Plasmon waveguides were fabricated by coating sol-gel copolymer templated nanoporous TiO<sub>2</sub> films on gold layers sputtered on glass substrates, and they were used to construct wavelength-interrogated plasmon waveguide resonance (PWR) sensors with Kretschmann configuration. The cross-sectional image of the plasmon waveguide obtained with scanning electron microscope indicates that the gold and nanoporous TiO<sub>2</sub> films are about 40 nm and 290 nm thick. The resonance wavelength (λ<sub>R</sub>) of the PWR sensor at a given incident angle is determined from either the reflected light intensity spectrum or the absorption spectrum. The porosity of TiO<sub>2</sub> film was determined to be ca. 0.42 by a comparison between simulation and experimental results. The PWR sensor operates with transverse electric mode. The in-situ and ex-situ responses of the PWR sensor to glutathione (GSH) adsorption were investigated theoretically and experimentally. The simulation results show that with either in-situ or ex-situ measurement the resonance wavelength linearly increases with increasing GSH concentration and the slope with ex-situ method is 6 times larger than that with in-situ method. The PWR sensor’s response to GSH adsorption from the 100 μmol/L solution was measured to be Δλ<sub>R</sub> = 31 nm with ex-situ detection and Δλ<sub>R</sub> = 6.1 nm with in-situ detection. Both the experimental and simulation investigations reveal that the ex-situ detection sensitivity is much higher than the in-situ one for the PWR sensor. The work suggests that the ex-situ detection method can offer the PWR sensor a lower detection limit in contrast with the in-situ method.
Thin films of nanoporous gold (NPG) have both localized and propagating surface plasmon resonance (SPR) effects. The propagating SPR effect of NPG film combined with its huge internal surface area makes it applicable as an evanescent wave sensor with high sensitivity. In this work, NPG films with controlled thicknesses were fabricated on glass substrates by sputtering deposition of AuAg films followed by dealloying in nitric acid. By using of the NPG films as the sensing layer, a broadband wavelength-interrogated SPR sensor was prepared for chemical and biological detection. The propagating SPR absorption band in the visible-near infrared region was clearly observed upon exposure of the NPG film to air, and this band was detected to move to longer wavelengths in response to adsorption of molecules within the NPG film. Simulations based on Fresnel equations combined with Bruggeman approximation were carried out for optimizing the propagating SPR property of NPG film. The sensor’s performance was investigated using both bisphenol A (BPA) and lead (II) ions as analytes. According to the experimental results, the detection limits of the sensor are 5 nmol·L<sup>-1</sup> for BPA and 1 nmol·L<sup>-1</sup> for lead (II) ions. The work demonstrated the outstanding applicability of the NPG film based SPR sensor for sensitive environmental monitoring.
A wavelength-interrogated surface plasmon resonance (SPR) sensor overlaid with a Teflon AF2400 film was prepared for rapid and sensitive detection of Benzo[a]pyrene (BaP) in water. The thickness of the Teflon AF 2400 film is much larger than the penetration depth of plasmon field, making the SPR sensor insensitive to refractive index (RI) of bulk solution and particle adsorption on the film surface. The sensor is only responsive to changes in RI of the Teflon film. The Teflon AF 2400 film is highly hydrophobic, enabling to effectively absorb nonpolar BaP molecules in water. Since BaP is a high-RI (n = 1.887) compound, its enrichment in the Teflon film can result in a considerable increase of the film RI. Consequently, the SPR sensor operating in the visible-near infrared reflection (NIR) wavelength range can be used to detect very low concentration of BaP in water. According to the simulation results, the thickness of the Teflon film should exceed 1000 nm to eliminate the SPR sensitivity to RI of bulk solution. The experimental results indicate that the resonance-wavelength shift (ΔλR) of the SPR sensor linearly increases with increasing the BaP concentration from C = 20 nmol·L<sup>-1</sup> up to 100 nmol·L<sup>-1</sup>. ΔλR is about 0.9 nm at C = 20 nmol·L<sup>-1</sup>, which is very close to the minimum ΔλR detectable with the CCD spectrometer used. The resonance wavelength stabilized 6 seconds after the sample injection, indicating that the diffusion of BaP molecules in the Teflon film is quite quick, which is attributable to the nanoporous structure of the Teflon film. It is anticipated that the sensitivity of SPR sensor to BaP and its detection limit can be further improved by optimization of the thickness of the Teflon film.