Hydrogen peroxide (H2O2) is a member of reactive oxygen species (ROS) and has substantial physiological roles in living organisms. Currently, advanced techniques facilitate imaging of living systems with gadolinium based contrast agents and intense oxidation regions may be detected with convenient molecular design. Therefore, gadolinium complex of phenylboronic acid functionalized 4,5-diazafluorene (Gd-SCL) was designed for quantitative and colorimetric detection of H2O2. UV?vis, FTIR, 1H NMR, HPLC and ESI-MS methods were used in the characterizations. Conversion of phenylboronic unit to the phenol due to reaction with H2O2 was occurred in 35 min under alkaline aqueous medium with the color change from orange-yellow to transparent colorless and proved by mass analyses. Designed sensor exhibited a good response to H2O2 in the range of 0.5?500 ?M with 0.41 ?M limit of detection (LOD) in the prepared standard samples. The sensor showed an excellent selectivity for H2O2 in presence of interfering species and the optimum pH range for operation of the sensor was determined in the range of 7?12. Cosmetic product (hair bleach) with 9% peroxide content was used in real sample analyses and LOD was found as 3.83 nM. Considering the magnetic capability of Gd lanthanide, successful H2O2 detection with phenylboronic acid and presuming this research as pre-trial, Gd-SCL complex may be a candidate for magnetic resonance imaging (MRI) agent after further investigations.