ACS PRF | ACS
All e-Annual Reports
44161-GB5
Photopolymerization by Evanescent Waves and Spectroscopic Study of Mechanochemical Response of the Polymer Network on an Optical Fiber
The major objective of this study is to fabricate a polymer network of nanometer thickness on the cylindrical side-surface (not the distal end) of an optical fiber core. A such environmentally responsive polymer network of nanometer thickness (a prerequisite for a supported 2D-single macromolecule) is of great potential for interfacing between living matter (cells, organelles, and their liquid media) and inorganic electronic and spectroscopic probes (microchips) and for detecting microscopic biochemical processes if network is specifically modified or combined with other biomimetic layers (membrane).
Using the light guided by optical fiber as a source of optical energy, polymerization on the side-surface of a fiber core has been initiated by evanescent waves. Since we have already proved the main concept of the project, our efforts have been concentrated on the following subgoals during the first year: optimization of polymerization parameters to obtain polymer network film of reproducible thickness and shape; design a spectroscopic scheme to provide a method for the nanofilm detection and its thickness control; polymerization within the interior of a soft microreactor formed by phospholipid bilayer to understand the potentials and limitations of fabricating multilayered hydrogel/membrane structures on the side surface of fiber core; and proton transport in and out of synthetic (hydrogel microparticles) and natural (bacterial spores) ionic reservoirs to understand how a fiber core itself may serve as a spectroscopic probe for swelling/de-swelling ability of ionic nanofilm in different environments.
In order to control photopolymerization, the composition of the hydrogel forming solution, the proper photoinitiator, and the transparence of fiber optics at the wavelength of light source were optimized using bulky hydrogel preparation procedure as a model. Hydrogel forming solutions containing N-isopropylacrylamide (monomer), methylenebisacrylamide (cross-linker), and photoinitiator with concentration ranged from 0.01 to 5 mol% were prepared and tested. Six photoinitiators exhibiting the maximum of absorbance at different wavelengths were examined: 2,2-dimethoxyacetophenone, 2,2-diethoxy-2-phenylacetophenone, 4,4-bis(dimethylamino)benzophenone, 4,4-bis(diethylamino)benzophenone, 4-(dimethylamino)benzophenone, and camphorquinone. The evidence of polymerization was obtained for two photoinitiators: the changes in integral intensity and spectral composition of the light passing through the fiber follow the polymerization kinetics.
A unique reactor-container was designed allowing us to control the polymerization process both spectroscopically (absorbance and fluorescence) and microscopically. Kinetics of polymerization using UV-VIS Spectroscopy revealed that the time of polymerization (the thickness of hydrogel layer) depends on the type and concentration of photoinitiator used.
To provide polymerization within microscopic volumes surrounded by lipid bilayers, the giant unilamellar vesicles (GUV, ~5-300 µm) have been used as soft microreactors. Two methods for preparation of concentrated suspensions of giant vesicles were examined: gentle hydration and solvent evaporation. The “liposomes-within-giant-vesicle” structures have been obtained and visualized by optical microscopy. The effect of alcohol on the elasticity of lipid bilayers of giant vesicles was studied. GUVs prepared by the solvent evaporation method were filled with a hydrogel forming solution and exposed to UV light to provide polymerization inside the GUV microreactors. The giant lipobeads were eventually fabricated. The ionic- and temperature-sensitivities of poly(N-isopropylacrylamide-co-1-vinylimidazole) (PNIPA-VI) hydrogel was entrapped inside GUVs. Temperature- and ionic-sensitivity of the lipobeads and microgels thus prepared are under characterization.
Two types of hydrogels were studied: PNIPA-VI is sensitive to pH and temperature and PNIPA is sensitive only to temperature. Hydrogel particles of micrometer size (~20-200 µm) were prepared.
The time-resolved measurements of pH external to the hydrogel particles in aqueous suspensions of different concentrations were performed. Time-variations of pH in the exterior to the PNIPA-VI particles revealed that the hydrogel releases and consumes protons in response to the changes in temperature from 25 to 37C.
Time-resolved pH measurements were also carried out in suspensions of natural gel-like multilayered structures (bacterial spores). Proton uptake kinetics was a multi-step process involving a number of successively ~10-fold slower steps of proton penetration into the bulk and their binding to the ionizable groups within different layers. Almost infinite ionic reservoir was capable of accumulating billions of protons (N~2x10^10 per spore). Carboxyl groups were discovered to be the major ionizable groups fixed in a spore matrix. Proton equilibrium binding within the spore matrix obeys the fundamental law of the Langmuir isotherm. The effective diffusion coefficient for hydrogen ions within the spore core was up to 3 orders of magnitude lower than that within the other spore compartments indicating that the inner membrane is a major permeability barrier for protons.
Three oral talks have been presented by the undergraduate students involved into these studies at 55th ACS Undergraduate Research Symposium (Manhattan College, May 2007): Dwight Campbell'08, The concept of polymerization by evanescent waves on the side-surface of an optical fiber. Jessica Sproul'07, Hydrogels and dormant spores are ionic reservoirs. Andrea Aquilato'07, Giant liposomes and lipobeads.
One manuscript has been submitted to the Journal of Physical Chemistry. Acknowledgements are made to the Donors of the American Chemical Society/Petroleum Research Fund for support of this research.
