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Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editors: Andrew Dunn and Anthony Durkin
  • Vol. 6, Iss. 8 — Aug. 26, 2011
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Nanoporous gold plasmonic structures for sensing applications

G. Ruffato, F. Romanato, D. Garoli, and S. Cattarin  »View Author Affiliations


Optics Express, Vol. 19, Issue 14, pp. 13164-13170 (2011)
http://dx.doi.org/10.1364/OE.19.013164


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Abstract

The fabrication, characterization and functionalization of periodically patterned nanoporous gold layers is presented. The material shows plasmonic properties in the near infrared range, with excitation and propagation of surface plasmon polaritons. Functionalization shows a marked enhancement in the optical response in comparison with evaporated gold gratings, due to a great increase of the active surface. Due to its superior response, nanoporous gold patterns appear promising for the realization of compact plasmonic platforms for sensing purposes.

© 2011 OSA

1. Introduction

NPG may be formed by a spontaneous pattern-forming instability during the chemical etching of silver from gold-silver alloys [4

4. N. A. Senior and R. C. Newman, “Synthesis of tough nanoporous metals by controlled electrolytic dealloying,” Nanotechnology 17(9), 2311–2316 (2006). [CrossRef]

]. The leaching of the less noble metal gives rise to a bicontinuous sponge-like structure of nanopores and gold ligaments [5

5. T. Fujita, L. H. Qian, K. Inoke, J. Erlebacher, and M. W. Chen, “Three-dimensional morphology of nanoporous gold,” Appl. Phys. Lett. 92(25), 251902 (2008). [CrossRef]

] whose geometric features depend on the alloy composition and on the experimental conditions of the dealloying process [4

4. N. A. Senior and R. C. Newman, “Synthesis of tough nanoporous metals by controlled electrolytic dealloying,” Nanotechnology 17(9), 2311–2316 (2006). [CrossRef]

8

8. E. Detsi, M. van de Schootbrugge, S. Punzhin, P. R. Onck, and J. T. M. De Hosson, “On tuning the morphology of nanoporous gold,” Scr. Mater. 64(4), 319–322 (2011). [CrossRef]

]. The nanoporous structure affects also the optical properties: the plasma frequency ωp exhibits red-shift due to the lower density in comparison with bulk gold and the material shows metallic behaviour for wavelengths above the near-IR range [9

9. A. I. Maaroof, A. Gentle, G. B. Smith, and M. B. Cortie, “Bulk and surface plasmons in highly nanoporous gold films,” J. Phys. D Appl. Phys. 40(18), 5675–5682 (2007). [CrossRef]

].

2. Experimental

Nanoporous gold optical response was measured with a J. A. Woollam Co. VASE ellipsometer with angular and wavelength spectroscopic resolution of 0.005° and 0.3 nm respectively. Spectroscopic Ellipsometry with rotating polarizer analysis was recorded in the wavelength range between 300 and 2400 nm (10 nm step) at three different angles of incidence (50°, 60°, 70°) on the no-patterned area of NPG sample. Focused Ion Beam (FIB) lithography was performed by means of the ion source of the dual beam system using 30 kV of accelerating voltage and a beam probe current of 280 pA. Taking into account the lower plasma frequency of NPG and the resulting shift of metallic behavior in the IR range (see section 3 “Results and Discussion”), a grating about 50 nm thick with a period of 1000 nm (duty cycle 0.5) has been patterned (Fig. 1(a)
Fig. 1 SEM micrographs of the FIB pattern on the nanoporous gold surface (a) and cross-section (b).
) over an area 640 μm x 640 μm. A cross section of the grating pattern shows that the typical amplitude of the grating is confined within the first 70 nm of the surface NPG (Fig. 1(b)). For comparison with the gold NPG case, a grating with period of about 400 nm was patterned on a typical gold bulk film of 80 nm thickness evaporated over a silicon substrate.

