Abstract
We have studied the controlled adsorption of molecules of volatile compounds on thin LangmuirBlodgett films of porphyrins deposited onto the surface of a quartz microbalance. The modification ofthe filmopticalanisotropy hasbeenmeasuredbyreflectanceanisotropy spectroscopy (RAS), and simultaneously, the amount of adsorbed mol- ecules has been monitored. Optical spectra measured after adsorption of single analytes are interpreted straightforwardly in terms of anisotropies related to the H-like and J-like aggregation modes of porphyrins. By an appropriate selection of analytes, we have then investigated the effect of the exposure to controlled vapor mixtures. The experimental RAS line shape variation with respect to the signal measured before exposure is the sum of the variations produced by single analytes. This shows that the identification of types and even quantities of gases and vapors in gas sensing experiments is possible, and the responses of the sensing layer to single analytes could be listed in a library.
Highlights
Chemical sensors are logically split in two main components: the sensing layer and the transducer device, necessary to convert the presence of molecules in the environment into a detectable signal
The optical anisotropy of the as grown LB film was almost insensitive to analytes: despite the evident variation of the simultaneously monitored quartz microbalance (QMB) signal, reflectance anisotropy spectroscopy (RAS) measured a nearly constant signal during exposure
The adsorption isotherms recorded by QMB before and after activation are identical: further adsorption processes are unaffected by activation
Summary
Chemical sensors are logically (and in many cases practically) split in two main components: the sensing layer and the transducer device, necessary to convert the presence of molecules in the environment into a detectable signal. Minimum readable signals are obtained by a multitude of individual sensing events: in a typical quartz microbalance about 1 pmol of adsorbed molecule is necessary to get a measurable frequency shift (typically about 1 Hz change with respect to a 20 MHz signal frequency). This produces a hiatus between the chemical design of sensing units and the effective use in a device. In the anisotropy signal read-out after exposure to mixture of analytes, the contributions of single analytes can be singled out
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