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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 5 — Mar. 5, 2007
  • pp: 2592–2597
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High sensitivity sensors made of perforated waveguides

Koichi Awazu, Carsten Rockstuhl, Makoto Fujimaki, Nobuko Fukuda, Junji Tominaga, Tetsuro Komatsubara, Takahiro Ikeda, and Yoshimichi Ohki  »View Author Affiliations


Optics Express, Vol. 15, Issue 5, pp. 2592-2597 (2007)
http://dx.doi.org/10.1364/OE.15.002592


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Abstract

Sensors based on surface plasmons or waveguide modes are at the focus of interest for applications in biological or environmental chemistry. Waveguide-mode spectra of 1 μm-thick pure and perforated silica films comprising isolated nanometric holes with great aspect ratio were measured before and after adhesion of streptavidin at concentrations of 500 nM. The shift of the angular position for guided modes was nine times higher in perforated films than in bulk films. Capturing of streptavidin in the nanoholes is at the origin of that largely enhanced shift in the angular position as the amplitude of the guided mode in the waveguide perfectly overlaps with the perturbation caused by the molecules. Hence, the device allows for strongly confined modes and their strong perturbation to enable ultra-sensitive sensor applications.

© 2007 Optical Society of America

1. Introduction

Fig. 1. Experimental arrangement for the measurement in Kretschmann geometry.
Fig. 2. (a). Calculated reflectivity as a function of the angle of incidence and the polarization for a typical sample as employed in the present investigations. Dips in the reflectivity are associated to be guided modes in the waveguide or to surface plasmon. (b). Amplitudes of excited guided modes. Glass prism, gold film, silica glass and water are lined from left to right. Each boundary is presented by a green line. Scale bars of the field strengths were shown on each right hand side.

A higher sensitivity of such sensors is already achieved, by increasing the surface of the detector. Porous anodic alumina films are commonly employed to enlarge the surface because of their well-ordered nanostructure self-organizes when aluminum films are anodized in an acid electrolyte causing pores at sufficient diameters. These pores run straight through the film parallel to each other. The pore sizes are dictated by a combination of the applied voltage used for the anodization and the solutions employed. For example, self-ordering takes place at 25 V in sulfuric acid solution, at 40 V in oxalic acid solution, and at 195 V in phosphoric acid solution, leading to pore sizes of 65 nm, 100 nm, and 500 nm, respectively [2

2. H. Masuda, K. Yada, and A. Osaka, “Self-ordering of cell configuration of anodic porous alumina with large-size pores in phosphoric acid solution,” Jpn. J. Appl. Phys. 37, L1340–L1342 (1998). [CrossRef]

]. Disadvantage of such an approach is the impossibility to obtain pores with arbitrary diameters. It has been reported that porous aluminum films may be employed simultaneously as deposition templates and as highly sensitive detectors of processes within the pores [3

3. K. H. A. Lau, L. S. Tan, K. Tamada, M. S. Sander, and W. Knoll, “Highly sensitivive detection of processes occurring inside nanoporous anodic alumina templates: A waveguide optical study,” J. Phys. Chem. B 108, 10812–108181 (2004). [CrossRef]

, 4

4. S. G. Cloutier, A. D. Lazareck, and J. Xu, “Detection of nano-confined DNA using surface-plasmon enhanced fluorescence,” Appl. Phys. Lett. 88, 013904-1–013904-3 (2006). [CrossRef]

]. Also porous silicon has been used as a bio-sensing material [5

5. J. J. Saarinen, S. M. Weiss, P. M. Fauchet, and J. E. Sipe, “Optical sensor based on resonant porous silicon structures,” Opt. Express 10, 3754–3764 (2005). [CrossRef]

].

