Measuring non-radiative relaxation time of fluorophores with biomedical applications by intensity-modulated laser-induced photoacoustic effect.

Modulated tone-burst light was employed to measure non-radiative relaxation time of fluorophores with biomedical importance through photoacoustic effect. Non-radiative relaxation time was estimated through the frequency dependence of photoacoustic signal amplitude. Experiments were performed on solutions of new indocyanine green (IR-820), which is a near infrared dye and has biomedical applications, in two different solvents (water and dimethyl sulfoxide (DMSO)). A 1.5 times slower non-radiative relaxation for the solution of dye in DMSO was observed comparing with the aqueous solution. This result agrees well with general finding that non-radiative relaxation of molecules in triplet state depends on viscosity of solvents in which they are dissolved. Measurements of the non-radiative relaxation time can be used as a new source of contrast mechanism in photoacoustic imaging technique. The proposed method has potential applications such as imaging tissue oxygenation and mapping of other chemophysical differences in microenvironment of exogenous biomarkers.