2-D periodic structures were considered insufficient to couple the incident light and to form the enhanced electric field derived from the grating-coupled surface plasmon resonance (GC-SPR). The resonance condition of GC-SPR is described as [10, 11]
where spp, ph, and g are the wavenumber vector of the surface plasmon, incident light, and grating proportional to the inverse of pitch, respectively. m is integer and θ is an incident angle corresponding to the surface plasmon resonance angle. As shown in Fig. 1(a), when an incident plane is parallel to the grating vector in 1-D grating, the azimuthal angle is 0 and the resonance angle is assumed to be X0 degrees. As the azimuthal angle ψ is larger, the resonance angle Xψ is also larger than X0 degrees as found from Eq. (1) [11] [Fig. 1(b)]. At further larger azimuthal angle, the resonance condition cannot be satisfied in 1-D grating, and the illumination light cannot be sufficiently available for plasmon coupling under the microscope. In a 2-D grating, the effective range of azimuth angles is revised because of two orthogonal , although the coupling efficiency depends on the position within 2-D arrays and may be slightly worse compared with that for 1-D. On the one hand, in the center of Bull’s eye structure, illumination lights coming from the full azimuthal angles are available to the resonance (coupling), and even in the grooves except for the center, the coupling efficiency is expected to be equivalent to that in 1-D grating; therefore, mean fluorescence intensity is expected to be larger within Bull’s eye pattern compared with that for 1-D. The Bull’s eye structure has been developed as a plasmonic antenna [16–21]. In the Bull’s eye with 200-nm-diameter hole under the illumination from rear panel [18], the Bull’s eye structures show directional emission patterns, which result from far-field interference of direct emission and emission funneled into plasmons and subsequently outcouples at the antenna grooves, in contrast to single holes with no grating structure showing angularly isotropic emission. Plasmons propagate radially from the centered hole to the grooves, and scatter out in a doughnut by diffraction at the grooves.
Fig. 1 Schematic view of wavenumber vectors which satisfy the resonance conditions in the grating-coupled SPR for the azimuth angle of (a) 0 and (b) ψ: (Upper) sum of wavenumber vectors shown in xy-plane and (bottom) incident plane and shown in 3-D. A red solid arrow, a void arrow, and a broken arrow correspond to the |kph|sinθ, kg, and kspp vectors, respectively.
In this study, we fabricated a new type of Bull’s eye-plasmonic chip, and it was applied to fluorescence microscopy. The fluorescence enhancement in the Bull’s eye-plasmonic chip was compared with those obtained in 1-D and 2-D plasmonic chips and the improvement of fluorescence enhancement by a periodic structure pattern was studied. The pitch and wavelength dependences of the fluorescence enhancement were also studied. Furthermore, the Bull’s eye-plasmonic chip was applied to single-nanoparticle detection, which is an important technique in cell imaging. The fluorescence enhancement factor and the spatial resolution were evaluated in the fluorescence microscopic imaging and the availability of a plasmonic chip is discussed.
2. Fabrication of a plasmonic chip
As described in our previous papers [5, 6], the plasmonic chip was fabricated by two processes.
The first process was the preparation of replicas by a UV-nanoimprint method using three types of mold (specially made, NTT-AT) and the next process was metal coating using the rf-sputtering method. The three kinds of plasmonic chips, 1-D (line and space), 2-D (hole array), and Bull’s eye-periodic structure, were measured by AFM (Fig. 2). 1-D and 2-D have 480 nm pitch. The groove depths of all the plasmonic chips were 30 ± 5 nm. In the Bull’s eye, there were three kinds of concentric circle patterns with different pitches, 400, 480, and 600 nm, and the diameter of the Bull’s eye pattern was 100 μm, as shown in Fig. 3. Replicas were coated in order with thin films of Ti, Ag, Ti, SiO2. Each film thickness was <1, 130 + 10, <1, and 25 + 5 nm. Finally, the surface of the plasmonic chip was modified with 3-aminopropyl triethoxysilane (APTES) aq. solution. Under the microscopic observation, nanospheres were necessary to be fixed to the chip surface without washing away with buffered saline solution. Therefore, they were fixed on the plasmonic chips and the glass slides by the intermolecular interaction between a carboxyl group of nanospheres (as shown in 3.1) and an amino group of surfaces modified with APTES. 1% APTES solution was dropped onto the chips. After incubation for 1 h at room temperature, the chip surface was fully rinsed with milli-Q water. As for the control, the glass slide was also modified with APTES using the same protocol.
