Fibre-optic sensors with molecular coatings

摘要

The intrinsic stability of fibre optic based sensing systems offer a platform that is suited tohazardous waste detection in a wide range of environments. Over the last few yearsCranfield University has been working on the development of chemical sensors using opticalfibres in combination with a group of chemical recognition molecules called calixarenes.Calixarenes semi-selectively with a range of solvents of interest makes them useful forchemical detection systems. This work has primarily been focused on the use of calixarenesin sensing benzene and other hazardous solvents. However, this approach could potentiallybe 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 atCranfield and explore approaches to improve and extend the performance of the sensorsystem. 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 experimentalerror associated with the use of dry Wilhelmy plates to monitor the surface pressure of theLangmuir film. Dry filter paper plates take up to 2 hours to give stable data, with a drift of upto 10% in the measured surface pressure. It is shown that this problem can be avoided byusing pre-soaked plates. To provide an alternative to the Wilhelmy plate surface pressuresenor, an optical fibre surface pressure sensor was developed, measuring changes in themeniscus forming properties of a liquid. The sensor consists of a tapered single mode silicafibre, mounted with a small curvature and positioned with the tapered region of the fibreimmersed in the water. The performance of the fibre optic sensor is comparable with that ofthe conventional Wilhelmy plate surface pressure sensor showing linearity of greater than0.9.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 uponcompression measured. Long chain-substituted resorcinarenes gave more stablemonolayers than their short chain analogues. The incorporation of long chain surfactants ledto large increases in surface area, demonstrating that both resorcinarenes and surfactantsare located at the water surface, except for one system where a bilayer structure ispotentially formed. Further work on the behavior of the materials involved the alteration of thedipole-dipole interaction of the monolayer materials with the subphase. The modification ofthis interaction through the introduction of dipole altering additives, including alcohols andhydrogen peroxide, to the aqueous subphase was investigated. The resulting isotherms ofthe materials showed a reduction in the surface pressure and area per molecule required inorder for the monolayer to reach its point of collapse. This ability to shift the point of collapsehas application in the optimisation of Langmuir-Blodgett coating of surfaces.Within this project the sensing properties of a fibre sensor were also modelled extensively inorder to determine the theoretical sensing limits of a fibre optic vapour sensor. The modelshowed that the sensing goals of 1ppm originally envisaged for this project wereunobtainable 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 watershow how significantly more sensitive the system is to toluene contamination in water than itis to toluene vapour. These results demonstrate the utility of the developed system for manypollutant-sensing applications, include crude oil detection.