Quantitative Determination of the Carboxyl Groups on Individual Nanoparticles by Acid‐Base Titrimetry

Quantification of the active groups on nanoparticle surface greatly facilitates the downstream functionalization, which is essential for their application. Among the common active groups, carboxyl group is one of the most popular groups. Hence, we used an improved gravimetric method to obtain the accurate number of the nanoparticles and further determined the number of the carboxyl groups on individual nanoparticles with titrimetry. In this work, polymer nanoparticles with carboxyl groups on their surfaces were taken as a model to validate this determination. Firstly, by using the total weight of the nanoparticles divided by the weight of one nanoparticle, their number concentration was determined. The total weight was obtained by measuring the dry mass of the nanoparticles, and the unit weight was calculated by the volume of a single nanoparticle multiplied with its density, in which the volume was characterized by transmission electron microscopy (TEM), and the density was determined by isopycnic gradient centrifugation. Directly using the real density of nanoparticles achieved more accurate quantification than the current gravimetric method which used the density of their bulk material counterparts for calculation. Then, a back titration was used to determine the carboxyl groups: Certain amount of nanoparticles were first to react with excess NaOH standard solution, and after complete reaction, the residual NaOH were measured by titration with HCl standard solution, from which the amount of the carboxyl groups on individual nanoparticles could be calculated.