Fig. 2 AFM images of Bull’s eye: a top view and a cross section for (a) 400, (b) 480, and (c) 600 nm pitch.
Fig. 3 Schematic of Bull’s Eye pattern: Pitch sizes were 400, 480, and 600 nm for 5 circles (green) in the left side, 7 circles (orange) in a middle line, and 5 circles (red) in the right side, respectively.
3. Fluorescence imaging
3.1 Preparation of nanospheres
Fluorescence images of microsphere and nanosphere (F8816, F8807, F8783, F8789, and F8786; FluoSphere®, ThermoFisher) were taken with an upright fluorescence microscope. Samples are summarized in Table 1. All nanospheres were modified with a carboxyl group. The particle concentrations were prepared as 4.5 × 1011 particles/ mL for observation with x10 objective, and 4.5 x 108 particles/ mL for a single- nanosphere observation with x 100 objective. The solutions of particles were injected into the space between and a substrate, i.e., a plasmonic chip or a glass slide, and a cover glass attached to the substrate with a double-sided tape. The solutions were rinsed with a phosphate buffered saline solution after incubation of 5 min. The particles fixed to the substrate were observed in a solution with a microscope.
Table 1. Samples of fluorescent nanospheres.
3.2 Microscopy
Fluorescence images were taken with an EM-CCD camera (iXon; Andor) mounted on an upright microscope (XI51; Olympus). Cy5 (λEx = 605 − 650 nm, λEm = 670 − 715 nm) and Cy3 (λEx = 510 − 555 nm, λEm = 570 − 615 nm) filter units were used for Crimson, Dred and Red samples, respectively. A halogen lamp was used for taking a bright-field image and a fluorescence image with a low magnification objective such as x 10 (NA = 0.3). A mercury lamp was used for taking a fluorescence image with a higher magnification objectives of x 100 (NA = 0.95). The single nanoparticle was observed with the same microscope.
4. Results
4.1 Fluorescence images of particles densely fixed to the Bull’s eye-plasmonic chip
The fluorescence enhancement factor was evaluated in the glass slide and three kinds of plasmonic chips, 1-D, 2-D, and Bull’s eye. ϕ 20-nm particles were densely fixed to each substrates and fluorescence images were taken with x 10 objective (Fig. 4). The fluorescence enhancement factor was defined as the ratio of fluorescence intensity on the plasmonic chip to that on the glass slide. As the illumination light showed a radial distribution in the brightness at the substrate surface, i.e., the center part of an illumination area was brighter than the edge of it, all the fluorescence enhancement factors must be evaluated at the same position around center part of an image for any fluorescence images. In the Bull’s eye, the mean fluorescence intensity was evaluated over the entire Bull’s eye-patterns including a bright center part. The enhancement factors were 18, 23, and 36 within +/− 20% error for 1-D, 2-D, and Bull’s eye, respectively. Evaluation of the enhancement factor theoretically predicted is complicated because it is necessary to be considered the coupling efficiency of illumination light with the azimuth angular distribution, the incident angular distribution, and the wavelength distribution based on the fluorescence filter unit used here. For the wavelength of 625 nm, the coupling efficiency between plasmon and illumination light is qualitatively considered under the microscope with x10 objective (NA = 0.3). NA = 0.3 corresponds to 0−17 degrees of the illumination angle. Every point within illumination spot on the 1-D chip has only one g and the range of azimuth angle under the resonance condition [Fig. 1(b)] was 0−65 degree corresponding to around 70% of entire angles. 2-D chip have two orthogonal g and the all azimuth angles of 0−90 degree were available. For Bull’s eye-pattern, mean coupling efficiency included the antenna effect in the center part utilizing the illumination light and emission from all azimuth angles and it was considered to be larger than that in 1-D.
Fig. 4 Fluorescence images on the glass slide (a) and plasmonic chip of 1-D (b), 2-D (c), and Bull’s eye (d). Bar correspond to 200 μm.
