Simplification and Evaluation of a Gold-deposited SPR Optical Fiber Sensor

Abstract

New sensor systems using surface plasmon resonance (SPR) have been developed for measuring the refractive index (RI) of an analyte. 1,2 The systems use an optical phenomenon in which incident light excites a charge-density wave at the interface between a metal and a dielectric. 1,2 An SPR sensor allows the realization of quick determination of the RI of a sample with high accuracy. However, the usual SPR sensor systems utilize bulk optical configurations that are limited by the use of a coupling prism. Furthermore, since the SPR system requires a spectrometer or a rotation stage, the resulting systems are relatively large and expensive, are inapplicable for remote sensing applications. 3 Optical fiber sensors based on the SPR phenomenon have been developed. These systems replace a coupling prism with an optical fiber, and the SPR phenomenon is applicable to analyses of liquid or gas samples. The systems have a sensing element of a deposited metal film on the core of an optical fiber and must scan the wavelength 3,4 or change the angle of incident light. [5][6][7][8][9][10][11][12][13][14][15] Another type of the sensor based on SPR in a thin deposited metal film on the core of an optical fiber implanted in a silica block and polished has been reported. [16][17][18][19][20][21][22][23] A gold island SPR sensor has also been investigated. [24][25][26][27][28][29] Most theoretical analyses for the optical fiber sensors using SPR are based on the traditional Kretschmann configuration and the response curves have been calculated. 3,6,12,14,[16][17][18][19][20][21][22][23][30][31][32][33][34] However, these analyses are made for the sensors in a restricted range of the refractivity (less than 1.45 RI units). Thus, a theoretical investigation of the response of the gold-deposited optical fiber sensor over a wider range of refractivity is necessary to understand the mechanism of the response.An optical fiber sensor system based on SPR without scanning the wavelength or changing the angle of incident light is desired for remote sensing, continuous analysis, and in situ monitoring using a small and inexpensive sensing element. We have already developed a small and simple gold-deposited optical fiber sensor system without scanning the wavelength or changing the angle of incident light. [35][36][37][38][39][40] The sensing element has a thin (30 -60 nm) gold film deposited all around the bare core of an optical fiber and a sample solution flows around it. When light with an intensity distribution is introduced into the optical fiber of the sensor with a focusing lens, the SPR occurs at the interface between the gold film and the sample solution. A part of the incident light disappears by the SPR and the intensity of the transmitted light varies depending on the refractivity of the sample solution. [35][36][37][38][39][40] We have performed the analyses of various alcohol solutions [35][36][37][38][39][40] and some esters in ethanol 36 and have demonstrated the usefulness of this sensor system. Theoretical analyses for the ...

A Gold-Deposited Surface Plasmon Resonance-based Optical Fiber Sensor System Using Various Light-Emitting Diodes

“…The full width at half maximum (FWHM) (0.062 ± 0.003 RI units) of the response curve of the sensor with the red (660 nm) LED is almost equal to that (0.065 ± 0.009 RI units) of the response curve of the sensor with a He-Ne laser (632.8 nm). 49 This agreement shows that the broadening of the response curve due to the width (25 nm) of the wavelength of the LED used is negligible in this sensor system.…”

“…The deposited Au film has a geometric thickness distribution in which the maximum value (45 nm) represents the thickness of the deposited metal. [49][50][51][52] The uncertainty of the Au thickness was less than 0.2 nm. The sensor element and Teflon tubes for sample inlets and outlets were fixed in a glass tube (12 cm long and 0.3 cm in diameter) with resin to form a sensor cell.…”

“…[43][44][45][46][47][48] We then simplified the structure of the sensor element of the Au-deposited optical fiber sensor; the Au film was deposited on only half of the core of the optical fiber and the deposited metal film had a geometric thickness distribution. 49 This simplified sensor element exhibited better properties than those of our previous sensor elements. 49 Furthermore, we also fabricated metal-deposited optical fiber sensors using Cu and Al and reported the response curves and sensor properties for the first time.…”

“…49 This simplified sensor element exhibited better properties than those of our previous sensor elements. 49 Furthermore, we also fabricated metal-deposited optical fiber sensors using Cu and Al and reported the response curves and sensor properties for the first time. 50 The sensor properties of Al-deposited and Ag-deposited SPR optical fibers were studied thoroughly along with the surface characterization of these metal films by atomic force microscopy and X-ray photoelectron spectroscopy.…”

“…The preparation of the sensor elements of the metal-deposited optical fibers has previously been reported. [49][50][51][52] A part (10 cm long) of the clad and the jacket of a step-index multi-mode optical fiber (Showa Electric Wire & Cable NJ-PF200/300) was removed by immersing the optical fiber in concentrated sulfuric acid. A Au film was deposited on half of the core (200 μm in diameter) of the optical fiber by evaporation of Au (Ishifuku Metal Industry, >99.99%) at a rate of 1.0 nm/s in a high vacuum (< 6 × 10 -4 Pa) at room temperature.…”