Localized surface plasmon resonance sensor based on gold-coated tapered optical fiber for refractive index measurement

Abstract

The tapered optical fiber (TOF) sensor based on localized surface plasmon resonance (LSPR) was experimentally demonstrated for determination of surrounding liquid refractive index (RI) changes. The reduction of tapered waist diameter was done by flame-brushing method which results in the diameter range of 6 μm to 18 μm. The sensing mechanism is based on the transmission shift in the monitored output spectra. Key parameter optimization of the TOF sensor has been investigated particularly on the coating time of gold nanoparticles (AuNPs) onto fiber substrate. Functionalization of AuNPs by self-assemble monolayer was carried out through dip coating technique. Five different coating periods of 24, 48, 72, 96 and 120 hours have been performed on the TOF and characterized by using field emission scanning electron microscopy (FESEM). Histogram analysis was extracted from the FESEM images in order to determine the average particle’s size while the coating thickness were measured based on the cross-section image analysis. The highest sensitivity of the sensor was recorded at 18.49314 nm/RI corresponding to 48 hours of coating time. The optimum thickness of coating layer was calculated around 53.11 nm along the average particle’s size of 15 nm. Energy dispersive X-ray (EDX) characterization shows 22.4% of gold element on deposited fiber surface which proved the existence of the gold on the tapered structure. The optimized sensor was further examined for its overall performance in terms of stability, reversibility and repeatability over RI range from 1.3324 to 1.4254. Stability test of the TOF sensor suggests that the output signals are stable at low and medium of RI but a little variation on highest RI was recorded around ±0.00594 a.u. For the reversibility check, the TOF sensor acquired good response with little variation of ±0.008 a.u. between ascending and descending measurement order.