Water contamination has become more severe as modern industrial technology has progressed over the years. Among water contaminants are heavy metal ions such as Fe3+, which is commonly used in industries such as mining, chemical processing, and battery manufacturing. Fe3+ is the principal contaminant of concern in acid mine drainage from coal mines, causing siderosis and organ damage. The current study prepared highly photoluminescent nitrogen-doped functionalized carbon spheres (N-CSs) from corncob residue using a facile, green, and low-cost microwave synthesis. The as-prepared N-CSs exhibited an excitation-dependent fluorescence with a maximum emission and excitation at 420 and 340 nm, respectively and showed good selectivity and sensitivity towards the detection of Fe3+ with a 70nM limit of detection.