Graphene oxide functionalized side-polished sampled fiber grating biosensor for highly sensitive detection of SARS-CoV-2 spike protein

A graphene oxide functionalized side-polished sampled fiber grating (GO-SPSFG) biosensor is proposed for highly sensitive detection of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The sensing structure is obtained by combining femtosecond laser line-by-line scanning technology and precise mechanical side-polishing. The fabricated sampled grating exhibits characteristics of both Bragg gratings and long-period gratings for effectively decoupling temperature in the sensing measurement. The generated evanescent field can be further enhanced by asymmetrically polishing the cladding surrounding the grating region and forming a D-shaped fiber structure with optimized residual cladding thickness. The side-polished fiber surface is further modified with graphene oxide, which can provide effective binding sites for biomolecule immobilization due to its abundant oxygen-containing functional groups as well as a large specific surface area. This approach not only significantly improves the sensitivity but also successfully eliminates temperature interference for biomolecule detection. It has been demonstrated that when the residual thickness of the side polished fiber is 70 μm, the sensor achieves a detection limit as low as 0.055 nM for the SARS-CoV-2 spike protein. This biosensor holds broad application potential in medical diagnostics, environmental virus monitoring, biomedical research, food safety screening, and biosecurity warning systems.