Novel hollow carbon dots from cactus cladodes Peel as a qualitative ‘turn-on’ fluorescence and naked-eye detection of the flammable cyclohexane gas sensor

Abstract In this study, we present a sustainable and cost-effective approach for the synthesis of hollow nitrogen-doped carbon dots derived from red cactus cladodes peel (CCP@CDs) for the detection of flammable cyclohexane gas. The CCP@CDs, prepared via a simple microwave irradiation method, exhibited bright fluorescence with a complex multi-component emission profile. The sensor demonstrated a dual-mode response to cyclohexane, with a visible color change from opaque brown to transparent yellow and a dramatic fluorescence enhancement. Our results showed a significant increase in fluorescence intensity, with the order of intensity being CCP@CDs after contact with cyclohexane > pH 2 > pH 12 > pH 7. Quantitative analysis revealed that the fluorescence color shifted, with the CIE coordinates moving from (0.175, 0.459) for the pristine solution to (0.148, 0.401) after exposure to the gas. These experimental findings were substantiated by DFT calculations, which confirmed a strong electronic interaction. Specifically, we observed a decrease in the HOMO-LUMO energy gap (E g ​) from 0.3199 eV to 0.2633 eV and a substantial increase in the dipole moment (µ) from 1.76 Debye to 5.30 Debye upon the formation of the CCP@CDs@cyclohexane complex. Molecular electrostatic potential maps (ESPMs) further provided visual evidence of this charge redistribution. Our findings highlight the potential of using biomass waste for the production of advanced, dual-mode chemical sensors for environmental and safety applications.