A General Photonic Crystal Sensing Motif: Creatinine in Bodily Fluids

We developed a new sensing motif for the detection and quantification of creatinine, which is an important small molecule marker of renal dysfunction. This novel sensor motif is based on our intelligent polymerized crystalline colloidal array (IPCCA) materials, in which a three-dimensional crystalline colloidal array (CCA) of monodisperse, highly charged polystyrene latex particles are polymerized within lightly cross-linked polyacrylamide hydrogels. These composite hydrogels are photonic crystals in which the embedded CCA diffracts visible light and appears intensely colored. Volume phase transitions of the hydrogel cause changes in the CCA lattice spacings which change the diffracted wavelength of light. We functionalized the hydrogel with two coupled recognition modules, a creatinine deiminase (CD) enzyme and a 2-nitrophenol (2NPh) titrating group. Creatinine within the gel is rapidly hydrolyzed by the CD enzyme in a reaction which releases OH-. This elevates the steady-state pH within the hydrogel as compared to the exterior solution. In response, the 2NPh is deprotonated. The increased solubility of the phenolate species as compared to that of the neutral phenols causes a hydrogel swelling which red-shifts the IPCCA diffraction. This photonic crystal IPCCA senses physiologically relevant creatinine levels, with a detection limit of 6 μM, at physiological pH and salinity. This sensor also determines physiological levels of creatinine in human blood serum samples. This sensing technology platform is quite general. It may be used to fabricate photonic crystal sensors for any species for which there exists an enzyme which catalyzes it to release H+ or OH-.