Ultrahigh-sensitivity label-free optical fiber biosensor based on a tapered singlemode- no core-singlemode coupler for Staphylococcus aureus detection

•Developed tapered no-core fiber coupler for ultrahigh-sensitivity biosensing.•Provide “additional” stability due to the coupler structure.•Demonstrated limit of detection of Staphylococcus aureus as low as 3.1 CFU/mL, the best value reported so far.•The sensor structure has potential for other applications such as disease,medical diagnostics and food safety inspection.