Two kinds of carbon dots with the maximum fluorescence peak of 492 nm (named as G-CDs) and 607 nm (named as R-CDs) were synthesized. In the presence of MoO₄²⁻ ions, the fluorescence of R-CDs at 607 nm can be quenched, which can probably be assigned to their aggregation caused by MoO₄²⁻, while that of G-CDs at 492 nm remained unchanged. For the first time, a ratiometric fluorescence probe was developed for MoO₄²⁻ ions detection. In the range 0.25 ~ 100 μM, the fluorescence ratio (F₄₉₂/F₆₀₇) of the probe was linearly related to MoO₄²⁻ concentration, and the detection limit was 61.5 nM, which fully meets the minimum detection requirements of MoO₄²⁻ ions in drinking water. On the other hand, when MoO₄²⁻ was introduced, a significant fading phenomenon of R-CDs can be observed with the naked eye; thereby, the colorimetric method can also be proposed. Based on above, the ratiometric fluorometric/colorimetric dual-mode sensing method was established for MoO₄²⁻ anion quantification. Compared with the traditional analysis methods, the results obtained by multimodal sensing can be mutually verified, which effectively improves the accuracy and reliability. The dual-mode assay proposed in this work provides an alternative scheme to meet the need of sensing target compounds in complex matrices.

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