Fluorescence-Based Detection of Hg(II) and Cr(VI) Using Nitrogen-Sulfur Codoped Carbon Nanodots in their Pristine and Aminosilica-Embedded Forms

In this study, sulfur-nitrogen-codoped carbon nanodots (N,S-doped CNDs) are synthesized both in their soluble pristine form and incorporated into aminosilica particles. These materials, are utilized for the fluorometric detection of Hg(II) and Cr(VI). Both the soluble N,S-doped CNDs and the aminosilica/N,S-doped CNDs exhibit two distinct emission spectral bands when the excitation wavelength is varied. The fluorescence of soluble N,S-doped CNDs at λ_ex/λ_em = 390 nm/470 nm is quenched in the presence of both Hg(II) and Cr(VI); however, only Hg(II) quenches the fluorescence at λ_ex/λ_em = 450 nm/553 nm. In contrast, only Cr(VI) quenches the fluorescence of aminosilica/N,S-doped CNDs at λ_ex/λ_em = 380 nm/463 nm, while the fluorescence at λ_ex/λ_em = 440 nm/538 nm remains unaffected. By exploiting the fluorescence quenching behavior of free and aminosilica-embedded N,S-doped CNDs, fluorescence-based probes are developed to selectively detect Hg(II) and Cr(VI). The limits of detection, defined as the concentrations corresponding to a signal-to-noise ratio of 3, are determined to be 0.04 and 0.06 μM for Hg(II) and Cr(VI), respectively. Further investigations reveal distinct quenching mechanisms for each system: the fluorescence quenching effect on N,S-doped CNDs by Hg(II) is attributed to a static mechanism, and the quenching of aminosilica/N,S-doped CNDs by Cr(VI) is ascribed to the inner filter effect.