A ratiometric fluorescent probe based on peanut-shell-derived CDs and CuNCs for smartphone-assisted detection of methyl orange.

Methyl orange (MO), a refractory azo dye, can persist in aqueous environments, creating a need for rapid and reliable monitoring methods. In this work, naturally O/N-containing peanut-shell biomass was utilized as an agricultural solid-waste precursor to synthesize oxygen-containing carbon dots (CDs) via a hydrothermal method. The CDs were mixed with D-penicillamine (DPA)/bovine serum albumin (BSA)-stabilized copper nanoclusters (CuNCs) at a controlled volume ratio to form a dual-emission ratiometric fluorescent probe. TEM was used to characterize the nanoscale morphologies, while XPS confirmed the elemental composition of the system. Fluorescence lifetime analysis and spectral overlap indicated that the inner filter effect was the major contributor. Under excitation at 345 nm, the probe exhibited emission peaks at 430 and 630 nm. MO preferentially attenuated the blue CD emission, whereas the red CuNC emission remained relatively stable as an internal reference. The probe showed a linear response to MO over 10-200 µmol L-1, with a detection limit of 0.68 µmol L-1. The ratiometric signal showed substantially smaller variation than the individual emission intensities during refrigerated storage. Selectivity and interference studies showed that the MO response remained readily distinguishable in the presence of representative coexisting species. Recoveries in Mizone beverage, liquor, sugar solution, and tap water ranged from 97.04% to 99.96%, with RSD values below 2.18%. Smartphone-assisted B/R analysis enabled visual semi-quantitative detection using three smartphone models, supporting the feasibility of the platform for on-site MO screening.