<p>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&#xa0;nm excitation, the UCNPs exhibit characteristic green emission at ~ 543&#xa0;nm and red emission at ~ 655&#xa0;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&#xa0;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.</p> Graphical Abstract <p></p>