Intracellular pH sensing using polymeric micelle containing tetraphenylethylene-oxazolidine

A novel amphiphilic copolymer consisting of tetraphenylethylene-oxazolidine (TPE-OX) as a pH-sensitive chromophore and poly(ethylene glycol) methyl ether methacrylate (PEGMA) as a water-soluble chain has been well designed and synthesized. A self-assembled polymeric micelle based on the copolymer exhibited clear dual-emission switching between cyan and red with a decrease of pH value in Tris-HCl buffer solution due to the extended conjugation and emerging intramolecular charge transfer effect when opening the spiro-ring of OX moiety. Fluorescence imaging of HepG2 cells stained by the polymeric micelle shows switched luminescence from cyan to red with high selectivity and contrast, indicating that the polymeric micelle was an effective probe for intracellular pH detection. Additionally, the red emission of the polymeric micelle in lysosomes can reversibly switch back to the original cyan emission in response to the two lysosomal activity inhibitors chloroquine and bafilomycin, which demonstrates that the polymeric micelle has potential applications in detecting the activity of lysosomal and further autophagy in cancer cells.