Experimentally and theoretically observed native pH shifts in a nanochannel array

Abstract

Lab-on-a-chip (LOC) technology provides a powerful platform for simultaneous separation, purification, and identification of low concentration multicomponent mixtures. As the characteristic dimension of LOC devices decreases down to the nanoscale, the possibility of containing an entire lab on a single chip is becoming a reality. This research examines one of the unique physical characteristics of nanochannels, in which native pH shifts occur. As a result of the electrical double layer taking up a significant portion of a 100 nm wide nanochannel, electroneutrality no longer exists in the channel causing a radial pH gradient. This work describes experimentally observed pH shifts as a function of ionic strength using the fluorescent pH indicator 5-(and-6)-carboxy SNARF®-1 and compares it to a model developed using Comsol Multiphysics. At low ionic strengths (~ 3 mM) the mean pH shift is approximately 1 pH unit whereas at high ionic strengths (~ 150 mM) the mean pH shift is reduced to 0.1 pH units. An independent analysis using fluorescein pH indicator is also presented supporting these findings. Two independent non-linear simulations coupling the Nernst-Planck equation describing transport in ionic solutions subjected to an electric field and Poisson's equation to describe the electric field as it relates to the charge distribution are solved using a finite element solver. In addition, the effects of chemical activities are considered in the simulations. The first numerical simulation is based on a surface ζ-potential which significantly underestimates the experimental results for most ionic strengths. A modified model assuming that SNARF and fluorescein molecules are able to diffuse into the hydrolyzed SiO 2 phase, and in the case of the SNARF molecule, able to bind to neutral regions of the SiO 2 phase agrees quantitatively with experimental results.

Engineering multi-stage nanovectors for controlled degradation and tunable release kinetics

“…Previous reports have demonstrated that nanochannels can experience significant pH shifts due to properties of within the loading milieu [43,44]. In our case, the acidic nature of the carboxyl groups on QD can reduce the local pH within the pores resulting in slower dissolution of MSV, as observed in other pSi materials [31].…”

A method to measure pH inside mesoporous particles using protein-bound SNARF1 fluorescent probe

“…Recent studies [24] in silica slits showed that the pH reported by SNARF1 depends on its radial position in the nanometer channels, so the pH reported by SNARF1/BSA may well depend on its radial distribution in the MPS pores because they have similar dimensions (Fig. S-1).…”

On-demand in situ generation of oxygen in a nanofluidic embedded planar microband electrochemical reactor

“…The forward rate constants for the acid–base reactions were initially set to an arbitrary large value, and the reverse rate constants were calculated, with the constraint that their ratio produced the known equilibrium constant according to Eq. S1, similar to an approach used previously (Bottenus et al 2009). …”