Solvent-triggered dual-emission switching of carbon dots for ratiometric discrimination of methanol and ethanol.

Developing robust methods to discriminate methanol from ethanol is critical for industrial safety and quality control. Herein, we report a novel strategy of two-step solvothermal method to synthesize dual-emission carbon dots (p-PD CDs) from p-phenylenediamine. The p-PD CDs exhibit unique, solvent-dependent fluorescence: in ethanol, they display a dominant blue emission at 445 nm with a weaker red emission at 598 nm; while in methanol, the blue emission is significantly quenched and red-shifted to 465 nm. This distinct solvatochromic response provides a ratiometric luminescent signal (I445/I598) for highly selective methanol detection in ethanol across two linear ranges (1-60 vol% and 60-100 vol%), achieving a detection limit of 0.38 vol%. Comprehensive characterization using transmission electron microscopy (TEM), femtosecond transient absorption (fs-TA), and ultraviolet photoelectron spectroscopy (UPS) revealed that the emission differences originate from methanol-caused aggregation of p-PD CDs. This aggregation is driven by weaker hydrophobic interactions and stronger hydrogen bonding compared to ethanol, leading to aggregation-caused quenching (ACQ) and an accelerated non-radiative decay pathway for the blue emission, as confirmed by time-resolved and transient absorption spectroscopy. The practical utility was successfully demonstrated by accurately quantifying methanol in industrial ethanol and a commercial cleaner with excellent recovery rates (99.5-101.7%), highlighting its significant potential for real-world application.