Concentration Polarization Induced Precipitation and Ionic Current

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Concentration Polarization Induced Precipitation and Ionic Current Oscillations with Tunable Frequency Elif Turker Acar, Preston Hinkle, and Zuzanna S. Siwy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12265 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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The Journal of Physical Chemistry

Concentration Polarization Induced Precipitation and Ionic Current Oscillations with Tunable Frequency Elif Turker Acar,1,2 Preston Hinkle,1 Zuzanna S. Siwy1,3,4* 1

Department of Physics and Astronomy, University of California, Irvine

2

Department of Chemistry, Faculty of Engineering, Istanbul University, Avcılar-Istanbul, Turkey 3

Department of Chemistry, 4Department of Biomedical Engineering, University of California, Irvine, CA 92697

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 two orders of magnitude, which we believe stems from the 30 nm length of the pores examined here, versus ~10 µm long pores reported before.

*

Corresponding Author: [email protected], Tel. 949-824-8290

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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 ultra-small 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 electroosmosis,1 which transport charged and neutral species through the pore; these reactions can lead to non-linear 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 a 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 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 few Hz.12,13,16,17 Another nanopore system, which exhibited ion current instabilities and oscillations, included a glass pipette placed in contact with dissimilar electrolytes, e.g. 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 2 ACS Paragon Plus Environment

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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 of water or/and formation of nanobubbles.

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In this

manuscript 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 nm 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 Due to 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 manuscript show frequencies of ion current oscillations that reach even 200 Hz, two 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 the manuscript were obtained by a process of dielectric breakdown as reported before.28,29 Figure S1a shows our experimental set-up: a 30 nm thick silicon nitride film (SPI Supplies, 0.05 mm width window) was placed between two chambers of a home-made 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. By changing the time between the initial current increase or ‘breakdown’ and the voltage shut off time, 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, ∆I, 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 -1V and +1V (Figure S1e). Due to 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 4 ACS Paragon Plus Environment

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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 mM 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 assure 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 210-9 m2/s. Diffusion coefficients of Co2+ and ions from the phosphate buffer (H2PO4and HPO42-) were taken as 110-9 m2/s.

Results and Discussions Figure 1 shows example I-V curves of a nanopore in a 30 nm thick SiN film prepared by the process of dielectric breakdown.28,29 The pore had an opening diameter of 15 nm, as predicted from ion current measurements performed in 1 M KCl directly after the breakdown occurred (Figure S1b-e). The pore exhibited weak rectification properties with positive currents larger than negative currents. The SiN nanopore featured stable ion current signals in time studied in the voltage range between -1V and +1V (Figure 1b,c) in 100 mM KCl, PBS buffer pH 8; 60 or 100 s time series was recorded at each voltage. Addition of 0.1 mM CoCl2 in the presence of 2 mM phosphate buffer, pH 8, however induced ion current fluctuations and oscillations, which became especially pronounced for positive voltages above 200 mV. Note that the ion current fluctuations occurred for the voltage polarity which produced higher currents before adding cobalt (Fig. 1a). We hypothesized that the current instabilities observed could be caused by tiny precipitates of CoHPO4 formed in the pore or/and its entrance as a result of local enhancement of ionic concentrations; note that the pore volume is of the order of 1023

m3. In the bulk, the product of cobalt and HPO42- concentrations remained below the

salt CoHPO4 solubility constant (10-6 M2),33 and indeed all prepared solutions studied here were clear with no sign of precipitates visible. We believe the local increase of ionic concentrations could originate from the finite surface charge of the pore wall, which in combination with electric field would lead to concentration polarization.31 Silicon 5 ACS Paragon Plus Environment


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.

(a)

(b)

(c)

(d)

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.

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 (

Concentration Polarization Induced Precipitation and Ionic Current

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