“…So far, various optical [ 5 , 6 , 7 , 8 , 9 , 10 , 11 ], colorimetric [ 12 , 13 ], and electrochemical [ 14 , 15 ] sensors have been developed for detecting glucose. Among these, the optical sensors based on photonic crystals (PCs) have received considerable attention because they can detect target analytes with the naked eye, without sophisticated equipment and professional operators [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. In particular, with the composites of hydrogel and PCs as sensors, interactions between the hydrogel matrix and an analyte result in the volumetric change of the hydrogel, leading to a change of periodicity of the embedded PCs.…”