Self-assembled monolayers of three straight-chain alkanethiols [CH,(CH,),SH, n = 11, 15, 171 have been formed on a gold substrate from cyclohexane solution and characterised using IR spectroscopy. Over a period of ca. six months, the relative intensities of the C-H stretching bands in the IR spectra were observed to change. These changes are explained by a tilting of the molecules from their initial position away from the surface normal upon oxidation of the thiolate root. This temporal dependence of the tilt angle has implications for the stability of self-assembled monolayers.

Novel phthalocyanine (Pc) molecules have been designed and synthesized to incorporate a thiol moiety for the formation of monolayer films onto gold-coated optical waveguides via the technique of self-assembly. The thiol moiety is connected to the Pc macrocycle by either a (CH2)11 or (CH2)3 hydrocarbon chain. Both Pc SAMs have been characterized on the metal surface using reflection absorption infrared (RAIR) spectroscopy. The RAIR spectra suggest that the two Pc molecules lie in a different orientation with respect to the gold surface. This behavior can be rationalized with consideration of the different structures (the different alkyl connecting chain lengths and different macrocyclic peripheral side chains) of the two Pc molecules. The fluorescence emission spectrum of each of the Pc SAMs has been obtained by exciting each monolayer along the longitudinal axis of the optical waveguide via laser-induced evanescent wave stimulation. The use of the longer mercaptoalkyl connecting chain appears to inhibit quenching of the electronically excited state through energy transfer to the metal layer. It is thought that the presence of peripheral alkyl side chains (six hexyl or seven decyl chains, respectively) minimizes self-quenching in the single-component SAM film. It has also been shown that the fluorescence signal of the (CH2)11 mercaptoalkyl Pc SAM can be reversibly quenched by exposure to gaseous nitrogen dioxide (NO2). The decrease in fluorescence intensity has been related to increasing NO2 concentration by a Stern−Volmer relationship. The Pc SAM can detect concentrations of NO2 down to 10 ppmv with no interferent effect seen from other environmentally relevant gases:  carbon monoxide and carbon dioxide. These results suggest that SAMs of specifically designed materials, such as Pc's, may be used as sensitive receptor molecules to form the basis of a novel technology for the development of optical chemical sensors for gaseous species.

A diphthalocyanine disulfide (Pc) molecule has been deposited as a monolayer on gold-coated substrates through the process of self-assembly. To establish the molecular orientation of the Pc molecule on the gold surface the two complementary techniques of transmission IR and reflection absorption IR (RAIR) spectroscopies were used. The appearance of IR absorption bands associated with the Pc nucleus in the transmission spectrum, and their absence in the RAIR spectrum, suggests that the Pc self-assembled monolayer (SAM) is orientated with the macrocycle parallel to the metal surface. The Pc SAM has been used in conjunction with surface plasmon resonance (SPR) to establish the utility of combining these techniques for optical gas sensing. The SPR reflectivity curves for the gold substrate and the Pc SAM on the gold substrate have been obtained. On exposure of the Pc SAM to the environmentally important NO2 gas, changes of the reflectivity signal were obtained in proportion to the concentration of the analyte gas. The results obtained show that the monolayer deposition technique of self-assembly is an ideal method for the production of chemically sensitive substrates which can be combined with surface plasmon resonance for the optical sensing of gaseous species.

No abstract