Recently, great efforts have been made to use biosensors for the early diagnosis of cancer. Specifically, using a biomarker to detect H 2 O 2 in physiological conditions is of great significance for understanding the signal transduction pathways and achieving early cancer diagnosis. In this work, we report an innovative H 2 O 2 sensor that was fabricated by trimetallic AuPtPd nanocomposites platform on reduced graphene oxide (rGO) nanosheets with the modification of the rGO and trimetallic AuPtPd nanoparticles on a glassy carbon electrode (GCE) by physical adsorption. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were utilized to characterize and identify these unique nanocomposites. In addition, the electrochemical properties of the proposed sensor were evaluated by cyclic voltammetry and chronoamperometry. Electrochemical research has demonstrated that the AuPtPd/rGO-modified GCE showed excellent electrocatalytic activity towards the reduction of H 2 O 2 , including a wider linear range from 0.005 μM to 6.5 mM, a low detection limit of 2 nM, good selectivity and acceptable repeatability. Moreover, the sensor can monitor the release of H 2 O 2 release from living cancer cells. Therefore, this study not only improves simplicity, sensitivity and quantitatively for detection H 2 O 2 in cells at nM level but also provides a foundation for the biological and biomedical applications such as the early diagnosis of cancer. [ABSTRACT FROM AUTHOR]