One-pot synthesis of thermal responsive QDs-PNIPAM hybrid fluorescent microspheres by controlling the polymerization temperature at two different polymerization stages

In the present study, for the first time, without the help of any amphiphilic polymers or polymerizable surfactants, 3-mercaptopropionic acid (MPA) capped CdTe quantum dots (QDs) could be one-pot copolymerized into poly(N-isopropylacrylamide) (PNIPAM) microspheres during the monomer polymerization process just by controlling the synthesis temperature at two different polymerization stages. In the first stage, NIPAM monomers were initiated by potassium persulfate (KPS) below the lower critical solution temperature (LCST) to form PNIPAM networks with CdTe QDs distributed throughout. In the second stage, when the polymerize temperature was raised above the LCST, the PNIPAM networks collapsed to form spheres and the CdTe QDs were entrapped into the PNIPAM microspheres (denoted as CdTe-PNIPAM microspheres). Transmission electron microscopy (TEM) images showed that CdTe QDs have been successfully entrapped within the PNIPAM matrix. According to the dynamic light scattering (DLS) results, the CdTe-PNIPAM hybrid microspheres had a significantly narrow size distribution and their mean hydrodynamic diameter was about 267 nm. Compared with the original CdTe QDs, the CdTe-PNIPAM microspheres exhibited not only a prominent red-shift of their emission wavelength (as much as 36 nm), but also temperature dependent on-off fluorescence properties. Around the LCST, when the temperature was increasing, the fluorescence intensity decreased sharply; when cooling the solution, the fluorescence intensity would restore. Moreover, after repeated heating and cooling cycles, the photoluminescence (PL) quenching and enhancement processes were fully reversible around the LCST region.