Fluorescence Confocal Laser Scanning Microscopy for pH Mapping in a Coaxial Flow Microreactor: Application in the Synthesis of Superparamagnetic Nanoparticles

Fluorescence confocal laser scanning microscopy (CLSM) has been applied to quantify pH distribution in a coaxial flow microreactor. The methodology consists of utilizing a micromolar concentration of the common dye fluorescein, whose fluorescence in solution strongly increases above pH 6.2, in order to map the pH changes in the central jet stream where an acid-base reaction proceeds. This experimental method has been calibrated thanks to a theoretical study. The latter was based on a numerical solution of ion transport equations in solution which takes proper account of the electrostatic and chemical phenomena. The experimental fluorescence profiles determined by CLSM have been found to be in excellent agreement with the theoretical predictions and to be highly sensitive to both the initial bulk solution pH and the applied volumetric rate ratio (alpha) of the Outer stream to the inner stream. The simulated pH gradients have been compared for different alpha. This latter has been tuned during the synthesis of the superparamagnetic nanoparticles in order to study its effect on the produced nanoparticles in terms of particles mean size, size distribution, and yield in magnetic material.