Aouani et al., showed the 120-fold fluorescence enhancement in the nanoaperture (diameter: 135 nm) with plasmonic corrugations covered with a gold film [20], which consists with the enhancement factor of less than 150 theoretically predicted [21]. They also showed 14-fold enhancement without corrugations and therefore, the plasmonic corrugation effect is considered to be 5-10 times. In our Bull’s eye structure coated with silver film, the enhancement factor includes the reflection interference effect at the metal surface considered as threefold enhancement. That is, for the 36-fold enhancement in the Bull’s eye chip observed in Fig. 4 (d), plasmon effect is considered to be around 12-times. It is almost in accordance with the value of corrugation effect and it is reasonable even if the difference between gold and silver is considered. On the other hand, in the center of the concentric circles, the fluorescence enhancement was larger than that in the pattern edge and the value in the center was evaluated as 42. This is considered to be an antenna effect due to the superposition of the electric field in the center [16–19]. In the Bull’s eye pattern used in this study, the center has a concave shape with the width of a half pitch as shown in Fig. 2. A larger enhancement may be obtained by changing the center shape from concave to convex or changing the width at the center. Furthermore, the contribution of the center part can be larger by minimization of a pattern area up to around 10 grooves. To improve the fluorescence enhancement in the center, further studies are required.
The pitch and wavelength dependence of the fluorescence enhancement were studied with Dred-20 and Red-20 nanospheres fixed on the Bull’s eye-plasmonic chip. As shown in Fig. 5 and Table 2, the pattern with the 600 nm pitch provided the sixfold enhanced fluorescence for both particles with Cy5 and Cy3 filter units. By the reflection interference effect on the metal surface [9], above threefold fluorescence enhancement compared with that on the glass slide was considered, and therefore, below twofold enhancement was regarded as the surface plasmon effect. The patterns with 400 nm and 480 nm pitches provided the 36-fold enhanced fluorescence for Red-20 and Dred-20 with Cy3 [Fig. 5 (b)] and Cy5 [Fig. 5 (a)] filter units, respectively. The 23-fold enhancement of Dred-20 on the 400 nm pitch was smaller than that on the 480 nm pitch and the 11-fold enhancement of Red-20 on the 480 nm pitch was smaller than that on the 400 nm pitch. In the GC-SPR under vertical illumination, i.e., at an incident angle of 0, the resonance wavelengths were evaluated from Eq. (1) as 570, 670 and 820 nm for the 400, 480, and 600 nm pitches, respectively. The resonance conditions under the vertical illumination for excitation wavelengths in Cy3 (510 − 555 nm) and Cy5 (605 – 650 nm) filters were almost satisfied with the 400 nm pitch and 480 nm pitch, respectively. Therefore, fluorescence images for Cy3 and Cy5 were brighter on the 400 nm and 480 nm pitches, respectively. Furthermore, the recoupled surface plasmon condition between fluorescence and surface plasmons also affected the fluorescence enhancement factors. Resonance conditions for emission wavelengths of Cy3 (570 − 615 nm) and Cy5 (670 – 715 nm) agreed with the resonance wavelengths for 400 and 480 nm pitches, respectively. In the fluorescence image with a plasmonic chip, the agreement of excitation/emission wavelengths with resonance wavelengths depending on pitch can provide larger fluorescence enhancement factors. In future, the fluorescence enhancement factor can be discussed by Finite-difference time-domain (FDTD) simulation method quantitatively.
Fig. 5 Fluorescence images of (a) Dred-20 with Cy5 filter unit and (b) Red-20 with Cy3 filter unit. Bars correspond to 100 μm.
Table 2. Fluorescence enhancement factor of nanospheres on the plasmonic chip.
