Structural study of meso-octaethylcalix[4]pyrrole Langmuir–Blodgett films used as gas sensors

Abstract

Monolayers of meso-octaethylcalix[4]pyrrole (MOP) have been prepared at the air/water interface by the Langmuir–Blodgett (LB) technique. LB isotherms indicate that during the barrier compression the MOP monolayer slowly goes towards a closely packed system consisting of molecules whose molecular area is consistent with a calix-like shape. A significant molecular area loss, suggesting the occurrence of self-assembling phenomena, has been observed by: (a) increasing the molecular concentration; (b) performing cyclic compression–decompression curves; (c) recording constant-pressure relaxation curves. By means of the LB dipping method, MOP thin films have been built-up by transferring four and eight layers on solid surfaces. Model-flat silicon oxide and nanostructured silver surfaces, both hydrophobized by three LB layers of cadmium arachidate, have been used as substrates. Scanning force microscopy (SFM) images performed on these systems showed surfaces constituted by the ensemble of MOP ellipsoidal aggregates tens of nanometers in size. The films transferred on silver have been found to be very useful for the molecular sensing of alcohol vapours by using the surface plasmon resonance (SPR) spectroscopy as transduction method. The film structure has been correlated to the SPR performance.