Teaching experiment design for the preparation of an optical fiber cholesterol sensor based on surface plasmon resonance

Optical fiber surface plasmon resonance (SPR) sensors have received widespread attention in the fields of early disease diagnosis, biomolecule detection, and environmental monitoring due to their advantages of high sensitivity, label-free status, real-time monitoring, and strong anti-interference ability. Cholesterol is an important human biomolecule, and abnormal levels are closely related to major diseases such as cardiovascular disease and Alzheimer’s disease. The existing cholesterol detection methods have drawbacks such as high cost, complex operation, and limited sensitivity. Based on this, a teaching experiment integrating scientific research achievements for the design and preparation of an optical fiber SPR cholesterol sensor is introduced into an optical-fiber sensor course, helping students understand the cutting-edge dynamics of the discipline, cultivate scientific research thinking, and enhance their comprehensive professional competencies.

The experiments used a reflective structure of multimode fiber–single-mode fiber fusion as the optical-fiber SPR sensing structure, with cholesterol oxidase as the sensitive material. The fabrication of the sensing probe consists of three main steps: structure splicing, gold film coating, and cholesterol oxidase functionalization. The optical fiber structure fusion is accomplished using an optical fiber knife and S179 fusion. The gold film coating is completed by a small ion sputtering instrument JS1600, with a sputtering current of 7 mA, a sputtering time of 80–160 s, and a chamber vacuum of 0.1 mbar/Pa. Thus, a uniform and firm gold coating of approximately 30–70 nm is sputtered onto the fiber sensing area. The cholesterol oxidase functionalization is accomplished through covalent bonding. Firstly, the gold-coated optical-fiber probe is immersed in an 11-mercaptoundecanoic acid solution to achieve carboxylation of the gold film surface. Then, an N-hydroxysuccinate/1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride mixed solution is used to activate the carboxyl groups on the gold film surface, to reduce the binding energy between the amino and carboxyl groups. Finally, the probe is immersed in the cholesterol oxidase solution. Through the covalent bonding between the amino groups in the cholesterol oxidase and the carboxyl groups on the gold film surface, the cholesterol oxidase is uniformly and firmly modified on the surface of the optical fiber. During the detection process, the cholesterol oxidase undergoes an oxidation–reduction reaction with the cholesterol molecules in the test environment, thereby decomposing cholesterol into 4-cholesterene-3-one and H₂O₂. This changes the refractive index of the sensing probe surface, which in turn shifts the SPR wavelength. Therefore, by tracking the resonance wavelength shift in the SPR spectrogram, cholesterol concentration detection can be achieved.

Experiments confirmed that the optimal gold plating time for the sensing probe is 140 s, at which point the sensing probe has the highest refractive index sensitivity of 2678 nm/RIU. Scanning electron microscopy characterization of the fiber end face showed that when the coating time is 140 s, the thickness of the gold film is about 50 nm. In addition, experiments also confirmed that the optimal sensing area length of the sensing probe is 4–8 mm. By modifying cholesterol oxidase on the surface of the optical fiber, sensitive determination of cholesterol concentration can be achieved. As the cholesterol concentration increases, the SPR resonance wavelength gradually redshifts. Within the range of 0–50 nM, there is a good linear fit between the cholesterol concentration and the resonance wavelength, with a linear sensitivity of 0.182 nm/nM. Furthermore, the cholesterol sensor exhibits excellent stability in cholesterol solutions and good specificity for cholesterol molecules, without being affected by other biomolecules.

This teaching experiment design is characterized by its cutting-edge nature, comprehensiveness, and practicality. It not only enables students to deeply understand the principles and practical applications of optical fiber sensors but also helps them master the basic methods of nanomaterial preparation, sensor construction, and performance testing. Additionally, it cultivates students' scientific research thinking and innovative practical abilities, promotes the integration of interdisciplinary knowledge, and provides a feasible solution for teaching reform in the optical fiber sensor course.