Probing third-order nonlinearity in serotonin: A Z-scan study.

Serotonin (5-hydroxytryptamine, 5-HT) is a crucial endogenous monoamine neurotransmitter that modulates neurotransmission, gastrointestinal motility, hemostasis, and cardiovascular integrity. There have been numerous attempts to study the biochemical and photophysical properties of serotonin to carry out its molecular imaging and quantitative estimation. Here, we investigate the properties of serotonin at physiological concentration and pH using a continuous wave (CW) laser excitation closed-aperture (CA) Z-scan technique. Serotonin is packaged at high concentration inside the acidic environment of vesicles, and upon release gets diluted at the release sites in a neutral pH environment. Our solution-based measurements indicate that serotonin showed negative refractive nonlinearity and positive absorptive nonlinearity at a neutral pH. However, in the acidic medium, it showed negative refractive nonlinearity and mostly negative absorptive nonlinearity. The effect of excitation laser power on the observed nonlinearity is also verified. We attribute the origin of the nonlinearity in serotonin to the thermal lensing effect. Our robust and straightforward strategy to probe the monoamine neurotransmitter properties will provide new avenues to investigate serotonergic processes.