Probe-free negatively charged dual-emission NaYF₄:Er³⁺, Yb³⁺@SiO₂ sensor for sensitive Rhodamine B detection

Rhodamine B (RhB) is a synthetic dye that is frequently used illicitly in processed food products, posing significant risks to human health. To address this challenge, we developed a probe-free, dual-emission sensor with a core-shell structure, consisting of NaYF₄:Er³⁺, Yb³⁺ upconversion microparticles as the core and a silica shell, denoted as UCMPs@SiO₂. These materials were synthesized via a hydrothermal method at 180 °C for 24 h and subsequently modified with a Stöber silica coating to enhance colloidal stability and particle dispersibility, while protecting the luminescent core with a negatively charged surface. Co-doping of Er³⁺ and Yb³⁺ ions into the NaYF₄ lattice enables Yb³⁺ to efficiently absorb near-infrared (NIR) light (∼980 nm) and transfer energy to Er³⁺, thereby generating intense green emission in the 520-540 nm range. At the same time, the SiO₂ shell effectively suppresses surface quenching and chemical degradation. The sensor operates via a ratiometric sensing mechanism, in which the red-to-green (red/green) emission ratio changes markedly upon RhB exposure due to spectral overlap between RhB absorption and the Er³⁺ green emission at 540 nm. As a result, the green is selectively quenched through the inner filter effect (IFE), while the red channel remains stable as an internal reference. This design achieves an estimated detection limit of 12.80 ng mL^-1, a linear range from 0.30 to 0.001 ppm, high repeatability, excellent anti-interference performance, and outstanding selectivity. These results demonstrate that UCMPs@SiO₂ represent a promising probe-free platform for sensitive food safety monitoring, offering a simple, environmentally benign, and versatile strategy for the detection of various organic dyes and toxic pollutants.