Surface functionalisation of magnetic nanoparticles: Quantification of surface to bulk amine density

Work has been conducted to adapt a colourimetric assay previously used on flat surfaces for the assay of amine group density on nanoparticles silanised with 3-(aminopropyl) triethoxysilane. The new assay was rapid, easy to perform, and linear in the range of optical density (OD_{282 nm}) values of 0.080-1.6 for particle suspension densities of between 0.5 and 7.0-mg/ml. In addition, the same materials, as well as the ones activated using 3-(aminopropyl) diethoxy methyl silane, were investigated for their elemental compositions by, combustion carbon-hydrogen-nitrogen (CHN) analysis and results from both approaches together have permitted the accurate calculation of the ratio of surface to total amine density for the materials when activated in water. This value can in turn be used as an indication of a surface amino structure (i.e. mono or multilayer). The aminosilanisation processes were further characterised by DNA-binding/elution and zeta potential measurement. This combination of approaches provides a fast, convenient and effective means of measuring surface amine densities on particles and yields information about the structure of the surface aminosilanes layers.