Photochemical kinetics and fluorescence spectroscopy in photonic crystal fibres

Abstract

This thesis describes work carried out to demonstrate the use of photonic crystal fibres for the study of photochemistry reaction kinetics and fluorescence spectroscopy. Photonic crystal fibre allows the guidance of light, in a well-defined mode, over long path lengths. When the fibre’s microstructure is filled with a sample solution this, therefore, provides a greatly increased measurement path length and greater light-sample interaction than is possible in conventional spectroscopic systems, leading to enhanced sensitivity whilst greatly reducing the required sample volumes.

The use of photonic crystal fibre as a micro reaction chamber for carrying out photochemical reactions and the study of their kinetics was achieved through monitoring the photoisomerisation of two azobenzene-based dyes, Disperse Red 1 and Disperse Orange 1, using real-time UV/Vis absorption spectroscopy. Both the 488 nm excitation laser and the broadband light source for the measurements were co-coupled through the fibre, giving perfect overlap of both with the sample. The fibre used for the measurements was a hollow core kagomé-type fibre with a core diameter of 19μm, giving a sample volume of 2.8 nL cm⁻¹. The 30 cm path-length of the fibre allowed the use of sample concentrations down to 5×10⁻⁶ M, over an order of magnitude lower than in a conventional 1cm cuvette, with a sample volume of 90 nl in the core, a reduction of five orders of magnitude over conventional measurements.

The kinetics of the photoisomerisation from the trans to the cis isomers of the dyes and the thermally driven cis-to-trans isomerisation could be tracked on the ms timescale, using a grating spectrometer which recorded the entire absorption spectrum of the dye. The data were numerically fitted using a custom model to take into account the properties of the fibre system. This led to the calculation of rate constants for the isomerisation processes in good agreement with those previously measured for these dye systems in bulk solution. Furthermore, the measurement of the dyes in pentane, in which they are highly insoluble, could be achieved due to the low concentrations that could be used; such measurements have not previously been reported.

For the study of photonic crystal fibre as a system for the excitation and collection of fluorescence, two types of fibre were used; the same kagomé hollow-core fibre used for the photochemistry absorption measurements and a suspended-core “Mercedes” fibre. This allowed for the excitation of fluorophores in two contrasting environments. In the kagomé fibre fluorophores in bulk solution are excited whilst, in the Mercedes fibre, only fluorophores either on or in close proximity to the silica core interact with the evanescent field of the excitation light. The Fluorescein fluorophore was used initially to measure the detection limits in both fibre types and limits of 2x10⁻¹¹ M in the kagomé and 10⁻⁹ M in the Mercedes fibre were obtained. This equates to 10⁶ molecules in the kagomé fibre, which displays the lower detection limit due to greater light-sample interaction. Two-photon excitation of the Fluorescein fluorophore was then carried out using a mode-locked Ti-Sapphire laser as an excitation source, demonstrating the ability of the fibre system to sustain two-photon excitation of a long (30 cm) path length. The two-photon measurements showed remarkable detection sensitivity allowing detection of fluorescence from 10⁻⁹ M solutions of Fluorescein, showing the potential of using PCF for two-photon based experiments which are of particular interest in fields such as photodynamic therapy.

A further study was carried out, using the two fibre types, for measurement of the fluorescence lifetime of the Rhodamine B fluorophore. Unperturbed lifetimes could be measured in the fibres showing no interference from the fibre. The measurements confirmed, in reference to known lifetime values, that in the kagomé fibre the excited fluorophores are in the bulk solution with only a minor influence from surface effects, whilst in the Mercedes fibre all of the excited molecules experience interaction with the surface of the silica core. This, therefore, gives a method of locating the fluorophores with respect to the fibre surface and the ability to choose between measurement of bulk solution and long path-length evanescent field-induced fluorescence.