Fabrication And Application Of Stimuli-responsive Photonic Hydrogels

Photonic crystals are composed of periodic dielectric nanostructures thataffect the propagation of electromagnetic waves (EM) in the same way as theperiodic potential in a semiconductor crystal affects the electron motion.Similarly to the X-ray diffraction in crystals, visible light can be diffracted inmaterials with periodic refraction index whose periods are comparable withthe wavelengths of visible light. The discovery and development of photoniccrystals have attracted much attention because of their bio-inspired functionsand highly stable color. The Science magazine has named photonic crystals asone of the six future research focuses in1999, indicating the broad prospectsof the research on phtonic crystals in21st century.Through the combination of photonic crystals and smart materials, theconcept of responsive photonic crystals was proposed. The diffracted light ofthe responsive photonic crystals varies with the external stimuli, which couldrealize the optical sensing to the external stimuli.Responsive photonic crystals have been extensively researched duringthe past decade, from3-D polymerized colloidal crystal array (PCCA), tomicrogel colloidal crystals, to inverse opal hydrogels films, to inverse-opalineheads and the recently2-D array photonic crystals. These methods have beenapplied to simple and fast sensing for analytes such as pH, electric field,pressure, ions and biomolecules, most of which were solvent, solute andphysical stimuli. To the best of our knowledge, most of these responsivephotonic crystals were based on weak polyelectrolytes and polymers of LCSTproperties, while few responsive photonic crystals based on strongpolyelectrolytes were reported. In addition, besides a brief report on oxygenphotonic sensor and some humidity sensors, almost no gas sensors based on photonic hydrogels were developed. Further more, as most of the polymericresponsive photonic crystals were based on hydrogel swelling and shrinkingmechanism, almost none of them responsed to the stimuli by the wettingphenomenon of the photonic crystal structures.According to the research status of the responsive photonic crystals, thisdissertation mainly focuses on developing three systems of the responsivephotonic crystals: the strong polyelectrolyte photonic hydrogels, gasresponsive photonic hydrogels and wettability-controlled inverse opalhydrogels. This dissertation can be further categorized into five parts:1) Strong polyelectrolyte inverse opal hydrogels were synthesized viacolloidal crystal template method. The experimental results showed that thesolvation degree of the counterions counterions, also known ashydrophobicity and hydrophilicity, dominated the sensitive polyelectrolytefilm to display different colors. The sensitive concentration for anions andcations sensing were10-4M-10-2M with the normalized wavelength changedby0.11and0.08, respectively. For amino acids sensing, the sensitiveconcentration was about10-2M with the normalized wavelength changed by0.06.2) Heavy metal ion sensitive photonic hydrogels were successfullyprepared by modifying specific ligands into the strong polyelectrolytescation-sensitive photonic hydrogels. The experimental results showed that thesensitive concentration for Cu2+, Pb2+and Ag+sensing were10-8M-10-3M,10-10M-10-3M,10-8M-10-4M with the normalized wavelength changed by0.110.13and0.065, respectively. Selectivities of the photonic hydrogels werebased on the complexation constants of the specific ligands. When the targetheavy metal ions were absorbed into the hydrogels, crosslinking could formvia coordination of the ligand with the heavy metal ions as well as the ionicgroups, resulting in the hydrogel shinking and the blue-shifts of the photonichydrogels.3) Gas-responsive inverse opal hydrogel for CO2were successfullyprepared using dimethyl aminopropyl methacrylamide as a functional monomer. This CO2gas sensitive photonic hydrogel can discriminate CO2over the whole concentration range (0-100vol%) by full-color response with0.15ml of gas sample, as well as zero interference from humidity. When thetotal amount of the gas mixture was increased to1mL, the sensitive regionfor CO2concentration changed to0-5vol%with the normalized wavelengthchange of0.30. The CO2response is mainly resulted from hydrogel swellingand shrinking, more precisely, by donnan potential caused by the increase offixed charge in the hydrogel and the change of polymer-solvent interactionparameter.4) A wettability sensing method based on stimuli-responsive polymericmaterials was proposed by controlling of hydrophobicity on the inverse opalhydrogels. The critical contact angle of22ofor wetting the inverse opalhydrogels was determined.5) Wettability-controlled inverse opal wetting sensing systems wereconstructed via the ion exchange process of the strong polyelectrolytephotonic hydrogels. Such wettability gradient photonic hydrogels showeddifferent sensitivity to different kinds of surfactants: zwitterionic andanti-charge surfactants had strong wettability on wettability gradient photonichydrogels while non-ionic surfactants and surfactants with the same chargehad little effects on the wettability degree of the photonic hydrogels. Furthermore,10-5M-10-4M small molecular weight thiols and NADPH weresuccessfully detected in aqueous solution utilizing responsive Geminisurfactants to control the wettability on the photonic hydrogels.