Surface-Charged Zirconia Nanoparticles Prepared by Organophosphorus Surface Functionalization with Ammonium or Sulfonate Groups.

Organophosphorus coupling agents bearing permanently charged functional groups (either cationic quaternary ammonium or anionic sulfonates) were synthesized and used for the modification of zirconia nanoparticles with a diameter <10 nm. Surface functionalization was confirmed by FTIR and solid-state NMR spectroscopy. Surface coverages up to 2.3-2.4 molecules/nm2 were achieved for modification with these charged coupling agents. The pH-dependent charge measurements of homogeneously modified particles showed stable surface charges over a wide range of pH for both ammonium- and sulfonate-functionalized particles. Surface charge measurements of particles co-functionalized with charged coupling molecules and uncharged methyl phosphonic acid revealed a decreasing charge density with increasing amount of uncharged coupling agent. Thus, an adjustment of charges by co-functionalization was obtained on the particle surface. The thus-formed surface-charged colloids were used in a second step for electrostatic-driven aggregation phenomena necessary for layer-by-layer processes. Sulfonate-modified negatively charged SiO2 submicrometer particles of 506 nm in diameter were decorated with ammonium-modified ZrO2 nanoparticles. In addition, a layer-by-layer deposition of alternating charge-modified TiO2 nanoparticles was proven by optical spectroscopy. Due to the broad applicability of organophosphorus coupling agents for surface modification, particularly for transition-metal oxides, the shown route represents a general method for the creation of almost pH-independent charges on the surface of nanoparticles.