In this work, flower-like ZnO nanoparticles (ZnONPs) were synthesized using zinc nitrate (Zn(NO3)2 6H2O) as a precursor with KOH. The morphology of the ZnONPs was controlled by varying the synthesis temperature at 50, 75 and 95°C. The morphology and structure of ZnONPs were characterized using scanning electron microscopy (SEM), and X-ray diffraction (XRD) and Brunauer-Emmett Teller (BET) analysis. ZnONPs were successfully synthesized by a simple chemical precipitation method. A synthesis temperature of 75°C produced the most suitable flower-like ZnONPs, which were combined with graphene nanoplatelets (GNPs) to develop a label-free electrochemical immunosensor for the detection of the colon cancer biomarker carcinoembryonic antigen (CEA) in human serum. Under optimum conditions, the developed immunosensor showed a linear range of 0.5 to 10 ng mL-1 with a limit of detection (LOD) of 0.44 ng mL-1. The label-free electrochemical immunosensor exhibited good selectivity, reproducibility, and repeatability, and recoveries were excellent. The immunosensor is used with a Near-Field Communication (NFC) potentiostat connected to a smartphone to facilitate POC cancer detection in low-resource locations.