A molecularly imprinted colloidal array as a colorimetric sensor for label-free detection of p-nitrophenol

We report on the synthesis of a label-free p-nitrophenol (PNP) responsive crystalline colloidal array (CCA) based on the combination of a photonic crystal and the molecular imprinting technique. This novel sensing material was prepared by a self-assembly approach using PNP imprinted colloidal spheres and was characterized by a three-dimensional (3D) ordered opal structure in which numerous recognition sites were created during imprinting. The PNP recognition swelled the colloidal spheres, leading to a red shift of the diffraction wavelength of the CCA due to the lattice spacing change and the effective diffractive index change. The relationship between the diffraction wavelength of the CCA and the size of the colloidal spheres were studied and the size of the molecularly imprinted colloidal spheres was optimized to 200 (±5) nm by adjusting the recipe composition during the emulsifier-free emulsion polymerization. As a result, color change due to the diffraction light shift which is related to PNP concentrations can be observed. The results showed that the diffraction wavelength of the molecularly imprinted colloidal array (MICA) red-shifted more than 50 nm in response to 30 mM of PNP with a detection limit of 1 mM PNP. The color change of the MICA from green to red was observed. The imprinting efficiency of the molecular imprinting, the effect of buffer pH and the selectivity were also investigated. We achieved a facile colorimetric detection method for PNP without sample treatment.