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A dual-channel ratiometric method is presented for improved colorimetric and fluorometric visualization of the fungicide cymoxanil (CYM). It is based on the use of a mixture of dually emitting carbon dots (CDs) and citrate-stabilized silver nanoparticles (AgNPs). The CDs, under photoexcitation at 350 nm, display dual (blue and green) fluorescence, with peaks at 435 and 520 nm. In mixed aqueous suspension of CDs and AgNPs, the intensity of blue fluorescence of CDs is reduced due to internal filter effect (IFE). This is due to the spectral overlap between the emission of CDs and the absorption of yellow AgNPs. After the addition of CYM to the mixture, CYM triggers the aggregation of AgNPs due to electrostatic attraction and hydrogen bonding interactions. The aggregated AgNPs have an orange color with an absorption whose maximum is shifted to around 510 nm. Hence, it overlaps the green emission of CDs. This causes an IFE on the green fluorescence, while the blue fluorescence is recovered. The colorimetric is performed by ratioing the absorbances at 515 and 390 nm. The ratiometric fluorometric assay is based on ratioing the emissions at 435 and 520 nm. The assay has a wide detection range (0.01–0.55 μΜ) and a low limit of detection (2 nM at S/N = 3). The assay was applied to the determination of CYM in spiked real samples (natural river water, soil and plant epidermis). Recoveries ranged between 97 and 105%. The method enables assays to perform on-site and visual detection by observing fluorescence color shades in either aqueous solutions and on wetted filter paper strips. [ABSTRACT FROM AUTHOR]