Highly selective fluorescence detection of L-selenium-methylselenocysteine in selenium-enriched Cardamine violifolia

A feasible and practicable "off-on" type of fluorescence strategy for highly selective screening of L-selenium-methylselenocysteine (L-SeMC) in selenium-enriched Cardamine violifolia was developed using g-C₃N₄-MnO₂ nanocomposites as fluorescent probes. The g-C₃N₄ nanosheets can emit blue fluorescence at 320 nm excitation wavelength with a fluorescence quantum yield of 28%. When MnO₂ was deposited onto g-C₃N₄ nanosheets, the fluorescence of the g-C₃N₄ nanosheets was quenched due to fluorescence resonance energy transfer (FRET). After the addition of L-SeMC, MnO₂ was reduced to Mn²⁺, which eliminated FRET and fluorescence was restored. Based on this, a quantitative method for the determination of L-SeMC was established. The fluorescence intensity of g-C₃N₄-MnO₂ nanocomposites showed a good linear relationship with the concentration of L-SeMC in the range of 0-45 μg mL^-1, the limit of detection (LOD, 3σ/K) was 8.25 ng mL^-1 and the correlation coefficient was 0.9904. Common selenium compounds such as SeO₂, Na₂SeO₃, SeMet and SeCys caused weak fluorescence intensity, which means that the developed method is highly selective to detect L-SeMC in a series of selenium compounds. Meanwhile, the technique was evaluated by spiking L-SeMC standards in C. violifolia extraction solutions and with 9 C. violifolia extraction specimens, receiving excellent accordance with results from the commercially available atomic fluorescence spectroscopy method.