Development of Photonic Crystal Hydrogel Sensors for Organophosphates and Ammonia

We developed several photonic crystal hydrogel sensors based on our Polymerized Crystalline Colloidal Array (PCCA) technology. Highly-charged colloidal particles ~ 100 nm in size self-assemble in low ionic strength solutions to form an electrostatically stabilized crystalline colloidal array (CCA). These CCA occur as face-centered cubic (fcc) lattice structures which Bragg diffract light in the visible, near IR and UV regions of the spectrum.The CCA ordering is preserved by locking it into place within polymer hydrogels, forming a polymerized crystalline colloidal array (PCCA). The Asher group utilizes the hydrogel's sensitivity for its environment to develop sensors for specific analytes by functionalizing the hydrogel with molecular recognition agents which are specific for an analyte. Analyte recognition causes volume phase transitions in the hydrogel which shrink or swell the network, changing the lattice spacing of the embedded CCA. The wavelength of Bragg diffracted light shifts corresponding to the concentration of analyte present.We developed sensors for organophosphorus (OP) nerve agents utilizing enzymes as molecular recognition agents. The first sensor utilizes the enzyme acetylcholinesterase, which irreversibly binds the OP, creating a charged species. The resulting Donnan potential swells the hydrogel and red-shifts the diffraction. The sensor functions as a dosimeter and displays ultra-trace detection levels (4 fM) for OP species in low-ionic strength media.The second OP sensor utilizes the enzyme organophosphorus hydrolase (OPH) and the pH-sensitive group 3-aminophenol as recognition agents. OPH hydrolyzes OPs at basic pH and produces protons. These protons in turn create a pH gradient inside the hydrogel which titrates the phenolates, lowering the free-energy of mixing and blue-shifting the Bragg diffracted light wavelength proportional to the OP concentration. The sensor is reversible, functions in high-ionic strength media, and has a 0.2 ìM OP detection limit in aqueous media.We also fabricated a sensor for ammonia which functions in human serum. Phenols on the hydrogel backbone are cross-linked through reaction of ammonia with added hypochlorite. The cross-linking causes an increase in the elastic constant of the hydrogel which forces the gel to shrink, blue-shifting the wavelength of Bragg diffracted light proportional to the concentration of ammonia present in solution. The sensor functions within the clinically relevant ammonia interval with a detection limit ~ 50 ìM NH3 in 1:1 serum/buffer solutions