Silica surface modification <i>via</i> diazotization of <i>p</i>-phenylenediamine: a stationary phase for HPLC

A phenylenediamine-derived layer was grafted onto superficially porous silica particles through the diazotization of <i>p</i>-phenylenediamine (<i>p</i>-PDA), circumventing traditional silica silanization reactions. The polyphenylene-type layer was characterized using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), and elemental analysis. XPS revealed a surface layer containing approximately one nitrogen atom for every three aryl rings. In addition, the layer on silica included azo-linkages of aryl groups and amine functionalities, with one amine group per four to five benzene rings. Elemental analysis indicated that surface coverage of the aryl groups could be modulated by adjusting the reaction conditions, thereby enabling control over surface loading while reducing potential pore obstruction. A <i>p</i>-PDA-derived layer having 2-4 rings per nm² proved effective as a stationary phase for high-performance liquid chromatography (HPLC) and exhibited excellent hydrolytic stability under relatively harsh acidic conditions (0.5% TFA at 80 °C). Furthermore, beyond small molecules, the new phase showed promise for peptide separation.