Inverse opal hydrogels with adjustable band gaps tuned by polyethylene glycol

Inverse opal hydrogels of polyacrylamide (PAM) with adjustable band gaps (P-IOHspam) were fabricated by changing the molecular weight of polyethylene glycol (PEG), and the amount of PEG and N,N′-methylenebisarcylamide (BIS) in the monomer precursors. After the PEG was removed, mesopores were left in the PAM hydrogels and these caused changes in the band gap of the P-IOHspam. Compared with inverse opal hydrogels of PAM (IOHspam) that were prepared without PEG, the reflection peaks shifted to a longer wavelength which provides a wider usable visible wavelength range. P-IOHspam show rapid shifts in the reflection peak in response to chemicals, such as PEG, glycol, glucose and L-lysine. The shift of the reflection peak is greater for the P-IOHspam made from monomer precursors containing more PEG and for higher molecular weights of PEG. The shifts are caused by changes in two factors: the average refractive index of the P-IOHpam material and the degree of equilibrium swelling of the PAM hydrogel, both of which are sensitive to chemicals. Such sensitive materials could be used as chemical sensors.