Time-domain NMR relaxometry as an alternative method for analysis of chitosan-paramagnetic ion interactions in solution

Chitosan (CHI)-paramagnetic ion interactions in aqueous solution as affected by pH were studied using time-domain NMR (TD-NMR) relaxometry. Longitudinal (T1) and transverse (T2) relaxation times of CHI solutions with Fe3+, Cu2+, and Mn2+ were measured using a single-shot pulse sequence named CP-CWFPx-x. The results indicate that CHI interacted with Fe3+ ions or iron oxide nanoparticles, maintaining the metal ion in solution at pH ranging from 3 to 5. Above this pH, CHI coagulated and removed Fe ions or nanoparticles, resulting in a clear supernatant. CHI probably interacted with Cu ions at pH 6.2 through the deprotonated CHI-NH2 groups maintaining the relaxation effect in alkaline solution, whereas Mn2+ did not interact with CHI strongly enough to produce a complex of high stability. The proposed method can also be further exploited in the study of interactions paramagnetic ions with others CHI and CHI derivatives as well as of other water-soluble polysaccharides.