(394g) Supporting Electrolyte Gradients in a 1 Mm DC Electric Field Microchannel

When an electric field is applied across an electrolyte solution, solute anions and cations electrophoretically migrate to the anode and cathode, respectively. As a concentration gradient starts to form, diffusional ion transport begins to counter electrophoretic motion, although this is potentially negligible since diffusional ion mobilities are an order of magnitude or more smaller than the electrophoretic mobilities achieved in microfluidic devices. The electric field catalyzes reactions at the electrodes in an aqueous medium; H⁺ and OH^- ions are generated at anode and cathode surfaces, respectively. The association/dissociation of solute species is another mechanism that affects ionic concentrations at a given locality. This especially becomes important when the abovementioned electrode reactions cause a pH gradient. In that case, the extent of buffer species dissociation varies spatially, in accordance with the pH gradient. Thus, the pK values of the solute species is a factor in determining the ion concentration gradients in microfluidic devices under an electric field. Microdevices typically contain <10 nL volumes such that movements of only 1 nanomole of ions can cause significant shifts in local concentrations. Recent results revealed that the H⁺ concentrations in a phosphate buffer can go from pH 8 to pH 3 in under 10 mins in a 100 V/cm electric field across a 1 cm microfluidic channel designed for protein iDEP, and under 1 min in a 3000 V/cm field in the same system. Such concentration changes can have a profound effect; for instance, it was discovered that the reported pH changes enabled protein iDEP by increasing particle size through protein aggregation.

The study presented here aims to complement the abovementioned work by elucidating the K⁺ and Cl^- concentration behaviors in similar electrokinetically driven microdevices. K⁺ is present as a supporting electrolyte and buffer ion in the phosphate buffer saline while Cl^- is present only as a supporting electrolyte. Therefore, due to the buffer association/dissociation mechanism mentioned above, it is expected that the changes in K⁺ and Cl^- concentrations do not mirror each other. K⁺ and Cl^- concentrations were quantified with fluorescence microscopy using potassium green and MQAE (n-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide), respectively. The electric fields were applied across a 1 mm gap between Pt thin film electrodes in a microfluidic channel. Fluorescence intensity profiles across the gap were analyzed at 535 (Potassium green) nm and 460 (MQAE) nm, then correlated to concentration profiles. 0.1 S/m phosphate buffer solutions were spiked with KCl at concentrations ranging from 0 to 100 mM and the ion concentration gradient formation was observed under 100 and 3000 V/cm electric fields. The ion concentration gradient formation behavior provides insight into the transient responses of microfluidic devices, the buffering reactions occurring dynamically within the devices, and underscores the substantial differences between the solutions loaded into devices and the rapidly changing solutions within electric fields.