Lock-in Based Fibre-Optic Fluorimetric Sensors for Water- and Airborne Analytes

Thumbnail Image
Journal Title
Journal ISSN
Volume Title
This thesis describes the modifications that made to an evanescent wave fibre optic transducer with a lock-in detection for colorimetry to be a fluorimetric transducer. This includes ‘side illumination’ as a superior excitation mode compared with the evanescent wave excitation of fluorescence. Specifically, side illumination results in a strong fluorescent signal and very low (negligible) exciting light coupling into the optical fibre, therefore, it serves mostly as a fluorescent signal source rather than noise. The modified transducer is designed to be versatile; easily adapted for either waterborne or airborne analyte sensing. In addition, it can be also used to detect changes in fluorescence from either a sensitiser film or solution and can be applied for sensing applications even when the sensitiser degrades under the exciting light. Moreover, the transducer does not require a fluorescence spectrometer. As a test of the transducer and to prove its versatility, it was applied in airborne and waterborne sensing, with minimal adaption between these media. This was done by exposing the spray-coated stripped section of the glass optical fibres with polymer poly(phenylene-vinylene) derivative poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as a fluorophore, to the nitro-aromatic explosive dinitrotoluene (2,4-DNT) as an analyte, in both air and water media. Limit of detections (LoDs) of 0.48 ppb and 56 ppb were obtained for airborne and waterborne DNT respectively. These LoDs are ~ 62 times below previously reported values for DNT detection using PPV derivatives, as well as 3.3 times below LoDs for DNT sensing with different sensitisers. A pyrazolyl chromene derivative known as Probe 1 was successfully used as the fluorescent sensitiser in a solid film form, thereby enabling its use for the detection of waterborne Cu2+ and thus avoiding the exposure to harmful solvent during sensing experiments. Specifically, a Probe 1 film was prepared via its immobilisation in a plasticised polyvinyl chloride (PVC) acting as a phase transfer membrane. It was then dip-coated onto the stripped section of an optical fibre before its exposure to a range of waterborne Cu2+ concentrations. The LoD of Cu2+ was 0.43 μM, below the potability limit, and is also 3.7 times lower than the LoD measured by a conventional spectrofluorimeter based on the same sensitiser. The good solubility and long-term stability of Morin derivative NaMSA known as an ‘off → on’ fluorescent sensitiser for Al3+ in water, enabled it to be used in aqueous media avoiding the need for phase transfer membrane preparation. The dissolved NaMSA was used in conjunction with our fibre optic transducer to detect Al3+ in drinking water below the potability limit. vi | P a g e Moreover, the concentration of Al3+ was reliably quantified in a range of samples by using the standard addition method. It is known that morin-based Al3+ cation sensors can be selectively recovered by being exposed to fluoride (F-) anions. This method was further developed to enable complementary sensing of either fluoride anions, or aluminium cations using the same dye in both cases and achieving sub-micromolar LoDs. The transducer demonstrates a high figures-of-merit compared with previous reports on both aqueous Al3+ cation and F- anion sensing.