Upconversion color tuning in Ce(3+)-doped LiYF(4):Yb(3+)/Ho(3+)@LiYF(4) nanoparticles towards ratiometric fluorescence detection of chromium(III).

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 Ce-doped LiYF:Yb/Ho@LiYF upconversion nanoparticles (UCNPs), which features two emission peaks in the green (at 540nm) and red (at 640nm) region under the excitation of 980nm near infrared laser. Importantly, the red/green intensity ratio can be regulated by changing the doping level of Ce to modulate output colors. Furthermore, a Cr-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 Cr into the probe solution, the absorption peak of CRD at 560nm 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 Cr. This method is successfully applied to practical detection of Cr ion in industrial waste water. The work reported here demonstrates a useful way to construct color-based visual assays.