We describe a new approach to fluorescence sensing which relies on visual determination
the polarization. The sensing device consists of a fluorescent probe, which changes
intensity in responses to the analyte, and an oriented fluorescent film, which is
not affected by the analyte. An emission filter is selected to observe the emission
from both the film and the sensing fluorophore. Changes in the probe intensity result
in changes in the polarization of the combined emission from the sensor and reference.
The degree of polarization can be detected visually using a dual polarizer with adjacent
sections oriented orthogonally to each other. The emission passing through the dual
polarizer is viewed with a second analyzing polarizer. This analyzer is rotated manually
to yield equal intensities from both sides of the dual polarizer. This approach was
used to measure the concentration of RhB in intralipid and to measure pH using 6-carboxyfluorescein.
The analyzer angle is typically accurate to 1 degree, providing pH values accurate
to +/- 0.1 pH unit at the midpoint of the titration curve. We also describe a method
of visual polarization sensing that does not require an oriented film and that can
use the same fluorophore for the sample and reference. These approaches to visual
sensing are generic and can be applied to a wide variety of analytes for which fluorescent
probes are available. Importantly, the devices are simple, with the only electronic
component being the light source.