Ultra-sensitive fiber optic heparin sensor based on phase-modulated configuration with electrostatic self-assembly

Heparin is a highly negatively charged polysaccharide found in nature, and its content will affect the anticoagulant ability of human body. We propose a novel fiber optic sensor based on phase modulation for heparin detection. This is the first time that optical fiber interferometry is used to detect heparin. The sensor's structure is formed by splicing a single-mode fiber, coreless fiber and thin-core fiber (TCF) in sequence via electric arc splicer. Cationic polymer, polyethyleneimine (PEI), is introduced onto the partially etched TCF's cladding to create a PEI-modified TCF. The strong electrostatic interaction between PEI and heparin molecules can cause changes in the refractive index of the TCF's cladding. Consequently, different concentrations of heparin can be detected by observing changes in the reflective spectrum. Our experimental results have demonstrated that the sensor has ultra-low detection limit (4.0 × 10⁻³ ng/mL) with wider detection range (10⁻¹² to 10⁻⁵ g/mL). Notably, the sensor has a detection time of only 3.0 min, and its reflective structure can effectively detect heparin in real-time and in-situ. Therefore, the sensor is a practical and promising tool for monitoring heparin in clinical care settings.