Individualization and Stabilization of Zinc Oxide Nanorods by Covalent Functionalization with Positively Charged...

We present the formation of individualized and stabilized zinc oxide (ZnO) nanorods by functionalization with positively charged catechol derivatives by means of ligand exchange reactions. The electrosteric stabilization of ZnO nanorods was studied using two catechol derivatives, introducing either three (1) or six (2) pH independent positive charges per molecule and sterically demanding groups onto the surface. ZnO nanorods providing initially acetate (Ac) or 2-[2-(2-methoxyethoxy)-ethoxy]acetic acid (TODA) ligands on their surface were used. The ligand exchange was performed by using mono and mixed functionalization approaches, utilizing either exclusively the positively charged catechols or mixtures of the latter with small commercially available catechol derivatives, namely 4-methylcatechol (Me-cat), 4-tert-butylcatechol (tBu-cat), and dopamine hydrochloride (Dop). Using a combination of various analytical methods such as zeta potential, dynamic light scattering (DLS), UV/Vis, and scanning electron microscopy (SEM) measurements we found that the initial surfactants on the nanorods surface, the number of positive charges per molecule, the steric demand, and the added amount of the catechol derivative strongly influence the colloidal behavior of the nanorods. Stable suspensions containing individualized ZnO nanorods were successfully formed upon functionalization of ZnO-TODA nanorods with 30 monolayers (MLs) of the higher charged catechol (2), as well as using mixtures of 20/10 and 18/10 MLs of 2/Dop.