The synthesis procedure shown here was adapted by Katie Cadwell, Steve Ng and Ken Gentry from
work by Kun-Lin Yang, Katie Cadwell, and Nicholas Abbott, J. Phys. Chem.
B, 108,20180-20186 (2005).
A liquid crystal applied over a dilute copper perchlorate solution dried on a glass slide responds to dimethyl methylphosphonate.
Dissolve 0.18g copper(II) perchlorate in 100 mL ethanol to give a
5 mM solution.
Clean the cut glass slides in ethanol.
Soak the clean slides in the copper perchlorate solution for several minutes.
Let the slides dry without touching them.
Cut small pieces of plastic wrap.
Sandwich the plastic wrap at both ends between the two slides. Use
tweezers and do not touch the center faces of the slides. Clamp both ends
of the slides with binder clips. The clips should clamp over the plastic
wrap leaving an empty volume between the slides.
Load the capillary tube with a small amount of liquid crystal.
Warm liquid crystal and a capillary tube with a blow dryer and transfer
liquid crystal to the space between plates.
Remove handles from binder clips.
Obtain a petri dish with a polarizing filter attached to each plate.
Put the holder in the petri dish. Rotate the cover to get maximum
darkness (polarizers oriented at 90° with respect to each other.)
The liquid crystal is in a homeotropic orientation because it is bound
to the copper salts on the slides. The liquid crystal will not bend the
light in this orientation and the crossed polarizers prevent light from
exiting the Petri dish.
Add a drop of dimethyl methylphosphonate and immediately replace the cover.
A light band will appear at the edges of the sensor.
The dimethyl methylphosphonate has bound to the copper salt on the slide,
displacing the liquid crystal. The liquid crystal is free to take a planar
orientation, which bends the light. (The movie shows 5 seconds out of every
minute for a total elapsed time of 20 minutes. Drag the slider bar
to make the change more noticeable.)
A black line will appear at the edge of the sensor after several
minutes. The dimethyl methylphosphonate concentration has increased in
the sensor and the LC is now isotropic. It will not bend light in this
orientation. (The move shows 5 seconds out of every two minutes for a
total elapsed time of an additional 10 minutes. Drag the slider bar to
make the change more noticeable.)
