Fluorescent carbon dots (CDs) were efficiently synthesized by a one-step microwave-assisted method using diphenylamine as a carbon precursor. The obtained CDs exhibit high stability and strong water solubility. Under UV irradiation, these CDs could emit bright green photoluminescence. These synthesized CDs have an average diameter of 1.8 nm (±0.46) and quantum yield (QY) as high as 44.69% using rhodamine-B as a reference. The CDs’ intensity can be quantitatively quenched by Hg²⁺ and Fe³⁺ ions with high sensitivity and low LOD about 9.58 nM and 22.27 nM, respectively, indicating that the CDs sensors can be potentially applied for Hg²⁺ and Fe³⁺ detection in aqueous solutions.

Using orange peels as a biowaste, fluorescent N-CDs were prepared simply and rapidly through a one-step microwave-assisted method and urea as a nitrogen source. The synthesized N-CDs exhibited a high QY value of 47.12% compared to CDs prepared using different methods. Moreover, the N-CDs have good pH and thermal stability. N-CDs exhibited high sensitivity toward Fe(III), Hg(I), and Hg(II) ions with low LOD values of about 0.0555, 0.15379, and 0.02505 μM, respectively. This approach is hopeful for the large-scale formation of N-CDs and could encourage their utilization as fluorescent chemosensors due to their affordability, simplicity, high efficiency, and environmental friendliness.