Controllable Growth of Straight Nanorods and Nanowires in the Langmuir Films of a Bolaamphiphilic PAR Derivative

Abstract

A novel bolaamphiphilic compound, 1, 10-bis[3'-hydroxy-4'-(2''-pyridylazo)phenoloxy] decane [(PAR)2C10], was synthesized and its spreading film and in situ coordination with metal ions in the Langmuir monolayer at the air/water interface were investigated. It was found that (PAR)2C10 could be spread on the water surface, and the Langmuir film showed a phase transition from a flat conformation to a U-shaped conformation upon compression. Interfacial coordination between (PAR)2C10 and metal ions could occur in situ in the Langmuir monolayer. Depending on the concentration of the subphase, different coordination modes were observed. A 2:1 (ligand-PAR-to-metal-ion) complex was formed at a lower concentration of the subphase, while a 1:1 complex was obtained on the subphase with a higher concentration of the salt. Interestingly, very straight nanowires, extending to several micrometers, were observed in the AFM images of the films transferred from the subphase containing CuCl2 in higher concentrations. However, tortuous nanowires were observed under the subphase containing Cu(CH3COO)2. The formation of such kind of wire-like structures was only observed for the combination of the bolaamphiphilic PAR derivative with the Cu(II) ions. Other metal ions or the single-chain PAR derivative did not show this behavior. A possible growth mechanism of the nanowires was proposed based on the coordination of the bolaamphiphilic (PAR)2C10 with the Cu(II) ions as well as with the counteranions.