  Electromagnetic embedded eigenstates, also known as bound states in the
continuum (BICs), hold a great potential for applications in sensing, lasing,
enhanced nonlinearities and energy harvesting. However, their demonstrations so
far have been limited to large-area periodic arrays of suitably tailored
elements, with fundamental restrictions on the overall footprint and
performance in presence of inevitable disorder. In this work, we demonstrate a
BIC localized in a single subwavelength resonator obtained by suitably
tailoring the boundaries around it, enabling a new degree of control for
on-demand symmetry breaking. We experimentally demonstrate how boundary-induced
BICs open exciting opportunities for sensing by tracing the dissolution of NaCl
in water, determining evaporation rates of distilled and saltwater with a
resolution of less than 1e-6 L using a tabletop experimental setup.
