Turn-on pH nano-fluorosensor based on imidazolium salicylaldehyde ionic liquid-labeled silica nanoparticles

Abstract

A new simple pH-sensitive fluorescent probe, methylimidazolium salicylaldehyde ionic liquid (MeIm-Sal-IL), was designed and successfully synthesized from commercially available salicylaldehyde (Sal) via an extremely simple efficient two-step synthetic protocol. This new pH sensor has prominent advantages over traditional fluorescent pH probes, such as a large Stokes shift (∼125 nm) and extreme aqueous solubility along with simultaneous color and fluorescence profile changes as a result of pH alteration. The fluorescence behavior of this new probe provides a new state-of-the-art pH sensing method. Fluorescence turn-on is observed with increasing pH (≈5 to 11), accompanied by a bathochromic shift of 70 nm from (λ_max emission 435 to 505 nm; λ_max absorption 323 to 378 nm). This bathochromic shift may be ascribed to the tautomeric equilibrium involving excited-state intramolecular proton transfer (ESIPT). Meanwhile, the fluorescence intensity enhancement at higher pH is attributable to the internal charge transfer (ICT) process. Also, the new probe was labeled with silica nanoparticles (SiNPs) in order to produce a novel nano-fluorosensor for pH measurements. The nanosensor was fabricated by covalent labeling of chloro-modified SiNPs with the new probe. The emission spectra of the new nanosensors are bathochromically shifted by about 65 nm (λ_max emission 435 to 500 nm) at low pH (3 to 5). Moreover, the fluorescence intensity of the nanosensor is increased by ≈60 fold in the pH range from 5 to 9. The sensitivity of the new nanosensor is highly applicable within the pH range of 5 to 9, which suggests a broad variety of applications in the physiological pH range.