A tapered microfiber sensor based on multiwalled carbon nanotubes (MWCNTs) for detecting the concentration of alizarin red S dye is proposed and verified by experiments. MWCNTs fixed on the surface of optical fiber can adsorb the alizarin red S dye molecule due to its porous structure. The surface refractive index (RI) change caused by MWCNTs coated with the fiber surface adsorbing of red dye can be captured by optical microfiber, which converted into macroscopic wavelength drift in the interference spectrum. The sensor has a high RI sensitivity of 1942.25 nm/RIU and a temperature sensitivity of
−
0.03
n
m
/
∘
C
, which has a low temperature cross-sensitivity. The sensor provides rapid detection of alizarin red S dye in the concentration range of 0–1000 mg/L, and the sensitivity is 0.412 nm/(mg/L) in the low concentration range of 0–25 mg/L. The sensor is expected to be a high-quality platform for safety considerations in environmental protection.
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