Synthesis of Core-shell Silica Fluorescent and the Effect of Size of the Particles on pH Sensing

In this research, core-shell particles of silica/fluorescein isocyanate were prepared by Stober method. By changing the pH of the reaction, the particles were produced in two different sizes. The results of infrared Fourier transform spectroscopy showed that the molecules of fluorescein isocyanate were attached to the silica by a silane coupling agent. Scanning and transmission electron microscopy images showed that relatively uniform particles with average diameter of 326 and 176 nm have been produced. The results showed that the fluorescence intensity of the fluorescent core-shell nanoparticles with greater diameter was more than three times greater than that of fluorescein isocyanate. Besides, the fluorescence intensity of particles with larger diameter was higher than that of particles with smaller diameter. The fluorescence intensity of nanoparticles in the solutions of phosphate buffer with different pH showed that the fluorescence intensity of the nanoparticles was linearly increased by increasing the pH of solution. The slope of the fitted line was greater for the larger particles; which indicated that the larger nanoparticles had higher sensitivity to the changes in the pH of the environment.