Polymerization Initiated by Inherent Free Radicals on Nanoparticle Surfaces: A Simple Method of Obtaining Ultrastable (ZnO)Polymer Core–Shell Nanoparticles with Strong Blue Fluorescence

Luminescent semiconductor nanoparticles, owing to their unique properties and promising applications, have attracted intensive investigation in the past decade. Because these nanoparticles are apt to aggregate and grow spontaneously, and some of them are unstable in air, water, or sunlight, their photoluminescence (PL) spectra red-shift, broaden, and weaken continuously during storage. Many efforts have been made to stabilize these nanoparticles, including employing organic ligands, coating nanoparticles with inorganic shells, and initiating polymerization on nanoparticle surfaces. However, because of coordination equilibria between nanoparticles and ligands, ligand-modified nanoparticles in solution are usually unstable if the temperature or concentration changes. Coating nanoparticles with inorganic shells to form crystalline core–shell nanocrystals do protect the nanocores effectively, but organic ligands are also needed by the shells and thus there still remains the lability caused by ligand– nanoparticle coordination equilibria. So far, the third method seems to be the optimal choice to stabilize nanoparticles for a long period of time. Under isolation of the polymer shell, particle growth and the decomposition induced by air or water are thoroughly suppressed, because the solid polymer matrix is so dense that ions and molecules cannot penetrate. Unfortunately, since most of the polymerization products are prepared by either dissolving initiators in monomers or grafting initiators on nanoparticle surfaces, the products are bulk materials or films but no longer nanometer-scale materials. As a result, further processing or assembly for application in photonic or optoelectronic devices is severely restricted. Therefore, a method of producing luminescent nanoparticles with long-term stability for practical use, especially in solution, remains a challenge for researchers in this field. As far as ZnO quantum dots are concerned, the traditional synthetic route through hydrolyzing zinc acetate by LiOH in ethanol produces green-light-emitting colloids, but the colloids become yellow-light-emitting within days at room temperature. Only recently were blue-light-emitting ZnO nanoparticles prepared in highly dilute solutions at 0 °C or in polymer matrixes, but the former were not stable at room temperature while the latter were bulk materials. We also reported blue-light-emitting polyether-grafted ZnO colloids with 30 % quantum yield that are stable for weeks at room temperature, but the colloids became green-light-emitting when heated at 60 °C for several days. To the best of our knowledge, a quantum yield above 30 % for ZnO nanoparticles has not been reported yet. Since the ZnO visible fluorescence originates from electrons being trapped in random surface holes, not from electron transitions from the conductance band to valence band, the luminescent properties of CdSe or CdTe are definitely better than those of ZnO quantum dots. Nevertheless, ZnO is a stable, non-poisonous, and cheap luminescent material for practical uses, which makes it more attractive than its rivals. Hence, to obtain luminescent ZnO nanoparticles with high quantum yield and stability for practical application, their surface structures must be adjusted and additional luminescent mechanisms should be included. In this communication, we present a simple method to initiate polymerization by virtue of the inherent free radicals pre-existing on ZnO nanoparticle surfaces, without any added initiators. Such surface-initiated radical polymerization is precisely controlled to form a thin polymer shell around each ZnO nanoparticle. Even after refluxing in ethanol at 80 °C for a month, the (ZnO)polymer core–shell nanoparticles were stable and their PL spectra showed almost no change except for a slight decrease in intensity. Furthermore, the as-prepared (ZnO)polymer nanoparticles have strong blue-light emission at about 420 nm, and their quantum yield exceeds 80 %, which is remarkable at present. Figure 1 is a high-resolution transmission electron microscopy (HRTEM) image of the as-prepared (ZnO)PMAA–PMMA core–shell nanoparticles, where PMAA is poly(methacrylic acid) and PMMA is poly(methyl methacrylate), with an average ZnO core diameter of 2.1 nm. These nanoparticles are uniform and monodisperse, even after refluxing in ethanol. Furthermore, the present nanoparticles are steady under the intense electron beams used for HRTEM measurements, indicating a firm C O M M U N IC A TI O N S