Molecularly imprinted photonic crystals for the direct label-free distinguishing of L-proline and D-proline

Novel molecularly imprinted photonic crystals (IPCs) for the highly sensitive label-free detection of L-proline and for the chiral recognition of L/D-proline were reported. A series of L-proline imprinted polyacrylamide photonic crystals (PAM-LPIPCs) and poly(acrylamide-co-acrylic acid) photonic crystals (PAM-co-AA-LPIPCs) were fabricated via the in situ polymerization of polystyrene opal. The PAM-LPIPCs exhibit good molecular response in L-proline solutions and can be visualized by the naked eye much like a pH test paper. The concentration of imprinted molecules (L-proline) in aqueous solution can be detected by the chromatic signal (structural color) or the optical signal (λmax). Furthermore, the responsivity and sensitivity of the PAM-co-AA-LPIPCs can be improved by increasing the amount of the imprinted content or the proportion of AA, or by decreasing the ratio of the cross-linking agent. When all these factors were balanced, a PAM2-co-AA0.4-LP0.5 IPC with good strength, high responsivity, high sensitivity and specific molecular recognition was obtained. It is found that the presented crystals can show obvious response to L-proline solution even at a low concentration of 1%. The PAM2-co-AA0.4-LP0.5 IPC not only very selectively distinguishes between L-proline and nicotinic acid, but it is also good at chiral recognition between L-proline and D-proline. What is more, the response is rapid and reversible and the IPC is recyclable.