Laser-induced fluorescence studies on collagen, cholesterol, and chlorophyll a

Results on laser induced fluorescence (LIF) properties of collagen, cholesterol and chlorophyll a (chl a) when excited at 337.1 nm are reported. The collagen and cholesterol fluorescence was studied in connection with atherosclerotic plaques diagnostics and/or removal within the cardiovascular system. The fluorescence signal excitation and collection were made using optical fibers, for liquid samples at concentrations between 0.1 mg/ml and 5.0 mg/ml for both collagen and cholesterol, their fluorescence lifetime being 5 nsec and 6 nsec, respectively. Chl a LIF concerned the stimulated emission of radiation considering the pigment as laser active medium. Chl a concentrations varied, in different solvents between 5 multiplied by 10 ^-3 M/l and 10 ^-5 M/l. For chl a at 0.62 mM/l and pumping photon densities higher than 5 multiplied by 10 ¹⁶ photons/cm ² and pulse, fluorescence linewidth narrowing from 28 nm to 5 nm in ethanol and from 25 nm to 4 nm in DMSO were obtained. Lasing maxima at 672 nm in acetone, 674 nm in ethanol and 678 nm in DMSO were observed. At lasing conditions for chl a in DMSO (1 mM/l) the measured fluorescence lifetime was 2 nsec instead of 5 nsec for normal fluorescence. The results on chl a stimulated emission show a lower lasing threshold at 10 ¹⁶ photons/cm ² and pulse, than previously reported, if DMSO solvent is used.