A Near-Infrared Excited FRET-Based Fluorescent Sensor Using Core–Shell Upconversion Nanoparticles Functionalized with Rhodamine B Derivative for Highly Selective Detection of Hg²⁺

A near-infrared (NIR)-excitable ratiometric fluorescent nanosensor, CS-UCNP@mSiO₂–RBPA, was developed for the selective and sensitive detection of Hg²⁺. The probe consists of core–shell NaYF₄:Yb,Er upconversion nanoparticles (UCNPs) coated with mesoporous silica and functionalized with a rhodamine B-derived chemosensor (RBPA). Upon 980 nm excitation, the UCNPs emit green (~ 543 nm) and red (~ 658 nm) luminescence. In the presence of Hg²⁺, RBPA undergoes spirolactam ring-opening, generating a yellow-emitting species that accepts energy from the UCNP donor via Förster resonance energy transfer (FRET). This results in a concentration-dependent decrease in green emission and a concomitant increase in yellow fluorescence at ~ 584 nm, while the red emission serves as an internal reference. The sensor exhibits excellent selectivity for Hg²⁺ over other common metal ions and operates effectively at neutral pH. A linear response was observed in the Hg²⁺ concentration range of 0–70 µM by UV–vis absorption, and a remarkably low limit of detection of 14 nM was achieved by ratiometric fluorescence (I₅₈₄/I₆₅₈). The combination of NIR excitation, ratiometric output, and high specificity makes this nanoplatform promising for environmental monitoring and potential biological applications.