Optical systems and methods of probing biological fluids

A method for determining a viscosity of a fluid medium containing a fluorescent species capable of emitting a fluorescent signal, the method comprising:

• directing a light source at the fluid medium, the light source providing radiation of a fixed intensity and having an excitation geometry when directed at the fluid medium, to excite the fluorescent species at an excitation area of the fluid medium to produce the fluorescent signal and photobleach the fluorescent species, the fluorescent species diffusing in and out of the excitation area of the fluid medium at a rate determined by the excitation geometry of the light source, a diffusivity of the fluorescent species, and the viscosity of the fluid medium;
• detecting the fluorescent signal with a detector over time to generate a fluorescence decay curve of the fluorescent species in the fluid medium;
• and
• determining the viscosity of the fluid medium based on a numerical comparison of the fluorescence decay curve of the fluorescent species in the fluid medium with a fluorescence decay curve of the fluorescent species in a reference medium of known viscosity.

A system for determining a viscosity of a fluid medium containing a fluorescent species capable of emitting a fluorescent signal, the system consisting of:

• a light source providing radiation of a fixed intensity and having an excitation geometry when directed at the fluid medium, the light source capable of exciting the fluorescent species at an excitation area of the fluid medium to produce the fluorescent signal and photobleaching the fluorescent species, and the fluorescent species diffusing in and out of the excitation area of the fluid medium at a rate determined by the excitation geometry of the light source, a diffusivity of the fluorescent species, and the viscosity of the fluid medium;
• a detector capable of detecting the fluorescent signal over time to generate a fluorescence decay curve of the fluorescent species in the fluid medium;
• and
• optionally, one or more of:
• a filter utilized in connection with the detector;
• a beam splitter disposed such that both the radiation of the light source and the fluorescent signal pass through the beam splitter, the beam splitter directing the fluorescent signal to the detector;
• and
• one or more lenses disposed on an optical pathway with the light source or the detector,
• wherein the viscosity of the fluid medium is determined based on a numerical comparison of the fluorescence decay curve of the fluorescent species in the fluid medium with a fluorescence decay curve of the fluorescent species in a reference medium of known viscosity.