Novel cytocompatible intracellular pH-imaging fluorescence probe composed of quantum dot and phospholipid polymer

Abstract

  To determine the location of molecules during uptake process by cells, a new nanoparticle-type fluorescence probe whose fluorescence behavior can change in response with surrounding pH was prepared. The nanoparticles was composed of quantum dot (QD) core and block-type water-soluble phospholipid polymer with pH responsible poly(2-diethylamino methacrylate (DEAEMA)) segment as a shell. A small amount of fluorescent dye bounded to the polymer. For detecting the pH circumstances, fluorescence resonance energy transfer (FRET) mechanism between QD as a donor and a fluorescent dye as an accepter was used. The block-type phospholipid polymer was synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization method to control molecular weight and polymer sequence. After solubilizing QD into water by the polymers to form nanoparticles, Alexa 594 cadaverine was immobilized on the nanoparticles. It was observed the change in fluorescence spectra dramatically in the pH range between pH 6.6 and pH 7.4. This was due to the morphology of the block-type phospholipid polymer chains was dramatically changed in this pH range. The poly(DEAEMA) segment in the polymer was shrunken at pH 7.4, while the segment was stretched at pH 5.0 related to the protonation of the polymer. The nanoparticles were added in the culture medium under HeLa cell culturing, the nanoparticles were accepted cell-uptake. During this process, the fluorescence property was changed. We considered that this fluorescence change is corresponded to releasing of the nanoparticles from endosome to cytoplasm. Thus, the nanoparticles are good pH probe to detect the location of molecules inside of cells.