Development and application of compact, low-cost multispectral time-resolved fluorometric fibre-optic probes for in vivo diagnosis and study of disease

Introduction Fluorescence is a natural molecular phenomenon that can be described by the absorption of light by a molecule at one wavelength followed by the emission of light at a longer wavelength [1], [2]. The fluorescence can be characterised by its excitation and emission spectra, quantum yield, polarisation state of the emission and also by the characteristic fluorescence decay following an infinitely short δtype excitation, known as lifetime [3]. Particularly, the fluorescence lifetime (FL) can be sensitive to any energy transfer between an excited molecule and its environment. Hence, since the lifetime is independent of the concentration of the molecule, it represents a direct approach to quantify factors that affect the fluorophore’s environment, such as temperature [4], viscosity [5] or pH [6]. Biological tissue contains a number of molecules that exhibit naturally occurring fluorescence, commonly referred to as autofluorescence. Autofluorescence lifetime (AFL) measurements are used to characterize tissue components and provide label-free contrast and information about structural and metabolic state of tissue without the need for the application of exogenous fluorescent labels and the associated concerns of toxicity and pharmacokinetics. Up to now, AFL measurements have been widely undertaken in cuvette based instruments, partly due to instrumentation limitations and to the complex nature of the fluorescence signal. However, the growing clinical interest has stimulated the development of new FL instrumentation [7], [8], [10].