Fibre-optic sensors with molecular coatings

The intrinsic stability of fibre optic based sensing systems offer a platform that is suited to hazardous waste detection in a wide range of environments. Over the last few years Cranfield University has been working on the development of chemical sensors using optical fibres in combination with a group of chemical recognition molecules called calixarenes. Calixarenes semi-selectively with a range of solvents of interest makes them useful for chemical detection systems. This work has primarily been focused on the use of calixarenes in sensing benzene and other hazardous solvents. However, this approach could potentially be expanded for use in a wide range of chemical and even biological recognition systems.

The initial aim of this project was to build on the previous work in fibre optic sensing at Cranfield and explore approaches to improve and extend the performance of the sensor system. The project first focused on improving the techniques used in the Langmuir-Blodgett (LB) deposition of calixarenes. Initial studies in this area highlighted one critical experimental error associated with the use of dry Wilhelmy plates to monitor the surface pressure of the Langmuir film. Dry filter paper plates take up to 2 hours to give stable data, with a drift of up to 10% in the measured surface pressure. It is shown that this problem can be avoided by using pre-soaked plates. To provide an alternative to the Wilhelmy plate surface pressure senor, an optical fibre surface pressure sensor was developed, measuring changes in the meniscus forming properties of a liquid. The sensor consists of a tapered single mode silica fibre, mounted with a small curvature and positioned with the tapered region of the fibre immersed in the water. The performance of the fibre optic sensor is comparable with that of the conventional Wilhelmy plate surface pressure sensor showing linearity of greater than 0.9.

Following the analysis of the experimental systems used in the construction of the sensors, the project then focused on the chemistry of the materials and their suitability for LB coating. A variety of these materials were spread as Langmuir monolayers and their behavior upon compression measured. Long chain-substituted resorcinarenes gave more stable monolayers than their short chain analogues. The incorporation of long chain surfactants led to large increases in surface area, demonstrating that both resorcinarenes and surfactants are located at the water surface, except for one system where a bilayer structure is potentially formed. Further work on the behavior of the materials involved the alteration of the dipole-dipole interaction of the monolayer materials with the subphase. The modification of this interaction through the introduction of dipole altering additives, including alcohols and hydrogen peroxide, to the aqueous subphase was investigated. The resulting isotherms of the materials showed a reduction in the surface pressure and area per molecule required in order for the monolayer to reach its point of collapse. This ability to shift the point of collapse has application in the optimisation of Langmuir-Blodgett coating of surfaces.

Within this project the sensing properties of a fibre sensor were also modelled extensively in order to determine the theoretical sensing limits of a fibre optic vapour sensor. The model showed that the sensing goals of 1ppm originally envisaged for this project were unobtainable due to the low number of gas molecules interacting with the sensor. However, this led to the proposal of a new application of the system in sensing contaminants in water, where the same limitations would not apply. The results for the sensor system tested in water show how significantly more sensitive the system is to toluene contamination in water than it is to toluene vapour. These results demonstrate the utility of the developed system for many pollutant-sensing applications, include crude oil detection.