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Issue 2, 2009
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Experimentally and theoretically observed native pH shifts in a nanochannel array

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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 SiO2 phase, and in the case of the SNARF molecule, able to bind to neutral regions of the SiO2 phase agrees quantitatively with experimental results.

Graphical abstract: Experimentally and theoretically observed native pH shifts in a nanochannel array

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Publication details

The article was received on 26 Feb 2008, accepted on 17 Sep 2008 and first published on 24 Oct 2008

Article type: Paper
DOI: 10.1039/B803278E
Lab Chip, 2009,9, 219-231

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    Experimentally and theoretically observed native pH shifts in a nanochannel array

    D. Bottenus, Y. Oh, S. M. Han and C. F. Ivory, Lab Chip, 2009, 9, 219
    DOI: 10.1039/B803278E

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