N-Doped Carbon Dots for Visual Recognition of 4-Nitroaniline and Use in Fluorescent Inks

Carbon dot (CD)-based fluorometric detection of the industrial pollutant, 4-nitroaniline (4-NA), is a very challenging research arena. We designed two N-doped CDs (NCDs) with distinct photophysical characteristics by varying the heteroatom percentage. The CD emission color shifts from blue (NCD-1) to green (NCD-2) in water as the heteroatom percentage increases from 8.3% to 15.8%. The NCDs also slightly differ in size with diameters of 2.5 and 3.6 nm, respectively. In comparison to NCD-2, only NCD-1 exhibits bright green luminescence (521 nm) in the solid state. The as-prepared NCDs showed excellent solvent-dependent emission behavior; NCD-2 shows a higher spectral shift (76 nm) than NCD-1 (41 nm) as the solvent medium changes from tetrahydrofuran (THF) to water. NCD-2 displays blue emission in DMSO and green emission in water. Both NCDs perform excellently for discriminative recognition of 4-NA by fluorescence quenching. Detailed investigation revealed that quenching mechanisms depend on the distinct fluorescence properties of NCDs and the solvent medium. In water, NCD-1 fluorescence mainly quenches through inner filter effect (IFE) and photoinduced electron transfer (PET), while a combination of Förster resonance energy transfer (FRET) and PET account for the fluorescence quenching of NCD-2. In the DMSO medium, the primary fluorescence quenching pathways of NCD-2 were FRET and IFE, along with a small contribution of PET. Furthermore, an inexpensive portable paper strip was constructed to demonstrate a fast and easy on-site sensing of 4-NA. Additionally, newly developed NCDs were ideal for application in the arena of invisible ink.