  In this work, a refractive index (RI) sensor with an effective integration of
colorimetric detection and optical sensing capabilities has been developed.
Colorimetric detection relies on the sensitivity of the structural color of
photonic crystal (PC) substrates to the changes in background RI, while the
optical sensing is performed by measuring the magnification abilities of the
dielectric microspheres, which depends on the position of the photonic nanojet.
Based on this concept, we have successfully assembled 35 {\mu}m-diameter barium
titanate glass microspheres, 4.9 {\mu}m-diameter polystyrene and silica
microsphere monolayers on 1D or 2D PC substrates to perform RI sensing in
various liquids. In addition, the developed RI sensor is highly compatible with
commercial optical microscopes and applicable for RI sensing in areas as small
as tens of square microns.
