A near-infrared excited Förster resonance energy transfer-based fluorescent sensor using core-shell upconversion nanoparticles functionalized with rhodamine B derivative for highly selective detection of Hg

A near-infrared (NIR)-excitable ratiometric fluorescent nanosensor, CS-UCNP@mSiO2–RBPA, was developed for the selective and sensitive detection of Hg2+. The nanosensor is constructed by grafting a rhodamine B-derived probe (RBPA) onto mesoporous silica-coated core–shell NaYF4:Yb,Er upconversion nanoparticles (UCNPs). Upon 980 nm excitation, the UCNPs exhibit characteristic green emission at ~ 543 nm and red emission at ~ 655 nm. In the presence of Hg2+, the spirolactam ring of RBPA opens specifically, producing a yellow-emitting species that enables efficient Förster resonance energy transfer (FRET) from the UCNP donor. This leads to a Hg2+ concentration-dependent quenching of green emission and a synchronous enhancement of yellow fluorescence at ~ 584 nm, with the red emission acting as a stable internal reference. The nanosensor shows outstanding selectivity toward Hg2+ against other competing metal ions and performs optimally at neutral pH. It presents a good linear response in the Hg2+ concentration range 0–70 µM via UV–vis absorption, and achieves an ultra-low detection limit of 14 nM based on the ratiometric fluorescence signal (I584/I655). Benefiting from NIR excitation, ratiometric self-calibration and excellent specificity, this fluorescent nanoplatform holds great potential for practical applications in environmental monitoring and biological analysis.