4.2 Fluorescence images of a single nanosphere
As for four kinds of a fluoSpheres described in Table 3, a nanosphere was individually fixed onto the glass slides and observed with an epi-fluorescence microscope. As shown in Fig. 6, the fluorescence intensity decreased with the diameter of the nanospheres and it was difficult to observe the fluorescence images for nanospheres with ϕ = 40 and 20 nm [Figs. 6(c) and 6(d)]. Using the same protocol as with the glass slide, fluoSpheres were fixed on the plasmonic chip. The single fluoSphere fixed to the Bull’s eye-pattern area except for the center of Bull’s eye was selected for observation and then, the sample stage was adjusted as it was arranged around a center of an illumination spot. As for a microparticle with ϕ 1 μm, regardless of the pitch size in the plasmonic chip, sevenfold fluorescence intensities were obtained compared with that on the glass slide [Figs. 6(a) and 7(a)]. The value of sevenfold enhancement was considered to include the reflection interference effect and a plasmon-field little contributed to the excitation of a particle because of a particle size over the decay length of an enhanced electric-filed. As the largest fluorescence intensity was shown in the pitch of 480 nm of the plasmonic chip, fluorescence intensities for the other nanospheres were measured on the pitch of 480 nm. The nanoparticle with ϕ 200 nm showed a 15 times enhanced fluorescence image [Fig. 7(b)]. The enhancement factors were not evaluated for ϕ 20 and 40 nm particles because the fluorescence intensity on the glass slide was too weak to be measured. That is, a plasmonic chip makes fluorescence detection possible for a single nanosphere with ϕ 20 nm. The diffraction limit is important to discuss the spatial resolution in the fluorescence imaging of a single nanosphere.
Table 3. Fluorescence intensities and diameters determined from the fluorescence images of nanospheres.
Fig. 6 Fluorescence images of Crimson and Dred nanospheres on the glass slides: Diameters of naospheres were (a) 1000 nm, (b) 200 nm, (c) 40 nm, and (d) 20 nm. Bars correspond to 2 μm.
Fig. 7 Fluorescence images of Crimson and Dred nanospheres on the plasmonic chip: Diameters of nanospheres were (a) 1000 nm, (b) 200 nm, (c) 40 nm, and (d) 20 nm. Bars correspond to 2 μm.
Diffraction limit is described by 0.61 λ / NA, in which λ and NA are emission wavelength detected in the imaging and the numerical aperture of the objective, respectively. Therefore, the spatial resolution for the emission wavelength of Dred was expected to be 440 nm, which corresponds to 2.75 pixels in the EM-CCD camera with a pixel size of 160 nm. The diameters of single nanospheres except for the microsphere were evaluated as 560 nm from the full width at half maximum (FWHM) corresponding to 3.5 pixels (Table 3). A pixel number below three pixels is difficult to be regarded as FWHM in the fluorescence peak. The spatial resolution of 560 nm was considered to be almost the same to the diffraction limit. If a CCD camera composed of a smaller pixel size could be used in microscopic imaging, spatial resolution can be improved. The propagated surface plasmon mode generally shows the tail in the direction of a propagation. However, the intensity of the tail was too small and therefore, it didn’t broaden the FWHM in fluorescence image of a nanosphere. Spatial resolution expected in the conventional fluorescence imaging can be obtained even with a Bull’s eye-plasmonic chip.
4.3 Improvement of fluorescence images by aperture stop
The most enhanced fluorescence was obtained at a pitch of 480 nm for the Bull’s eye-plasmonic chip. In order to improve the enhancement, illumination angles were controlled by inserting a pinhole at the aperture stop. The resonance angle for 480 nm pitch was around 8 degrees for the water interface [8]. The illumination angle range for the x 100 objective was 0 − 71 degrees. Incident light at higher angles was considered to be out of resonance. Inserting the pinholes of ϕ 1 mm, donut-type with ϕ 0.5 − 1.5 mm, and donut-type with ϕ 1.5 − 2.5 mm correspond to changing the illumination angles by 0 − 12, 6 − 18, and 18 − 31 degrees, respectively. Fig. 8 shows the reflection image for the Bull’s eye-plasmonic chip under the microscope with a different pinhole at the aperture stop. The Cy5 filter unit removing an emission filter was used. Figs. 8(a)-8(d) correspond to images observed without a pinhole (fully open aperture), with ϕ 1 mm pinhole, with a donut-type pinhole of ϕ 0.5−1.5 mm, and with a donut-type pinhole of ϕ 1.5−2.5. The mean coupling efficiency was evaluated from
in which Rgrating and Rflat are the mean reflectivities inside the grating area and outside the grating, i.e., flat metal area, respectively, depicted in squares of Fig. 8. Each coupling efficiency was 0.12, 0.19, 0.12, and 0.08 for Figs. 8(a)-8(d) and the best coupling was under the pinhole of 1 mm. It was considered that the resonance angle was included in the illumination angles and the most effective coupling was performed.Fig. 8 Reflection image for Bull’s eye-plasmonic chip filled with water (a) without pinhole, (b)with ϕ1 mm-pinhole at aperture stop, (c) with ϕ 0.5-1.5 mm donut-type pinhole, and (d) with ϕ 1.5-2.5 mm-pinhole. Bars correspond to 15 μm.
