Interactions between Organized, Surface-Confined Monolayers and Vapor-Phase Probe Molecules. 11. Synthesis, Characterization, and Chemical Sensitivity of Self-Assembled Polydiacetylene/Calix[n]arene Bilayers

We report the synthesis and characterization of self-assembled polydiacetylene (PDA)/4-tert-butylcalix[n]arene (where n = 4 and 6, abbreviated as CA[4] and CA[6], respectively) bilayers on Au surfaces. The PDA/CA[n] bilayers act as chemically sensitive interfaces that are resistant to high thermal stresses and extreme electrochemical potentials, both of which may be encountered in technologically significant sensing applications. Moreover, these simply prepared bilayer materials are excellent protosystems for studying a range of important sensor-related issues. Here we discuss the linking chemistry that leads to calixarene immobilization and examine the role calixarenes play in vapor-phase sensing applications using surface acoustic wave (SAW) mass balances and Fourier transform infrared−external reflectance spectroscopy (FTIR-ERS). FTIR-ERS spectra indicate that CA[n] reacts with the acid chloride-terminated PDA self-assembled monolayers (SAMs) to form ester linkages. Using irreversible adsorption of vapor-phase n-butylamine onto the CA[n] bilayer-coated SAW devices as an indirect nanogravimetric probe of surface coverage, we calculate that CA[4] and CA[6] cover 58% and 61%, respectively, of the PDA surface. The irreversibly bound n-butylamine serves a secondary purpose by effectively filling voids between calixarenes, which hinders access to them by the vapor-phase molecules. Dosing the bilayer-coated SAW devices with VOCs after exposure to n-butylamine permits a better understanding of the role of the calixarene cavities relative to nonspecific adsorption at, for example, defect sites between CA[n] molecules. Additional control experiments involving PDA/CA[n] analogs, such as phenyl-terminated PDA, show that VOC adsorption is enhanced only on the CA[n] surfaces. This strongly suggests the presence of a specific interaction between the CA[n] cavities and VOCs rather than simple nonspecific adsorption.