Persistent Luminescent Nanoparticles (PLNPs) emit prolonged light after excitation, allowing getting images with high signal‐to‐noise ratio, making them particularly beneficial for in vitro biodetection. In this study, we report the preparation of a library of gallium and zinc‐based Persistent Luminescent Nanoparticles doped with chromium (ZnGa 2 O 4 :Cr 3+ ) through two distinct syntheses conducted at 120 °C for 12 hours or 24 hours, followed by a calcination at 500 °C or 750 °C. Using centrifugation, and starting from polydisperse samples, we demonstrated the ability to achieve different sizes in a monodisperse manner. Furthermore, our research revealed that ZnGa 2 O 4 :Cr 3+ (ZGO) nanoparticles react differently with hydrogen peroxide (H 2 O 2 ) based on their size, synthesis duration, and calcination temperature. We observed the most significant amplification of persistent luminescence for ZGO particles prepared at 120 °C for 12 hours and calcinated at 500 °C, with a size of approximately 100 nm, in the presence of 100 mM H 2 O 2 (later named ZGO‐2). This study enabled the detection of H 2 O 2 in serum without autofluorescence using the developed PLNPs, with a detection limit of up to 2.1 μM and a recovery rate of around 99 %. Additionally, we designed a glucose test for based on the responsiveness of ZGO PLNPs to H 2 O 2 . Using glucose oxidase, the glucose detection limit in serum was established around 0.13 μM, with a detection range between 0–5 μM. These findings could open new avenues to further control the size, synthesis, calcination temperature, and persistent luminescence of PLNPs, thus enhancing their utility in the development of new in vitro biosensors.