The main objective of this research was to use potentiometric titration to characterize biomolecular conjugation density on the surface of large (diameter = 2.5 µm) poly N-isopropylacrylimde -co- acrylic acid (PNIPAM -co- AAc) microgel particles. Microgels are colloidal gel particles, of macromolecular networks that swell when exposed to a good solvent. They are typically synthesized with BIS [n,n-methylenebis (acrylamide)] crosslinker (2-10 molar %) which increases mechanical integrity for desired applications. Properties such as colloidal stability, solvent retention, and biocompatibility give rise to applications in biomedicine, personal care products, and separation technology. In biomedical applications, microgels must be surface functionalized with biomolecules to promote surface interaction among cells and other extra cellular matrix proteins. This work focused on using zero-length carbodiimide bioconjugation chemistry to attach albumin bovine serum (BSA) and cyanines to the AAc groups in the microgel particles. To evaluate the extent of conjugation, potentiometric titration was employed. Potentiometric titration is typically used to evaluate microgel charge density. In this work, the technique was used to characterize biomolecular attachment density by comparing the equivalence point of microgel suspensions before and after carboiimide conjugation. The microgels were conjugated via the EDC/sulfo-NHS bioconjugation reaction. Potentiometric titration experiments were carried out with 0.1N sodium hydroxide and 0.1N hydrochloric acid solutions, using an autotitration apparatus. The dose response curves collected showed a decrease in equivalence point with increasing biomolecule concentration during EDC/sulfo-NHS conjugation. The decrease indicates that less titrant was needed to neutralize the unconjugated AAc counter ions, as more biomolecule was conjugated to the microgel surface. Nuclear magnetic resonance (NMR) Spectroscopy verified evidence of conjugation. These results will help explain the unique phase behavior that microgels display, which will support better designed materials for future biomedical applications.

Lauryn Norris

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