Meso-porous membrane of noble metal for surface plasmon resonance gas sensors

Abstract

Since surface plasmon resonance (SPR) provides quick response and high sensitivity to environmental substances, it is recognized as a powerful technique for toxic gas detection system [1–3]. Although already reported SPR gas sensors have achieved a high sensitivity at the ppm level, the adsorption of vapor molecules relies only on the affinity of the planar surface of the sensitive membrane. In terms of adsorption efficiency, a planar adsorber element is less advantageous because the specific surface area is small. It is expected that the sensitivity of the SPR gas sensor could be improved by introducing a porous structure to the sensor surface. However, in order to excite SPR at optical frequencies, materials are limited to Ag or Au on account of the dielectric permittivity to achieve wavenumber matching. These noble metals are stable both chemically and physically, and fabrication of a nano-sized porous structure on these metals has been difficult. In this letter, a meso-porous membrane of a noble metal on which SPR can be excited is reported. The adsorption of water vapor on the membrane is observed from the SPR signal, and the dimension of the porous structure is validated by the Kelvin’s capillary condensation theory. The meso-porous structure proposed in this study is based on the surface profile of a two-dimensional colloidal crystalline structure (CCS) of mono-sized nanoparticles. By utilizing this as a substrate for metallic vapor deposition, a metal film with a surface profile consisting of a number of nano-sized pores and grooves is realized. The crystalline structure is fabricated utilizing the self-assembling behavior of colloidal polymer nanoparticles spread on a hydrophilic glass slide [4]. First, a glass slide is washed carefully with a sponge containing surfactant, and then rinsed couple of times with pure water and organic solvent for degreasing. After rinsing, it is dried by blowing N2 gas over it. Due to the cleaning, the surface of the glass slide obtains a hydrophilic property. In the next stage, a colloidal aqueous solution of polystyrene nanospheres is dropped onto the cleaned surface of the stationary glass slide. As the thickness of the suspension layer becomes smaller than the diameter of the colloidal particles, owing to the evaporation, the particles are forced to attract each other due to capillary force. When the solvent is completely evaporated, the particles form a closely packed hexagonal crystalline structure on the surface of the glass slide. The crystalline structure of the nanoparticles intrinsically possesses nano-sized grooves and cavities of high aspect ratio at the interface of constitutive particles. As illustrated in Fig. 1a, each particle in the crystalline structure has contact points with the neighboring particles every 60 of the azimuth angle. The cross-sectional profile of the contact point of the two particles, expressed by the line A–B, shows a cycloidal surface profile as depicted in Fig. 1b. The width of the wedge-shaped groove gradually decreases and finally becomes zero numerically in the limit. This means that an ultimate porous structure is realized at the contact points. In addition, a small quasi-triangular cavity is T. Numata (&) Y. Otani N. Umeda Faculty of Engineering, Tokyo University of Agriculture and Technology, Nakacho 2-24-16, Koganei, Tokyo 184-8588, Japan e-mail: t-numata@cc.tuat.ac.jp J Mater Sci (2007) 42:1050–1053 DOI 10.1007/s10853-006-1285-z