Quinoline-functionalized cross-linked poly(vinyl acetate) and poly(vinyl alcohol) nanoparticles as potential pH-responsive luminescent sensors

• Cross-linked pH-responsive organic nanoparticles were successfully synthesized via emulsion polymerization and subsequent hydrolysis of vinyl acetate.
• Covalently attached chromophores based on phenyl-quinoline and pyridine-quinoline derivatives were evaluated.
• A gradual shift or two distinct bands at ∼420 nm or ∼480 nm were detected, corresponding to the unprotonated or the protonotated form of the quinoline derivatives.
• Fast pH-responsive photoluminescence of the nanoparticles embedded in poly(lactic acid) and negligible leaching were observed.

A series of cross-linked organic nanoparticles (NPs) have been prepared through emulsion copolymerization of vinyl acetate (VAc) and of luminescent quinoline based monomers in the presence of divinylbenzene. Furthermore, hydrolysis of VAc led to the respective cross-linked poly(vinyl alcohol) nanoparticles. The two luminescent vinylic quinoline derivatives employed, namely 2,4-diphenyl-6-(4-vinylphenyl)quinoline (SDPQ) or 2-(pyridin-2-ylo)-6-styrene-4-phenylquinoline (QPy), were used at feed compositions of 1 mol% and 2 mol%. The chemical composition and the success of hydrolysis have been assessed through infrared spectroscopy, while scanning electron microscopy verified that the spherical shape and the size (∼50–100 nm) of the NPs are practically unaffected by the hydrolysis process. The pH-sensitive luminescence behavior of the NPs, arising from the protonation/deprotonation equilibrium of the quinoline groups, was investigated in aqueous dispersions or in solid state after incorporation of the NPs in a poly(lactic acid) (PLA) matrix. Depending on the system, either two distinct bands at ∼420 nm and ∼480 nm, corresponding to the unprotonated and the protonated form of the quinoline derivative, or a gradual shift from ∼420 nm to ∼480 nm were observed upon decreasing pH. The photoluminescence inspection of the surrounding aqueous environment confirmed that the leaching of the NPs from the PLA matrix is very limited or negligible. Moreover, our initial studies suggest that the photoluminescence response of the solid systems is quite fast (a few minutes), indicating that they are promising for the design of adequate solid pH-responsive luminescent sensors.