Microwave-Assisted Synthesis of Nitrogen-Doped Carbon Dots for Sensitive Fluorescence-Based Urea Detection in Soil

Abstract This study presents a rapid, green, and highly sensitive fluorescence-based method for detecting urea in soil using nitrogen-doped carbon dots (CDs) synthesized via a microwave-assisted process. Citric acid and urea were used as carbon and nitrogen precursors, respectively, with optimal synthesis achieved at a 1:1 weight ratio and 9 min of microwave irradiation. The structural properties of nitrogen-doped carbon dots (CDs) were studied using X-ray Diffraction (XRD). Transmission Electron Microscopy (TEM) technique was used to observe the shape and size of the CDs, providing insight into their morphology. To understand the chemical composition, bonding states, and surface functionalities, X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR) analyses were carried out. The performance of the nitrogen-doped-CDs in detecting urea, including their sensitivity and selectivity, was evaluated using fluorescence spectroscopy. The resulting CDs exhibited enhanced fluorescence properties and a limit of detection (LOD) as low as 143 mg/gm. The method demonstrated high selectivity toward urea even in the presence of interfering metal ions, and its effectiveness was validated in soil samples under varying pH conditions. This approach provides a cost-effective, scalable, and environmentally friendly solution for real-time monitoring of soil nutrients, supporting sustainable agricultural practices through improved nitrogen management.