Fabry-Perot (FP) optical fiber sensors are reported to be highly sensitive for detecting various physical, chemical, and biological objects. In this study, an FP-based Phase Shifted Bragg-Grating Lab-on-Fiber (PSBG-LOF) is presented to determine ultralow glucose concentrations in liquids by using a novel PSBG at the end facet of a single-mode fiber (SMF). The proposed LOF consists of an intermediate silica layer sandwiched between two identical PSBGs formed by 4.5 pairs of siliconoxynitrite (SiON) doped silicon (Si), which are newly synthesized silica (SiO2) thin films, all deposited by the plasma enhanced chemical vapor deposition (PECVD) method. The SiON-doped Si molecule group was used for the first time as PSBG structures and LOF of the glucose in liquids. Our findings with the proposed sensors revealed that the sensitivity value was 14904 nm/RIU (4.3 pm/ppm and 4.29 nm/(mg/ml)) and the detection limit was calculated as 1.98 × 10−6 RIU. In addition, the proposed sensor is insensitive to temperature changes in the range of 25°C-45°C. The results are very promising for the in-vivo biosensing applications comprising temperature unresponsive LOF. © 2023 Elsevier Ltd