Upconversion Nanoprobes for the Ratiometric Luminescent Sensing of Nitric Oxide

By taking advantage of the optical properties of upconversion nanoparticles (UCNPs), we have designed a luminescence ratiometric nanosensor for measuring nitric oxide (NO) in biological fluids, live cells, and tissues. This nanoconjugate consists of a UCNP core with two strong fluorescence emission peaks at 540 and 656 nm as the upconversion fluorophore, NO-reactive rhodamine B-derived molecules (RdMs) encapsulated within the mesopores of the mSiO₂ shell, and a β-cyclodextrin (βCD) layer on the exterior of the particle. Reaction of the analyte with the O-phenylenediamine of the RdM induces opening of the spiro-ring and is accompanied by an appearance of a strong rhodamine B (RdB) absorption band between 500 and 600 nm, which has spectral overlap with the green emission (540 nm) of the UCNPs. This results in an increase in the I₆₅₆/I₅₄₀ ratio and quantitatively correlates with [NO]. The assay is validated under clean buffer conditions as well as inserum and liver tissue slices obtained from mouse models.