The fluorescence image of a single ϕ 200 nm particle was observed with the above conditions, as shown in Fig. 9. The fluorescence intensity normalized to the illumination light intensity was 10% improved at Figs. 9(b) and 9(c). In the case of Fig. 9(d), the normalized fluorescence intensity is almost the same as that without a pinhole. Application of an appropriate pinhole improved the fluorescence enhancement.
Fig. 9 Single ϕ 200 nm particle was observed at the same condition as pinholes described in Fig. 8. Bars correspond to 2 μm.
5. Conclusions
The Bull’s eye-plasmonic chip provided enhanced fluorescence intensity compared with 1-D and 2-D-plasmonic chips. The center part showed the brightest area, maybe because of superposition of the plasmon field. The Bull’s eye pattern can be a powerful tool for fluorescence microscopy with a plasmonic chip. In the fluorescence image with a plasmonic chip, the agreement of excitation/emission wavelengths with resonance wavelengths depending on pitch was found to provide the larger fluorescence enhancement factors. In the fluorescence imaging of a single nanosphere, a fluorescence image that was undetectable on the glass slide was observed on the Bull’s eye-plasmonic chip due to the fluorescence enhancement. The spatial resolution was found to be evaluated as almost the diffraction limit without broadening due to the propagated plasmon mode. Further sensitive fluorescence observation of a single nanosphere was performed with an appropriate pinhole at an aperture stop. The Bull’s eye-plasmonic chip can be applied to highly sensitive fluorescence imaging and it can greatly contribute to a single-nanosphere observation in cell imaging.
Funding
JSPS KAKENHI (JP15H0110, JP16H02092, JP16H06504).
Acknowledgments
K.T. and S.I. thank Toyo Gosei for providing the UV-curable resin PAK-02-A and Mr. Masashi Sumiya for experimental assistance. This work was supported by JSPS KAKENHI Grant Numbers JP15H0110 in Scientific Research on Innovation Areas “Photosynergetics,” JP16H02092 in Scientific Research (A), and JP16H06504 in Scientific Research on Innovation Areas “Nano-Material Optical-Manipulation.”
References and links
1. S. Schultz, D. R. Smith, J. J. Mock, and D. A. Schultz, “Single-target molecule detection with nonbleaching multicolor optical immunolabels,” Proc. Natl. Acad. Sci. U.S.A. 97(3), 996–1001 (2000). [CrossRef] [PubMed]
2. S. Kühn, U. Håkanson, L. Rogobete, and V. Sandoghdar, “Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna,” Phys. Rev. Lett. 97(1), 017402 (2006). [CrossRef] [PubMed]
3. H. Wang, T. B. Huff, D. A. Zweifel, W. He, P. S. Low, A. Wei, and J.-X. Cheng, “In vitro and in vivo two-photon luminescence imaging of single gold nanorods,” Proc. Natl. Acad. Sci. U.S.A. 102(44), 15752–15756 (2005). [CrossRef] [PubMed]
4. B. Masters, “Three-dimensional microscopic tomographic imagings of the cataract in a human lens in vivo,” Opt. Express 3(9), 332–338 (1998). [CrossRef] [PubMed]
5. K. Tawa, H. Hori, K. Kintaka, K. Kiyosue, Y. Tatsu, and J. Nishii, “Optical microscopic observation of fluorescence enhanced by grating-coupled surface plasmon resonance,” Opt. Express 16(13), 9781–9790 (2008). [CrossRef] [PubMed]
6. K. Tawa, M. Umetsu, H. Nakazawa, T. Hattori, and I. Kumagai, “Application of 300× enhanced fluorescence on a plasmonic chip modified with a bispecific antibody to a sensitive immunosensor,” ACS Appl. Mater. Interfaces 5(17), 8628–8632 (2013). [CrossRef] [PubMed]
7. M. Tsuneyasu, C. Sasakawa, N. Naruishi, Y. Tanaka, Y. Yoshida, and K. Tawa, “Sensitive detection of interleukin-6 (IL-6) on a plasmonic chip by grating-coupled surface-plasmon-field-enhanced fluorescence imaging,” Jpn. J. Appl. Phys. 53(6S), 06JL05 (2014). [CrossRef]
8. X. Q. Cui, K. Tawa, K. Kintaka, and J. Nishii, “Enhanced fluorescence microscopic imaging by plasmonic nanostructrues: From 1D grating to 2D nanohole array,” Adv. Funct. Mater. 20(6), 945–950 (2010). [CrossRef]
9. K. Tawa, C. Yasui, C. Hosokawa, H. Aota, and J. Nishii, “In situ sensitive fluorescence imaging of neurons cultured on a plasmonic dish using fluorescence microscopy,” ACS Appl. Mater. Interfaces 6(22), 20010–20015 (2014). [CrossRef] [PubMed]
