Novel carbon dots-based system for "on-off-on" fluorescence consecutive sensing of Au³⁺ and L-Cys

In this study, a novel carbon dots-based system was developed for the sequential quantification of Au³⁺ and L-cysteine (L-Cys). The system comprises N,F-doped carbon dots (N,F-CDs), a custom-designed miniaturized fluorimeter, and test strips. The N,F-CDs exhibit outstanding optical properties, including a large Stokes shift (127 nm) and high fluorescence intensity. The test strips offer low-cost, low-volume sample consumption, while the miniaturized fluorimeter provides high sensitivity in a compact format. The system demonstrated excellent analytical performance, including high sensitivity (limits of detection: 0.048-0.488 μmol/L for Au³⁺ and 8.7-40.9 nmol/L for L-Cys), minimal matrix interference (within ±20 %), high accuracy (97.5 % to 104.5 % for Au³⁺, 97.6 % to 107.5 % for L-Cys), good precision (2.9 % to 4.5 % for Au³⁺, 1.2 % to 7.0 % for L-Cys), robust stability, and strong inter/intra-day reproducibility (relative standard deviations ≤2.1 %). Furthermore, it showed high selectivity for Au³⁺ and L-Cys in complex matrices such as tap water, milk, and lemon-flavored beverages.