Thermoswitchable fluorescent nanoparticles
Résumé
In this work, we developed a new process to covalently graft a thermo-responsive polymer on the surface of fluorescent nanocrystals in order to synthesize hybrid materials that combine both responsive and fluorescent properties. Nontoxic ZnO quantum dots (QDs) formed the core because usually QDs shows excellent fluorescence properties. These dots were synthesized by a sol-gel method followed by stabilization with organosilane. For the first time, poly(N-isopropylacrylamide) (PNIPAM) and poly(ether oxide) derivatives copolymers were grown by Activator ReGenerated by Electron Transfer-Atom Transfer Radical Polymerization (ARGET-ATRP) from ZnO quantum dots by surface-initiated polymerization. This process allowed the formation of fluorescent and responsive ZnO/PNIPAM core/shell QDs and ZnO/(co-)polymer while only requiring the use of ppm amount of copper for the synthesis. Each step of the synthesis process was monitored by the combination of various techniques such as UV measurements, fluorescence spectroscopy, dynamic light scattering and transmission electron microscopy. More particularly, thermo-responsive polymers are generally characterized by either lower critical solution temperature (LCST) or upper critical solution temperature (UCST). The LCST represents the temperature above which the polymer phase is separated from water, and below the LCST this polymer is miscible in water. We demonstrated that aqueous dispersible core/shell ZnO@PNIPAM QDs could be prepared by growing PNIPAM chains by ARGET-ATRP from silanized ZnO surfaces. The influence of the nature of the silanized layer and the PNIPAM polymerization time on the properties of the final nanomaterials were investigated. Results clearly evidence that the nanomaterial displays temperature-dependent optical properties and sizes. Both the PNIPAM layer thickness and the temperature affected the luminescence properties of the core/shell nanoparticles. In addition, the PNIPAM layer, when it is thick enough, could stabilize the QDs optical properties. Copolymer with different length chain and proportions of 2-(2-methoxyethoxy) ethyl methacrylate (MEO2MA) and hydroxyl-terminated oligo (ethylene glycol) methacrylate (HOEGMA) were also used as thermoswitchable copolymers. The nanomaterials exhibit temperature-dependent optical and sizes properties which depend mainly on the content of each monomer. By tuning the amount of each monomer, LCST ranging from 20 to 50°C can be obtained. Then, drug loading and release capacity of ZnO/(co-)polymer as well as their cytotoxicity towards cancer cells were evaluated. Results demonstrate the impact of the co-polymer structure on quantum dots properties in view of the application in cancer therapy.