10. H. Raether, “Surface plasmons on smooth and rough surfaces and on gratings,” Springer-Verlag, (1988).
11. W. Knoll, “Interfaces and thin films as seen by bound electromagnetic waves,” Annu. Rev. Phys. Chem. 49(1), 569–638 (1998). [CrossRef] [PubMed]
12. R. Matsuura, K. Tawa, Y. Kitayama, and T. Takeuchi, “A plasmonic chip-based bio/chemical hybrid sensing system for the highly sensitive detection of C-reactive protein,” Chem. Commun. (Camb.) 52(20), 3883–3886 (2016). [CrossRef] [PubMed]
13. K. Tawa, M. Sumiya, M. Toma, C. Sasakawa, T. Sujino, T. Miyaki, H. Nakazawa, and M. Umetsu, “Interleukin-6 detection with a plasmonic chip,” J. Mol. Eng. Mater.in press.
14. K. Tawa, F. Kondo, C. Sasakawa, K. Nagae, Y. Nakamura, A. Nozaki, and T. Kaya, “Sensitive detection of a tumor marker, α-fetoprotein, with a sandwich assay on a plasmonic chip,” Anal. Chem. 87(7), 3871–3876 (2015). [CrossRef] [PubMed]
15. K. Tawa, S. Yamamura, C. Sasakawa, I. Shibata, and M. Kataoka, “Sensitive detection of cell surface membrane proteins in living breast cancer cells by using multicolor fluorescence microscopy with a plasmonic chip,” ACS Appl. Mater. Interfaces 8(44), 29893–29898 (2016). [CrossRef] [PubMed]
16. J.-M. Yi, A. Cuche, E. Devaux, C. Genet, and T. W. Ebbesen, “Beaming visible light with a plasmonic aperture antenna,” ACS Photonics 1(4), 365–370 (2014). [CrossRef] [PubMed]
17. H. Aouani, O. Mahboub, E. Devaux, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Plasmonic antennas for directional sorting of fluorescence emission,” Nano Lett. 11(6), 2400–2406 (2011). [CrossRef] [PubMed]
18. A. Mohtashami, C. I. Osorio, and A. F. Koenderink, “Angle-resolved polarimetry measurements of antenna-mediated fluorescence,” Phys. Rev. Appl. 4(5), 054014 (2015). [CrossRef]
19. H. E. Arabi, H.-E. Joe, T. Nazari, B.-K. Min, and K. Oh, “A high throughput supra-wavelength plasmonic bull’s eye photon sorter spatially and spectrally multiplexed on silica optical fiber facet,” Opt. Express 21(23), 28083–28094 (2013). [CrossRef] [PubMed]
20. H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Bright unidirectional fluorescence emission of molecules in a nanoaperture with plasmonic corrugations,” Nano Lett. 11(2), 637–644 (2011). [CrossRef] [PubMed]
21. J. Wenger, H. Aouani, D. Gérard, S. Blair, T. W. Ebbesen, and H. Rigneault, “Enhanced fluorescence from metal nanoapertures: physical characterizations and biophotonic applications,” Proc. SPIE 7577, 75770J (2010). [CrossRef]
