A new N-doped carbon quantum dots (N-CQDs) fluorescent probe was synthesized via a solvent-free solid-phase carbonization method. The sensing material was characterized using various instrumental techniques, including Fourier Transform Infrared (FTIR), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Dynamic Light Scattering (DLS). The designed probe selectively detected Hg ions in an ON-OFF manner and biothiols [Cysteine (Cys), Homocysteine (Hcy) and Glutathione (GSH)] in an OFF-ON mode in pure water, emitting a distinct blue-green fluorescence colour. The N-CQDs probe sensitively and selectively detected Hg ions in ultrapure water as the sensing medium. The detection and quantification limits for Hg ions were calculated from the standard calibration graph as 19.16 and 63.87 nM, respectively. The LOD/LOQ for Cys, Hcy and GSH were 34.81/116.35 nM, 29.67/98.91 nM, and 29.67/98.91 nM, respectively. The validation studies (selectivity, sensitivity, linearity range, precision, repeatability, intermediate precision and robustness) were performed for Hg ions sensing. Finally, the N-CQDs sensor was successfully applied to real sample applications in water, food, living cells and fish samples with satisfactory results. These results demonstrated that the N-CQDs-based fluorescent sensor is well-suited for real samples with minimal interference from coexisting metal ions. In addition, the N-CQDs probe has been effectively employed in the development of cotton swabs, paper-based test kits, and a smartphone-integrated platform for detecting these analytes in real samples.