Upconversion color tuning in Ce3+-doped LiYF4:Yb3+/Ho3+@LiYF4 nanoparticles towards ratiometric fluorescence detection of chromium(III)

- A dual-emission Ce3+-doped LiYF4:Yb3+/Ho3+@LiYF4 upconversion nanoparticles were synthesized.
- The output color of the nanoparticles can be precisely manipulated by modulating the doping-concentration of Ce3+ ions.
- Cr3+-responsive rhodamine derivative was assembled on the surface to fabricate the colorimetric probe by LRET process.
- A sequence of color change were obtained under 980 nm laser excitation, which could easily be visualized by the naked eye.
- This ratiometric probe is successfully applied to practical detection.

Ratiometric fluorescence sensor exhibits advantages of sensitive response, high anti-interference ability and naked eye visualization owing to multiple independent emission peaks utilized for results analysis. To achieve such multi-emission probe, the traditional methods have involved simple mixing of two emitters or tedious synthesis processes of hybrid material. However, these probes often have problems of inconstant emission strength ratio, low light-stability, and complicated synthetic process, which limit their applications in practical field. Herein, we report a single-structure Ce3+-doped LiYF4:Yb3+/Ho3+@LiYF4 upconversion nanoparticles (UCNPs), which features two emission peaks in the green (at 540 nm) and red (at 640 nm) region under the excitation of 980 nm near infrared laser. Importantly, the red/green intensity ratio can be regulated by changing the doping level of Ce3+ to modulate output colors. Furthermore, a Cr3+-responsive rhodamine derivative (CRD) was modified on the devised UCNPs surface to fabricate the colorimetric probe by luminescent resonance energy transfer (LRET) process. Upon addition of Cr3+ into the probe solution, the absorption peak of CRD at 560 nm is significantly enhanced, which greatly reduced the green emission, leading to an obvious color evolution from green to yellow to orange and to red with increasing the concentration of Cr3+. This method is successfully applied to practical detection of Cr3+ ion in industrial waste water. The work reported here demonstrates a useful way to construct color-based visual assays.

A ratiometric fluorescence sensor for Cr3+ detection has been developed based on luminescent resonance energy transfer in rhodamine derivative modified LiYF4:Yb3+/Ho3+/Ce3+@LiYF4 upconversion nanoparticles.168