Article Cite This: J. Phys. Chem. C 2018, 122, 3648−3654
pubs.acs.org/JPCC
Concentration-Polarization-Induced Precipitation and Ionic Current Oscillations with Tunable Frequency Elif Turker Acar,†,‡ Preston Hinkle,† and Zuzanna S. Siwy*,†,§,∥ †
Department of Physics and Astronomy, §Department of Chemistry, and ∥Department of Biomedical Engineering, University of California, Irvine, California 92697, United States ‡ Department of Chemistry, Faculty of Engineering, Istanbul University, 34320 Avcılar-Istanbul, Turkey S Supporting Information *
ABSTRACT: At the nanoscale, charges present at the surfaces of liquid−solid interfaces greatly influence the properties of ions and molecules present in the solution, and can lead to nanoscale effects such as ion selectivity, ion current rectification, and modulation of local ionic concentrations. Concentration polarization is another nanoscale phenomenon whereby ion concentrations are enriched at one opening of an ion-selective nanopore and depleted at the other. We show that when a nanopore is in contact with a weakly soluble salt present at a concentration below its solubility product, concentration polarization can lead to locally enhanced ionic concentrations and precipitation of the salt. Formed precipitates partially or fully occlude the nanopore’s opening as indicated by a measured transient decrease of the nanopore’s conductance. We have identified experimental conditions at which the locally created precipitate is either pushed through or dissolved, clearing the pore entrance and allowing the precipitation reaction to occur again. The dynamic process of precipitate formation and dissolution is observed as ion current fluctuations and oscillations with frequencies reaching 200 Hz. The frequency of the system operation exceeds other nanopore-based oscillators by 2 orders of magnitude, which we believe stems from the 30 nm length of the pores examined here, versus ∼10 μm long pores reported before.
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INTRODUCTION Single nanopores have been used in research focused on fundamental aspects of transport in nanoconfined volumes.1 Nanopores also became the basis of biological sensors, with nanopore-based DNA sequencing likely the most well-known application of applied nanopore research.2−5 Other types of nanopore sensors were developed as well, which utilized the sensitivity of the transmembrane current to the surface properties of the pore wall and the ultrasmall volume of nanopores.5−10 Nanopores can also be treated as femtoliter test tubes in which chemical reactions involving small quantities of material can occur often in highly dynamical conditions.11 When an electric field is applied across the membrane, some reactions are affected by the processes of electrophoresis and electro-osmosis,1 which transport charged and neutral species through the pore; these reactions can lead to nonlinear effects in observed transport properties. As an example, single conically shaped nanopores in polymer films12−14 and glass nanopipettes15 were shown to exhibit oscillatory ion current in conditions at which precipitation of a weakly soluble salt could occur. The pores were placed in contact with a high concentration of background electrolyte such as KCl, containing a calcium- or cobalt-based salt at concentrations at least an order of magnitude lower than the salt solubility product.12 Voltage-induced local modulations of ionic concentrations in the pore led to enhancement of ionic concentrations © 2018 American Chemical Society
of all ions so that precipitation occurred at the small opening of the pore observed as a current blockage. At a sufficiently high transmembrane potential, the pore after being blocked would open up for ionic transport either due to the precipitate dissolution or via clearance of the solid precipitate by electroosmotic flow.15 The cyclical process of precipitate formation and clearance led to oscillations of ion current and formation of the fastest reported electrochemical oscillator operating at frequencies of a few Hz.12,13,16,17 Another nanopore system, which exhibited ion current instabilities and oscillations, included a glass pipet placed in contact with dissimilar electrolytes, for example, aqueous salt solution on one side and a salt solution in an organic solvent on the other side.18,19 The precipitation and electrolyte gradient-induced current instabilities have so far been mainly observed in highly asymmetric conical pores, which rectified the current.20 Rectification of ion current at the nanoscale is linked to voltage modulation of ionic concentrations, and enhancement of ionic concentrations for one voltage polarity.21,22 Ion current fluctuations and oscillations were also reported for other nanopore systems, but resulted from different mechanisms such as hydrophobic gating, thus caused by spontaneous evaporation Received: December 13, 2017 Revised: January 23, 2018 Published: January 24, 2018 3648
DOI: 10.1021/acs.jpcc.7b12265 J. Phys. Chem. C 2018, 122, 3648−3654
Article
The Journal of Physical Chemistry C
Figure 1. (a) Current−voltage curves of a 15 nm in diameter nanopore in 100 mM KCl without and with 0.1 mM CoCl2 in the presence of 2 mM PBS buffer, pH 8. (b) Ion current time series for the same nanopore in 0.1 M KCl, 2 mM PBS buffer without the cobalt salt for positive voltages. (c,d) The same as in (b) but in the presence of 0.1 mM CoCl2, for positive and negative voltages.