A self-assembled monolayer (SAM) of benzenethiol (Thiophenol, C6H5SH) was deposited on the gold coated grating surfaces at room temperature. The substrates were submerged in a 3-mM solution of benzenethiol in methanol for about 48 hrs, then rinsed thoroughly with ethanol for at least 5 minutes and dried in a nitrogen stream [17

17. R. L. Aggarwal, L. W. Farrar, E. D. Diebold, and D. L. Polla, “Measurement of the absolute Raman scattering cross section of the 1584-cm−1 band of benzenethiol and the surface-enhanced Raman scattering cross section enhancement factor for femtosecond laser-nanostructured substrates,” J. Raman Spectrosc. 40(9), 1331–1333 (2009). [CrossRef]

]. Measurements on gold gratings were performed in air environment in a θ/2θ symmetric reflectivity configuration, with θ scanned with step size of 0.2°, using ellipsometer 75 W Xe lamp, monochromatized at λ = 1400 nm for NPG grating and at λ = 600 nm for the evaporated gold (EVG) grating.

3. Results and discussion

In Fig. 2
Fig. 2 Dielectric permittivity of nanoporous gold. Comparison with tabulated buk gold values.
experimental optical constants of NPG are compared with those of bulk gold [18

18. E. D. Palik, ed., Handbook of Optical Constants of Solids (Academic Press, 1991).

]. It appears a weaker metallic behaviour in the optical regime with ε1 nearly close to zero. Above the 900 nm, ε2 of NPG is smaller than for the bulk case and ε1 starts to be sufficiently negative to show a metallic behaviour.

The behavior of the dielectric constants must be considered when a grating is designed in order to support propagating plasmonic modes. The geometry of the grating (duty cycle, period and thickness of the walls) was fixed in order to obtain a plasmonic resonance in the spectral range in which the material exhibits a metallic behavior. To understand the optical response, we examine the effective frequency-dependent dielectric function ε(ω) and describe it with a Lorentz-Drude model in the UV-VIS-nearIR range:
ε(ω)=ε+εUV(ω)+εD(ω)+εIR(ω)=εAUVω2ωUV2+iωωτ,UVωp2ω2+iωωτ,DAIRω2ωIR2+iωωτ,IR
(1)
where ε takes into account the constant contribution to polarization due to d band electrons close to the Fermi surface. εUV(ω) is a Lorentz oscillator that describes the 3d energy band-to-Fermi Level interband transition centered at a frequency ωUV in the UV range with a band-width ωτ,UV. The term εD(ω) represents the Drude contribution due to free s-electrons. The Lorentz contribution εIR(ω) considered in order to model the behavior of the dielectric response in the near IR range, enables excellent fits to dielectric constant of nanoporous films [9

9. A. I. Maaroof, A. Gentle, G. B. Smith, and M. B. Cortie, “Bulk and surface plasmons in highly nanoporous gold films,” J. Phys. D Appl. Phys. 40(18), 5675–5682 (2007). [CrossRef]

] and it is probably associated to the excitation of Localized Surface Plasmons.

Reflectivity analysis after FIB lithography shows the appearance of resonance dips in the near-IR in correspondence of the excitation of propagating surface plasmons on the porous gold surface (Fig. 3
Fig. 3 Reflectivity measurements for NPG grating before (solid line) and after (dashed line) functionalization at null azimuth (black line) and p-polarization (α = 0°), and at azimuth 40° (red line), polarization α = 140°. Green solid line: reflectivity data for no-patterned NPG surface.
). The excitation of SPPs on a grating is achieved when the on-plane component of the incident light wave-vector and the diffracted SPP wave-vector kSPP match the momentum conservation condition:
kSPP=kin||±nG
(3)
where kin||=2πλ(sinθin,0)and θin is the angle of incidence. The crystal momentum of the grating of Λ pitch is G=2πΛ(cosϕ,sinϕ) whose rotation is measured by ϕ (see inset picture in Fig. 4
Fig. 4 Resonance angle shift Δθ as a function of azimuth rotation after functionalization with a benzenethiol self-assembled monolayer. Experimental data of functionalized nanoporous gold (NPG) grating (period Λ = 1000 nm, incident wavelength λ = 1400 nm – red points) are compared with experimental shifts for an evaporated-gold (EVG) grating (Λ = 400 nm, λ = 600 nm – black points). Inset picture: scheme of the incidence mounting.
). Only the first diffraction order (n = 1) is effective because in our case, G is always greater than kin ||. Dip features such as width, depth and position strictly depend on the properties of the supporting material, i.e. optical constants of the nanoporous gold layer.

