Determination of swelling of responsive gels with nanometer resolution. Fiber-optic based platform for hydrogels as signal transducers.

A novel technique for detection of hydrogel swelling intended for use as a chemical or biological sensor, but also generally applicable for obtaining high-precision hydrogel swelling data, is described. The underlying design principle is that a hydrogel bound to the tip of an optical fiber constituting the environmental sensing element makes up a Fabry-Perot cavity for high-resolution detection of the optical length. The interference of light guided by the optical fiber and reflected at the two interfaces, fiber-gel and gel-solution, enables optical detection of the optical path length within the gel and degree of swelling of the gel. Acrylamide-based hydrogels with various molar fractions of the cationic monomer, N-(3-dimethylaminopropyl)acrylamide, were fabricated at the end of the fiber to demonstrate the feasibility of the approach. These sensors were investigated in solutions of varying ionic strength and pH. Relative gel length changes of the approximately 50-microm half-spherical gels were determined with a precision of approximately 2 nm. Moreover, the combination of good reproducibility and resolution of determination of swelling supports measurements of ionic strength changes in the millimolar range. Kinetic measurements for gel swelling induced by changes in ionic strengths had a time constant of approximately 2 s (half-spherical gel with 60-microm radius), whereas the time constants for gel swelling induced by changes in pH were observed in the range 90-130 s. Thus, different processes dictate the swelling rate in the two different cases. The results show that hydrogel equilibrium swelling and kinetics can be determined by the optical interference method with nanometer resolution, thus providing a unique platform for characterization of hydrogels swelling in general, and using functionalized hydrogels as biological sensors in particular.