A ratiometric upconversion nanoprobe for fluorometric turn-on detection of sulfite and bisulfite

A nanoprobe is described for the ratiometric fluorometric determination of sulfite ions. Upconversion nanoparticles (UCNPs) of the type β-NaYF₄:Yb(III),Er(III),Tm(III) were covalently modified with the molecular probe HIAN which is a hydroxynaphthalimide fluorophore modified with a (cationic) indolinium moiety. Under excitation at 980 nm, the green emission of the UCNPs (peaking at 543 nm) is almost totally quenched, while the NIR emission (peaking at 802 nm) remains unaffected. In the presence of sulfite or bisulfite (hydrogen sulfite), the green fluorescence is restored and can be visually observed. A ratiometric method was worked out by measurement of the ratio of the green and NIR emissions. The analytical range extends from 10 to 250 μM, the limit of detection is 0.14 μM, and the assay can be performed within 40 s. Graphical abstract Based on the use of a molecular probe for sulfite and hydrogen sulfite, and by exploiting an inner filter effect (IFE), an assay for sulfite/hydrogen sulfite was developed by using upconversion nanoparticles (UCNPs). Addition reaction of sulfite/bisulfite with the material results in weakened IFE and enhanced green fluorescence of the UCNPs at excitation/emission wavelengths of 980/543 nm.