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Glucose-selective optical sensors were fabricated by incorporating 3-phenylboronic acid and a tertiary amine, dimethylaminopropylacrylamide, into a hydrogel matrix. Determination of glucose in solution is based on the glucose-induced contraction of the hydrogel. The gel was fabricated on the end of an optical fiber, and the optical length was measured by an interferometric technique. Previously it was found the gel could be tuned for enhanced glucose sensitivity and selectivity by varying the 3-phenylboronic acid/tertiary amine ratio. The properties of the responsive hydrogel as a glucose sensor were determined in more detail with respect to swelling kinetics and equilibrium swelling degree. Temperature effects, size variation, molecular interference, and reversibility were addressed. Results showed there was a good degree of reversibility, both for equilibrium swelling and swelling kinetics. Fabricated hydrogel sensors with slight differences in size yielded an overlapping relative response indicating an excellent degree of sensor reproducibility. The sensor proved to be temperature-dependent; by increasing the temperature from 25 to 37 degrees C, the swelling was about 4-fold more rapid, and a concomitant decrease in equilibrium swelling was seen. Identified interference from other analytes with determination of glucose was used a basis for selecting ethylenediaminetetraacetic acid (EDTA) as an anticoagulant for in vitro determination of glucose concentration in blood plasma. Glucose measurements performed in blood plasma were promising, showing that the sensor is capable of measuring physiological glucose levels in blood with a minimal effect from interfering molecules. The obtained results indicate that the developed sensor is a candidate for continuous monitoring of glucose in blood.

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.

Development of an oligonucleotide functionalized hydrogel integrated on a high resolution interferometric readout platform capable of determining changes in optical length of the hydrogel with 2 nm resolution is described. The hydrogels were designed with hybridized dioligonucleotides grafted to the polymer network making up a network junction point in addition to the covalent cross-links. The hybridized dioligonucleotide network junctions were made with a 10 basepair complementary region flanked by additional basepairs that could aid in destabilizing the junction points in competitive displacement hybridization by the added probe oligonucleotides. The probe oligonucleotide destabilizing the junction point thus induces swelling of the functionalized hydrogel that is sensitive to the concentration of the probe, the sequence, and matching length between the probe and sensing oligonucleotide. This design yields a molecular amplification of the change in the optical length of the gel at least 5-fold compared to a hydrogel where sensing functionality is based on hybridization with a grafted oligonucleotide that is not a part of a network junction. Concentration sensitivity applied for specific label-free detection of oligonucleotide is estimated to be in the nanomolar region. Applications of the resulting oligonucleotide imprinted hydrogel for label-free sensing of probe oligonucleotide sequences or taking advantage of the oligonucleotide sequences designed with aptamer functionalities for determination of other types of molecules are discussed.

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