<p>Nitrogen doped Carbon Quantum Dots (NCQDs) have been synthesized using most economical and easiest hydrothermal process. Here, N-phenyl orthophenylenediamine and citric acid were utilised as a source of nitrogen and carbon for the preparation of NCQDs. The synthesized NCQDs were characterized using experimental techniques like UV − Vis absorption, FT-IR, transmission electron microscopy (TEM), X-ray Diffraction (XRD), EDX, dynamic light scattering (DLS), fluorimeter and time resolved fluorescence spectroscopy. Measured quantum yield of the NCQDs was ~ 50.5%. TEM image represented that the NCQDs were quasi-spherical shaped with average size of 3.5&#xa0;nm. This nitrogen doped carbon quantum dots have been used to study their bactericidal activity against representative Gram-negative (<i>E. coli</i> and <i>P. aeruginosa</i>) and Gram-positive (<i>B. subtilis</i> and <i>S. aureus)</i> bacterial strains using the agar well diffusion method. Results demonstrated that synthesized Nitrogen doped carbon quantum dots have been found to exhibit maximum antibacterial activity against <i>S. aureus</i> with good inhibitory effect with inhibition range from 2&#xa0;mg mL^− 1 to 3&#xa0;mg mL^− 1. These Nitrogen doped carbon quantum dots have also been used as fluorescence probe for sensitive and selective detection of mercury. The emission intensity of carbon quantum dots has drastically quenched by Hg²⁺ ion. Observed limit of detection (LOD) was found to be 4.98 nM, much below than the approved limit prescribed by Environmental Protection Agency. Hence the synthesized NCQDs play an important role in monitoring the antibacterial effect as well as water quality.</p> Graphical Abstract <p></p>