Molecular recognition and removal properties of Langmuir–Blodgett films of nucleolipid amphiphiles bearing thymine headgroup

Three different types of nucleolipid amphiphiles bearing thymine moiety in the headgroup, N-octadecyl thymine-1-acetamide (OTA), octadecyl ammonium thymine-1-acetate (OATA) and N,N-didodecyl thymine-1-acetamide (DDTA) were synthesized, and the molecular recognition capabilities of the corresponding Langmuir (L) and Langmuir–Blodgett (LB) films for melamine (MELA), 2,4,6-triaminopyrimidine (TAP) and 2,6-diaminopyridine (DAP) were investigated. The π–A isotherm revealed that an L film of OTA requires two-dimensional expansion along with molecular recognition due to the tight packing nature of the headgroups of the film. However, such expansion was not observed in the case of OATA and DDTA, and this may be due to conformational relaxation based on ionic bonding between the headgroup and the alkyl chain in the OATA, and due to sufficient space around the headgoup of the DDTA. In addition, molecular recognition, removal and re-recognition properties were investigated for the LB films of the amphiphils. Although, any LB film deposited from a pure water subphase did not exhibit recognition ability at room temperature, the OTA LB film deposited from a guest molecule aqueous solution followed by removing the embedded guest molecule from the film was found to recognize the corresponding guest molecule. Therefore, it is reasonable to conclude that both sufficient space for migration and location of the guest molecules in the film and dynamic stability of the film versus the recognition and removal process should be required for fabricating a chemical sensing device based on reversible molecular recognition and removal.