A two-dimensional photonic hydrogel sensor driven by coordination competition for detecting histidine

A Cu²⁺-coordination crosslinked two-dimensional photonic crystal hydrogel (PN-2DPCH-Cu²⁺) was constructed, in which a polystyrene two-dimensional photonic crystal (PS-2DPC) was embedded in a Cu²⁺-crosslinked poly(acrylamide-co-acrylic acid-co₋N-vinyl imidazole) hydrogel network. The introduction of Cu²⁺ ions was mainly based on the coordination reaction of Cu²⁺ and N atoms of the imidazole rings on the hydrogel chains. The constructed PN-2DPCH-Cu²⁺ smart photonic hydrogel sensor achieved highly sensitive and simple detection of histidine (His) just using a laser pointer and a ruler. The coordination competition of Cu²⁺ to His and N atoms of the imidazole rings drove the hydrogel's response. In the presence of His, the PN-2DPCH-Cu²⁺ sensor swells obviously because the stronger coordination between Cu²⁺ ions and histidine compared to imidazole rings removed the coordination-crosslinked Cu²⁺ from the hydrogel, thus decreasing its crosslinking density. The swollen hydrogel increased the microsphere spacing of the embedded PS-2DPC, which can be acquired by measuring the Debye diffraction ring diameter of PS-2DPC. The microsphere spacing increments were linearly changed when the His concentrations were in the ranges 0.01-0.03 µM, 0.05-1 µM and 10-500 µM, and the limit of detection was 9.6 nM. The sensor showed high selectivity to His, good anti-interference and stability, and the response can be achieved in 20 min. The good recoveries and relative standard deviations from determining His contents in milk and beverage using our constructed PN-2DPCH-Cu²⁺ sensor demonstrated its accuracy and reliability. The coordination competition strategy of driving hydrogel to response can extend to develop other analytes sensitive hydrogel sensors by choosing suitable polymerizable monomers and metal ions.