Hongxin Zhang

Cholesterol and blood pH are crucial markers for human health. While fiber optic sensors offer a promising detection method, temperature fluctuations pose a significant challenge. In this paper, we proposed a novel Fiber Bragg Grating (FBG) combined a tapered singlemode fiber-multimode fiber-singlemode fiber(TSMS) sensor to achieve the detection of cholesterol, pH, and temperature for the first time. By exploiting the distinct sensitivities of different interference patterns within a Mach-Zehnder interferometer, the sensor detects cholesterol and pH. In order to solve the cross-sensitivity problem between temperature, pH and cholesterol, FBG was selected as the temperature detection device, and finally the measurement of cholesterol concentration, pH and temperature was achieved. Half of the interferometer sensing surface is coated with polyacrylic acid/chitosan film to achieve pH detection, and the other part is coated with β-cyclodextrin to achieve specific detection of cholesterol. Experimental results show that the sensor has a cholesterol detection sensitivity of 1.24 nm/uM, a relatively sensitive pH sensitivity of 0.12 nm/pH, and a temperature sensitivity is 0.01 nm/°C. Because these parameters can be demodulated using a sensitivity matrix, the proposed sensor has the potential to simultaneously detect cholesterol temperature and pH. In addition, the experiment results indicate that the cholesterol sensor has good selectivity compared with acetylcholine, cysteine, glutamic acid, fructose, potassium ion, sodium ion, urea, and glucose. This paves the way for fiber optic sensors to become robust tools for multi-parameter biological analysis.

A sensitivity-improved fiber-optic sensor based on surface plasmon resonance is proposed and a strategy (using polyacrylic acid 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 65% compared with the uncoated sensor. β-cyclodextrin (β-cd) can recognize cholesterol molecules. Firstly, the traditional 11-MERCAPTOUNDECANOICACID (MUA) method was used to fix β-cd on the sensor, 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 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.

Tributyrin is widely used in medical treatment, drug development, and other fields. Lipase can decompose one tributyrin into one glycerol and three butyric acids, causing the effective refractive index (RI) and pH to change. Detecting the RI change or pH change during the experiment is usually used to determine the tributyrin in the sample. However, these methods encounter limitations at low or high concentrations. The RI difference detection method is difficult to implement in low-concentration detection, but the pH detection method is difficult to apply in high-concentration detection. In this article, we present an optical fiber sensor with dual channels based on surface plasmon resonance (SPR) to achieve the measurement of tributyrin. Polyacrylic acid (PAA)/chitosan (CS) pH-sensitive material is coated on half of the SPR sensor as channel 1 to monitor tributyrin-induced pH changes, and the lipase is bonded on the polydopamine (PDA)/maleic acid (MA) coated on the fiber as channel 2 to decompose tributyrin. Experimental results show that channel 1 realizes high pH sensitivity (34.52 nm/pH) and the tributyrin sensitivity reaches 12.8 nm/mM with a concentration between 0.1 and 1 mM. In the concentration range of 1–4 mM, channel 2 exhibits a tributyrin sensitivity of 4.85 nm/mM. In addition, the sensor proposed in this article has excellent stability and selectivity. Thus, the dual-channel optical fiber SPR sensor holds significant potential for drug development and biomedicine.

As a low-density lipoprotein, tributyrin plays a vital role in food safety and human health. In this work, we first proposed a novel high-performance tributyrin sensor made by supporting zeolitic imidazolate framework-8 (ZIF-8) and lipase onto the optic fiber sensor based on the surface plasmon resonance principle. Adsorption of lipase within the ZIF-8 framework, through a rapid aqueous synthesis method, a thin and dense ZIF-8 film can be uniformly formed and stably and tightly bonded to the surface of the optical fiber sensor. ZIF-8 is one of the Metal-organic frameworks (MOFs), which not only has the high porosity and specific surface area of MOFs but also shows good affinity with lipase. The experimental results show that the proposed sensor can achieve the specific detection of tributyrin in the linear range of 0.1-3 mM, the sensitivity is 14.75 nm/mM, with a detection limit of 0.096 mM. The sensor proposed in this article for the first time using ZIF-8/lipase composite material to detect tributyrin has an accuracy of more than 83% in sample detection at the range of 0.1-3 mM. In addition, the stability test of the proposed sensor is as good as 0.47 nm, and the selectivity (in the presence of glycine, acetylcholine, glucose, and urea) of the sensor was analyzed. The sensor proposed in this paper has the advantages of low sample consumption of 100 uL, high sensitivity, simple structure, and label-free measurement. This method provides a new route for the rapid and reliable detection of tributyrin, which has potential applications in medical research and clinical diagnosis.

As a low-density lipoprotein, tributyrin plays an essential role in food safety and human health. In this study, a novel lipase-conjugated carbon nanotube (CNT) surface plasmon resonance (SPR) fiber-optic sensor is used to specifically detect tributyrin for the first time. In this work, CNTs can be used as an amplifying material to significantly increase the sensitivity of SPR sensors due to their high refractive index and large surface area. CNTs can also be used as an enzyme carrier to provide abundant carboxyl groups for the specific binding of lipases. Covering the surface of the sensor with CNTs can not only enhance the performance of the sensor, but also provide sufficient detection sites for subsequent biomass detection, reduce the functionalization steps, and simplify the sensor preparation process. The experimental results demonstrate that the refractive index sensitivity of the traditional multimode fiber (MMF)–single mode fiber (SMF)–MMF transmissive optical fiber sensor is 1705 nm RIU⁻¹. After covering the sensor with CNTs, the sensitivity is 2077 nm RIU⁻¹, and the sensitivity has been improved very well. In addition, there are abundant functional groups on CNTs, which can provide abundant binding sites. Conjugating lipase on carbon nanotubes helps to achieve linear detection in the range of 0.5 mM to 4 mM tributyrin, with a sensitivity of 4.45 nm mM⁻¹ and a detection limit of 0.34 mM, which is below the 2.26 mM detection standard and meets food safety monitoring requirements. Compared with other sensors, the optical fiber biosensor proposed in this study expands the concentration detection range of tributyrin. Furthermore, the sensor also has good stability, anti-interference performance and specificity. Therefore, the sensor proposed in this paper has good application prospects in the fields of food safety and biomedicine.

Fiber-optic biosensors based on localized surface plasmon resonance (LSPR) have the advantages of great biocompatibility, label-free, strong stability, and real-time monitoring of various analytes. LSPR fiber-optic biosensors have attracted extensive research attention in the fields of environmental science, clinical medicine, disease diagnosis, and food safety. The latest development of LSPR fiber-optic biosensors in recent years has focused on the detection of clinical disease markers and the detection of various toxic substances in the environment and the progress of new sensitization mechanisms in LSPR fiber-optic sensors. Therefore, this paper reviews the LSPR fiber-optic sensors from the aspects of working principle, structure, and application fields in biosensors. According to the structure, the sensor can be divided into three categories: traditional ordinary optical fiber, special shape optical fiber, and specialty optical fiber. The advantages and disadvantages of existing and future LSPR fiber-optic biosensors are discussed in detail. Additionally, the prospect of future development of fiber-optic biosensors based on LSPR is addressed.