Improving the label-free rapid semi-quantification of E.coli by AgNPs-decorated hydrogel inverse opal photonic crystals

Using photonic crystal materials for colorimetric sensing, the influences of structure and composition on the photonic bandgap (PBG) of materials are always vital to detection. In this work, the effects of pores in the structure of polyethylene diacrylate-based inverse opal photonic crystal (PEGDA-based IOPC) and the concentration of silver nanoparticles (AgNPs) used for decoration of the IOPC by chemical reaction on the shift of PBG center (λ_rp) have been studied for rapid semi-quantification of Escherichia coli (E. coli). We found that pores in the structure of PEGDA-based IOPC significantly increased the amount of AgNPs attached to this material compared with the structure created by the ordered self-assembly of spherical SiO₂ particles of the parent template, leading to a pronounced red-shift of λ_rp. At the concentration of 20 µg/L AgNPs (C_AgNPs) used for decoration, the shift of λ_rp reached 80 nm due to the increase in the average refractive index (n_aver.) of the material. It thus enabled us to observe changes in the color of the reflected light with the naked eye. In addition, since the increase in the concentration of E.coli (C_E.coli) also caused a red-shift of the λ_rp, the C_AgNPs, thus, could be reduced but still give a sufficiently strong shift of λ_rp to detect E.coli at a high level. The spectral position of the reflectance peak changed from green to red with increasing the C_E.coli from 50 cfu/mL to 10⁶ cfu/mL at the C_AgNPs = 10 µg/L. These results indicated the potential application of AgNPs decorated PEGDA-based IOPC in rapid semi-quantification of E.coli by the naked eye.