# Real-Time Precise Prediction Dispersion Turning Point of Optical Microfiber Coupler Biosensor with Ultra-High Sensitivity and Wide Linear Dynamic Range

## TL;DR

This paper introduces a new method to accurately predict and control the dispersion turning point in optical microfiber biosensors, enabling ultra-sensitive and wide-range detection of biological substances.

## Contribution

A novel method is proposed to predict the dispersion turning point in liquid environments using interference peaks counted in air, enabling precise control and enhanced performance.

## Key findings

- A maximum sensitivity of 1.17 × 10⁵ nm/RIU is achieved in refractive index sensing.
- Wide dynamic range detection of cardiac troponin I is realized across three concentration ranges.
- The method balances sensitivity and linear dynamic range for practical biosensing applications.

## Abstract

Optical microfiber biosensors demonstrate exceptionally ultra-high sensitivity at the dispersion turning point (DTP). However, the DTP is highly susceptible to variations in dimensional and external environmental factors, and the spectral response is mismatched from preparation in air to application in a liquid environment, making the DTP difficult to control effectively. In this work, we propose a method that bridges the relationship between the interference spectra of air and aqueous environments. By counting the interference peaks in air, we can accurately predict the DTP position in liquids. Meanwhile, it provides a new balance between sensitivity and wide linear dynamic range, achieving wide dynamic range detection across various concentrations. The optical microfiber coupler (OMC) is fabricated using the hydrogen–oxygen flame melting tapering method. In addition, the concentration, temperature, and solvent used for the sensor’s biofunctional layer are optimized. Finally, in refractive index sensing, a maximum sensitivity of 1.17 × 105 ± 0.038 × 105 nm/RIU is achieved. For biosensing, a wide dynamic range detection of cardiac troponin I (cTnI) is realized at concentrations of 12–48 ng/mL, 120–480 pg/mL, and 120–480 fg/mL.