Rhodamine B Doped ZnO Monodisperse Microcapsules: Droplet-Based Synthesis, Dynamics and Self-Organization of ZnO Nanoparticles and Dye Molecules.

Najla Ghifari,Abdel I El Abed,Bertrand Cinquin,Adil Chahboun

doi:10.3390/nano10122351

Abstract

In the present work, droplet-based microfluidics and sol-gel techniques were combined to synthesize highly monodisperse zinc oxide (ZnO) microspheres, which can be doped easily and precisely with dyes, such as rhodamine B (RhB), and whose size can be finely tuned in the 10–30 μm range. The as-synthesized microparticles were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and confocal microscopy. The results reveal that the microspheres exhibit an excellent size monodispersity, hollow feature, and a porous shell with a thickness of about 0.6 μm, in good agreement with our calculations. We show in particular by means of fluorescence recovery after photobleaching (FRAP) analysis that the electric charges carried by ZnO nanoparticles primary units play a crucial role not just in the formation and structure of the synthesized ZnO microcapsules, but also in the confinement of dye molecules inside the microcapsules despite a demonstrated porosity of their shell in regards to the solvent (oil). Our results enable also the measurement of the diffusion coefficient of RhB molecules inside the microcapsules ( cm/s), which is found two order of magnitude smaller than the literature value. We attribute such feature to a strong interaction between dye molecules and the electrical charges carried by ZnO nanoparticles. These results are important for potential applications in micro-thermometry (as shown recently in our previous study), photovoltaics, or photonics such as whispering gallery mode resonances.

Highlights

Zinc oxide (ZnO) is a very promising semiconductor material
We investigated the structure of the shell of the microspheres at the microscale using scanning electron microscopy (SEM) and confocal microscopy in the presence of a charged dye and used transmission electron microscopy (TEM) to have better insight into the organization of the primary ZnO nanocrystals building blocks of the microcapsules
We investigated the structure of the shell of the microspheres at the microscale using mainly scanning electron microscopy and confocal microscopy in the presence of a charged dye and used transmission electron microscopy to have a better insight into the organization of the primary ZnO nanocrystals building blocks of the microcapsules

Summary

Introduction

Zinc oxide (ZnO) is a very promising semiconductor material. It is appealing because of its outstanding optical and electrical properties [1]. Our droplet-based microfluidics approach enables the production of well-defined monodisperse zinc oxide microcapsules with a high control of size and chemical composition according to the use of highly monodisperse droplets as soft-templates for their synthesis. Such a feature may be used efficiently in the future as whispering gallery modes (WGMs)-based optical micro-resonators for biosensing applications, owing to the well controlled spherical shape of the synthesized microparticles and the high refractive index of ZnO (e.g., 2), as demonstrated few years ago by Moirangthem et al [27]

Experimental Section

Structural Properties of Synthesized ZnO Microparticles

Qd and tsq scales

Effect of Electrical Charges on Droplets Stability and Microparticles Size

Conclusions and Perspectives