Assessing the hydration state of agarose-based hydrogels using terahertz spectroscopy

Agarose is a gel-forming polysaccharide extracted from marine red algae. This biopolymer is an important analytical separation medium, stabilizer and thickener in the food and pharmaceutical industries, and phantom material for biomedical research. The distribution of water to the different hydration zones in the gel can significantly influence the functional properties of agarose-based hydrogels. In this work, the application of terahertz spectroscopy in assessing the hydration state of biomolecules was extended from aqueous solutions and lipid-water emulsions to polysaccharide gels. The agarose gel system, whose structure is constituted by water confined in a complex network of polymer assemblies, was investigated. THz time-domain spectroscopy (THz-TDS) measurements were performed in transmission mode. The behavior of the absorption and dielectric loss spectra of the gels relative to the polymer concentration indicate that the agarose network acts as a kosmotropic agent that favors the formation of the regular hydrogen bond structure. The hydration number equation of Hishida and Tanaka was modified to account for heterogeneity and confinement effects using effective medium theory. The hydration number of the agarose monomers was found to decrease with increasing polymer concentration. This trend in hydration number can be attributed to the increase in fiber density as a result of increasing agarose concentration which causes the thinning of the hydration shells and the sequestration of water molecules to the inside cavity of the agarose double helices.