  A new type of nontoxic low-cost sensor is reported here, whose
photoluminescence (PL) intensity is instantly responsive to the external strain
applied over a large range (up to 250% strain). Highly stretchable fluorescent
composites of carbon dots (CDs) and polyurethane (PU) are fabricated via a
scalable green chemistry method by conveniently dispersing CDs in the aqueous
solution of PU. It is discovered that, upon tensile deformation, the PL peak of
the CD-PU film remains non-shifted but displays varied intensity. The observed
PL responses to strain are ascribed to the enlarged inter-particle distance of
CDs along the tensile direction (z axis), although a higher degree of
aggregation is resulted in the other two axes. The PL-dependence on the
anisotropic patterns of CDs in solid state points to a new mechanism to
overcome aggregation-induced quenching by controlling the distribution
behaviors of the fluorescent species.
