Solution concentration and refractive index sensing based on polymer microfiber knot resonator

We report a simple and highly sensitive solution concentration and refractive index (RI) sensor based on a microfiber knot resonator (MKR). The sensor was assembled using a flexible poly(trimethylene terephthalate) microfiber of 1.3 µm diameter. The resonance peak shifts due to the variation of the concentration of the ambient medium surrounding the MKR were detected. In the mass concentration range of 0.6-1.2%, the concentration sensitivity and resolution of the sensor are 1.7 nm/% and 1.2 × 10−3%, respectively. The highest measured sensitivity of the sensor is about 95.5 nm/RIU (RI unit) and the RI detection limit is about 2.1 × 10−5 in the RI range from 1.39 to 1.41.