We will report here on a strategy to perforate dielectric waveguides to enhance significantly the sensitivity of the sensor as firstly, the perturbation of the modes by adsorbed molecules is enlarged and secondly, their adhesion surface is increased. Silica coat is mechanically and environmentally stable. The holes in the insulator are created in the following way: the passage of an atomic particle through an insulator results in the creation of a cylindrical, latent nuclear track that extends along a straight line corresponding to the path of the atomic particle [6

6. K. Awazu, S. Ishii, K. Shima, S. Roorda, and J. L. Brebner, “Structure of latent tracks created by swift heavy ion bombardment of amorphous SiO2,” Phys. Rev. B 62, 3689–3698 (2000). [CrossRef]

]. The presence of tracks is revealed after etching in a suitable solution. However, the shape of the etched hole is often not cylindrical, but conical, with the largest diameter being near the surface. This is a consequence of the etching rate in the latent track being comparable to the general etching rate in the unirradiated region. Musket et al. have used vapor etching of ion tracks to create high aspect ratio, isolated cylindrical holes through ~600 nm-thick films of thermally grown silica on silicon [7

7. R. G. Musket, J. M. Yoshiyama, R. J. Contolini, and J. D. Porter, “Vapor etching of ion tracks in fused silica,” J. Appl. Phys. 91, 5760–5764 (2002). [CrossRef]

]. In the present study, we have adopted the vapor etching technique to create cylindrical holes through silica waveguides. Excitation of the surface plasmon and waveguide modes are experimentally observed as very sharp dips in reflectivity when measured as a function of the incident angle. Au must be needed, otherwise we cannot observe waveguide mode. The adhesion of streptavidin molecules onto the surface and into the holes inside the waveguide causes a strong shift in the measured angular position of the modes. We found a nine times increase of the shift for the perforated waveguides as compared to bulk waveguides at the expense of only slightly larger line widths. The line width of those modes is only negligible larger due to enhanced scattering at the surface of the holes increasing the guiding losses. As the diameter of the holes was in the order of a few tens of nanometers only, the modes experience effective material parameters for the waveguide material and will not interact sensitive on the fine details of the holes. Incorporation of streptavidin causes a significant change in the optical properties of the waveguide material.

2. Experimental

As glass substrate and prism, we used S-LAH66 glass (Ohara Co., Ltd.) with a high refractive index of 1.769 [8]. To ensure adhesion of a 50 nm Au layer on the substrate, first a ~0.8 nm Cr layer was deposited on the surface. On top of the gold film again a ~0.8 nm Cr layer was deposited by thermal evaporation (R-DEC Co., Ltd., Japan). Finally, a 500nm thick-waveguide layer of a-SiOx film was deposited on the Cr/Au/Cr films. To improve the optical performance of the film, the structure was tempered at 600 °C for 24 h. Rutherford backscattering measurement revealed the x in a-SiOx to be in the vicinity of 2. Thus, hereinafter we call the film silica.

Ion bombardment to perforate the waveguide was performed at room temperature at a residual pressure below 1 × 10-4 Pa. The 12 UD Pelletron tandem accelerator at the University of Tsukuba was employed with 150 MeV Au14+ ions. To avoid overlap of the individual cylindrical holes, very low ion fluency was required. This was achieved by diffusing the ion beam through an aluminum foil in forward scattering geometry. 150 MeV Au14+ was reduced to 137 MeV Au30+ by passage through the Al foil. We performed vapor etching experiments with 20% hydrofluoric acid (HF). Influence of various liquid TL and sample TS temperatures was examined and TL = 19 °C and TS = 30 °C were obtained as a best condition. Changing the temperature allowed for control over the hole sizes.

The substrate glass was mounted on a prism to form an optically contiguous medium. A He-Ne laser operating at 632.8 nm was used in subsequent measurements and a polarizer selected for either TE- or TM-polarization. We also clamped a Teflon flow cell onto the exposed film to permit injection and extraction of the desired dielectric medium/reagents. To observe the optical waveguide modes, reflectivity was measured at a constant temperature of 20.8°C against incident angle.

Figure 3 illustrates the chemical surface modification procedure to ensure adhesion of molecules. At first, the surface of silica was modified by silanol groups. It has been reported that the number of surface silanol groups is increased in an alkali solution [9

9. B. R. Midmore, “Effect of aqueous phase composition on the properties of a silica-stabilized w/o emulsion,” J. Colloid Interface Sci. 213, 352–359 (1999). [CrossRef] [PubMed]

