The charging of micellar nanoparticles in electrospray ionization

Charging of nanoparticles through electrospray has scarcely been explored. Spherical nanometer-sized amphiphilic block copolymer nanoparticles with diameters ranging from ∼65 to ∼150 nm were electrosprayed and analysed by charge detection spectrometry. Herein, we explore the charging of these micellar nano-objects by conducting a thorough study in different solvents, including pure water, and upon the addition of "supercharging" agents. The charge (z) of micellar nanoparticles electrosprayed from water solution is compared to the Rayleigh's limiting charge (z(R)) of a charged water droplet of the same dimensions. An average ratio (z/z(R)) of 0.6-0.65 is observed for the micellar macro-ions, supporting the charge residue mechanism, where the number of charges available to the micellar macro-ion is limited by the number of charges on the nanodroplet, which is a function of the surface tension of the solvent. Also we show the possibility of increasing the charging of micellar nanoparticles in the negative mode by adding organic bases (in particular piperidine) to water/methanol solutions.