A label-free biosensor for pepsin detection based on graphene oxide functionalized micro-tapered long period fiber grating

•Proposed a GO functionalized MTLPFG biosensor for label-free pepsin detection.•Illustration of nonlinear adsorption process and analyzed by Langmuir adsorption model.•Analyzed the performance of MTLPFG and GO-MTLPFG for pepsin detection, and verified the corresponding Langmuir adsorption model. The rapid and sensitive detection of pepsin plays an important role in clinical and medical practice. A label-free and sensitive micro-tapered long-period fiber grating (MTLPFG) sensor that functionalized by graphene oxide (GO) was proposed for pepsin detection. MTLPFG was fabricated with CO2 laser heating and tapered to form a series of periods, and the GO that activated oxygen groups by EDC/NHS was deposited onto MTLPFG surface. On account of large specific surface area and oxygen containing groups of GO, the biomolecules can be adsorbed on the GO surface through amide groups and π-π stacking. The spectrum variation trend during sensing process obeys Langmuir absorption mode, which illustrates biomolecule adsorption on GO layer. The bare MTLPFG and GO-MTLPFG separately demonstrate the limit of detection (LOD) of 104.6 ng/ml and 25.79 ng/ml, which corresponding to the effectively detection range of 1–1000 ng/ml. Combination of GO and optical fibers exhibit great adaptive capacity to the biosensors, and provides inspirations for biochemical sensing applications. [Display omitted]