(505c) Electrokinetics of Heterogeneous Ion-Exchange Membranes

Ion selective systems [1] such as nanochannels, ion exchange particles or ion exchange membranes exhibit very interesting electrokinetic behavior that is increasingly used in microfluidic systems for applications related to point-of-care diagnostics such as sample pretreatment, biomolecule preconcentration or sensing [2]. All these systems are characterized with a nonlinear current voltage curve which shows three distinctive regions denoted as: (i) underlimiting, (ii) limiting and (iii) overlimiting. We showed that some heterogeneous ion exchange membranes can interact with specific biomolecules possessing the ability to change the current voltage curves in the limiting and overlimiting region above all. Based on this membrane response to the presence of the biomolecules, we developed a sensor for specific detection of single stranded nucleic acids. Interestingly, the response of ion exchange particles which are functional parts of heterogeneous membranes, to the same molecules did not result in quantitatively same behavior. This observation raised a question about particular features of the membranes, especially those related to the structure of the membrane, that render the membrane sensitive to the biomolecules.

To investigate the effect of membrane structure on the exhibited behavior, we developed a system that integrates a small piece of ion exchange membranes and allows to (i) reconstruct swollen heterogeneous membranes by means of micro-computed tomography and (ii) study their electrochemical and electrokinetic behavior when connected in DC field. In this contribution, we will show the effect of ionic strength of water solutions on swelling and shrinkage of membranes and associated irreversible changes in the membrane structures. This will be followed by experimental analysis of the electrokinetic behavior of these membranes when connected in overlimiting region and discussion about the role of structure in appearance of this region and thus sensitivity to the presence of biomolecules.

[1] Slouka, Z., Senapati, S., Chang, H.C., Microfluidic systems with ion-selective membranes, Annual Review Analytical Chemistry, 2014; 7:317-335

[2] Senapati, S., Slouka, Z., Shah, S. et al. An ion-exchange nanomembrane sensor for detection of nucleic acids using a surface charge inversion phenomenon, Biosensor&Bioelectronics, 2014, 60:92-100