]. Thus, 3-aminopropyltriethoxysilane (3APT) was bound with a silanol groups by immersing the silica waveguide substrates in 0.5 vol% 3APT ethanol solution for 3 h. The 3APT-modified substrate was rinsed with ethanol and dried in a stream of nitrogen gas and then immersed in 0.5 mM 5-[5-(N-succinimidyloxycarbonyl)pentylamido]hexyl D-biotinamide (biotin-(AC5)2-OSu) solution in 1/15 M PBS buffer (pH 7.4) for amide-coupling with the 3APT layer. Biotin-(AC5)2-OSu) was used for streptavidin capture in the presence of phosphate buffer solution (PBS) [10

10. N. Fukuda, M. Fujimaki, K. Awazu, K. Tamada, and K. Yase, “High sensitive optical detection of bio-chemicals onto a silicon oxide surface based on waveguide mode,” Mater. Res. Symp. Proc. 900, O12-39.1–6 (2006).

].

Fig. 3. Schematic of the supermolecule architecture of a silica film in preparation for streptavidin capture. A hydrophilic surface of a silica film was obtained in alkali solution. 3-aminoprophyltriethoxysilane (3APT) was used as the silane coupling agent. The 3APT-modified substrate was immersed in 0.5 mM 5-[5-(N-succinimidyloxycarbonyl) pentylamido]hexyl D-biotinamide (biotin-(AC5)2-OSu) solution in 1/5 M PBS buffer for amide coupling with the 3APT layer.

3. Results and discussion

SEM images of the fabricated samples provided sufficient evidence that cylindrical holes were etched into the 500nm-thick silica films (Fig. 4.) These holes with a diameter of ~ 50 nm resulted from vapor etching followed by rinsing in distilled water and drying. The hole density was estimated to be 5.0 (± 0.8) ×108 cm-2 as deduced from SEM images, which is consistent with the experimentally counted fluence of 5.0 (± 0.2) ×108 cm-2 using a Faraday cup.

Waveguide modes can be excited in the silica waveguide. The waveguide mode spectrum shifts when the silica waveguide film is exposed to a solution containing biotin of 500 μM. Figure 5(a) shows reflectivity against incident angle in the vicinity of the TE0 mode obtained on the as-prepared silica film before (black circles) and after (red circles) immersion in biotin solution. The central line shift in the as-prepared sample case was less than 0.01° and merely noticeable. The angular resolution in this experiment was 0.01°. Figure 5(b) shows the reflectivity vs. angle around the same mode obtained on porous silica film with a number of 1 × 1010 cm-2 holes, 500nm deep and 50 nm in diameter. The black and red circles indicate the spectra before and after immersion in biotin solution. The central line shift was dramatically increased up to 0.14°. It was found that the remaining dip intensity was increased by the creation of holes and the line width was likewise increased slightly. Both observations are easily attributed to an increase of the guiding losses of the modes due to additional light scattering at the etched holes and also due to the chrome on the metal film and the APT modification of silica. Reduction of refractive index with increase of holes in waveguide introduced wider dips and much smaller contrast of in off- and on-resonance reflectance. Better spectrum can be obtained with controlling thickness of waveguide and metals.

Fig. 4. Top (left) and cross-sectional (right) view of a particular silica films. Here 137 MeV Au30+ ion bombardment was performed at a fluence of 5× 108 cm-2 followed by vapor etching.
Fig. 5. (a). TE0 mode of the waveguide before (black) and after (red) modification with 500-μM biotin. Shift of the dip position was less than 0.01°. (b). TE0 mode of the silica waveguide substrate with a density of 1 ×1010 cm-2 holes before and after modification with 500-μM biotin.

Figure 6 shows angular dependent reflectivity for a TE0 mode before and after capturing streptavidin of 500 nM. For the silica waveguide before creation of pores, dip shift was estimated to be 0.06 °. The sample having 1×1010 holes/cm2 was fragile and it was not successful to measure the spectrum after biotin-streptavidin bindings. When a number of 7 × 109 cm-2 holes were created in the silica waveguide, central position of the dip shifted dramatically by 0.53 °. Materials may not reach the bottom of the hole but the vicinity of the maximum of field distributions of the guided modes that are concentrated inside the waveguide. Additional waveguides with increased hole densities were fabricated and the TE0 spectra were measured. The relationship between dip shift due to capturing of streptavidin and hole density was examined. Change in reflectivity at a constant angle (ΔR/R) could be estimated from the TE0 spectra. For example, reflectivity at 53.56° was estimated from Fig. 6(b) to be 0.51 before and 0.78 after capturing of streptavidin. Based on those values, we evaluated the relative change in reflectivity (ΔR/R) with respect to the unexposed sample. ΔR/R values are plotted against hole-density in Fig. 7. It was found that the dip shift increases with number of holes. The ΔR/R value reached a maximum above 3 × 109 cm-2. Saturation is observed, as the line width of the unexposed compared to the exposed spectra are no more overlapping. Such a shift is hardly to achieve with conventional plasmon based sensors.

