If you are a fan of Mission Impossible, you will remember that Jim
Phelps (played by Peter Graves) received orders on a tape that
self-destructs in a puff of smoke.  In the real world, a material which
could store information for a prescribed period of time is also very
desirable in secure communications applications.

Researchers from Northwestern University have recently created a class of
self-erasable and rewritable materials in which information is written via
“inks.”  These inks are gold or silver nanoparticles coated with mixed
self-assembled monolayers (SAMs) of dodecylamine and photoswitchable
azobenzene–terminated thiol 4-(11-mercaptoundecanoxy)azobenzene (MUA),
embedded in thin flexible organogel films.  Upon ultraviolet irradiation,
the azobenzene groups of MUA undergo a rapid trans-to-cis isomerization,
which causes a significant increase in the dipole moment leading to
aggregation of metal NPs into metastable clusters.

Due to the corresponding redshift of surface plasmon resonance band, the
colors of the nanoparticles change; multiple colors can be achieved
according to the degree of aggregation.  These metastable clusters
disintegrate in the absence of UV light.  The “write” and “erase” time can
be tuned by controlling the intensity of light, fractional surface
coverage of the MUA ligands, and heating.  After “erased,” the information
could be rewritten multiple times without material fatigue.  To make the
materials environmentally friendly, the authors are seeking an alternative
water–based material such that the organogel support medium is no longer
needed.