Rational design of multicolor-emissive carbon dots via aromatic diamines for solvent-responsive fluorescent sensing

This study presents a novel synthesis and comprehensive characterization of nitrogen-doped carbon dots (CDs) derived from p-phenylenediamine (p-CDs), benzidine (b-CDs), and 1,5-diaminonaphthalene (n-CDs), highlighting their fluorescence properties for advanced sensing applications. By systematically varying the synthesis temperature (140–200 °C) and reaction time (12–24 h), we observed distinct precursor-dependent fluorescence responses. Increasing temperature induced a gradual redshift in p-CDs (438–450 nm) and n-CDs (397.8–403.8 nm), whereas b-CDs exhibited a pronounced blueshift (524–426 nm), reflecting differences in conjugation degree, carbonization behavior, and surface-state evolution. The CDs exhibit partially graphitized crystalline domains, with lattice spacings of 0.21–0.23 nm, and particle sizes ranging from 2.72 ± 1.63 nm (n-CDs) to 10.76 ± 4.15 nm (p-CDs). X-ray photoelectron spectroscopy (XPS) analysis reveals distinct nitrogen configurations (pyridine N, pyrrole N) and atomic compositions (e.g., 75.04% C1s in p-CDs), which critically influence their electronic structure. Solvent-dependent fluorescence studies revealed emission peaks spanning 404.8–620.0 nm. Dual-emission behavior was observed for p-CDs dispersed in benzene and chloroform, and for n-CDs dispersed in benzene, dimethylformamide (DMF), and acetonitrile, where strong π–π interactions and solvent-induced surface-state modulation generated secondary emission bands. Notably, acetonitrile enhances b-CDs’ fluorescence intensity by 1.5-fold, while ethanol maximizes n-CDs’ emission. These findings highlight the potential of the synthesized CDs as solvent-responsive fluorescence probes for environmental monitoring. Graphical abstract