Highly sensitive and rapid optical fiber bio-sensor for endotoxin detection using anti-resonance effect

Endotoxin detection is critical to human health. Reported endotoxin assays usually rely on multistep enrichment or specific modification techniques to improve sensitivity, resulting in long response times (∼1 h) and greater sample consumption (>1 mL). By now, it is still challenging to implement a highly sensitive and ultra-fast response (∼seconds) biosensor for optical endotoxin assays with only micro/nanoliter volume. Here, a new approach is proposed to break the challenges by using an anti-resonance (AR) fiber-optic biosensors with the length-independent and volume-independent sensitivity. The AR fiber-optic biosensor is composed of a side-polished single-mode/hollow-core/single-mode fiber (SP-SHSF) structure, capable of achieving high sensitivity (∼7394 nm/RIU@RI=1.33–1.385 and 33,657.14 nm/RIU@RI=1.436–1.446) with a lowest sample volume limit of down to sub-nanoliter (∼600 pL). A standard Limulus Amebocyte Lysate (LAL) assay was combined to achieve refractive index (RI) increases during LAL gel formation, and endotoxin concentrations were quantified by fixing the response time required for a 5 nm resonance dip drift. The limit of detection (LOD) of endotoxin for the biosensor was measured to be 10⁻⁴ EU/mL with a fast response time of 344 s and only 5 μL consumption of LAL reagent per test. Compared with the traditional LAL test, the fiber-optic biosensor has more than two orders of magnitude lower LOD, about 9 times faster respond and 20 times less sample consumption. The proposed fiber-optic biosensor has the advantages of small size, high sensitivity, reusability, and online detection, which is of great significance for detecting endotoxin contamination in products for biopharmaceuticals and medical devices.