In order to compare the sensitivity of the plasmonic gratings, reflectivity data have been also compared with the optical response of an evaporated gold (EVG) grating after the same functionalization process. EVG surface has been patterned with a period Λ = 400 nm in order to excite SPPs in the visible range, where the angular response for EVG is greater since the sensitivity to a thin coating layer decreases with wavelength [2

2. J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108(2), 462–493 (2008). [CrossRef] [PubMed]

]. For incident λ = 600 nm and azimuth ϕ = 45° (α = 135°) we measure a shift ΔθEVG(45°) = 0.82°.

The choice of evaluating the EVG and NPG gratings at different wavelength has been motivated to compare them at the respective maximum of sensibility. The greater dip shift in the case of patterned NPG is explained by the enhanced binding surface per unit area of the nanopores. Analyte molecules in fact, not merely bind in the form of a thin coating layer on the outer surface, but, in the case of NPG, penetrate into the pores and bind to the inner surface, inducing a greater change of the effective index of the plasmonic support.

4. Conclusions

In summary, we presented our work of fabrication, characterization and design of periodically patterned nanoporous gold surfaces that support excitation of propagating surface plasmons. Thanks to a greatly enhanced surface-to-volume ratio, nanoporous gold reveals benefits for better reaction efficiency and detection sensitivity. Functionalization shows an enhancement in the optical response in comparison with evaporated gold gratings. Thus nanoporous gold represents a useful and promising material for the realization of compact sensitive devices based on SPP plasmonic substrate for sensing purposes in a large variety of fields: environmental protection, biotechnology, medical diagnostics, drug screening, food safety and security.

Acknowledgments

This work has been supported by a grant from “Fondazione Cariparo” – Surface PLasmonics for Enhanced Nano Detectors and Innovative Devices (SPLENDID) – Progetto Eccellenza 2008 and from University of Padova – Progetto di Eccellenza “Platform”.

References and links

1.

S. A. Maier, ed., Plasmonics— Fundamentals and Applications (Springer, 2007).

2.

J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108(2), 462–493 (2008). [CrossRef] [PubMed]

3.

J. Biener, G. W. Nyce, A. M. Hodge, M. M. Biener, A. V. Hamza, and S. A. Maier, “Nanoporous plasmonic metamaterials,” Adv. Mater. (Deerfield Beach Fla.) 20(6), 1211–1217 (2008). [CrossRef]

4.

N. A. Senior and R. C. Newman, “Synthesis of tough nanoporous metals by controlled electrolytic dealloying,” Nanotechnology 17(9), 2311–2316 (2006). [CrossRef]

5.

T. Fujita, L. H. Qian, K. Inoke, J. Erlebacher, and M. W. Chen, “Three-dimensional morphology of nanoporous gold,” Appl. Phys. Lett. 92(25), 251902 (2008). [CrossRef]

6.

X. Lu, E. Bischoff, R. Spolenak, and T. J. Balk, “Investigation of dealloying in Au-Ag thin films by quantitative electron probe microanalysis,” Scr. Mater. 56(7), 557–560 (2007). [CrossRef]

7.

X. Y. Lang, P. F. Guan, L. Zhang, T. Fujita, and M. W. Chen, “Characteristic length and temperature dependence of surface enhanced Raman scattering of nanoporous gold,” J. Phys. Chem. C 113(25), 10956–10961 (2009). [CrossRef]

8.

E. Detsi, M. van de Schootbrugge, S. Punzhin, P. R. Onck, and J. T. M. De Hosson, “On tuning the morphology of nanoporous gold,” Scr. Mater. 64(4), 319–322 (2011). [CrossRef]

9.

A. I. Maaroof, A. Gentle, G. B. Smith, and M. B. Cortie, “Bulk and surface plasmons in highly nanoporous gold films,” J. Phys. D Appl. Phys. 40(18), 5675–5682 (2007). [CrossRef]

10.

F. Yu, S. Ahl, A. M. Caminade, J. P. Majoral, W. Knoll, and J. Erlebacher, “Simultaneous excitation of propagating and localized surface plasmon resonance in nanoporous gold membranes,” Anal. Chem. 78(20), 7346–7350 (2006). [CrossRef] [PubMed]

11.

