There is a need for chemical sensors for monitoring volatile organic compounds (VOCs) in air. Acute and chronic inhalation of toxic VOCs can cause adverse health effects in humans, so monitoring these analytes is important for ensuring that their concentrations are maintained below maximum permissible levels. Chemical sensors using polydimethylsiloxane (PDMS) to extract VOCs with partial selectivity, coupled with label-free optical detection methods based on refractive index, can overcome the limitations of conventional VOC detection methods. A variety of tunable and high refractive index PDMS materials were developed by incorporating a range of titanium and zirconium concentrations (2.5 – 30 mol % and 2.5 – 15 mol %, respectively) using a simple sol-gel synthesis and by incorporating a range of titanium concentrations (2.5 – 10 mol %) into naphthyl-functionalized PDMS. These materials ranged in refractive index from 1.4023 ± 0.0002 to 1.5663 ± 0.0001 at 635 nm and 1.3942 ± 0.0003 to 1.5510 ± 0.0007 at 1550 nm. The ability to use tunable refractive index PDMS films to differentiate between m-xylene and cyclohexane was demonstrated by monitoring changes in refractive index and thickness following absorption of these analytes using a refractometer at 1550 nm. The sensitivity of the refractive index response to an analyte using a particular PDMS film was dependent upon the difference between the refractive index of the analyte and film, as well as the film-air partition coefficient of the analyte. The detection limits for m-xylene and cyclohexane were 81 ppm and 4940 ppm, respectively, using PDMS-titanium-oxo nanocomposites with 5 and 10 mol % Ti, respectively. A simple planar waveguide sensor with an input grating coupler was developed to monitor changes in refractive index of the cladding through shifts in peak resonance wavelength. Using high refractive index PDMS materials as the waveguide core, we monitored changes in refractive index arising from absorption of VOCs into the grating. Here, the sensitivity of the waveguide response was dependent upon the difference in refractive index of the analyte and polymer, as well as the film-air partition coefficient of the analyte. The detection limits for m-xylene and cyclohexane were 1980 ppm and 18000 ppm, respectively. / Thesis (Master, Chemistry) -- Queen's University, 2013-08-24 11:45:57.642

Chemical detection, including analysis of gases and liquids, is a large field in environmental research and industry. It requires sensitive, rapid, and inexpensive chemical sensors. Many industrial materials such as coatings and adhesives readily absorb chemical analytes, which may result in changes of their chemical, mechanical, and optical properties. This uptake of volatile organic compounds either from the gas phase or from an aqueous solution into a thin film is frequently accompanied by a change in material refractive index and film thickness. While the undesired swelling of thin film coatings and their refractive index changes affect their use in harsh environments, the sensitivity of some polymers to solvent vapours can also be exploited for sensing applications. In this project, a method is reported for real-time monitoring of vapour uptake by simultaneous detection of the refractive index, n, and thickness, d, of thin transparent films with a precision of 10-4 for refractive index and 100 nm for thickness. The setup combines a total internal reflection refractometer with an interferometric imaging method. Two setups using 1550 nm and 635 nm measurement wavelengths were developed, with a detection rate of 1 second per measurement. Two processing methods using a fast Fourier transform algorithm to calculate n and d are applied to the experimental results and compared. Both methods could extract n and d simultaneously from each image captured by the refractometer. The results show that the setup is capable of monitoring film RI and thickness change in real-time. The partitioning of volatile organic compound vapours into polydimethylsiloxane (PDMS) and PDMS-polydiphenylsiloxane (PDPS) copolymers is described. The system is also suited for characterization of other solid and liquid films like SU-8 photoresist and crude oil. It shows great potential in commercial applications of thin film characterization. / Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2013-09-19 22:21:38.836