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, 9 (2), 219-31

Experimentally and Theoretically Observed Native pH Shifts in a Nanochannel Array

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Experimentally and Theoretically Observed Native pH Shifts in a Nanochannel Array

Danny Bottenus et al. Lab Chip.

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 (approximately 3 mM) the mean pH shift is approximately 1 pH unit whereas at high ionic strengths (approximately 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 zeta-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.

Figures

Fig. 1
Fig. 1
Structure of the EDL and the potential distribution as a function of distance from the channel wall. In the Stern layer, immobilized solvated positive ions are attracted to the negative surface charge from the dissociated silanol groups. In the diffuse layer, which is similar to the bulk except electroneutrality does not exist, there is an influx of positive solvated ions compared to negative solvated ions. Together, the Stern layer and diffuse layer make up the EDL which has a length defined by the Debye length, k−1, and also a potential difference, ψ, across the region.
Fig. 2
Fig. 2
Cross-sectional SEM images of nanochannels. (a) Cross-sectional images of nanochannels after plasma etching. The Si substrate is demarcated by the arrows. (b) Cross-sectional images after thermal oxidation showing the 100 nm thick SiO2 phase on sidewalls and bottom of the nanochannel. (c) Cross-sectional images after anodically bonding Pyrex coverslip to nanochannel array. The Pyrex coverslip is shown by the region above the dotted line and the Si/SiO2 region below the dotted line.
Fig. 3
Fig. 3
Schematic of domain solved using Comsol v3.4. The domain includes two bulk reservoirs where the concentrations of species are equal to their bulk value, the nanochannel region, and a phase of thermal SiO2 on each side of the nanochannel. B1–B12 represents the boundaries. Each SiO2 phase and the nanochannel region have a thickness of 100 nm.
Fig. 4
Fig. 4
(a) Chemical structure of protanated and deprotanated 5-(and-6)-carboxy SNARF®-1 drawn in ChemDraw Ultra 10.0. Note: In the protanated form there is an electrostatic positive region and an electrostatic negative region. (b) Chemical structures of fluorescein based on different pH. Fluorescein is a cation at pH < 2.08, neutral at 2.08 < pH < 4.31, an anion at 4.31 < pH < 6.43, and a dianion at pH > 6.43. (c) Chemical structure of Rhodamine B which will be discussed in the Results section. Note: The similarities between the Rhodamine B and SNARF molecule.
Fig. 5
Fig. 5
(a) Experimental setup. An argon ion laser transmits a 488 nm beam into the nanofluidic device. SNARF solution in the nanochannel array is excited and the emission spectrum is collected by an optical fiber coupled to a UV/VIS spectrophotometer. (b) Experimental setup to monitor pH shift in nanochannels with fluorescein, using MIR-FTIRS.
Fig. 6
Fig. 6
Nanochannel array filling by capillary action. At t = 0 s, the nanochannel is being filled with buffered solution. At t = 15 s, the fluid has traveled approximately half way through the nanochannel array and at t = 30 s, the fluid has completely filled the nanochannel array. Note: The boron gate electrode on the top-center part of the chip which could be used for pH modulation.
Fig. 7
Fig. 7
An example of fluorescence emission spectrum of 5-(and-6)-carboxy SNARF®-1 measured in solutions of various pH used for the calibration curve in Fig. 9. Based on the intensity ratio (R) of 580 nm to 640 nm, the mean pH can be determined from eqn (31). An isosbestic point is observed at approximately 610 nm indicating the formation of two different species above and below 610 nm.
Fig. 8
Fig. 8
Calibration curve of the pH dependence of 5-(and-6)-carboxy SNARF®-1. The ratio (R) of fluorescence intensities at 580 nm and 640 nm was measured in a divot in a glass slide covered with a Pyrex coverslip for different pH values at the same ionic strength. The calibration curve was then used to determine the pH in the nanochannels.
Fig. 9
Fig. 9
A comparison between experimentally observed pH shift and that predicted by a ζ-potential based model. The (■) data points are the raw experimental data obtained indirectly from the spectrophotometer and the (•) data points are the experimental data that have been corrected to account for where the SNARF molecule is located in the channel and the local pH at that position. The model undershoots the experimental data by approximately a factor of two to four for most ionic strength solutions.
Fig. 10
Fig. 10
(a) Cross-sectional plot of the potential profile across the nanochannel. x = 0 is the center of the channel. As the ionic strength decreases, the potential increases in the SiO2 phase and eventually decays to 0 at the center of the nanochannel. The overshoot that is seen is most likely due to numerical noise in the simulation. (b) Numerical simulation of the pH profile with an ionic strength of ~3 mM. The pH profile strongly shows the EDL in the nanochannel. In the SiO2 phase the pH is low because hydronium ions are attracted to the active silanol groups. As you move away from the SiO2 phase the pH rapidly drops off to its bulk value of pH 7.4. (c) Cross-sectional plot of the pH profile across the nanochannel. As the ionic strength decreases, the pH approaches its bulk value.
Fig. 11
Fig. 11
Experimental to modified simulation comparison of the pH shift. Both sets of experimental data have been corrected to account for where the SNARF molecule is located in the channel and the local pH at that position. The raw experimental data from the spectrophotometer can be found in Fig. 9. Without SNARF binding the experimental results indicate a pH shift slightly higher than what the model predicts. With SNARF binding and a K = 0.05 parameter from the Langmuir isotherm, the experimental results match the model. The MIR-FTIR analysis and fluorescein data (3 data pts) is also shown here and agrees with the model predictions except at low ionic strength.
Fig. 12
Fig. 12
MIR-FTIR analysis of fluorescein in pH 4 and pH 8 buffer solution in nanochannels.
Fig. 13
Fig. 13
The calibration curve that is plotted by the intensity of 1580 cm−1 to 1600 cm−1 in different pH buffer solutions from pH 4 to 8.

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