Potentiometric Investigation of Protonation Reactions at Aqueous−Aqueous Boundaries within a Dual-Stream Microfluidic Structure

Abstract

The laminar flow regime prevailing in pressure-driven flow through a Y-shaped microfluidic channel was utilized to create a stable boundary between two aqueous liquids. Transverse transport of ions between these two liquids gave rise to a diffusion potential, which was monitored by measurement of the open circuit potential. In this report, the influence on the cross-channel potential distribution of protonation reactions occurring in the boundary zone between the two co-flowing liquids is presented. The proton source was present in one of the co-flowing streams, and an uncharged proton acceptor was present in the other aqueous stream. The time-dependent transport equation for diffusion and migration was augmented by chemical reaction terms and was solved for all species present in both streams as a theoretical basis for the analysis. Within this model, the system was assumed to be homogeneous along the channel height, and effects of nonuniform velocity profiles were neglected. A reduction in potential by several millivolts was predicted for a protonation reaction occurring close to the boundary between the two aqueous streams, provided that the mobility of the protonated species was lower than the mobility of the co-cation in the background electrolyte (alkali metal cation in this case). The magnitude of the decrease in the potential was greater for protonated molecules with lower mobility or if the mobility of the background electrolyte cation was increased. Experimental results are presented for imidazole and D-histidine as proton acceptors present in 10 mM KCl, 10 mM NaCl, or 10 mM CsCl solution and co-flowing with a stream of 10 mM hydrochloric acid, which served as the proton source. Decreases in measured potential, in line with the predicted diminished potential, were obtained.

Several ways to simulate time dependent liquid junction potentials by finite differences

“…It could be a membrane between two well stirred solutions. Experimentally, the situation of a watery boundary can also be be realised in microchannels, involving dual-stream laminar flow of two aqueous solutions along each other carrying different electrolyte salts at various concentrations [18][19][20][21][22] forming a flowing junction [17]. potential using the Henderson equation [20] and numerical solutions of the Nernst-Planck equation [21,22] and the measured open-cicuit potentials has been found.…”

Pharmaceutical modulation of diffusion potentials at aqueous–aqueous boundaries under laminar flow conditions

“…This enabled in situ measurement of potentials in the microfluidic cell using off-chip electrodes and hence simplified the fabrication procedure compared to a previous device we studied that employed integrated electrodes [31]. We recently extended this to the measurement of chemical interactions occurring at the aqueous-aqueous boundary [32,33].…”

Compatibility of organic solvents for electrochemical measurements in PDMS-based microfluidic devices

“…In this way, the electrochemical microfluidics has been used to lower detection limits in electrochemical sensing devices (Pumera et al 2006;Swensen et al 2009), to improve the performance of biofuel cells (Kjeang et al 2009) and in fundamental research like determination of electrocatalytic (Dumitrescu et al 2012) or enzymatic reaction kinetics, (Han et al 2009) investigation of molecular interactions (Strutwolf et al 2010) or studying electrochemically modulated charge transfer processes at soft interfaces between two co-flowing liquid phases (MacDonald et al 2007;Kaluza et al 2013Kaluza et al , 2014.…”