Ratiometric fluorescent sensor with large pseudo-Stokes shifts for precise sensing and imaging of pH without interferential background fluorescence

Abstract

Endowing fluorescent pH sensors with large Stokes shifts promises to resolve interferential background fluorescence in practice, and yet few such method has been reported, owing to lack of luminescent materials with large Stokes shifts used in fluorescent sensors. Herein, we elaborately designed NaGdF₄:Ce@NaGdF₄:Nd@NaYF₄:Eu core-double shells (CDS) lanthanide-doped fluoride nanoparticles (LFNPs), employing Gd³⁺-mediated energy migration and interfacial energy transfer to realize intense red and NIR emissions under 254 nm irradiation, and pseudo-Stokes shifts of which reached up to striking 361 nm and 610 nm, respectively. The CDS LFNPs collaborated with absorption-based pH indicator bromocresol green to from a novel fluorescent sensor film, and employing low-cost dual chip RGB-NIR camera to precisely record luminescence signals. On the basis of inner-filter effects, this senor system enabled accurate ratiometric read-out of pH value ranging from 5 to 6 (pK_a ± 0.5), according to intensity ratios of pH-sensitive red emissions and referenced NIR emissions, avoiding common errors (e.g., fluctuant light sources). Notably, the large pseudo-Stokes shifts allowed red and NIR emissions far from the interfering background fluorescence possessing relatively small Stokes shifts, ensuring elevated signal-to-noise ratio and accurate pH determination. Therefore, the devised pH sensor system based on the CDS LFNPs exhibited sufficient accuracy in autofluorescent real samples (e.g., algae, serum), revealing a novel way of employing large pseudo-Stokes shifts to realize the background-free pH measurement and 2D imaging.

Keywords: Background-free pH sensing; Inner-filter effects; Lanthanide-doped fluoride nanoparticles; Large pseudo-Stokes shifts; RGB-NIR imaging; Ratiometric fluorescent sensor.