With high sensitivity and with wide-dynamic-range localized surface-plasmon resonance sensor for volatile organic compounds

To detect volatile organic compounds (VOCs), optical sensors based on the localized surface-plasmon resonance (LSPR) are widely used because they are simple and inexpensive to fabricate. Although LSPR spectroscopy has been generally applied to biological molecules such as proteins and DNA, small molecular sensing has been difficult because of the very small changes induced in the refractive index. We describe a LSPR gas sensor with high and wide range sensitivity. These characteristics are obtained from the improved peak shift that results from using an Au-dot pattern coated with a porous silica film. The 400-nm-diameter, 800-nm-wide Au dots were patterned by electron beam lithography (EBL) on a quartz glass substrate. The LSPR peak of the isolated Au-dot pattern occurs at 1230 nm in the near isolated area bringing enough refractive index changes. In addition, coating the Au-dot pattern with a thin porous silica film increases the adsorption of VOCs, thereby increasing the refractive-index change. The response of the sensor to VOC vapors was measured in a dynamic-flow system. The relative intensity decreases with toluene vapor exposure but completely recovers to the initial baseline after exposure to clean air. The real-time response allows detection of toluene vapors with concentrations ranging from 1 to 26,000 ppm, and the limit of detection for toluene is calculated to be 0.4 ppm. Eighteen VOC vapors were tested and each type exhibited different calibration curves, but these differences depend on the adsorption characteristics of porous silica rather than on the refractive index of the VOCs.