initial current increase or “breakdown” and the voltage shut-off time is changed, the pore’s resistance, related to its diameter, can be accurately tuned.28,29 The dielectric breakdown was performed with 12 V applied with two Ag/AgCl pellet electrodes. Figure S1d shows the predicted dependence of the pore diameter on the current increase, ΔI, recorded at 12 V after the dielectric breakdown had occurred. Pore-opening diameter was subsequently measured through current−voltage recordings, and relating the system’s resistance (determined as inverse slope of I−V curves) with the nanopore geometry and access resistance;32 the I−V curve measurements were performed in the voltage range between −1 and +1 V (Figure S1e). Because of a high KCl concentration of 1 M and acidic conditions of pH 1.7 used for the pore preparation and sizing, we assumed that the conductivity of the medium in the pore was equal to the conductivity of the bulk solution. We found close agreement between the predicted size of the pore determined by the breakdown current and the final measured diameter of the pore determined through I−V curves (Figure S1d). Recording of Ion Current in Time. Recordings of ion current were performed with Axopatch 200B and Digidata 1322A (Molecular Devices, Inc.) using sampling frequency of 10 kHz. All signals were filtered with low-pass Bessel filter of 1 kHz. Two pellet Ag/AgCl electrodes were used for current measurements. Recordings were performed in 100 mM KCl, buffered to pH 8 with potassium based phosphate buffer (PBS). Some solutions, as indicated in the text, also contained 0.1 or 0.4 mM CoCl2. Numerical Modeling. Numerical solutions of the Poisson−Nernst−Planck equations were found using the commercially available Comsol Multiphysics 4.3 package.36 The mesh was adjusted to ensure convergence of the model to the point when no change in the observed concentration profiles and currents was observed upon further mesh decrease. The diffusion coefficient for potassium and chloride ions was assumed to be 2 × 10−9 m2/s. Diffusion coefficients of Co2+ and ions from the phosphate buffer (H2PO4− and HPO42−) were taken as 1 × 10−9 m2/s.
of water or/and formation of nanobubbles.23−27 In this article we describe 30-nm-long nanopores in silicon nitride, which can produce current oscillations and instabilities via precipitation induced by concentration polarization. We considered nanopores with opening diameters between ∼2 and 15 nm prepared by the process of dielectric breakdown.28,29 All structures examined exhibited either Ohmic behavior or weak rectification, in agreement with recent report on nanopores prepared by the dielectric breakdown process.30 Because of low-aspect ratio of the pores, ionic concentrations in a pore and its entrance are enhanced via concentration polarization on one side of the membrane,31 making precipitation possible. We found the amplitude of current fluctuations and oscillations was strongly dependent on the pore-opening diameter. The experiments presented in this article show frequencies of ion current oscillations that reach even 200 Hz, 2 orders of magnitude higher than what was previously observed with nanopore oscillators.12−15 The fact that our shorter nanopores exhibit much higher oscillation frequencies compared to the longer pores suggests that pore length provides yet another tuning parameter for the behavior of the system.
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EXPERIMENTAL AND METHODS SECTION Pore Preparation. All nanopores presented in this article were obtained by a process of dielectric breakdown as reported before.28,29 Figure S1a of the Supporting Information shows our experimental setup: a 30-nm-thick silicon nitride film (SPI Supplies, 0.05-mm-width window) was placed between two chambers of a homemade conductivity cell. The process of dielectric breakdown was shown before to occur when both chambers were filled with 1 M KCl either of strongly acidic or basic pH, or when there was a pH gradient established across the film.29 For a 30-nm-thick silicon nitride film we found the acidic conditions of pH 1.7 (adjusted with 1 M HCl) rendered the pore size most controllable. In the dielectric breakdown process, a transmembrane potential is applied until a point at which the measured ionic current suddenly increases (Figure S1b,c); this increase has been shown to correspond to the formation of single nanopores. When the time between the 3649
DOI: 10.1021/acs.jpcc.7b12265 J. Phys. Chem. C 2018, 122, 3648−3654
Article
The Journal of Physical Chemistry C
Figure 2. Recordings of ion current through a single silicon nitride nanopore with an opening diameter of 2.5 nm in the presence of 0.1 M KCl, with 0.4 mM CoCl2 and 2 mM PBS buffer. (a) Overview of time series recorded in the voltage range between −2 V and +2 V. (b) Example recording for 1 V with zoomed-in window in (c).
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with electric field would lead to concentration polarization.31 Silicon nitride nanopores were indeed shown to be negatively charged due to the presence of silanol groups.34,35 As expected, increasing the cobalt concentration to 0.4 mM CoCl2 increased the amplitude of the ion current oscillations/fluctuations, from ∼7% of the baseline current in 0.1 mM to ∼15% in 0.4 mM CoCl2 (Figure S2) at 1 V. We hypothesize that the lower conductance states were recorded with a precipitate partly occluding the pore; the higher conductance states would occur when the precipitates would dissolve or detach from the pore opening, lowering the system resistance. The type of ion current oscillations that were induced by the presence of the cobalt salt and phosphate buffer were however mostly influenced by the pore-opening diameter. The current fluctuations seen in the smallest pores studied (
