Molecular specificity in photoacoustic microscopy by time-resolved transient absorption.

We have recently harnessed transient absorption, a resonant two-photon process, for ultrahigh resolution photoacoustic microscopy, achieving nearly an order of magnitude improvement in axial resolution. The axial resolution is optically constrained due to the two-photon process unlike traditional photoacoustic microscopy where the axial resolution is inversely proportional to the frequency bandwidth of the detector. As a resonant process, the arrival time of the two photons need not be instantaneous. Systematically recording the signal as a function of the delay between two pulses will result in the measurement of an exponential decay whose time constant is related to the molecular dynamics. This time constant, analogous to the fluorescence lifetime, but encompassing nonradiative decay as well, can be used to differentiate between molecular systems with overlapping absorption spectra. This is frequently the situation for closely related yet distinct molecules such as redox pairs. In order to enable the measure of the exponential decay, we have reconfigured our transient absorption ultrasonic microscopy (TAUM) system to incorporate two laser sources with precisely controlled pulse trains. The system was tested by measuring Rhodamine 6G, an efficient laser dye where the molecular dynamics are dominated by the fluorescence pathway. As expected, the measured exponential time constant or ground state recovery time, 3.3±0.7  ns, was similar to the well-known fluorescence lifetime, 4.11±0.05  ns. Oxy- and deoxy-hemoglobin are the quintessential pair whose relative concentration is related to the local blood oxygen saturation. We have measured the ground state recovery times of these two species in fully oxygenated and deoxygenated bovine whole blood to be 3.7±0.8  ns and 7.9±1.0  ns, respectively. Hence, even very closely related pairs of molecules may be differentiated with this technique.