Sulfur and Nitrogen‐Doped Carbon Quantum Dots for Selective Sensing of Mercury Ions and Its Application in Breast Cancer Cell Imaging

ABSTRACT Here, a straightforward and environmentally friendly fluorescent S‐ and N‐doped carbon quantum dots (S,N‐CQDs) with a high blue fluorescence emission are synthesized for the ultrasensitive Hg 2+ metal ion detection. The carbonization method uses urea and L‐cysteine as carbon sources. UV–visible, fluorescence, and fourier‐transform infrared spectroscopic are employed to study the optical characteristics of the synthesized S,N‐CQDs. UV–Vis spectrum reported a shoulder at about 340 nm indicating the formation of CQDs. The fluorescence spectrum displayed a prominent emission peak centered at around 423 nm, confirming the blue fluorescent CQDs. FTIR revealed various functional groups and chemical bonds present, which established the successful synthesis of the S,N‐CQDs. X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy are employed to study the nanostructure and morphology that support the formation of the S,N‐CQDs. The stability and surface charge of as‐synthesized S,N‐CQDs are shown by zeta potential testing. The BET analysis reported the surface area and total pore volume of the synthesized S,N‐CQDs as 2.74 and 0.004 cm 3 g −1 , respectively, with the average pore diameter of 2.43 nm, confirming the micro/mesoporous nature of the synthesized S,N‐CQDs. The Hg 2+ metal ion sensing was investigated using fluorimetry that reported limit of detection (LOD) 10 nM with a linear sensing range of 0–600 ppb. Finally, the fluorescence imaging experiment using breast cancer cells exhibited its practical utility in biological samples.