Abstract:
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.