Chemiluminescence, a widely used optical method in forensic science and in vitro diagnosis, faces sensitivity limitations. This study introduces plasmonic enhanced chemiluminescence (PEC) using tannic acid-functionalised silver nanoparticles, achieving up to 175-fold increase in luminol chemiluminescence intensity. Systematic investigation deconvolutes key factors including nanoparticle size and surface functionalisation. A wider dynamic range and lower detection limit of Fe ions were demonstrated, even in anaemia situations. These findings advance ultrasensitive detection in chemiluminescence biosensors through PEC.