Shift of SPR peak position due to biotin-streptavidin bindings was reported as 360ng/cm2 per 0.01° [11

11. Y. Terasaka, Y. Arima, and H. Iwata, “Surface plasmon resonance-based highly sensitive immunosensing for brain natriuretic peptide using nanobeads for signal amplification,” Anal. Biochem. 357, 208–215 (2006). [CrossRef]

]. When surface was filled with streptavidin molecules of 5nmϕ, mass of streptavidin can be estimated as 400ng/cm2, which is close to the value reported in [11

11. Y. Terasaka, Y. Arima, and H. Iwata, “Surface plasmon resonance-based highly sensitive immunosensing for brain natriuretic peptide using nanobeads for signal amplification,” Anal. Biochem. 357, 208–215 (2006). [CrossRef]

]. It is impossible to compare the displacement of the angle per biotin-streptavidin bindings between SPR and TE0 mode because SPR cannot be excited in the silica waveguide. Moreover, surface area of porous silica in the laser beam spot is 7.88cm2 which is much larger than 0.03cm2 of non-porous silica. The relative change in reflectivity (ΔR/R) of SPR and TE0 mode were calculated at 0.2% and 2% per 0.01° shift, respectively. From the calculation, it was firmed up that approach of the present work was better than SPR. We expect the sensor chips to be able to detect impurities of ppb order.

Fig. 6. TE0 modes of the waveguide modified with biotin. Black and red circles denote reflectivity before and after capture of 500 nM streptavidin. (a). No holes. (b). 7×109 holes cm-2.
Fig. 7. Relative change in reflectivity (ΔR/R) of TE0 against hole density in the waveguide.

4. Summary

Acknowledgments

C. Rockstuhl, would like to thank the Canon-Foundation Europe for financial support.

References and links

1.

E. Kretschmann, “Die Bestimmung optischer Konstanten von Metallen durch Oberflächenplasmaschwingungen,” Z. Physik 241, 313–324 (1971). [CrossRef]

2.

H. Masuda, K. Yada, and A. Osaka, “Self-ordering of cell configuration of anodic porous alumina with large-size pores in phosphoric acid solution,” Jpn. J. Appl. Phys. 37, L1340–L1342 (1998). [CrossRef]

3.

K. H. A. Lau, L. S. Tan, K. Tamada, M. S. Sander, and W. Knoll, “Highly sensitivive detection of processes occurring inside nanoporous anodic alumina templates: A waveguide optical study,” J. Phys. Chem. B 108, 10812–108181 (2004). [CrossRef]

4.

S. G. Cloutier, A. D. Lazareck, and J. Xu, “Detection of nano-confined DNA using surface-plasmon enhanced fluorescence,” Appl. Phys. Lett. 88, 013904-1–013904-3 (2006). [CrossRef]

5.

J. J. Saarinen, S. M. Weiss, P. M. Fauchet, and J. E. Sipe, “Optical sensor based on resonant porous silicon structures,” Opt. Express 10, 3754–3764 (2005). [CrossRef]

6.

K. Awazu, S. Ishii, K. Shima, S. Roorda, and J. L. Brebner, “Structure of latent tracks created by swift heavy ion bombardment of amorphous SiO2,” Phys. Rev. B 62, 3689–3698 (2000). [CrossRef]

7.

R. G. Musket, J. M. Yoshiyama, R. J. Contolini, and J. D. Porter, “Vapor etching of ion tracks in fused silica,” J. Appl. Phys. 91, 5760–5764 (2002). [CrossRef]

8.

http://www.ohara-inc.co.jp/en/product/optical/list/s-lah.html.

9.

