In the recent years, nanotechnology became a promising field in cancer research by using hyperthermia behavior of magnetic nanoparticles in cancer treatment as well as the usage of mesoporous silica (SiO2) nanoparticles as biocompatible drug carrier. Thereby, surface functionalization is one of the most important techniques enhancing the specific uptake of cancer cells and their treatment. Nevertheless, surface group and ligand quantification is still one of the biggest challenges in nanoparticle modification but indispensable in regard of their possible medical applications. Nearly all known quantification methods of surface-bound ligands including direct and indirect methods are affected by size or charge distributions of particles, resulting in scattering of measurement results.
To close this knowledge gap, we report a novel quantification method of freely accessible hydroxide groups located on the particle surface of SiO2 nanoparticles, utilizing the electrokinetic sonic amplitude (ESA) effect supported by simultaneous conductivity measurements. ESA techniques itself became one of the most innovative methods for analyzing particle-ligand interactions. Thereby, the zeta potential of charged particles located in an alternating electric field is calculated from the resulting sound wave, opening insights into the surface chemistry of nanoparticles. In this context, a reversible titration was performed for shifting the equilibrium between deprotonated and re-protonated hydroxide groups. Furthermore, the degree of surface functionalization of phase-pure quasi-cubic hematite (α Fe2O3) particles having a zeta potential of about +50 mV was monitored through their reaction with 10-undecynoic acid and citric acid as ligand molecules, paving a profound way of ligand quantification. Thereby, all results highlight the usage of ESA techniques by its high potential as quantification method in nanoparticle surface modification.