Coupling gas-phase electrophoresis with infrared spectroscopy for controlled microfluidic synthesis of hydrogel nanoparticles.

A continuous-flow analytical platform integrating droplet microfluidic synthesis with horizontal attenuated total reflectance Fourier transform infrared spectroscopy (HATR-FTIR) and electrospray-differential mobility analysis (ES-DMA) was demonstrated for real-time quantification of chitosan (CS) hydrogel formation via ionic crosslinking with sodium tripolyphosphate (TPP). The hyphenated system enables quantitative monitoring of spectroscopic signatures with particle size evolution during ionic crosslinking of the hydrogel nanoparticles, which was validated orthogonally by the image analysis using atomic force microscopy. Correlating HATR-FTIR-derived concentration measurements with ES-DMA particle quantification enabled estimation of the number of CS molecules per hydrogel particle, providing quantitative mechanistic insight into network growth. Increasing crosslinker concentration promoted progressive hydrogel growth. This resulted in larger mobility diameter (dp,m), higher apparent molecular mass (Mm,n,avg), and reduced particle number concentrations. The pH-dependent studies revealed that acidic conditions enhanced protonation of CS amine groups and facilitated stronger electrostatic interactions with TPP, resulting in larger hydrogel structures (dp,m increasing from 19.2 nm at pH 4.5 to 23.6 nm at pH 2.5) and greater crosslinking efficiency (Mm,n,avg increasing from 92.58 kDa to 150.52 kDa). This work presents the first integration of HATR-FTIR and ES-DMA within a continuous microfluidic droplet reactor for controlled hydrogel synthesis via ionic crosslinking, establishing a powerful analytical framework for real-time mechanistic investigation and precise engineering of functional polymeric colloidal nanomaterials.