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We have developed a selective upconversion switching method for the ratiometric fluorescence detection of nitrite using upconversion nanoparticles (UCNPs) and an efficient nitrite reaction. The green emission (λ(em) = 539 nm) of NaYF4:Yb(3+),Er(3+) nanoparticles can be selectively quenched by the neutral red (NR) dye due to the spectral overlap between the emission at 539 nm and the absorption of NR, while its red emission (λ(em) = 654 nm) remains unchanged. Nitrite reacts specifically and strongly with NR to form diazonium salt and lose the diazonium group, which sharply decreases the absorption of NR. Thus, the green emission of NaYF4:Yb(3+),Er(3+) can be recovered by increasing the amount of nitrite, leading to visible color changes from red to orange-yellow and finally green under excitation at 980 nm. The increase in the ratio of emission intensities (I539/I654) is quantitatively correlated to the concentration of nitrite ions. Moreover, the developed method has been successfully applied to nitrite detection in real samples such as drinking water, natural water and meat foods. In particular, the upconversion sensors can efficiently avoid background optical interference and thus show potential for the detection of nitrite salts in complex samples.

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In this work, for the first time, novel polyaniline/carbon dots (PANI/Cdots) nanocomposites have been synthesized by a simple method. Meanwhile, the bright fluorescence of Cdots is effectively quenched by PANI through fluorescence resonance energy transfer. With the addition of Hg 2+ , the strong binding ability between Hg 2+ and amino groups of PANI makes the Cdots be released break away from the PANI, resulting in restoration of Cdots fluorescence, which constructs a fluorescent probe for Hg 2+ detection. 10 This fluorescence "off-on" signal is sensitive to the concentration of Hg 2+ , and there is a good linear relationship between the fluorescence intensity of Cdots and the Hg 2+ concentrations in the range from 0.05 to 1.0 µM. The detection limit for Hg 2+ at a concentration is 0.8 nM. Moreover, it turns out that the nanoprobes represent a rather high selectivity for Hg 2+ detection. We believe the PANI/Cdots composites will emerge a new class of fluorescence materials that could be very likely to be suitable for practical 15 application. 75 quenching the donor emission and consequently provides good