Functional solid-state photonic droplets with interpenetrating polymer network and their applications to biosensors

Complete solid-state cholesteric liquid crystal (CLC_solid) balls with an semi interpenetrating polymer network (IPN) structure (CLC_solid-IPN) were fabricated using uniformly sized CLC_solid balls intertwined with poly(acrylic acid) (PAA). The CLC_solid balls were produced from a reactive mesogen mixed with a nonreactive chiral dopant using a microfluidic device after ultraviolet curing and dopant extraction. The CLC_solid-IPN balls were prepared by allowing AA monomers to infiltrate the space remaining after dopant removal. The CLC_solid balls could be applied to a small strain sensor because they exhibited excellent elastic properties which led to the reflected color change with the magnitude of the blueshift proportional to the applied strain. The CLC_solid-IPN balls showed pH responsive color change, which property could be applied to a small pH sensor. The urease-immobilized CLC_solid-IPN (CLC_{solid-IPN-urease}) balls could be applied to a small urea sensor after urease immobilization. The enzymatic reaction with urea redshifted the reflected color of the CLC_{solid-IPN-urease} balls because the increase in local pH induced by the enzymatic reaction led to swelling of the intertwined PAA. The highly swollen KOH-treated CLC_solid-IPN (CLC_{solid-IPN-KOH}) balls were used to sense divalent metal ions. The replacement of K⁺ ions with divalent metal ions (M) led to a blueshift in the reflected color due to the bridged −COO–M−COO– structure. Thus, the proposed photonic CLC_solid-IPN balls, which are not affected by the viewing angle, represent a potential new sensor platform that can offer simple functionalization, requires only small test samples and does not need sophisticated analytical instruments.