Detection of radiological hazards in the solution phase using conventional means is considerably more
difficult than in the gas phase. A new approach is required to provide a reliable, specific and low cost
method of protecting sensitive national assets, such as water supplies, from a terrorist dirty bomb attack.
Fibre optic sensors provide the required speed of response, the optical platforms are mature and of
relatively low cost with proven reliability in the field. This paper describes the combination of a low cost
sensor platform and smart sensor molecule (Isoamethyrin) for the selective determination of uranyl and
other actinide species in water at sub ppm levels. Isoamethyrin is a synthetic porphyrin which has been
demonstrated to show high selectivity for uranyl ions with an associated colour change on complexation.
Fibre optic sensors are created by revealing an evanescent wave in a section of the fibre and covalently
bonding the isoamethyrin to the fibre surface in this region. Colour changes occurring as a result of
interaction between isoamethyrin and uranyl ions are monitored over 3 wavelength ranges covering the
red, green and blue regions of the visible spectrum. Sensors created in this manner were found to be fast
responding (<5s), sensitive (detection threshold <500ppb), specific (response restricted to certain
actinides and lanthanides) and low cost.
A fibre optic platform has been fabricated for the field deployment of evanescent wave cavity ring-down spectroscopy with an absorbance sensitivity of 5 ppm. An optical cavity is fabricated by depositing high-reflectivity mirrors onto each end of the fibre and the evanescent field is exposed to the sample in a tapered region of the cavity. The decay time, τ, is dominated by the propagation loss of the radiation in the fibre optic and the loss of the tapered region. The multi-pass configuration can detect molecules adsorbing to the surface of the tapered region if they absorb radiation at the wavelength of the laser. An indicator molecule has been tethered to the glass surface to produce a colour change in response to the bulk pH producing an optical pH sensor with a sensitivity of 0.01 pH units. The fibre cavities have potential to form an optical sensor network to detect target molecules with presumptive detection on functionalised fibre surfaces.
Evanescent wave cavity ring-down spectroscopy (EW-CRDS) is used to observe the adsorption isotherm for hemoglobin (Hb) from controlled urine samples to assess the potential for rapid diagnosis in hemoglobinuria. The absorbance of Hb at 425 nm is monitored using an alexandrite laser-pumped, room temperature, LiF:F color-center pulsed laser. A minimum absorbance detection level of 2.57×10–4 is achieved, corresponding to a minimum detectable concentration of Hb in urea of 5.8 nM. A multilayered Hb biofilm is formed, and a minimum of eight layers are required to model the adsorption isotherm, allowing for cooperative binding within the layers and extending 56 nm into the interface. A binding constant for Hb to silica 18.23±7.58×106 M is derived, and a binding constant for Hb to Hb in subsequent layers is determined to be 5.631±0.432×105 M. Stoichiometric binding coefficients of 1.530±0.981 for layer one and 1.792±0.162 for subsequent layers suggest that cooperative binding both to the silica surface and between the layers of the biofilm is important.
A novel sensor for detecting very low concentrations of chemicals in water and other liquids is presented. A cavity ring-down spectrometer has been developed that can measure chemicals in solution in a harsh environment. The high Q Fabry Perot cavity is fabricated in an optical fibre with high reflectivity mirrors on each end. The cavity contains a fused fibre taper, with very low intrinsic loss, for coupling light in the cavity evanescently into a smart surface layer that is bound on to the fibre surface. Small changes in the absorption are detected by changes in the ring-down time of the resonant cavity. The low loss cavity results in ring down times of 1μs for a 2 m cavity, which is equivalent to 100 passes through the smart surface. The ring down time provides a very accurate measure of absorbance because it is independent of source and detector drift and the fibre cavity is unaffected by changes in temperature, vibration or bending. Absorption changes of 5x10<sup>-5</sup> dB can be detected with the current configuration and further improvements can be achieved by optimisation leading to detection of atto-molar chemical concentrations.