The effect of macrodipole orientation on the piezoelectric response of cyclic β-peptide nanotube bundles on gold substrates

A cyclic tri-β-peptide containing β-naphthylalanine (β-Ala(nap)), cyclo(β-Ala(nap)-β-Ala-β-Ala) (C3NAA), and its analog with an ethylenediamine-succinic acid sequence (ED-SA), cyclo(β-Ala(nap)-ED-SA) (C3NES) self-assemble into peptide nanotube bundles on gold, where the surfaces of the substrates are modified by the self-assembled monolayers (SAMs). The C3NAA nanotube generates a threefold larger macrodipole along the long axis of the nanotube than is present on the C3NES nanotube. The orientation of the peptide nanotube bundles change from horizontal to perpendicular on the substrates due to the methanol-vapor annealing process. The surface potential of the C3NAA nanotube bundle on the guanidium-terminated SAM is larger than that of the C3NES nanotube bundle due to the larger macrodipole of the C3NAA nanotube than the C3NES nanotube, but the converse piezoelectric responses are insignificant. On the other hand, the C3NAA nanotube bundles positioned horizontally along the gold substrate generate a relatively large surface potential, and the converse piezoelectric response is as high as approximately 9 pm/V. The piezoelectric response likely becomes larger when the macrodipole is perpendicular to the applied electric field and is sensitive to the extent to which the dipoles are arranged in a noncentrosymmetric structure.