F. Romanato, H. K. Kang, K. H. Lee, G. Ruffato, M. Prasciolu, and C. C. Wong, “Interferential lithography of 1D thin metallic sinusoidal grtings: accurate control of the profile for azimuthal angular dependent plasmonic effects and applications,” Microelectron. Eng. 86(4-6), 573–576 (2009). [CrossRef]

12.

F. Romanato, K. H. Lee, H. K. Kang, G. Ruffato, and C. C. Wong, “Sensitivity enhancement in grating coupled surface plasmon resonance by azimuthal control,” Opt. Express 17(14), 12145–12154 (2009). [CrossRef] [PubMed]

13.

F. Romanato, K. H. Lee, G. Ruffato, and C. C. Wong, “The role of polarization on surface plasmon polariton excitation on metallic gratings in the conical mounting,” Appl. Phys. Lett. 96(11), 111103 (2010). [CrossRef]

14.

Y. Sun and T. J. Balk, “Evolution of structure, composition, and stress in nanoporous gold thin films with grain-boundary cracks,” Metall. Mater. Trans., A Phys. Metall. Mater. Sci. 39(11), 2656–2665 (2008). [CrossRef]

15.

S. Cattarin, D. Kramer, A. Lui, and M. Musiani, “Preparation and characterization of gold nanostructures of controlled dimension by electrochemical techniques,” J. Phys. Chem. C 111(34), 12643–12649 (2007). [CrossRef]

16.

S. Cattarin, D. Kramer, A. Lui, and M. Musiani, “Formation of nanostructured gold sponges by anodic dealloying: EIS investigation of product and process,” Fuel Cells (Weinh.) 9(3), 209–214 (2009). [CrossRef]

17.

R. L. Aggarwal, L. W. Farrar, E. D. Diebold, and D. L. Polla, “Measurement of the absolute Raman scattering cross section of the 1584-cm−1 band of benzenethiol and the surface-enhanced Raman scattering cross section enhancement factor for femtosecond laser-nanostructured substrates,” J. Raman Spectrosc. 40(9), 1331–1333 (2009). [CrossRef]

18.

E. D. Palik, ed., Handbook of Optical Constants of Solids (Academic Press, 1991).

19.

M. C. Dixon, T. A. Daniel, M. Hieda, D. M. Smilgies, M. H. W. Chan, and D. L. Allara, “Preparation, structure, and optical properties of nanoporous gold thin films,” Langmuir 23(5), 2414–2422 (2007). [CrossRef] [PubMed]

20.

N. W. Ashcroft and N. D. Mermin, eds., Solid State Physics (Thomson Brookes – Cole, 1976).

OCIS Codes
(240.6680) Optics at surfaces : Surface plasmons
(160.4236) Materials : Nanomaterials
(280.4788) Remote sensing and sensors : Optical sensing and sensors

ToC Category:
Optics at Surfaces

History
Original Manuscript: April 29, 2011
Revised Manuscript: June 1, 2011
Manuscript Accepted: June 1, 2011
Published: June 22, 2011

Virtual Issues
Vol. 6, Iss. 8 Virtual Journal for Biomedical Optics

Citation
G. Ruffato, F. Romanato, D. Garoli, and S. Cattarin, "Nanoporous gold plasmonic structures for sensing applications," Opt. Express 19, 13164-13170 (2011)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-19-14-13164


