We have developed "multi-region", fiber-optic, gas-phase surface plasmon resonance sensors (SPRS) for the detection of corrosive and reactive off-gassing from materials confined in sealed systems. These sensors are single-ended requiring only one penetration of the system and can be placed in various regions of interest in the sealed system since they are fiber-based. In addition, multiple sensor regions allow speciation and quantization of several target species on a single fiber. To predict corrosion of sensitive electronics and electronic connections in sealed systems, we have demonstrated H2O, H2, and sulfur compound sensitivity on a single 600 Ám core optical fiber.
We have demonstrated a multi-region, single-ended, surface plasmon resonance sensor constructed on an optical fiber that allows long-term passive, primary sensing of concentration and species in a sealed system.1 Surface plasmon resonance spectroscopy (SPR) has been applied to a number of analytical problems due to its high sensitivity to variations in the electronic nature of a surface. In particular, reports of sensing of chemicals and of biological samples using functionalized surfaces are widespread in the literature.2,3,4,5,6,7 The majority of examples employ an open beam optical arrangement known as a Kretschmann configuration that has a single angle of incidence on a metallic layer and a single angle of exit to an optical spectrometer. The surface plasmon resonance is supported in a thin metallic layer and the chemistry of interest takes place on the side of the metal layer opposite the incident and reflected light. In practice this thin metallic layer is usually deposited on the diagonal facet of a right-angle prism allowing the thin film access to the chemical system while having structural support for the film provided by the prism.
A single-fiber version of the fiber-optic SPR sensor that employs a retro-reflecting metal film on the end of a multimode fiber similar in geometry (Figure 1) to the intensity-based “micromirror” sensors documented in the literature has been demonstrated. 8,9,10,11 While the geometry is similar to previous work, the previous work only sampled the reflectance of the reflecting mirror on the fiber. In our system, light is injected into the fiber that then travels by way of the optical coupler to the SPR coated fiber where it interacts with the sensing film. The light is then retro-reflected back through the fiber to the coupler where half returns to the light source and is lost and the other half is directed via the second leg of the coupler to the monochrometer where it is analyzed. The reflecting end serves the purpose of returning the light back through the fiber to the monochromator but does not participate in the sensing since the film is generally thick (100 nm) and each ray only interacts with the end film once as compared to multiple times for the axial coatings. This is accomplished by making the end from an inert noble metal such as Au or over coating the metal with a sealant.
Figure 1 . Schematic diagram of the SPR sensor showing the optical system. Broad band light is injected into the fiber which then is sent to the SPR end via a three-port coupler. The light is passed through the SPR section of the fiber and reflected back by a retro-reflecting end back through the coupler and is detected by the monochromator.
Since the fiber is single ended, it becomes practical to use this sensor to monitor the atmospheres of complex subsystems for contamination and aging effects. For example, deterioration of some important compounds found in packaging can lead to production of sulfur compounds that can corrode connectors and lead to electronic failures. In addition, batteries, transformers, thermal degradation of packaging can evolve H2 leading to metallic embrittlement, water formation, and explosive environments. Further, leakage of seals can allow moisture to infiltrate and condense on critical components contributing to corrosion and other failure mechanisms.
Multi Coating SPR Fiber
The ability to measure several contaminants of interest and to discriminate between them by monitoring a series of key wavelengths implies that a single fiber with multiple regions etched and coated with different metals could be used to produce a multi-component sensor (Figure 2). Such a sensor has been demonstrated that was coated with 12.5 mm long 20 nm Pd film followed by a 12.5 mm length 20 nm Ag film followed by a 12.5 mm length 20 nm Au/20 nm SiO2 film stack for detection of H2, sulfur compounds, and moisture respectively (Figure 2). Data from these tests were first acquired using the unexposed fiber to detect 0.2 Torr H2 (Figure 3 (a)) and then exposed to 300 μTorr of H2S (Figure 3 (b)). The Ag section of the fiber was then monitored until the non-reversible reaction was complete. The fiber was then re-exposed to 0.2 Torr H2 with the permanent H2S/Ag response normalized into the data to determine if the sulfur atmosphere poisoned the Pd film (Figure 3 (c)). The data indicate that the resonance peak that appears on exposure to H2 is essentially unchanged compared to Figure 3 (a) in shape and magnitude after the exposure of the Ag film to H2S. It should be noted that Figure 3 (a) and (b) are normalized to the unexposed spectrum. Figure 3 (c) is re-normalized after the H2S exposure and thus, includes the spectral features introduced on the exposure of the Ag film to H2S. Since the two metals have non-overlapping surface plasmon resonances, the responses to the two analytes can be separately resolved. The resulting response indicated that low background levels of sulfur exposure for several days do not appreciably degrade the response of the Pd section to H2. In all cases, a horizontal line at 1 would represent the reflectivity of the unexposed, normalized optical spectrum.
Figure 2 . Diagram of three section SPR sensor showing Au, Ag, and Pd regions on a 600 μm optical fiber core.
Figure 3 . Plots showing the response to (a)200 mTorr H2 in Pd, (b) 280 μTorr H2S on Ag, (c) 200 mTorr H2 in Pd, and (d) the response of the fiber when dryed from 0˚C dew point to -70˚C frost point on a single SPR fiber. The time progression is from top to bottom and the data is re-normalized after the H2S on Ag exposure since that change is not reversible. Thus, (c) and (d) are normalized to (b). The SPR wavelengths are separated with the minimum for Ag at approximately 450 nm, the minimum for Pd at about 550 nm, and the minimum for Au at about 600 nm. The dots are the raw data from the spectrometer and the solid lines are running averages of the data. The vertical dashed lines locate the minimum of the averaged data.
We have demonstrated multi-region, SPR using optical fibers. Several SPR supporting metals have been modeled and used to design sensors for testing including H2 sensing using Pd films, H2S sensing using Ag films, and H2O sensing using SiO2 films on an Au film. Data illustrate that different metals allow variation in the spectral response to various analytes allowing the manufacture of a three region SPR sensor that is sensitive to all three analytes using the single fiber and experimental system. It has been demonstrated that the H2 response is unchanged after an extended interval of exposure to H2S and that moisture can be detected at low frost points (-70°C) after repeated exposure to H2 and prolonged exposure to H2S. Thus, simplified fiber optic SPR geometry using a single-ended fiber has been demonstrated and shown to have multiple responses to several chemical vapors that can be present due to aging effects in sealed systems.
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