SPR is an optical phenomenon that enables detection of molecules adhering to a surface. SPR is often detected as a loss in reflected light intensity at a specific frequency or wavelength of light as the incident light energy is coupled into the surface plasmon wave. The wavelength dependence of this intensity loss is analyzed and can be used to quantitatively determine the molecular adsorption on the surface of the SPR active material. Most SPR experiments are performed using expensive benchtop instruments wth bulky optical components, making SPR difficult to employ in remote sensing applications.
The SPR sensor enables remote measurements in a stable, robust probe format, consisting of a commercially available attenuated total reflection (ATR) probe modified to support SPR. Thin layers of gold and silica are deposited on the curved apex of the ATR hemispherical lens. This modified probe allows for sensitive detection of gas molecules as they adsorb onto the silica surface.
The use of the hemispherical ATR probe as the starting optics allows for the sensor to be placed at the end of a fiber optic cable that carries both the incident light and the reflected signal back to the measuring instrument. The curvature of the hemisphere compensates for the divergence of the source fiber and refocuses the light back to a collection fiber. This fiber optic SPR probe has been show to produce extremely high resolution, low noise SPR spectra typical of expensive benchtop systems.
The SPR sensor probe is extremely sensitive to water vapor adsorption onto the probe surface. It has been applied to atmospheric moisture monitoring and has shown sub-ppm sensitivity over a range of ~0 to 100% relative humidity (0.750 to >20,000 ppm at 25°C, 1 atm). Although the current probe is configured as a moisture sensor, the technology, using different materials, is applicable to measurement of any gas phase species that can condense on the probe tip from some equilibrium concentration in the gas phase. The probe is especially useful in an environment where an electrical measurement is undesirable, or where there is electrical noise interference. In an alternate configuration, the SPR sensor can be used to make highly accurate measurements of the refractive index of a liquid.
- moisture analysis
- stable and robust
- high resolution
- low noise