Fabrication and optical sensing properties of mesoporous silica nanorod arrays

Abstract

In this study, a mesoporous silica nanorod (MSNR) array on a gold (Au) film was fabricated via the Stöber-solution growth method using a porous anodic alumina (PAA) membrane as a template. The typical length and diameter of the MSNRs are approximately 675 nm and 65 nm, respectively, which are in accordance with the characteristics of the PAA template. In the Kretschmann configuration, the MSNR array with the Au film exhibits typical optical waveguide (OWG) characteristics, and an obvious waveguide coupling dip corresponding to the TM₁ mode under p-polarized light was observed in the reflectivity spectrum. A resolution of the refractive index (RI) as high as 3.6 × 10⁻⁸ RIU was achieved in the fabricated device, which is unprecedented for OWG systems due to the unique structure of the waveguiding layer. As an example to demonstrate the sensitivity of the OWG sensor, (1-hexade)cyltrimethylammonium bromide (CTAB), a small surfactant molecule, is detected in water with a limit of detection (LOD) of 623 nM. Two heavy metal ions (Ag⁺ and Pb²⁺) were also detected using the OWG sensor after modification using 3-MPTMS, a LOD of 200 nM was determined, which is the lowest LOD achieved so far for a label free sensing system.