Label-Free Ultrasensitive Cholesterol Detection Based on SPR Optical Fiber Sensor

A sensitivity-improved fiber-optic sensor based on surface plasmon resonance (SPR) is proposed, and a strategy [using polyacrylic acid (PAA) and chitosan] to fix carbon nanotubes (CNTs) on the probe to detect cholesterol is established. CNTs can improve the sensitivity of the sensor. The sensitivity of the sensor after coating with CNTs is 2437 nm/RIU, which increases by 65% compared with the uncoated sensor. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula>-cyclodextrin (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula>-cd) can recognize cholesterol molecules. First, the traditional 11-mercaptoundecanoic acid (MUA) method was used to fix <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula>-cd on the sensor, and cholesterol sensitivity is 0.023 nm/nM in the range of 10–300 nM. Then, the Langmuir adsorption isotherm model was used to analyze the detection of cholesterol by the new method. Experiment results show that cholesterol sensitivity was 0.065 nm/nM at 10–400 nM (2.83 times than the MUA method), and the limit of quantification (LOQ) was 2.34 nM. In addition, the sensor has expanded detection range, improved the sensitivity of the sensor, and also has good stability and selectivity, paving the way for the future use of fiber sensors in cholesterol detection.