N-doped carbon dots: triple mode excitation–emission features for dual channel sensing and logic gate application

Carbon dots (CDs) have congregated massive opportunities in basic and applied research streams owing to their versatile properties. Herein, a unique N-doped CD (NCD) was synthesized via the hydrothermal technique using 3,5-diaminobenzoic acid and hydrazine hydrate as precursors. The NCDs exhibit exceptional optical properties; they emit very different colors under different excitation windows. The fluorescence maxima were at 404 nm (blue), 506 nm (green), and 605 nm (red), respectively, when the excitation wavelengths fall within 300–340 nm, 380–460 nm, and 500–540 nm. The emission features were drastically different from more straightforward CDs (CDBlank) prepared using 3,5-diaminobenzoic acid only via the same hydrothermal route without hydrazine hydrate. We found that the 506 nm and 605 nm emissions originate from various surface states, while the 404 nm emission comes from the NCD core states. The triple emissive NCDs were implemented for sensing; Fe3+ effectively quenches the green and red emissions through turn-off quenching, while ascorbic acid (AA) successfully recovers the quenched emission in a turn-on manner. Both static and dynamic mechanisms were responsible for quenching green and red emissions and were accounted for by complexation-induced aggregation and electron transition from the electron-rich NCDs to the vacant d orbital of Fe3+, respectively. The detection limits for the green and red regions were 7.9 nM and 12.2 nM, respectively. The AA restores both the emission signals by reducing Fe3+ to Fe2+. The calculated detection limits for AA recognition were 0.55 μM and 1.05 μM, respectively, for the green and red emissions. For real-life practicability, some actual samples were examined under the excitation wavelengths (420 nm and 500 nm) with satisfactory outcomes. Furthermore, a multi-input logic gate was constructed to convert the molecular information as fluorescence signal outputs.