The present finding describes an efficient facile approach for the fabrication of nitrogen-doped carbon dots (N-CDs) as a “fluorescent nanoswitch”. Highly fluorescent blue-light-emitting N-CDs have been synthesized via a simpler hydrothermal method using 2,2′-(ethylenedioxy)-bis(ethylamine) and malic acid as the precursors. N-CDs showed excitation-dependent and pH-independent emission along with a quantum yield of ∼25%. The blue fluorescent emission of N-CDs has been selectively “turned off” (quenching of fluorescence (FL)) during the sensing of Cr(VI) with 0.02 μM limit of detection and further been selectively “turned on” (restoration of FL) on sensing ascorbic acid, compared with other metal cations and biomolecules tested. For testing the practical applicability of N-CDs, the switchable reversibility of the fluorescent nanoswitch has been tested for up to four cycles on the basis of FL “on–off–on”. Furthermore, the toxicological test showed the antibacterial effect of the N-CDs on the tested Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli cells. Additionally, these N-CDs can also be used as a fluorescent ink for imaging purposes.