B. R. Midmore, “Effect of aqueous phase composition on the properties of a silica-stabilized w/o emulsion,” J. Colloid Interface Sci. 213, 352–359 (1999). [CrossRef] [PubMed]

10.

N. Fukuda, M. Fujimaki, K. Awazu, K. Tamada, and K. Yase, “High sensitive optical detection of bio-chemicals onto a silicon oxide surface based on waveguide mode,” Mater. Res. Symp. Proc. 900, O12-39.1–6 (2006).

11.

Y. Terasaka, Y. Arima, and H. Iwata, “Surface plasmon resonance-based highly sensitive immunosensing for brain natriuretic peptide using nanobeads for signal amplification,” Anal. Biochem. 357, 208–215 (2006). [CrossRef]

OCIS Codes
(160.6030) Materials : Silica
(240.6680) Optics at surfaces : Surface plasmons
(240.6690) Optics at surfaces : Surface waves
(260.3910) Physical optics : Metal optics
(310.2790) Thin films : Guided waves

ToC Category:
Optics at Surfaces

History
Original Manuscript: November 17, 2006
Revised Manuscript: January 12, 2007
Manuscript Accepted: January 21, 2007
Published: March 5, 2007

Virtual Issues
Vol. 2, Iss. 4 Virtual Journal for Biomedical Optics

Citation
Koichi Awazu, Carsten Rockstuhl, Makoto Fujimaki, Nobuko Fukuda, Junji Tominaga, Tetsuro Komatsubara, Takahiro Ikeda, and Yoshimichi Ohki, "High sensitivity sensors made of perforated waveguides," Opt. Express 15, 2592-2597 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-5-2592


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References

  1. E. Kretschmann, "Die Bestimmung optischer Konstanten von Metallen durch Oberflächenplasmaschwingungen," Z. Physik 241, 313-324 (1971). [CrossRef]
  2. H. Masuda, K. Yada, and A. Osaka, "Self-ordering of cell configuration of anodic porous alumina with large-size pores in phosphoric acid solution," Jpn. J. Appl. Phys. 37, L1340-L1342 (1998). [CrossRef]
  3. K. H. A. Lau, L. S. Tan, K. Tamada, M. S. Sander, and W. Knoll, "Highly sensitivive detection of processes occurring inside nanoporous anodic alumina templates: A waveguide optical study," J. Phys. Chem. B 108, 10812-108181 (2004). [CrossRef]
  4. S. G. Cloutier, A. D. Lazareck, and J. Xu, "Detection of nano-confined DNA using surface-plasmon enhanced fluorescence," Appl. Phys. Lett. 88, 013904-1-013904-3 (2006). [CrossRef]
  5. J. J. Saarinen, S. M. Weiss, P. M. Fauchet, and J. E. Sipe, "Optical sensor based on resonant porous silicon structures," Opt. Express 10, 3754-3764 (2005). [CrossRef]
  6. K. Awazu, S. Ishii, K. Shima, S. Roorda, and J. L. Brebner, "Structure of latent tracks created by swift heavy ion bombardment of amorphous SiO2," Phys. Rev. B 62, 3689-3698 (2000). [CrossRef]
  7. R. G. Musket, J. M. Yoshiyama, R. J. Contolini, and J. D. Porter, "Vapor etching of ion tracks in fused silica," J. Appl. Phys. 91, 5760-5764 (2002). [CrossRef]
  8. http://www.ohara-inc.co.jp/en/product/optical/list/s-lah.html
  9. B. R. Midmore, "Effect of aqueous phase composition on the properties of a silica-stabilized w/o emulsion," J. Colloid Interface Sci. 213, 352-359 (1999). [CrossRef] [PubMed]
  10. N. Fukuda, M. Fujimaki, K. Awazu, K. Tamada, and K. Yase, "High sensitive optical detection of bio-chemicals onto a silicon oxide surface based on waveguide mode," Mater. Res. Soc. Symp. Proc. 900, O12-39.1-6 (2006).
  11. Y. Terasaka, Y. Arima, and H. Iwata, "Surface plasmon resonance-based highly sensitive immunosensing for brain natriuretic peptide using nanobeads for signal amplification," Anal. Biochem. 357, 208-215 (2006). [CrossRef]

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