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References

  1. S. A. Maier, ed., Plasmonics— Fundamentals and Applications (Springer, 2007).
  2. J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108(2), 462–493 (2008). [CrossRef] [PubMed]
  3. J. Biener, G. W. Nyce, A. M. Hodge, M. M. Biener, A. V. Hamza, and S. A. Maier, “Nanoporous plasmonic metamaterials,” Adv. Mater. (Deerfield Beach Fla.) 20(6), 1211–1217 (2008). [CrossRef]
  4. N. A. Senior and R. C. Newman, “Synthesis of tough nanoporous metals by controlled electrolytic dealloying,” Nanotechnology 17(9), 2311–2316 (2006). [CrossRef]
  5. T. Fujita, L. H. Qian, K. Inoke, J. Erlebacher, and M. W. Chen, “Three-dimensional morphology of nanoporous gold,” Appl. Phys. Lett. 92(25), 251902 (2008). [CrossRef]
  6. X. Lu, E. Bischoff, R. Spolenak, and T. J. Balk, “Investigation of dealloying in Au-Ag thin films by quantitative electron probe microanalysis,” Scr. Mater. 56(7), 557–560 (2007). [CrossRef]
  7. X. Y. Lang, P. F. Guan, L. Zhang, T. Fujita, and M. W. Chen, “Characteristic length and temperature dependence of surface enhanced Raman scattering of nanoporous gold,” J. Phys. Chem. C 113(25), 10956–10961 (2009). [CrossRef]
  8. E. Detsi, M. van de Schootbrugge, S. Punzhin, P. R. Onck, and J. T. M. De Hosson, “On tuning the morphology of nanoporous gold,” Scr. Mater. 64(4), 319–322 (2011). [CrossRef]
  9. A. I. Maaroof, A. Gentle, G. B. Smith, and M. B. Cortie, “Bulk and surface plasmons in highly nanoporous gold films,” J. Phys. D Appl. Phys. 40(18), 5675–5682 (2007). [CrossRef]
  10. F. Yu, S. Ahl, A. M. Caminade, J. P. Majoral, W. Knoll, and J. Erlebacher, “Simultaneous excitation of propagating and localized surface plasmon resonance in nanoporous gold membranes,” Anal. Chem. 78(20), 7346–7350 (2006). [CrossRef] [PubMed]
  11. F. Romanato, H. K. Kang, K. H. Lee, G. Ruffato, M. Prasciolu, and C. C. Wong, “Interferential lithography of 1D thin metallic sinusoidal grtings: accurate control of the profile for azimuthal angular dependent plasmonic effects and applications,” Microelectron. Eng. 86(4-6), 573–576 (2009). [CrossRef]
  12. F. Romanato, K. H. Lee, H. K. Kang, G. Ruffato, and C. C. Wong, “Sensitivity enhancement in grating coupled surface plasmon resonance by azimuthal control,” Opt. Express 17(14), 12145–12154 (2009). [CrossRef] [PubMed]
  13. F. Romanato, K. H. Lee, G. Ruffato, and C. C. Wong, “The role of polarization on surface plasmon polariton excitation on metallic gratings in the conical mounting,” Appl. Phys. Lett. 96(11), 111103 (2010). [CrossRef]
  14. Y. Sun and T. J. Balk, “Evolution of structure, composition, and stress in nanoporous gold thin films with grain-boundary cracks,” Metall. Mater. Trans., A Phys. Metall. Mater. Sci. 39(11), 2656–2665 (2008). [CrossRef]
  15. S. Cattarin, D. Kramer, A. Lui, and M. Musiani, “Preparation and characterization of gold nanostructures of controlled dimension by electrochemical techniques,” J. Phys. Chem. C 111(34), 12643–12649 (2007). [CrossRef]
  16. S. Cattarin, D. Kramer, A. Lui, and M. Musiani, “Formation of nanostructured gold sponges by anodic dealloying: EIS investigation of product and process,” Fuel Cells (Weinh.) 9(3), 209–214 (2009). [CrossRef]
  17. R. L. Aggarwal, L. W. Farrar, E. D. Diebold, and D. L. Polla, “Measurement of the absolute Raman scattering cross section of the 1584-cm−1 band of benzenethiol and the surface-enhanced Raman scattering cross section enhancement factor for femtosecond laser-nanostructured substrates,” J. Raman Spectrosc. 40(9), 1331–1333 (2009). [CrossRef]
  18. E. D. Palik, ed., Handbook of Optical Constants of Solids (Academic Press, 1991).
  19. M. C. Dixon, T. A. Daniel, M. Hieda, D. M. Smilgies, M. H. W. Chan, and D. L. Allara, “Preparation, structure, and optical properties of nanoporous gold thin films,” Langmuir 23(5), 2414–2422 (2007). [CrossRef] [PubMed]
  20. N. W. Ashcroft and N. D. Mermin, eds., Solid State Physics (Thomson Brookes – Cole, 1976).

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